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Structure and function

Your two kidneys are complex organs. They regulate the amount of water lost from the body and get rid of waste products, especially a nitrogen-containing compound called urea. About 180 litres of water filters through your kidneys every day, but only about 1.5 litres finally leaves your body as urine.

Where are the kidneys?
	Your kidneys are just under your ribcage above the small of your back. The blood arrives through the renal artery and leaves in the renal vein. The kidneys produce urine which is carried to the bladder along the ureter. The diagram below shows the structure of the kidneys.

Picture 2. A cross section through a kidney.

Picture 3. Kidney structure and funtion.

Question 1

a) What are the main functions of the kidney?

b) How do we control the passage of urine from our bodies?

c) i. About how much water is processed by the kidney daily?

ii. What percentage leaves the body as urine?

Cortex

The outer cortex region is a darker colour. It is covered by a fibrous capsule and contains many renal capsules inside each of which is a glomerulus (knot of capillaries).

Medulla

The inner lighter medulla is composed of tubules, collecting ducts and blood capillaries grouped together to form a series of pyramid structures (medullary pyramids).

Renal artery

The vessels which carry oxygenated blood to the kidneys from the aorta.

Renal vein

The vessels which return blood to the vena cava from the kidneys. This blood is deoxygenated and waste products such as urea have been filtered out in the kidneys.

Vena cava

Blood from the renal veins passes into the inferior vena cava (great vein) which then conveys it back to the right atrium of the heart.

Aorta

The main artery of the body which leaves the left ventricle of the heart and conveys high pressure oxygenated blood to the other arteries such as the renal artery.

Ureter

One of a pair of tubes which carry waste substances filtered out of the blood in the kidneys to the bladder.

Bladder

Sac-like structure in which urine is temporarily stored. The wall of the bladder contains smooth muscle and also cells which are sensitive to stretching as the bladder fills. Contraction of the smooth muscle forces urine out of the bladder.

Sphincters

Rings of muscle which close the exit from the bladder. Stimulation of stretch receptors in the wall of the bladder results in a reflex relaxation of the sphincters, so allowing urine to leave the bladder.

Urethra

The tube through which urine is expelled from the bladder and out of the body.