What is the countercurrent principle?In the human body, the countercurrent principle is particularly relevant for the metabolism in kidney tissue.
The biological countercurrent principle has different meanings. For the animal world, the functional principle plays a role, especially in thermoregulation. In the human body it is particularly relevant for the metabolism in the kidney tissue. An opposite direction of flow in adjacent tissues ensures the efficiency of mass transfer.
The countercurrent systems in the human kidney tissue serve in particular the substance and energy conservation. The Henle loop within the nephron represents in the human body a prime example of the principle of operation of the opposing flow stream in adjacent anatomical structures. As Henle loop is located in the renal medulla loop portion of the renal tubule system, which is mainly the concentration of Harns serves.
The Henle loop and thus one of the most important Gegenstromprinzipe of humans takes place within the outer marrow zone. The principle is all-important for diuresis or urination and consists of three different components with opposite directions of flow.
Sharks and other fish also use the counterflow principle for breathing. They have a countercurrent exchanger, in which oxygen-poor blood meets an oxygen-rich medium. In gas exchange, there is contact between blood and oxygen-rich medium to maintain the oxygen partial pressure difference and promote further uptake of O2 from the medium.
Function & Task
The countercurrent system of the human kidneys consists of three different components. The first of these is the thin descending leg of the so-called Henle loop, the second element is the thick rising leg of the loop and the third element corresponds to the interstitium located between the first two components.
The thin, descending portion of the Henle loop is permeable to water. The thick, ascending loop part is not. Within the ascending Henle loop portion, sodium ions migrate from the urine to the adjacent interstitium. This hike is done by active transport. The water does not migrate to the interstitium, but remains in the urine. Unlike the sodium, it is not possible for the water to reach the interstitium because of the impermeable parts of the Henle loop. For this reason, the fluid becomes hypotonic, while the interstitium receives hypertonicity.
The hypertonic interstice is finally filled with water from the descending thin part of the Henle loop. Because in this part of the loop, the wall is water permeable. In this way, the primary urine is concentrated: the concentration takes place within the descending portion of the loop without additional energy input. Water is removed from the primary urine when concentrated by the countercurrent principle.
Thanks to the principle, water recovery in the kidneys is passively possible and coupled with the reabsorption of sodium. This procedure is extremely energy efficient.
The Henle loop has several floors, all of which are involved in the process at the same time. Due to the simultaneous flow of the described principle in all floors of the Henle loop, a fractionated concentration of urine sets. The concentration of electrolytes is highest in the apical part of the Henle loop, because in this part water was extracted from the primary urine over the entire length of the thinly descending leg. The countercurrent principle has thus contributed to the energy-efficient concentration of Hans by the opposite direction of flow of the adjacent tissues in the Henle loop of the kidneys.
Diseases & complaints
When the Henle loop of the kidneys is affected by diseases, sometimes disturbances of the countercurrent principle and thus urinary concentration occur. A relatively rare hereditary disease of the Henle loop is the Bartter syndrome. More precisely, this disease affects the thickly rising branch of the loop. The cause of the disease is a defect in the Na + / K + / 2Cl cotransporter, which is said to be furosemide-sensitive. Other variants of the disease are associated with a defect of the apical K + channel or are due to a defect of 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 disturbed cooperation between cotransporter and channels, it is no longer possible to resorb enough sodium ions. Due to the reduced reabsorption, the blood pressure of the patients drops. Because of the dangerously far-reaching blood pressure, the pressoreceptors in the wall of the aorta initiate a catecholamine release.
In addition, the drop in blood pressure also leads to a reduced perfusion of vasa afferentia. This reduced blood flow stimulates the release of renin. Hyperreninemic hyperaldosteronism is the result. In Type IV of the disease there is a defect in Barttin that corresponds to the essential β subunit in the ClC-K channel. This subunit is involved not only in the baso-lateral loop loop membrane but also in the basolateral inner ear membrane. For this reason, this subset of the disease is characterized not only by a disturbed countercurrent principle, but also by deafness.
All other diseases of the renal medulla zone can disturb the countercurrent principle, such as kidney cancer or necrosis of the kidney tissue located there. In addition, disorders of urinary concentration and its functional principle may be due to numerous mutations. For the Barrter syndrome alone, a total of five causative mutations have been documented.