Chapter 19

The heart is a hollow, four-chambered muscular organ roughly the size of a closed fist, acting as a high-pressure dual-pump system. Its function is crucial for maintaining homeostasis, underscored by the following data:

  • Hemodynamic Metrics:

    • Average Rate: 7575 beats per minute (bpm).

    • Stroke Volume (SVSV): The volume of blood ejected per beat (approx. 70 mL70\text{ mL}).

    • Cardiac Output (COCO): Total blood pumped per minute. Calculated as CO=HR×SVCO = HR \times SV. In a resting adult, this is approx. 5.25 L/min5.25\text{ L/min}.

    • Ejection Fraction (EFEF): The percentage of blood pumped out of the ventricle with each contraction. Calculated as EF=(SVEDV)×100EF = (\frac{SV}{EDV}) \times 100. Normal range is 55%−70%55\%-70\%.

Precise Anatomy and Positioning

  • Mediastinum: The heart is located in the thoracic cavity within the middle mediastinum, medially between the pleural cavities of the lungs.

  • Orientation:

    • The Base: Superior surface, at the level of the 3rd3\text{rd} costal cartilage. It serves as the attachment point for the great vessels (Aorta, Pulmonary Trunk, and Venae Cavae).

    • The Apex: The inferior, pointed tip directed anteriorly, inferiorly, and to the left. It is located in the 5th5\text{th} intercostal space at the midclavicular line.

  • Cardiac Notch: A depression in the medial surface of the left lung that accommodates the heart's leftward deviation.

Heart Wall and Membranous Layers

  • Pericardium: A triple-layered protective sac.

    1. Fibrous Pericardium: Outer layer of dense irregular connective tissue. Prevents overfilling and anchors the heart.

    2. Serous Pericardium: Double-layered (Parietal and Visceral). The Pericardial Cavity between them contains serous fluid to reduce friction.

  • The Layers of the Heart Wall:

    • Epicardium: The visceral layer of the serous pericardium; often contains adipose tissue.

    • Myocardium: The thickest layer, composed of cardiomyocytes. Arranged in spiral/circular bundles to allow for more efficient wringing-like contraction.

    • Endocardium: Innermost layer of simple squamous epithelium (endothelium). It is continuous with the lining of blood vessels and covers the heart valves.

Microscopic Anatomy: Cardiac Muscle Features

  • Cardiomyocytes: Short, branched, and striated. Contain high volumes of mitochondria (approx. 25%−35%25\%-35\% of volume) to remain resistant to fatigue.

  • Intercalated Discs: Specialized junctions between cells containing:

    • Desmosomes: Physical anchors that prevent cells from pulling apart during contraction.

    • Gap Junctions: Electrical synapses that allow ions to flow between cells, creating a functional syncytium (the cells contract as a single unit).

Internal Anatomy and Surface Features

  • Atria (Receiving Chambers):

    • Separated by the interatrial septum containing the fossa ovalis.

    • Pectinate Muscles: Parallel ridges of muscle on the inner surface of the right atrium and the auricles.

  • Ventricles (Pumping Chambers):

    • Separated by the interventricular septum.

    • Trabeculae Carneae: Irregular muscular ridges on the internal walls.

    • Moderator Band: A muscular ridge in the right ventricle that carries part of the conduction system to the papillary muscles.

  • Valvular Mechanics:

    • Atrioventricular (AV) Valves: Tricuspid (Right) and Mitral/Bicuspid (Left). Tethered by chordae tendineae to papillary muscles to prevent prolapse during ventricular systole.

    • Semilunar (SL) Valves: Pulmonary and Aortic. These open when ventricular pressure exceeds arterial pressure and close when ventricles relax.

Coronary Circulation

  • Right Coronary Artery (RCA): Supplies the right atrium and both ventricles. Branches into the Marginal Artery and Posterior Interventricular Artery (PDA).

  • Left Coronary Artery (LCA): Branches into the Anterior Interventricular Artery (LAD) (the "widow maker") and the Circumflex Artery.

  • Venous Drainage: The Coronary Sinus collects deoxygenated blood from the cardiac veins and empties directly into the right atrium.

The Conduction System and ECG

  1. SA Node: The "natural pacemaker" (70−100 bpm70-100\text{ bpm}).

  2. AV Node: Delays the impulse for 0.1 sec0.1\text{ sec} to allow atrial emptying.

  3. AV Bundle (Bundle of His): Only electrical connection between atria and ventricles.

  4. Bundle Branches: Conduct through the interventricular septum.

  5. Purkinje Fibers: Excite the cardiomyocytes from the apex upward.

  • ECG Waves:

    • P Wave: Atrial depolarization.

    • QRS Complex: Ventricular depolarization (atrial repolarization is masked).

    • T Wave: Ventricular repolarization.

Cardiac Physiology and Regulation

  • Frank-Starling Law: States that within physiological limits, the heart pumps all the blood that comes to it without allowing excessive damming of blood in the veins. Increased Preload (stretch) leads to increased contraction force.

  • Factors Affecting Stroke Volume:

    • Preload: Degree of stretch (related to End Diastolic Volume, EDVEDV).

    • Contractility: Contractile strength at a given muscle length; increased by sympathetic stimulation (Ca2+Ca^{2+} influx).

    • Afterload: The back pressure exerted by arterial blood (Systemic Vascular Resistance).

  • Autonomic Regulation:

    • Sympathetic: Cardioacceleratory center in medulla triggers norepinephrine release → ++ Inotropic and ++ Chronotropic effects.

    • Parasympathetic: Cardioinhibitory center via Vagus Nerve (CN X) triggers acetylcholine release → Decreased HR primarily via the SA and AV nodes.