Mass transport and the mammalian heartEdexcel A-Level Biology A: Revision notes

Small organisms such as a flatworm exchange gases and nutrients with their surroundings by diffusion alone. Diffusion is fast over short distances and when the surface area to volume ratio is large.

As an organism gets larger, its volume increases faster than its surface area, so the surface area to volume ratio falls and the diffusion distance to inner cells increases. Diffusion alone then becomes too slow to meet the requirements of the cells, especially when the metabolic rate is high.

Large, active animals therefore need a mass transport system: a heart that acts as a pump and a circulation of blood vessels. Mass flow moves oxygen, glucose and other substances rapidly over long distances and removes carbon dioxide and wastes.

The heart is a muscular double pump with four chambers. The right side receives deoxygenated blood from the body through the vena cava and pumps it to the lungs via the pulmonary artery. The left side receives oxygenated blood from the lungs through the pulmonary vein and pumps it to the body via the aorta. The septum keeps the two sides apart.

Valves ensure one-way flow: atrioventricular valves (tricuspid on the right, bicuspid on the left) between atria and ventricles, and semilunar valves at the base of the aorta and pulmonary artery. Tendons (valve tendons) stop the atrioventricular valves inverting. The walls of the ventricles are thicker than those of the atria, and the left ventricle is thickest because it pumps blood at high pressure around the body. Coronary arteries supply the cardiac muscle itself.

One heartbeat is the cardiac cycle, with three phases.

1. Atrial systole: the atria contract and push the remaining blood through the open atrioventricular valves into the ventricles.
2. Ventricular systole: the ventricles contract, ventricular pressure exceeds atrial pressure so the atrioventricular valves close, then exceeds arterial pressure so the semilunar valves open and blood is forced into the aorta and pulmonary artery.
3. Cardiac diastole: the atria and ventricles relax, the semilunar valves close as arterial pressure exceeds ventricular pressure, and blood flows from the veins into the atria and on into the ventricles.

Valves open and close passively, in response to pressure differences.

Always describe valve action by comparing pressures. The atrioventricular valve is open when atrial pressure is greater than ventricular pressure and closed when ventricular pressure is greater. The semilunar valve is open when ventricular pressure exceeds the pressure in the artery and closed when arterial pressure is greater.

The thick left ventricular wall develops a higher pressure (about 16 kPa in the example data) than the right, which suits the longer systemic circuit. The thin wall of the right ventricle gives lower pressure, which protects the lung capillaries.

For a heart rate calculation, use $\text{heart rate} = \dfrac{60}{\text{cycle time in s}}$. A cycle of 0.8 s gives 75 beats per minute.

A heart dissection (usually of a sheep or pig heart from an abattoir) lets you relate structure to function.

• Use gloves, a lab coat and a sharp scalpel on a dissecting board, cutting away from your hands; disinfect surfaces afterwards.
• Identify the chambers, the vessels (the aorta has a thick elastic wall) and the coronary arteries.
• Cut across the ventricles and measure wall thickness: the left wall is thicker than the right.
• Open the chambers to see the atrioventricular and semilunar valves and the tendons that anchor them.

Limitations: the tissue is dead so you cannot see valve action or pressure changes, and one heart may not be typical.