Kidney

Kidney. The human kidneys, two in number, lie at the back of the abdomen, behind the peritoneal sac on either side of the spinal column, at the junction of the dorsal and lumbar vertebra?. Each kidney weighs a little less than 5 oz. There is an outer investing capsule, which euvelops the gland and becomes continuous at the inner concave margin or hiliim of the kidney with the outer coats of the vessels which enter the gland at that point. These vessels consist of the renal artery and vein and the ureter; down the last-named tube the urine secreted by the kidney passes on its way to the bladder. If the ureter be traced upwards, it will be found to expand on reaching the inner margin of the kidney into what is known as the pelvis. The pelvis divides into a number of little cups called calyces, each of which receives the pointed extremity of one or more pyramids of the kidney. If a longitudinal section be.made through a kidney, it will be found to consist of an outer or iiirtieal'portion and an inner, which is called the pyramidal portion, being made up of seine dozen little pyramids the apices of which abut as aforesaid upon the several calyces into which the pelvis subdivides. The kidney substance comprises the ramifications of innumerable minute tubules. These tubules commence by a blind extremity in the cortex of the kidney, pursue n tortuous course through the cortex, and then a straight course through the pyramids, which latter derive their striated appearauce from being composed of bundles of tubules. As the tubules approach the apex of each pyramid they open into a series of ducts known as collecting tubules, which empty themselves into the calyces. The tubules of the kidneys are lined throughout by a single layer of epithelial cells. These cells secrete from the blood a fluid, the urine, which passes along the course of the tubules, finds its way into the calyces, and so into the pelvis of the kidney, and thence down the ureter into the bladder.

The character of the epithelial cells lining the tubules varies in different parts. It will suffice to direct attention, first, to the blind extremity of the tubule in the cortex, and, secondly, to those portions of the tubules which, from the wavy course they pursue through the substance of the kidney, are known as the convoluted portions of the tubules. At its extremity in the cortex each tubule expands, forming a kind of cap, which envelops a loop of capillary blood vessels, forming what is known as a glomerulus or Malpighian corpuscle. These latter bodies may be seen with the naked eye as minute red points dotted over the cortex of the'kidney. The little cap which envelops the loop of blood-vessels is lined with flattened epithelial cells, and certain fluid constituents of the blood find their way from the capillaries through the layer of epithelium, and reach the interior of the urinary tubule. In this manner it is supposed that the bulk of the water of the urine is abstracted from the blood. To pass now to those parts of the urinary tubules which are convoluted, here the epithelium is of the kind known as secreting epithelium, and, the convoluted tubule being surrounded by a close network of capillary blood-vessels, this secreting epithelium extracts from the blood certain of its constituents.

It is supposed that the more important constituents of the urine are secreted and passed into the system of tubules by the epithelium of these convoluted tubules.

It remains to say something concerning the peculiar arrangement of the blood-vessels of the kidney. The renal artery breaks np into a number of branches, many of which find their way towards the cortex of the kidney, and ultimately break up into a series of minute arterioles, each of which finally enters a Malpighian corpuscle. There the arteriole breaks up into the little cluster of capillaries which have been already alluded to as being invested by the blind extremity of a urinary tubule. The blood from these capillaries is collected again into a little vein which emerges from the Malpighian corpuscle at the point where the little arteriole entered it. This little vein, however, breaks up again into a second system of capillaries, which ramify around the convoluted portion of one of the urinary tubules; the blood from this second system of capillaries is again collected into a little vein, which unites with other veins and finally empties itself into the renal vein, which passes out at the hilum of the kidney. It will thus be noted that there is a double system of capillaries in the case of the kidneys, one set of capillaries being found in the Malpighian corpuscle and another set investing the convoluted portions of the tubules. For diseases of the kidneys, see Bright's Disease, Hamaturia. - A stone or calculus is sometimes formed in the kidney, a serious condition which usually calls for operative treatment.

Surgical kidney is the term applied to the condition sometimes met with as the result of backward pressure caused by obstruction to the flow of urine through the urethra or bladder. The kidney may be affected by tubercular or malignant disease.