Lecture 17 : Physiology of the Renal System II: Glomerular Filtration

Question 1

Volume of blood flow to kidney per minute?

Answer 1

1.1 L / min, so renal plasma flow rate of 600 ml / min

Question 2

Glomerulus Structure

Answer 2

Proximal tubule – Reabsorbs water, ions, and nutrients from the filtrate back into the blood. Also secretes substances into the filtrate

Bowman’s capsule – Surrounds the glomerulus and collects the filtrate that passes out of the capillaries

Capillary tuft (Glomerulus) – A network of capillaries where blood is filtered under pressure to form filtrate

Afferent arteriole – Brings blood into the glomerulus. Its diameter influences glomerular filtration rate (GFR)

Efferent arteriole (portal vessels) – Carries filtered blood away from the glomerulus and helps maintain pressure needed for filtration

Question 3

What is and isn’t filtered during ultrafiltration in the glomerulus

Answer 3

Question 4

What drives ultrafiltration in glomerulus

Answer 4

Hydrostatic pressure :
- Higher Hydrostatic pressure ( 50mmHg ) in capillaries drives fluid out becoming filtrate

Osmotic / Oncotic Pressure :
- Higher osmotic pressure in capillaries
- As fluid leaves, plasma proteins (albumin mainly) remain in the blood in the capillaries
- Pulling water back into the capillaries, opposing filtration and regulating how much fluid is filtered out of blood into bowmans capsule

If hydrostatic pressure > osmotic / oncotic pressure, filtration continues

Osmotic Pressure Equation

Question 5

Ultrafiltration process

Answer 5

Question 6

Glomerular barrier structure - 3 layers

Answer 6

1. Endothelial cells in capillaries of glomerulus
   - small, 60nm fenestrations
   - Negatively charged glycocalyx creating a charge barrier than is effective for blocking proteins
2. Glomerular Basement Membrane
   - Also negatively charged due to collagen so blocks proteins
3. Podocytes are the epithelial cells in the Bowman’s Capsule
   - Podocytes only allow small molecules to pass through
   - Pedicels are little projections that form barrier
   - Block large proteins from entering filtrate

Question 7

Bulk flow vs Diffusion

Answer 7

Question 8

Filtrate and GFR

Answer 8

Molecules mostly less than 10kDa in size

GFR is 120ml / min, 180L / day

Each nephron filters 30-50nL / min

Question 9

What is the filtration fraction

Answer 9

Question 10

Control of glomerular hydrostatic pressure

Answer 10

Question 11

How can we measure GFR?

Answer 11

Question 12

Notes for empirical estimates of GFR

Answer 12

Take 1 urine sample and assume rate of production of creatinine is fairly constant per given person, and correlated to sex, mass and age.

At EQBM, rate of creatinine production = rate of creatinine loss in kidney

Only appropriate for adults, we lose nephrons growing up, causing Creatinine to rise

Question 13

Proteinuria

Answer 13

Presence of unusual concentration of protein in urine, indicating kidney damage especially to glomerular filtration barrier.

Question 14

Congenital Nephrotic Syndrome

Answer 14

Rare genetic disorder involving podocytes where glomerulus is more permeable to plasma proteins and so more protein is found in urine