Chapter 15:Cardiac System: Part 1Structure of the Heart

Course Structure and Administrative Context

• The course curriculum covers Human Anatomy and Physiology II (2402 / '82) and does not proceed strictly sequentially through textbook chapters. Instead, content is organized around functional and physiological connections between different organ systems.

• Chapter 15 is selected as the entry point for studying the cardiovascular system.

• The lecture utilizes the 13th edition of the textbook due to its high-quality print illustrations of physiological systems, though students utilize the 15th edition on the Connect online platform. Structural and factual content between the 13th and 15th editions remains substantially identical.

• The cardiovascular lecture series is divided into three distinct modules:

  • Video 1: Structural anatomy of the human heart.

  • Video 2: Physiology and functional mechanics of the heart.

  • Video 3: Blood vessels and vascular distribution.

Overview of the Cardiovascular System

• The cardiovascular system consists of the heart ("cardio") and the complete network of blood vessels ("vascular") distributing blood from the heart to all body tissues and returning it back to the heart.

• Structural and Functional Distribution:

  • Blood vessels permeate the entire body to supply every living tissue.

  • Distance Threshold: No cell in the human body can survive if it is located greater than $100\,\mu\text{m}$ ($100\,\text{microns}$) away from a blood capillary. Cells situated beyond this threshold, such as epidermal skin cells moving away from the vascularized dermis, undergo cell death.

  • Arteries: Conventionally depicted in red, these vessels carry blood away from the heart.

  • Veins: Conventionally depicted in blue, these vessels return blood back toward the heart.

• Quantitative Hemodynamic Statistics:

  • Daily Pumping Volume: The heart pumps approximately $7{,}000\,\text{liters}$ ($1{,}842\,\text{gallons}$) of blood through the body each day.

  • Volumetric Conversion: $1\,\text{gallon}$ is equivalent to $3.8\,\text{liters}$.

  • Total Blood Volume: An average adult human possesses between $4.5\,\text{liters}$ and $5.0\,\text{liters}$ of total blood.

  • Lifetime Cardiac Contractions: The heart contracts approximately $2{,}500{,}000{,}000$ ($2.5 \times 10^9$) times in an average human lifetime.

  • Onset of Cardiac Activity: Contractions begin at $3\,\text{weeks}$ post-fertilization and continue uninterrupted until death.

Anatomical Location, Orientation, and Dimensions of the Heart

• Position and Boundaries:

  • Located within the thoracic cavity in the central tissue region known as the mediastinum.

  • Medial to the left and right lungs.

  • Posterior to the sternum and anterior to the vertebral column (situated directly between the sternum and the spine).

  • Positioned superiorly to the diaphragm; the heart rests directly above the diaphragm but is not physically attached to it.

  • Deviated asymmetrical orientation: The body of the heart lies predominantly on the left side of the thoracic midline.

• Anatomical Landmarks:

  • Base: The broad superior portion of the heart, situated directly beneath the $2\text{nd}$ rib.

  • Apex: The pointed inferior tip of the heart, directed toward the left, situated at the level of the $5\text{th}$ intercostal space.

• Dimensions and Morphology:

  • The heart is a hollow, four-chambered, highly muscular, cone-shaped pump.

  • Physical dimensions average approximately $14\,\text{cm}$ in length and $9\,\text{cm}$ in width.

  • Heart size varies proportionately with an individual's total body size and is roughly equivalent to the size of that individual's closed fist.

• Radiological Visualization:

  • Plain Radiography (X-ray): Displays skeletal markers (ribs, clavicle, sternum) surrounding the distinct silhouette of the heart and the prominent arch of the aorta.

  • Computed Tomography (CT Scan): A transverse view looking inferiorly reveals the posterior thoracic vertebrae, the anterior sternum, and the heart impression positioned toward the left side of the chest cavity.

Pericardial Membranes and Layers of the Heart Wall

• Pericardial Coverings (Serous and Fibrous Membranes):

  • Fibrous Pericardium: The outermost layer composed of tough, dense connective tissue that anchors the heart within the mediastinum.

  • Parietal Pericardium: The outer serous membrane layer lined directly against the fibrous pericardium.

  • Pericardial Cavity: The fluid-filled space situated between the parietal pericardium and visceral pericardium, containing serous fluid that reduces friction during continuous cardiac contractions.

