Lecture 17 - Renal II (Glomerular Filtration)

Question 1

What is the nephron composed of?

Answer 1

A renal corpuscle and a renal tubule.

Question 2

What does the renal corpuscle consist of?

Answer 2

A tuft of capillaries called a glomerulus and a cup-shaped structure called Bowman’s capsule. The renal tubule extends from the capsule.

Question 3

What is the combined blood flow to the kidneys?

Answer 3

1.1L per minute

Question 4

What is the kidney function reliant on?

Answer 4

Highly dependent upon sufficient blood pressure in the glomeruli. the arteries and arterioles that provide blood flow to the kidneys must maintain sufficient blood flow to keep the tissues of the kidneys alive and also maintain sufficient blood pressure to allow waste to be separated from the blood.

Question 5

What are the two forces which drive movement in capillary beds?

Answer 5

1. Hydrostatic: higher hydrostatic pressure (50mmHg) in the capillaries drives fluid out much higher than in most capillaries.
2. Osmotic/oncotic: higher osmotic pressure in capillaries impedes the flow.

Question 6

How can osmotic pressure be estimated?

Answer 6

The morse equation
Osmotic pressure=nCRT
nC=osmolality
R=ideal gas constant
T=temperature (K)

Question 7

Describe the filtration pressure along the glomerular capillary.

Answer 7

Once the efferent capillary leaves the glomerulus, it enters a portal vein and travels to a secondary capillary bed surrounding the loop of hence –> here the hydrostatic pressure is much more similar to a systemic capillary, while the osmotic (oncotic) pressure is much higher.

Question 8

What are the three layers that separate the blood from the lumen of Bowman’s capsule?

Answer 8

1. Endothelial cells of the glomerular capillaries
2. Glomerular basement membrane
3. Epithelial cells of Bowman’s capsule

Question 9

How do the three layers of filtration in Bowman’s capsule work?

Answer 9

• The endothelial cells have small holes (fenestrations) between them.
• The endothelial cells have a negatively charged glycocalyx, which creates a charge barrier that is particularly effective for protein.
• The basement membrane also consists of fixed, negatively charged proteins (e.g., collagen).

Question 10

What are podocytes?

Answer 10

These are the epithelial cells of the Bowman’s capsule. From these cells, small processes (pedicels; foot processes) project and interdigitate with their neighbours to the movement of fluid.

Question 11

Describe bulk flow in filtration

Answer 11

• Most of the movement of substances through the filtration barrier is by bulk flow.
• The movement of solutions from an area of high pressure to an area of low pressure. As the solvent moves it carries any solutes dissolved in it, this process is sometimes called solvent drag.

Question 12

Describe diffusion in filtration

Answer 12

The movement of a substance from an area of higher concentration to lower concentration as a result of Brownian motion. The glomerular barrier is sufficiently tortuous, and the distance sufficiently great, that diffusion alone cannot explain glomerular filtration.

Question 13

What is found in the filtrate?

Answer 13

• Molecules less than 10kDa in size (Na+, K+, Mg2+/Ca2+, Cl-, HCO3-, glucose and urea)
• Larger molecules may be found, particularly if positively charges and in the case of damage to the glomeruli making them ‘leaky’

Question 14

What is typical glomerular filtration rate (GFR)?

Answer 14

~ 120ml per min

180L per day

Question 15

How much does the nephron filter per minute?

Answer 15

30-50nL per min

Question 16

What is the filtration fraction?

Answer 16

This is the proportion fo the plasma flow that is filtered by the glomerulus

Question 17

How do we calculate filtration fraction?

Answer 17

Glomerular flow rate/ Renal plasma flow

=120ml/min / 600ml/min =0.2

Question 18

What are the two ways to locally increase the glomerular capillary pressure?

Answer 18

1. Dilate the afferent arteriole

2. Contract the efferent arteriole

Question 19

What happens to glomerular capillary pressure if proximal/afferent constriction?

Answer 19

Pressure downstream falls (decrease flow)

Question 20

What happens to glomerular capillary pressure if distal/efferent constriction?

Answer 20

Pressure upstream increases (increase flow)

Question 21

What happens to glomerular capillary pressure if proximal/afferent dilation?

Answer 21

Pressure downstream increases (increase flow)

Question 22

What happens to glomerular capillary pressure if distal/efferent dilation?

Answer 22

Pressure upstream falls (decrease flow)

Question 23

How can GFR be measured?

Answer 23

• Using substances that are freely filtered, but neither secreted nor reabsorbed over the length of the tubules.
• Such substances can be injected intravenously (insulin) or produced by the body at a steady rate (creatinine).

Question 24

What is meant by proteinuria?

Answer 24

As the glomerulus is the first and key barrier to protein loss, protein in the urine suggests glomerular dysfunction. thus, this features as a key set of renal failures, which when sufficiently severe constitute nephrotic syndrome.
- Set conditions where there is a predominant dysfunction of the glomerulus are called glomerulonephritis.

Question 25

What is congenital nephrotic syndrome?

Answer 25

• A rare genetic disorder involving a component of the glomerular barrier between podocytes (either nephrin or podocin).