0527 - Kidney function and control of fluid balance - EG

Question 1

List some roles of the kidney.

Answer 1

excretory role; fluid homeostasis/blood pressure control; electrolyte balance; acid-base balance; endocrine organ - renin, erythropoietin

Question 2

What is the approximate daily volume turnover of water? List ways in which water enters and exits the body.

Answer 2

~2.5L/d. Enters through water of oxidation, food, beverages, Lost through excretion in faeces, respiration, perspiration and as urine

Question 3

what is the blood flow rate to the kidney?

Answer 3

1.2 L/min, i.e. 20-25% of CO, making it the best perfused organ of the body by ~4x

Question 4

Where does the kidney source most of its energy from?

Answer 4

fatty acids and ketone bodies

Question 5

Define ultrafiltration and state what is ultrafiltrated in the kidney.

Answer 5

the movement of water and solute across a semi-permeable membrane due to a pressure gradient. Incorporates starlings law of the capillary with both pressure differences and colloid-osmotic differences between capillary and interstital space. Plasma is ultrafiltrated by the pressure of blood (BP from heart) through the subendothelial basal membrane of the glomerulus.

Question 6

To what molecular size can the kidneys filter?

Answer 6

Anything under 37 Angstrom, hence limits albumin, but ions, sugars, metabolites and free drugs are filterable.

Question 7

What percentage of plasma liquid volume (PLV) is filtered as blood passes through the glomerulus? How much ultrafiltrate is produced per day? (L)

Answer 7

20% of PLV filtered as pass through, leads to 180 L/d of ultrafiltrate, worked out as: CO is 5L/min; 25% blood goes to kidney = 1.2L/min is 1’800 L/day, of which plasma makes up roughly half = 900 L/day and of this 20% is filtered, therefore 180 L/d of ultrafiltrate.

Question 8

What is the normal amount of urine produced per day?

Answer 8

0.5-2.0 L/day

Question 9

What amount of urine is produced per day in polyuria?

Answer 9

> 2.5 L/day

Question 10

What amount of urine is produced per day in eu-uria?

Answer 10

0.5-2.0 L/day

Question 11

What amount of urine is produced per day in oliguria?

Answer 11

0.1 - 0.4 L/day

Question 12

What amount of urine is produced per day in anuria?

Answer 12

<0.1 L/day

Question 13

What is the normal pH of urine?

Answer 13

~6.0 (RR 5.6-7.0)

Question 14

What are the three mechanisms by which substances are moved into/out of the kidney tubule?

Answer 14

filtered; secreted; resorbed with osmosis drive through active pumping of Na+ ions

Question 15

Define clearance.

Answer 15

The virtual volume of blood that would be totally cleared of substance in a given time (ml/min). Clearance = ([urine]x urine volume in mL)/([plasma]x1440 min)

Question 16

List properties of good clearance indicators.

Answer 16

freely filterable; must not be metabolised; must not alter renal function; must not change due to resorption and or secretion (i.e. be filtered only)

Question 17

What is the property of a substance that enables it to indicate glomerular filtration rate (GFR) and give an example of such a substance.

Answer 17

substance filtered only (neither absorbed or secreted), examples include inulin, endogenous creatinine, mannitol

Question 18

What is the feature of a substance that enables it to indicate total renal plasma flow (RPF) and give an example of such a substance.

Answer 18

substance is filtered and secreted to be totally cleared from the plasma, examples include p-aminohippuric acid (PAH).

Question 19

If the clearance of a substance (x) is less than that of the clearance of inulin, what is possibly occurring?

Answer 19

The substance (x) is either being resorbed or may not be filtrated at all (for example by protein binding)

Question 20

If the clearance of a substance (x) is more than that of the clearance of inulin, what is possibly occurring?

Answer 20

Secretion of the substance (x)