Case 7 Flashcards
1
Q
what are the three roles of the nephron?
A
- Filtration – takes place in glomerulus – ball of capillaries at beginning of tubule
- Selective reabsorption
- Secretion – there are some substances, like potassium and hydrogen ions, that we need to get rid of a greater rate than filtration alone with accomplish, so those can be actively transported into the tubule fluid at the later stages of the nephron, to top up whatever has been filtered
2
Q
what are the functions of the kidney?
A
Maintenance of Extracellular Fluid Volume (ECFV) – sodium and water (therefore maintaining blood pressure) (normally amount of salt water you take in is same as what you lose – you’re in balance)
Acid-base balance regulation - therefore normally preventing acidosis/alkalosis
Excretion of metabolic waste – urea and creatinine (a waste product that comes from the normal wear and tear on muscles of the body – everyone has it in their bloodstream)
Endocrine secretion
- Renin-angiotensin system (for sodium regulation of blood pressure)
Erythropoietin (for RBC production and regulation) (centre for this because the kidneys have a very high demand for oxygen and therefore, they monitor blood oxygen levels)
Vitamin D (for calcium regulation) (calcitriol)
3
Q
what is the nephron divided up into? what does each section do?
A
Glomerulus - filtration (renal corpuscle = production of filtrate)
Proximal Convoluted Tubule – selective reabsorption of water, ions, and all organic nutrients
Descending Limb of Loop of Henle – further selective reabsorption of water
Ascending Limb of Loop of Henle – selective reabsorption of sodium and chloride ions
Distal Convoluted Tubule – secretion of ions, acids, drugs, toxins/ variable reabsorption of water sodium and calcium ions (under hormonal control)
Collecting Duct – variable reabsorption of water and reabsorption or secretion of sodium, potassium, hydrogen and bicarbonate ions
- Papillary Duct - delivery of urine to minor calyx
4
Q
what is the blood supply of the kidney like? (blood flow)
A
- The average cardiac output is 5 litres/min. The kidneys receive 20% of this (1 litre/min).
- The renal blood flow (RBF) is about 10-50 times greater than other the blood supply of other organs.
- RBF exceeds O2 requirements of kidneys (which reflects its function as a filter)
- RBF not regulated metabolically
5
Q
what is the primary means for eliminating waste products of metabolism? what are these products?
A
- The kidneys are the primary means for eliminating waste products of metabolism that are no longer needed by the body.
- These products include urea (from the metabolism of amino acids), creatinine (from muscle creatine), uric acid (from nucleic acids), bilirubin (from Hb breakdown), and metabolites of various hormones.
6
Q
apart from waste products of metabolism, what else is removed from the kidneys?
A
The kidneys also eliminate most toxins and other foreign substances that are either produced by the body or ingested, such as pesticides, drugs, and food additives.
7
Q
what does the glomerulus do? what can pass through it?
A
- The glomerulus allows for filtration of contents of the blood into the proximal convoluted tubule (PCT).
- Proteins larger than the size of albumin can’t pass into the PCT.
8
Q
what layers must fluid cross to get through glomerulus to proximal convoluted tubule?
A
Wall of glomerular capillary
Basement membrane
Inner layer of Bowman’s capsule
(Podocytes, Pedicels, Filtration slits)
9
Q
the glomerulus provides what kind of barrier? what does it allow through?
A
- The glomerulus provides a size and a charge barrier.
- It allows small positive molecules through.
- Large or negatively charged molecules are repelled.
- Size and charge barrier
- Size = anything bigger than albumin cannot pass through in the normal healthy glomerulus
- Water, electrolytes and other small molecules can pass through, but albumin is the cut off barrier
- Anything bigger than that – larger proteins, red blood cells – should not get through
- Charge = a layer on the extracellular matrix called glycocalyx – it contains a number of negatively charged ions so it will repel negatively charged ions within the plasma – but encourages positive ions to come through
- The other barrier is the extracellular matrix itself – it prevents larger material passing through
10
Q
what is the equation for GFR?
A
GFR = Kf . [P(GC) - (P(BC) + pi(GC))
11
Q
what factors affect the GFR?
A
Kf = filtration coefficient (remains constant)
P(GC) = glomerular capillary hydrostatic pressure (favours filtration)
pi(GC) = glomerular capillary oncotic pressure (opposes filtration)
Oncotic pressure is a form of osmotic pressure exerted by proteins, notably albumin, in a blood vessel that pulls water into the circulatory system.
P(BC) = Bowman’s capsule hydrostatic pressure (opposes filtration)
- Shouldn’t be a protein in the Bowman’s capsule to exert an oncotic pressure so not included in equation
= 3 forces working
12
Q
what is autoregulation?
A
the process by which the RBF and GFR are maintained despite changes in systemic pressure (blood pressure changes throughout day)
13
Q
does GFR change?
A
not without pathology
14
Q
what happens to vascular resistance when blood pressure increases? and of what? what does this do?
A
when the blood pressure increases, the vascular resistance of the afferent arteriole increases too
- this maintain the RBF and the GFR
- As renal arterial pressure increases, the resistance of the afferent arteriole increases (they constrict)
- Under normal circumstances, the efferent arteriole doesn’t change
- This means that glomerular capillary pressure doesn’t change as renal arterial pressure increases
- Therefore, renal blood flow does not change
- And GFR does not change as systemic and renal arterial pressure fluctuates
- Each glomerulus regulates itself and maintains its GFR at a steady level
15
Q
therefore what is autoregulation what how does it occur
A
- the increased vascular resistance
Myogenic – vascular smooth muscle responds to stretch by vasoconstricting = narrow lumen and increase resistance – so pressure downstream is maintained
Tubuloglomerular feedback – distal tubular flow regulates vasoconstriction.
-contents of tubule are monitored and sends signal back to glomerulus to say there’s too much or too little fluid coming through
-each nephron communicates with its glomerulus and tells it how much fluid is passing through and whether flow needs to be increased or decreased
16
Q
what is the macula densa?
A
a collection of densely packed epithelial cells at the junction of the thick ascending limb (TAL) and distal convoluted tubule (DCT)
17
Q
tubuloglomerular feedback
- what does this involve
- what does this allow
- what happens
- what is indicative of what
A
- This process involves the macula densa.
- The macula densa is a collection of densely packed epithelial cells at the junction of the thick ascending limb (TAL) and distal convoluted tubule (DCT).
- As the TAL ascends through the renal cortex, it encounters its own glomerulus, bringing the macula densa to rest at the angle between the afferent and efferent arterioles.
- The macula densa’s position enables it to rapidly alter glomerular resistance in response to changes in the flow rate through the distal nephron.
- The macula densa uses the composition of the tubular fluid as an indicator of GFR.
- A large sodium chloride concentration is indicative of an elevated GFR.
- A low sodium chloride concentration indicates a depressed GFR.
18
Q
describe the mechanism of tubuloglomerular feedback for increased GFR
A
• Increased arterial pressure causes increased glomerular pressure and plasma flow.
• This increases the GFR.
The plasma colloid osmotic pressure increases to limit the increased GFR. (but then increase plasma colloid osmotic pressure as more fluid has left it – becomes more concentrated)
• The increased GFR increases the tubular flow to the proximal convoluted tubule
This leads to increased reabsorption of water and ions in the proximal convoluted tubule and the loop of Henle (glomerulotubular balance)
• The increased GFR increases the tubular flow to the early distal convoluted tubule.
There is increased osmolarity of the tubular fluid (i.e. increased NaCl). (flow related increase osmolality or [NaCl] (monitoring flow by detecting osmolality or Na+ conc. or Cl- conc.))
• This is sensed by the macula densa by an apical Na-K-2Cl cotransporter (NKCC2).
- : (i) sensor mechanism (ii) transmitter (in walls of distal tubule that are adjacent to the glomerulus of the same nephron) (tubuloglomerular feedback)
• The juxtaglomerular cells in the macula densa secrete renin, which results in afferent arteriole constriction.
• This increases the preglomerular resistance, thus decreasing the GFR and keeping it maintained at a steady level. (decrease the glomerular pressure & plasma flow)
• This is known as TUBULOGLOMERULAR FEEDBACK.
19
Q
what is a measurement of GFR?
A
• ‘Renal Clearance’ – volume of plasma which is cleared of substance x per unit time
- Can applied to anything that is filtered through the kidneys
- But if applied to a marker of glomerular filtration rate then it can be used to measure GFR
20
Q
what is the equation for renal clearance?
