Anatomy and Physiology of the Heart

Statistics and Vital Importance of the Heart

The beating heart is an organ of incredible capability, serving as the central engine for life. On a daily basis, the heart contracts approximately $100,000$ times. Over the course of a year, this activity equates to an estimated $35 \times 10^6$ contractions. This continuous mechanical work allows the heart to pump approximately $7000\,L$ of blood through a vast network of blood vessels totaling roughly $100,000\,km$ in length every single day.

While the ancient Greeks once proposed that the heart was the seat of central intelligence, modern science recognizes it as the vital pump of the cardiovascular system. However, the heart does not function in isolation. It works in conjunction with miles of blood vessels that act as the fundamental links between the cardiovascular system and the cells of the body. This system ensures a continuous circulation of blood to fulfill homeostatic needs, specifically providing tissue cells with a constant supply of nutrients and oxygen while simultaneously removing metabolic wastes.

Systemic and Pulmonary Circulation

The cardiovascular system is divided into two distinct circuits: the pulmonary circulation and the systemic circulation. The right side of the heart is responsible for the pulmonary circuit. It receives oxygen-depleted blood from the body tissues and pumps it to the lungs. In the lungs, the blood offloads carbon dioxide and picks up oxygen. The left side of the heart governs the systemic circuit. It receives freshly oxygenated blood returning from the lungs and pumps it throughout the entire body to provide oxygen and nutrients to all tissues.

The systemic circulation is categorized as a high-pressure circuit. Blood leaving the left side of the heart is pumped at pressures of approximately $120/80\,mmHg$ to travel throughout the body. To generate the necessary force for this high-pressure demand, the wall of the left ventricle is significantly thicker than that of the right ventricle. In contrast, the pulmonary circulation is a low-pressure circuit. Blood pumped from the right ventricle into the lungs requires pressures of only about $25/8\,mmHg$ . Higher pressures in this circuit would be hazardous to the delicate and highly permeable lung tissues involved in gas exchange, potentially forcing fluid into the lungs.

Despite the discrepancy in pressure, the volume of blood leaving both the right and left ventricles remains equal. This equality is maintained because the pulmonary capillaries can dilate to reduce pulmonary vascular resistance. Because the heart is part of a closed circuit, both the stroke volume ( $SV$ ) and the cardiac output ( $CO$ ) will be equal on both the right and left sides.

Blood pressure measurements consist of the systolic pressure, which is the pressure during ventricular contraction, and the diastolic pressure, which represents the pressure during ventricular relaxation or filling. These pressures are at their highest in the arteries located closest to the heart.

Anatomy, Size, and Location of the Heart

A healthy adult heart is roughly the size of a closed fist and weighs between $250\,g$ and $350\,g$ . It is situated within the mediastinum, the medial cavity of the thorax. It is positioned posterior to the sternum and anterior to the vertebral column. Approximately two-thirds of the heart's mass lies to the left of the mid-sternal line.

The heart's orientation is characterized by its base and apex. The base of the heart is level with the second ( $2\text{nd}$ ) rib. The apex points inferiorly and toward the left hip. The apical pulse can be felt behind the fifth ( $5\text{th}$ ) intercostal space, just below the left nipple in the mid-clavicular line (the line descending from the middle of the clavicle). The heart rest on its right side upon the superior surface of the diaphragm. A notable external landmark is the sternal angle, found at the junction of the manubrium and the body of the sternum, which lies anterior to the aortic arch and the fourth thoracic vertebra ( $T4$ ).

The Layers of the Heart and the Pericardium

The heart is enclosed in a double-walled sac known as the pericardium. The outermost layer is the fibrous pericardium, a tough, dense connective tissue that protects the heart, prevents over-distension, and anchors it to surrounding structures. Deep to the fibrous pericardium is the serous pericardium, a two-layered slippery membrane. The parietal layer of the serous pericardium lines the internal surface of the fibrous pericardium. At the base of the heart, this layer turns inferiorly to cover the external heart surface as the visceral layer, also known as the epicardium.

The pericardial cavity is the space between the parietal and visceral layers and is filled with serous fluid. This fluid acts as a lubricant, allowing the membranes to glide past each other and enabling the heart to beat in a friction-free environment. The heart wall itself consists of three distinct layers:

Epicardium: The most superficial layer, identical to the visceral layer of the serous pericardium; it is often infiltrated with fat.
Myocardium: The middle layer and the bulk of the heart, composed primarily of cardiac muscle. The muscle cells are arranged in spiral or circular bundles by connective tissue fibers, linking all parts of the heart together.
Endocardium: The innermost layer, consisting of a thin sheet of endothelium resting on connective tissue. It lines the heart chambers, covers the fibrous skeleton of the valves, and is continuous with the endothelial linings of the blood vessels.

Clinical Conditions of the Heart Layers

Inflammation of the heart's layers can lead to significant clinical complications:

Pericarditis is the inflammation of the pericardium. This condition roughens the serous membrane surfaces, creating a "friction rub" that can be heard with a stethoscope as the heart rubs against the sac. Symptoms include deep sternal pain, coughing, and fever. It can be caused by viral or bacterial infections or a myocardial infarct. In severe cases, it leads to cardiac tamponade, where inflammatory fluid accumulates in the pericardial cavity and compresses the heart. Treatment involves draining the fluid with a syringe.

