The Renal System

Excretion and the kidneys

Structure of the kidney

Human kidneys are about 12cm long by about 7cm wide.They are part of a set of organs known as the urinary system.

The kidneys are involved in excretion. This is the removal from the body of unwanted substances.

Excretion is an important part of homeostasis (keeping the internal environment constant).

The most poisonous of all waste products is ammonia. This is converted by the liver immediately into urea which is released into the blood.

The kidneys extract urea from the blood and excrete it from the body as part of a liquid called urine.

A thin tube the ureter comes out of the concave side of each kidney and extends downwards to a single bag called the bladder.

The kidney tissue consists of many capillaries and tiny tubes called nephrons. A section through the kidney shows a darker outer region the cortex and a lighter inner zone the medulla.

Detailed structure

The renal artery divides up into a great many arterioles and capillaries. Each arteriole leads to a glomerulus which is a capillary repeatedly divided and coiled. The glomerulus is surrounded by a cup shaped organ called a Bowmans capsule.

The capillaries of the glomerulus offer resistance to the flow of blood so that a high pressure is set up. The pressure causes fluid to filter out through the capillary walls and collect in the Bowmans capsule. Glucose and some of the salts and as much of the water are reabsorbed back into a network of capillaries. The remaining liquid is now called urine contains only the waste products such as inactive hormones, urea and excess salts and water. Osmoregulation - the control of osmotic potential in the body. Blood and tissue fluid must be kept at a constant osmotic pressure to avoid an unnecessary movement of water in and out of cells by osmosis.

Control of excretion - homeostasis

The kidneys play an important part in the homeostasis of the body. The kidneys vary the amount of water and dissolved substances which are re-absorbed back into the blood.

Osmoregulation is under hormonal control - by antidiuretic hormone. This brings about changes in the permeability of the walls of the tubules of the kidney nephrons so that more water is reabsorbed from the urine and thus retained by the body.

If a lot of liquid is drunk less water is reabsorbed by the kidney tubules. The result is a large amount of dilute urine.

If too little is drunk the kidney tubules reabsorb a maximum amount of water.