A
(Ux) V / Px
• Ux = urinary concentration of ‘x’
• V = urine volume per unit time
• Px = plasma concentration of ‘x’
21
Q
what are the features of a good marker of GFR?
A
Freely filtered in glomerulus – small enough to get through the glomerular capillaries
Not reabsorbed in PCT
Not secreted out of DCT
Excreted in urine
If whatever is filtered all ends up in the urine, the rate of clearance will be exactly proportional to GFR and therefore can be used to measure GFR
22
Q
what are different markers of GFR? what marker of GFR is used in clinical practice? what is it affected by?
A
creatinine
- by-product of muscle breakdown
- affected by diet (how much protein you eat), age (older people tend to have muscle wasting), gender and ethnicity
- 51Cr-EDTA (radioactive – so not used routinely)
- 125I-iothalamate (radioactive – so not used routinely)
- 99mTc-DTPA (radioactive – so not used routinely)
- Inulin
- ‘gold standard’
- not endogenous so not used routinely in clinical situations
- have to introduce it into circulation and establish steady circulation
- takes several hours
- Cystatin C: the (clinical) future?
- used clinically
- many cases better than creatinine
- but for some reason creatinine is what’s used
23
Q
sodium regulation
- how much do we reabsorb per day
- how much do we excrete per day
- what does plasma sodium concentration determine
- why better than active water transport
- what is it linked to
A
- We reabsorb about 1.5kg of Na+ ions a day.
- We excrete about 9g of Na+ ions a day.
- 1.5 kg salt filtered per day
- 9 g salt excreted per day
- (you excrete almost exactly what you take in in your diet)
- Vast majority of filtered sodium is reabsorbed
• Plasma [Na+] determines
Extracellular fluid volume (and therefore your blood volume and therefore blood pressure)
Arterial blood pressure
• Less “expensive” than active water transport. This is because it is easier to transport Na+ ions and allow other things (like water and glucose) to follow. This way we don’t expend excess amounts of ATP.
• Linked to most other renal transport processes e.g. glucose reabsorption.(most other things involved in the kidney gets a free ride with the movement of sodium)
- Spend energy on sodium and everything else moves passively or through secondary transport
24
Q
where is sodium reabsorbed? through what?
A
• Proximal convoluted tubule – 67% Na+ reabsorbed (bulk – irrespective of whether you need to lose or retain sodium)
• Loop of Henlé – 25% Na+ reabsorbed
This occurs via the Na+-K+-Cl- Cotransporter (NKCC2) in the ascending loop of Henle.
• Distal convoluted tubule & collecting duct – 8% Na+ reabsorbed (fine tuning – hormonally regulated – depending on whether need to retain or lose sodium)
25
describe bulk sodium reabsorption
| - where does this take place
Proximal convoluted tubule:
1. The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
2. This allows the Na+-H+ exchanger on the apical membrane to take up Na+ ions from the urine.
3. The anion-Cl- exchanger allows the uptake of HCO3- ions in exchange for Cl- ions.
Therefore, both Na+ ions and HCO3- ions are reabsorbed.
26
describe fine tuning reabsorption of sodium
Distal convoluted tubule & collecting duct:
1. The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
2. Aldosterone combines with a cytoplasmic receptor.
3. Hormone-receptor complex initiates transcription in the nucleus.
4. New protein channels (ENaC – epithelium sodium channel) and pumps are made.
5. Aldosterone-induced proteins modify existing proteins.
6. Result is increased Na+ reabsorption and K+ secretion.
27
what are the three transport protein families involved in glucose transport?
```
1. SLC – solute carrier family
SLC5: sodium-linked cotransporters
2. SGLT
SGLT1 - transports 1 glucose: 2 Na
SGLT2 – transports 1 glucose: 1 Na
3. GLUT
GLUT1 and GLUT2
(between these two glucose transporters, you can take up glucose from the fluid and get it back into the blood)
- GLUT gene family
- Facilitated diffusion
```
28
describe glucose reabsorption in the early proximal convoluted tubule
- how much reabsorption here
- how
- how glucose get to blood
- what affinity like
- what capacity like
The early proximal convoluted tubule is involved in the mass reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT2 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT2 protein allows passage of glucose from inside the cell into the blood.
These have a low-affinity but high capacity because there is a lot of glucose available and these transport proteins allow for mass reabsorption of glucose.
- SGLT2 (take glucose and sodium in from tubule)
- GLUT2 (glucose moves out into blood)
= both low-affinity but high-capacity – grabs all glucose passing by and chucks it back into blood
29
describe glucose reabsorption in the late proximal convoluted tubule
- how much reabsorption
- how
- how glucose get to blood
- what affinity like
- what capacity like
The late proximal convoluted tubule is involved in the fine reabsorption of glucose.
The Na+-K+ pump on the basolateral membrane pumps Na+ ions into the blood, thus lowering the Na+ concentration in the cell.
This allows the Na+-glucose SGLT1 cotransporter on the apical membrane to take up Na+ and sodium from the urine.
Next, the GLUT1 protein allows passage of glucose from inside the cell into the blood.
These have a high-affinity but low capacity because glucose has already been mass absorbed and so there is less glucose left in the tubular fluid. These transport proteins allow for fine-tuning reabsorption of glucose.
- SGLT1 (take glucose and sodium in from tubule)
- GLUT 1 (glucose moves out into blood)
= both high-affinity but low-capacity – a lot less glucose present but they are able to mop up remaining amount of glucose because they have a high affinity for it
- So, by the time the fluid leaves the proximal tubule there should be no glucose left in the fluid – it reabsorbs all of filtered glucose under normal circumstances
30
glucose excretion
- what is fasting glucose level
- what is GFR
- what is filtered glucose rate
- what is transport maximum
- what happens when Tm is reached
- what happens to rest of glucose
* Fasting glucose ~ 5 mmol/L and GFR = 125 ml/min
* Filtered glucose = 5 x 0.125 = 0.63 mmol/min
* Transport maximum (Tm) ~ 1.25 mmol/min Plasma glucose ~ 10 mmol/L
* This graph shows that once the Tm is reached, no more glucose can be reabsorbed.
* The excess glucose must be excreted (the point of intersection of the lines).
- So, you can normally reabsorb all the glucose that you filter
- But there is a limit to how much glucose they can transport (this is known as transport maximum) – problem in people with diabetes
- Not until you get to a plasma glucose of about 10 mmol/L or higher that you are filtering more than you can reabsorb
- Under normal circumstances, the amount of glucose filtered matches the amount reabsorbed
- Only when exceeds the amount that you can reabsorb that you get excretion
- Relationship not perfectly linear because not every nephron has the same transporting capacity
31
how is the kidney a source of glucose? how much of all glucose in the body? what happens to it? what happens in diabetes?
• The kidney itself is a source of glucose via gluconeogenesis.
• The kidney makes around 20% of all glucose in the body, but it then breaks it back down.
- 300% increase in diabetes
32
what does water reabsorption occur?
in the descending loop of Henle
| • The longer the loop of Henle, the greater the amount of water that is reabsorbed.
33
if you need to reabsorb more water, what is this mediated by?
the effect of ADH/ Vasopressin
34
how does ADH lead to water reabsoption? what is reabsorption dependent on?
• ADH inserts aquaporins (AQP2) in the apical membrane of the cells in the late distal convoluted tubule and the cells of the collecting tubule.
This allows water to be reabsorbed from the cells back into the body.
The water will only flow through these channels in the presence of an osmotic gradient caused by Na+ ions.
35
which aquaporins are already present on the basolateral membrane of these cells?
AQP3/4
36
what is the effect of aldosterone?
allows reabsorption of water via ENaC
37
what is the descending limb impermeable to? why?
| what is the ascending limb impermeable to? why?
- thin descending limb is permeable to water; impermeable to solutes
- thick ascending limb is impermeable to water; active solute transport
38
how does concentration change through the nephron? what is the maximum gradient in the ascending limb?
PCT = 300 mOsm/kg
bottom of loop of Henle = 1200 mOsm/kg
DCT = 100 mOsm/kg
leaving the CD = 1200 mOsm/kg
(maximum gradient in ascending limb = 200 mOsm/kg)
39
where does calcium and magnesium ion reabsorption take place?
• PCT & Loop of Henlé
91% Ca2+ reabsorbed – paracellular route (passive reabsorption)
89% Mg2+ reabsorbed – paracellular route (passive reabsorption)
• DCT
3-7% Ca2+ reabsorption
5-6% Mg2+ reabsorption
40
how are calcium ions reabsorbed in the PCT and loop of Henle?
passively reabsorbed (paracellular route)
41
how are calcium ions reabsorbed in the DCT?
