The Heart: Anatomy, Valves, and Circulation

Heart Anatomy and Circulation

The Pulmonary and Systemic Circuits

The heart functions as a dual pump, maintaining two separate circuits: the pulmonary circuit and the systemic circuit. The right side of the heart receives oxygen-poor blood from the body and pumps it to the lungs for oxygenation (pulmonary circuit), while the left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body (systemic circuit).

• Pulmonary Circuit: Right atrium → Right ventricle → Pulmonary arteries → Lungs

• Systemic Circuit: Left atrium → Left ventricle → Aorta → Body tissues

• Key Point: The heart ensures unidirectional blood flow and separates oxygenated from deoxygenated blood.

• Example: Blood returning from the body enters the right atrium, is pumped to the lungs, returns to the left atrium, and is then pumped to the body.

Size, Location, and Orientation of the Heart

The heart is approximately the size of a fist and weighs less than one pound. It is located in the mediastinum, between the second rib and the fifth intercostal space, resting on the superior surface of the diaphragm. Two-thirds of the heart lies to the left of the midsternal line, anterior to the vertebral column and posterior to the sternum.

• Base: Leans toward the right shoulder.

• Apex: Points toward the left hip; the apical impulse can be palpated between the fifth and sixth ribs, just below the left nipple.

Coverings and Layers of the Heart

Pericardium and Heart Wall

The heart is enclosed in a double-walled sac called the pericardium. The superficial fibrous pericardium protects and anchors the heart, while the deep serous pericardium consists of two layers (parietal and visceral) separated by a fluid-filled pericardial cavity that reduces friction.

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Cardiac muscle layer responsible for contraction; contains the cardiac skeleton for structural support and electrical insulation.

• Endocardium: Innermost layer; lines heart chambers and covers valves.

Chambers and Associated Great Vessels

Internal and Surface Features

The heart has four chambers: two superior atria (receiving chambers) and two inferior ventricles (pumping chambers). The interatrial septum separates the atria, while the interventricular septum separates the ventricles. Surface features include the coronary sulcus (atrioventricular groove), anterior interventricular sulcus, and posterior interventricular sulcus.

• Right Atrium: Receives deoxygenated blood from the body via the superior and inferior vena cava and the coronary sinus.

• Left Atrium: Receives oxygenated blood from the lungs via four pulmonary veins.

• Right Ventricle: Pumps blood into the pulmonary trunk.

• Left Ventricle: Pumps blood into the aorta.

• Trabeculae carneae: Irregular ridges of muscle in the ventricles.

• Papillary muscles: Anchor chordae tendineae attached to heart valves.

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow through the heart. They open and close in response to pressure changes. There are two major types:

• Atrioventricular (AV) Valves: Located between atria and ventricles; prevent backflow into atria when ventricles contract.

• Semilunar (SL) Valves: Located between ventricles and major arteries; prevent backflow into ventricles.

Atrioventricular (AV) Valves

• Tricuspid Valve: Right AV valve, three cusps, between right atrium and ventricle.

• Mitral (Bicuspid) Valve: Left AV valve, two cusps, between left atrium and ventricle.

• Chordae tendineae: Anchor valve cusps to papillary muscles, preventing eversion.

Semilunar (SL) Valves

• Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

• Aortic Semilunar Valve: Between left ventricle and aorta.

• Each SL valve has three half-moon-shaped cusps.

Pathway of Blood Through the Heart

Right and Left Sides

Blood flows through the heart in a specific sequence, ensuring separation of oxygenated and deoxygenated blood.

• Right Side: Superior/inferior vena cava → Right atrium → Tricuspid valve → Right ventricle → Pulmonary semilunar valve → Pulmonary trunk → Pulmonary arteries → Lungs

• Left Side: Pulmonary veins → Left atrium → Mitral valve → Left ventricle → Aortic semilunar valve → Aorta → Systemic circulation

Equal volumes of blood are pumped to both circuits, but the systemic circuit is longer and higher pressure, reflected in the thicker walls of the left ventricle.

Coronary Circulation

Arteries and Veins

Coronary circulation provides the heart muscle (myocardium) with its own blood supply. The left and right coronary arteries arise from the base of the aorta and branch to supply the heart. Cardiac veins collect deoxygenated blood and return it to the right atrium via the coronary sinus.

• Left Coronary Artery: Branches into the anterior interventricular artery and circumflex artery.

• Right Coronary Artery: Branches into the right marginal artery and posterior interventricular artery.

• Great, middle, and small cardiac veins drain into the coronary sinus.

Microscopic Anatomy of Cardiac Muscle

Structure and Function

Cardiac muscle cells are striated, short, branched, and interconnected. They contain one or two central nuclei and numerous mitochondria, making them highly resistant to fatigue. Intercalated discs connect cells via desmosomes (mechanical strength) and gap junctions (electrical coupling), allowing the heart to function as a coordinated unit (functional syncytium).

• Sarcomeres: Contain Z discs, A bands, and I bands.

• T tubules: Wider but less numerous than in skeletal muscle.

• Sarcoplasmic reticulum: Simpler, no triads.

Comparison of Cardiac and Skeletal Muscle

Key Differences

• Self-excitability: Some cardiac cells (pacemaker cells) can spontaneously depolarize; skeletal muscle requires neural stimulation.

• Contraction as a unit: All cardiac muscle cells contract together; skeletal muscle fibers contract independently.

• Calcium sources: Cardiac muscle uses both extracellular and SR Ca2+; skeletal muscle uses only SR Ca2+.

• Refractory period: Cardiac muscle has a longer absolute refractory period, preventing tetanic contractions and allowing the heart to relax and fill efficiently.

• Metabolism: Cardiac muscle relies almost exclusively on aerobic respiration and has more mitochondria than skeletal muscle.

Additional info: Table 18.1 in the source provides a detailed comparison of skeletal and cardiac muscle properties.