VMBO Year 2 · HAVO/VWO Years 2–3 · Circulation · Section 3.1

Blood

The composition of blood

Blood is approximately 55% plasma and 45% blood cells and platelets, although proportions vary. It transports nutrients, oxygen, heat, carbon dioxide and other waste substances.

Blood plasma

Plasma is a yellowish fluid containing roughly 91% water, 7% proteins and small amounts of other dissolved substances, including salts.

One plasma protein, fibrinogen, is involved in clotting. Plasma transports nutrients, waste products, antibodies and a small amount of dissolved oxygen.

Red blood cells

Mature human red blood cells have no nucleus and are shaped like discs with a thinner centre. A typical count is about five million per cubic millimetre of blood.

They contain haemoglobin, the protein giving blood its red colour. Haemoglobin binds oxygen in the lungs and releases it in tissues, making red blood cells essential for oxygen transport.

White blood cells

White blood cells have a nucleus and can change shape. Some can leave small blood vessels and move into tissues. A typical count is around 7,000 per cubic millimetre, but it varies.

They defend against pathogens such as bacteria and viruses. Some types engulf and destroy microorganisms. In some infections, many of these defensive cells also die.

Platelets

Platelets are small fragments shed from larger cells and have no nucleus. A typical count is around 300,000 per cubic millimetre.

They help stop bleeding by forming a plug and supporting the reactions that produce a clot at a damaged blood vessel.