- what through apical
- what in cell
- what through basolateral
using transport proteins
using transport proteins
• Ca2+ ions enter the cell via TRPV5 transport protein channels.
• Because Ca2+ is an intracellular signalling molecule, we can’t have free Ca2+ in the cell as it would trigger other signalling pathways.
• Therefore, Ca2+ needs to be chaperoned from the apical membrane to the basolateral membrane where they would then enter blood.
• Ca2+ ions bind to an intracellular protein called Calbindin-D28K.
• This allows Ca2+ ions to move to the basolateral membrane.
• Once at the basolateral membrane, these ions can exit the cell into the bloodstream via two transport proteins: (1) Na+/Ca2+ exchanger [NCX1] and (2) plasma-membrane-calcium-ATPase-pump [PMCA1b].
- Calcium channel (TRPV5) in apical membrane – bring calcium in
- Calbindin-D28K – binding protein in cell
- Calcium pump – PMCA1b – principal route of calcium moving into blood – energy requiring – pumping out against a gradient
- Ca2+/Na+ exchanger – NCX1 – calcium out into blood and sodium in
42
what can TRPV5 (calcium channel) be regulated by?
Parathyroid hormone (PTH)
Vitamin D – the kidneys activate vitamin D which stimulates TRPV5 in the DCT
Sex hormones
Klotho (higher up in tubule) – this is a protein that’s associated with longevity.
- Channel in apical membrane is opened by the above
43
how is Mg2+ absorbed in the DCT? what stimulated by?
• Mg2+ reabsorption is less known.
• There is a ROMK potassium channel on the apical membrane of these cells.
• This causes the movement of K+ ions out of the cell and into the tubular fluid.
• This makes the tubular fluid positively charged.
• This favours the movement of Mg2+ ions into the cell via TRPM6 transport protein channels.
This is activated by epidermal growth factor.
• There is a Mg2+ exchanger on the apical membrane but we don’t know what Mg2+ is exchanged for.
- Lot less known about this
- Magnesium channel – TRPM6 – apical
- Buffer protein?
- Mg2+ exchanger in basolateral?
- Potassium leaks out in potassium channel in apical
- This makes tubular fluid more positive
- Which makes Mg2+ ions move from tubule into cell
- Magnesium channel (TRPM6) can be opened by epidermal growth factor
44
we absorb potassium from our diet. what happens to it next? how taken up into cells?
* It enters the ECF.
| * It is taken up by cells by a Na+-K+ pump.
45
what is the Na+/K+ pump activated by?
insulin
46
how much of the potassium ions are taken up by the cells and how much remains in the ECF?
98% of the potassium ions is taken up by the cells and 2% remains in the ECF.
47
where are K+ ions reabsorbed?
* Proximal Convoluted Tubule takes up 65% of K+ ions.
* Loop of Henlé takes up 25% of K+ ions.
* Distal Convoluted Tubule & collecting duct have variable K+ reabsorption and secretion. Here, you either get net reabsorption or net secretion.
48
where is K+ excreted? and what percentage where?
* 92% of the potassium is excreted by the kidneys.
| * 8% of the potassium is excreted by the colon.
49
why does potassium need to be carefully regulated? what can hyper and hypokalaemia lead to?
* Potassium needs to be carefully regulated because both hypokalaemia and hyperkalaemia can be fatal.
* Hypolkalaemia causes excess hyperpolarisation which leads to paralysis and so death.
* Hyperkalaemia causes excess depolarisation which also leads to paralysis and so death.
- Hypokalaemia -> hyperpolarisation -> paralysis -> death
- Hyperkalaemia -> depolarisation -> paralysis -> death
50
potassium regulation in the PCT
• On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions. (from blood) - high blood glucose concentration in glomerulus
- Glucose Tm exceeded
- Glucose present in tubular fluid in collecting duct
- > 1200 mOsm/kg
- = osmotic diuresis -> thirst
* These can leave via a K+ channel on the basolateral membrane.
* The net effect is the recycling of K+ ions.
* On the apical membrane there is a K+ channel.
* This allows the outflow of K+ ions, into the urine.
* As the tubular fluid continues through the PCT, there is a gain in the charge of the fluid because of the secretion of positively charged ions into the tubular fluid.
* This causes K+ ions to diffuse through the tight junctions between cells, back into the blood (down an electrochemical gradient).
* This is unregulated.
* In this way, most of the K+ ions are secreted into the tubular fluid, however, some diffuse back into blood.
- Na+/K+-ATPase pump in basolateral side (blood)
- Potassium ion channel in apical side going into lumen of proximal tubule
- K+ ions makes the fluid go from negative to positive charge
- This then favours the movement of potassium through a paracellular route -> into extracellular fluid where it is reabsorbed into the blood
- Pretty much passive reabsorption – due to build up of positive charge – pushes the ions out
51
potassium regulation in the (thick) ascending limb of loop of Henle
* On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
* These can leave via a K+ channel on the basolateral membrane.
* The net effect is the recycling of K+ ions.
* On the apical membrane there are two K+ transport proteins.
* The NKCC2 is a sodium-potassium- 2 chlorine cotransporter.
* It allows the entry of potassium ions into the cell.
* The ROMK2 potassium channel causes an outflow of K+ ions.
* This causes recycling of K+ ions on the apical membrane, similar to that on the basolateral membrane.
* The net movement of potassium ions favours the influx via NKCC2.
- Na+/K+-ATPase transporter in basolateral side (lowers intracellular sodium which causes sodium to move in through the next transporter)
- NKCC2/SLC12A1 (K+/Na+/2Cl- transporter – all moving into cell across apical membrane) – sodium and potassium reabsorption (potassium hitches a ride)
- ROMK2 – apical membrane potassium channel (potassium recycling) (potassium into lumen)
- Potassium channel on basolateral membrane (potassium can then be reabsorbed into blood)
- Both potassium channels (basolateral and apical membranes – both potassium cell leaving cell) help reabsorption of potassium through NKCC2
52
what two types of cells does the collecting duct have? which are the vast majority?
1. principal (vast majority is this)
| 2. intercalated
53
what are principal cells involved in?
the secretion of K+ ions into urine
54
what are intercalated cells involved in?
the reabsorption of K+ ions from the urine
55
describe potassium secretion in principal cells
| - what does aldosterone do? what stimulates aldosterone release?
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
There is another K+ channel that allows the uptake of K+ ions form the blood into the cell.
On the apical membrane, there is a K+/Cl- cotransporter which causes the secretion of both K+ and Cl- ions into the urine.
There is also a K+ channel (ROMK1) that allows secretion of K+ ions.
Principal cells:
- Na+/K+-ATPase (K+ in) and K+ (out but can do either direction) channel in basolateral membrane
- Na+ channel (sodium in) and K+ channel (potassium out) (ROMk1 and 3) and Cl-/K+ cotransporter (both out) in the apical membrane
- Due to potassium channel on both apical and basolateral side we have a route for getting rid of (apical) and a route for retaining potassium (basolateral)
- Aldosterone can be activated: opens sodium channel, activate potassium channel on apical membrane, and reverse the direction the potassium is going in on basolateral membrane (so the channel brings potassium into the cell instead of going into blood)
- Hyperkalaemia can stimulate aldosterone release
56
what can the ROMK1 channel be stimulated by?
by aldosterone or high plasma K+ (hyperkalaemia)
57
describe potassium reabsorption in the intercalated cells
On the basolateral membrane, the Na+/K+ pump allows intake of K+ ions.
On the apical membrane, there is a K+/H+ exchanger which causes the reabsorption of K+ ions.
There is also another H+ channel which causes the outflow of H+ ions.
Intercalated cells:
- K+/H+ cotransporter in apical membrane (potassium in, hydrogen out)
- H+ pump in apical membrane (channel) (out)
- If we have acidosis or low plasma K+ conc., the cotransporter kicks in – the system in the principal cell that loses potassium becomes less dominant
- Na+/K+-ATPase in basolateral membrane
58
what activates the intercalated cells?
acidosis or hypokalaemia
59
what are the different sources of acid? what happens to it?
• Normally, there is around 15,000mmol of CO2 produced per day. This is ‘potential acid’, but it usually isn’t a problem as it is efficiently excreted by the lungs.
• Metabolism also produces ~40 mmol H+ per day (‘non-volatile acids’: sulphuric, phosphoric, organic acids).
• There is also a net uptake of ~30 mmol H+ per day by GI tract
• So the kidney has to:
Excrete ~70 mmol H+ per day
Reabsorb all the filtered HCO3 -
60
when excess H+ ions excreted in the urine, what needs to happen and why?
