ECF Volume Regulation

Question 1

What are the major ECF and ICF osmoles?

Answer 1

ECF- Na and Cl ions

ICF- K salts

Question 2

How do changes in sodium content of the ECF affect blood pressure/

Answer 2

Changes in sodium content of the ECF causes changes in the ECF volume and therefore will affect the volume of blood perfusing the tissues, which is also the effective circulating volume and so will affect blood pressure

Question 3

How do the kidneys respond to a low ECF volume?

Answer 3

Decreased ECF volume causes increased sympathetic discharge, which causes increased renal VC nerve activity, causing increased renal arteriolar constriction and an increase in renin. This causes an increase in angiotensin II, causing a decrease in peritubular capillary hydrostatic pressure and an increase in sodium absorbed from the proximal tubule. Increased angiotensin II also causes increased aldosterone which increases distal tubule sodium reabsorption.

Question 4

What causes an increase in sodium reabsorption?

Answer 4

An increase in reabsorptive forces in the peritubular capillaries

Question 5

How is aldosterone secretion controlled?

Answer 5

Through reflexes involving the kidneys

Question 6

What is the juxtaglomerular apparatus?

Answer 6

Smooth muscle of the media of the afferent arteriole, just before entering the glomerulus has become specialised, containing large epithelial cells with many granules. These are known as juxtaglomerular cells and are closely associated with a histologically specialised loop of the distal tubule known as the macula densa. The two together form the juxtaglomerular apparatus

Question 7

What hormone do the juxtaglomerular cells produce?

Answer 7

Renin

Question 8

How does renin convert angiotensin into angiotensin II?

Answer 8

It splits off the decapeptide angiotensin I which is then converted by enzymes (angiotensin coverting enzyme) to angiotensin II

Question 9

How does angiotensin II stimulate aldosterone release?

Answer 9

Angiotensin II stimulates the aldosterone-secreting cells in the zona glomerulosa of the adrenal cortex

Question 10

What factors stimulate release of renin?

Answer 10

Pressure in afferent arteriole at the level of the juxtaglomerular cells is decreased
Increased sympathetic nerve activity via beta-1 effect
Decreased NaCl delivery at the macula densa

Question 11

What factors inhibit release of renin?

Answer 11

Angiotensin II feeds back to inhibit renin

ADH inhibits renin release

Question 12

What are the two factors affecting ADH secretion and when is each prominent?

Answer 12

Osmolarity (usually main determinant)

Volume (If sufficient volume change to compromise brain perfusion)

Question 13

What hormones promote sodium reaborption and excretion?

Answer 13

Aldosterone promotes sodium reabsorption

Atiral natriuretic peptide (ANP) promotes sodium excretion

Question 14

What would happen if aldosterone was given to someone on an adequate sodium diet?

Answer 14

Sodium retention

Potassium loss

Question 15

What are the effects of ANP in response to increased ECF volume?

Answer 15

Causes natriuresis, loss of sodium and water in urine

Question 16

Describe how osmotic diuresis can cause a hyperglycaemic coma

Answer 16

Glucose remains in the tubule and exerts an osmotic effect to retain water in the tubule. [Sodium] in the lumen is decreased because the sodium is present in a larger volume. Since sodium gains access ti proximal tubule cells by passive diffusion, sodium reabsorption will be decreased. This leads to a decreased ability to reabsorb glucose since it shares a symport with sodium. In the descending limb of the LoH, movement of water from the tubule into the interstitium is reduced so the fluid in the descending limb is not so concentrated. This means that the fluid delivered to the ascending limbs is less concentrated, and so medullary interstitial gradient is much less. This means there is a reduction in the volume of NaCl and H2O reabsorbed from the loops of Henle so a large volume of NaCl and H20 is delivered to the distal tubule and the interstitial gradient is gradually abolished. The macula densa will detect the high rate of delivery of NaCl that renin secretion will be suppressed and sodium reabsorption at the distal tubule will be decreased. A large volume of nearly isotonic urine will be excreted and the decreased PV (?) will stimulate ADH release via baroreceptors but cannot be effective due to run down interstitial gradient. Hypotension can cause a hyperglycaemic coma due to inadequate blood flow to the brain.