(a) Cardiac function and cardiac output.           

• The volume of blood pumped through each ventricle per minute is the cardiac output.
• Cardiac output is determined by heart rate and stroke volume.
• (CO = HR x SV)
• The left and right ventricles pump the same volume of blood through the aorta and pulmonary artery.

(b) The cardiac cycle

• During diastole blood returning to the atria flows into the ventricles.
• Atrial systole transfers the remainder of the blood through the atrio-ventricular (AV) valves to the ventricles.
• Ventricular systole closes the AV valves and pumps the blood out through the semi lunar (SL) valves to the aorta and pulmonary artery.
• In diastole the higher pressure in the arteries closes the SL valves.
• The opening and closing of the AV and SL valves are responsible for the heart sounds heard with a stethoscope.

(c) The structure and function of cardiac conducting system

• The heart beat originates in the heart itself.
• The auto-rhythmic cells of the sino-atrial node (SAN) or pacemaker, located in the right atrium, set the rate at which cardiac muscle cells contract.
• The timing of cardiac cells contracting is controlled by the impulse from the SAN spreading through the atria causing atrial systole.
• They then travel to the atrio-ventricular node (AVN), located in the centre of the heart.
• Impulses from the AVN travel down fibres in the central wall of the heart and then up through the walls of the ventricles, causing ventricular systole.
• Impulses in the heart generate currents that can be detected by an electrocardiogram (ECG).
• The medulla regulates the rate of the sino-atrial node through the antagonistic action of the autonomic nervous system (ANS).
• A sympathetic nerve releases noradrenaline which increases the heart rate, whereas a parasympathetic nerve releases acetylcholine which decreases the heart rate.

(d) Blood pressure changes, in response to cardiac cycle, and its measurement

• Blood pressure changes in the aorta during the cardiac cycle.
• These changes can be measured using a sphygmomanometer.
• An inflatable cuff stops blood flow, in the artery, and deflates gradually.
• The blood starts to flow (detected by a pulse) at systolic pressure.
• The blood flows freely through the artery (and a pulse is not detected) at diastolic pressure.
• A typical reading for a young adult is 120/80 mmHg.
• Hypertension (high blood pressure) is a major risk factor for many diseases including coronary heart disease.