Countercurrent Principle: Function, Tasks, Role & Diseases

The countercurrent principle is a biological operating principle involved in the thermoregulation of many animals, in the respiration of fish such as sharks, and in processes such as human urinary concentration. Diuresis in humans occurs largely in the so-called loop of Henle in the renal medulla and is characterized by systems of opposite flow direction. One related disorder is hereditary and mutational Barrter syndrome.

What is the countercurrent principle?

In the human body, the countercurrent principle is particularly relevant to the exchange of substances in kidney tissue. The biological countercurrent principle has different meanings. For the animal world, the functional principle plays a role mainly in thermoregulation. In the human body, it is particularly relevant for the exchange of substances in kidney tissue. A countercurrent flow direction in neighboring tissues ensures the efficiency of substance exchange. The countercurrent systems in human kidney tissue serve in particular to conserve substances and energy. In the human body, the loop of Henle within the nephron represents a prime example of the functional principle of counterflow in adjacent anatomical structures. The loop of Henle is the term used to describe the loop section of the renal tubule system located in the renal medulla, which serves primarily to concentrate urine. The loop of Henle, and thus one of the most important countercurrent principles in humans, occurs within the outer medullary zone. The principle is all-important for diuresis or urine formation and consists of three distinct components with opposing flow directions. Sharks and other fish also use the countercurrent principle for respiration. They have a countercurrent exchanger in which oxygen-poor blood meets an oxygen-rich medium. During gas exchange, there is contact between the blood and the more oxygenated medium to maintain the oxygen partial pressure difference and promote further uptake of O2 from the medium.

Function and Purpose

The counterflow system of the human kidneys consists of three distinct components. The first of these is the thin descending limb of the so-called loop of Henle, the second element is formed by the thick ascending limb of the loop, and the third element corresponds to the interstitium, which is located between the first two components. The thin, descending portion of the loop of Henle is permeable to water. The thick, ascending portion of the loop is not. Within the ascending portion of the loop of Henle, sodium ions migrate from the urine to the adjacent interstitium. This migration occurs by active transport. Water does not migrate into the interstitium, but remains in the urine. Unlike sodium, water is not able to reach the interstitium at all because of the impermeable portions of the loop of Henle. For this reason, the fluid becomes hypotonic while the interstitium acquires hypertonicity. Water finally flows into the interstitium, which has become hypertonic, from the descending thin portion of the loop of Henle. This is because in this portion of the loop the wall is permeable to water. In this way, the primary urine is concentrated: the concentration occurs within the descending portion of the loop without additional energy expenditure. Water is removed from the primary urine during concentration by the countercurrent principle. Water recovery in the kidneys is possible passively thanks to the principle, and is thereby coupled to the reabsorption of sodium. This procedure is extremely energy efficient. The Henle loop has several stages, all of which are simultaneously involved in the process. The simultaneous operation of the principle described above in all stages of the loop of Henle results in a fractional concentration of the urine. The concentration of electrolytes is highest in the apical part of the loop of Henle, because in this part water has been removed from the primary urine over the entire distance of the thin descending limb. Thus, the countercurrent principle contributed to the energy-efficient concentration of Hans by the opposite direction of flow of the adjacent tissues in the loop of Henle of the kidneys.

Diseases and ailments

When the loop of Henle of the kidneys is affected by disease, disturbances of the countercurrent principle and thus of urine concentration sometimes occur. A relatively rare inherited disease of the loop of Henle is Bartter syndrome.This disease, more specifically, affects the thick ascending branch of the loop. The cause of the disease is a defect in the Na+/K+/2Cl- cotransporter, which is thought to be furosemide sensitive. Other variants of the disease are associated with a defect in the apical K+ channel or result from a defect in the baso-lateral Cl- channel. These channels cooperate with Na+/K+/2Cl- cotransport in NaC1 reabsorption in the dilution segment and contribute significantly to the functioning of the countercurrent principle in the ascending branch of the loop in a healthy kidney. Due to the impaired cooperation between cotransporters and channels, insufficient sodium ions can be reabsorbed. Due to the reduced reabsorption, the patients’ blood pressure drops. Because of the alarming drop in blood pressure, the pressoreceptors in the wall of the aorta initiate catecholamine release. In addition, the drop in blood pressure also leads to decreased blood flow to the vasa afferentia. This decreased blood flow stimulates the release of renin. Hyperreninemic hyperaldosteronism is the result. In type IV disease, there is a defect in Barttin, which corresponds to the essential β-subunit in the ClC-K channel. This subunit is involved not only in the baso-lateral loop membrane of Henle but also in the baso-lateral inner ear membrane. For this reason, this subtype of the disease is characterized not only by a disturbed countercurrent principle, but additionally by deafness. All other diseases of the renal medullary zone can also disturb the countercurrent principle, such as renal cancer or necrosis of the renal tissue located there. In addition, disorders of urinary concentration and its functional principle can be caused by numerous mutations. For Barrter syndrome alone, a total of five causative mutations have been documented.