Overview
This lecture covers the anatomy and physiology of the heart, focusing on its structure, blood supply, muscle layers, valves, and the pathway of blood flow.
Anatomy and Location of the Heart
- The heart is the main pumping organ, located in the mediastinum between the lungs above the diaphragm.
- The upper part of the heart is called the base; the lower part is the apex (point of maximal impulse).
- The apex is best heard at the fifth intercostal space, midclavicular line.
Heart Rate and Electrical Conduction System
- Normal heart rate is 60–100 beats per minute; >100 is tachycardia, <60 is bradycardia.
- The SA (sinoatrial) node initiates normal heartbeat; called the pacemaker of the heart.
- If the SA node fails, the AV (atrioventricular) node takes over, producing 40–60 beats per minute.
- Electrical conduction is managed by the autonomic nervous system: sympathetic increases, parasympathetic decreases heart rate.
Blood Supply to the Heart
- The heart is supplied by the coronary arteries: right and left.
- Right coronary artery supplies the right side; left divides into circumflex (supplies left atrium/ventricle except anterior) and left anterior descending artery (supplies anterior left ventricle).
Muscle Layers of the Heart
- Three layers: endocardium (inner), myocardium (middle), pericardium (outer).
- Infections in the endocardium are mainly caused by Group A beta-hemolytic streptococci.
- Myocardium is susceptible to infarction (myocardial infarction).
- Pericardium has fibrous (attaches heart to walls) and serous layers (produces pericardial fluid).
Pericardial Fluid and Related Disorders
- Normal pericardial fluid volume is 30 ml; excess (>30 ml) causes cardiac tamponade, less causes pericarditis.
- Pericarditis presents with a pericardial friction rub (leather-like sound).
Heart Valves and Sounds
- Four valves: tricuspid, mitral (AV valves), pulmonary, aortic (semilunar valves).
- AV valves prevent backflow to atria; semilunar valves prevent backflow to ventricles.
- Valvular insufficiency causes heart murmurs (swishing sound).
- S1 (“lub”): AV valve closure. S2 (“dub”): semilunar valve closure.
- Extra heart sounds: S3 (ventricular gallop, rapid ventricular filling) and S4 (atrial gallop, resistance to ventricular filling).
Heart Valve Landmarks
- Auscultation points: aortic (2nd ICS, right), pulmonic (2nd ICS, left), Erb’s (3rd ICS, left), tricuspid (4th ICS, left), mitral (5th ICS, midclavicular).
- Use the bell of the stethoscope for heart sounds.
Blood Flow Through the Heart
- Deoxygenated blood enters right atrium via superior/inferior vena cava → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs.
- Oxygenated blood returns via pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.
- All veins carry deoxygenated blood except pulmonary veins; all arteries carry oxygenated blood except pulmonary arteries.
Key Terms & Definitions
- Tachycardia — Heart rate >100 bpm.
- Bradycardia — Heart rate <60 bpm.
- SA node — Heart’s primary pacemaker.
- AV node — Backup pacemaker.
- Mediastinum — Space between the lungs housing the heart.
- Coronary arteries — Blood vessels supplying the heart muscle.
- Endocardium — Inner heart muscle layer.
- Myocardium — Middle, muscular layer.
- Pericardium — Outermost layer with fibrous and serous parts.
- Cardiac tamponade — Heart compression from excess pericardial fluid.
- Pericarditis — Inflammation of the pericardium.
- Valvular insufficiency — Faulty valve closure causing murmurs.
- S1/S2/S3/S4 — Heart sounds (lub-dub, extra gallops).
Action Items / Next Steps
- Practice identifying heart sounds at the five valve auscultation points.
- Memorize the pathway of blood flow through the heart.
- Review definitions and clinical significance of all key terms.