It consists a pair of bean shaped organs called kidneys, a pair of ducts called ureters which arise from each kidney, a muscular sac like organ the urinary bladder and a tubule called urethra which arises from bladder and empties urine outside the body. The bladder end of urethra has a sphincter valve, which controls release of urine from the bladder.
Structure of kidney:
Each kidney is bean shaped organ which is about 10 cm long. They lie in the abdominal cavity being attached with dorsal body wall on either side of vertebral column. Each kidney is enclosed by a thin membranous covering called peritoneum and located on the top of each kidney is adrenal gland. The kidneys receive blood through paired renal arteries which directly arise from dorsal aorta. The blood is drained out from each kidney by a renal vein. Both renal arteries enter and renal veins leave kidney at the site called hilns.
A longitudinal section of kidney shows that it consists of two distinct regions, an outer darker region, the cortex and an inner lighter region, the medulla. Medulla consists of many cone shaped structures called pyramids. Urine is dropped continuously from the tips of pyramids into funnel shaped spaces called pelvis of the ureter.
Nephron:
Each kidney is composed of about a million microscopic tubules or nephrons. It is regarded as structural and functional unit of kidney which performs osmoregulation as well as excretion.
Each nephron begins with a spherical structure called malpighian body. It lies in the cortex of kidney. Malpighian body is composed of cup like Bowman’s capsule. Inside this cup lies a dense network of capillaries known as glomerulus. The Bowman’s capsule gives out a coiled tubule known as proximal convoluted tubule. It lies in the cortex region. A network of capillaries is also attached with proximal convoluted tubule. Proximal convoluted tubule narrows and descends down into the medulla, makes a u turn and comes back in the cortex. This narrow U-shape part of tubule is known as loop of cortex. Thus it has a descending and an ascending limb. The ascending limb of the loop of Henle becomes larger in diameter in the cortex and forms a coil called distal convoluted tubule.
Distal convoluted tubule finally opens into common collecting duct. The latter receives urine from a number of nephrons and drains into the pelvis of the kidney.
Blood supply to the glomerulus of a nephron begins as an afferent arteriole which arises from renal arteriole. From the glomerulus blood is carried by the efferent arteriole to two capillary networks. One of the network called peritubular capillaries is associated with proximal and distal convoluted tubules while the other one called vasa recta runs straight parallel besides the limbs of the loop of Henle.
Function:
Kidney performs regulatory functions by different processes viz filtration, re-absorption, tubular secretions and counter current exchange. These processes are carried on at the level of each nephron.
Kidneys play vital role in osmoregulation as well as excretion of nitrogenous wastes.
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