The components of blood
Blood is a tissue made of cells suspended in a straw-coloured liquid called plasma. Plasma transports dissolved substances such as glucose, amino acids, carbon dioxide, urea, hormones and mineral ions. Red blood cells contain the red pigment haemoglobin, which combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. Red cells are biconcave discs with no nucleus, giving a large surface area and more room for haemoglobin. White blood cells defend the body: phagocytes engulf pathogens and lymphocytes make antibodies. Platelets are cell fragments that help blood to clot.
Key idea
Red cell adaptations: no nucleus (more space for haemoglobin), biconcave shape (large surface area for gas exchange) and being small and flexible to squeeze through capillaries.
The three types of blood vessel
Arteries carry blood away from the heart at high pressure; they have thick, muscular, elastic walls and a narrow lumen. Veins return blood to the heart at low pressure; they have thinner walls, a wide lumen and valves to prevent backflow. Capillaries are one cell thick, allowing substances to diffuse quickly between blood and cells.
Worked example
An artery has a thick elastic wall and no valves; a vein has a wide lumen and valves. If a vessel has a wall one cell thick and permeable to glucose, it must be a capillary, because only capillaries allow exchange with tissue cells.
Remember
- Arteries = Away from the heart.
- Veins carry blood towards the heart and have valves.
- Capillary walls are one cell thick for fast diffusion.