Overview
This lecture covers normal cardiac function, including heart anatomy, the cardiac cycle, electrical conduction, muscle physiology, and regulation of heart activity.
Heart Anatomy & Structure
- The heart is in the mediastinal cavity with a superior base and an inferior apex.
- Four chambers: right/left atria (upper), right/left ventricles (lower).
- Valves: Tricuspid (right AV), Mitral/Bicuspid (left AV), Pulmonic (right ventricle to pulmonary trunk), Aortic (left ventricle to aorta).
- Pericardium surrounds the heart: serous (lubrication) and fibrous (protection from overfilling).
- The left ventricle has thicker walls for pumping blood into the systemic circuit.
- Blood enters the right atrium via superior/inferior vena cava and leaves left ventricle via the aorta.
Blood Flow & Circulation
- Pulmonary circuit: right heart pumps deoxygenated blood to lungs, left heart receives oxygenated blood.
- Systemic circuit: left heart pumps oxygenated blood to body, returns as deoxygenated blood to right heart.
Coronary Circulation
- Coronary arteries (left/right) branch from the aorta to supply heart muscle.
- Coronary veins drain into the coronary sinus, returning blood to the right atrium.
The Cardiac Cycle & Heart Sounds
- Systole = contraction, Diastole = relaxation of heart muscle.
- AV valves close at the start of ventricular systole (S1, "lub" sound).
- Semilunar valves close at the start of ventricular diastole (S2, "dub" sound).
- Stroke volume = End Diastolic Volume (EDV) – End Systolic Volume (ESV).
- Ejection fraction (normal 60-80%) = percentage of blood ejected from ventricle per beat.
Cardiac Muscle Physiology
- Cardiac myocytes (muscle cells) contract; autorhythmic cells conduct electrical impulses.
- Muscle cells are striated, branched, and connected by intercalated discs (gap junctions and desmosomes).
- Myofibrils contain thick (myosin) and thin (actin) filaments for contraction.
- Calcium from sarcoplasmic reticulum initiates contraction; ATP is required both for contraction and relaxation.
Cardiac Electrophysiology
- Resting membrane potential is negative inside the cell due to ion concentration differences.
- Cardiac action potential has three phases: rising (Na+ influx), plateau (Ca2+ influx/K+ efflux), falling (K+ efflux).
- Longer action potential ensures sustained contraction for effective pumping.
Cardiac Conduction System & EKG
- SA node is the pacemaker; electrical signals travel SA → AV node → bundle of His → bundle branches → Purkinje fibers.
- EKG waves: P (atrial depolarization), QRS (ventricular depolarization & hidden atrial repolarization), T (ventricular repolarization), U wave (sometimes seen in hypokalemia or slow heart rate).
Regulation of Cardiac Function
- Parasympathetic (vagus nerve) slows heart rate; sympathetic increases rate and contraction strength (via beta-1 receptors).
- Cardiac output = stroke volume × heart rate (normal: 5-6 L/min).
- Bainbridge reflex: increased atrial stretch increases heart rate.
- Baroreceptors regulate heart rate via blood pressure sensing.
- Frank-Starling law: increased preload (filling) increases contraction strength and stroke volume.
- Heart releases atrial and B-type natriuretic peptides in response to stretch, lowering blood pressure.
Key Terms & Definitions
- Pericardium — Double-layered sac around the heart (serous for lubrication, fibrous for protection).
- Systole — Period of heart muscle contraction.
- Diastole — Period of heart muscle relaxation.
- Stroke Volume — Blood ejected by ventricle per contraction.
- Ejection Fraction — Percentage of ventricular blood ejected per beat.
- Autorhythmic Cells — Heart cells generating/conducting electrical impulses.
- Intercalated Discs — Connections between cardiac muscle cells allowing synchronized contraction.
- SA Node — Sinoatrial node, heart's primary pacemaker.
- EKG/ECG — Recording of heart’s electrical activity.
Action Items / Next Steps
- Review diagrams of heart anatomy, conduction pathways, and EKG waveforms.
- Practice calculating cardiac output and ejection fraction with provided formulas.
- Read next chapter or assigned material on cardiac pathophysiology.