Week 2 - B - Physiology 3(B) - Plasma clearance

Question 1

What is a measure of how effectively the kidneys can ‘clean’ the blood of a substance?

Answer 1

The plasma clearance

Question 2

Each substance that is handled by the kidney will have it’s own specific plasma clearance value What is the plasma clearance of a substance measured in?

Answer 2

As it is the volume of a substance cleaned per minute Measured in ml/min

Question 3

For the plasma clearance of a substance, you require the rate of excretion of a substance and the plasma concentration of the substance How is the rate of excretion f the substance calculated?

Answer 3

Concentration of the substance in the urine and the urine flow rate

Question 4

What is the equation for calculating plasma clearance?

Answer 4

Plasma clearanc e = ([X]urine x Vurine) / [X]plasma

Question 5

For certain substances, the plasma clearance = GFR What is the exogenous compound that could be used to measure the glomerular filtration rate?

Answer 5

Inulin - freely filtered at the glomerulus and there is no tubular modification Can see how GFR = Inulin clearance

Question 6

What else is inulin used to measure?

Answer 6

Used to measure the extracellular fluid volume (ECV)

Question 7

What is a natural body product which can be used to estimate the glomerular filtration rate?

Answer 7

Creatinine

Question 8

Why is creatinine measurement only an estimation of the glomerular filtration rate? (eGFR)

Answer 8

Creatinine is a waste product produced by skeletal muscle and is filtered into the nephron tubules However there is a little bit of tubular secretion of creatinine also

Question 9

What is an example substance that is filtered, completely reabsorbed and not secreted? What is its plasma clearance?

Answer 9

Example is gluocse Plasma clearance is 0 in normal physiological conditions Plasma clearance = [X]urine x Vurine (rate of urine flow) divided by [X]plasma If the concentration of glucose in the urine is 0 then the plasma clearance would be zero

Question 10

If clearance GFR then substance is SECRETED into tubule Give an example of each substance?

Answer 10

Clearance GFR - hydrogen ions

Question 11

What is the inulin clearance value normally?

Answer 11

125ml/min

Question 12

What compound is used to measure the renal plasma flow?

Answer 12

This is para-aminohippuric acid (PAH)

Question 13

What is the renal plasma flow volume?

Answer 13

650ml/min

Question 14

Ideally, what should a GFR and a RPF marker have? (with regards to filtration and tubular modification)

Answer 14

GFR marker should be freely filtered with no tubular secretion or reabsorption RPF marker should be filtered and completely secreted Ideally any substance used as a clearance marker should have the following properties: (1) Non-toxic (2) Inert (i.e. not metabolised) (3) Easy to measure

Question 15

Filtration fraction is the fraction of plasma flowing through the glomeruli that is filtered into the tubules How is this calculated?

Answer 15

Filtration fraction = GFR/RPF = 125ml/min / 650ml/min = 0.2 = 20% i.e. ~20% of the plasma that enters the glomeruli is filtered. The remaining 80% moves on to the peritubular capillaries.

Question 16

How does the renal plasma flow differ from the renal blood flow?

Answer 16

Renal plasma flow is the volume of plasma delivered to the kidneys per unit time Renal blood flow is the volume of blood delivered to the kidney per unit time

Question 17

As the cardiac ouput is approx 5litres (4.8 litres for easiness), what is the renal blood flow? Remember the percentage of blood the kindey recieves from the cardiac output

Answer 17

Recieves 25% so 1200ml/min is the renal blood flow

Question 18

Define cardiac output?

Answer 18

Volume of blood pumped by the ventricles of the heart per minute