  • Visceral Pericardium (Epicardium): The innermost serous layer adherent directly to the surface of the heart; it cannot be mechanically separated from the underlying cardiac muscle tissue.

• Layers of the Heart Wall:

  • Epicardium (Visceral Pericardium): Forms the protective outer layer of the heart wall. Composed of serous connective tissue that secretes serous fluid. Associated with prominent deposits of epicardial adipose tissue (fat) that cushion, protect, and thermoregulate the coronary blood vessels.

  • Myocardium: The middle and thickest layer of the heart wall, composed entirely of cardiac muscle tissue. Functions as the primary contractile engine and pumping force of the heart.

  • Endocardium: The innermost layer lining the heart chambers and covering the heart valves. Composed of a smooth single layer of simple squamous epithelium supported by connective tissue, preventing resistance to intra-cardiac blood flow.

Internal Chambers, Septa, and Surface Features

• The Four Cardiac Chambers:

  • Right Atrium: Superior receiving chamber that collects deoxygenated blood from three systemic sources:

  • Superior Vena Cava (SVC): Drains systemic blood returning from the head, brain, shoulders, and upper extremities.

  • Inferior Vena Cava (IVC): Drains systemic blood returning from the lower body extremities and abdomen.

  • Coronary Sinus: Drains deoxygenated venous blood returning from the myocardium itself.

  • Right Ventricle: Inferior pumping chamber with relatively thin muscular walls. Functions as a low-pressure pump, sending blood across the short distance of the pulmonary circuit to the lungs.

  • Left Atrium: Superior receiving chamber that receives oxygenated blood returning from the lungs via four pulmonary veins ($2$ originating from the left lung and $2$ from the right lung).

  • Left Ventricle: Inferior pumping chamber with extremely thick muscular walls. Functions as a high-pressure pump to generate force needed to push blood throughout the entire systemic circulation.

• Internal Septa:

  • Interatrial (Interauricular) Septum: Muscular wall separating the right atrium from the left atrium.

  • Interventricular Septum: Thick muscular wall separating the right ventricle from the left ventricle.

• Internal and External Anatomical Features:

  • Auricles: Ear-shaped flap-like muscular appendages projecting from the exterior surface of each atrium, increasing atrial capacity.

  • Pectinate Muscles: Distinct fan-shaped parallel muscular ridges located along the internal walls of the auricles and atria.

  • Trabeculae Carneae: Irregular, ridged muscular columns and folds lining the internal surfaces of both the right and left ventricles.

  • Papillary Muscles: Specific cone-shaped muscular projections rising from the ventricular myocardium.

  • Chordae Tendineae: Tough fibrous cords ("heartstrings") that anchor the cusps of the atrioventricular valves directly to the papillary muscles, preventing valve prolapse into the atria during ventricular contraction.

  • External Cardiac Sulci: Grooves on the external cardiac surface containing major coronary blood vessels and protective adipose tissue:

  • Anterior Interventricular Sulcus: Grooved boundary between the right and left ventricles on the anterior surface.

  • Posterior Interventricular Sulcus: Grooved boundary between the ventricles on the posterior surface.

  • Coronary Sulcus (Atrioventricular Sulcus): Encircles the heart between the upper atria and lower ventricles; houses the coronary sinus on its posterior side.

Structural Mechanics of Heart Valves and Cardiac Skeleton

• Atrioventricular (AV) Valves:

  • Positioned between the atria and ventricles to enforce one-way blood flow and prevent backflow into the receiving chambers during ventricular systole.

  • Tricuspid Valve: Positioned between the right atrium and right ventricle; composed of three leaf-like cusps.

  • Bicuspid Valve (Mitral Valve): Positioned between the left atrium and left ventricle; composed of two leaf-like cusps.

• Semilunar (SL) Valves:

  • Positioned at the exit origins of the major outflow arteries to prevent arterial blood from backflowing into the ventricles during ventricular diastole.

  • Pulmonary Valve: Located at the base of the pulmonary trunk as it exits the right ventricle.

  • Aortic Valve: Located at the base of the ascending aorta as it exits the left ventricle.

• Spatial Orientation of Valves (Transverse Plane):

  • The pulmonary valve occupies the most anterior anatomical position.

  • The tricuspid valve lies posterior and to the right of the pulmonary valve.

  • The bicuspid (mitral) valve lies posterior and to the left of the pulmonary valve.

