Glomerular Filtration

Question 1

What are the basic renal processes? Brief descriptions too

Answer 1

• Filtration: formation of an essentially protein free filtrate at the glomerular capillaries
• Reabsorption: reabsorb what needs to be kept from filtrate
• Secretion: get rid of the rest

Question 2

What is the glomerular filtration rate?

Answer 2

Volume of the plasma from the glomerular capillaries filtered into the Bowmans capsule per unit time

Question 3

What is the approximate normal glomerular filtration rate? What does this allow the body to do?

Answer 3

• 180 L/day

- Allows monitoring of ECF volume and efficient elimination of wastes

Question 4

Where does reabsorption occur? What is absorbed where?

Answer 4

• In the proximal and distal tubules of the nephron

Proximal tubule: AA’s, sugars, NaCl and water reabsorbed
Distal tubule: NaCl and water reabsorbed

Question 5

Where does secretion occur in the kidneys? What are the main substances secreted at each site?

Answer 5

• Proximal and distal tubules of the nephron

Proximal: Organic ions and drugs
Distal: K & H ions

Question 6

Why are the kidneys particularly vulnerable to vascular disease?

Answer 6

Because they receive such a high CO (20-25% of the total), at relatively high pressure.

Question 7

What is the approximate average value for GFR? What percentage of plasma flowing through the corpuscle does this account for?

Answer 7

GFR usually about 125mL/min
Kidneys receive a BF of about 1200mL/min, 55% of which is plasma, so about 660mL/min (renal plasma flow)

125/660 = 19% (filtration fraction) - so about 19% of renal plasma flow becomes filtrate

Question 8

What forces determine the amount of glomerular filtration?

Answer 8

Starlings Forces

• Hydrostatic force promoting filtration
• Oncotic pressure favouring reabsorption

Question 9

Give a brief description of how permeable the glomerular barrier is to certain molecules

Answer 9

• Completely permeable to ions, simple sugars, water and urea
• Significantly less permeable to proteins depending on charge and size, ranging from around 98% permeability for insulin, to <1% for albumin

Question 10

Describe the different layers of the filtration membrane and what each stops from entering the Bowmans capsule

Answer 10

1. (inner) Fenestrations of glomerular endothelial cells allows all components of blood plasma except blood cells through
2. Basal lamina of the glomerulus: prevents filtration of larger proteins
3. (outer) Slit membranes between pedicels of podocyte cells prevents filtration of medium sized proteins

Question 11

How does the glomerular capillary pressure compare to the capillary pressure elsewhere in the body? Why is this?

Answer 11

• Significantly higher pressure in glomerular capillaries to keep hydrostatic pressure high, for filtration
• This is because the afferent arteriole is short and wide, offering little resistance to flow

Question 12

How do the efferent and afferent arterioles compare?

Answer 12

• Afferent: short and wide, offering little resistance

- Efferent: long and narrow, offering high post capillary resistance

Question 13

How does the efferent arteriole contribute to the high hydrostatic pressure at the glomerular capillaries?

Answer 13

• Having high resistance causes hydrostatic pressure upstream to be increased, and hydrostatic pressure downstream to be decreased

Question 14

What are the different pressure values that determine the net filtration gradient at the corpuscle?

Answer 14

• Hydrostatic pressure from blood (55mmHg)
• Oncotic pressure from proteins in plasma (30mmHg)
• Fluid pressure from fluid in Bowmans capsule (15mmHg)

Net: 55 - 30 - 15 = 10mmHg favouring filtration

Question 15

What are some factors that affect the GFR via altering the diameter of afferent / efferent arterioles?

Answer 15

• Sympathetic VC nerves: afferent and efferent constriction, afferent arteriole more sensitive
• Circulating catecholamines (adrenalines): cause constriction of the afferent arteriole
• Angiotensin II: Constriction of efferent at {low], constriction of both at [high]

Question 16

Describe the autoregulation of GFR in response to changes in BP

Answer 16

• If there is an increase in BP there is constriction of the afferent arteriole causing a reduction in glomerular capillary pressure
• In there is a decrease in BP the inverse occurs

This allows GFR to remain relatively constant over a range of blood pressure

Question 17

What range of blood pressure is glomerular autoregulation effective over?

Answer 17

60 - 130 mmHg

Question 18

Is glomerular autoregulation dependent of nerve / hormone input?

Answer 18

Autoregulation is independent of nerves or hormones, occurs in denervated and in isolated perfused kidneys.

Question 19

How is kidney blood flow altered in times where blood volume is severely compromised, as in a haemorrhage? How is this achieved?

Answer 19

• Blood flow to the kidneys can be redirected to other more immediately important organs. Can allow for as much as 800mL of blood to be redistributed
• Stimulation of sympathetic vasoconstriction nerves causes this to occur

Question 20

Approximately what percentage of plasma that enters the kidneys is reabsorbed to the systemic blood flow?

Answer 20

99%

Barely any fluid volume is actually excreted

Question 21

How does the efferent arteriole facilitate reabsorption in the peritubular capillaries?

Answer 21

Its long and narrow shape provides significant resistance, which lowers hydrostatic pressure - making the oncotic and fluid pressure dominate causing reabsorption

Question 22

Why is oncotic pressure much higher in the peritubular capillaries?

Answer 22

• Because there is a loss of 20% plasma volume at the corpuscle, therefore the protein in the blood is even more concentrated than usual resulting in higher oncotic pressure

Question 23

How much water, glucose, Na and urea filtered at the glomerulus are reabsorbed? Where are they mainly reabsorbed?

Answer 23

99% H2O, 100% glucose, 99.5% Na+, 50% urea filtered at the glomerulus are reabsorbed, mainly at the proximal convoluted tubule