1B renal regulation of water and acid-base balance

Question 1

Define osmosis

Answer 1

Flow of water from area of low solute conc to area of high solute conc across a semi-permeable membrane

Question 2

What is the driving force for osmosis?

Answer 2

Osmotic (aka oncotic) pressure which depends upon number (not size) of solute particles

Question 3

How do we calculate osmolarity?

Answer 3

Osmolarity (Osm/L or mOsm/L) = conc x no. of dissociated particles

Question 4

Calculate the osmolarity for 100 mmol/L glucose and 100 mmol/L NaCl

Answer 4

• Osmolarity for glucose = 100 x 1 = 100 mOsm/L
• Osmolarity for NaCl = 100 x 2 = 200 mOsm/L → this is because NaCl’s dissociated particles is both Na+ and Cl-

Question 5

How much of body weight is fluid?

Answer 5

60%

Question 6

Describe the body’s fluid distribution in different compartments (in %)

Answer 6

• 2/3 Intracellular fluid
• 1/3 extracellular fluid (ECF)
  
  • 1/4 of this is intravascular (plasma in bloodstream)
  • 3/4 of this is extravascular
    
    • 95% of this is interstitial fluid (that surrounds and bathes cells)
    • 5% of this is transcellular fluid (including cerebrospinal fluid, peritoneal fluid)- very important though

Question 7

What are ways of unregulated water loss?

Answer 7

• Sweat
• Faeces
• Vomit
• Water evaporation from respiratory lining and skin

Question 8

What is the regulated way of losing water?

Answer 8

Renal regulation through urine production

It does this through positive and negative water balance

Question 9

Describe the steps for positive water balance

Answer 9

High water intake → increases ECF volume (that is the first place water goes when it enters body) → lowers Na+ conc → lowers osmolarity → kidney produces hypoosmotic urine to lose water → osmolarity of ECF normalises

Question 10

Describe the steps for negative water balance

Answer 10

Low water intake → lowers ECF volume → increases Na+ conc → increases osmolarity → leads to hyperosmotic urine production (compared to plasma) → osmolarity of ECF normalises

Question 11

What happens at PCT water-wise?

Answer 11

67% of water is reabsorbed at PCT
(facilitated by sodium absorption)

Question 12

What happens in the descending loop of Henle?

Answer 12

• Water passively reabsorbed
• NaCl isn’t reabsorbed

Question 13

What happens in the ascending loop of Henle?

Answer 13

• NaCl is reabsorbed passively in the thin ascending limb
• NaCl is also reabsorbed actively in the thick ascending limb
• Water can’t be reabsorbed

Question 14

What happens at the DCT and collecting duct?

Answer 14

• There’s a variable amount of water reabsorbed depending on body’s needs
• Action of ADH kicks in here to modulate aquaporin channels (open and closing them) to vary amount of water reabsorption

Question 15

When it comes to water reabsorption from kidney, how and why is it done passively?

Answer 15

• Water is reabsorbed through the passive process of osmosis and requires a gradient
• This is done because body doesn’t want to spend too much energy absorbing water
• The medullary interstitium needs to be hyperosmotic for water reabsorption to occur from Loop of Henle and collecting duct

Question 16

Describe how the process of countercurrent multiplication works in steps

Answer 16

1) Filtrate arrives at loop of Henle at 300 mOsm/L which is isoosmotic with the plasma (300 mOsm/L too)

2) Active salt reabsorption occurs- in the thick ascending loop, salt is actively reabsorbed into interstitium so osmolarity in tubular filtrate of ascending loop decreases 300 → 200 and medullary interstitium osmolarity rises 300 → 400 because salt is being added

3) Passive water reabsorption occurs- since interstitium osmolarity is higher, water from descending loop moves into interstitium through osmosis to equilibrate the osmolarity- this causes descending loop osmolarity to increase 300 → 400

These 2 steps above basically repeat themselves over and over again now:

4) More filtrate arrives at descending loop (at 300 mOsm/L) and pushes rest of filtrate along loop which changes up the osmolarities along the loop

5) Active salt reabsorption occurs- salt gets reabsorbed from thick ascending loop which increases osmolarity of interstitium and osmolarity in ascending loop falls

6) Passive water reabsorption occurs- water flows out from descending limb into interstitium so descending loop osmolarity increases til its = to interstitium osmolarity

Gradient in medullary interstitium already developing from outer medulla to inner medulla

This process repeats again and again (hence multiplication process) to achieve a proper gradient down medulla

It’s called counter current because filtrate flows in opposite directions in ascending and descending loops of Henle

Question 17

How does countercurrent multiplication help us?

