Structure of the Heart and Cardiovascular System

Cardiovascular System Overview

The cardiovascular system consists of three primary components: the heart, blood vessels, and blood. Its main function is to transport oxygen, nutrients, hormones, and waste products throughout the body. This chapter focuses on the heart's intricate structure, while its functions and physiological processes will be covered in later discussions.

Size and Location of the Heart

• The heart's size can be estimated by observing a person's closed fist, typically measuring about 12 cm long, 9 cm wide, and 6 cm thick. The average heart mass ranges from 250-300 grams in adults.

• The apex of the heart, the pointed inferior tip formed by the left ventricle, rests on the diaphragm and is directed anteriorly, inferiorly, and to the left. Its palpable pulsation, known as the point of maximal impulse (PMI), is typically felt in the fifth intercostal space, just medial to the midclavicular line.

• The heart is located in the mediastinum, the central area of the thoracic cavity between the lungs, extending from the sternum to the vertebral column.

• Anatomically, approximately two-thirds of the heart's mass is positioned to the left of the body's midline, with about one-third to the right, nestled obliquely within the chest.

Anatomical Features of the Heart

1. Apex of the Heart

   • The apex is primarily formed by the inferior lateral portion of the left ventricle, pointing downward and to the left. Clinically, it's significant as the point where the heartbeat is most easily heard or felt (PMI).

2. Base of the Heart

   • The base is the widest part of the heart, located superiorly and posteriorly, largely made up of the left atrium and a portion of the right atrium. It is the site where the great vessels (superior vena cava, inferior vena cava, pulmonary arteries, pulmonary veins, and aorta) enter and exit the heart. The terms "atria" (plural) and "atrium" (singular) are often used.

3. Surfaces of the Heart

   • Anterior (Sternocostal) Surface: Lies deep to the sternum and ribs, primarily formed by the right ventricle and part of the right atrium, providing protection.

   • Inferior (Diaphragmatic) Surface: Located between the apex and the right surface, resting mainly on the central tendon of the diaphragm. It is formed by both the right and left ventricles.

   • Right Pulmonary Surface: Faces the right lung, primarily formed by the right atrium.

   • Left Pulmonary Surface: Faces the left lung, primarily formed by the left ventricle.

4. Pericardium

   • The heart is enveloped in a protective sac known as the pericardium, which has several vital functions:

     • Protects the heart from infection and physical trauma.

     • Securely anchors the heart in place within the mediastinum.

     • Allows the heart to fill and empty flexibly without overstretching or overfilling by constraining its expansion.

   • The pericardium consists of two main layers:

     • Fibrous Pericardium: This tough, inelastic outer layer is made of dense irregular connective tissue. It prevents overstretching of the heart during diastole, provides protection, and anchors the heart to the diaphragm inferiorly and to the sternum anteriorly.

     • Serous Pericardium: A thinner, more delicate serous membrane composed of two layers which form a closed sac:

       • Parietal Layer: This outer sub-layer of the serous pericardium is fused to the inner surface of the fibrous pericardium.

       • Visceral Layer (Epicardium): This inner sub-layer is fused directly to the surface of the myocardium (heart muscle). It is one of the three layers of the heart wall.

   • The space between the parietal and visceral layers of the serous pericardium is the pericardial cavity, which normally contains a thin film of pericardial fluid (approximately 15-50 mL). This serous fluid acts as a lubricant, reducing friction between the layers as the heart beats.

   • Common clinical conditions affecting the pericardium include pericarditis, an inflammation of the pericardium. This condition is characterized by sharp chest pain, often due to reduced pericardial fluid or inflammation causing friction between the roughened pericardial layers, and can lead to increased fluid accumulation (pericardial effusion).

Layers of the Heart Wall

The heart wall is composed of three distinct layers:

1. Epicardium (or Visceral Layer of the Serous Pericardium)

   • This outermost layer, also known as the visceral layer of the serous pericardium, consists of a mesothelium (simple squamous epithelium) underlain by a layer of fibroelastic tissue and variable amounts of adipose tissue.

   • It provides a protective covering for the heart and houses the major coronary blood vessels, lymphatic vessels, and nerves that supply the myocardium.

2. Myocardium

   • This is the thickest and most crucial layer of the heart wall, composed of cardiac muscle tissue. It is directly responsible for the pumping action of the heart through rhythmic contractions.

   • Cardiac muscle fibers are organized in complex spiral and circular bundles that run diagonally around the heart. This intricate arrangement allows for efficient wringing-like contractions, maximizing the ejection of blood from the chambers.

   • The thickness of the myocardium varies significantly between chambers; the left ventricle has the thickest myocardium, reflecting its role in pumping blood throughout the systemic circulation at high pressure.

