The heart and cardiac cycleAQA A-Level Biology: Revision notes

Mammals have a double circulation. The right side of the heart pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the body.

• Right atrium receives blood from the body through the vena cava. Right ventricle pumps it to the lungs through the pulmonary artery.
• Left atrium receives blood from the lungs through the pulmonary vein. Left ventricle pumps it to the body through the aorta.
• The renal arteries carry blood from the aorta to the kidneys; the renal veins return it to the vena cava.
• The coronary arteries branch from the aorta and supply the heart muscle with oxygen and glucose. A blockage can cause a myocardial infarction (heart attack).

The heart has four chambers. Atrioventricular (AV) valves lie between atria and ventricles; semilunar valves lie at the start of the aorta and pulmonary artery. The left ventricle wall is thickest, because it pumps blood at higher pressure around the whole body.

In each cycle the pressure and volume of the chambers change, and the valves open and close passively as pressures change, so blood flows in one direction only.

1. Atrial systole: the atria contract, pressure rises and blood is forced into the ventricles through the open AV valves.
2. Ventricular systole: the ventricles contract, ventricular pressure exceeds atrial pressure, so the AV valves close. Both sets of valves are shut briefly while pressure rises. When ventricular pressure exceeds aortic (or pulmonary artery) pressure, the semilunar valves open and blood is ejected.
3. Diastole: the ventricles relax, pressure falls below that in the arteries and the semilunar valves close. The atria fill with blood, atrial pressure exceeds ventricular pressure, and the AV valves open.

The pressure in the left ventricle rises to about 16 kPa, much higher than in the right ventricle, because the left side pumps blood further.

Cardiac output is the volume of blood pumped by one ventricle per minute.

Cardiac output = stroke volume × heart rate

Stroke volume is the volume pumped per beat (cm³) and heart rate is in beats per minute. Cardiac output is in cm³ min⁻¹.

Worked example: a stroke volume of 75 cm³ and a heart rate of 68 beats per minute give a cardiac output of 75 × 68 = 5100 cm³ min⁻¹. If training raises stroke volume, the same output can be achieved with a lower heart rate.

When analysing pressure data, read the times at which pressures cross to find when valves open and close, and use 60 ÷ cycle time to find heart rate.

Cardiovascular disease (CVD) includes coronary heart disease and stroke. Risk factors increase the chance of developing it:

• High blood pressure
• Smoking
• High blood cholesterol (particularly LDL) and diets high in saturated fat or salt
• Lack of exercise and obesity
• Age, sex and genetic factors

Data on risk factors are often given as the incidence (number of new cases per head of population per year). Compare groups using figures, and quote values from the data.

A correlation is a relationship between two variables. A causal relationship means one variable directly produces a change in the other. A correlation does not prove cause, because a third confounding variable may affect both (for example, people with high saturated fat intake may also smoke more).

A causal link is more likely if there is a plausible mechanism, a dose-response pattern, and the effect remains when other variables are controlled.

To evaluate conflicting evidence, compare: sample size (larger is less affected by chance), duration (long enough for disease to develop), control of variables, whether the difference is statistically significant, and who funded the research.