THE KIDNEYS 



1223 



The renal tubules, commencing in the renal corpuscles, present, during their course, many 

 changes in shape and direction, and are contained partly in the medullary and partly in the 

 cortical substance. At their junction with the glomerular capsule they exhibit a somewhat 

 constricted portion, which is termed the neck. Beyond this the tubule becomes convoluted, 

 and pursues a considerable course in the cortical substance constituting the proximal convoluted 

 tube. After a time the convolutions disappear, and the tube approaches the medullary sub- 

 stance in a pore or less spiral manner; this section of the tubule has been called the spiral tube. 

 Throughout this portion of their course the renal tubules are contained entirely in the cortical 

 substance, and present a fairly uniform, caliber. They now enter the medullary substance, 

 suddenly become much smaller, quite straight in direction, and dip down for a variable depth 

 into the pyramids, constituting the descending limb of Henle's loop. Bending on themselves, 

 they form what is termed the loop of Henle, and reascending, they become suddenly enlarged, 

 forming the ascending limb of Henle's loop, and reenter the cortical substance. This portion 

 of the tubule ascends for a short distance, when it again becomes dilated, irregular, and angular. 

 This section is termed the zigzag tubule ; it ends in a convoluted tube, which resembles the 

 proximal convoluted tubule, and is called the distal convoluted tubule. This again terminates 

 in a narrow junctional tube, which enters the straight or collecting tube. 



The straight or collecting tubes commence in the radiate part of the cortex, where they receive 

 the curved ends of the distal convoluted tubules. They unite at short intervals with one another, 

 the resulting tubes presenting a considerable increase in caliber, so that a series of comparatively 

 large tubes passes from the bases of the rays into the renal pyramids. In the medulla the tubes 

 of each pyramid converge to join a central tube (duct of Bellini) which finally opens on the summit 

 of one of the papillae; the contents of the tube are therefore discharged into one of the calyces. 



Structure of the Renal Tubules. The renal tubules consist of a basement membrane lined with 

 epithelium. The epithelium varies considerably in different sections of the tubule. In the neck 

 the epithelium is continuous with that lining the glomerular capsule, and like it consists of 

 flattened cells each containing an oval nucleus (Fig. 1132). The two convoluted tubules, the 

 spiral and zigzag tubules and the ascending limb of Henle's loop, are lined by a type of epithelium 

 which is histologically the same in all. The cells are somewhat columnar in shape and dovetail 

 into one another of their lateral aspect. Each has a striated border next the lumen of the tube, 

 its inner part is granular and its outer portion vertically striated. The nucleus is spherical and 

 situated about the center of the cell. In the descend- 

 ing limb of Henle's loop the epithelium resembles that 

 found in the glomerular capsule and the commence- 

 ment of the tube, consisting of flat, clear epithelial 

 plates, each with an oval nucleus (Fig. 1131). The 

 nuclei alternate on opposite surfaces of the tubule so 

 that the lumen remains fairly constant. 



In the straight tube the epithelium is clear and 

 cubical: in its papillary portion the cells are distinctly 

 columnar and transparent (Fig. 1132). 



The Renal Bloodvessels. The kidney is plentifully 

 supplied with blood (Fig. 1133) by the renal artery, a 

 large branch of the abdominal aorta. Before it enters 

 the kidney, each artery divides into four or five 

 branches which at the hilum lie mainly between the 

 renal vein and ureter, the vein being in front, the 

 ureter behind; one branch usually lies behind the 

 ureter. Each vessel gives off some small branches to 

 the suprarenal glands, to the ureter, land to the sur- 

 rounding cellular tissue and muscles. Frequently a 

 second renal artery, termed the inferior renal, is given 

 off from the abdominal aorta at a lower level, and supplies the lower portion of the kidney, 

 while occasionally an additional artery enters the upper part of the kidney. The branches 

 of the renal artery, while in the sinus, give off a few twigs for the nutrition of the surround- 

 ing tissues, and end in the arterise propriae renales, which enter the kidney proper in the 

 renal columns. Two of these pass to each renal pyramid, and run along its sides for its 

 entire length, giving off in their course the afferent vessels of the renal corpuscles in the renal 

 columns. Having arrived at the bases of the pyramids, they form arterial arches or arcades 

 which lie in the boundary zone between the bases of the pyramids and the cortical arches, and 

 break up into two distinct sets of branches devoted to the supply of the remaining portions 

 of the kidney. 



The first set, the interlobular arteries (Fig. 1128), are given off at right angles from the side 

 of the arterial arcade looking toward the cortical substance, and pass directly outward between 

 the medullary rays to reach the fibrous tunic, where they end in the capillary net-work of this 

 part. These vessels do not anastomose with each other, but form what are called end-arteries. 



FIG. 1131. Longitudinal section of de- 

 scending limb of Henle's loop. a. Membrana 

 propria. 6. Epithelium. 



