Anatomy & Physiology: The Heart – Structure, Function, and Circulation

The Cardiovascular System

Overview

The cardiovascular system is responsible for transporting blood, nutrients, gases, and wastes throughout the body. It consists of three main components: the heart, blood vessels, and blood.

• Heart: A muscular organ that pumps blood through the circulatory system.

• Blood vessels: Tubular structures (arteries, veins, capillaries) that carry blood to and from all body parts.

• Blood: The fluid that transports oxygen, nutrients, and waste products.

Heart Size and Location

Physical Characteristics

• The heart is roughly the size of a closed fist and is conical (triangular) in shape.

• Average weight: about 10 ounces (males), 8 ounces (females).

• The apex (pointed end) of the heart points downward and to the left, resting on the diaphragm.

• Located in the mediastinum within the thoracic cavity, protected by the rib cage.

Heart Coverings and Wall Structure

Pericardium

• The pericardium is a double-walled sac that surrounds and protects the heart.

• Fibrous pericardium: The tough, outer layer that anchors the heart and prevents overfilling.

• Serous pericardium: A thinner, inner membrane with two layers:

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

  • Visceral layer (epicardium): Covers the external heart surface.

• The pericardial cavity between these layers contains serous fluid, reducing friction during heartbeats.

Heart Wall Layers

• Epicardium: Outer layer (same as visceral pericardium).

• Myocardium: Middle, thickest layer composed of cardiac muscle tissue; responsible for contraction.

• Endocardium: Inner layer of simple squamous epithelium (endothelium), continuous with blood vessels entering and leaving the heart.

Cardiac Muscle Tissue

Characteristics

• Striated, involuntary muscle with branching fibers and usually a single nucleus per cell.

• Contains intercalated discs with desmosomes (cell adhesion) and gap junctions (ion passage for electrical coupling).

• Functions as a functional syncytium—cells contract together as a unit.

Types of Cardiac Cells

• Cardiac pacemaker cells (autorhythmic cells): Self-excitable, generate and conduct electrical impulses, do not contract.

• Contractile cardiac muscle cells: Responsible for the heart's pumping action.

Note: Cardiac muscle does not require neuronal stimulation to contract.

Chambers of the Heart

Internal Structure

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

• Atria: Receive blood returning to the heart; have auricles (ear-like flaps) to increase volume.

• Ventricles: Pump blood out of the heart; have thick muscular walls.

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

• Interventricular septum: Separates the ventricles; contains trabeculae carneae (muscular ridges).

External Landmarks

• Coronary sulcus (atrioventricular groove): Encircles the heart, separating atria from ventricles.

• Anterior and posterior interventricular sulci: Separate the right and left ventricles on the heart's surface.

Heart Valves

Types and Functions

• Atrioventricular (AV) valves: Between atria and ventricles.

  • Right AV valve (tricuspid): Three cusps.

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

  • Valves are anchored by chordae tendineae to papillary muscles.

• Semilunar (SL) valves: Between ventricles and major arteries.

  • Pulmonary valve: Right ventricle to pulmonary trunk.

  • Aortic valve: Left ventricle to aorta.

Valve Function

• AV valves open when atrial pressure exceeds ventricular pressure; close during ventricular contraction to prevent backflow.

• SL valves open when ventricular pressure exceeds arterial pressure; close to prevent backflow into ventricles.

Mitral valve prolapse (MVP): When part of the mitral valve bulges into the atrium during contraction, sometimes causing a murmur.

Rheumatic fever: Immune-mediated damage to heart valves, often affecting mitral and aortic valves.

Major Blood Vessels of the Heart

Great Vessels

• Superior vena cava (SVC): Returns deoxygenated blood from upper body to right atrium.

• Inferior vena cava (IVC): Returns deoxygenated blood from lower body to right atrium.

• Pulmonary trunk: Carries deoxygenated blood from right ventricle to lungs (splits into left and right pulmonary arteries).

