Regulation of ECF Volume I Flashcards

Question

Consider a person suffering from severe diarrhoea, who has lost 3l of salt and water (from ECF) and drinks 2 l of pure water: There will be opposing inputs to ADH secreting cells: ECF osmolarity → inhibition of ADH via osmoreceptors ↓ ECF volume → ↑ADH via baroreceptors what would happen in this situation?

Answer 1

V.I.F. Volume considerations have primacy if ECV is compromised, so that ADH will ↑because of the baroreceptors, even though this is associated with hypoosmolarity.

Answer 2

Normally, osmolarity is the main determinant of [ADH], but if sufficient volume change to compromise brain perfusion, then volume becomes the primary drive, so to conserve volume, tolerate disturbed osmolarity. Once volume is restored in hypovolaemia, then osmolarity will be normalized and again becomes main determinant of ADH.

Answer 3

infuse or drink saline - lose water, replace salt and water

Answer 4

Na+ reabsorption

Answer 5

Na+ excretion

Answer 6

If Aldosterone is given to normal subjects on an adequate Na+ diet, there will be Na+ retention and K+ loss. There will be a weight gain of 2-3kg due to the Na+ and H2O retention. After a couple of days, a spontaneous diuresis occurs 2° to volume expansion, although K+ loss persists

Answer 7

hyperaldosteronism, due to a tumour of the adrenal cortex, they are K+ depleted, but not hypernatraemic. ANP is secreted by atrial cells in response to expansion of ECF volume and causes natriuresis, loss of Na+ and H2O in urine. Actions may be to inhibit secretion of renin, generally oppose the actions of angiotensin II.

Answer 8

Illustrates how renal function can be disrupted and is most important clinically in explaining the effects of uncontrolled diabetes mellitus in producing hyperglycaemic coma. In uncontrolled DM, where [BG] is not kept within strict control, the high plasma glucose level exceeds the maximum reabsorptive capacity in the proximal tubule

Answer 9

1. Glucose remains in the tubule and exerts an osmotic effect to retain H2O in the tubule 2. ∴ [Na+] in the lumen is decreased because the Na+ is present in a larger volume. Since Na+ gains access to the proximal tubule cells by passive diffusion down a concentration gradient created by the active transport out of the basolateral surfaces, Na+ reabsorption will be ↓ → ↓ ability to reabsorb glucose since it shares a symport with Na+. 3. In the descending limb of the loop of Henle, movement of H2O out of the tubule into the interstitium is reduced because the glucose and excess Na+ exert an osmotic effect to retain H2O. ∴ fluid in the descending limb is not so concentrated. 4. This means that the fluid delivered to the ascending limb is less concentrated. Since the NaCl pumps in ascending limb are gradient limited, medullary interstitial gradient is much less. ∴ there is a considerable reduction in the volume of NaCl and H2O reabsorbed from the loops of Henle, so a large volume of NaCl and H2O is delivered to the distal tubule. AND the interstitial gradient is gradually abolished. 5. Under normal conditions, a large volume of NaCl and H2O delivered to the distal tubule means there is excess ECF volume and ∴ need to get rid of NaCl and H2O. The macula densa will detect the high rate of delivery of NaCl so that renin secretion will be suppressed and ∴ Na+ reabsorption at the distal tubule will be decreased.

Answer 10

The ↓ PV will stimulate ADH release via baroreceptors but cannot be effective because the interstitial gradient has run down. Patients with uncontrolled DM can produce urine volumes of up to 6-8 l/day, causing severe salt and water depletion. If ingestion is not adequate, a raging thirst is one of the first signs of DM, then the hypotension may be so severe as to cause a hyperglycaemic coma. This is due to inadequate BF to the brain whereas a hypoglycaemic coma is due to inadequate glucose for the brain.