ECF Volume Regulation Flashcards
What are the major ECF and ICF osmoles?
ECF- Na and Cl ions
ICF- K salts
How do changes in sodium content of the ECF affect blood pressure/
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
How do the kidneys respond to a low ECF volume?
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.
What causes an increase in sodium reabsorption?
An increase in reabsorptive forces in the peritubular capillaries
How is aldosterone secretion controlled?
Through reflexes involving the kidneys
What is the juxtaglomerular apparatus?
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
What hormone do the juxtaglomerular cells produce?
Renin
How does renin convert angiotensin into angiotensin II?
It splits off the decapeptide angiotensin I which is then converted by enzymes (angiotensin coverting enzyme) to angiotensin II
How does angiotensin II stimulate aldosterone release?
Angiotensin II stimulates the aldosterone-secreting cells in the zona glomerulosa of the adrenal cortex
What factors stimulate release of renin?
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
What factors inhibit release of renin?
Angiotensin II feeds back to inhibit renin
ADH inhibits renin release
What are the two factors affecting ADH secretion and when is each prominent?
Osmolarity (usually main determinant)
Volume (If sufficient volume change to compromise brain perfusion)
What hormones promote sodium reaborption and excretion?
Aldosterone promotes sodium reabsorption
Atiral natriuretic peptide (ANP) promotes sodium excretion
What would happen if aldosterone was given to someone on an adequate sodium diet?
Sodium retention
Potassium loss
What are the effects of ANP in response to increased ECF volume?
Causes natriuresis, loss of sodium and water in urine
Describe how osmotic diuresis can cause a hyperglycaemic coma
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.
