The Heart: Structure, Function, and Physiology

The Heart: Structure, Function, and Physiology

Functions of the Heart

The heart is a muscular organ responsible for pumping blood throughout the body, supplying oxygen and nutrients, and removing waste products.

• Circulatory Pump: Maintains blood flow through the pulmonary and systemic circuits.

• Homeostasis: Regulates blood pressure and volume, supporting tissue health.

• Endocrine Role: Secretes atrial natriuretic peptide (ANP) to regulate blood volume.

• Example: The heart increases cardiac output during exercise to meet increased metabolic demands.

Size, Position, and Orientation of the Heart

The heart is roughly the size of a fist and is located in the mediastinum of the thoracic cavity, slightly left of the midline.

• Size: About 250-350 grams in adults.

• Position: Posterior to the sternum, between the lungs, with the apex pointing left and inferiorly.

• Orientation: The base is superior and the apex is inferior.

• Additional info: The heart rests on the diaphragm.

Pericardium: Structure, Location, and Function

The pericardium is a double-walled sac that surrounds and protects the heart.

• Fibrous Pericardium: Tough, outer layer; prevents overfilling and anchors the heart.

• Serous Pericardium: Thin, inner layer; divided into parietal and visceral layers.

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

• Visceral Pericardium (Epicardium): Covers the heart surface.

• Pericardial Cavity: Space between parietal and visceral layers; contains serous fluid to reduce friction.

Tissues of the Heart Wall

The heart wall consists of three layers, each with distinct structure and function.

• Epicardium (Visceral Pericardium): Outer layer; provides protection.

• Myocardium: Middle, muscular layer; responsible for contraction.

• Fibrous Skeleton: Dense connective tissue; supports valves and electrical insulation.

• Endocardium: Inner layer; lines chambers and valves, reduces friction.

Chambers and Valves of the Heart

The heart has four chambers and four main valves that regulate blood flow.

• Chambers: Right atrium, right ventricle, left atrium, left ventricle.

• Valves: Tricuspid (right AV), bicuspid/mitral (left AV), pulmonary semilunar, aortic semilunar.

• Function: Valves prevent backflow and ensure unidirectional blood flow.

Major Blood Vessels Associated with the Heart

Blood vessels carry blood to and from the heart, connecting to specific chambers.

• Vessels to the Heart: Superior and inferior vena cava (right atrium), pulmonary veins (left atrium).

• Vessels from the Heart: Pulmonary artery (right ventricle), aorta (left ventricle).

Blood Flow Through the Heart: Pulmonary and Systemic Circuits

Blood flows through two main circuits: pulmonary (to lungs) and systemic (to body).

• Pulmonary Circuit: Right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium.

• Systemic Circuit: Left ventricle → aorta → body tissues → vena cava → right atrium.

• Additional info: The circuits operate in series, ensuring oxygenation and nutrient delivery.

Coronary Circulation

Coronary circulation supplies the heart muscle with oxygen and nutrients.

• Coronary Arteries: Branch from the aorta; supply myocardium.

• Coronary Veins: Drain blood from myocardium into the coronary sinus.

• Anastomosis: Connections between vessels; provide alternate routes.

• Collateral Circulation: Backup blood supply via anastomoses.

• Ischemic: Reduced blood supply.

• Angina Pectoris: Chest pain from temporary ischemia.

• Infarction: Tissue death from prolonged ischemia (e.g., myocardial infarction).

Cardiac Muscle: Structure, Function, Metabolism, and Contraction

Cardiac muscle is specialized for continuous, rhythmic contraction.

• Structure: Striated, branched cells with intercalated discs.

• Function: Pumps blood via coordinated contraction.

• Metabolism: High mitochondrial content; relies on aerobic respiration.

• Contraction: Involuntary, regulated by intrinsic and extrinsic factors.

Autorhythmic vs. Contractile Cardiac Cells

Cardiac cells are classified as autorhythmic (pacemaker) or contractile.

• Autorhythmic Cells: Located in SA node, AV node, etc.; initiate and conduct action potentials.

• Contractile Cells: Make up most myocardium; respond to action potentials and contract.

• Action Potentials: Autorhythmic cells have spontaneous depolarization; contractile cells have rapid depolarization and plateau phases.

• Ion Permeability: Changes in Na+, Ca2+, and K+ drive action potentials.

Intrinsic Conduction System of the Heart

The heart's conduction system coordinates contraction.

• Structures: SA node (right atrium), AV node (interatrial septum), AV bundle (interventricular septum), bundle branches, Purkinje fibers (ventricular walls).

• Pathway: SA node → AV node → AV bundle → bundle branches → Purkinje fibers.

Electrocardiogram (ECG)

An ECG records the electrical activity of the heart.

• Deflection Waves: P wave (atrial depolarization), QRS complex (ventricular depolarization), T wave (ventricular repolarization).

• Function: Diagnoses arrhythmias and heart function.

Arrhythmias

Arrhythmias are abnormal heart rhythms.

• Sinus Tachycardia: Fast heart rate.

• Sinus Bradycardia: Slow heart rate.

• Heart Block: Impaired conduction between atria and ventricles.

• Atrial Flutter: Rapid, regular atrial contractions.

• Atrial Fibrillation: Rapid, irregular atrial contractions.

• Ventricular Fibrillation: Rapid, irregular ventricular contractions; life-threatening.

• PVC: Premature ventricular contraction.

• Atrioventricular (Heart) Block: Disrupted conduction through AV node.

Mechanical Events of the Cardiac Cycle

The cardiac cycle includes systole (contraction) and diastole (relaxation).

• Duration: Systole (~0.3 s), diastole (~0.5 s).

• Pressure Changes: Drive blood flow and valve actions.

• Volume Changes: End diastolic volume (EDV), end systolic volume (ESV).

• Valve Actions: Open/close in response to pressure.

• Heart Sounds: First sound (AV valves close), second sound (semilunar valves close).

• Stenosis: Narrowing of valves.

• Incompetent (Insufficient): Leaky valves.

• Murmur: Abnormal heart sound.

Key Cardiac Terms

Understanding cardiac function requires knowledge of several key terms.

• Cardiac Output (CO): Volume of blood pumped per minute.

• Stroke Volume (SV): Volume of blood pumped per beat.

• End Diastolic Volume (EDV): Volume in ventricle at end of diastole.

• End Systolic Volume (ESV): Volume in ventricle at end of systole.

• Contractility: Force of contraction.

• Venous Return: Blood returning to heart.

• Cardiac Reserve: Difference between resting and maximal CO.

• Preload: Degree of stretch before contraction.

• Afterload: Resistance heart must overcome to eject blood.

Key Equations:

• Cardiac Output: $CO = HR \times SV$

• Stroke Volume: $SV = EDV - ESV$

Regulation of Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output is regulated by intrinsic and extrinsic factors.

• Heart Rate: Influenced by autonomic nervous system, hormones, and fitness.

• Stroke Volume: Affected by preload, contractility, and afterload.

• Additional info: Sympathetic stimulation increases heart rate and contractility.

Congestive Heart Failure (CHF): Causes and Consequences

CHF occurs when the heart cannot pump effectively, leading to fluid buildup.

• Causes: Coronary artery disease, hypertension, valve disorders, myocardial infarction.

• Consequences: Pulmonary edema, systemic edema, reduced organ perfusion.

• Example: Left-sided CHF causes fluid accumulation in the lungs.