Anatomy and Physiology of the Heart: Structure, Function, and Circulation

The Pulmonary and Systemic Circuits

Overview of Heart Circulation

The heart is a transport system that operates with two pumps, each responsible for a distinct circulatory circuit: the pulmonary and systemic circuits.

• Right Side: Receives oxygen-poor blood from body tissues and pumps it to the lungs to remove carbon dioxide and pick up oxygen. This is known as the Pulmonary Circuit.

• Left Side: Receives oxygen-rich blood from the lungs and pumps it to the body tissues. This is called the Systemic Circuit.

Heart Chambers

• Receiving Chambers:

  • Right Atrium: Receives blood returning from the systemic circuit.

  • Left Atrium: Receives blood returning from the pulmonary circuit.

• Pumping Chambers:

  • Right Ventricle: Pumps blood through the pulmonary circuit.

  • Left Ventricle: Pumps blood through the systemic circuit.

Size, Location, and Orientation of the Heart

Physical Characteristics

• Size: Approximately the size of a fist; weighs less than a pound.

• Location:

  • Located in the mediastinum between the second rib and fifth intercostal space.

  • Sits on the superior surface of the diaphragm.

  • About two-thirds of the heart lies to the left of the midline (midsternal line).

  • Oriented anteriorly (in front) to the vertebral column and posteriorly (behind) to the sternum.

  • Apical Pulse: Palpated between the fifth and sixth ribs, just below the left nipple.

Coverings of the Heart

Pericardium

The pericardium is a double-walled sac that surrounds the heart, providing protection and anchoring it to surrounding structures.

• Superficial Fibrous Pericardium: Protects, anchors the heart, and prevents overfilling.

• Deep Two-Layered Serous Pericardium:

  • Parietal Layer: Lines the internal surface of the fibrous pericardium.

  • Visceral Layer (Epicardium): Covers the external surface of the heart.

  • Layers are separated by the pericardial cavity, which contains fluid to decrease friction.

Clinical Homeostatic Imbalances

• Pericarditis: Inflammation of the pericardium, causing roughened membrane surfaces and a pericardial friction rub (creaking sound).

• Cardiac Tamponade: Excess fluid in the pericardial space compresses the heart, impairing its ability to pump. Treatment involves removing the fluid, usually with a syringe.

Layers and Surface Features of the Heart

Heart Wall Layers

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Circular or spiral bundles of contractile cardiac muscle cells; contains the cardiac skeleton (crisscrossing, interlacing connective tissue).

• Endocardium: Innermost layer, continuous with the endothelial lining of blood vessels; lines heart chambers and covers cardiac skeleton of valves.

Internal and Surface Features

• Internal Features:

  • Four chambers: two superior atria and two inferior ventricles.

  • Interatrial Septum: Separates atria; contains the fossa ovalis (remnant of fetal foramen ovale).

  • Interventricular Septum: Separates ventricles.

• Surface Features:

  • Coronary Sulcus (Atrioventricular Groove): Encircles the junction of atria and ventricles.

  • Anterior Interventricular Sulcus: Marks the anterior position of the interventricular septum.

  • Posterior Interventricular Sulcus: Landmark on the posteroinferior surface.

Atria: The Receiving Chambers

• Small, thin-walled chambers; contribute little to propulsion of blood.

• Auricles: Appendages that increase atrial volume.

• Right Atrium: Receives deoxygenated blood from the body via:

  • Superior vena cava

  • Inferior vena cava

  • Coronary sinus

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

• Pectinate Muscles: Found only in auricles.

Ventricles: The Discharging Chambers

• Make up most of the heart's volume; have thicker walls than atria.

• Actual pumps of the heart.

• Right Ventricle: Most of anterior surface; pumps blood into the pulmonary trunk.

• Left Ventricle: Posteroinferior surface; pumps blood into the aorta (largest artery in the body).

• Trabeculae Carneae: Irregular ridges of muscle on ventricular walls.

• Papillary Muscles: Project into ventricular cavity; anchor chordae tendineae attached to heart valves.

Heart Valves

Valve Function and Types

Heart valves ensure unidirectional blood flow through the heart, opening and closing in response to pressure changes.

• Atrioventricular (AV) Valves: Located between atria and ventricles.

  • Tricuspid Valve: Between right atrium and ventricle; three cusps.

  • Mitral Valve (Bicuspid): Between left atrium and ventricle; two cusps.

  • Chordae Tendineae: Anchor AV valve cusps to papillary muscles, preventing valve flaps from inverting.

• Semilunar (SL) Valves: Located between ventricles and major arteries.

  • Each valve has three half-moon-shaped cusps.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

• No valves are found between major veins and atria; not a problem due to inertia of incoming blood and compression of venous openings during heart contraction.

