34 Regulation of Water Balance

Question 1

Explain the renin-angiotensin-aldosterone axis

Answer 1

• Renin catalyzes the conversion of angiotensinogen to angiotensin I (ANG I)
• Angiotensin-converting enzyme (ACE) converts the inactive ANG I to physiologically active ANG II
• ACE is abundantly present in the lungs (but if asked, say it’s everywhere!)

Question 2

what is angiotensinogen?

Answer 2

an α2-globulin renin substrate synthesized by the liver responsible for blood pressure and fluid balance

Question 3

what is renin?

Answer 3

an enzyme, produced and stored in the granular cells of the juxtaglomerular apparatus (JGA), that promotes the production of the protein angiotensin.

Question 4

what is angiotensin II?

Answer 4

• Stimulates aldosterone release
• Vasoconstricts renal vessels
• Enhances tubuloglomerular feedback
• Enhances Na-H exchange
• Causes renal hypertrophy
• Stimulates thirst and AVP release

Question 5

what is the main concern of Renin?

Answer 5

to look at how much NaCl is left behind on the urine side

Question 6

what are the other effector pathways discussed?

Answer 6

Renal sympathetic nerve activity

Arginine vasopressin (antidiuretic hormone)

Atrial natriuretic peptide

Question 7

what is the function of the Renal sympathetic nerve activity as an effector pathway?

Answer 7

↑↑ renal sympathetic activity increases renal vascular resistance, renin release, and tubule reabsorption of Na+

Question 8

what is the function of arginine vasopressin as an effector pathway?

Answer 8

AVP increases distal-nephron water permeability → ↑↑ water retention

Question 9

what is the function of the atrial natriuretic peptide as an effector pathway?

Answer 9

ANP promotes natriuresis → ↑↑ Na+ excretion

Question 10

how does the AVP system work? name example when this would take effect?

Answer 10

Large reductions in effective circulating volume (5-10%) are required to stimulate AVP release

Hemorrhagic shock & hypovolemic shock

note that Reduced effective circulating volume can lead to an inappropriate increase in AVP levels → congestive heart failure

Question 11

what is AVP interested in?

Answer 11

osmolality and tries to deal with the problem from the water perspective

Question 12

what is standard plasma osmolality?

Answer 12

290 mOsm

The threshold for AVP release is ~280 mOsm