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Anatomy and Physiology

NCLEX Review Guide: Cardiovascular Anatomy and Physiology

Heart Location and Structural Layers

Position and Covering

  • The heart lies in the mediastinum, resting on the diaphragm with about two-thirds of its mass to the left of the midline. Its apex points down and to the left at approximately the fifth intercostal space, the site where the point of maximal impulse (PMI) is normally palpated.
  • The heart is enclosed by the pericardium, a double-walled sac. The outer fibrous pericardium anchors the heart, while the inner serous pericardium (parietal and visceral layers) secretes a small amount of pericardial fluid that reduces friction during contraction.

Key Points

  • Normal pericardial fluid is 15-50 mL; excess fluid can cause cardiac tamponade.
  • The three layers of the heart wall, outer to inner, are the epicardium, myocardium, and endocardium.

Wall Layers

  • The myocardium is the thick middle muscular layer responsible for the pumping action; the left ventricle has the thickest myocardium because it ejects blood against high systemic pressure. The endocardium lines the chambers and is continuous with the endothelium of the great vessels.

Memory Aid: "Every Man Eats" from outside in - Epicardium → Myocardium → Endocardium.

Chambers and Valves

Four Chambers

  • The two upper atria are low-pressure receiving chambers; the two lower ventricles are the pumping chambers. The right side handles deoxygenated blood bound for the lungs, and the left side pumps oxygenated blood to the body.

Four Valves

  • The atrioventricular (AV) valves - tricuspid (right) and mitral/bicuspid (left) - prevent backflow into the atria during ventricular contraction. The semilunar valves - pulmonic and aortic - prevent backflow into the ventricles during relaxation.

Key Points

  • "S1" (lub) = closure of AV valves; "S2" (dub) = closure of semilunar valves.
  • AV valves are anchored by chordae tendineae and papillary muscles that prevent valve prolapse.

Memory Aid: "Try Pulling My Aorta" for the valve sequence blood passes - Tricuspid → Pulmonic → Mitral → Aortic.

Blood Flow Through the Heart

Sequential Pathway

  • Deoxygenated blood returns via the superior and inferior vena cava into the right atrium, passes the tricuspid valve into the right ventricle, and is ejected through the pulmonic valve into the pulmonary artery to the lungs.
  • Oxygenated blood returns through the pulmonary veins into the left atrium, crosses the mitral valve into the left ventricle, and is pumped through the aortic valve into the aorta for systemic delivery.

Key Points

  • The pulmonary artery is the only artery that carries deoxygenated blood; the pulmonary veins are the only veins that carry oxygenated blood.

Coronary Circulation

Arterial Supply

  • The coronary arteries branch from the base of the aorta. The left coronary artery divides into the left anterior descending (LAD) and circumflex branches, supplying the anterior wall, septum, and lateral left ventricle. The right coronary artery (RCA) supplies the right ventricle, inferior wall, and in most people the SA and AV nodes.
  • The myocardium is perfused mainly during diastole; therefore, sustained tachycardia shortens filling time and can reduce coronary perfusion.

Key Points

  • LAD is often called the "widow-maker" because occlusion causes a large anterior MI.
  • RCA supplies the SA node in ~60% of people, so inferior MIs frequently cause bradycardia and heart block.

Cardiac Conduction Overview

Intrinsic Pacemakers

  • The SA node is the primary pacemaker (60-100 bpm). The impulse travels to the AV node (40-60 bpm backup), then through the Bundle of His, bundle branches, and Purkinje fibers to trigger ventricular contraction. Detailed dysrhythmia interpretation is covered in the Cardiac Dysrhythmias chapter.

Memory Aid: Pacemaker rates step down - SA 60-100 → AV 40-60 → Ventricles 20-40 bpm.

Cardiac Cycle and Output

Systole and Diastole

  • During systole the ventricles contract and eject blood; during diastole the ventricles relax and fill. Most coronary filling and ventricular filling occur in diastole.

Cardiac Output

  • Cardiac output (CO) = heart rate × stroke volume, normally about 4-8 L/min. Stroke volume is determined by preload (venous return/stretch), afterload (resistance the ventricle pumps against), and contractility.
  • The Frank-Starling law states that, within limits, greater ventricular stretch (preload) produces a stronger contraction and larger stroke volume.

Key Points

  • Ejection fraction (normal 55-70%) reflects the percentage of end-diastolic volume ejected each beat.
  • Increased afterload (e.g., hypertension) raises cardiac workload and can reduce stroke volume over time.

Memory Aid: Stroke volume rests on three legs - Preload, Afterload, Contractility ("PAC").

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