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..the kidneys are the principal excretory organs? This means, the kidneys are the main organs responsible for wastes leaving the body, not necessarily the alimentary canal. The undigested food material which passes through the alimentary canal is not considered to be an excretory product because it has not been produced by the cells of the body.

All vertebrates have two kidneys. They look a lot like large, reddish-brown beans, hence the name given to kidney beans. Their main function is to clean the blood and filter out waste matter that the body does not need. Also, the kidneys help prevent excess water from leaving the body.

Cortex: This is the outer layer of the kidney. Extensions of the cortex protrude into the medulla of the kidneys, and are called renal columns.

Medulla: This is located inside the cortex, and within the medulla are between eight and eighteen triangular structures called the renal pyramids.

Renal pelvis: This is the upper end of the ureter. It is funnel shaped, and collects the waste (urine) produced by the nephrons of the kidney.

Renal arteries: Carries blood to the kidneys at a rate of about 1.2 litres per minute (one-fifth of the cardiac output). Each kidney contains about one million nephrons, which are minute tubules, vital to the cleansing of the blood.

Calyces: Urine formed in the kidney passes as tiny droplets into the minor calyces, and then into a major calyx. The urine then travels to the renal pelvis and into the ureter, which carries it to the urinary bladder.

Ureter: A tube which collects the urine formed by the kidneys and empties it into the bladder, where it remains until emptied via the urethra.

Nephrons: These act as a filtration system, extracting the fluid form the blood that enters them, and altering its composition to form urine. In this way, waste products, such as urea, are excreted, while other substances necessary in the blood are conserved and returned to the blood stream.

Apart from excreting waste products, the kidney plays an important role in regulating the amount of water and salt in the body.

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