• Pulmonary veins: Carry oxygenated blood from lungs to left atrium.

• Aorta: Largest artery, carries oxygenated blood from left ventricle to systemic circulation.

Pathway of Blood Through the Heart

Circulatory Circuits

• Pulmonary circuit: Right side of heart pumps deoxygenated blood to lungs (short, low-pressure circuit).

• Systemic circuit: Left side of heart pumps oxygenated blood to body tissues (long, high-pressure circuit).

Stepwise Blood Flow

1. Deoxygenated blood enters right atrium via SVC, IVC, and coronary sinus.

2. Passes through tricuspid valve to right ventricle.

3. Pumped through pulmonary valve into pulmonary trunk and arteries to lungs.

4. Oxygenated blood returns via pulmonary veins to left atrium.

5. Passes through mitral valve to left ventricle.

6. Pumped through aortic valve into aorta and systemic circulation.

Fetal Circulation Remnants

• Foramen ovale: Fetal opening between atria; becomes fossa ovalis after birth.

• Ductus arteriosus: Fetal vessel connecting pulmonary trunk to aorta; becomes ligamentum arteriosum after birth.

Coronary Circulation

Arterial Supply

• Right coronary artery: Supplies right atrium and most of right ventricle.

• Left coronary artery: Branches into:

  • Circumflex artery: Supplies left atrium and posterior left ventricle.

  • Anterior interventricular artery: Supplies anterior left ventricle, apex, and interventricular septum.

• Anastomosis: Connections between arteries providing alternate routes for blood flow.

Venous Drainage

• Cardiac veins (great, middle, small) collect deoxygenated blood from myocardium.

• Most veins drain into the coronary sinus, which empties into the right atrium.

Clinical Correlations

Ischemic Heart Disease

• Myocardial ischemia: Reduced blood flow to heart muscle, causing hypoxia and chest pain (angina pectoris).

• Myocardial infarction (MI): Heart attack; death of cardiac muscle due to prolonged ischemia, resulting in scar tissue.

• Coronary artery disease (CAD): Plaque buildup (atherosclerosis) narrows arteries, leading to ischemia and MI.

• Arteriosclerosis: General hardening and thickening of arteries.

• Atherosclerosis: Specific form of arteriosclerosis due to plaque formation.

Diagnostic and Therapeutic Procedures

• Percutaneous coronary angiography: Imaging technique using dye to detect blockages in coronary arteries.

• Percutaneous transluminal coronary angioplasty (PTCA): Balloon catheter used to open narrowed arteries; often followed by stent placement.

• Coronary artery bypass graft (CABG): Surgical procedure using a vessel graft to bypass blocked coronary arteries.

Summary Table: Heart Valves

Valve	Location	Structure	Function
Tricuspid (Right AV)	Between right atrium and right ventricle	3 cusps	Prevents backflow into right atrium
Bicuspid/Mitral (Left AV)	Between left atrium and left ventricle	2 cusps	Prevents backflow into left atrium
Pulmonary (SL)	Between right ventricle and pulmonary trunk	3 semilunar cusps	Prevents backflow into right ventricle
Aortic (SL)	Between left ventricle and aorta	3 semilunar cusps	Prevents backflow into left ventricle

Key Terms and Definitions

• Myocardium: Muscular middle layer of the heart wall.

• Endocardium: Inner lining of the heart chambers.

• Epicardium: Outer layer of the heart wall; also called visceral pericardium.

• Intercalated discs: Specialized connections between cardiac muscle cells.

• Syncytium: A group of cells functioning as a unit.

• Anastomosis: Connection between blood vessels providing alternate routes.

Relevant Equations

• Cardiac Output (CO): The volume of blood pumped by each ventricle per minute.

$CO = HR \times SV$

• Where HR = heart rate (beats per minute), SV = stroke volume (mL per beat).

Additional info: Some details, such as the cardiac output formula and expanded definitions, were inferred and added for completeness and clarity.