Minute Anatomy.—The renal tubules , of which the kidney is for the most part made up, commence in the cortical substance, and after pursuing a very circuitous course through the cortical and medullary substances, finally end at the apices of the renal pyramids by open mouths, so that the fluid which they contain is emptied, through the calyces, into the pelvis of the kidney. If the surface of one of the papillæ be examined with a lens, it will be seen to be studded over with minute openings, the orifices of the renal tubules, from sixteen to twenty in number, and if pressure be made on a fresh kidney, urine will be seen to exude from these orifices. The tubules commence in the convoluted part and renal colum
ns as the renal corpuscles, which are small rounded masses of a deep red color, varying in size, but of an average of about 0.2 mm. in diameter. Each of these little bodies is composed of two parts: a central glomerulus of vessels, and a membranous envelope, the glomerular capsule (capsule of Bowman------------------------->) which is the small pouch-like commencement of a renal tubule.
The glomerulus is a lobulated net-work of convoluted capillary bloodvessels, held together by scanty connective tissue. This capillary net-work is derived from a small arterial twig, the afferent vessel, which enters the capsule, generally at a point opposite to that at which the latter is connected with the tubule; and the resulting vein, the efferent vessel, emerges from the capsule at the same point. The afferent vessel is usually the larger.The glomerular or Bowman’s capsule, which surrounds the glomerulus, consists of a basement membrane, lined on its inner surface by a layer of flattened epithelial cells, which are reflected from the lining membrane on to the glomerulus, at the point of entrance or exit of the afferent and efferent vessels. The whole surface of the glomerulus is covered with a continuous layer of the same cells, on a delicate supporting membrane. Thus between the glomerulus and the capsule a space is left, forming a cavity lined by a continuous layer of squamous cells; this cavity varies in size according to the state of secretion and the amount of fluid present in it. In the fetus and young subject the lining epithelial cells are polyhedral or even columnar.
The glomerulus is a lobulated net-work of convoluted capillary bloodvessels, held together by scanty connective tissue. This capillary net-work is derived from a small arterial twig, the afferent vessel, which enters the capsule, generally at a point opposite to that at which the latter is connected with the tubule; and the resulting vein, the efferent vessel, emerges from the capsule at the same point. The afferent vessel is usually the larger.The glomerular or Bowman’s capsule, which surrounds the glomerulus, consists of a basement membrane, lined on its inner surface by a layer of flattened epithelial cells, which are reflected from the lining membrane on to the glomerulus, at the point of entrance or exit of the afferent and efferent vessels. The whole surface of the glomerulus is covered with a continuous layer of the same cells, on a delicate supporting membrane. Thus between the glomerulus and the capsule a space is left, forming a cavity lined by a continuous layer of squamous cells; this cavity varies in size according to the state of secretion and the amount of fluid present in it. In the fetus and young subject the lining epithelial cells are polyhedral or even columnar.
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 substance in a more 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 reënter 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 papillæ; 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.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 descending limb of Henle’s loop the epithelium resembles that found in the glomerular capsule and the commencement of the tube, consisting of flat, clear epithelial plates, each with an oval nucleus.The nuclei alternate on opposite surfaces of the tubule so that the lumen remains fairly constant.
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 papillæ; 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.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 descending limb of Henle’s loop the epithelium resembles that found in the glomerular capsule and the commencement of the tube, consisting of flat, clear epithelial plates, each with an oval nucleus.The nuclei alternate on opposite surfaces of the tubule so that the lumen remains fairly constant.
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