Answer 17

It helps water passively reabsorb into body without spending a lot of energy

Question 18

In collecting duct cells what side does the basolateral cell membrane face?

Answer 18

The side with the blood capillaries

Question 19

What side does the apical cell membrane face?

Answer 19

The lumen of the collecting duct (the inside of the tube)

Question 20

What is the vasa recta?

Answer 20

A series of blood capillaries that surround nephron mainly in medullary region

Question 21

What happens to urea after being filtered through Bowman’s capsule?

Answer 21

1) It travels through nephron and reaches collecting duct

2) Through UT-A1 and UT-A3 transporters the urea is transported out into medullary interstitium (conc of urea in interstitium can be as high as 600 mmol/L)

3) Urea in interstitium can now either:

• go into vasa recta through UT-B1 transporter which surrounds nephron so urea circulates medullary region
• go into descending limb of loop of Henle through UT-A2 transporter where it goes back through nephron and some exits collecting duct back into interstitium again (recycling)

Question 22

What is the purpose of the recycling of urea?

Answer 22

To increase interstitium osmolarity which:

• Allows urine concentration to occur- water moves from collecting duct into interstitium (cuz osmolarity in interstitium is higher)
• Urea excretion requires less water- this is because when filtrate reaches inner medullary collecting duct it equilibrates with urea in inner medullary interstitium (which could be as high as 600 mmol/L so conc of urea in collecting duct could also go up to as high as this)- this urea then requires less water to excrete

Question 23

What does vasopressin do to urea filtration at the Bowman’s capsule?

Answer 23

Helps boost UT-A1 and UT-A3 numbers to increase collecting duct’s permeability for urea to aid urea reabsorption

Question 24

What is Vasopressin?

Answer 24

Protein hormone with length of 9 amino acids

Question 25

What is the main function of vasopressin?

Answer 25

Promote water reabsorption from collecting duct

Question 26

What functions other than water reabsorption does vasopressin have?

Answer 26

- Helps in urea reabsorption - Helps in sodium reabsorption

Question 27

Where is vasopressin produced?

Answer 27

In hypothalamus by neurones in supraoptic and paraventricular nuclei

Question 28

Where is vasopressin stored?

Answer 28

Once produced it’s packaged into granules and sent to posterior pituitary for storage

Question 29

How does plasma osmolarity affect ADH production and release?

Answer 29

- Increase in plasma osmolarity is detected by osmoreceptors in hypothalamus (are sensitive to even 2-3% change) this stimulates ADH production and release to open aquaporin channels in collecting duct to reabsorb water - Decrease in plasma osmolarity inhibits ADH production and release to keep aquaporin channels closed because we have excess fluid in plasma which we want to lose

Question 30

What is plasma osmolarity in a healthy adult?

Answer 30

275-290 mOsm/kg H2O

Question 31

How does hypovolemia affect ADH production and release?

Answer 31

Hypovolemia means low blood volume → decreases blood pressure → detected by baroreceptors → info transmitted to hypothalamus → body wants to conserve fluid so this stimulates ADH production and release to open aquaporins to reabsorb water

Question 32

How does hypervolemia affect ADH production and release?

Answer 32

Hypervolemia means high blood volume → increases blood pressure → detected by baroreceptors → info transmitted to hypothalamus → body wants to get rid of excess fluid so inhibits ADH release to keep aquaporins closed to lose water

Question 33

How much of a change in blood pressure is needed for baroreceptors to detect it?

Answer 33

5-10%

Question 34

What factors other than plasma osmolarity and hypo/hypervolemia stimulate ADH production and release?

Answer 34

- Nicotine - Nausea - Angiotensin II

Question 35

What factors other than plasma osmolarity and hypo/hypervolemia inhibit ADH production and release?

