Chapter 18: The Heart – Structure, Function, and Cardiac Physiology

Cardiac Physiology Overview

Foundational Concepts

Cardiac physiology is essential for understanding how the heart functions as a pump and how its muscle tissue generates and coordinates contractions. A strong grasp of skeletal muscle physiology, ion flow in action potentials, and the autonomic nervous system (ANS) is necessary for mastering cardiac physiology.

• Skeletal muscle vs. cardiac muscle: Both use the sliding filament mechanism for contraction, but cardiac muscle has unique features such as intercalated discs and functional syncytium.

• Action potentials (APs): Cardiac muscle APs differ from those in skeletal muscle and neurons, especially in ion flow and refractory periods.

• Relevant chapters for review: Membrane potentials (Ch. 3), muscular tissues (Ch. 4), skeletal muscle histology and contraction (Ch. 9), action potentials (Ch. 11), and ANS regulation (Ch. 14).

Gross Anatomy of the Heart

Location in the Mediastinum

The heart is located in the mediastinum, the central compartment of the thoracic cavity. It is about the size of a closed fist and sits 2/3 to the left of the midsternal line.

• Base: Superior aspect, attached to pulmonary arteries/veins, aorta, and vena cavae.

• Apex: Inferior pointed portion, directed toward the left hip.

• Orientation: The heart is twisted, so different chambers are more visible from anterior or posterior views.

Pericardial Layers

Structure and Function

The heart is enclosed in a double-walled sac called the pericardium, which protects and anchors the heart while reducing friction during movement.

• Fibrous Pericardium: Tough, collagenous outer layer that limits heart size and attaches to great vessels.

• Serous Pericardium: Thin, secretory membrane with two layers:

  • Parietal Pericardium: Lines the fibrous pericardium.

  • Visceral Pericardium (Epicardium): Covers the heart surface.

• Pericardial Cavity: Fluid-filled space between parietal and visceral layers, reducing friction.

Layers of the Heart Wall

Epicardium, Myocardium, Endocardium

The heart wall consists of three layers, each with distinct functions:

• Epicardium: Also called visceral pericardium; outermost layer.

• Myocardium: Middle layer of cardiac muscle responsible for contraction and generation of action potentials. Contains a fibrous skeleton for structural support and electrical insulation.

• Endocardium: Innermost layer; simple squamous epithelium lining the heart chambers and valves, continuous with blood vessel endothelium.

Heart Chambers

Structure and Blood Flow

The heart is a hollow organ with four internal chambers:

• 2 Atria (Right and Left): Superior, small chambers that receive blood from outside the heart. Separated by the interatrial septum, which contains the fossa ovalis (remnant of fetal foramen ovale).

• 2 Ventricles (Right and Left): Inferior, large chambers that receive blood from the atria and pump it out of the heart. Separated by the interventricular septum.

• 4 Valves: Ensure unidirectional blood flow:

  • Atrioventricular (AV) valves: Tricuspid (right) and bicuspid/mitral (left).

  • Semilunar (SL) valves: Pulmonary (right) and aortic (left).

Atria

Function and Features

• Right atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and from the myocardium via the coronary sinus.

• Left atrium: Receives oxygenated blood from the lungs via pulmonary veins.

• Auricles: Thin-walled, muscular ridges (pectinate muscles) in anterior areas.

• Low pressure chambers: Blood flows passively from atria to ventricles through AV valves.

Ventricles

Function and Features

• High pressure chambers: Thick myocardium for forceful contraction.

• Trabeculae carneae: Muscular ridges in ventricular walls.

• Papillary muscles: Protrude from walls and attach to chordae tendineae, which anchor AV valve flaps.

• Right ventricle: Pumps deoxygenated blood to lungs via pulmonary trunk and arteries (pulmonary circulation).

• Left ventricle: Pumps oxygenated blood to the body via the aorta (systemic circulation); has the thickest myocardium.

Atrioventricular (AV) Valves

Mechanism and Function

• Ensure one-way flow: From atrium to ventricle.

• Open during ventricular diastole: Allow ventricles to fill with blood.

