15. control of body fluid volume

Question 1

what is renin?

Answer 1

• Renin is an enzyme that is synthesized and stored in the JGA in the kidneys. A fall in plasma Na+ leads to a fall in ECF volume, causing the release of renin

Question 2

how is renin release stimulated?

Answer 2

• Increased sympathetic innervation - due to low BP detected by baroreceptors in carotid sinus
- stimulate Granular cells of the JGA to increase renin
release.

• Reduced perfusion pressure in the kidney detected by baroreceptors in the afferent arteriole.
- which stimulate renin release

• Decreased Na+ to the macula densa

• If less NaCl reaches the macula densa, the macula densa is stimulated to secrete prostaglandins
• This acts on granular cells causing renin release

Question 3

what is the action of renin?

Answer 3

Angiotensinogen → Angiotensin I → Angiotensin II

Question 4

what is the action of angiotensin 2?

Answer 4

• Directly vasoconstricts arterioles within the kidney
(efferent>afferent)
• Stimulates the zona glomerulosa of the adrenal
cortex to release aldosterone
- Directly increases Na+ reabsorption from the PCT
- Releases ADH
- Stimulate thirst

Question 5

how do the Starlings forces in the PCT affect sodium reabsorption?

Answer 5

• Amount of Na+ reabsorbed into peritubular capillaries from the PCT depends on the rate and amount of uptake from the intercellular spaces into the capillaries
• Changes in body fluid volume alter plasma hydrostatic and oncotic pressure
e.g. if low plasma volume, decreased hydrostatic pressure and increased oncotic pressure so↑NaCl intake mirrored by ↑ECF volume.
• This ↑hydrostatic pressure and ↓oncotic pressure, so NaCl and water reabsorption by the PCT decreases

Question 6

what is the effect of prostaglandin and when is it released?

Answer 6

• A decrease in effective circulating volume stimulates cortical prostaglandin (PG) synthesis.

result in
• Vasodilators – preventing excessive vasoconstriction
• Renin release

Question 7

how does Atrial natureric peptide (ANP) affect sodium reabsorption?

Answer 7

• Produced by cardiac atrial cells in response to an increase in ECF volume.

ANP acts to:
• Inhibit Na+/K+ ATPase and close Na+ channels of the collecting ducts and PCT, reducing Na+ reabsorption. Thus, Na+ and water excretion by the kidney is increased
• Vasodilate afferent arterioles, thereby increasing GFR
• Inhibit aldosterone secretion
• Inhibit ADH release
• Decrease renin release