Answer 35

- Ethanol - Atrial natriuretic peptide

Question 36

Describe how ADH works at the collecting duct

Answer 36

1) ADH binds to V2 receptor on basolateral membrane of principal cells of collecting duct 2) This triggers G-protein mediated signal cascade in cell 3) This activates protein kinase A 4) This increases secretion of aquaporin 2 channels in vesicle form which are inserted into apical cell membrane 5) Water then absorbed through aquaporin 2 into cell then through aquaporin 3 and 4 in basolateral cell membrane into blood vessel **Overall ADH up/downgrades both AQP2 (apical membrane) and AQP3 (basolateral membrane) numbers as required**

Question 37

What is diuresis?

Answer 37

Increased excretion of dilute urine

Question 38

Describe the mechanism for diuresis occurring in the nephron

Answer 38

1) **ADH amount is 0 or small** 2) Blood filtered at Bowman’s capsule and filtrate enters nephron 3) Filtrate passes into PCT where 67% of water and 67% of NaCl are reabsorbed into body 4) Isoosmotic fluid reaches descending limb of loop of Henle where water passively moves into interstitium 5) Fluid enters ascending limb of loop of Henle where NaCl is reabsorbed into interstitium leaving hypoosmotic fluid 6) This hypoosmotic fluid enters DCT where AQP2 channels are absent since there’s negligible ADH, however NaCl is actively reabsorbed again- this makes the fluid even more hypoosmotic 7) Hypoosmotic fluid enters collecting duct where more NaCl is reabsorbed 8) When it reaches inner medullary region, some water is also reabsorbed since some aquaporins are always present and water moves via other pathways e.g. paracellularly between epithelial cells 9) This leaves the fluid even more hypoosmolar to create urine at 50 mOsm/L compared to plasma which is 300 mOsm/L

Question 39

At a cellular level how is NaCl reabsorbed in thick ascending limb?

Answer 39

1) Na+ K+ ATPase pump pumps Na+ into blood creating low conc of Na+ in cell 2) Through Na+ K+ 2Cl- symporter, Na+ moves from region of high Na+ (in tubular fluid) to low Na+ in cell which releases energy which K+ and Cl- use to get transported through this symporter into the cell 3) Once in cell K+ and Cl- then exit cell into blood through K+ Cl- symporter

Question 40

At a cellular level how is NaCl reabsorbed in the DCT?

Answer 40

1) Through **Na+ K+ ATPase pump**, Na+ is pumped into blood 2) NaCl enters cell from tubular filtrate through **Na+ Cl- symporter** 3) KCl leaves cell into blood via **K+ Cl- symporter**

Question 41

At a cellular level how is Na+ reabsorbed in collecting duct?

Answer 41

Through Na+ channels and Na+ K+ ATPase pump

Question 42

What is antidiuresis?

Answer 42

Concentrated urine excreted in low volumes- could be due to dehydration or disease

Question 43

Describe the mechanism for antidiuresis in the nephron

Answer 43

1) There is a high amount of ADH 2) Fluid filtered through Bowman’s then passes through PCT where 67% water and 67% NaCl is reabsorbed leaving isosmotic fluid 3) Water moves out of fluid in descending limb and NaCl moves out of fluid in ascending limb of loop of Henle leaving hypoosmolar fluid 4) In DCT then collecting duct NaCl is again actively reabsorbed 5) In DCT and collecting duct, AQP2 are present so water is reabsorbed 6) In collecting duct there is a gradient of osmolarity which increases as you go down it meaning more and more water is reabsorbed into interstitium from outer medullary to inner medullary regions 7) By the time urine leaves kidneys the osmolarity could be as high as 1200 mOsm/L and urine volume as low as 500ml per day

Question 44

What does ADH do in terms of Na+ reabsorption (specific transporters)?

Answer 44

Supports it in: - Thick ascending limb- increased Na+ K+ 2Cl- symporters - DCT- increased Na+ Cl- symporters - Collecting duct- increased Na+ channels

Question 45

List some ADH related clinical disorders

Answer 45

- AVP deficiency - AVP resistance - Symptom of inappropriate ADH secretion (SIADH)

Question 46

What are the causes of AVP deficiency?

