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The Nephron and Urine Formation

The functional unit of the kidney is called the nephron, and each kidney contains about a million nephrons (see Figure 16.4). Renal arteries supply each kidney with blood; after entering the kidney, they branch and subdivide so that each nephron is supplied with arterial blood through a tiny afferent arteriole. The arteriole feeds a capillary bed referred to as the glomerulus. Constriction of the arterioles causes the blood pressure to be especially high in this region, and the glomerular capillaries are extremely permeable, causing a significant portion of the plasma, along with its dissolved substances, to be forced out of the arteriole.


This filtrate collects in Bowman’s capsule, a cuplike structure which leads to the renal tubule. As the filtrate passes through the tubule, its composition will be altered, ultimately forming urine. During the process of filtration, blood cells and large protein molecules cannot pass through the capillary walls, and thus do not leave the blood, but instead leave the capillary bed through an efferent (leaving) arteriole. This vessel subdivides again into a set of peritubular capillaries, which surround and interact with the renal tubule. This complex arrangement allows the filtrate in the tubule to exchange materials with the blood as urine formation proceeds. The peritubular capillaries merge to form a venule which ultimately joins many others and exits the kidney as a renal vein.
The renal tubule has three main sections: the proximal convoluted tubule; a long, “U”-shaped section called the Loop of Henle; and the distal convoluted tubule, which empties into the collecting duct, the tube through which urine ultimately leaves the nephron. As the filtrate moves through the tubule, two important processes occur simultaneously. During secretion, some of the molecules that did not enter the filtrate but are still in the blood are actively transported from the peritubular capillaries into the renal tubule. Many drugs and toxic substances are removed from the blood by this method. Reabsorption is the other major process that occurs, by which a majority of the materials in the filtrate, including most of the water, glucose, and other nutrients, are returned to the blood. Whatever remains in the renal tubule after these transport processes occur becomes the urine, and leaves via the collecting duct. The urine usually consists of water which contains urea (a waste formed by the metabolism of amino acids) as the major solute. Also present in lesser quantities may be uric acid (which results from nucleotide metabolism), creatinine (from the metabolism of creatine), ketones (from the metabolism of fatty acids), and a variety of other substances normally present in trace amounts.

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