3. Endocardium

   • The innermost layer, the endocardium, is a thin, smooth lining of endothelium (simple squamous epithelium) overlying a thin layer of connective tissue. This layer is continuous with the inner lining of the blood vessels (tunica intima) and covers the heart valves.

   • Its smooth surface minimizes friction and resistance, allowing for the efficient and unimpeded movement of blood through the heart chambers, thus preventing blood clot formation.

Heart Chambers and Valves

• The heart has four chambers: two superior receiving chambers called atria (right and left) and two inferior pumping chambers called ventricles (right and left). Each atrium features distinct structures:

  • Auricles: Small, wrinkled, ear-like pouches that protrude from each atrium, slightly increasing their blood capacity.

  • The internal division of the atria is by the interatrial septum, a thin wall separating the right and left atria. Within this septum, specifically in the right atrium, is the fossa ovalis, a shallow depression. This fossa is a remnant of the foramen ovale, an opening present in the fetal heart that allowed blood to bypass the pulmonary circulation by flowing directly from the right atrium to the left atrium.

  • Pectinate muscles: These are muscular ridges found in the anterior wall of the right atrium and the auricles of both atria.

• The internal walls of the ventricles are characterized by:

  • Trabeculae carneae: Irregular ridges and folds of cardiac muscle that project from the inner ventricular walls.

  • Papillary muscles: Cone-shaped muscle projections that extend into the ventricular lumens. These muscles are connected to the atrioventricular (AV) valves by thin, fibrous cords called chordae tendineae.

• Valves: The heart's four valves ensure unidirectional blood flow, preventing backflow:

  • Atrioventricular (AV) Valves: Located between the atria and ventricles, they prevent backflow into the atria during ventricular contraction (systole).

    • The tricuspid valve (right AV valve) separates the right atrium from the right ventricle and has three cusps.

    • The bicuspid (mitral or left AV valve) separates the left atrium from the left ventricle and has two cusps. These valves are anchored by chordae tendineae to papillary muscles, which contract during ventricular systole to prevent the valve cusps from prolapsing into the atria.

  • Semilunar (SL) Valves: Located at the base of the great arteries exiting the ventricles, they prevent backflow into the ventricles during ventricular relaxation (diastole).

    • The pulmonary valve separates the right ventricle from the pulmonary trunk.

    • The aortic valve separates the left ventricle from the aorta. Both semilunar valves have three crescent-shaped cusps and open when ventricular pressure exceeds arterial pressure, and close passively when blood tries to flow back.

• The right ventricle pumps deoxygenated blood through the pulmonary valve to the pulmonary trunk, which then divides to supply the lungs. The left ventricle, with its much stronger myocardial wall, ejects oxygenated blood through the aortic valve into the aorta, which distributes blood to the rest of the systemic circulation.

Sulci and Structural Features

• Sulci are external grooves or depressions on the heart surface that mark the boundaries between chambers. These grooves are filled with fat and accommodate the major coronary blood vessels that supply the heart muscle. Key sulci include:

  • Coronary Sulcus (Atrioventricular Sulcus): A deep groove that encircles most of the heart, acting as the boundary between the atria and the ventricles.

  • Anterior Interventricular Sulcus: A depression on the anterior surface, separating the right and left ventricles.

  • Posterior Interventricular Sulcus: A groove on the posterior surface, likewise separating the ventricles.

• Fibrous Skeleton of the Heart: This complexring-like fibrous structure, composed of dense connective tissue, interconnects the heart valves and surrounds the bases of the great arteries. It provides several critical functions:

  • Forms a structural foundation for the heart valves by attaching to the valve cusps.

  • Serves as a point of insertion for cardiac muscle fibers.

  • Acts as an electrical insulator between the atria and ventricles, preventing direct electrical transmission between their muscle masses. This insulation forces electrical impulses to pass only through the specialized conduction system (AV bundle), ensuring coordinated atrial and ventricular contraction.

Coronary Circulation

The myocardium, being a highly active muscle, requires a continuous and ample supply of oxygenated blood and nutrients through its own dedicated circulation, known as the coronary circulation. The rapid movement of blood through the heart's chambers prevents direct nutrient and gas exchange. It is named 'coronary' for its crown-like (corona) shape around the heart. Coronary arteries branch off from the ascending aorta, just distal to the aortic valve, when the heart is relaxing (diastole). Key components include:

• Right Coronary Artery (RCA): Arises from the right side of the aorta. It supplies blood to the right atrium, most of the right ventricle, and parts of the left ventricle and conduction system. Its major branches include:

  • Marginal branches: Supply the lateral wall of the right ventricle.

  • Posterior Interventricular Branch (Posterior Descending Artery, PDA): Extends inferiorly in the posterior interventricular sulcus, supplying the posterior walls of both ventricles and the interventricular septum.

