the process of producing urine in the kidneys. Uropoiesis ensures that excess Na, K, Ca, Mg, and other salts are excreted from the body, as well as water, foreign substances, and the end products of metabolism. Uropoiesis, along with other factors, regulates the stability of the body’s internal environment and the volume and pH of the extracellular fluid.

Figure 1. Schematic illustration of the passage of fluid through the kidney

The uropoietic process involves three principal stages: the filtration of blood plasma in the renal glomeruli, the reabsorption of some of the filtrate into the blood, and tubular secretion (see Figure 1). Plasma filters through the wall of the capillaries of the renal glomeruli as a result of the balance of opposing pressures: the hydrostatic pressure of the blood in the glomerular capillaries (7.6 kilonewtons/m², or 57 mm Hg) is counteracted by the hydrostatic pressure in Bowman’s capsule and by the oncotic pressure (the pressure exerted by proteins on the plasma membrane) in the plasma. Human kidneys produce about 125 ml of glomerular filtrate per minute. The filtrate contains all the constituents of the blood except trace elements and large protein molecules. The rate of total blood flow through the kidneys is about 1,200 ml/min. The heart provides the driving force that causes the filtrate to pass from the plasma into Bowman’s capsule; thus, filtration is dependent on blood pressure. The amount of filtrate is also regulated by the constriction and dilation of the arterioles that enter and emerge from the glomeruli. The filtrate is sharply decreased in volume, and its composition is changed as it passes through the renal tubules. The epithelial cells of the tubules reabsorb and return to the bloodstream a substantial part of the water, all the glucose, all the amino acids, and other substances that are needed by the body.

The arteriole that emerges from a glomerulus (efferent arteriole) is not directly connected with a vein; rather, the efferent arteriole branches out into a second capillary network, which surrounds the proximal and distal renal convoluted tubules. Thus, blood in the kidney encounters two capillary systems during its passage from artery to vein. It is at the second capillary system that reabsorption of essential substances occurs. The reabsorption rate varies with the body’s internal state. When an excess of water occurs, the normal passive reabsorption of water in the distal tubules decreases or is entirely curtailed, and the excretion of urine increases to 20 ml/min. Meanwhile, active reabsorption of Na continues and even intensifies. If the body is dehydrated, 99 percent of the water in the blood is reabsorbed in the distal tubules, while the quantity of reabsorbed Na remains unchanged or decreases. As a result, a small amount of urine with a high osmotic concentration is excreted, and fluid is retained in the body.

The cells of the renal tubules not only return substances from the filtrate to the blood but also excrete a quantity of unnecessary, chiefly foreign, substances. When the filtration pressure falls below a certain level, filtration and the production of urine cease. The final step in the conversion of the glomerular filtrate into urine occurs as the fluid reaches the end of the collecting tubules. The urine does not undergo any changes as it continues its passage through the renal pelvis, ureter, and urinary bladder.

The epithelial cells of the renal tubules expend a great deal of energy in order to effect selective reabsorption from the filtrate; this energy is obtained from the oxidation of carbohydrates. (Some parts of renal tissue consume more oxygen per hour than an equivalent mass of myocardial tissue.)

In order to maintain osmotic and volume homeostasis, renal activity is regulated by central nervous system reflex mechanisms that cause certain organs to secrete the hormones vasopressin and aldosterone. These hormones directly influence water-salt metabolism by regulating the rate of reabsorption of water and electrolytes in the renal tubules. (SeeWATER-SALT EXCHANGE and EXCRETORY SYSTEM.)