The kidney filters large quantities of plasma, reabsorbs substances that the body must conserve, and leaves behind and/or secretes substances that must be eliminated. The two kidneys in humans produce together ~120 mL/min of ultrafiltrate, yet only 1 mL/min of urine is produced. 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 has ~1 X106 nephrons. The structure of the nephron and the site of action of different classes of diuretics are shown in Figure 28—1.
The proximal tubule of the nephron 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; 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 becomes the thick ascending limb (TAL), which consists of three segments: a medullary portion (MTAL), a cortical portion (CTAL), and a postmacular segment. Together the proximal straight tubule, DTL, ATL, MTAL, CTAL, and postmacular segment are known as the loop of Henle. The DTL is highly permeable to water but relatively impermeable to NaCl and urea. In contrast, the ATL is permeable to NaCl and urea but 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 via a cluster of specialized columnar epithelial cells known as the macula densa. The macula densa is strategically located to sense concentrations of NaCl leaving the loop of Henle. If the concentration of NaCl is too high, the macula densa sends a chemical signal (perhaps adenosine or ATP) to the afferent arteriole of the same nephron, causing it to constrict. This, in turn, causes a reduction in glomerular filtration rate (GFR). This homeostatic mechanism, known as tubuloglomerular feedback (TGF), serves to protect the organism from salt and volume wasting. The macula densa also regulates renin release from the adjacent juxtaglomerular cells in the wall of the afferent arteriole.
Approximately 0.2 mm past the macula densa, the tubule becomes the distal convoluted tubule (DCT). The postmacular segment of the TAL and the DCT often are referred to as the early distal tubule. Like the TAL, the DCT actively transports NaCl and is impermeable to water. Since these characteristics impart the ability to produce a dilute urine, the TAL and the DCT are collectively called the diluting segment of the nephron, and the tubular fluid in the DCT is hypotonic regardless of hydration status. However, unlike the TAL, the DCT does not contribute to the countercurrent-induced hypertonicity of the medullary interstitium (see below).
The collecting duct system (connecting tubule, initial collecting tubule, cortical collecting duct, and outer and inner medullary collecting ducts) is an area of fine control of ultrafiltrate composition and volume and is where final adjustments in electrolyte composition are made. In addition, vasopressin (also called antidiuretic hormone; see Chapter 29) modulates water permeability of this part of the nephron.
The more distal portions of the collecting duct pass through the renal medulla, where the interstitial fluid is markedly hypertonic. In the absence of vasopressin, the collecting duct system is impermeable to water, and a dilute urine is excreted. In the presence of vasopressin, the collecting duct system is permeable to water, which is reabsorbed down a steep concentration gradient that exists between the tubular fluid and the medullary interstitium.
The hypertonicity of the medullary interstitium plays a vital role in the ability to concentrate urine, which is accomplished via a combination of the unique topography of the loop of Henle and the specialized permeability features of the loop's subsegments. The "passive countercurrent
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