University:
Felician University
Course:
BIO-102 | Human Biology
Academic year:

2019
Views:

347

Pages:

1
Author:

StellarSymphony

THE HEART Heart Structure Mechanism of Heart Beat Superior Vena Cava Aorta Left Atrium 2 Right Atrium Inferior Vena Cava 3 1 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 Pulmonary Artery Pulmonary Vein Left 4 Ventricle 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 Right Ventricle Valves: 1. Tricuspid valve (right) 2. Bicuspid valve (left) 3. Pulmonary valve (right) 4. Aortic valve (left) Cardiac Cycle 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 Atrial 120 Ventricular Systole Diastole Aortic valve opens Aortic valve closes 90 Aorta Ventricle 60 30 0 0 AV valve opens AV valve closes Atrium 100 200 300 400 Pressure (mm Hg) Coronary Heart Disease Risk Factors 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) 500 600 Time (ms)