You are running late for physiology class, and instead of cutting through a yard

Question

You are running late for physiology class, and instead of cutting through a yard where there is a dangerous cat, you go the long way and run all the way to school. It is very hot and you sweat excessively and become dehydrated. You lose proportionately more water than salts.

a) Will your levels of aldosterone increase or decrease in response to dehydration? Explain.

b) What is the direct stimulus for increased release of aldosterone when MAP is low?

c) How specifically does aldosterone cause increased absorption of water?

d) You drink 2 L of water to replace the fluids that you lost, and restore normal mean arterial pressure. What happens to the osmolarity of your blood?

e) You start to experience muscle weakness due to low plasma potassium levels. What condition are you suffering from?

Explanation / Answer

a) Dehydration causes a net loss of water from the body tissues. The water lost is ultimately taken from the blood plasma. As blood gets concentrated, the thirst response is triggered. The Osmoreceptors in the thirst center in the hypothalamus are activated. The hypothalamus will release anti-diuretic hormone. ADH travels to the kidney to cause reabsorption of water.

Sodium depletion during dehydration will trigger the production of Aldosterone by the adrenal glands in response to angiotensin II. Aldosterone will cause increased sodium and water absorption in the kidneys by increasing number of Na+/K+ pumps. Hence, urine will be concentrated to prevent loss of water.

b) Aldosterone is secreted in response to low mean arterial blood pressure (MAP), blood volume or blood sodium. Low MAP will trigger baroreceptors in blood vessels. The juxtaglomerular cells in the kidneys release the enzyme called Renin, which transform angiotensinogen to angiotensin I. Angiotensin I is converted to angiotensin II in the lungs. Angiotensin II will rigger synthesis and release of Aldosterone from the adrenal glands.

c) Aldosterone will increase the absorption of water from the distal convoluted tubule and collecting ducts of the nephrons in the kidney. There is reabsorption of sodium out of the renal tubule filtrate into blood. There is simultaneous loss of potassium into the filtrate and ultimately through urine. Along with sodium, water is reabsorbed through the tubules in to blood. As a result, the blood pressure will increase.

d) Drinking two liters of water will increase the mean arterial pressure. Drinking water will only increase the volume of water in blood. The salt concentration is not increased as water lacks solutes. As a result, blood will become hypoosmolar due to decrease in osmolarity. Most of the water will ultimately be lost via urine. This can lead to hyponatremia (lesser sodium in blood).

e) Low levels of potassium can cause muscle weakness and fatigue due to disturbed functioning of smooth muscle. This condition is known and hypokalemia. Low levels of K+ are a result of loss of K+ through urine and increased Aldosterone during dehydration.

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