Answer 46

- Decreased/negligent production and release of ADH - Can be genetic or acquired due to e.g. infection or trauma

Question 47

What are the clinical features of AVP deficiency?

Answer 47

- Polyuria- large urine volume - Polydipsia- thirst

Question 48

What is the treatment for AVP deficiency?

Answer 48

External ADH

Question 49

What is the cause for AVP resistance?

Answer 49

Correct amount of ADH produced but something going wrong at collecting duct - Fewer/mutant AQP2 - Mutant V2 receptors

Question 50

What are the clinical features for AVP resistance?

Answer 50

- Polyuria - Polydipsia

Question 51

What treatments are given for APV resistance?

Answer 51

- Thiazide diuretics- reduce filtration rate at Bowman’s capsule so less blood filtered so less urine produced - NSAIDs

Question 52

What is the cause of SIADH?

Answer 52

Increased production and release of ADH

Question 53

What are the clinical features of SIADH?

Answer 53

- Hyperosmolar urine - Hypervolemia - Hyponatremia

Question 54

What treatment is given for SIADH?

Answer 54

**Non peptide inhibitor** of ADH receptor (conivaptan and tolvaptan)

Question 55

How are acids and bases added to our body?

Answer 55

Through our diet and metabolism

Question 56

What happens to the acids and bases added in our diet and metabolism?

Answer 56

- A lot of base is excreted in faeces - There is however a net addition of metabolic acid to our fluid compartments (50-100 mEq/day)

Question 57

Why is it important to neutralise excess metabolic acid?

Answer 57

Otherwise it’ll impact blood pH

Question 58

How is excess metabolic acid neutralised?

Answer 58

- By different buffer systems, mainly by bicarbonate buffer system - This produces sodium salts and CO2

Question 59

How much bicarbonate do we have in ECF compartment?

Answer 59

350mEq or 24mEq/L

Question 60

Why is bicarbonate in the ECF compartment important?

Answer 60

If the body just continuously used bicarbonate to neutralise metabolic acids without replenishing it, the bicarbonate would run out in 4-7 days

Question 61

How do kidneys come in regulating HCO3-?

Answer 61

Kidneys make sure HCO3- is replenished by: - Helps with **secretion and excretion H+** - There is a fine balance because if body secretes more H+ than metabolic acids coming in, there will be an alkalosis - If less H+ is secreted then acidosis will occur - **Reabsorption of 100% of HCO3-** - **Production of new HCO3-**

Question 62

What is the equation important in acid-base balancing?

Answer 62

Question 63

What does the equation in acid-base balancing highlight?

Answer 63

The bicarbonate ion buffer system is a unique buffer system which is managed by both lungs (through CO2) and kidneys (through HCO3-)

Question 64

What does the Henderson-Hasselbalch equation help us study?

Answer 64

- The role that PCO2 and conc of HCO3- plays on blood pH - It shows that if PCO2 rises in body, this increases H+ ion conc causing acidosis and if it falls it causes an acidosis - It also shows inverse relationship of H+ conc with HCO3- conc → if HCO3- rises it’ll cause alkalosis and if it falls it’ll cause an acidosis

Question 65

What is an acid-base disorder due to changes in PCO2 called?

Answer 65

Respiratory disorder

Question 66

What is an acid-base disorder due to changes in HCO3- conc called?

Answer 66

Metabolic disorder

Question 67

How much HCO3- is reabsorbed in nephron at different areas?

Answer 67

- 80% in PCT - 10% in loop of Henle - 6% in DCT - 4% in collecting duct

Question 68

Describe how HCO3- is reabsorbed at PCT

Answer 68

1) CO2 enters cell from tubular fluid by diffusion 2) CO2 reacts with H2O in presence of carbonic anhydrase to make H+ and HCO3- 3) H+ can leave cell into tubular fluid through 2 methods: - Through **Na+ H+ antiporter**- using downward energy from Na+ travel into cell from tubular fluid, H+ moves into tubular fluid - Through **H+ ATPase pump** which pumps H+ ion 4) HCO3- from the reaction is reabsorbed from cell into blood through **Na+ HCO3- symporter** 5) The H+ that left the cell into tubular fluid then reacts with HCO3- in fluid to form H2CO3 which splits into H2O and CO2, the CO2 of which moves back into cell to start from step 1) with the main final step being the HCO3- leaving cell into blood

