The Circulatory System

The Circulatory System

• The heart pumps blood through blood vessels (arteries, arterioles, veins & capillaries) to reach different parts of the body.
• Blood transports respiratory gases, products of digestion, metabolic wastes and hormones round the body.
• There are two circuits. One takes blood from the heart to the lungs, then back to the heart and the other takes blood around the rest of the body.
• The heart has its own blood supply - left and right coronary arteries.
• 

Blood Vessels

• Arteries
• Carry blood from the heart to the rest of the body
• Thick muscular walls
• Elastic tissue and folded endothelium allows for stretch and recoil as the heart beats - maintains high pressure
• All carry oxygenated blood except for the pulmonary arteries - take deoxygenated blood to the lungs
• Arterioles
• Form a network throughout the body
• Blood flow controlled by muscles inside which contract to restrict the blood flow or relax to allow full blood flow
• Veins
• Take blood back to the heart - under low pressure
• Wider lumen with little elastic or muscle tissue
• Valves to stop blood flowing backwards
• All carry deoxygenated blood except for the pulmonary veins - carry oxygenated blood to the heart from the lungs
• Capillaries
• Adapted for efficient diffusion
• Short diffusion pathway - near cells in exchange tissues
• Walls = one cell thick
• Large number of capillaries - increases surface area

Tissue Fluid

The Heart

• 
• Right side pumps deoxygenated blood to the lungs
• Left side pumps oxygenated blood to the whole body
• The left ventricle has thicker walls than the right ventricle as it needs to contract more powerfully to pump blood around the whole body. The right ventricle only needs to get blood to the lungs.
• Ventricles have thicker walls than the atria as they have to push blood out of the heart whereas atria only have to pump blood to the ventricles.
• AV valves stop blood flowing back into atria when ventricles contract.
• SL valves stop blood flowing back into heart after ventricles contract.
• Cords attach AV valves to ventricles to stop them being forced up into atria when ventricles contract.

The Cardiac Cycle

• 
• Ventricles contract, atria relax
• The ventricles are relaxed.
• Atria contract decreasing the volume of the chambers and increasing the pressure in the chambers.
• Blood is pushed into the ventricles.
• Ventricular pressure and chamber volume increase as the blood is received by the ventricles from the contracting atria.

The Cardiac Cycle 2

• 
• Ventricles contract, atria relax
• Atria relax.
• Ventricles contract - decreasing their volume and increasing pressure.
• Pressure becomes higher in ventricles than atria which forces AV valves shut to prevent back flow.
• Pressure in the ventricles is also higher than in the aorta and pulmonary artery which forces open SL valves and blood moves into the arteries.

The Cardiac Cycle 3

• 
• Ventricles relax, atria relax
• Ventricles and atria both relax.
• Higher pressure in pulmonary artery and aorta closes SL valves.
• Blood returns to the heart and atria fill again due to high pressure in the vena cava and pulmonary vein.
• Pressure of atria increases.
• Ventricles continue to relax - pressure is lower than atria so AV valves open.
• Blood flows from atria to ventricles.
• Atria contract and the whole process begins again.