Glomerular filtration Flashcards
What are the three main processes performed by the nephron?
1) Filtration
2) Reabsorption
3) Secretion
How do you calculate urinary excretion rate?
Urinary excretion rate = filtration rate + secretion rate - reabsorption rate
Why is a high rate of filtration needed?
High rate of filtration needed to help clear waste products efficiently
What is the blood flow to the kidney?
Filtration requires good blood flow to kidney
Kidneys receive ~20% cardiac output
∴ Renal blood flow ~1 litre / min (haematocrit ~0.4 400ml/min)
∴ Renal plasma flow ~600ml / min
Define Glomerular Filtration Rate
The volume of filtrate formed by all the nephrons in both kidneys per unit time
What determines GFR?
1) Glomerular capillary filtration coefficient, Kf
2) Net filtration pressure (NFP)
GFR = Kf x NFP
What does the glomerular capillary filtration coefficient Kf reflect?
1) Surface area available for filtration (a small sieve vs big sieve)
2) Hydraulic conductivity (‘permeability’) of the filtration barrier (how big are the holes in the sieve?)
Changes in Kf aren’t the major part of the physiological regulation but may be affected in disease processes
How do you calculate Net Filtration Pressure (NFP)?
NFP = PG – PB – πG + πB
How does most physiological regulation of GFR occur?
By changing glomerular hydrostatic pressure (PG)
What does glomerular hydrostatic pressure (PG) depend on?
1) Arterial pressure
2) Afferent arteriole resistance
3) Efferent arteriole resistance
How can you vary glomerular hydrostatic pressure (PG) independently of arterial pressure?
By varying the resistance of the afferent and efferent arterioles
Would afferent arteriole dilation and efferent arteriole constriction increase or reduce GFR?
Increases GFR
Would afferent arteriole constriction and efferent arteriole dilation increase or reduce GFR?
Reduces GFR
How are pressures in the peritubular capillary different from the glomerular capillary?
Peritubular hydrostatic pressure is lower and colloid osmotic pressure is higher meaning that peritubular capillaries favour absorption rather than filtration
Name two mechanisms of auto regulation of GFR
1) Myogenic response
2) Tubuloglomerular feedback
Describe the stages of myogenic autoregulation of GFR
Myogenic response = inherent ability of smooth muscle in afferent arterioles to respond to changes in vessel circumference by contracting or relaxing
1) Increase in arterial blood pressure
2) Increased renal blood flow and increased GFR
3) Increased stretch of afferent arteriole (AA) smooth muscle cells
4) Opens Ca2+ channels
5) Reflex contraction of AA smooth muscle
6) Vasoconstriction of AA
7) Increased resistance to flow
8) Prevents changes in renal blood flow and GFR
Describe the stages of tubuloglomerular feedback in autoregulation of GFR if blood pressure increased
1) Increase in arterial blood pressure
2) Increased renal blood flow and increased GFR
3) Increased [NaCl] delivered to macula densa cells
4) Release of paracrine factors (eg. adenosine)
5) Constriction of AA smooth muscle
6) Vasoconstriction of AA
7) Increased resistance to flow
8) Restores renal blood flow and GFR
Describe the stages of tubuloglomerular feedback in autoregulation of GFR if blood pressure decreased
1) Decrease in arterial blood pressure
2) Decreased renal blood flow and decreased GFR
3) Decreased [NaCl] delivered to macula densa cells
4) Release of renin and angiotensin II
5) Increased EFFERENT arteriolar resistance
What happens to GFR in kidney diseases?
GFR is directly related to the function of the nephrons and declines in all forms of progressive kidney diseases. GFR is usually accepted as the best overall index of kidney function in health and disease.
