The Heart: Structure, Function, and Physiology

The Heart: Structure, Function, and Physiology

The Pulmonary and Systemic Circuits

The heart is a dual pump that circulates blood through two distinct circuits: the pulmonary circuit and the systemic circuit.

• 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 receives oxygen-rich blood from the lungs and pumps it to the rest of the body.

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

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

Example: Blood flows from the right atrium to the right ventricle, then to the lungs; from the left atrium to the left ventricle, then to the body.

Size, Location, and Orientation of the Heart

The heart is a hollow, cone-shaped organ about the size of a fist, weighing less than a pound.

• Location: In the mediastinum, between the second rib and fifth intercostal space, on the diaphragm, two-thirds left of the midsternal line.

• Base: Directed toward the right shoulder.

• Apex: Points toward the left hip; apical impulse felt below the left nipple.

Coverings of the Heart

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

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

• Serous Pericardium: Deep, double-layered; parietal layer lines fibrous pericardium, visceral layer (epicardium) covers the heart.

• Pericardial Cavity: Fluid-filled space between layers reduces friction.

Clinical Note: Pericarditis is inflammation of the pericardium, causing friction rub and potentially cardiac tamponade (compression of the heart by fluid).

Layers of the Heart Wall

The heart wall consists of three layers:

• Epicardium: Visceral layer of serous pericardium.

• Myocardium: Bulk of the heart; composed of contractile cardiac muscle cells arranged in circular and spiral bundles.

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

Cardiac Skeleton: Dense network of collagen and elastic fibers; anchors muscle fibers, supports valves, and limits spread of action potentials.

Chambers and Associated Great Vessels

The heart has four chambers separated by septa and surface grooves.

• Atria: Superior, thin-walled receiving chambers; right atrium receives blood from superior/inferior vena cava and coronary sinus; left atrium receives blood from four pulmonary veins.

• Ventricles: Inferior, thick-walled discharging chambers; right ventricle pumps blood into pulmonary trunk; left ventricle pumps blood into aorta.

• Surface Features: Coronary sulcus, anterior and posterior interventricular sulci mark chamber boundaries.

Internal Features: Interatrial septum (fossa ovalis), interventricular septum, trabeculae carneae, papillary muscles, chordae tendineae.

Heart Valves

Valves ensure unidirectional blood flow by opening and closing in response to pressure changes.

• Atrioventricular (AV) Valves: Tricuspid (right) and mitral (left) valves prevent backflow into atria; anchored by chordae tendineae to papillary muscles.

• Semilunar (SL) Valves: Pulmonary (right) and aortic (left) valves prevent backflow from arteries into ventricles; each has three crescent-shaped cusps.

• No valves: Between major veins and atria; blood inertia and contraction compress venous openings.

Clinical Note: Valve disorders include incompetence (backflow) and stenosis (narrowing); may require replacement.

Blood Flow Through the Heart

Blood flows from atria to ventricles, then to lungs or body.

• Systemic Circuit: Left ventricle → aorta → body tissues → systemic veins → right atrium.

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

• Ventricular Workload: Left ventricle pumps with greater pressure due to higher resistance in systemic circuit.

Coronary Circulation

The heart's own blood supply is provided by coronary arteries and veins.

• Coronary Arteries: Arise from aorta; left coronary artery branches into anterior interventricular and circumflex arteries; right coronary artery branches into right marginal and posterior interventricular arteries.

• Anastomoses: Junctions provide alternate routes but cannot compensate for complete occlusion.

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

Clinical Note: Blockage causes angina pectoris (pain) or myocardial infarction (heart attack).

Microscopic Anatomy of Cardiac Muscle

Cardiac muscle cells are striated, short, branched, and interconnected.

• Intercalated Discs: Junctions containing desmosomes (mechanical strength) and gap junctions (electrical coupling).

• Functional Syncytium: Myocardium acts as a single coordinated unit.

• Mitochondria: Numerous, providing resistance to fatigue.

• Sarcomeres: Contain Z discs, A bands, I bands; SR is simpler than in skeletal muscle.

Comparison: Skeletal vs. Cardiac Muscle

Key structural and functional differences between skeletal and cardiac muscle:

Feature	Skeletal Muscle	Cardiac Muscle
Structure	Striated, long, cylindrical, multinucleate	Striated, short, branched, one or two nuclei
Gap Junctions	No	Yes
Contraction	Independent (motor units)	As a unit (syncytium)
T tubules	Abundant	Fewer, wider
Sarcoplasmic Reticulum	Elaborate, terminal cisterns	Less elaborate, no terminal cisterns
Ca2+ Source	SR only	SR and extracellular fluid
Pacemaker Cells	No	Yes
Tetanus Possible	Yes	No
ATP Supply	Aerobic/anaerobic	Aerobic only

Pacemaker Cells and the Intrinsic Conduction System

The heart's rhythm is set by pacemaker cells and the intrinsic conduction system.

