The Heart

Overview of the Cardiovascular System

The cardiovascular system consists of the heart, blood vessels, and blood. The heart acts as a muscular pump, divided into right and left sides, each serving a distinct circulatory circuit. The right side pumps blood to the lungs (pulmonary circuit), while the left side pumps blood to the rest of the body (systemic circuit).

• Right atrium: Receives deoxygenated blood from the body.

• Left atrium: Receives oxygenated blood from the lungs.

• Right ventricle: Pumps deoxygenated blood to the lungs.

• Left ventricle: Pumps oxygenated blood to the body.

Memory clue: Atria receive blood; ventricles pump blood.

Location and Basic Anatomy

The heart is located in the mediastinum of the thoracic cavity, slightly left of center, within the pericardial cavity. It is cone-shaped, about the size of a fist (250–350 g), with the apex pointing toward the left hip and the base facing posteriorly.

• Atrioventricular sulcus: Groove between atria and ventricles.

• Interventricular sulcus: Groove between ventricles.

• Great vessels: Superior/inferior venae cavae, pulmonary trunk/arteries, pulmonary veins, aorta.

Pulmonary and Systemic Circuits

• Pulmonary circuit: Right ventricle → lungs → left atrium (lower pressure)

• Systemic circuit: Left ventricle → body → right atrium (higher pressure)

The atria also secrete atrial natriuretic peptide (ANP), which reduces sodium and water retention, helping to lower blood pressure.

Coronary Circulation

• Coronary arteries: Right and left coronary arteries branch from the ascending aorta.

• Left coronary artery: Branches into the anterior interventricular (LAD) and circumflex arteries.

• Coronary veins: Drain into the coronary sinus, which empties into the right atrium.

Clinical connection: Coronary artery plaques can cause ischemia (low oxygen); blocked blood flow may result in myocardial infarction (heart attack).

Heart Coverings and Wall Structure

The pericardium is the heart's protective sac, consisting of several layers:

• Epicardium: Outer layer.

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

• Endocardium: Inner endothelial lining.

Fibrous skeleton: Dense connective tissue supporting valves and septa; provides electrical insulation.

Clinical terms:

• Cardiac tamponade: Excess fluid in pericardial cavity compresses the heart.

• Myocarditis: Inflammation of myocardium, reducing function.

Memory: MYO = muscle, ENDO = inside, EPI = on top.

Chambers, Valves, and Blood Flow

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

• Semilunar valves do not have chordae tendineae.

Blood flow sequence: Body → venae cavae → right atrium → tricuspid valve → right ventricle → pulmonary valve → lungs → pulmonary veins → left atrium → mitral valve → left ventricle → aortic valve → aorta → body.

Memory clue: RIGHT → LUNGS; LEFT → BODY. Arteries carry blood AWAY from the heart.

Cardiac Muscle Properties

Cardiac muscle exhibits autorhythmicity, meaning it can generate its own rhythm without nervous input. Specialized pacemaker cells initiate action potentials, while contractile cells produce force.

• Striations: Organized contractile proteins for sliding-filament contraction.

• Short, branching cells: Typical of cardiac myocytes; usually one nucleus.

• Myoglobin & mitochondria: Support oxygen and energy needs.

• Intercalated discs: Connect adjacent cells via desmosomes (mechanical) and gap junctions (electrical).

• Functional syncytium: Heart cells contract as a coordinated unit.

Electrical terms:

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Return toward resting potential.

• Hyperpolarization: Membrane becomes more negative than resting.

Pacemaker vs. contractile cells:

• Pacemaker potential: HCN channels cause slow depolarization; calcium influx completes depolarization; potassium efflux repolarizes.

• Contractile cell action potential: Rapid sodium influx → initial repolarization → calcium plateau → repolarization. The plateau prevents rapid, repeated contractions.

Memory: Intercalated discs = desmosomes (hold together) + gap junctions (communicate).

Conduction System and ECG

The heart's electrical pathway ensures coordinated contraction:

• SA node → AV node → AV bundle → right/left bundle branches → Purkinje fibers

• SA node: Primary pacemaker (~60–70+ bpm), located in right atrium.

• AV node: Slower rate (~40–50 bpm); delay (~0.13 s) allows ventricular filling.

• Purkinje system: Paces at ~20 bpm; rapidly spreads signals through ventricles.

ECG (Electrocardiogram): Records electrical activity of the heart.

Memory: P = atria, QRS = ventricles contract, T = ventricles reset.

Heart Sounds and Cardiac Cycle

The heart produces two main sounds:

• S1 (LUB): Closure of AV valves (tricuspid and mitral).

• S2 (DUB): Closure of semilunar valves (pulmonary and aortic).

Heart murmur: Abnormal sound from turbulent blood flow, often due to defective valves or septal defects.

Cardiac cycle: One complete heartbeat, consisting of systole (contraction) and diastole (relaxation).

Memory: S1 = AV valves shut; S2 = semilunar valves shut.

Heart Rate and Cardiac Output

Key formulas for cardiac function:

• Stroke Volume (SV):

• Cardiac Output (CO):

• Ejection Fraction (EF):

Definitions:

• HR: Heart rate (beats per minute)

• SV: Blood pumped per beat

• CO: Blood pumped per minute

• EDV: End-diastolic volume (after filling)

• ESV: End-systolic volume (after contraction)

Example: If EDV = 120 mL, ESV = 50 mL, HR = 70 bpm:

• SV = 120 − 50 = 70 mL/beat

• CO = 70 × 70 = 4,900 mL/min = 4.9 L/min

• EF = 70 ÷ 120 × 100 = 58.3%

Normal EF: 50–65%

Factors Affecting Stroke Volume

Heart Rate Control

• Positive chronotropic: Increases HR

• Negative chronotropic: Decreases HR

• Positive inotropic: Increases contractile force

• Sympathetic nerves: Norepinephrine increases HR and contractility

• Parasympathetic (vagus) nerves: Acetylcholine decreases HR

• Hormones: Epinephrine/norepinephrine increase HR and contractility; aldosterone/ADH increase blood volume; ANP reduces blood volume

Quick Review and Practice

• Heart location: Mediastinum, pericardial cavity; apex toward left hip

• Heart coverings/layers: Fibrous + serous pericardium; epicardium, myocardium, endocardium

• Four chambers: Right atrium (RA), right ventricle (RV), left atrium (LA), left ventricle (LV)

• Four valves: Tricuspid, pulmonary, mitral, aortic

• Pulmonary vs. systemic: Right → lungs; left → body

• Cardiac muscle: Striated, branching, intercalated discs, autorhythmicity

• Conduction pathway: SA → AV → AV bundle → branches → Purkinje

• ECG waves: P = atria depolarize; QRS = ventricles depolarize; T = ventricles repolarize

• Heart sounds: S1 = AV close; S2 = SL close

• Cardiac output: CO = HR × SV; SV = EDV − ESV

• SV controls: Preload, contractility, afterload

• HR controls: SA node, autonomic nerves, hormones, temperature, electrolytes

Practice Questions (with answers):

1. The thickest heart wall layer is the myocardium.

2. The valve between the left atrium and ventricle is the mitral (bicuspid) valve.

3. The normal pacemaker is the SA node.

4. The ECG T wave represents ventricular repolarization.

5. S1 occurs when the AV valves close.

6. If HR = 80 beats/min and SV = 70 mL/beat, CO = 5.6 L/min.

Additional info: This guide is based on Chapter 17 of a standard Anatomy & Physiology textbook and covers all major learning objectives for the heart, including structure, function, conduction, and clinical relevance.