Countercurrent multiplier system

Countercurrent multiplier system is a mechanism that expends energy to create a concentration gradient. It is found widely in nature and especially in mammalian organs. The mechanism involves a long loop of Henle which is a part of a nephron, the functional unit of the kidney. The countercurrent multiplier system serves to maintain a high salt concentration in the medulla of the kidney.

Mechanism

The countercurrent multiplier system works by having fluid flow in opposite directions in two adjacent segments of the same tube with an active transport pump in one of the segments. The active transport pump (in the ascending limb of the loop of Henle) pumps sodium ions out of the ascending limb. This creates a concentration gradient with the interstitial fluid in the medulla of the kidney. The descending limb of the loop of Henle is permeable to water but not to sodium, so water moves out of the descending limb by osmosis. This concentrates the fluid inside the descending limb, which then moves into the ascending limb. The active transport pump in the ascending limb pumps out sodium, which maintains the concentration gradient. The net effect is to move sodium from the cortex of the kidney to the medulla, creating a high concentration of sodium in the medulla.

Function

The primary function of the countercurrent multiplier system is to deliver solutes and water in the appropriate proportions to the collecting duct for further processing. This system allows the kidney to reabsorb water while simultaneously secreting waste products into the urine. The high concentration of sodium in the medulla of the kidney draws water out of the collecting duct by osmosis, concentrating the urine.

Clinical significance

The countercurrent multiplier system is crucial for the kidney's ability to produce concentrated urine, thus allowing the body to retain water. Disruptions to this system can lead to conditions such as diabetes insipidus, which is characterized by excessive thirst and excretion of large amounts of dilute urine.

See also

• Loop of Henle
• Nephron
• Osmosis
• Diabetes insipidus

This physiology-related article is a stub. You can help WikiMD by expanding it.