Glomerular filtration, renal blood flow, and their control

Question 1

What are the 4 determinants of GFR?

Answer 1

1. glomerular hydrostatic pressure (pro filtration)
2. colloid osmotic pressure from plasma proteins in the in capillary (reduces filtration)
3. Bowman’s capsule hydrostatic pressure (reduced filtration)
4. colloid osmotic pressure from plasma proteins Bowman’s capsule (negligible)

Question 2

What are 3 ways that can decrease the GFR?

Answer 2

1. decrease in filtration coefficient. Ex. increased thickness of glomerular capillary membrane or number of functional capillaries in some disease conditions
2. Increased Bowman’s capsule pressure. Ex. urinary obstruction increases the Bowman’s capsule hydrostatic pressure
3. Increased glomerular colloid pressure.
   - arterial colloid osmotic pressure
   - fraction of plasma filtered by glomerular capillaries –> increased with a decrease in renal blood flow

Question 3

What are 3 ways that can increase the glomerular hydrostatic pressure?

Answer 3

1. arterial pressure. Higher the arterial pressure, greater the hydrostatic pressure – but it’s normally buffered by autoregulation
2. afferent resistance. Higher the afferent resistance, lower the glomerular hydrostatic pressure
3. efferent resistance. Higher the efferent resistance, higher the glomerular hydrostatic pressure (as long as renal blood flow is not severely compromised)

Question 4

What are the determinants of renal blood flow?

Answer 4

The total pressure from arteries, arterioles, capillaries, and veins
Most of the renal vascular resistance come from 1. interlobular arteries, 2. afferent arterioles, and 3. efferent arterioles

Question 5

How does increased renal blood flow increase GFR even with a constant glomerular hydrostatic pressure?

Answer 5

Increase in renal blood flow –> lower fraction of the plasma is initially filtered out of the glomerular capillaries –> slower rise in glomerular capillary colloid pressure –> less inhibition effect on GFR

Question 6

What are some neurohormonal mechanisms that influence GFR?

Answer 6

1. norepinephrine/ epinephrine. Afferent and efferent arteriole constriction –> reduces renal blood flow & GFR
2. Endothelin. Peptide released from damaged endothelial cells. Vasoconstricts afferent arterioles –> reduces renal blood flow & GFR
3. Angiotensin II. Increases the efferent arterioles vasoconstriction more than the afferent –> increases the hydrostatic pressure while decreasing the renal blood flow. In times of hypotension/ hypovolemia, Angiotensin II will help to decrease the reduction of GFR by increasing efferent resistance.
4. Endothelium derived nitric oxide. Reduces renal vascular resistance –> increased renal blood flow and GFR
5. Prostaglandin (esp PGE2 and PGI2). Dampens the effects of sympathetic vasoconstriction or from Angiotensin II

Question 7

Describe the tubuloglomerular feedback/ autoregulation of GFR.

Answer 7

• This system is uses NaCl concentration at the macula densa.
• 2 components: afferent arteriolar and efferent arteriolar feedback
• macula densa cells are in the initial portion of the distal tubule & juxtaglomerular cells are in the walls of the afferent and efferent arterioles
• the macula densa cell senses a low NaCl concentration –> juxtaglomerular cells releases renin –> increases formation of angiotensin I –> converted to angiotensin II –> efferent arteriole vasoconstriction –> increases GFR
• the juxtaglomerular cells also decreases renal blood flow in the afferent arterioles –> increases the hydrostatic pressure –> increases the GFR

Question 8

How is NaCl influenced by GFR?

Answer 8

decrease GFR –> slows the flow rate in the loop of Henle –> increase reabsorption of NaCl

Question 9

How does myogenic mechanism contribute to GFR?

Answer 9

• with hypertension, it increases vascular resistance to prevent overstretching and excessive increase in renal blood flow and GFR
• with hypotension, it will decrease the vascular resistance to promote renal blood flow and GFR
• the myogenic response is probably more important in protecting the kidneys from hypertension induced injury

Question 10

How does high protein diet, hyperglycemia, glucocorticoids, fever, and age influence GFR?

Answer 10

1. High protein diet increases GFR. Amino acid is reabsorbed in the proximal tubule via co-transported with Na –> macula densa senses low Na –> increases GFR.
2. Hyperglycemia increases GFR. Same mechanism as high protein diet.
3. Glucocorticoids increases GFR. It decreases vascular resistance
4. Fever increases GFR. It decreases vascular resistance
5. Aging reduces GFR due to decrease in functional nephrons