Blood is important in multicellular organisms because it flows around the body, transporting substances to and from cells.

The composition of blood

Blood transports materials and distributes heat around the body. It also helps to protect against disease. Blood contains a liquid called plasma. Suspended in the plasma are cells and cell fragments, such as:

  • red blood cells
  • white blood cells
  • cell fragments called platelets
Diagram showing the composition of blood

This table explains the functions of various blood components.

PlasmaTransporting carbon dioxide, digested food molecules, urea and hormones; distributing heat
Red blood cellsTransporting oxygen
White blood cellsIngesting pathogens and producing antibodies
PlateletsInvolved in blood clotting

Transporting substances in plasma

Plasma is made primarily of water. Many of the molecules the body needs to transport, such as urea, carbon dioxide and glucose, are soluble in water. This means that a large number of substances can be transported around the body in plasma at any one time. This ability to transport important substances around makes plasma well adapted to meet its function.

Adaptations of red blood cells for oxygen transport

Red blood cells transport the oxygen required for aerobic respiration in body cells.

They must be able to absorb oxygen in the lungs, pass through narrow blood capillaries, and release this oxygen to respiring cells.

Red blood cells have adaptations that enable them to carry a maximum amount of oxygen:

 \text{haemoglobin} + \text{oxygen} \xrightarrow {at~the~lungs} \text{oxyhaemoglobin}

 \text{haemoglobin} + \text{oxygen} \xleftarrow {at~the~cells} \text{oxyhaemoglobin}

Haemoglobin can combine reversibly with oxygen. This is important - it means that it can combine with oxygen as blood passes through the lungs, and release the oxygen when it reaches the cells.

  • They have no nucleus - they lose it during their development - so they can pack in more haemoglobin.
  • They are small and flexible so that they can fit through narrow blood capillaries.
  • They have a biconcave shape - they are the shape of a disc that is curved inwards on both sides - to maximise their surface area for oxygen absorption.
  • They are thin, so there is only a short distance for the oxygen to diffuse to reach the centre of the cell.
Red blood cells
Red blood cells

Looking at blood cells

You may be given some prepared slides of blood to examine with the microscope.

Many types of blood cell are 10 μm in diameter or less. You will need to use a high magnification to examine them.

Diagram showing red and white blood cells. There are more red than white

The slides will have been stained to show the cells, and cell features. The micrograph shows many red blood cells and three white blood cells.