The circulatory system

• The circulatory system has two major functions: transport of absorbed food molecules (glucose, amino acids, CO2, urea, hormones) and defence against disease
• Red blood cells carry oxygen from the lungs to the tissues and cells of the body
• RBCs are Biconcave- large surface area for diffusion of oxygen
• RBCs contain haemoglobin- contains iron which helps carry oxygen
• RBCs have no nucleus- more space for haemoglobin
• White blood cells- Phagocytes engulf and digest microbes (phagocytosis)
• White blood cells- Lymphocytes produce antibodies in response to foreign cells/microbes
• Phagocytes have a lobed nucleus are fewer in number than RBCs but larger
• Platelets assist in clotting at the site of wounds and forming scabs.
• Platelets convert fibrinogen (soluble protein in plasma) into fibrin (insoluble mesh) which traps RBCs, these blood cells make a clot and a scab is formed
• the structure of platelets- cell fragments
• Plasma is fluid which contains water, minerals, proteins (eg. fibrinogen) waste products (urea and CO2) dissolved foods (eg. Glucose) and body hormones
• Function of plasma- Transport eg. Cells, heat and dissolved molecules
• Arteries carry blood under high pressure away from the heart
• Arteries have a thick elastic muscular wall and a small lumen
• Veins carry blood under low pressure towards the heart
• Veins have thin walls and a large lumen ad contain valves to prevent backflow of blood
• Capillaries have a thin wall (one cell thick) which is permeable to allow diffusion of oxygen, carbon dioxide, dissolved food and urea into and out I the blood
• Capillaties carry blood under low pressure and they are found in large quantities in organs
• Double circulation- blood travels though the heart twice in one complete circuit of the body