• Close during ventricular systole: Prevent backflow into atria; closure produces the first heart sound (“lub”).

• Papillary muscles: Contract during systole to prevent valve prolapse.

• Right AV (tricuspid) valve: 3 cusps.

• Left AV (bicuspid/mitral) valve: 2 cusps.

Semilunar (SL) Valves

Mechanism and Function

• Ensure one-way flow: From ventricle out of heart.

• Open during ventricular systole: Allow blood to exit ventricles.

• Close during ventricular diastole: Prevent backflow; closure produces the second heart sound (“dupp”).

• Pulmonary SL valve: At pulmonary trunk exit.

• Aortic SL valve: At ascending aorta exit.

Blood Flow Through the Heart

Systemic and Pulmonary Circuits

Blood flows through the heart in a circular path, passing through both systemic and pulmonary circuits:

• Systemic circuit: Left ventricle → aorta → body → right atrium.

• Pulmonary circuit: Right ventricle → pulmonary trunk/arteries → lungs → left atrium.

Coronary Circulation

Arteries and Veins

• Coronary arteries: Supply oxygenated blood to heart wall; arise from ascending aorta and branch into left and right coronary arteries.

• Left coronary artery: Anterior interventricular (LAD) and circumflex branches.

• Right coronary artery: Right marginal and posterior interventricular branches.

• Arterial anastomoses: Provide alternate blood flow routes if a branch is blocked.

• Cardiac veins: Collect deoxygenated blood from myocardium; drain into coronary sinus, which empties into right atrium.

Coronary Artery Blockage

Clinical Implications

• Angina: Chest pain due to reduced oxygen supply to myocardium.

• Myocardial infarction (heart attack): Tissue death from prolonged oxygen deprivation; replaced by scar tissue.

• Bypass surgery: Reroutes blood around blocked arteries using vessels such as the great saphenous vein, internal mammary artery, or radial artery.

Anatomy of Cardiac Muscle Cells

Cellular Structure and Connections

• Short, branched, striated cells: Uni- or binucleate; contract via sliding filament mechanism.

• Intercalated discs: Specialized junctions containing gap junctions and desmosomes.

• Gap junctions: Allow direct electrical communication; enable functional syncytium.

• Desmosomes: Provide strong adhesion during contraction.

Functional Syncytium

Coordinated Contraction

• All cardiac muscle cells contract together: Due to gap junctions, atrial cells contract as a unit, followed by ventricular cells.

• Electrical connection: Atrial and ventricular regions are connected only by specialized autorhythmic cells traversing the coronary sulcus.

Comparison Table: Cardiac vs. Skeletal Muscle

Feature	Cardiac Muscle	Skeletal Muscle
Cell shape	Short, branched	Long, cylindrical
Nuclei	Uni- or binucleate	Multinucleate
Intercalated discs	Present	Absent
Gap junctions	Present	Absent
Contraction	Involuntary, rhythmic	Voluntary, variable
Tetanus	No	Yes

Key Terms and Definitions

• Epicardium: Outer layer of heart wall (visceral pericardium).

• Myocardium: Middle, muscular layer responsible for contraction.

• Endocardium: Inner lining of heart chambers and valves.

• Intercalated disc: Specialized junction between cardiac muscle cells.

• Functional syncytium: Coordinated contraction of cardiac muscle cells as a unit.

• Papillary muscle: Muscles in ventricles that anchor chordae tendineae.

• Chordae tendineae: Tendinous cords attaching AV valve flaps to papillary muscles.

Example: Blood Flow Pathway

Deoxygenated blood enters the right atrium via the superior and inferior vena cava, passes through the tricuspid valve into the right ventricle, and is pumped through the pulmonary semilunar valve into the pulmonary trunk and arteries to the lungs. Oxygenated blood returns via pulmonary veins to the left atrium, passes through the bicuspid valve into the left ventricle, and is pumped through the aortic semilunar valve into the aorta for systemic circulation.

Additional info:

• Cardiac muscle cells have abundant mitochondria (25-35% of cell volume) for high energy demand.

• Cardiac muscle has less developed sarcoplasmic reticulum and no triads, relying more on extracellular Ca2+ for contraction.