The Cardiovascular System: The Heart

Overview of Heart Function and Circuits

The heart is a muscular organ that serves as the central pump of the cardiovascular system, circulating blood throughout the body via two main circuits: the pulmonary and systemic circuits.

• Pulmonary Circuit: The right side of the heart receives oxygen-poor blood from the body and pumps it to the lungs for oxygenation.

• Systemic Circuit: The left side of the heart receives oxygen-rich blood from the lungs and pumps it to the rest of the body.

• Receiving Chambers: Right atrium (from systemic circuit), left atrium (from pulmonary circuit).

• Pumping Chambers: Right ventricle (to pulmonary circuit), left ventricle (to systemic circuit).

Size, Location, and Orientation of the Heart

The heart is roughly the size of a fist and is located in the mediastinum, between the second rib and fifth intercostal space, resting on the diaphragm. About two-thirds of the heart lies to the left of the midsternal line, with the apex pointing toward the left hip.

Coverings of the Heart

The heart is enclosed in a double-walled sac called the pericardium:

• Fibrous Pericardium: Outermost layer; protects, anchors, and prevents overfilling.

• Serous Pericardium: Two layers (parietal and visceral/epicardium) with a fluid-filled pericardial cavity between them to reduce friction.

Layers of the Heart Wall

The heart wall consists of three layers:

• Epicardium: Visceral layer of serous pericardium.

• Myocardium: Thick, muscular middle layer responsible for contraction; contains spiral and circular bundles of cardiac muscle.

• Endocardium: Thin, inner endothelial layer lining the chambers and valves.

Chambers and Associated Great Vessels

Internal and Surface Features

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

Atria: The Receiving Chambers

• Right Atrium: Receives deoxygenated blood from the superior vena cava, inferior vena cava, and coronary sinus. Contains pectinate muscles and the fossa ovalis.

• Left Atrium: Receives oxygenated blood from four pulmonary veins. Pectinate muscles are only in the auricles.

Ventricles: The Discharging Chambers

• Right Ventricle: Pumps blood into the pulmonary trunk.

• Left Ventricle: Pumps blood into the aorta; has the thickest walls.

• Both ventricles contain trabeculae carneae and papillary muscles attached to chordae tendineae.

Heart Valves and Blood Flow

Heart Valves

Four valves ensure unidirectional blood flow:

• Atrioventricular (AV) Valves: Tricuspid (right) and mitral/bicuspid (left) valves between atria and ventricles. Prevent backflow into atria during ventricular contraction.

• Semilunar (SL) Valves: Pulmonary (right) and aortic (left) valves between ventricles and major arteries. Prevent backflow into ventricles during relaxation.

Function of AV and SL Valves

• AV valves open when atrial pressure exceeds ventricular pressure; close when ventricles contract.

• SL valves open when ventricular pressure exceeds arterial pressure; close when ventricles relax.

Blood Flow Through the Heart

Blood flows from atria to ventricles, then to either the lungs (right side) or the rest of the body (left side). Both circuits move equal volumes of blood, but the left ventricle works harder due to higher resistance in the systemic circuit.

Coronary Circulation

The heart's own blood supply is provided by the coronary arteries and drained by cardiac veins. Blockage can lead to angina or myocardial infarction (heart attack).

• Coronary Arteries: Left and right coronary arteries branch to supply the heart muscle.

• Cardiac Veins: Great, middle, and small cardiac veins drain into the coronary sinus, which empties into the right atrium.

Microscopic Anatomy of Cardiac Muscle

Cardiac muscle cells (myocytes) are short, branched, and interconnected. They contain intercalated discs with desmosomes (mechanical connection) and gap junctions (electrical connection), allowing the heart to function as a syncytium.

• Abundant mitochondria for aerobic metabolism.

• Myofibrils with sarcomeres (Z discs, A bands, I bands).

Comparison: Skeletal vs. Cardiac Muscle

Intrinsic Conduction System and Pacemaker Cells

The heart's rhythm is set by pacemaker cells in the sinoatrial (SA) node, which generate spontaneous action potentials. The intrinsic conduction system coordinates the heartbeat through a sequence:

1. Sinoatrial (SA) node

2. Atrioventricular (AV) node

3. AV bundle (bundle of His)

4. Right and left bundle branches

5. Purkinje fibers (subendocardial conducting network)

Action Potentials and ECG

Cardiac muscle action potentials have a plateau phase due to Ca2+ influx, preventing tetanus. The electrocardiogram (ECG) records the electrical activity of the heart, with key features:

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization (and atrial repolarization)

• T wave: Ventricular repolarization

The Cardiac Cycle

The cardiac cycle includes all events in one heartbeat:

• Systole: Contraction phase

• Diastole: Relaxation phase

• Phases: Ventricular filling, isovolumic contraction, ventricular ejection, isovolumic relaxation

Regulation of Cardiac Output

Cardiac output (CO) is the amount of blood pumped by each ventricle per minute:

• CO = Heart Rate (HR) × Stroke Volume (SV)

• Normal adult CO ≈ 5.25 L/min

• Stroke volume is determined by preload, contractility, and afterload.

Developmental and Clinical Aspects

The heart develops from mesoderm and forms a four-chambered structure by day 35 of embryonic development. Congenital heart defects are common and may involve septal defects or valve abnormalities. Age-related changes include valve thickening, reduced cardiac reserve, and increased risk of atherosclerosis.