MAVO Year 4 · Transport and immune defence · Section 13.1

Blood

Composition of blood

Blood consists of plasma, typically about 55% of its volume, and formed elements, mainly red cells, making up roughly 45%. These proportions vary. White blood cells and platelets occupy only a small part of the formed-element volume.

Blood plasma

Plasma is mostly water, approximately 90–92%, with proteins and other dissolved substances. Plasma proteins include albumin, antibodies and fibrinogen. Nutrients, hormones, waste products and mineral ions are also transported in it. Plasma contributes to transport, heat distribution, fluid balance, defence and clotting.

Red blood cells and haemoglobin

Mature human red blood cells are biconcave discs without nuclei. They live about 120 days, are produced in red bone marrow and are removed mainly in the spleen and liver. Their haemoglobin, an iron-containing protein, binds oxygen and gives them their red colour. Iron from old cells can be recycled.

Anaemia means insufficient haemoglobin for normal oxygen transport. Iron deficiency is one cause, but blood loss, vitamin deficiencies and other conditions can also cause it.

White blood cells

White blood cells have nuclei and perform several immune functions. Some engulf microbes or remove debris through phagocytosis. Others, including lymphocytes and their descendants, recognise specific targets or make antibodies. Many can leave small blood vessels and enter tissues. Pus can contain dead white cells, microbes and damaged tissue material.

Leukaemia

Leukaemia is cancer affecting blood-forming tissues. Abnormal blood cells can multiply and interfere with normal cell production. Depending on the type, this can increase infection risk, reduce oxygen transport through anaemia and cause bleeding because too few normal platelets are available.

Platelets and clotting

Platelets are small cell fragments without nuclei, produced from large cells in bone marrow. At a damaged vessel, they adhere and form a plug. A clotting cascade converts soluble fibrinogen into fibrin fibres. These trap cells and reinforce the plug, reducing blood loss. A clot forming inappropriately inside a vessel can cause thrombosis and obstruct blood flow.