The nephron is the functional unit of the kidney. There are about 2 million nephrons in each human kidney. They are responsible for the production of urine and correspond to the secretory part of other glands. The collecting tubules, responsible for the final concentration of the urine, are analogous to the ducts of exocrine glands. The nephron is made up of the renal corpuscle and the renal tubule. The renal corpuscle consists of the glomerulus, a tuft of 10 to 20 capillary loops, surrounded by a double-layered epithelial cup, the renal or Bowman’s capsule. The glomeru-lar capillaries are supplied at the vascular pole of Bowman’s capsule by an afferent arteriole and drained by an efferent arteriole that leaves Bow-man’s capsule at the vascular pole and then branches to form a new capillary network to supply the kidney tubules. The opposite pole of Bowman’s capsule, the urinary pole, is where the filtrate leaves the renal capsule. The tubular parts of the nephron are the proximal thick segment (consisting of the proximal convoluted tubule and the proximal straight tubule), the thin segment, which constitutes the thin limb of the loop of Henle, and the distal thick segment, consisting of the distal straight tubule and the distal convoluted tubule. The loop of Henle is the U-shaped portion of the nephron consisting of the thick straight portions of the proximal and distal tubules and the thin segment between them. The distal convoluted tubule joins the collecting tubule. The nephron and the collecting tubule constitute the uriniferous tubule.
Cortex, kidney, human, H&E ×60.
The renal cortex can be divided into regions referred to as the cortical labyrinth (CL) and the medullary rays (MR). The cortical labyrinth contains the renal corpuscles (RC ), which appear as relatively large spherical structures. Surrounding each renal corpuscle are the proximal and distal convoluted tubules. They are also part of the cortical labyrinth. The convoluted tubules, particularly the proximal, because of their tortuosity, present a  variety of profiles, most of which are oval or circular; others, more elongate, are in the shape of a letter J, a C, or even an S. The medullary rays are composed of groups of straight tubules oriented in the same direction and appear to radiate from the base of the pyramid. When the medullary rays are cut longitudinally, as they are in this figure, the tubules present elongated profiles. The medullary rays contain proximal straight tubules (thick segments; descending limb of Henle’s loop), distal straight tubules (thick segments; ascending limbs of Henle’s loop), and collecting tubules.
Cortex, kidney, human, H&E ×120.
This figure presents another profile of the renal cortex, at a somewhat higher magnification, cut in a plane at a right angle to the section in figure above. The peripheral part of the micrograph shows the cortical labyrinth in which the tubules display chiefly round and oval profiles but also some that are more elongate and curved. The appearance is the same as the cortical labyrinth areas of figure above. A renal corpuscle (RC) is also present in the cortical labyrinth. In contrast, the profiles presented by the tubules of the medullary ray in this figure are quite different from those seen in figure above. All of the tubules bounded by the dashed line belong to the medullary ray (MR), and all are cut in cross section.
A general survey of the tubules within the medullary ray reveals that several distinct types can be recognized on the basis of the size of the tubule, shape of the lumen, and size of the tubule cells. These features as well as those of the cortical labyrinth are considered in Plate 76.
PLATE 75 KEY CL, cortical labyrinth MR, medullary ray RC, renal corpuscle dashed line, approximate boundary of the medullary ray