Clinical Homeostatic Imbalances

• Incompetent Valve: Blood backflows, causing the heart to repump the same blood repeatedly.

• Valvular Stenosis: Stiff flaps constrict opening, requiring the heart to exert more force to pump blood.

• Defective valves can be replaced with mechanical, animal, or cadaver valves.

Pathway of Blood Through the Heart

Right Side of the Heart

1. Superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus

2. Right atrium

3. Tricuspid valve

4. Right ventricle

5. Pulmonary semilunar valve

6. Pulmonary trunk

7. Pulmonary arteries

8. Lungs

Left Side of the Heart

1. Pulmonary veins

2. Left atrium

3. Mitral valve

4. Left ventricle

5. Aortic semilunar valve

6. Aorta

7. Body tissues

Pulmonary vs. Systemic Circuits

• Pulmonary Circuit: Short, low-pressure circulation.

• Systemic Circuit: Long, high-friction circulation.

• Ventricular Anatomy: Left ventricle walls are three times thicker than right, pumping with greater pressure.

Coronary Circulation

Blood Supply to the Heart Muscle

• Shortest circulation in the body; delivered when the heart is relaxed.

• Left ventricle receives most of the coronary blood supply.

• Both left and right coronary arteries arise from the base of the aorta and supply arterial blood to the heart, encircling it in the coronary sulcus.

• Arteries contain many anastomoses (junctions), providing additional routes for blood delivery but cannot compensate for coronary artery occlusion.

• Heart receives 1/20th of the body's blood supply.

Major Coronary Arteries and Veins

• Left Coronary Artery: Supplies interventricular septum, anterior ventricular walls, left atrium, and posterior wall of left ventricle. Branches:

  • Anterior interventricular artery

  • Circumflex artery

• Right Coronary Artery: Supplies right atrium and most of right ventricle. Branches:

  • Right marginal artery

  • Posterior interventricular artery

• Cardiac Veins:

  • Great cardiac vein (anterior interventricular sulcus)

  • Middle cardiac vein (posterior interventricular sulcus)

  • Small cardiac vein (inferior margin)

  • Several anterior cardiac veins empty directly into right atrium.

Clinical Homeostatic Imbalances

• Angina Pectoris: Thoracic pain caused by fleeting deficiency in blood delivery to myocardium; cells are weakened.

• Myocardial Infarction (Heart Attack): Prolonged coronary blockage; areas of cell death are repaired with noncontractile scar tissue.

Cardiac Muscle Fibers

Microscopic Anatomy

• Cardiac Muscle Cells:

  • Striated, short, branched, fat, interconnected, one nucleus.

  • Contain numerous large mitochondria (25-35% of cell volume) for resistance to fatigue.

  • Rest of volume composed of sarcomeres (Z discs, A bands, I bands).

  • T tubules are wider but less numerous; enter cell only once at Z disc.

  • Sarcoplasmic reticulum (SR) is simpler than in skeletal muscle; no triads.

• Intercalated Discs: Connecting junctions between cardiac cells that contain desmosomes (hold cells together) and gap junctions (allow ions to pass between cells), enabling the heart to function as a functional syncytium (single coordinated unit).

• Endomysium: Intercellular space between cells containing connective tissue matrix, numerous capillaries, and connections to the cardiac skeleton.

Comparison: Skeletal vs. Cardiac Muscle Physiology

• Muscle contraction is preceded by depolarizing action potential.

• Depolarization wave travels down T tubules, causing SR to release Ca2+.

• Excitation-contraction coupling occurs as Ca2+ binds to troponin, causing filaments to slide.

• Cardiac Muscle Cells: Some are self-excitable (pacemaker cells initiate depolarization of the entire heart).

• Heart contracts as a unit (all cardiomyocytes contract together), ensuring effective pumping.

• Skeletal muscles contract independently.

• Influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR (skeletal muscles do not use extracellular Ca2+).

• Cardiac muscle fibers have a longer absolute refractory period, preventing tetanic contractions and allowing the heart to relax and fill efficiently.

• Cardiac muscle relies almost exclusively on aerobic respiration and cannot function without oxygen.

• Cardiac muscle is more adaptable to other fuels, including lactic acid, but must have oxygen.

Key Differences between Skeletal and Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Cell Shape	Long, cylindrical, multinucleate	Short, branched, one nucleus
Intercalated Discs	Absent	Present
Pacemaker Cells	Absent	Present
Contraction	Independent	As a unit (syncytium)
Source of Ca2+	Sarcoplasmic reticulum only	SR and extracellular fluid
Refractory Period	Short	Long
Energy Requirement	Can use anaerobic respiration	Requires aerobic respiration

Additional info: Some details were expanded for clarity and completeness, including definitions and physiological context.