Question

Explain in detail the primary mechanism that determines urine concentration and how urine concentration is regulated (ok to ignore the role of aldosterone). Be sure to include: (1) a description of the mechanism that allows concentrated urine to be produced, including the structures involved, and the specific roles they have in the mechanism. (2) Describe the primary hormone that determines urine concentration, and the negative feedback system regulates its secretion (include all components). (3) Lastly, describe two scenarios, one when the hormone levels would be high, and one when the hormone levels would be low and explain how the system helps maintain homeostasis during these conditions. Discus each separately.

Answer 1

1.

The three segments of the loops of Henle have different characteristics that enable countercurrent multiplication:

• The thin descending limb is passively permeable to both water and small solutes such as sodium chloride and urea. As active reabsorption of solutes from the ascending limb of the loop of Henle increases the concentration of solutes within the interstitial space (space between cells), water and solutes move down their concentration gradients until their concentrations within the descending tubule and the interstitial space have equilibrated. As such, water moves out of the tubular fluid and solutes to move in. This means, the tubular fluid becomes steadily more concentrated or hyperosmotic (compared to blood) as it travels down the thin descending limb of the tubule.
• The thin ascending limb is passively permeable to small solutes, but impermeable to water, which means water cannot escape from this part of the loop. As a result, solutes move out of the tubular fluid, but water is retained and the tubular fluid becomes steadily more dilute or hyposmotic as it moves up the ascending limb of the tubule.
• The thick ascending limb actively reabsorbs sodium, potassium and chloride. this segment is also impermeable to water, which again means that water cannot escape from this part of the loop. This segment is sometimes called the “diluting segment”.

2. Regulation of Urine Concentration and Volume. Antidiuretic hormone (ADH) is produced by the pituitary gland to control the amount of water that is reabsorbed through the collecting ducts.

The concentration of urine is controlled by antidiuretic hormone, which helps the kidneys to conserve water. Its main effects in the renal tubules is to increase water permeability in the late distal tubule and collecting ducts, increase active transport of sodium chloride in the thick ascending limb of the loop of Henle, and enhance countercurrent multiplication and urea recycling, all of which increase the size of the osmotic gradient.

3. Scenario-1. Very high ADH levels may be dangerous because they can cause fluid imbalances that lead to seizures or cerebral edema. A person may also have high ADH levels if they have heart failure. This can result in excess fluid building up in the body.

Scenario-2. Low levels of anti-diuretic hormone will cause the kidneys to excrete too much water. Urine volume will increase leading to dehydration and a fall in blood pressure.

When water levels in the blood are optimal, they are considered to be in a state of homeostasis. When homeostasis exists, the hypothalamus tells the pituitary gland to release the normal amount of ADH, telling the kidneys to retain and excrete the right amount of water necessary to retain balance.

ADH is antidiuretic hormone and is also called vasopressin. It's responsible for regulating the amount of water in the blood and is released from the pituitary gland based on signals from the hypothalamus, which detects the water levels of the blood.