* Excess H+ ions in the urine can cause the urine to become very acidic (pH = 1.3), thus painful.
* Therefore, in the urine, the H+ ions needs to be buffered.
61
what do carbonic anhydrases contain?
Zn
62
which carbonic anhydrases are there? what do they do?
• These are enzymes that contain Zn.
• There are at least 16 isoforms, but two important isoforms reside in the kidneys:
CA II – soluble cytoplasmic (found freely dispersed in the cytoplasm)
CA IV – extracellular, linked to cell membrane (by a GPI anchor)
• They catalyse the hydration of CO2. But, this is what actually happens…
• Carbonic anhydrase (CA) catalyses the second reaction (CO2 + OH-…)
H2O H+ + OH-
CO2 + OH- HCO3- (this reaction)
May see this shorted as H2O + CO2 -> H2CO3 -> H+ + HCO3- (but this isn’t really what’s happening)
63
reasbsorption of filtered HCO3- (general overview)
* HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
* The CO2 diffuses into the cell.
* The OH- ions combine with the H+ ions in the tubular fluid to form water.
* This water diffuses into the cell via osmosis.
* Once in the cell, the water breaks down again into H+ and OH- ions.
* The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
* At the basolateral membrane, the HCO3- ions exit the cell and enter the blood.
* The net movement of HCO3- ions is from the tubular fluid into the blood.
* This process uses a lot of ATP.
- H+ secretion at apical membrane reclaims HCO3- -> CO2 + H2O (carbonic anhydrase IV (CAIV) – apical membrane)
- CO2 can freely diffuse across apical cell membrane
- In cell, CO2 + H2O -> H+ + HCO3- (CAII)
- H+ recycled and resecreted into lumen
- HCO3- extruded at basolateral membrane
- Net transfer of HCO3- from lumen to interstitium/blood
64
what are the sites of HCO3- reabsorption?
• HCO3- ions are reabsorbed throughout the nephron:
PCT = 80% (same as reabsorption of NaCl and water)
Thick ascending loop of Henle (TAL) = 10%
DCT = 6%
Collecting duct = 4%
65
how much HCO3- is excreted?
Less than 0.01% is excreted.
66
describe how HCO3- is reasborbed in the PCT.
| what happens to everything else?
* HCO3- in the tubular fluid is converted to CO2 and OH- ions as a result of CA IV.
* The CO2 diffuses into the cell.
* The OH- ions combine with the H+ ions in the tubular fluid to form water.
* This water diffuses into the cell via osmosis.
* Once in the cell, the water breaks down again into H+ and OH- ions.
* The CO2 combines with the OH- (via CA II) ions to from HCO3- ions.
* At the basolateral membrane, the HCO3- ions exit the cell via a ‘kidney variant of Na+/ HCO3- cotransporter’ (kNBCe1) and enter the blood.
* The H+ ions are secreted from the cell via a H+-ATPase pump and via a Na+/H+ exchanger (NHE3).
* The net movement of HCO3- ions is from the tubular fluid into the blood.
* This process uses a lot of ATP.
- Acid secreted across apical membrane into lumen
- NHE3 (sodium-hydrogen exchanger) (very common to have exchangers using sodium) (sodium is moving along its concentration gradient and hydrogen ions moving against concentration gradient (secondary active transport?))
- H+-ATPase – primary active transport into lumen
- H+ ions combine with bicarbonate (CAIV) -> CO2 + H2O
- CO2 -> diffuse into cell -> break back down into H+ and bicarbonate
- In PCT, transporter that’s responsible for transporting HCO3- across basolateral membrane = kNBCe1 (kidney sodium-bicarbonate cotransporter) – 3HCO3- and 1Na+ -> both transporter out of cell (unusual for having Na+ moving out of cell in cotransport because that’s against sodium’s natural gradient) (however it is able to function like this due to its unusual stoichiometry – moving three negative charges and one positive charge in the same direction – net movement of charge = 2 negative = transporter is electrogenic – this net movement of charge allows sodium to move in unusual direction)
• NHE3 is dominant in proximal tubule (majority of H+ secreted is by NHE3 rather than H+-ATPase)
• large capacity (the transporter – trying to reabsorb 80% of bicarbonate and to do this we need to secrete acid) but limited gradient generation (only down to pH 6-ish in the lumen (if we just had the NHE3 transporter))
• V-type (vacuolar) H+-ATPase can generate a bigger gradient (down to pH 4 or 5) (this becomes really important in the collecting duct)
• more important later in the tubule where lumen is more acidic
• 1:3 stoichiometry of kNBCe1 makes it electrogenic
• allows HCO3- efflux from the cell because of extra drive from membrane potential
• unusual to have Na+ leaving the cell on a cotransporter
NHE3 = apical membrane
kNBCe1 = basolateral membrane
67
where is the NHE3 dominant? what is capacity like? why?
in PCT (Na+/H+ exchanger)
* The NHE3 is dominant in proximal tubule. It has a large capacity but limited gradient generation (from 7.35 to 6 pH). This is because the Na+ ions move over a small gradient and so only small amounts of H+ are exchanged (secreted), therefore the pH only decreases by a small amount.
* V-type (vacuolar) H+-ATPase can generate a bigger gradient (from 7.35 to 4/5 pH).
68
what makes the kNBCe1 electrogenic? what does this mean for efflux?
• 1:3 stoichiometry of kNBCe1 makes it electrogenic.
It allows HCO3 - efflux from the cell because of extra drive from membrane potential.
This is because there is a net efflux of 2- charge. This gives the protein the extra drive.
69
what is proximal renal tubular acidosis?
* Rare autosomal-recessive disease
* Impaired HCO3- reabsorption in PCT
* Severe metabolic acidosis (PCT responsible for absorbing 80% of bicarbonate)
* Not treatable by HCO3- supplementation (80% of bicarbonate is reabsorbed in PCT)
* Attributed to mutations in kNBCe1 (reabsorbing 3 bicarbonate ions)
* Ocular abnormalities too because of kNBCe1 and pBNCe1 expression there too
70
how are HCO3- ions transported at the basolateral membrane in the thick ascending limb and DCT?
* The transport protein for HCO3- ions at the basolateral membrane differs to that in the PCT.
* Here, it is an anion-exchanger (AE2).
* It exchanges HCO3- ions for Cl- ions.
- same processes but slightly different combination of transporters
- H+ generated from CO2 + H2O as usual (CAII)
- Luminal CAIV less important (slower here)
- Still have Na-H exchanger (NHE3) and H+-ATPase but there is a different transporter on the basolateral membrane
- No kNBCe1 here, just AE2 (carbonate-chloride exchanger) for HCO3- exit at the basolateral membrane
- 1 bicarbonate ion out of cell for 1 chloride ion into cell
71
alpha-intercalated cells of the collecting tubule and duct cells - what pumps, what transport?
* The H+ pump at the apical membrane is electrogenic.
* The H+/K+ -ATPase pump at the apical membrane is not electrogenic, therefore allowing transport of H+ ions up a steep gradient.
* The kAE1 exchanger protein at the basolateral membrane exchanges HCO3- ions for Cl- ions.
- Different isoform of the same transporter on the basolateral membrane
- kAE1 rather than AE2 – slightly different protein structure
- kAE1 has shorter N-terminus than eAE1
- Here is the main site of V-type H+-ATPase activity – bulk of hydrogen ion secretion here
- Instead of NHE3, there is H+,K+-ATPase (hydrogen-potassium exchanger) – main role is in the reabsorption of K+ rather than secretion of H+ ions
* V-type H+-ATPase in apical membrane of α-intercalated cells – transporter very cell-specific, even within the collecting duct
* absent from principal cells
72
what is distal renal tubular acidosis?
* Defective H+ excretion by distal segment of nephron • Inability to acidify urine (acid not secreted) – serious systemic consequences (metabolic acidosis)
* May be incomplete (compensatory mechanisms of proximal tubule)
* Treatable with HCO3- supplementation
* Dominant and recessive patterns
* Several transporter mutations – mainly affecting the α – intercalated cells: kAE1, V-type H+ -ATPase, CAII (proximal effects too)
73
excretion of H+ as 'titratable acid' (TA)
| - what happens to H+ ions
* The water that diffused into the cell during reabsorption of HCO3- ions split into H+ and OH- ions.
* These H+ is secreted back out of the apical membrane into the tubular fluid.
* Once in the tubular fluid, the H+ ions are buffered by filtered phosphate (HPO42-) into H2PO4-.
