Chapter 10 Urinary system
If the metabolic processes of the body are to function effectively, the chemical composition and volume of the tissue fluid must be kept constant. The most important function of the urinary system – and principally that of the kidney – is to maintain this constant internal environment, described as homeostasis.
The urinary system lies in the abdominal and pelvic cavities. It is anatomically linked with the genital or reproductive system and may be referred to as the urogenital system. Both systems share the urethra which runs through the penis of the male and joins the vagina of the female.
There are two kidneys lying in the cranial abdominal cavity, one on each side of the midline ventral to the lumbar hypaxial muscles (Fig. 10.1). Each kidney is closely attached to the lumbar muscles by a covering of parietal peritoneum. There is no mesenteric attachment, as seen in other abdominal organs, and the kidney is described as being retroperitoneal. The right kidney lies slightly cranial to the left because the stomach has evolved to lie on the left side of the abdomen, pushing the left kidney out of position. Lying close to the cranial pole of each kidney are the ovaries of the female and the adrenal glands (Fig. 10.2).
The kidneys of the cat and dog have a characteristic bean shape and the indented area is known as the hilus. This is the point at which blood vessels, nerves and the ureters enter and leave the kidney. The kidneys are normally a deep reddish-brown but the colour may be affected by any substance filtering through them. On a lateral radiograph of the abdomen, a normal kidney can be seen to be equivalent in size to approximately 2.5 vertebrae (Fig. 10.3). The outer surface may be surrounded by a layer of fat, which acts as an energy reserve and protects the kidney from external damage.
Arterial blood is carried from the aorta in a single renal artery to each kidney (Fig. 10.4). This carries 20% of cardiac output. Within the tissue of the kidney, the renal artery divides into several interlobar arteries, which pass between the renal pyramids and into the cortex. Here capillaries supply the renal tubules and also give off numerous capillary networks known as glomeruli (sing. glomerulus) (Fig. 10.5). Each glomerulus supplies an individual nephron. The capillaries then recombine to form interlobar veins, which enter the single renal vein. This carries venous blood to the caudal vena cava.
Blood entering the kidney carries oxygen, nutrients and waste products from the tissues of the body; blood leaving the kidney carries carbon dioxide produced by the kidney tissues, but the nitrogenous waste products have been removed via the glomeruli.
The functional unit of the kidney is the nephron (Fig. 10.5). Each kidney contains about a million nephrons, which are closely packed together. They are responsible for the filtration of blood and the production of urine. Each nephron is a long tubule divided into several parts:
Blood is filtered by the kidneys and the resulting filtrate undergoes a series of modifications within the renal tubules to produce urine. This urine is very different in composition and volume from the original filtrate. For every 100 L of fluid filtered from the blood only 1 L is produced as urine – 99% of the original filtrate is reabsorbed back into the blood. The changes made to the filtrate reflect the status of the extracellular fluid (ECF) and in particular that of the blood plasma.