6.22 The Heart

Superior Vena Cava Inferior Vena Cava Pulmonary artery Pulmonary vein Right Atrium Right Ventricle Left Atrium Left Ventricle Valves: 1. Tricuspid valve (right) 2. Bicuspid valve (left) 3. Pulmonary valve (right) 4. Aortic valve (left)

A heart beat is myogenic (contraction initiated by the heart) * Electrical signals are initiated by a sinoatrial (SA) node * This pacemaker stimulates the atria to contract and also relays signals to an atrioventricular (AV) node * The AV node sends signals to ventricular Purkinje fibres (via a Bundle of His within the wall of the septum) * The Purkinje fibres cause the ventricular walls to contract The SA node maintains a normal sinus rhythm (60-100 bpm) * The pacemaker is regulated by the medulla oblongata * Sympathetic nerves release noradrenaline (⇧ heart rate) * Parasympathetic nerves release acetylcholine (⇩ heart rate) * Heart rate may also be increased via hormonal action (via the release of adrenaline / epinephrine) * Adrenaline will cause a more sustained elevation in heart rate than that achieved by the action of the brainstem

The cardiac cycle describes the events of a heart beat Systole (contraction) * As atria contract, atrial pressure exceeds ventricular pressure (AV valves open → blood flows to ventricles) * As ventricles contract, ventricular pressure exceeds atrial pressure (AV valves close → 1st heart sound) * Pressure builds (isovolumetric contraction) until the ventricular pressure exceeds the arterial pressure * Semilunar valves open and blood flows into arteries Diastole (relaxation) * As blood flows into arteries, ventricular pressure drops * Backflow closes semilunar valves → 2nd heart sound * When ventricular pressure drops below atrial pressure, the AV valves will open and cardiac cycle is repeated

Coronary thrombosis is caused by clots within the coronary arteries * Vessels are damaged by cholesterol deposition (atherosclerosis) * The deposits reduce vessel diameter and increase blood pressure * The stress damages arterial walls (and is repaired with fibrous tissue) * The vessel wall loses elasticity and forms atherosclerotic plaques * If a plaque ruptures, blood clotting is triggered, forming a thrombus * If the thrombus blocks blood flow, a myocardial infarction results * These events are collectively described as coronary heart disease

Risk factors for CHD include: * Genetics (e.g. hypertension) * Obesity (overweight = risk) * Diseases (e.g. diabetes) * Diet (e.g. ⇧ trans fats) * Exercise (inactivity = risk) * Smoking (⇧ blood pressure) * Sex (males = higher risk)