The urine is secreted by the kidney, whence it passes successively through the renal pelvis, ureter, bladder, and urethra into the outer world.
The kidney is made up of homologous parts or lobes, which are readily distinguished in early life by the superficial furrows marking their lines of junction. In later years these depressions on the surface of the kidney disappear. Each of the lobes corresponds to one of the papillae of the kidney and the pelvic calix that embraces it. In some of the lower animals e. g., the rabbit the kidney has but one papilla, so that the whole renal pelvis in those animals corresponds to a single calix in man.
The kidney is a compound tubular gland of peculiar construction, the tubules taking origin from little spherical bodies, called Malpighian bodies, instead of from simple blind extremities, and, after running a definite and somewhat complicated course, uniting successively with several others to form the excretory ducts, called the “collecting tubules” which open into the calices near the tips of the papillae.
If a section of the organ be made through its convexity down to the pelvis, the papillae will be seen projecting into the calices of the pelvis, and it will be noticed that each papilla forms the apex of a pyramidal portion of tissue having a different tint and texture from the rest of the kidney. These pyramids form the "medulla" of the organ.
The blood vessels supplying nearly all its substance enter the kidney near the bases of the pyramids, having approached the organ through the fat that lies around the calices. Within the kidney they break up into branches that run along the base of each pyramid in that portion of the organ which is called the "boundary zone." Between that zone and the convex surface of the kidney the tissue is known as the "cortex".
The arrangement of the renal tubules, which make up the chief bulk of the kidney, can be most easily understood if they are traced back from their openings at the apex of the pyramid to their origins in the Malpighian bodies. The different portions of the tubules present somewhat different characters, and have received special names.
The collecting tubes, which open into the calix at the apex of the pyramid, are straight, and lie nearly parallel to each other and to the axis of the pyramid, and, therefore, nearly perpendicular to the base of the pyramid. As they are followed from the apex, in a direction the reverse of that taken by the urine in flowing through them, they branch dichotomously, and the branches become progressively smaller. At the base of the pyramid these straight tubules are collected into bundles that radiate toward the convex surface of the kidney, and are called the "medullary rays”. In these, and in the part of the pyramid that is near the boundary- zone, the collecting tubes are associated with other straight portions of the tubules, "Henle's tubes", which will be described presently. From the medullary rays the tubules pass into the region between those rays in the cortical portion of the kidney. This region of the cortex is known as the "labyrinth". Here the tubules lose their straight character and become much contorted, forming the "second convoluted tubules". They then re-enter the medullary rays, which they descend for a variable distance into the pyramid, constituting the "ascending branches of Henle's tubes", which make a sharp turn, " Henle's loop," and then retrace their course up the medullary rays into the cortical portion of the kidney, "descending branches of Henle's tube". They then pass again into the labyrinth and form the "first convoluted tubules", which finally merge into the structure of the Malpighian bodies, also situated in the labyrinth. In consequence of the passage of tubules from them into the surrounding labyrinth the medullary rays become smaller as they are followed from the base of the pyramid, and eventually disappear before the capsule of the kidney is reached. They are completely surrounded by the labyrinth.
If we now follow the course of the urine in its way from the Malpighian body to the outlet of the tubule, we shall find that it passes through the following divisions of the tubule : 1, the first convoluted tubule ; 2, the descending branch of Henle's tube ; 3, Henle's loop ; 4, the ascending branch of Henle's tube ; 5, the second convoluted tubule ; 6, the collecting tube. Of these, the two convoluted tubules are situated in the labyrinth; all the rest in the medullary rays and pyramid. All of the portions, with the exception of the convoluted tubules and the loop, are straight and lie parallel to each other.
Before entering more particularly into the structure of the renal tubule, it will be best to complete this general sketch by considering the course of the blood vessels.
As has already been said, the vessels enter the kidney between the calices and pyramids and are distributed in branches that lie parallel to the bases of the latter, and, therefore, to the convex surface of the organ, and are situated in the boundary-zone. The arterial branches in this location form the "arterial arcade". From this arcade perpendicular branches, the "interlobular arteries pass toward the capsule, taking a straight course through the labyrinth be- tween the medullary rays. In this course they give off branches, the "afferent vessels", which go to the Malpighian bodies.
The main artery becomes smaller in giving off these branches, and finally ends in terminal afferent vessels.
Within the Malpighian body the afferent vessel divides abruptly into a number of capillary loops, which are compacted together to form a globular mass, called the "glomerulus". These loops rejoin to form the "efferent" vessel, which is somewhat smaller than the afferent vessel, and leaves the Malpighian body at a point close to that at which the afferent vessel enters it.
