Glomerular Filtration Rate (Friday 03/11/17)

Question 1

Structure of the Glomerulus

Answer 1

For filtrate to cross the glomerulus to Bowman’s capsule, it must first cross the fenestrated (holes) endothelium of the glomerular capillaries. It lets everything smaller than a whole cell through.

The fenestrarted endothelium sits on a basement membrane made of collagen fibril mesh. Large proteins can’t get through the holes in the collagen fibres.

The final layer are podycytes. The gaps between adjacent podycytes are called filtration slits and are 7 by 14nm.

Filtrate is protein free plasma. The protein is not part of the filtrate and remains in the blood.

Question 2

Clinical Relevance

Answer 2

Protein in urine: = damage to glomeruli.

Diabetic nephropathy: The glomeruli die, and GFR drops of.

Question 3

Filtration of molecules depends on their size and charge.

Answer 3

If a molecule is negatively charged, they can’t be filtered. For example proteins (remain in blood).

Question 4

What drives GF?

Answer 4

In glomerular capillary, molecules are always driven out. In glomerular capillary, the hydrostatic pressure stays constant, but the osmotic gradient/ pressure rises until they almost equal (because as water leaves, capillary, protein is left, increasing the osmotic pressure).

In contrast in systemic capillary…

Question 5

Measuring GFR

Answer 5

For a molecule to be used as a meaure of GFR function, it must meet the following criteria:

It must be filtered at the glomerulus, not re-absorbed or secreted by the nephron, not be subject to metabolism and not alter GFR. (Basically amount fitered must be equal amount secreted). Works for inulin or creatinine.

Can measure inulin in urine, and rate of urine production (eg two urine samples over two hours). See equation: GFR = (urinary inulin x urinary production) divided by plasma inulin. If a patients GFR falls over months, they have chronic kidney failure.

Another way is: take a blood sample, find creatinine level, compare it to the graph.

Question 6

4 factors affecting GFR

Answer 6

1. Permeability Co-efficient:
2. Rate of blood flow through the glomerulus: Atrial Naturetic Peptide- more blood flows, more filtrate
3. Hydrostatic Pressure:
4. Colloid Osmotic Pressure Difference: