The tubular fluid that has been adjusted in concentration, volume of fluid, and concentration of ions and other materials now passes into a hairpin-shaped portion of the tubule called the loop of Henle. The loop of Henle adjusts the volume of filtrate. A hormone is produced by the brain (in the hypothalamus) that is capable of altering the permeability of the cells of the loop of Henle to water. The substance, a protein hormone called anti-diuretic hormone (ADH), causes the reabsorption of water from the loop. Cells of the hypothalamus respond to the concentration of particles in the blood (its osmotic pressure) and adjust the amount of water that is reabsorbed from the tubule by secreting more or less ADH as necessary to maintain a constant concentration in the blood. The range of adjustment is remarkable. The volume of urine produced can range from about 0.5 liter to more than 30 liters per day, depending upon the need. If water is administered or restricted, the water concentration of the body changes. This causes a change in the production of ADH, which in turn increases or decreases the excretion of water to return the level to normal.
Sweating also causes water loss and thus decreases urine flow. Intake of large amounts of fluid will dilute the body fluids and cause an increased urine output. There is a constant adjustment, because water is lost by breathing, through the skin, and through excretions, and the kidney must make the proper corrections. Losses have been reduced to a minimum in animals such as the kangaroo rat, which lives in the desert and must conserve water. All of its water intake is from seeds and other foods containing some water, and excretion is almost zero. The desert rat is able to concentrate urine to a level about five times that of the human.
As the urine passes from the loop of Henle, it enters upon a final adjustment in the latter portion of the tubule. Volume, concentration of material, and the like are adjusted to maintain homeostasis. Other alterations of the fluid are also made in the passage down the tubule. If the body becomes acidic, the cells of the tubule exchange sodium ions, which are neutral, for acid ions (H+), thus causing the body to lose acid. Conversely, if the body becomes basic (lacks acid or hydrogen ions), the reverse is true.
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