Question

1. Regarding countercurrent mechanisms:
   1. List which structure(s) is/are responsible for establishing the concentration gradient in the renal medulla? Describe how this is accomplished. (5 points)
      1. Note: You do not need to describe the entire positive feedback loop, I am looking more simply for structures or structural modifications and functions of those structures. Do mention how the tubular fluid concentration changes and the interstitial fluid of the medulla is affected as a result.
   2. List which structure(s) is/are responsible for maintaining the concentration gradient in the renal medulla? Describe how this is accomplished. (2 points)
   3. ACE inhibitors (angiotensin converting enzyme inhibitors) are a popular class of drug that treats hypertension. Use your knowledge of the renin-angiotensin-aldosterone system to describe the physiological mechanism of how this medication works at the nephron and kidney level to reduce blood pressure. (3 points)
   4. Predict how ADH hypersecretion would affect the sodium concentration of the urine, and explain why. (2 points)

Answer 1

a) The descending and ascending loop and vasa recta form a countercurrent multiplier system, this is known as countercurrent mechanism because Descending and asscending limb goes in opposite direction,. the basic mechanism involved is that the asscending limb reabsorbs ion like sodium, potassium and chlorine which will make the medula hypertonic (salty), this in turn will make the descending limb absorb water passively

b) When the filterate passes through descending loop water leaves the filterate and diffuses out into the surrounding tissues via auquaporins by passive diffusion, this happens because the surrounding are highly hyperosmatic due to presence of sodium ions, th efilterate than enters the thin asscending loop which is impermeable to water but permeable to ions, ions than passively diffuses out, the thick asscending loop is also impermeable to water but it actively transports sodium and potassium into the surrounding tissues , this produces an electrochemical gradient, where the outside is more positive than inside because of this chlorine ion leaves the loop, thich asscending loop has potassium leak channels which allows potassium ion leak back into the loop, this creates an electrochemical gradient which makes other important ion slike calcium and magnessium to leave the thich asscending loop.

c) Angiotensin causes blood vessels to constrict which in turn increases the blood pressure, in addition to this it stimulates adrenal gland to release aldosterone which will act on the kidney to retain sodium and water so that it will increase the blood pressure, Ace inhibitors act by reducing or blocking the action of angiotensin, which will cause blood vessel dilation and relaxation resulting in lowering of blood pressure.

d) Too much ADH would result in massive reabsorption water making the urine highly concentrated, there will be lots sodium in the urine, but in the extracellular fluid the reverse happens the excess water will reduce the osmolarity of the blood and the concentration of sodium will be low causing hyponatremia