* At the same time, HCO3- enters the tubular fluid after being filtered by the glomerulus, which further neutralises it.
- CO2 + H2O -> H+ + HCO3- (CAII)
- H+ secreted from apical membrane
- HPO42- + H+ -> H2PO4- in lumen
- This acid is effectively buffered and we can excrete acid in the form of H2PO4- rather than excreting free H+ ions
- Secreted H+ is mostly buffered by filtered phosphate – also creatinine, urate, etc.
- ‘New’ HCO3 (essentially brings more acid into cell) exits across basolateral membrane -> enters circulation
- HCO3- neutralises acidity HCO3- + H+ -> CO2 + H2O -> CO2 freely diffuses across basolateral membrane into cell (this means there’s a net movement of acid being moved from intersitium into filtrate)
- The acid in the blood forms CO2 -> forms H+ in the cell -> buffered in filtrate -> urine
74
excretion of H+ as NH4+
- when
- what happens
* An alternate way of secreting H+ ions is to utilise the ammonium (NH4+) ions synthesised in the cell during glutamine metabolism.
* Glutamine metabolism produces NH4+ ions and OH- ions.
* The OH- ions can combine with CO2 (via CA II) to form HCO3- ions that can enter the blood.
* The NH4+ ions split into ammonia (NH3) and H+ ions.
* NH3 and H+ ions are secreted into the tubular fluid where they both combine again to form NH4+ for excretion in urine.
- Glutamine metabolism -> NH4+ + OH- (NH4+ is synthesised by the kidney)
- NH4+ -> NH3 + H+
- H+ secretion across apical membrane
- NH3 freely passes across apical membrane
- Once in the filtrate, NH4+ is formed again -> acid excreted in this form
- OH- + CO2 (diffuses into cell from interstitium) -> HCO3- (CAII) -> transported across basolateral membrane
- ‘New’ HCO3- enters circulation and neutralises acidity
- HCO3- + H+ -> CO2 (diffuses through basolateral membrane) + H2O
75
where are sites of net acid secretion? how much excreted?
• Acid (H+ ions) is secreted throughout the nephron:
PCT = 40mmol NH4+ and 15mmol titratable acid (TA)
DCT = 5mmol TA
Collecting duct = 10mmol TA
Small amounts of this acid is recycled in the medulla of the kidneys.
Excreted = 40mmol NH4+ and 30mmol TA.
- Most of the NH4+ is secreted in the proximal tubule (40 mmol NH4+)
- Some NH4+ recycles in the medulla
- Hyperosmotic gradient in medullary interstitium, created by the loop of Henle due to secretion of NaCl, urea and NH4+ - therefore some secretion and reabsorption of NH4+ in thick ascending limb
- Re-secretion into collecting duct and excreted there
- 50% of titratable acids are secreted in proximal tubule (15 mmol TA)
- 5 mmol TA secreted in DCT
- 10 mmol TA secreted in collecting duct
40 mmol NH4-
30 mmol TA
= 70 mmol acid secreted by kidneys (based on someone that’s 70kg)
76
describe NH4+ handling in the kidney
- most is secreted in PCT
- some re-enters tDLH
- partly reabsorbed in TAL
- a bit lost to liver
- most re-secreted in collecting duct
77
describe NH4+ reabsorption in the thick ascending limb
- why can NH4+ be selected for
- what goes into blood
* On the apical membrane, there is a Na+/K+/Cl- cotransporter (NKCC1).
* There is a lack of selectivity for the K+ ion and instead NH4+ can be selected for.
* Also, there is a ROMK2 channel on the apical membrane.
* This is a K+ channel but can also select for NH4+ ions.
* Once in the cell, NH4+ splits into H+ ions and NH3.
* NH3 can now diffuse into the blood.
(reabsorption from the lumen into the interstitium) (due to NH4+ role in creating hyperosmotic gradient in medulla)
- NH4+ in lumen can pass through potassium channels (ROMK2) into cell
- NH4+ can also replace K+ in the NKCC1 (1Na+, 1K+, 2Cl-) transporter – transporter into cell
- Cells will also be producing NH4+ through glutamate metabolism
- NH4+ -> NH3 + H+
- NH3 -> diffusion across basolateral membrane (this membrane over apical membrane due to nature of plasma membrane – more permeable on basolateral membrane than apical membrane )
- NH3 + H+ -> NH4+ in interstitium
• Via ROMK2 channel, and NKCC1 accepts NH4+ in place of K+
• Low NH3 permeability at apical membrane, so it leaves across basolateral membrane
78
respiratory acidosis
- cause
- compensation
- cause = hypoventilation/increased PCO2
| - compensation = increase HCO3- reabsorption (to react with excess H+), kidneys excrete additional H+
79
respiratory alkalosis
- cause
- compensation
- cause = hyperventilation/decreased PCO2
| - compensation = decreased HCO3- reabsorption (to increase HCO3- levels in urine), and excrete less H+
80
metabolic acidosis
- cause
- metabolic
- cause = decreased HCO3- reabsorption (causes a build-up of H+ ions in the blood as HCO3- can't react with it)
- compensation = hyperventilation/decreased PCO2 (to get rid of excess H+ ions)
81
metabolic alkalosis
- cause
- compensation
- increased HCO3- reabsorption
82
what is a common cause of nephropathy? why? what happens?
```
damage to the glomerular filtrate
this can be damage to the:
Endothelium (pre-eclampsia)
Glomerular basement membrane
Podocytes – this is due to genetic mutations. The podocytes ‘drop off’ the basement membrane and flow in the blood until they are removed from the system.
```
* This damage leads to a faulty barrier, which allows larger molecules such as albumin and glucose to pass through into the kidneys.
* Chronic damage to the glomerular filter leads to kidney dysfunction.
83
what is nephrotic syndrome?
this is a disease where there is loss of protein (e.g. proteinurea, hypoalbuminaemia)
84
what is proteinuria?
presence of protein in the urine
85
protein: creatinine ratio (PCR) in the urine
- what is normal
- what is nephrotic range
<20mg/mmol (normal)
| 200mg/mmol (nephrotic range)
86
what is nephritic syndrome?
this is a disease where there is loss of blood (e.g. haematourea etc)
87
albumin: creatinine ratio (ACR) in the urine
- what is normal
- what is microalbuminuria
- albuminuria
<30 μg/mg (normal)
30-300 μg/mg (microalbuminuria)
>300 μg/mg (albuminuria)
88
epidemiology of diabetic nephropathy
- what percentage of those with diabetes develop nephropathy
- how common cause of kidney failure
2025: 300 million with diabetes (WHO)
40% develop nephropathy – Genetic susceptibility
Commonest cause of kidney failure worldwide
89
how does the type of diabetes affect pattern for nephropathy?
both share same pattern
90
out of all of those that develop nephropathy, who has the worst prognosis?
diabetics
91
what does albuminuria persistency increase the risk of?
developing a heart attack
92
what are the stages of injury in nephropathy?
```
Hyperfiltration
Microalbuminuria
Macroalbuminuria
Proteinuria
Declining renal function
```
93
describe the glomerular compartment
```
Endothelial cells
Glomerular basement membrane
Podocytes
Mesangial cells
Mesangial cell deposit
```
94
what is the pathology of diabetic nephropathy?
GBM thickening
Mesangial expansion
Nodular sclerosis
Advanced renal sclerosis
95
what is treatment for diabetic nephropathy?
• Aims of treating Diabetic Nephropathy:
Glycaemic control
Blood pressure control – these drugs help to delay the progression of kidney diseases by blocking the renin-angiotensin aldosterone system(RAAS):
o ACE inhibitors
o Angiotensin-2 receptor blockers (ARB) - Losartan
o Renin-inhibitors – Aliskiren
These drugs delay the progression because they lower the blood pressure which in turn lowers the GFR.
• Secondary treatments:
Lipid lowering
Reduce other CV risks
96
diuretics
- what do they do
- how
- who's effects do they augment
- what does choice of diuretic agent depend on
• Reduce extracellular fluid volume
• Lower blood pressure
• Augment effects of RAAS inhibitors
• Choice of diuretic agents depends on renal function:
1) Osmotic Diuretics (e.g. mannitol) – these work on the late PCT and the Descending loop of Henle.
2) Loop diuretics (e.g. furosemide) – these work on the ascending Loop of Henle
3) Thiazide diuretics (e.g. bendroflumethiazide) – these work on the DCT
4) Potassium-sparing – these work on the late DCT and early collecting tubule.
97
dialysis
- what is removed
- what is concentration in dialysis fluid like
- what happens
* Waste, excess fluids and salts are removed from the body by passing the blood over a dialysis membrane.