• Left Coronary Artery (LCA): Arises from the left side of the aorta and is typically shorter than the RCA. It quickly branches into two major arteries:

  • Anterior Interventricular Branch (Left Anterior Descending Artery, LAD): Often called the "widowmaker," it courses along the anterior interventricular sulcus, supplying the anterior walls of both ventricles and the interventricular septum.

  • Circumflex Branch: Travels posteriorly in the coronary sulcus, supplying the left atrium and the posterior wall of the left ventricle.

• Coronary Veins: After delivering oxygen, deoxygenated blood is collected by cardiac veins, which generally follow the paths of the coronary arteries.

  • The largest coronary vein is the Coronary Sinus, a large venous channel located in the posterior coronary sulcus, which collects most of the deoxygenated blood from the myocardium and empties directly into the right atrium.

• Myocardial Ischemia and Infarction: Blockage in a coronary artery (e.g., due to atherosclerosis) can lead to myocardial ischemia (reduced blood flow) and, if prolonged, myocardial infarction (heart attack), which is tissue death due to lack of oxygen.

Cardiac Muscle Tissue

1. Differences from Skeletal Muscle:

   • Cell Structure: Cardiac muscle fibers (cells) are shorter, branched, and typically mononucleated (one nucleus per cell), unlike the long, unbranched, multinucleated fibers of skeletal muscle.

   • Intercalated Discs: Connections between adjacent cardiac fibers are highly specialized structures called intercalated discs. These discs contain:

     • Desmosomes: Strong cell junctions that hold cardiac cells together, preventing them from pulling apart during contraction.

     • Gap junctions: Pores that allow direct passage of ions and small molecules between adjacent cells, enabling rapid electrical communication and unified contraction of the muscle (functional syncytium).

   • Energy Production: The cardiac muscle relies almost exclusively on aerobic respiration for ATP production, efficiently utilizing fatty acids, glucose, and to a lesser extent, lactic acid. This is supported by its exceptionally high mitochondrial content (about 25-35\% of cell volume).

   • Calcium's Role: Excitation-contraction coupling in cardiac muscle is critically dependent on extracellular calcium ions. Depolarization involves a slow inward movement of Ca^{2+} through voltage-gated slow Ca^{2+} channels, leading to a prolonged plateau phase of the action potential and triggering further Ca^{2+} release from the sarcoplasmic reticulum (calcium-induced calcium release), which is essential for muscle contraction.

2. Autorhythmic Fibers (Pacemaker Cells): Specialized cardiac muscle fibers, comprising approximately 1\% of myocardial cells, possess the unique ability to generate action potentials spontaneously, setting the rhythm for heart contractions. They have an unstable resting potential that gradually depolarizes until threshold is reached.

   • The primary pacemaker of the heart is the SA (Sinoatrial) Node, located in the superior portion of the right atrium. It generates impulses at the fastest rate (typically 60-100 beats/minute), controlling the overall heart rate. Its spontaneous depolarization is mainly due to a slow influx of sodium ions (Na^{+}) and a subsequent influx of calcium ions (Ca^{2+}).

Cardiac Conduction System

• The intrinsic electrical conduction system ensures that the heart beats in a coordinated and efficient manner. The impulse for contraction begins at the SA node, which spontaneously depolarizes. This electrical impulse quickly spreads through the atrial muscle fibers via gap junctions, leading to atrial contraction (atrial systole).

• The impulse then reaches the AV (Atrioventricular) node, located in the interatrial septum, where it pauses for approximately 0.1 seconds. This delay is crucial as it allows the atria to complete their contraction and empty blood into the ventricles before ventricular contraction begins.

• From the AV node, the impulse travels rapidly down the AV bundle (Bundle of His), which is the only electrical connection between the atria and ventricles through the fibrous skeleton. The AV bundle then divides into the right and left bundle branches within the interventricular septum.

• These bundle branches extend towards the apex of the heart and then give rise to numerous smaller fibers called Purkinje fibers. These fibers rapidly distribute the impulse throughout the ventricular myocardium, leading to almost simultaneous contraction of both ventricles (ventricular systole), effectively pumping blood into the pulmonary trunk and aorta.

Electrocardiogram (ECG)

An Electrocardiogram (ECG or EKG) is a graphic recording of the electrical activity of the heart over time, detecting the summation of action potentials generated by heart muscle cells. It is composed of three primary waves and several intervals/segments:

• P Wave: Represents atrial depolarization, which corresponds to the electrical excitation of the atria causing them to contract (atrial systole).

• QRS Complex: This large, rapid deflection indicates ventricular depolarization, signifying the electrical activation of the ventricles that precedes ventricular contraction (ventricular systole). Atrial repolarization (relaxation) also occurs during this time but is usually masked by the much larger electrical signal of ventricular depolarization.