Soon after leaving the Malpighian body the efferent vessel breaks up into a second set of capillaries, which lie among the convoluted tubules of the labyrinth and also penetrate into the medullary rays, to be distributed between the tubules composing them. This capillary network extends also into the pyramid, in which the capillaries run, for the most part, parallel to the renal tubules, with comparatively few transverse anastomosing branches. For this reason they have been called the "vasa recta." They also receive blood from little twigs given off from the arterial arcade.
The blood from the intertubular capillaries is collected in veins, which run a course parallel to that of the arteries and lie in close proximity to them. They have received names similar to those of the corresponding arteries : "interlobular veins”, "venae rectae", and "venous arcade". Relatively large veins also leave the kidney from beneath the capsule on the convex surface of the organ. They are called the "stellate veins".
The Malpighian body is enclosed by a thin fibrous capsule (Bowman's capsule), which is perforated at two opposite points to permit the passage on the one hand of the afferent and efferent vessels, and on the other hand to allow of a communication between its cavity and the beginning of the uriniferous tubule. When distended with blood the glomerulus nearly fills this capsule, but when collapsed it is retracted toward the attachment formed by the vessels that pierce the capsule. It is covered by a single layer of epithelial cells, which is reflected at that attachment and forms a lining for the inner surface of the capsule to the point where its cavity opens into the lumen of the renal tubule. Here the epithelial lining becomes continuous with that of the tubule.
The different portions of the uriniferous tubule differ in their external diameters, the diameters of their lumina, and the character of their epithelial linings. The appearance of the epithelial cells differs, however, in accordance with their state of functional activity.
The first convoluted tubule is relatively large, and is lined with large epithelial cells, which project into the tubule about one-third of its diameter. The cells have round nuclei situated near their centers, and are granular, with an appearance of radiate striation in their deeper halves when charged with secretion.
The descending branch of Henle's tube has a smaller diameter, but its lumen is wide in consequence of the thinness of the clear epithelial cells lining it. In the ascending branch the lumen is again smaller, although the diameter of the tube is larger, because the lining cells are thicker, somewhat resemble those of the first convoluted tubule. The transition from the character of the descending to that of the ascending branch does not always take place exactly at the loop.
The second convoluted tubule is a little smaller than the first, and is lined with cells that are not quite so granular and a little more highly refracting.
The collecting tubules are lined with columnar epithelium, the cells of which become longer as the diameter of the tube increases in its progress toward the apex of the pyramid.
The epithelial lining throughout the course of the renal tubule is said to rest upon a thin, homogeneous basement-membrane interposed between it and the interstitial fibrous tissue. The latter is present in small amount, and partakes of the character of an areolar tissue, holding the tubules and blood vessels in place. It is rather abundantly supplied with lymphatics.
For the study of the uriniferous tubules sections made trans- verse to the course of the straight tubules will be found very useful. In the cortex the medullary rays, with their descending and ascending branches of Henle's tubes and their collecting tubules, will appear surrounded by the labyrinth, made up of the convoluted tubules, Malpighian bodies, and larger vessels, the latter in cross-section. Near the apex of the pyramid cross-sections of the larger collecting tubes and of the vasa recta will be seen; and near its base the smaller collecting tubes and the two limbs of Henle's tube, with, possibly, here and there a "loop" in nearly longitudinal section, will appear. Among all these sections of the tubules the interstitial tissue with its capillaries and lymphatics will complete the picture.
The nerves of the kidney are small and apparently not abundant. Their larger branches follow the courses of the arteries.
The external surface of the kidney is covered with a capsule of fibrous tissue, which on its deeper surface becomes continuous with the interstitial tissue, so that its vascular supply communicates with the capillaries in the superficial portions of the kidney.
The fibrous capsule of the kidney becomes continuous at the hilum of that organ with the fibrous coats of the calices and pelvis, and, through these, with those of the ureter and bladder.
The columnar epithelium lining the collecting tubes is continuous with a layer of similar cells covering the papillae.
The watery constituent of the urine is secreted in the Malpighian body, where it passes from the blood through the capillary walls of the glomerulus into the cavity of Bowman's capsule. Under nor- mal conditions it is free from albumin, and, therefore, is unlike the serum that passes through the walls of the capillaries in other parts of the body. It has been thought that this difference was attributable to the functional action of the endothelium in the glomerulus, though morphologically it is similar to that throughout the body. It is more probable that the epithelium covering the glomerulus has something to do with the prevention of a loss of albumin. In disease of the kidney, alterations in the glomerulus and, perhaps, in other parts of the kidney permit albumin to pass into the secretion.
The epithelium lining the uriniferous tubules discharges its secretion into the lumen of the tubules, whence it is carried by the stream flowing from the Malpighian bodies. The epithelial cells lining the convoluted tubules and the ascending branches of Henle's tubes appear to be those most active in carrying on the eliminative function of the kidney.