* This allows the exchange of substances between the blood and the dialysis fluid, which has the same concentration of substances as blood plasma.
* Substances diffuse from both sides to create the correct concentration of substances.
98
what are the positives of peritoneal dialysis?
* Immediate use reduces fluid overload.
* No anticoagulation.
* Cheaper and can be used at home.
* Continuous
* Least likely to cause fluid shifts and hypotension.
99
what are the negatives of haemodialysis?
* Specialist nursing care
* Tertiary units
* Need for good central venous access
* High and efficient solute clearance
* Anticoagulation (heparin) required.
* Intermittent: not tolerated when haemodynamically unstable.
* Continuous Hemofiltration
100
what are transplantation options for diabetic nephropathy?
* Kidney
* Combined kidney and pancreas – in the case of severe diabetes
* Islet cell
101
what are the percentage of obese adults and children in the UK?
In 2012:
25% of adults (>16) in England were obese - an overall increase from 15% in 1993.
19% of children (yr 6) were obese
102
where is adipose tissue deposited? what happens in each place?
Subcutaneous fat – storage
| Visceral/omental fat – endocrine tissue
103
what is adipose tissue used for?
energy balance and for monitoring appetite
104
what does visceral fat secrete?
1. Inflammatory mediators:
TNFa – induces insulin resistance by promoting serine-phosphorylation of IRS-1, which impairs insulin signalling
Resistin – neutralising antibodies reduces insulin resistance
IL-6 – direct correlation between IL-6 and insulin resistance/ visceral fat secrets 2x more IL-6 than subcutaneous fat
2. Adiponectin (Acrp30) – this stimulates adipose tissue to function correctly. Obesity reduces Acrp30.
3. Leptin
This signals the adipose tissue mass size to the CNS (hypothalamus).
Increased leptin = reduces food intake + increases energy expenditure.
Obesity increases adipose tissue mass.
105
what are the effects of leptin?
Brain:
• Leptin secretion sends signals to the hypothalamus.
• This causes inhibition of feeding and increased sympathetic output.
• The increase of sympathetic output causes B-cells to decrease insulin synthesis and secretion.
• This leads to increased lipolysis and decreased lipogenesis.
Liver:
• Leptin decreases gluconeogenesis.
• Leptin increases glycogenolysis.
• Leptin increases B-oxidation.
Muscle:
• Leptin increases glucose uptake.
• Leptin increases glycogenolysis.
106
what is the link between leptin and obesity?
Obese people are leptin resistant. This means that they have an increased appetite and they don’t expend much energy. This means that they get fatter and secrete more TNF-a and so more insulin resistance manifests, increasing the risk of insulin resistance.
107
is insulin resistance and diabetes an inflammatory condition?
• High doses of salicylates (aspirin)
Reverse insulin resistance and diabetes
Preserve β-cell function
• High-fat diets or obesity activate NF-κB and increase production of IL- 6, IL- 1β and TNFα.
When these reach their targets (e.g. liver) they cause a further increase in NK-KB.
• Activation of NF-κB in liver results in liver and muscle insulin resistance and diabetes.
• Antibody-mediated neutralization of IL- 6 in high-fat fed animals partially restores insulin sensitivity.
108
what is NF-kB? what has its incorrect regulation been linked to?
a protein that controls transcription of DNA. It plays a key role in regulating the immune response to infection. Incorrect regulation of NF-kB has been linked to inflammatory diseases. NF-kB is the protein responsible for cytokine production and cell survival.
109
what do thiazolidinediones do? what are they? what do they lead to?
• These are a form of drug used to treat type 2 diabetes.
• They work by increasing the sensitivity of cells to insulin (that are insulin resistant).
• These provide:
Improvement of fasting plasma glucose.
Improvement of lipid profile – lower NEFA/ TG/ cholesterol
Improvement of B-cell function
110
mechanism of actions of thiazolidinediones?
* These drugs are PPARϒ agonists.
* PPARϒ are nuclear hormone receptors that cause an increase in adipogenesis in fat cells.
* This causes an increase in glucose uptake for adipogenesis, thus reducing the blood glucose level, without the need of insulin secretion.
* These drugs also activate AMP-Kinase.
* Once activated, AMPK switches on catabolic pathways that generate ATP, while switching off ATP-consuming processes such as biosynthesis and cell growth and proliferation.
111
what is advanced glycation end products (AGE)?
* AGEs are substances that can be a factor in the development or worsening many degenerative diseases, such as diabetes.
* These compounds can affect nearly all types of cells and molecules in the body.
* They are thought to play a causative role in the blood vessel complications of diabetes.
* AGEs are seen as speeding up oxidative damage to cells and in altering their normal behaviour.
112
AGE formation in diabetes
* Normally, AGE is produced in the body due to the combination of proteins and glucose.
* In type II diabetes, a state of hyperglycaemia develops intracellularly because on increased uptake of glucose (due to the increased levels of insulin), even though the intracellular mechanisms are abnormal (insulin resistance).
* This leads to increased levels of NADH and FADH.
* This increases the proton gradient in the oxidative phosphorylation.
* This results in mitochondrial production of reactive oxygen species which damage the DNA.
* These reactive oxygen species also cause accumulation of metabolites, which activate multiple pathogenic mechanisms.
* One of these mechanisms includes increased production of AGEs.
113
what are the intracellular effects of AGE?
* Causes activation of NF-kB, which results in the production of cytokines, and therefore is proinflammatory.
* Main targets are endothelial cells and smooth muscle.
114
what are the extracellular effects of AGE?
• Crosslinking key basement membrane molecules.
• Targets:
collagen I and IV, vitronectin, laminin, elastin
Lipid linked LDL
• Consequences:
Increased vascular stiffness
Increased synthesis of several ECM components
Reduced endothelial cell adhesion
Reduced NO (muscle relaxant) production and clearance of LDL
115
AGE and the macrophage
* Basement membrane AGEs inhibit monocyte migration - “apoptaxis.”
* Soluble AGEs activate monocytes.
* AGE binds to receptors on monocytes.
* This causes increased expression of macrophage scavenger receptor (MSR) class A receptors and CD36 receptors on the monocyte cell membrane.
* As a result more Oxidised LDL (OxLDL) is taken up, thus increasing the concentration of LDL inside the cell.
* This leads to foam cell formation.
116
AGE and AGE receptor summary
• AGE generation by hyperglycaemia underpins development of diabetic complications, such as kidney disease and atherosclerosis.
• Glucose forms adducts, causes protein crosslinking and changes in structure.
• AGE will:
Target vascular tissue and beta cells
Trigger inflammatory reactions
Trigger atherosclerosis
Induce apoptosis
Change cell adhesion properties
Cause vascular stiffness and vasoconstriction
Change basement membrane properties – kidney function
• Overall, in diabetes, AGE is responsible for the chronic effects:
Vascular damage
Kidney dysfunction
B-cell damage
117
how many medulla in the average human kidney?
8 to 12
118
what are the pyramids?
where the nephrons come together to drain into the pelvis
119
how many nephrons per kidney? what is their link with cardiovascular disease risk?
- There’s a range of 600,000 to 2.5 million nephrons per kidney
- The fewer nephrons you have, the more likely you are to have raised blood pressure in later life
- So, link between number of nephrons and risk of cardiovascular disease
120
where are all the glomeruli?
in the cortex, so filtration takes place in cortex
121
what lets and prevents substances moving through the glomerulus?
- On glomerular capillaries – podocyte cells – they have foot processes – sat on outside of endothelium – but they move around on the surface of capillaries
- Filtration slits between the foot processes of the cells let and prevent stuff moving through – e.g. red blood cells are too big to get through
122
what happens in disease in terms of podocytes?
they break off
123
where are the epithelial cells and where are the endothelial cells in the renal corpuscle?
- Epithelial cells in the Bowman’s capsule
| - Endothelial cells in the capillaries
124
what percentage of plasma that enters the capillary is filtered?
About 20% of plasma that enters the capillary is filtered, the remaining 80% remains in the blood
125
what is the blood like in the efferent arteriole compared to the afferent?
will be more viscous because it’s lost 20% of plasma – so higher concentration of red blood cells
126
what is the net pressure favouring filtration?
- Net pressure favouring filtration is only 10 mmHg (pressure small really)
- But it’s enough to cause significant volumes of fluid to be filtered
127
what is the normal GFR? does GFR change normally?
- Net pressure favouring filtration is only 10 mmHg (pressure small really)
- But it’s enough to cause significant volumes of fluid to be filtered
GFR = 20% RPF (renal plasma flow?) = 125 ml/min = 180 L/day
= 60 times plasma volume!!