Myocarditis is the inflammation of the myocardium. It can cause heart failure, chest pain, and arrhythmias, often resulting from viral infections or autoimmune reactions. Treatments include inotropes, which are drugs that increase the force of heart contraction, and diuretics, which help decrease blood volume to reduce pressure on the heart.

Endocarditis involves the inflammation of the endocardium, heart valves, chordae tendineae, and interventricular septum. It is typically caused by bacteria from other body parts (such as the mouth) spreading through the bloodstream and attaching to damaged heart areas. This can lead to leaky valves and blood clots. Treatment requires antibiotics or surgical valve replacement.

Internal Anatomy and the Cardiac Skeleton

The heart contains four chambers: two atria (receiving chambers) and two ventricles (discharging chambers). The atria are separated by the interatrial septum, while the ventricles are separated by the thick interventricular septum. The position of this internal septum is marked externally by the anterior interventricular sulcus.

The cardiac skeleton is a dense network of connective tissue fibers that provides internal reinforcement to the myocardium. It serves several roles: anchoring cardiac muscle fibers, preventing the valves and vessels from stretching under the stress of pulsing blood, and acting as an electrical insulator. Because connective tissue is not electrically excitable, the cardiac skeleton ensures that action potentials ( $AP$ s) only spread through specific pathways. The arrangement of muscle bundles allows for a "wringing" action that squeezes blood upward from the apex during ventricular contraction.

Major Vessels and External Features

Deoxygenated blood enters the right atrium through three major veins:

Superior Vena Cava (SVC): Returns blood from body regions superior to the diaphragm.
Inferior Vena Cava (IVC): Returns blood from regions below the diaphragm.
Coronary Sinus: Collects blood draining from the myocardium itself.

The right ventricle pumps this deoxygenated blood into the pulmonary trunk, which branches into the pulmonary arteries leading to the lungs. Oxygenated blood returns via the pulmonary veins into the left atrium. The left ventricle then ejects the oxygenated blood into the aorta, the largest artery in the body. Auricles are small, ear-like appendages overlying the atria that function to increase atrial volume and surface area.

Heart Valves and the Mechanism of Unidirectional Flow

Four valves ensure that blood flows through the heart in a single direction, opening and closing in response to pressure changes. These valves are made of connective tissue covered by endocardium.

Atrioventricular (AV) Valves prevent backflow into the atria during ventricular contraction. The right AV valve is the tricuspid valve (featuring three cusps). The left AV valve is the mitral valve or bicuspid valve (featuring two cusps). These valves are anchored by collagen cords called chordae tendineae (the "heartstrings"), which connect the valve cusps to papillary muscles protruding from the ventricular walls. When ventricles contract, the papillary muscles contract to keep the chordae tendineae taut, preventing the valves from prolapsing into the atria.

Semilunar (SL) Valves guard the bases of the large arteries exiting the ventricles. The aortic valve sits between the left ventricle and the aorta, while the pulmonary valve sits between the right ventricle and the pulmonary trunk. Each SL valve has three pocket-like, crescent-shaped cusps. They open when ventricular pressure exceeds arterial pressure and snap shut when ventricles relax and blood starts to flow backward, filling the cusps.

Heart sounds are generated by the closing of these valves. The first sound ( $S1$ ) occurs when the AV valves close. The second sound ( $S2$ ) occurs when the SL valves close.

Coronary Circulation

The myocardium is too thick for oxygen to diffuse from the blood within the chambers; therefore, it requires its own blood supply through the coronary circulation. The right and left coronary arteries arise from the base of the aorta.

Arterial Supply:

The Left Coronary Artery (LCA) branches into the anterior interventricular artery and the circumflex artery.
The Right Coronary Artery (RCA) branches into the right marginal artery and the posterior interventricular artery.

There are often junctions between these branches called anastomoses, which provide collateral routes for blood flow. However, these are usually insufficient if a major artery is suddenly and completely blocked. Blood flow to the myocardium is intermittent; it occurs only when the ventricles are relaxed and is restricted when the myocardium contracts and compresses the vessels.

Venous Drainage: Venous blood is collected by cardiac veins. The coronary sinus is the largest vessel for venous return. Smaller veins include the great cardiac vein (located in the anterior interventricular sulcus), the middle cardiac vein (in the posterior interventricular sulcus), and the small cardiac vein (along the inferior margin).

Myocardial Ischaemia and Infarction

Angina pectoris refers to thoracic or chest pain resulting from myocardial ischaemia, which is an inadequate blood supply to the heart muscle. If the ischaemia is brief, the cells do not die. However, a prolonged blockage results in a myocardial infarction ( $MI$ ), or heart attack, where muscle cells die and are replaced by non-contractile scar tissue, decreasing heart function. Regular exercise is beneficial because it encourages the development of collateral vessels or anastomoses, which can provide alternative blood routes and lessen the likelihood of sudden death during a myocardial infarction.