The Cardiovascular System: The Heart – Structure, Function, and Physiology

Heart Anatomy

Location of the Heart

The heart is a muscular organ located in the mediastinum, resting on the superior surface of the diaphragm. Approximately two-thirds of the heart lies to the left of the midsternal line, positioned anterior to the vertebral column and posterior to the sternum. It is roughly the size of a fist and is enclosed within the pericardium, a double-walled sac that provides protection and anchorage.

Pericardium

The pericardium consists of two main layers:

• Superficial fibrous pericardium: Protects, anchors, and prevents overfilling of the heart.

• Deep serous pericardium: Composed of two layers separated by a fluid-filled pericardial cavity, which reduces friction during heart activity.

  • Parietal layer: Lines the internal surface of the fibrous pericardium.

  • Visceral layer (epicardium): Covers the external surface of the heart.

Layers of the Heart Wall

The heart wall is composed of three layers:

• Epicardium: The visceral layer of the serous pericardium.

• Myocardium: Consists of spiral bundles of cardiac muscle cells and the fibrous skeleton of the heart, which anchors muscle fibers, supports vessels and valves, and limits the spread of action potentials.

• Endocardium: A thin endothelial layer lining the heart chambers and continuous with blood vessel endothelium.

Chambers of the Heart

Atria: The Receiving Chambers

The heart contains four chambers: two atria and two ventricles. The atria are separated by the interatrial septum and are externally marked by auricles, which increase atrial volume. The right atrium receives blood from the superior and inferior vena cava and the coronary sinus, while the left atrium receives blood from the right and left pulmonary veins.

Ventricles: The Discharging Chambers

The ventricles are separated by the interventricular septum and are externally marked by anterior and posterior interventricular sulci. The right ventricle pumps blood into the pulmonary trunk (to the lungs), and the left ventricle pumps blood into the aorta (to the body). The walls of the ventricles are ridged by trabeculae carneae, and papillary muscles project into the ventricular cavities.

Pathway of Blood Through the Heart

Pulmonary and Systemic Circuits

The heart functions as two side-by-side pumps:

• Right side: Pumps blood through the pulmonary circuit (to and from the lungs).

• Left side: Pumps blood through the systemic circuit (to and from body tissues).

Blood pathway: Right atrium → tricuspid valve → right ventricle → pulmonary semilunar valve → pulmonary trunk → pulmonary arteries → lungs → pulmonary veins → left atrium → bicuspid (mitral) valve → left ventricle → aortic semilunar valve → aorta → systemic circulation.

Coronary Circulation

Arterial and Venous Supply

Coronary circulation provides the functional blood supply to the heart muscle itself. The right and left coronary arteries branch into several arteries, including the marginal, posterior interventricular, circumflex, and anterior interventricular arteries. Venous blood is collected by the small, anterior, and great cardiac veins, which join to form the coronary sinus.

Homeostatic Imbalances

• Angina pectoris: Thoracic pain due to transient deficiency in blood delivery to the myocardium; cells are weakened.

• Myocardial infarction (heart attack): Prolonged coronary blockage leads to cell death, which is replaced by noncontractile scar tissue.

Heart Valves

Atrioventricular (AV) Valves

AV valves prevent backflow into the atria when the ventricles contract. The right AV valve is the tricuspid valve, and the left is the mitral (bicuspid) valve. Chordae tendineae anchor the valve cusps to papillary muscles.

Semilunar (SL) Valves

SL valves prevent backflow into the ventricles when the ventricles relax. The aortic semilunar valve is located at the left ventricle, and the pulmonary semilunar valve is at the right ventricle.

Microscopic Anatomy of Cardiac Muscle

Cardiac Muscle Structure

Cardiac muscle cells are striated, short, branched, and interconnected. They contain numerous mitochondria and a connective tissue matrix (endomysium) that connects to the fibrous skeleton. Intercalated discs, containing desmosomes and gap junctions, anchor cells and allow for electrical coupling, enabling the heart to function as a syncytium.

