Chapter 21: The Cardiovascular System – The Heart (Anatomy & Physiology Study Notes)

Introduction to the Heart

Overview

The heart is a muscular organ essential for maintaining blood circulation throughout the body. Its continuous activity ensures the delivery of oxygen and nutrients to tissues and the removal of metabolic wastes.

• Heart Function: Keeps blood in motion; cessation leads to exhaustion of nutrient and oxygen supplies and accumulation of wastes.

• Heart Rate: Beats about 100,000 times per day (approximately 70 beats per minute).

• Blood Volume Pumped: About 1.5 million gallons per year (2.9 gallons per minute; 5–30 liters per minute depending on activity).

An Overview of the Cardiovascular System

Heart Structure and Circuits

The heart is about the size of a clenched fist and consists of four chambers: two atria and two ventricles. It pumps blood into two major circuits:

• Pulmonary Circuit: Carries blood to and from the lungs for gas exchange.

• Systemic Circuit: Delivers oxygenated blood to the rest of the body.

Blood Vessels

• Arteries: Transport blood away from the heart.

• Veins: Transport blood toward the heart.

• Capillaries: Interconnect arteries and veins, facilitating exchange of substances between blood and tissues.

The Pericardium

Structure and Function

The heart is enclosed by the pericardium, which protects and anchors it within the thoracic cavity.

• Fibrous Pericardium: Outer tough layer providing protection and structural support.

• Serous Pericardium: Inner layer with two sublayers:

  • Visceral Layer (Epicardium): Attached directly to the heart surface.

  • Parietal Layer: Adjacent to the fibrous pericardium.

• Pericardial Cavity: Space between serous layers containing pericardial fluid, which lubricates and reduces friction during heart movements.

Structure of the Heart Wall

Layers of the Heart Wall

The heart wall is composed of three distinct layers, each with specialized functions:

• Epicardium: External surface; consists of visceral pericardium.

• Myocardium: Middle layer; composed of cardiac muscle tissue, connective tissue, blood vessels, and nerves. Responsible for contraction.

• Endocardium: Inner endothelial surface lining the heart chambers.

Cardiac Muscle Tissue

• Striated Appearance: Due to organized myofibrils.

• Aerobic Respiration: High mitochondrial and myoglobin content for sustained activity.

• Extensive Circulatory Supply: Ensures continuous oxygen delivery.

• Involuntary Contraction: Cardiac muscle cells contract without CNS input.

• Intercalated Discs: Specialized junctions connecting cardiac muscle cells.

Intercalated Discs

• Desmosomes: Bind adjacent cell membranes, providing mechanical strength.

• Gap Junctions: Allow ions to move directly between cells, enabling rapid electrical communication.

• Functional Syncytium: All muscle cells contract as a single unit.

Cardiac Skeleton

• Each cardiac cell is wrapped in an elastic sheath.

• Muscle layers are wrapped in fibrous sheets, separating superficial and deep muscle layers.

• Fibrous sheets encircle the base of the pulmonary trunk, ascending aorta, and valves.

Functions of the Cardiac Skeleton

• Stabilizes cardiac cell and valve positions.

• Supports blood vessels and nerves in the myocardium.

• Distributes contraction forces and prevents overexpansion.

• Provides elasticity for recoil after contraction.

• Isolates atrial cells from ventricular cells.

Orientation and Superficial Anatomy of the Heart

Position and Borders

• The heart lies slightly left of the midsagittal plane, within the mediastinum.

• Base: Superior border.

• Apex: Inferior portion.

• Right Border: Formed by the right atrium.

• Inferior Border: Formed by the right ventricle.

Surfaces of the Heart

• Anterior Surface: Right atrium, right ventricle, left ventricle.

• Posterior Surface: Left atrium, small portion of right atrium.

• Diaphragmatic Surface: Right and left ventricles.

External Landmarks

• Interatrial Groove: Separates left and right atria.

• Coronary Sulcus: Separates atria and ventricles.

• Anterior/Posterior Interventricular Sulcus: Separates left and right ventricles.

Chamber Characteristics

• Atria: Superior to coronary sulcus, thin walls, expandable auricles.

• Ventricles: Inferior to coronary sulcus, thicker walls (left thicker than right).

Internal Anatomy and Organization of the Heart

Chamber Separation

• Interatrial Septum: Separates left and right atria.

• Interventricular Septum: Separates left and right ventricles.

• Atrioventricular Valves: Folds of endocardium between atria and ventricles.

