The Heart and Circulation

Why animals need a heart and circulation

• In a large animal diffusion is too slow and the diffusion distance is too great: cells are too far away from the gut and the gas exchange surface.
• In a small organism the cells will not be far from the blood, and it has a low metabolic rate.
• A double circulation keeps oxygenated and deoxygenated blood separate.

Blood vessels

• Lining of endothelial cells in arteries, capillaries and veins.
• More capillaries: more capillaries are in contact with the cells, which results in faster diffusion.
• A drawing of an artery in section shows the lumen and three layers: tunica externa, tunica media and tunica intima.

Flow and pressure along the vessels

• Blood in the aorta is under higher pressure as it has just been pumped out of the left ventricle.
• Pressure decreases the further the blood gets from the heart: the aorta branches into many arteries, and the friction between blood and vessel slows the blood.
• Velocity decreases as blood flows through the arterioles, is low as blood flows through the capillaries and increases as blood flows through the venules.
• Describe the velocity of the blood. Never write blood pressure falls through the capillaries to describe the flow.

Structure of the heart

• Oxygenated blood returns to the heart in the pulmonary vein; the left ventricle pumps it to the body. Never write oxygenated blood returns in the vena cava.
• Ventricle walls thicker than atrium walls.
• The right ventricle has thinner walls as blood is only pumped to lungs: lower pressure on the right side to prevent damage to alveoli.
• The mammalian heart has a septum and two ventricles, giving a double circulation; the left hand side and right hand side beat simultaneously.
• The coronary artery branches from the aorta, the closest vessel carrying oxygenated blood, and carries oxygenated blood to the heart muscle for aerobic respiration.
• The atrioventricular valve prevents backflow into atrium; the cords prevent valve from going inside out, and the papillary muscle pulls on cords.

The cardiac cycle

1. Atrial systole: the atria contract; increase in volume of the ventricles as the atria squeeze blood into them.
2. Ventricular systole: the ventricles contract; decrease in volume as blood is pumped out into the aorta and pulmonary artery.
3. Cardiac diastole: atria and ventricles relaxed; the atrioventricular valves are open, the semilunar valves are closed, and the ventricles start to fill.

• The atrium has higher pressure than the ventricle, so the atrioventricular valve opens.
• The ventricle has higher pressure than the atrium, so the atrioventricular valve closes.
• The ventricle has higher pressure than the aorta, so the semilunar valve opens.
• The aorta has higher pressure than the ventricle as the heart relaxes, so the semilunar valve closes.
• Write cardiac diastole for the stage when the whole heart is relaxed. Never name the stage ventricular diastole.
• The duration of diastole and systole decrease with increase in heart rate.