6) The structure and function of the heart

• 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.
• 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.
• The heartbeat originates in the heart itself.

  The auto-rhythmic cells of the sino-atrial node (SAN) or pacemaker, located in the wall of the right atrium, set the rate at which the heart contracts.
• 1. The timing of cardiac muscle cell contraction is controlled by impulses from the SAN spreading through the atria causing atrial systole.
  2. They then travel to the atrioventricular node (AVN), located in the centre of the heart.
  3. 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.
• Blood pressure increases during ventricular systole and Decreases during Diastole.
• Measurement of blood pressure using a sphygmomanometer

  1. An inflatable cuff stops blood flow, in the artery, and deflates gradually.
  2. The blood starts to flow (detected by a pulse) at systolic pressure.
  3. The blood flows freely through the artery (and a pulse is not detected) at diastolic pressure.
• A typical blood pressure 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.