25. Renal excretory function

Question 1

How is the renal pressure of the glomerulus maintained?

Answer 1

The efferent arteriole is under a greater pressure than the afferent arteriole

Question 2

What are the vasa recta?

Answer 2

Capillaries that branch off from the efferent arteriole and surround the loop of Henle - provide the blood supply

Question 3

What is the relevance of the vasa recta?

Answer 3

These each have a hairpin turn and carry blood at a very slow rate
Allows for the countercurrent exchange mechanism at the loop of Henle

Question 4

What are the structures through which components pass from the Bowman’s capsule to be filtered out in the urine?

Answer 4

Podocytes
Glomerular basement membrane
Fenestrations in the endothelium of the Bowman’s capsule

Question 5

What are the factors that determine the filtrate that is produced by the kidney?

Answer 5

Net filtration pressure
The fenestrations of the endothelium, glomerular basement membrane and podocytes
Size of the molecule
Charge of the molecule

Question 6

Why does the charge of the molecule determine the filtrate produced by the kidney?

Answer 6

The glomerular basement membrane has a negative charge due to the presence of glycoproteins
Restricts large, negatively charged proteins

Question 7

What is filtered through the glomerulus?

Answer 7

Free movement of small molecules e.g. water, electrolytes (Na+, K+, Cl-, phosphate, glucose), urea and amino acids
Restriction of larger solutes e.g. proteins and negatively charged proteins

Question 8

At what molecular mass are proteins no longer filtered through the glomerulus?

Answer 8

55,000

Question 9

What is meant by the glomerular filtration rate (GFR)?

Answer 9

This is the total amount of fluid that is filtered through the glomerulus - the clearance rate

Question 10

What is creatinine?

Answer 10

Metabolic waste produced by muscle metabolism that is excreted in the urine?

Question 11

What is the effect on creatinine of kidney failure?

Answer 11

This will result in a reduced excretion of creatinine and so the levels of creatinine in the blood will build up

Question 12

What is the relevance of creatinine in assessing renal function?

Answer 12

Creatinine is a naturally occurring substance which is completely filtered out by the kidney and not reabsorbed - so you can compared other components to this to measure the GFR
A raised creatinine level signifies damage to the kidney

Question 13

How can the creatinine levels be measured?

Answer 13

(urine concentration x urine volume)/plasma concentration

Question 14

What is the glomerular filtration rate?

Answer 14

The rate of fluid being filtered through the kidney

Question 15

What is the creatinine clearance rate and how does this relate to GFR?

Answer 15

This is the volume of blood that is cleared of creatinine per unit time
Can be used to estimate the GFR - eGFR

Question 16

How can the GFR be estimated for an individual? (eGFR)

Answer 16

Using the MDMR calculation

Question 17

What are the four variables used in the MDMR formula?

Answer 17

Serum creatinine, age, ethnicity and sex

Question 18

What is considered to be more useful for assessing kidney function out of the GFR and the serum creatinine?

Answer 18

GFR

Question 19

Why is GFR considered to be more useful that creatinine when assessing muscle function?

Answer 19

Creatinine is produced by muscle - muscular individuals will have a naturally raised serum creatinine so the eGFR will underestimate the true GFR
AND malnourished individuals will have a low serum creatinine so the eGFR will overestimate the true GFR
Some drugs inhibit the natural tubular secretion of creatinine

Question 20

What is the apical surface?

Answer 20

Surface facing the tubule lumen

Question 21

What is the baso-lateral surface?

Answer 21

Surface facing the vascular compartment’/interstitium (outside of the lumen)

Question 22

What is the transport mechanism at the apical surface of the glomerular tubules?

Answer 22

Carrier proteins

Tight junctions between cells

Question 23

What is the transport mechanism at the baso-lateral surface of the glomerular tubules?

Answer 23

Primary active transport - Na+/K+ ATPase pumps 3 sodiums out of the tubule for 2 potassiums entering the tubule
Creates an electrochemical gradient to allow sodium, phosphate, glucose and amino acids to enter the tubule cell via the apical membrane

Question 24

What is the function of the proximal convoluted tubule?

Answer 24

Bulk of reabsorption of solutes - 80%
Reabsorption of 65% of water
100% reabsorption of amino acids and low molecular weight proteins -

Question 25

How does the fluid in the descending and the ascending limbs of the loop of Henle differ to each other?

Answer 25

Descending limb - isotonic

Ascending limb - hypotonic

Question 26

What enters/leaves the descending loop of Henle?

Answer 26

Passive entry of water and NaCl into the descending loop of Henle

Question 27

What enters/leaves the ascending loop of Henle?

Answer 27

Active sodium and chloride transport out of the ascending loop - controlled