The urinary and the genital organs are intimately related in development, and remain closely related morphologically throughout the life of the animal. They are derived principally from mesoderm, but short end-sections of their duct systems originate also from ectoderm. The function of the URINARY ORGANS is excretion of fluid wastes, whereas the function of the male and female GENITAL ORGANS is reproduction.
Urinary Organs
General and ComparativeThe urinary organs consist of the KIDNEYS, which secrete urine, and the RENAL PEL-VIS, URETER, BLADDER, and URETHRA which function in transporting the urine to the outside. The urethra differs in the two sexes. In the male it is closely related to the genital organs.
KidneysThe kidneys (renes*) are paired excretory glands in which waste products are continuously eliminated from the blood. They regulate the fluid and salt balance of the body and thus maintain normal osmotic pressures in the blood and tissues. They are also capable of removing foreign substances from the blood. The kidneys, therefore, have considerable controlling and regulatory influence over the blood, and the amount of blood that, with certain fluctuations, flows continuously through them is large. In man it has been calculated that about 1,500 liters of blood must flow through the kidneys for the production of 1,500 ml. of urine; this occurs about once every 24 hours.The color of the kidneys varies from brownish red to dark bluish red depending upon the amount of blood they contain. They are basically bean-shaped. The carnivores (417-420) and· small ruminants (434, 435) have thick, well-rounded kidneys; the pig has rather flat ones (426, 427). Exceptions to this basic shape are the right kidney of the horse, which is heartshaped (437), and those of the ox, which are lobated irregular ovals (430, 431).The kidney has dorsal and ventral surfaces, a convex lateral and a concave medial border, and cranial and caudal extremities, or poles. The hilus is an indentation in the medial border, where the renal blood vessels, lymphatics, nerves, and the ureter enter the organ (417). The hilus leads into a recess (sinus renalis) which is situated in the center of the kidney and contains the renal pelvis and, embedded in fat, the proximal branches of the renal vessels and nerves (410).The kidneys lie in the lumbar area to the right and left of the median plane. The medial border of the right kidney is related to the caudal vena cava, and that of the left kidney is related to the aorta (439). The renal arteries and veins arise from these large vessels opposite the kidneys and pass to the hilus by the shortest route ( 11, 12). The right kidney is usually somewhat more cranial than the left, more so in the dog and horse (1; 248), and less so in the cat and pig (541; 545). Because of the presence of the rumen, the left kidney of the ruminants has become pendulous and almost entirely invested with peritoneum. The rumen has pushed * Also Gr. nepkros; hence nephritis, nephron, etc.R. Nickel et al., The Viscera of the Domesti...
