Table 1–1 lists the names and sequence of the various tubular segments. Physiologists and anatomists have traditionally grouped 2 or more contiguous tubular segments for purposes of reference, but the terminologies have varied considerably. Table also gives the combination terms used in this text.
The proximal tubule, which drains Bowman’s capsule, consists of a coiled segment—the proximal convoluted tubule—followed by a straight segment—the proximal straight tubule—which descends toward the medulla, perpendicular to the cortical surface of the kidney.
The next segment, into which the proximal straight tubule drains, is the descending thin limb of Henle’s loop (or simply the descending thin limb). The descending thin limb is in the medulla and is surrounded by an interstitial environment that is quite different from that in the cortex. The descending thin limb ends at a hairpin loop, and the tubule then begins to ascend parallel to the descending limb. The loops penetrate to varying depths within the medulla. In long loops (see later discussion), the epithelium of the first portion of this ascending limb remains
thin, although different from that of the descending limb. This segment is called the ascending thin limb of Henle’s loop (or simply the ascending thin limb). Beyond this segment, in these long loops, the epithelium thickens, and this next segment is called the thick ascending limb of Henle’s loop (or simply the thick ascending limb). In short loops (see later discussion), there is no ascending thin limb, and the thick ascending limb begins right at the hairpin loop. The thick ascending limb rises back into the cortex. Near the end of every thick ascending limb, the tubule returns to Bowman’s capsule, from which it originated, and passes directly between the afferent and efferent arterioles, as they enter and exit that renal corpuscle at its vascular pole. The cells in the thick ascending limb closest to Bowman’s capsule (between the afferent and efferent arterioles) are specialized cells known as the macula densa. The macula densa marks the end of the thick ascending limb and the beginning of the distal convoluted tubule. This is followed by the connecting tubule, which leads to the cortical collecting tubule, the first portion of which is called the initial collecting tubule.
From Bowman’s capsule through the loop of Henle to the initial collecting tubules, each of the 1 million nephrons in each kidney is completely separate from the others. However, connecting tubules from several nephrons merge to form cortical collecting tubules, and a number of initial collecting tubules then join end to end or side to side to form larger cortical collecting ducts. All the cortical collecting ducts then run downward to enter the medulla and become outer medullary collecting ducts and then inner medullary collecting ducts. The latter merge to form several hundred large ducts, the last portions of which are called papillary collecting ducts, each of which empties into a calyx of the renal pelvis.
Standard nomenclature for structures of the kidney (1988 Commission of the International Union of Physiological Sciences). Shown are a short-looped and a long-looped (juxtamedullary) nephron, together with the collecting system (not drawn to scale). A cortical medullary ray—the part of the cortex that contains the straight proximal tubules, cortical thick ascending limbs, and cortical collecting ducts—is delineated by a dashed line. 1, renal corpuscle (Bowman’s capsule and the glomerulus); 2, proximal convoluted tubule; 3, proximal straight tubule; 4, descending thin limb; 5, ascending thin limb; 6, thick ascending limb; 7, macula densa (located within the final portion of the thick ascending limb); 8, distal convoluted tubule; 9, connecting tubule; 9*, connecting tubule of a juxtamedullary nephron that arches upward to form a so-called arcade (there are only a few of these in the human kidney); 10, cortical collecting duct; 11, outer medullary collecting duct; 12, inner medullary collecting duct. (Reproduced with permission from Kriz W, Bankir L. Am J Physiol 1988;254LF:F1–F8.)
The pathway taken by fluids flowing within a nephron always begins in the cortex (in Bowman’s capsule), descends into the medulla (descending limb of the loop of Henle), returns to the cortex (thick ascending limb of the loop of Henle), passes down into the medulla once more (medullary collecting tubule), and ends up in a renal calyx. Each renal calyx is continuous with the ureter, which empties into the urinary bladder, where urine is temporarily stored and from which it is intermittently eliminated. The urine is not altered after it enters a calyx. From this point on, the remainder of the urinary system serves only to maintain the fluid composition established by the kidney.
As noted earlier, the tubular epithelium has a one-cell thickness throughout.
Before the distal convoluted tubule, the cells in any given segment are homogeneous and distinct for that segment. Thus, eg, the thick ascending limb contains only thick ascending limb cells. However, beginning in the second half of the distal convoluted tubule, 2 cell types are found intermingled in most of the remaining segments. One type constitutes the majority of cells in the particular segment, is considered specific for that segment, and is named accordingly: distal convoluted tubule cells, connecting tubule cells, and collecting-duct cells, the latter known more commonly as principal cells. Interspersed among the segment-specific cells in each of these 3 segments are individual cells of the second type, called intercalated cells. There are actually several types of intercalated cells; 2 of them are called type A and type B. (The last portion of the medullary collecting duct contains neither principal cells nor intercalated cells but is composed entirely of a distinct cell type called the inner medullary collecting-duct cells.)
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