The heart and mammalian circulationEdexcel A-Level Biology B: Revision notes

Arteries carry blood away from the heart at high pressure. The wall is thick, with elastic fibres that stretch and recoil to smooth the flow and maintain pressure, smooth muscle that adjusts the lumen, and a narrow lumen.

Veins return blood to the heart at low pressure. The wall is thin with little muscle or elastic tissue, the lumen is wide, and pocket valves prevent backflow. Skeletal muscle compresses the veins and pushes blood onwards.

Capillaries are the exchange vessels. The wall is a single layer of endothelial cells, so the diffusion distance is short. The lumen is only just wide enough for a red blood cell, which slows the flow, and capillary beds give a large total surface area.

The mammalian heart has four chambers. The right atrium receives deoxygenated blood from the vena cava; the right ventricle pumps it to the lungs through the pulmonary artery. The left atrium receives oxygenated blood from the pulmonary veins; the left ventricle pumps it through the aorta to the body.

The septum keeps the two sides separate. Atrioventricular valves (tricuspid on the right, bicuspid on the left) prevent backflow into the atria, and semilunar valves at the base of the aorta and pulmonary artery prevent backflow into the ventricles. The left ventricle has a much thicker muscular wall than the right because it pumps blood at a higher pressure around the whole body. The heart muscle is supplied by the coronary arteries.

A bony fish has a single circulation: blood passes through the heart once per circuit, goes through the gill capillaries (losing pressure) and then on to the body at low pressure.

A mammal has a double circulation: blood passes through the heart twice per circuit. The pulmonary circuit takes blood to the lungs, and the systemic circuit takes it to the body.

Advantages: blood returns to the heart after the lungs and can be pumped to the body at higher pressure, so tissues receive blood faster; and oxygenated and deoxygenated blood are kept separate, so the blood sent to the body is fully oxygenated, supporting a high metabolic rate.

The cardiac cycle has three stages, driven by pressure changes:

1. Atrial systole: the atria contract, atrial pressure rises above ventricular pressure, and blood is forced through the open atrioventricular valves into the ventricles.
2. Ventricular systole: the ventricles contract. Ventricular pressure exceeds atrial pressure, so the atrioventricular valves close. When it exceeds the pressure in the aorta or pulmonary artery, the semilunar valves open and blood is ejected.
3. Diastole: the heart relaxes. Ventricular pressure falls below arterial pressure, so the semilunar valves close; the atria refill from the veins, and when ventricular pressure falls below atrial pressure, the atrioventricular valves open and the ventricles fill.

Cardiac output = heart rate × stroke volume. A heart rate of 75 beats per minute gives a cycle of 60 ÷ 75 = 0.8 s.

Cardiac muscle is myogenic: it initiates its own contraction without nervous stimulation. The sinoatrial node (SAN) in the wall of the right atrium acts as the pacemaker and starts a wave of excitation that spreads over both atria, causing atrial systole.

The atria and ventricles are separated by non-conducting tissue, so the wave reaches the ventricles only through the atrioventricular node (AVN). The AVN delays the wave, allowing the atria to finish contracting and the ventricles to fill. The wave then passes down the bundle of His in the septum to the Purkyne fibres in the ventricle walls, so the ventricles contract from the apex upwards.

The nervous system and hormones, such as adrenaline, change the rate of the SAN but do not start the beat.

An ECG records the electrical activity of the heart. The P wave is atrial depolarisation (atrial systole), the QRS complex is ventricular depolarisation (ventricular systole) and the T wave is ventricular repolarisation.

Heart rate = 60 ÷ time between two successive R waves (in seconds). If the R-R interval is 0.8 s, the rate is 75 beats per minute. A rate above 100 beats per minute is tachycardia; below 60 is bradycardia. P waves with no QRS indicate that excitation is not reaching the ventricles (heart block).

In pressure graphs, the ventricle pressure curve crosses the atrial curve when the atrioventricular valve closes and crosses the aortic curve when the semilunar valve opens. Read each crossing point to identify what is happening.