Cardiac Muscle Contraction

Mechanism of Contraction

Cardiac muscle contraction is initiated by depolarization, which opens voltage-gated fast Na+ channels, reversing the membrane potential. The depolarization wave in T tubules causes the sarcoplasmic reticulum to release Ca2+, and slow Ca2+ channels in the sarcolemma prolong the depolarization phase (plateau). Ca2+ influx triggers further Ca2+ release from the SR, leading to contraction. Repolarization occurs as Ca2+ channels close and K+ channels open.

Heart Physiology: Electrical Events

Intrinsic Cardiac Conduction System

The intrinsic conduction system consists of autorhythmic cells that initiate and distribute impulses, coordinating heart depolarization and contraction. The sequence of excitation is as follows:

1. Sinoatrial (SA) node: Pacemaker, generates impulses (~75/min).

2. Atrioventricular (AV) node: Delays impulses (~0.1 s), allows atria to contract before ventricles.

3. AV bundle (bundle of His): Only electrical connection between atria and ventricles.

4. Right and left bundle branches: Carry impulses through the interventricular septum.

5. Purkinje fibers: Complete the pathway into the apex and ventricular walls.

Homeostatic Imbalances

• Arrhythmias: Irregular heart rhythms.

• Fibrillation: Rapid, irregular contractions; ineffective for pumping blood.

• Ectopic focus: Abnormal pacemaker activity.

• Heart block: Impaired conduction between atria and ventricles.

Extrinsic Innervation of the Heart

The autonomic nervous system (ANS) modifies heart rate and force. The cardioacceleratory center (sympathetic) increases heart rate and contractility, while the cardioinhibitory center (parasympathetic, via the vagus nerve) decreases heart rate.

Electrocardiography (ECG/EKG)

An ECG records the electrical activity of the heart. Key waves include:

• P wave: Depolarization of the SA node (atrial depolarization).

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

Heart Sounds

Two main heart sounds are produced by valve closure:

• First sound (lub): AV valves close (beginning of systole).

• Second sound (dup): SL valves close (beginning of diastole).

Heart murmurs are abnormal sounds, often indicating valve problems.

Mechanical Events: The Cardiac Cycle

Phases of the Cardiac Cycle

• Ventricular filling: AV valves open, blood flows into ventricles; atrial systole delivers remaining blood.

• Ventricular systole: Ventricles contract, AV valves close, SL valves open for ejection phase.

• Isovolumetric relaxation: Ventricles relax, SL valves close, brief rise in aortic pressure (dicrotic notch).

End diastolic volume (EDV): Volume in ventricles at end of diastole. End systolic volume (ESV): Volume remaining after contraction.

Cardiac Output (CO)

Cardiac output is the volume of blood pumped by each ventricle per minute:

$CO = HR \times SV$

Where:

• HR: Heart rate (beats per minute)

• SV: Stroke volume (volume pumped per beat)

Stroke volume is calculated as:

$SV = EDV - ESV$

Cardiac reserve is the difference between resting and maximal CO.

Regulation of Stroke Volume

Three main factors affect stroke volume:

• Preload: Degree of stretch of cardiac muscle before contraction (Frank-Starling law).

• Contractility: Contractile strength at a given muscle length, independent of preload.

• Afterload: Pressure that must be overcome to eject blood (mainly arterial pressure).

Regulation of Heart Rate

Heart rate is regulated by:

• Autonomic nervous system: Sympathetic stimulation increases HR (positive chronotropic effect); parasympathetic stimulation decreases HR (negative chronotropic effect).

• Other factors: Age, gender, exercise, and body temperature.

Homeostatic Imbalances and Heart Failure

• Tachycardia: Abnormally fast heart rate (>100 bpm).

• Bradycardia: Abnormally slow heart rate (<60 bpm).

• Congestive heart failure (CHF): Progressive condition where CO is too low to meet tissue needs, caused by coronary atherosclerosis, high blood pressure, multiple infarcts, or dilated cardiomyopathy.

Age-Related and Developmental Aspects

• Sclerosis and thickening of valve flaps

• Decline in cardiac reserve

• Fibrosis of cardiac muscle

• Atherosclerosis

• Congenital heart defects: Mixing of systemic and pulmonary blood, narrowed valves or vessels