The Right Atrium

• Receives oxygen-poor blood from the superior vena cava, inferior vena cava, and coronary sinus.

• Contains pectinate muscles (anterior wall and auricle).

• Fossa Ovalis: Remnant of fetal foramen ovale (allowed blood to bypass lungs).

The Right Ventricle

• Receives blood from right atrium via right atrioventricular (AV) valve (tricuspid valve).

• Blood exits through pulmonary valve (pulmonary semilunar valve) to pulmonary trunk and arteries.

• Papillary Muscles & Chordae Tendineae: Prevent valve inversion during contraction.

• Trabeculae Carneae: Muscular ridges on internal surface.

• Moderator Band: Muscular band unique to right ventricle; prevents overexpansion.

The Left Atrium

• Receives oxygenated blood from lungs via pulmonary veins.

• Pectinate muscles restricted to auricle.

• Blood passes through left atrioventricular (AV) valve (bicuspid/mitral valve).

The Left Ventricle

• Thickest wall for strong contractions (systemic circuit).

• No moderator band; prominent trabeculae carneae.

• Blood exits through aortic valve (aortic semilunar valve) to ascending aorta, aortic arch, and systemic circulation.

Structural Differences: Right vs. Left Ventricle

Feature	Right Ventricle	Left Ventricle
Wall Thickness	Thin	Thick
Contraction Strength	Weaker	Stronger (6–7x)
Moderator Band	Present	Absent

Heart Valves

• Four Valves: Two AV valves (tricuspid, bicuspid/mitral), two semilunar valves (aortic, pulmonary).

• AV valves consist of ring of connective tissue, cusps, chordae tendineae, and papillary muscles.

• Papillary muscles contract to prevent valve inversion.

• During ventricular contraction, AV valves close and semilunar valves open, preventing backflow.

Coronary Blood Vessels

Coronary Arteries

• Originate at base of ascending aorta; supply cardiac muscle tissue.

• Right Coronary Artery (RCA): Atrial branches, right marginal branch, posterior interventricular branch.

• Left Coronary Artery (LCA): Anterior interventricular branch, circumflex branch, left marginal branch, posterior left ventricular branch.

Coronary Veins

• Drain cardiac venous blood into right atrium.

• Great Cardiac Vein: Delivers blood to coronary sinus.

• Middle Cardiac Vein: Delivers blood to coronary sinus.

• Coronary Sinus: Drains into posterior right atrium.

• Posterior Vein of Left Ventricle: Parallels posterior left ventricular branch.

• Small Cardiac Vein: Parallels right coronary artery.

• Anterior Cardiac Veins: Branches from right ventricle.

The Coordination of Cardiac Contractions

Cardiac Cycle

• Systole: Contraction phase (atrial and ventricular).

• Diastole: Relaxation phase; chambers fill with blood.

Conducting System

• Nodal Cells: SA node and AV node; set rate of contractions (autorhythmic).

• Conducting Cells: Distribute contractile stimulus to myocardium.

Cardiac Conduction Pathway

1. Impulse originates at Sinoatrial (SA) Node (cardiac pacemaker; 80–100 action potentials/min).

2. Impulse travels to Atrioventricular (AV) Node (located in floor of right atrium).

3. Impulse conducted by internodal pathways; atrial contraction occurs.

4. AV node slows impulse; travels to AV bundle.

5. AV bundle divides into right and left bundle branches (interventricular septum).

6. Bundle branches conduct impulses to Purkinje fibers; ventricular contraction occurs.

Autonomic Control of Heart Rate

• SA node sets intrinsic heart rate; autonomic nervous system (ANS) can modify rate.

• ANS nerves innervate SA node, AV node, cardiac cells, and smooth muscle in cardiac vessels.

• Norepinephrine (NE): Sympathetic stimulation increases heart rate and contraction force.

• Acetylcholine (ACh): Parasympathetic stimulation decreases heart rate and contraction force.

• Cardiac centers in medulla oblongata regulate heart rate via cardioacceleratory (sympathetic) and cardioinhibitory (parasympathetic) centers.

Key Equations

• Cardiac Output (CO): The volume of blood pumped by the heart per minute. $CO = HR \times SV$ Where HR = heart rate (beats/min), SV = stroke volume (mL/beat).

Additional info: These notes are based on lecture slides and textbook images for a college-level Anatomy & Physiology course, focusing on the structure, function, and physiology of the heart within the cardiovascular system.