  • The aortic valve is located centrally and posteriorly relative to the pulmonary valve.

• Skeleton of the Heart:

  • Composed of dense connective tissue and rigid fibrous rings encircling all four heart valves.

  • Anchors the valve leaflets firmly in place, prevents structural deformation or over-expansion of the valve orifices, and provides an insertion foundation for cardiac muscle fibers.

Pulmonary and Systemic Circulatory Routes

• Systemic Circulation (High-Pressure Circuit):

  • Path of Blood Flow: Left Ventricle $\rightarrow$ Aortic Valve $\rightarrow$ Ascending Aorta / Aortic Arch $\rightarrow$ Systemic Arteries $\rightarrow$ Capillaries of body organs (brain, upper extremities, thoracic tissue, abdominal organs, lower extremities) $\rightarrow$ Systemic Veins $\rightarrow$ Superior Vena Cava, Inferior Vena Cava, and Coronary Sinus $\rightarrow$ Right Atrium.

  • Delivers oxygen- and nutrient-rich blood throughout the body system and returns deoxygenated, carbon dioxide-rich blood to the right side of the heart.

• Pulmonary Circulation (Low-Pressure Circuit):

  • Path of Blood Flow: Right Atrium $\rightarrow$ Tricuspid Valve $\rightarrow$ Right Ventricle $\rightarrow$ Pulmonary Valve $\rightarrow$ Pulmonary Trunk $\rightarrow$ Left and Right Pulmonary Arteries $\rightarrow$ Pulmonary Capillaries of the Lungs (gas exchange releases $\text{CO}_2$ and absorbs $\text{O}_2$) $\rightarrow$ Four Pulmonary Veins ($2$ left, $2$ right) $\rightarrow$ Left Atrium.

  • A short, low-resistance circuit dedicated entirely to oxygenating deoxygenated venous blood returned from the body.

Coronary Circulation: Arterial Supply and Venous Drainage

• Origin of Coronary Arteries:

  • The coronary circulation provides oxygenated blood and essential nutrients directly to the myocardium.

  • Right and left coronary arteries branch off the base of the ascending aorta immediately superior to the cusps of the aortic valve.

• Arterial Supply Network:

  • Right Coronary Artery (RCA):

  • Exits the right side of the ascending aorta and runs along the coronary sulcus.

  • Right Marginal Artery: Branches off the RCA to supply the lateral right ventricular wall.

  • Posterior Interventricular Artery (Right Posterior Ventricular Artery): Continuation of the RCA onto the posterior aspect within the posterior interventricular sulcus.

  • Left Coronary Artery (LCA):

  • Exits the left side of the ascending aorta and rapidly bifurcates into two main branches:

    1. Left Anterior Interventricular Artery (also termed Left Anterior Descending or LAD): Runs down the anterior interventricular sulcus; supplies the anterior walls of both ventricles and the interventricular septum. Gives rise to the Left Marginal Artery.

    • Clinical Relevance: This vessel is the most common site of clinical arterial occlusion causing acute heart attacks (myocardial infarction), frequently requiring interventional cardiac stent placement or coronary artery bypass grafting (CABG).

    1. Left Circumflex Artery: Curves around the left margin within the coronary sulcus to the posterior side of the heart; gives rise to the Left Posterior Ventricular Artery and Left Posterior Interventricular Artery.

• Venous Drainage Network:

  • Deoxygenated blood from cardiac muscle tissue drains through cardiac veins running parallel to the major coronary arteries:

  • Great Cardiac Vein: Runs alongside the left anterior interventricular artery along the anterior surface.

  • Small Cardiac Vein: Runs along the inferior/right margin alongside the right marginal branch.

  • Middle Cardiac Vein: Runs alongside the posterior interventricular artery in the posterior interventricular sulcus.

  • Left Marginal Vein and Right Marginal Vein: Drain lateral surfaces of the ventricles.

  • Coronary Sinus: A large venous channel situated on the posterior surface within the coronary (atrioventricular) sulcus; receives deoxygenated blood from all cardiac veins and empties directly into the Right Atrium.

• Perivascular Adipose Tissue:

  • Extensive adipose tissue surrounds the coronary arteries and veins in the sulci.

  • Serves essential structural and physiological functions: mechanical cushioning, shock absorption, protection against compression during heart contractions, and local thermoregulation.