The kidneys perform two major functions. First, they excrete most of the end prod ucts of bodily metabolism, and second, they control the concentrations of most of the constituents of the body fluids. The main goal of this chapter is to gain some under standing of the processes by which the urine is formed and waste products removed from the bloodstream. The control of the constituents of the body fluids is discussed only secondarily.The primary operating unit of the kidney is called a nephron, of which there are about a million in each kidney (Figs. 17.1 and 17.2). Each nephron is capable of form ing urine by itself. The entrance of blood into the nephron is by the afferent arteriole, located in the renal cortex, and the tubules of the nephron and the associated peritubu lar capillaries extend deep into the renal medulla. The principal functional units of the nephron are the glomerulus, through which fluid is filtered from the blood; the juxta-glomerular apparatus, by which glomerular flow is controlled; and the long tubule, in which the filtered fluid is converted into urine.
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