Red blood cells

Red blood cells transport oxygen around the body. They contain haemoglobin, a protein that binds oxygen. As the blood travels around the body, haemoglobin releases oxygen where it is needed by cells.

Recognising red blood cells

A mature human red blood cell is a small biconcave disc: it is thinner in the centre than at the edges, with a shallow dip on each side. In a photomicrograph, this often makes the centre look paler than the edge. In a diagram, it may be shown as a disc with a dip on both sides.

Mature human red blood cells have no nucleus. This helps distinguish them from white blood cells, which have a nucleus. When identifying cells in a photomicrograph or diagram, look for the combination of a small biconcave shape and the absence of a nucleus.

White blood cells

White blood cells help defend the body against pathogens, such as disease-causing bacteria and viruses. Different white blood cells defend the body in different ways.

How white blood cells attack pathogens

Some white blood cells surround and engulf a pathogen, taking it into the cell. They then digest and destroy it. This process is called phagocytosis.

Other white blood cells produce antibodies. These are proteins that bind to specific targets on pathogens, helping the body recognise and eliminate them.

Identifying white blood cells

White blood cells have a nucleus. In photomicrographs and diagrams of human blood, this distinguishes them from mature red blood cells, which do not have a nucleus.

Platelets and blood clotting

Platelets are small cell fragments carried in the blood. When a blood vessel is damaged, platelets gather at the damaged area and help a blood clot to form.

The clot seals the damaged area, reducing further blood loss. It also forms a barrier that reduces the entry of pathogens, such as bacteria, into the body.

Blood plasma

Blood contains red blood cells, white blood cells and platelets carried in a liquid called plasma. As blood flows around the body, the plasma transports these cells and platelets with it.

Plasma also carries substances dissolved in the liquid. These include ions, nutrients, urea, hormones and carbon dioxide. Plasma therefore transports both the suspended blood cells and platelets and the substances dissolved in it.

Fibrin mesh formation in blood clotting

When a blood vessel is damaged, a clot forms across the damaged area and seals it. The clot is held together by a mesh of fibrin.

Fibrin forms from fibrinogen, a soluble protein in the blood. During clotting, fibrinogen is converted into insoluble fibrin. Strands of fibrin form a mesh across the damaged area, providing the structure of the clot that seals it.