- Under normal circumstances, your GFR doesn’t change – it remains constant (so if you drink more water or something it doesn’t change it?) (IMPORTANT)
- This is because even a slight change in GFR could mean huge changes in volume, so you could potentially become dehydrated very quickly if there weren’t compensatory mechanisms
128
what causes changes in systemic pressure? how does this affect GFR?
- Sleep
- Exercise
- Chronic disease e.g. hypertension, renal, artery stenosis
Even through changes due to this, GFR does not change due to autoregulation
129
late proximal - what's being transported where?
- Na+/K+-ATPase pump (basolateral side)
- sodium out into blood and potassium in
- this is what requires the energy
- Na+/H+ exchanger
- sodium taken up from tubular fluid in proximal tubule (due to lower Na+ gradient inside cell now)
- H+ into proximal tubule which goes to urine
- exchanger = NHE-3/SLC9A3
- Cl-/anion exchanger
- Cl- taken in from proximal tubule
- anion given out
- Chloride ion channel
- chloride out into blood
- Na+/H+ exchanger
- sodium taken in from blood
- H+ given out into blood
- NHE-1/SLC9A1 (this is to do with acid regulation of cell itself – not to do with sodium reabsorption, but to do with cell’s own volume and pH)
130
what reabsorbs the majority of sodium?
NHE-3
131
what drives NHE-3?
Na+/K+-ATPase
132
late distal/collecting duct
- what type of regulation
- what mediated by
- what in basolateral membrane
- what ion channels
- how mediated
- how quick process
- Fine regulation
- Mediated by aldosterone – from adrenal gland
- Na+/K+-ATPase in basolateral membrane
- Na+ channel (ENaC (epithelium sodium channel)) taking in Na+ from urine
- K+ channel giving out K+ to urine
- Both channels present normally but aldosterone increases the production and the open probability of these channels – making the membrane much more leaky – allows a lot more sodium to come in if we need it
- Slow process – 24-48hrs – genomic effect
- However, it’s generally not an emergency because the amount of sodium we eat doesn’t vary that much
133
what can aldosterone also promote, apart from sodium uptake?
potassium loss
134
what are reasons that someone might have a positive dipstick result but didn't excess Tm?
- Loss of considerable nephron function – global GFR was 22 but a considerable proportion of her nephrons would not have been functioning, so GFR of each individual nephron would have been higher
- Splay – uraemic toxins – she was beginning to develop uraemia (retaining urea in blood) – toxins damaging the function of the remaining filtering nephrons – so their capacity to filter and reabsorb glucose would also be diminished
135
gluconeogenesis equation - where in kidney?
| glycolysis equation - where in kidney?
- Takes place in cortex?
Lactate pyruvate -> oxaloacetate -> phosphoenol pyruvate -> triose phosphatases -> glucose
- Glycolysis in medulla?
Lactate pyruvate
136
what is the counter-current multiplier?
loop of Henle
137
where is the counter-current exchanger?
vasa recta (capillary network surrounding tubule)
138
do cortical nephrons go into medulla?
just dip into medulla
139
which are the longer nephrons?
Juxtamedullary nephrons are much longer – they go virtually all the way to the ends of the pyramids – these are the ones that are important in your ability to concentrate urine (called that because the glomerulus is juxtaposed/close to the boundary with the medulla)
140
describe what happens in the loop of Henle and why? concentrations?
- Active transport of NaCl along the ascending thick limb results in the movement of water from the descending limb (due to high concentration of sodium and urea)
- As fluid moves down descending limb it gets more concentrated as water is leaving
- As passes up ascending limb, sodium removed, which reduces osmolality of fluid to point where it’s more dilute than it was when it started (100 mOsm/l compared with 300 mOsm/l)
- Maximum concentration gradient that Na+Cl- pumps can maintain between fluid inside and fluid in the extracellular space is 200 mOsm/kg (concentration higher in the extracellular fluid) (this is in the ascending limb)
- The outside will be 200 mOsm/kg higher than it is on the inside – that’s the most that those pumps can maintain
- Therefore, can be any lower than 100 mOsm/kg on the inside at the top of the ascending limb
141
what are the concentration gradients of Na+ into the cells from tubule and into blood from cells?
- Na+ concentration in the fluid in tubule higher than in the cell so it will go down concentration gradient - Na+/Cl-/K+ cotransporter on apical side takes the sodium into the cell
- Na+/K+-ATPase on basolateral side will pump it out into extracellular fluid against concentration gradient
142
how is urine concentrated? what is the maximal concentration?
- In the proximal tubule, water and sodium are reabsorbed in proportion
- So, the concentration of fluid that enters your loop of Henle is the same as plasma = 300 mOsm/kg
- Because as it goes down water can leave down water potential gradient, when you get to bottom of loop concentration is 1200 mOsm/kg
- As you go up loop, sodium transported out, lowering osmolality
- At distal tubule osmolality is 100 mOsm/kg
- So, by passing fluid through the loop we have diluted the fluid – large volume of diluted urine
- But often we need to concentrate urine, if we need to retain fluid
- This depends upon hormone ADH/vasopressin
- Under normal circumstances, collecting duct is not permeable to water
- The only way you can get water back and concentrate urine is to make collecting duct permeable to water
- When this happens, you can maximally concentrate your urine to 1200 mOsm/kg = MAXIMUM – determined by length of loop of Henle
143
how does aldosterone work? where?
LATE DISTAL/COLLECTING DUCT
- Aldosterone acts on collecting duct cells to open sodium channels on apical side
- There’s a Na+/K+-ATPase pump on basolateral membrane, pumping sodium out and so gradient for Na+ to come into cell down channel from tubule
- So, aldosterone allows sodium to enter the cell and be pumped out
- What you also need is ADH to bind to receptor on basolateral side, and stimulate the insertion of water channels – aquaporins (AQP2)
- When these are inserted into the apical membrane, the collecting duct cells become permeable and water can enter from the fluid in tubule
- So, aldosterone promotes sodium reabsorption and ADH promotes water reabsorption – aldosterone on its own doesn’t cause any water reabsorption
- Water than enters the cell can leave into extracellular fluid through AQP3/4 – allows it to get back into blood
144
which direction is blood flow in vasa recta?
Blood in vasa recta (from the efferent arteriole) is moving in the opposite direction to the direction of flow through the nephron (so blood moving down when fluid moving up ascending limb)
145
what happens when blood glucose is too high? what does it lead to?
- High blood glucose concentration in glomerulus
- Glucose Tm exceeded
- Glucose present in tubular fluid in collecting duct
- > 1200 mOsm/kg
- = osmotic diuresis -> thirst
146
reabsorption vs. secretion of K+
| - where
- Proximal
- 65% K+ reabsorbed
- Loop of Henle
- 25% K+ reabsorbed
- Distal & collecting duct
- variable K+ reabsorption or secretion
147
why give insulin to correct for hyperkalaemia?
- The way insulin is being used here is to stimulate the Na+/H+ exchanger in basolateral side
- So, more sodium pumped into cell, providing a gradient for the Na+/K+-ATPase, so potassium pumped into cell – reducing plasma levels
- This isn’t in kidney
148
disturbances of distribution of which ions occur frequently in people with chronic kidney disease?
Ca2+ & Mg2+
| because an inability to regulate particularly calcium properly -> bone mineral wasting
149
where are Ca2+ and Mg2+ distributed?
- Calcium – 99% in bone, 1% in extracellular fluid
| - Magnesium – 50% in bone, 50% in extracellular fluid
150
what do osteoclasts and osteoblasts do?
- Osteoblast cells = take up calcium and magnesium from ECF into bone
- Osteoclast cells = take calcium and magnesium out of the bone and into ECF
151
where do we get how much of our Ca2+ and Mg2+ from? what happens to it?