• T Wave: Represents ventricular repolarization, indicating the electrical recovery and relaxation of the ventricles (ventricular diastole).

• PR Interval: Measures the time from the start of atrial excitation to the start of ventricular excitation.

• ST Segment: Represents the time when the ventricular contractile fibers are fully depolarized, indicating the plateau phase of the ventricular action potential. An elevated or depressed ST segment can be indicative of myocardial ischemia or infarction.

• Each of these phases corresponds to different parts of the cardiac cycle, showing systole (contraction) and diastole (relaxation).

Blood Vessel Structure and Function

Blood vessels form a closed network of tubes that carry blood away from the heart, transport it to the tissues, and then return it to the heart. All blood vessels, except capillaries, share a common basic structure of three layers:

1. Layers of Blood Vessels:

   • The tunica interna (intima): The innermost layer, composed of endothelium (simple squamous epithelium) that provides a smooth surface for blood flow and plays an active role in vessel function.

   • The tunica media: The middle layer, typically the thickest, consisting mainly of smooth muscle cells and elastic fibers. This layer is responsible for regulating the diameter of the blood vessel and thus blood flow and pressure.

   • The tunica externa (adventitia): The outermost layer, composed primarily of collagen and elastic fibers. It anchors the vessel to surrounding structures and contains nerves and small blood vessels (vasa vasorum) that supply the outer parts of the vessel wall.

2. Arteries: Strong, elastic vessels designed to withstand high pressure and rapidly distribute blood away from the heart to the systemic and pulmonary circulations.

   • Elastic Arteries (e.g., aorta): Large arteries with abundant elastic fibers in their tunica media, allowing them to stretch and recoil with each heartbeat, maintaining blood pressure.

   • Muscular Arteries (distributing arteries): Smaller arteries with more smooth muscle and less elastic tissue, capable of powerful vasoconstriction and vasodilation to regulate blood flow to specific organs.

3. Veins: Thinner-walled, less elastic vessels that transport deoxygenated blood back to the heart (except pulmonary veins, which carry oxygenated blood). They are often referred to as capacitance vessels because their walls are more distensible, allowing them to accommodate large changes in blood volume. Veins feature valves (folds of tunica interna) to prevent the backflow of blood, especially against gravity, and aid venous return.

4. Capillaries: The smallest and most numerous blood vessels, forming vast networks at the tissue level. Their walls consist of a single layer of endothelial cells and a basement membrane, which is extremely thin to facilitate efficient exchange of gases (oxygen and carbon dioxide), nutrients, hormones, and waste products between the blood and interstitial fluid. There are three main types:

   • Continuous capillaries: Most common, with an uninterrupted endothelial lining, found in muscles, skin, lungs, and the central nervous system.

   • Fenestrated capillaries: Have pores (fenestrations) in their endothelial cells, allowing greater permeability for fluid and small solute exchange, found in kidneys, small intestine, and endocrine glands.

   • Sinusoids (discontinuous capillaries): Have large, irregular lumens and larger fenestrations, allowing for passage of large molecules and even blood cells, found in the liver, spleen, bone marrow, and some endocrine glands.

Fetal Circulation

Fetal circulation differs significantly from adult circulation because the fetus obtains oxygen and nutrients from the mother's blood via the placenta, and its lungs are not yet functional for gas exchange. Several unique shunts allow blood to bypass the fetal lungs and liver:

• The foramen ovale: An interatrial opening that allows most of the oxygenated blood entering the right atrium from the inferior vena cava to bypass the right ventricle and pulmonary circulation by flowing directly into the left atrium. This blood then enters the left ventricle and aorta, providing oxygenated blood to the developing brain and body.

• The ductus arteriosus: A short, muscular vessel connecting the pulmonary artery directly to the aorta, further shunting blood away from the non-functional fetal lungs. This ensures that the minimal blood that enters the pulmonary artery bypasses the lungs and goes directly into the systemic circulation.

• The ductus venosus: Bypasses the liver, allowing oxygenated blood from the umbilical vein to flow directly into the inferior vena cava.

• At birth, with the first breath, pulmonary vascular resistance drops, and systemic vascular resistance increases. These changes lead to the closure of these fetal shunts, which then anatomically remodel into adult structures:

  • The foramen ovale closes and forms the fossa ovalis.

  • The ductus arteriosus constricts and degenerates into the ligamentum arteriosum.

  • The ductus venosus becomes the ligamentum venosum.

Conclusion

The heart and blood vessels form a complex, efficient, and interconnected system designed to maintain constant circulation throughout the entire body. A thorough understanding of the intricate structure, specific functions, and anatomical relationships of these components, from the myocardial layers to the specialized conduction pathways and the unique fetal adaptations, is fundamentally crucial for comprehending how they work together in a coordinated manner to sustain life and respond to various physiological demands.