Overview
This lecture introduces the basic structure and function of the heart as part of the circulatory (cardiovascular) system, focusing on heart anatomy, blood flow, heart wall layers, valves, cardiac muscle, and the cardiac conduction system.
Circulatory System Overview
- The circulatory (cardiovascular) system includes the heart, arteries, veins, and capillaries that transport blood throughout the body.
- The heart's main function is to act as a pump for blood circulation.
Structure and Location of the Heart
- The heart sits in the mediastinum, a central cavity in the thorax.
- The pericardium, a double-walled sac, protects the heart and maintains its position.
Heart Circuits and Blood Flow
- The heart functions in two circuits: the pulmonary and systemic circuits.
- The right side of the heart pumps oxygen-poor blood to the lungs (pulmonary circuit) for oxygenation.
- The left side of the heart pumps oxygenated blood to the rest of the body (systemic circuit).
- Blood returns to the right atrium (systemic circuit), goes to lungs, then returns to the left atrium (pulmonary circuit).
Heart Chambers and Blood Vessels
- The heart has four chambers: right atrium, left atrium (receiving chambers), right ventricle, and left ventricle (pumping chambers).
- The interatrial septum separates the atria; the interventricular septum separates the ventricles.
- Blood enters the right atrium via the superior vena cava, inferior vena cava, and coronary sinus.
- Blood enters the left atrium via four pulmonary veins.
- The right ventricle pumps blood into the pulmonary trunk; the left ventricle pumps into the aorta.
Heart Wall Structure
- The pericardium includes fibrous and serous layers; the serous layer has parietal and visceral (epicardium) components.
- The heart wall consists of three layers: epicardium (outer), myocardium (middle, muscle), endocardium (inner lining).
- The cardiac skeleton provides structure and electrical insulation.
Heart Valves
- Atrioventricular (AV) valves separate atria from ventricles: tricuspid (right side) and mitral/bicuspid (left side).
- Semilunar valves (aortic and pulmonary) prevent backflow from arteries into ventricles.
- Valves ensure unidirectional blood flow and respond to pressure changes.
Cardiac Muscle and Conduction
- Cardiac muscle fibers are striated, short, branched, and interconnected via intercalated discs.
- Intercalated discs contain desmosomes (physical connection) and gap junctions (electrical coupling).
- Cardiac muscle cells exhibit automaticity (self-excitation) and contract in a coordinated manner as a unit.
- The heart’s refractory period is longer than skeletal muscle, preventing continuous contractions.
- Pacemaker cells make up the intrinsic cardiac conduction system, distributing impulses for coordinated contraction.
- Arrhythmias and fibrillation result from problems in the conduction system.
Key Terms & Definitions
- Circulatory System — the body system that circulates blood via the heart and vessels.
- Pericardium — protective double-walled sac enclosing the heart.
- Atria — upper receiving chambers of the heart.
- Ventricles — lower pumping chambers of the heart.
- Cardiac Skeleton — connective tissue network providing structural support and electrical insulation in the heart.
- Automaticity — ability of cardiac muscle cells to self-excite without nerve input.
- Pacemaker Cells — specialized heart cells that initiate and coordinate heartbeats.
- Arrhythmia — irregular heart rhythm.
- Fibrillation — rapid, uncoordinated heart contractions.
Action Items / Next Steps
- Review upcoming lectures on arteries and blood vessels.
- Prepare to study heart rate regulation and coronary vasculature in future sessions.