Urinkonzentrierung

Urinkonzentrierung refers to the kidney's ability to produce urine that is more concentrated than plasma. This process is essential for maintaining water balance in the body and preventing dehydration. The primary mechanism for urine concentration occurs in the renal medulla, specifically within the loops of Henle and the collecting ducts.

The countercurrent multiplier system in the loops of Henle establishes a concentration gradient in the renal medulla, with osmolality increasing progressively from the cortex to the inner medulla. As the glomerular filtrate passes through the ascending limb of the loop of Henle, sodium and chloride ions are actively transported out, while the limb remains impermeable to water. This action makes the interstitial fluid of the medulla hypertonic.

The collecting ducts then pass through this hypertonic medulla. Under the influence of antidiuretic hormone (ADH), also known as vasopressin, the permeability of the collecting ducts to water increases. ADH facilitates the insertion of aquaporin channels into the apical membrane of the principal cells in the collecting ducts. As the filtrate flows through the collecting ducts, water is reabsorbed by osmosis from the hypotonic filtrate into the hypertonic medullary interstitium, thereby concentrating the urine. The amount of ADH present, which is regulated by the body's hydration status, determines the degree of water reabsorption and thus the final concentration of urine. When the body is dehydrated, ADH levels rise, leading to increased water reabsorption and the production of concentrated urine. Conversely, when the body is overhydrated, ADH levels decrease, resulting in less water reabsorption and the production of dilute urine. Impairment of this process can lead to conditions such as nephrogenic diabetes insipidus.