The renal tubule begins at and leads out of Bowman’s capsule on the side opposite the vascular pole (the tubule pole, compare the Arctic pole and Antarctic pole of Earth). The tubule has a number of segments divided into subdivisions. Throughout its length the tubule is made up of a single layer of epithelial cells resting on basement membrane and connected by tight junctions that physically link the cells together.
Renal tubule segments include proximal convoluted tubule, proximal straight tubule, descending thin limb of the loop of Henle, the ascending thin limb of the loop of Henle (only long loop nephrons), ascending thick limb of the loop of Henle, distal convoluted tubule, connecting tubule, and collecting duct.
The proximal tubule is the first segment. It drains Bowman’s capsule and consists of a coiled segment – the proximal convoluted tubule – followed by a shorter straight segment – the proximal straight tubule. The coiled segment is entirely within the cortex, whereas the straight segment descends a short way into the outer medulla. Most of the length and functions of the proximal tubule are in the cortex. The next segment is the descending thin limb of the loop of Henle. The descending thin limbs of all nephrons begin at the same level, at the point where they connect to straight portions of proximal tubules in the outer medulla. This marks the border between the outer and inner stripes of the outer medulla.
In contrast, the descending thin limbs of different nephrons penetrate down to varying depths in the medulla. At their ending they abruptly reverse at a hairpin turn and become an ascending portion of the loop of Henle parallel to the descending portion. In long loops, the ones that have penetrated deep into the inner medulla, the epithelium of the first portion of the ascending limb remains thin, although different functionally from that of the descending limb. This segment is called the ascending thin limb of Henle’s loop, or simple the ascending thin limb. Further up the ascending portion the epithelium thickens, and this next segment is called the thick ascending limb. In short loops, there is no ascending think portion, and the thick ascending portion begins right at the hairpin loop.
All thick ascending limbs begin at the same level, which marks the border between the inner and outer medulla. Therefore, the thick ascending limbs begin at a slight deeper level in the medulla than do thin descending limbs. Each thick ascending limb rises back into the cortex right back to the very same Bowman’s capsule from which the tubule originated. Here it passes directly between the afferent and efferent arterioles at the vascular pole of Bowman’s capsule. The cell 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 duct, the first portion of which is called the initial collecting tubule. Connecting tubules from several nephrons merge to form a given cortical collecting duct.
All the cortical collecting ducts then run downward to enter the medulla and become outer medullary collecting ducts, and continue to become inner medullary collecting ducts. These merge to form larger ducts, the last portions of which are called papillary collecting ducts, each of which empties into a calyx of the renal pelvis. Each renal calyx is continuous with the ureter. The tubular fluid, now properly called urine, is not altered after it enters a calyx. Up to the distal convoluted tubule, the epithelial cells forming the wall of a nephrons in any given segment are homogeneous and distinct for that segment. For example, the thick ascending limb contains only thick ascending limb cells. However, beginning in the second half of the distal convoluted tubule the epithelium contains 2 intermingled cell types. The first constitutes the majority of cells in a particular segment and are usually called principal cells. Thus, there a segment-specific principal cells in the distal convoluted tubule, connecting tubule, and collecting ducts. Interspersed among the segment-specific cells in these regions are cells of a second type, called intercalated cells, that is, they are intercalated between the principal cells. 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.
Blood enters each kidney at the hilum via a renal artery. After several divisions into smaller arteries blood reaches arcuate arteries that course across the tops of the pyramids between the medulla and cortex. From these, cortical radial arteries project upward toward the kidney surface and give off a series of afferent arterioles (AAs), each of which leads to a glomerulus within Bowman’s capsule. These arteries and glomeruli are found only in the cortex, never in the medulla. In most organs, capillaries recombine to form the beginnings of the venous system, but the glomerular capillaries instead recombine to form another set of arterioles, the efferent arterioles (EAs). The vast majority of the EAs soon subdivide into a second set of capillaries called peritubular capillaries. These capillaries are profusely distributed throughout the cortex intermingled with the tubular segments. The peritubular capillaries then rejoin to form the veins by which blood ultimately leaves the kidney. EAs of glomeruli situated just above the corticomedullary border (juxtamedullary glomeruli) do not brach into peritubular capillaries the way most EAs do. Instead these arterioles descend downward into the outer medullar. Once in the medulla they divide many times to form buddies of parallel vessels called vasa recta. These bundles of vasa recta penetrate deep into the medulla.
Vasa recta on the outside of the vascular bundles “peel off” and give rise to interbundle networks of capillaries that surround Henle’s loops and the collecting ducts in the outer medulla. Only the center-most vasa recta supply capillaries in the inner medulla; thus, little blood flows into the papilla. The capillaries from the inner medulla re-form into ascending vasa recta that run in close association with the descending vasa recta within the vascular bundles. The structural and functional properties of the vasa recta are rather complex.