Question 69

Describe how HCO3- is reabsorbed at ascending limb of loop of Henle

Answer 69

Very similar to PCT

Question 70

Describe how HCO3- is reabsorbed at DCT and collecting duct

Answer 70

- **Alpha intercalated cell** - HCO3- reabsorption and H+ secretion - **Beta intercalated cell** - HCO3- secretion and H+ reabsorption

Question 71

What happens at alpha intercalated cells?

Answer 71

1) H2O + CO2 leads to H+ and HCO3- 2) H+ moves into tubular fluid via **H+ ATPase pump** and **H+ K+ ATPase** where it combines with HCO3- to form H2CO3 which splits into H2O and CO2, the CO2 of which diffuses back into cell to do step 1) 3) HCO3- moves into blood via **Cl- HCO3- antiporter**

Question 72

What happens at beta intercalated cells?

Answer 72

1) H2O + CO2 leads to H+ and HCO3- 2) Cl- HCO3- antiporter secretes HCO3- into tubular fluid (to excrete in the case of alkalosis in body due to excess HCO3-) 3) H+ is absorbed back into blood via H+ ATPase pump

Question 73

Describe how new HCO3- is produced at PCT

Answer 73

1) **1 glutamine molecule** gives: - 2 NH4+ molecules - 1 divalent ion (A2-) which gives rise to 2 HCO3- 2) The 2 NH4+ are excreted into tubular fluid through 2 methods: - Through Na+ H+ antiporter (NH4+ substitutes in place of H+) - Turns into NH3 and moves into tubular fluid where it combines with a H+ to form NH4+ again 3) NH4+ then excreted from body leaving us with a net gain of 2 HCO3-

Question 74

Why do we not want the two ammonia ions to enter blood?

Answer 74

If it does, it will go to liver and be split into NH3 and H+, the H+ of which will require 1 HCO3- to neutralise it, which will nullify the effect of the 2 HCO3- we just produced from splitting the glutamine

Question 75

Describe how new HCO3- is produced at DCT and collecting duct

Answer 75

- In alpha intercalated cell, when H+ is secreted into tubular fluid, instead of being neutralised by a HCO3-, it’s neutralised by a **non-bicarbonate buffer**- a phosphate ion H+ + HPO42- → H2PO4- - By using a non-bicarbonate buffer, the HCO3- that is produced in the alpha intercalated cell and reabsorbed into blood is a net gain of HCO3-

Question 76

How is metabolic acidosis characterised?

Answer 76

Decrease in HCO3- conc leading to decrease in pH - What is the compensatory response? - Increased (hyper)ventilation which kicks in first → PCO2 goes down so H+ conc goes down - Increased HCO3- conc reabsorption and production to compensate for decrease in conc

Question 77

How is metabolic alkalosis characterised?

Answer 77

Increased HCO3- conc leading to increased pH

Question 78

What is the compensatory response for metabolic alkalosis?

Answer 78

- Decreased (hypo)ventilation which kicks in first → PCO2 goes up so H+ conc goes up - Increased HCO3- conc excretion to compensate for increase in conc

Question 79

How is respiratory acidosis characterised?

Answer 79

Increased PCO2 leading to lower pH

Question 80

How is respiratory acidosis compensated?

Answer 80

- Acute- increased CO2 enters cells where it reacts with H2O in presence of carbonic anhydrase to produce a H+ and a HCO3- and the H+ is neutralised by cellular proteins so there’s a net gain of HCO3- which is transported back into blood to help with increasing pH - Chronic- increased HCO3- reabsorption and production in kidney along with increased H+ and NH4+ ion excretion to help normalise pH

Question 81

How is respiratory alkalosis characterised?

Answer 81

Decreased PCO2 leading to higher pH

Question 82

How is respiratory alkalosis compensated?

Answer 82

- Acute- shifting bicarbonate buffer reaction towards more carbonic acid production so less HCO3- produced - Chronic- decreased HCO3- reabsorption and production in kidney to normalise pH