- 40% of calcium is absorbed by the gut from food we eat
- 10-65% of magnesium is absorbed in the gut from food we eat
- The above enters the extracellular space through a paracellular route
- Then we lose 1-2% calcium and 3-5% magnesium in our urine
152
what does hypo/hypercalcaemia cause?
lower/raise depolarisation threshold
153
what does hypo/hypermagnesaemia cause?
raise/lower heart rate
154
give a summary on control of water, potassium, calcium and magnesium
- what water excretion dependent on and regulated by
- what potassium critical for
- what happens to potassium
- what control of calcium and magnesium and where
- Water excretion
- dependent on osmotic gradient in medulla
- regulated by aldosterone (Na+) and ADH/vasopressin (aquaporins)
- Potassium
- critical for excitable cells
- reabsorbed & secreted by the tubule
- Calcium & magnesium
- fine control of reabsorption in distal tubule
I MAY HAVE CALLED SOME OF THE PUMPS CHANNELS – MAYBE CHECK THAT WHAT I CALLED CHANNELS ARE ACTUALLY CHANNELS AND NOT PUMPS – CHANNELS ARE THE RECTANGES, PUMPS ARE CIRCLES
155
what is aldosterone, where produced, what does it do?
mineralocorticoid – produced by the zona glomerulosa of adrenal cortex in adrenal gland – essential for sodium conservation in the kidney, salivary glands, sweat glands and colon
156
what does the human tissue act 2004 allow and not allow?
- Permits donation post mortem – opt-in
- Allows family members to refuse if you have not registered on the Organ Donor Register (and occasionally even if you have)
- Permits (some) live organ donation to be targeted to specific individuals
- Makes paid donation illegal
- Allow 12-year-old + to make their own decisions
157
what is the organ donation (deemed consent) bill of 2019?
Amends Human Tissue Act 2004 on the issue of consent:
The person concerned is to be deemed, for the purposes of …(transplantation).. to have consented to the activity unless a person who stood in a qualifying relationship to the person concerned immediately before death provides information that would lead a reasonable person to conclude that the person concerned would not have consented.
158
what is the normal pH of blood? how regulated?
7.4 in arterial blood
159
what is arterial blood pH regulated by? what are sources of acid? what happens to it? what happens to HCO3-?
- regulation is dominated by the HCO3-/CO2 buffering system
CO2 + H2O H+ + HCO3- (catalysed by carbonic anhydrases)
- CO2 = potential acid – because if you increase concentration of CO2, that pushes equilibrium to other side, increasing H+ concentration – so CO2 has potential to become acidic load to our body
- Metabolism -> CO2 -> excretion by lung
- Metabolism & diet -> H+ -> excretion by kidney
- HCO3- - retained by kidney – freely filtered into glomerulus – all absorbed by kidneys in order to buffer acid in body fluids
- Urine is always slightly acidic
160
what is the Henderson-Hasselbalch equation? what can you work out from it?
pH = pK + log10 [HCO3-]/[CO2]
- we want to maintain pH at 7.4
- pK = 6.1 (constant)
- therefore, log1- [HCO3-]/[CO2] = 1.3
- log20 = 1.3, therefore, [HCO3-][CO2] ratio needs to = 20 so that we maintain pH at 7.4
- we get this ratio by having [HCO3-] = 24 mM (regulated by kidneys) and [CO2] = 1.2 mM (regulated by respiratory system)
161
describe the whole body acid-base balance
- 15,000 mmol CO2 produced per day
- ‘potential acid’ – not a problem – excreted by lungs
- Metabolism also produces about 40 mmol H+ per day
- ‘non-volatile acids’: sulphuric, phosphoric, organic acids
- Net uptake of about 30 mmol H+ per day by GI tract (some bicarbonate also absorbed but overall a lot more acid)
- So, the kidney has to:
a) Excrete about 70 mmol H+ per day
b) Reabsorb all the filtered HCO3- (equivalent to about 4,000 mmol H+ per day!! If we didn’t reabsorb the bicarbonate in order to buffer the acid – bodies wouldn’t be able to deal with this)
162
how can you excrete 70 mmol H+ painlessly?
- Typical daily urine output: 1.5 litres
- 70 mmol H+ in 1.5 litres is 47 mM
- Without any buggering that would be pH 1.3
- Ouch! – very acidic
- And, renal epithelia can only secrete H+ to a concentration of about pH 4.4. (i.e. a 1000-fold concentration gradient)
- Excreted H+ needs to be buffered!
163
in all three mechanisms for acid secretion/HCO3- reabsorption what happens?
- H+ secreted across apical membrane
| - HCO3- absorbed across basolateral membrane (using various different transporters)
164
what is renal tubule acidosis (RTA)? what are the different types?
- What happens when HCO3- reabsorption and H+ excretion go wrong
Four types
- Type 1 – Distal RTA
- Type 2 – Proximal RTA
- Type 3 RTA (combination of type 1 and 2)
- Type 4 – Hyperkalaemia RTA (Hypoaldosteronism) (this type of acidosis due to having an aldosterone insufficiency or inability to respond to aldosterone in kidneys)
165
describe NH4+ secretion in proximal tubule
- NH4+ produced by glutamine metabolism (occurs in mitochondria of proximal tubule cells)
- NH4+ -> NH3 + H+
- NH3 freely diffused into lumen
- H+ ions transported into lumen – either via H+-ATPase or NHE3
- NH4+ ions reformed and excreted in this form
- On basolateral membrane, NBC (sodium-bicarbonate cotransporter), secreting ‘new’ HCO3- across membrane
166
describe how NH4+ re-secreted in collecting duct
(NH4+ contributing to medullary gradient which is quite important but overall we want to be excreting acid in the form of NH4+, so we get some re-secretion of NH4+ in the collecting duct – so now we’re getting NH4+ moving back the other way)
- NH4+ can replace K+, which is thought to be in the Na+,K+-ATPase on basolateral membrane (2NH4+ in cell, 3Na+ out cell)
- Majority of ammonium will come from NH3 diffusing back into cell across basolateral membrane which will then pass through apical membrane
- NH4+ -> NH3 + H+
- H+-ATPase will secrete H+
- NH3 + H+ -> NH4+ in lumen
• Most crosses epithelium as NH3
• Maybe some NH4+ is carried by Na+,K+-ATPase
167
describe regulation of respiratory/metabolic acidosis
| - what happens if chronic
* PCO2 (resp.) or [HCO3-] (metab.)
* (after being detected by kidneys) both directly stimulate H+ secretion and NH4+ synthesis by proximal tubule
* chronic leads to expression of NHE3 and kNBCe1 (so can be excreting more acid and be reabsorbing more bicarbonates)
168
describe regulation of respiratory/metabolic alkalosis
| - what happens if chronic
* opposite changes in proximal tubule – excrete less H+ ions
* chronic leads to more β-intercalated cells (HCO3- secreting) in collecting tubule (doesn’t occur under normal conditions)
169
give a summary of acid-base balance
* Kidney reabsorbs all of the filtered HCO3-
* Excretes additional H+ as titratable acid (using filtered buffers) and NH4+ (synthesized)
* All achieved by apical H+ secretion via NHE3, V-type H+-ATPase and H+,K+-ATPase
* Basolateral HCO3- exit via kNBCe1 and AE2 replenishes plasma HCO3-
* Dependent on carbonic anhydrases II & IV
170
what is motivational interviewing?
- A skillful clinical style for eliciting from patients their own good motivations for making healthy behaviour changes by exploring and resolving ambivalence
- Not for tricking people into doing what they don’t want to!
- In general, growing evidence for the effectiveness of MI
- MI out outperformed traditional lifestyle advice in 80% of studies and was no more time consuming
‘We are generally better persuaded by the reasons we discover ourselves than by those given to us by others’
171
what is collaborative? what is evoking? what is supporting autonomy?
Collaborative: conversation between equals working together, using our empathy skills
Evoking: asking the person about their motives, ideas and resources – guiding and listening not telling
Supporting autonomy: respectful of their choice and control, so as not to create resistance
172
what are the 4 processes (structured approach) of MI?
- Engaging
- Focusing
- Evoking
- Planning
(i.e. the general path across one consultation, or several)
Engaging: settle into a helpful conversation
Focusing: find a useful direction
Evoking: draw out person’s good reasons for change
Planning: if they want to, help them plan how to change
173
what are the two types of talk?
Sustain talk:
- Talk from the patient about staying the same
- Listen out for it
- Can identify obstacles the person needs to problem solve or lack of confidence
Change talk:
- Talk from the patient about changing
- Listen out for it
- Can identify motivation and confidence
- Helping people voice ‘change talk’ and explore it further helps them move towards change
174
what are the key communication skills in all four processes?
OARS
- Open questions
- Affirmation of strengths/progress – ‘that’s great that you’ve come along to appointment’
- Reflections – ‘wow it sounds like you’re struggling/doing great’ – shows that you’re listening – better than always asking questions
- Summarising
175
what is the EPE framework?
ELICIT – PROVIDE – ELICIT (EPE) FRAMEWORK
Elicit:
- What do they already know?
- What do they want to know?
- Do you have their permission to give information?
Provide:
- One tailored chunk of information
Elicit:
- What do they think of your information?
