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Renal Physiology > Glomerular Filtration, Reabsorption, Secretion and Loop of Henle > Flashcards

Glomerular Filtration, Reabsorption, Secretion and Loop of Henle Flashcards

1
Q

How much blood flow does the kidneys receive? And how much of the CO?

A

1200mls/min

20-25% total CO

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2
Q

Normal GFR value

A

125mls/min

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3
Q

Factors affecting glomerular filtration

A

Balance between hydrostatic forces favouring filtration and

Oncotic pressure forces favouring reabsorption (starlings forces)

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4
Q

What determines the filterability of solutes across the glomerular filtration barrier?

A

Molecular size
Electrical charge
Shape

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5
Q

Layers of the glomerular barrier

A
  1. Fenerstrations/pores of glomerular endothelial cell; prevents filtration of blood cells but allows all components of blood plasma to pass through
  2. Basal lamina of glomerulus; prevents filtration of larger proteins
  3. Sit membrane between pedicles; prevents filtration of medium sized proteins
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6
Q

Why is the glomerular capillary pressure higher than in most capillaries of the body?

A

Because the afferent arteriole is short and wide and therefore offers little resistance to flow

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7
Q

How does the efferent arteriole of the glomerular capillaries have a high post capillary resistance?

A

Unique arrangement - Long and narrow

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8
Q

What is the golden rule of the circulation?

A

If you have high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased

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9
Q

Which from the afferent and efferent arterioles contribute to the very high glomerular capillary pressure?

A

Both

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10
Q

At the glomerular capillaries, hydrostatic pressure vs oncotic pressure

A

Hydrostatic - favouring filtration always exceeds the oncotic pressure

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11
Q

What is the only thing that occurs at the glomerular capillaries?

A

Filtration

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12
Q

What is responsible for reabsorption?

A

Peritubular capillaries

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13
Q

How much is filtered through the glomerulus into the renal tubule? How much of this is excreted as urine?

A

180 L/day

1 - 2L is excreted as urine

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14
Q

Why is only 1 - 2L of 180L / day excreted as urine?

A

An enormous amount of fluid must be reabsorbed back into the peritubular capillaries

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15
Q

What arteriole has important effects on starlings forces in the peritubular capillaries?

A

Efferent arteriole

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16
Q

Hydrostatic pressure in efferent arteriole

A

Very low

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17
Q

IIp and concentration of plasma proteins in the efferent afteriole and peritubular capillaries

A

Increased

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18
Q

The balance of pressures in the peritubular capillaries are entirely favour of what?

A

Reabsorption

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19
Q

How much of H20 filtered at the glomerulus is reabsorbed within the tubule?

A

99%

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20
Q

How much of glucose filtered at glomerulus is reabsorbed within the tubule?

A

100%

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21
Q

How much of Na+ filtered at the glomerulus is reabsorbed within the tubule?

A

99.5%

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22
Q

How much of urea filtered at the glomerulus is reabsorbed within the tubule?

A

50%

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23
Q

Where in the tubule are substances mostly reabsorbed?

A

Proximal convoluted tubule

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24
Q

Which occurs at the glomerular capillaries?

