Heart, blood vessels and the cardiac cycle

What you’ll learn

• Explain why large active animals require mass transport.
• Link artery, vein and capillary structures to their functions.
• Use pressure changes to explain the cardiac cycle and calculate cardiac output.

Understand the idea

As a body becomes larger, diffusion distances increase and surface area relative to volume decreases. Diffusion alone cannot supply all cells rapidly enough. A circulation system moves blood in bulk, while diffusion remains important over the short final distance between capillaries and cells. In the mammalian double circulation, the right side pumps to the lungs and the left side pumps to the body. Separate circuits allow a lower pressure at the lungs and a higher pressure for the systemic circulation.

Blood returns from the body in the venae cavae, passes through the right atrium and right ventricle, and leaves through the pulmonary artery. Pulmonary veins return blood to the left atrium; the left ventricle sends it into the aorta. The left ventricular wall is thicker because it must generate a greater pressure. Arteries have thick muscular and elastic walls; recoil helps sustain flow between beats. Veins have a relatively large lumen and often valves that support low-pressure return. Capillary walls are a single thin endothelial layer, providing a short exchange distance.

During diastole the chambers relax and blood enters the heart. Atrial systole completes ventricular filling. Ventricular contraction raises ventricular pressure, closing the atrioventricular valves; when it exceeds arterial pressure, semilunar valves open. As ventricles relax, the arterial pressure closes semilunar valves. Valves respond to pressure differences and prevent backflow; they do not actively pump blood. Heart rate multiplied by stroke volume gives the volume pumped per minute.

Key formulae

$$\text{cardiac output}=\text{heart rate}\times\text{stroke volume}$$

$$\text{stroke volume}=\text{end-diastolic volume}-\text{end-systolic volume}$$

Worked example

A heart beats 75 times per minute. Its left ventricle contains 125 cm³ before contraction and 55 cm³ after contraction. Calculate cardiac output in dm³ per minute.

1. Stroke volume is 125 − 55 = 70 cm³ per beat.
2. Output is 75 × 70 = 5250 cm³ per minute.
3. Divide by 1000 to convert cm³ to dm³.

Answer: 5.25 dm³ per minute.