• Humans have a four chambered heart and a well branched system of blood vessels. Vessels leading away from the heart are called arteries (contain mainly oxygenerated blood) , while those leading to the heart are called veins (contain mainly deoxygenerated blood). These branch to form very fine vessels called capillaries in organs and tissues.
• Human blood contains liquid plasma, blood cells and platelets. The plasma transports most materials. The red blood cell contains haemoglobin which is concerned with the transport of oxygen. White blood cells defend the body, whole platelets initiate clot formation at cuts and wounds on the skin.
• In the capillaries, some of the plasma leaks out carrying the substances that the body cells need. This fluid constitutes the intercellular fluid. Materials diffuse into the cells from the intercellular fluid; cell wastes and secretions diffuse into this fluid from the cells. Intercellular fluid re enter blood vessels or enter lymph vessels. The latter empty their contents into major blood vessels.
• In higher plants, vascular tissues carry out transport. Xylem tissue conducts water from roots to leaves. Root pressure, capillarity and transpiration pull help in the ascent of water in xylem vessels in stems. Phloem tissue conducts food from the leaves to all parts of the plant.
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