A

Filtration

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25
Mechanisms of reabsorption
1. Carrier mediated transport systems 2. Reabsorption of Na+ ions 3. Tubular secretion
26
What substances are reabsorbed by carrier mediated transport systems?
``` Glucose Amino acids Organic acids Sulphate ions Phosphate ions ```
27
How do carrier mediated transport systems work to reabsorb substances?
Carriers have a maximum transport capacity Tm which is due to saturation of carriers If Tm is exceeded, then the excess substrate enters the urine
28
What is the renal threshold?
The plasma threshold at which saturation occurs
29
Filtrationn of glucose
Freely filtered
30
What is the renal plasma glucose threshold?
10mmoles/l
31
What is responsible for glucose regulation?
Insulin | Counter regulatory hormones
32
Normal plasma glucose concentration
5 mmoles/l
33
What is the name for the presence of glucose in the urine?
Glycosuria
34
What is the most abundant ion in the ECF?
Na+
35
What % of Na+ ion reabsorption occurs in the proximal tubule?
65 - 75%
36
How are Na+ ions reabsorbed?
Active transport | - Na+ pumps
37
Active transport of Na+ ions
Active Na+ pumps are located on the basolateral surfaces where there is a high density of mitochondria This decreases Na+ in the epithelial cells, increasing the gradient (a secondary gradient) for Na+ ions to move into the cells passively across the luminel membrane. So links the passive transport on the tubule lumen to the active transport on the intralumineal membrane
38
Are Na+ ions permeable at cell membranes?
No
39
Why does the brush border of the proximal tubule cells have a higher permeability to Na+ ions than most other membranes in the body?
Enourmous surface area offered by the microvilli and the large number of Na+ ions channels, which facilitate this passive diffusion of Na+
40
What is the reabsorption of Na+ ions key to?
The reabsorption of the other components of the fitrate
41
How do negative ions diffuse across the proximal tubule membrane?
Passively across the proximal tubular membrane down the electrical gradient established and maintained by the active transport of Na+
42
What does the active transport of Na+ out of the tubule followed by Cl- create?
An osmotic force, drawing H20 out of the tubules
43
What does H20 removed by osmosis from the tubule fluid result in?
It concentrates all the substances left in the tubule (e.g. glucose, urea) creating outgoing concentration gradients
44
What does the rate of reabsorption of these non actively reabsorbed solutes depend on?
Amount of H20 removed (which will determine the extent of the concentration gradient for other substances). More concentrated = easier to transport substances The permeability of the membrane to any particular solute
45
Permeability of the tubule membrane to urea
Moderately permeable to urea so that only about 50% is reabsorbed, the remainder stays in the tubule
46
Permeability of the tublar membrane to insulin and mannitol
Impermeable
47
What establishes the gradients down which other ions, H20 and solutes pass passively?
Active transport of Na+
48
Anything which decreases active transport of Na e.g. decreased blood flow results in what?
Disruption of renal function
49
Importance of Na+ transport
1. Active transport of Na+ establishes the gradient down which other ions, H20 and solutes pass passively 2. Carrier mediated transport systems - substances such as glucose, amino acids etc share the same carrier molecule as Na+ (symport) 3. Na+ reabsorption is linked to HCO3- ion reabsorption
50
Effects of [Na] concentration in the tubule on glucose transport
High [Na+] = facilitates glucose transport | Low [Na+] = inhibits glucose transport
51
What drives the co transport of sodium and glucose?
ATP hydrolysis
52
How does tubular secretion work?
Secretory mechanisms transport substances FROM the peritubular capillaries INTO the tubule lumen and therefore provides a second route into the tubule
53
What molecules are tubular secretion important for?
Those that are protein bound | e.g. drugs
54
Where are all of the substances in tubular secretion secreted?
In the proximal tubule
55
What are the 3 basic renal processes?
Filtration Reabsorption Secretion
56
Factor affecting GFR
Pressure in the glomerular capillaries
57
What is the pressure in the glomerular capillaries dependent on?
The afferent and efferent arteriolar diameter and therefore the balance of resistance between them
58
Extrinsic control of pressure in the glomerular capillaries
Sympathetic VC nerves - afferent and efferent constriction - Greater sensitivity of afferent arteriole Circulating catecholamines - constrict primary afferent Angiotensin II - constriction of efferent at [low], both afferent and efferent at [high]
59
What is autoregulation independent of?
Nerves | Hormones
60
Autoregulatory range in men
60 - 130 mmHg
61
In what situations would autoregulation be overriden and why?
Blood volume / BP face serious compromise e.g. in haemorrhage. Liberates blood for more immediately important organs
62
What is the renal threshold?
The plasma threshold at which saturation occurs
63
What is the major cation in the cells of the body?
K+
64
Normal ECF [K+]
approx. 4 mmoles/l
65
Hyperkalaemia value
5.5 mmoles/l
66
Hypokalaemia value
< 3.5 mmoles/l
67
What does hyperkalaemia result in?
Decrease resting membrane potential of excitable cells and eventually ventricular fibrillation and death
68
What does hypokalaemia result in?
Increases resting membrane potential i.e. hyperpolarises muscle, cardiac cells -> cardiac arrythmias and eventually death
69
Where is K+ reabsorbed?
Primarily at the proximal tubule
70
What are changes in K+ excretion due to?
Its secretion in distal parts of the tubule
71
K+ secretion is regulated by what?
Adrenal cortical hormone aldosterone
72
An increase in [K+] in ECF bathing the aldosterone secreting cells does what?
Stimulates aldosterone release which circulates to the kidneys to stimulate increase in the renal tubule cell K+ secretion
73
What does aldosterone also do?
Stimulates Na+ reabsorption at the distal tubule but by a different reflex pathway
74
Where is the major site of reabsorption?
Proximal tubule
75
Parts of the Loop of Henle
Ascending Limb | Descending Limb
76
What is the maximum concentration of urine that can be produced by the human kidney?
1200 - 1400 mOsmoles/l
77
What is the minimum obligatory H20 loss needed (even if we do not drink) and why is this?
500mls Due to urea, sulphate, phosphate, other waste products and non waste ions (Na+ and K+) which must be excreted each day to approx. 600mOsmoles
78
Minimum urine concentration a man can produce
30 - 50 mOsmoles/l
79
What nephrons are the loops of Henle in?
Juxtamedullary nephrons
80
How are kidneys able to produce urine of varying concentration?
Because of the loops of Henle acting as counter-current multipliers
81
How does the counter current of Loop of Henle work?
Fluid flows down the descending limb and UP the ascending limb, in osmotic correlation to each other Fluid enters at the proximal and leaves at the distal tubule - concentrated fluid in the descending limb rounds the bend and delivers a high conc to the ascending limb - active NaCl removal - further concentrates the interstitium So concentrates fluid on the way down and promptly re-dilutes it on the way back up, NOT by adding H20 but by removing NaCl
82
What is a counter current?
Something that flows passed itself
83
What are the critical characteristics of the loops which makes them counter current multipliers?
1. The ascending loop actively co transports Na+ and Cl- ions out of the tubule lumen and into the interstitium 2. The descending limb is freely permeable to H20 but relatively impermeable to NaCl
84
What is the ascending limb impermeable to?
H20
85
What is the overwhelming significance of the counter current multiplier?
It creates an increasingly concentrated gradient in the interstitium Also deliverys hypotonic fluid to the distal tubule
86
What are the vasa recta?
The specialised arrangement of the peritubular capillaries of the juxtamedullary nephrons acting as counter current exchangers
87
Permeability of vasa recta
Permeable to H20 and solutes and therefore equilibrium with the medullary interstitial gradient
88
Functions of the vasa recta
Providing O2 for medulla In providing O2 must not disturb gradient Removes volume from the interstitium, up to the 36l/day
89
Where is the site of water regulation?
The collecting duct
90
What is the collecting duct permeability under control of?
ADH
91
What does ADH stand for?
Anti diuretic hormone
92
Another name for ADH
Vasopressin

Renal Physiology (7 decks)

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