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The basic urine-forming unit of the kidney is the nephron, which consists of a filtering apparatus, the glomerulus, connected to a long tubular portion that reabsorbs and conditions the glomerular ultrafiltrate. Each human kidney is composed of ~1 million nephrons. Figure 25–1 illustrates subdivisions of the nephron.

Figure 25–1

Anatomy and nomenclature of the nephron.

GLOMERULAR FILTRATION. In the glomerular capillaries, a portion of plasma water is forced through a filter that has 3 basic components: the fenestrated capillary endothelial cells, a basement membrane lying just beneath the endothelial cells, and the filtration slit diaphragms formed by epithelial cells that cover the basement membrane on its urinary space side. Solutes of small size flow with filtered water (solvent drag) into the urinary (Bowman's) space, whereas formed elements and macromolecules are retained by the filtration barrier.

OVERVIEW OF NEPHRON FUNCTION. The kidney filters large quantities of plasma, reabsorbs substances that the body must conserve, and leaves behind or secretes substances that must be eliminated. The changing architecture and cellular differentiation along the length of a nephron is crucial to these functions (see Figure 25–1). The 2 kidneys in humans produce together ~120 mL of ultrafiltrate per minute, yet only 1 mL/min of urine is produced. Therefore, >99% of the glomerular ultrafiltrate is reabsorbed at a staggering energy cost. The kidneys consume 7% of total-body oxygen intake despite the fact that the kidneys make up only 0.5% of body weight.

The proximal tubule is contiguous with Bowman's capsule and takes a tortuous path until finally forming a straight portion that dives into the renal medulla. Normally, ~65% of filtered Na+ is reabsorbed in the proximal tubule, and since this part of the tubule is highly permeable to water, reabsorption is essentially isotonic. Between the outer and inner strips of the outer medulla, the tubule abruptly changes morphology to become the descending thin limb (DTL), which penetrates the inner medulla, makes a hairpin turn, and then forms the ascending thin limb (ATL). At the juncture between the inner and outer medulla, the tubule once again changes morphology and becomes the thick ascending limb (TAL, with 3 segments noted in Figure 25–1). Together the proximal straight tubule, DTL, ATL, and TAL segments are known as the loop of Henle.

The DTL is highly permeable to water, yet its permeability to NaCl and urea is low. In contrast, the ATL is permeable to NaCl and urea but is impermeable to water. The TAL actively reabsorbs NaCl but is impermeable to water and urea. Approximately 25% of filtered Na+ is reabsorbed in the loop of Henle, mostly in the TAL, which has a large reabsorptive capacity. The TAL passes between the afferent and efferent arterioles and makes contact with the afferent arteriole by means of ...

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