Blood Composition

Blood is a tissue

Blood may look like a simple red liquid, but it is a tissue made from several specialised components working together. Its liquid component is plasma. Red blood cells, white blood cells and platelets are suspended in this plasma and carried through the blood vessels.

Each component has a different job. Plasma provides transport, red blood cells carry oxygen, white blood cells defend against disease, and platelets help stop bleeding. Together, they connect body systems: substances absorbed from the digestive system and oxygen obtained by the gas-exchange system can be delivered to cells, while waste substances can be carried away.

Plasma: the transport liquid

Plasma is a pale yellow liquid containing water. It transports the blood cells and platelets and carries dissolved substances, including:

• soluble products of digestion, such as glucose and amino acids, from the small intestine;
• carbon dioxide away from respiring cells;
• urea, produced when excess amino acids are broken down in the liver, towards the kidneys;
• hormones from the glands that produce them to their target organs.

The water in plasma is therefore a medium in which many substances can be transported rapidly around the body.

Red blood cells: transporting oxygen

Red blood cells carry oxygen from the lungs to body tissues. They contain haemoglobin, an iron-containing protein that binds reversibly to oxygen. Haemoglobin loads oxygen where its concentration is high, such as in the lungs, and releases it in body tissues. Cells can then use the oxygen in aerobic respiration.

Their structure is closely matched to this function:

• A red blood cell is a biconcave disc—thin in the centre and thicker around the edge. This produces a large surface-area-to-volume ratio and a short diffusion distance, so oxygen can diffuse into and out of it rapidly.
• A mature red blood cell has no nucleus, leaving more internal space for haemoglobin. More haemoglobin allows more oxygen to be carried.
• Red blood cells are small and flexible, helping them pass through narrow capillaries.

White blood cells: defence against disease

White blood cells form part of the immune system. Unlike mature red blood cells, they have a nucleus. They are generally larger and much less numerous than red blood cells.

Different types defend the body in different ways. Phagocytes can change shape to surround, engulf and digest pathogens; this process is called phagocytosis. Lymphocytes produce antibodies. An antibody has a shape complementary to a particular antigen on a pathogen, so it binds specifically and helps bring about the pathogen's destruction.

A white blood cell's nucleus is often the clearest feature in a stained blood smear. A phagocyte may have a lobed nucleus, whereas a lymphocyte often has one large, rounded nucleus.

Platelets and clotting

Platelets are tiny fragments of cells rather than complete cells, and they have no nucleus. When a blood vessel is damaged, platelets help start a sequence that converts soluble fibrinogen in the plasma into insoluble fibrin.

Fibrin forms a mesh across the wound. Blood cells and platelets become trapped in this mesh, producing a clot. The clot reduces blood loss and helps block pathogens from entering through the damaged tissue; it can later dry to form a scab.

Recognising blood components

A blood smear spreads a thin layer of blood across a microscope slide, and stains can make its components easier to distinguish.

Identify components using several clues together:

• Red blood cells: the most numerous cells; small, similarly sized biconcave discs with no visible nucleus. Their thin centres may appear paler.
• White blood cells: larger and much less numerous, with a conspicuous dark-stained nucleus.
• Platelets: extremely small cell fragments, appearing as tiny dots or irregular specks.
• Plasma: the liquid background surrounding and suspending the formed components.

Relative size and abundance are usually more reliable clues than colour alone because the appearance depends on the stain and image.