• Pacemaker Cells: Autorhythmic, spontaneously depolarize; initiate action potentials.

• Sequence of Excitation: SA node → AV node → AV bundle (bundle of His) → bundle branches → Purkinje fibers.

• SA Node: Primary pacemaker; sets sinus rhythm (~75 bpm).

• AV Node: Delays impulse (~0.1 s) for atrial contraction.

• Bundle of His: Only electrical connection between atria and ventricles.

• Purkinje Fibers: Rapidly distribute impulse to ventricles.

Clinical Note: Arrhythmias, fibrillation, heart block may require defibrillation or artificial pacemaker.

Extrinsic Innervation of the Heart

The autonomic nervous system (ANS) modulates heart rate and force.

• Sympathetic: Increases heart rate and contractility via cardioacceleratory center.

• Parasympathetic: Decreases heart rate via cardioinhibitory center (vagus nerve).

Action Potentials in Cardiac Muscle

Contractile cardiac muscle cells generate action potentials with a characteristic plateau phase.

• Depolarization: Fast Na+ channels open; rapid influx.

• Plateau: Slow Ca2+ channels open; Ca2+ influx prolongs depolarization.

• Repolarization: Ca2+ channels close; K+ channels open; rapid efflux.

Equation: $\text{Stroke Volume (SV)} = \text{End Diastolic Volume (EDV)} - \text{End Systolic Volume (ESV)}$

Benefit: Long refractory period prevents tetanus, ensures efficient blood ejection.

Electrocardiography (ECG/EKG)

An ECG records the heart's electrical activity, revealing normal and abnormal rhythms.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization and atrial repolarization.

• T wave: Ventricular repolarization.

• P-R interval: Start of atrial to start of ventricular depolarization.

• S-T segment: Ventricular myocardium depolarized.

• Q-T interval: Start of ventricular depolarization through repolarization.

Clinical Note: Abnormal ECG patterns indicate ischemia, arrhythmias, or chamber enlargement.

The Cardiac Cycle

The cardiac cycle describes the sequence of mechanical events during one heartbeat.

• Systole: Contraction phase.

• Diastole: Relaxation phase.

• Phases:

  1. Ventricular filling (diastole): AV valves open, blood flows into ventricles.

  2. Isovolumic contraction: All valves closed, ventricles contract.

  3. Ventricular ejection: SL valves open, blood ejected.

  4. Isovolumic relaxation: All valves closed, ventricles relax.

• Heart Sounds: 'Lub' (AV valves close), 'dup' (SL valves close).

Clinical Note: Heart murmurs indicate valve disorders (incompetence or stenosis).

Cardiac Output, Stroke Volume, and Heart Rate

Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.

• Formula: $\text{CO} = \text{HR} \times \text{SV}$

• Normal Values: HR = 75 bpm, SV = 70 ml/beat, CO ≈ 5.25 L/min.

• Cardiac Reserve: Difference between resting and maximal CO.

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

• Preload: Degree of stretch before contraction; Frank-Starling law.

• Contractility: Strength at a given length; increased by Ca2+, SNS, hormones.

• Afterload: Pressure ventricles must overcome; increased by hypertension.

Equation: $\text{Ejection Fraction} = \frac{\text{SV}}{\text{EDV}}$

Regulation of Heart Rate

Heart rate is regulated by autonomic input, hormones, ions, and other factors.

• Autonomic Regulation: SNS increases HR, PSNS decreases HR (vagal tone).

• Hormones: Epinephrine, norepinephrine, thyroxine increase HR.

• Ions: Ca2+ and K+ imbalances affect HR and contractility.

• Other Factors: Age (fetus fastest), sex (female faster), exercise, temperature.

Clinical Note: Tachycardia (>100 bpm), bradycardia (<60 bpm), arrhythmias, and heart block may indicate pathology or adaptation.

Homeostatic Imbalances of Cardiac Output

Congestive heart failure (CHF) is a progressive condition where CO is insufficient for tissue needs.

• Causes: Coronary atherosclerosis, high blood pressure, myocardial infarcts, dilated cardiomyopathy.

• Left-side failure: Pulmonary congestion and edema.

• Right-side failure: Peripheral congestion and edema.

• Treatment: Diuretics, antihypertensives, digitalis.

Developmental Aspects of the Heart

The heart develops from mesoderm, forming a four-chambered organ by day 35 of embryonic development.

• Fetal Circulation: Foramen ovale and ductus arteriosus bypass pulmonary circuit; close at birth.

• Congenital Defects: Septal defects, patent ductus arteriosus, coarctation of aorta, tetralogy of Fallot.

Heart Function Throughout Life

Regular aerobic exercise improves heart health; age-related changes include valve thickening, reduced cardiac reserve, fibrosis, and increased risk of atherosclerosis.

• Diet: Major contributor to cardiovascular disease.