A distal (DC) and two proximal (PC) convoluted tubules are marked in figure on left. The cells of the distal tubule are more crowded on one side. These crowded cells constitute the macula densa (MD) that lies adjacent to the afferent arteriole.
In figure on right, both the vascular pole and the urinary pole of the renal corpuscle are evident. The vascular pole is characterized by the presence of arterioles (A), one of which is entering or leaving (double-headed arrow) the corpuscle. The afferent arteriole possesses modified smooth muscle cells with granules, the juxtaglomerular cells (not evident in this figure). At the urinary pole, the parietal layer of Bowman’s capsule is continuous with the beginning of the proximal convoluted tubule (PC ). Here, the urinary space of the renal corpuscle continues into the lumen of the proximal tubule, and the lining cells change from simple squamous to simple cuboidal or low columnar with a brush border.
KEY A, arteriole BC, Bowman’s capsule (parietal layer) CT, collecting tubule D, distal straight tubule DC, distal convoluted tubule MD, macula densa P, proximal (straight tubule) PC, proximal convoluted tubule Pod, podocyte (visceral layer of Bowman’s capsule) asterisks, urinary space double-headed arrow, blood vessel at vascular pole of renal corpuscle
Renal corpuscles are restricted to the cortical labyrinth. The medulla contains the thick straight segments of proximal and distal tubules, along with their thin segments, the collecting tubules and ducts, and the blood vessels that run in parallel with them. These structures function as the countercurrent multiplier and countercurrent exchange systems that, ultimately, produce hypertonic urine. The final urine drains from the papillary ducts (of Bellini) into calyces that then empty into the renal pelvis.
Medulla, kidney, human, H&E ×240.
A section through the outer portion of the medulla is shown in this figure. This region contains proximal and distal thick segments, thin segments, and collecting tubules. All of the tubules are parallel, and all are cut in cross section; thus, they present circular profiles. The proximal straight tubules (P) display typical star-shaped lumina and a brush border (or the fragmented apical cell surface from which the brush border has been partially broken). These tubules have outside diameters that are generally larger than those of 734 the distal straight tubules (D). As mentioned previously and as shown here, the distal straight tubules display a larger number of nuclei than do comparable segments of proximal straight tubule cells. Note, also, that the lumen of the distal tubule is more rounded and the apical surface of the cells is sharper. The collecting tubules (CT ) have outer diameters that are about the same as those of the proximal tubules and larger than those of distal tubules. The cells forming the collecting tubules are cuboidal and smaller than those of proximal tubules; thus, they also display a relatively larger number of nuclei than do comparable segments of proximal tubule cells. Count them! Finally, boundaries between the cells that constitute the collecting tubules are usually evident (asterisks); this serves as one of the most dependable features for the identification of collecting tubules.