Chapter 18: The Heart

Learning Objectives

• Describe the major functions of the cardiovascular system.

• Locate and identify the position of the heart in the thoracic cavity.

• Identify major anatomical structures of the heart, including its layers, chambers, valves, and associated muscles.

• Describe the flow of blood through the heart and explain the importance of one-way flow and the role of valves.

• Differentiate between the oxygen content of blood in the right and left sides of the heart, and explain the physiological importance of the pulmonary and systemic circulations (including coronary circulation).

• Diagram the cardiac conduction system, state the function of each component, and explain the electrical events represented by each waveform in a normal electrocardiogram (ECG or EKG).

• Compare and contrast the cardiac muscle action potential and contraction duration with that of skeletal muscle.

• Explain the physiology of each event/phase of the cardiac cycle, including pressure and volume changes, valve function, heart sounds, and the relationship between electrical and mechanical events.

• Define and apply terms such as heart rate, stroke volume, end diastolic volume (EDV), end systolic volume (ESV), cardiac output, preload, afterload, venous return, and the Frank-Starling Law.

• Explain how various factors (autonomic nervous system, hormones, electrolytes, temperature) affect heart function.

Gross Anatomy of the Heart

Size, Location, and Orientation of the Heart

The heart is a muscular organ located in the thoracic cavity, specifically within the mediastinum, between the lungs. It is roughly the size of a fist and is oriented obliquely, with the apex pointing toward the left hip and the base toward the right shoulder.

• Aorta: The main artery carrying oxygenated blood from the left ventricle to the body.

• Parietal pleura: The outer membrane lining the thoracic cavity.

• Pericardium: The double-walled sac enclosing the heart.

• Apex of heart: The pointed end of the heart, directed downward and to the left.

Example: The heart's position allows it to efficiently pump blood to both the lungs and the systemic circulation.

Coverings of the Heart

The heart is enclosed by the pericardium, which consists of two main layers:

• Fibrous pericardium: Tough, outer layer that protects the heart and anchors it to surrounding structures.

• Serous pericardium: Thinner, inner layer composed of two sublayers:

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

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

• Pericardial cavity: Space between the parietal and visceral layers, filled with serous fluid to reduce friction.

Clinical terms:

• Pericarditis: Inflammation of the pericardium.

• Cardiac tamponade: Accumulation of excess fluid in the pericardial cavity, which can compress the heart.

Layers of the Heart Wall

• Epicardium: The outermost layer (visceral layer of serous pericardium).

• Myocardium: The thick, muscular middle layer responsible for contraction and pumping action.

• Endocardium: The innermost layer, lining the heart chambers and covering the valves.

Heart Chambers and Valves

Chambers of the Heart

• Atria (right and left): Upper chambers that receive blood returning to the heart.

• Ventricles (right and left): Lower chambers that pump blood out of the heart.

Heart Valves

Valves ensure one-way flow of blood through the heart:

• Atrioventricular (AV) valves: Located between atria and ventricles (tricuspid on right, bicuspid/mitral on left).

• Semilunar (SL) valves: Located at the exits of the ventricles (pulmonary and aortic valves).

Valve Disorders:

• Incompetent (insufficient) valve: Fails to close completely, allowing backflow of blood.

• Valvular stenosis: Fails to open completely, restricting blood flow and increasing cardiac workload.

Blood Flow Through the Heart

Pathway of Blood

Blood flows through the heart in a specific sequence, ensuring separation of oxygenated and deoxygenated blood:

1. Deoxygenated blood enters the right atrium from the superior and inferior vena cava.

2. Passes through the tricuspid valve into the right ventricle.

3. Pumped through the pulmonary valve into the pulmonary artery to the lungs (pulmonary circulation).

4. Oxygenated blood returns via pulmonary veins to the left atrium.

5. Passes through the mitral (bicuspid) valve into the left ventricle.

6. Pumped through the aortic valve into the aorta and systemic circulation.

Importance: One-way valves prevent backflow and ensure efficient circulation.

Pulmonary vs. Systemic Circulation

• Pulmonary circulation: Right side of the heart pumps deoxygenated blood to the lungs for oxygenation.

• Systemic circulation: Left side of the heart pumps oxygenated blood to the rest of the body.

Coronary circulation: Supplies blood to the heart muscle itself, primarily during relaxation (diastole).

Cardiac Muscle and Conduction System

Cardiac Muscle Tissue

• Cardiac muscle fibers: Striated, branched, and interconnected by intercalated discs, forming a functional syncytium (cells function as a unit).

• Comparison to skeletal muscle: Cardiac muscle has longer action potentials and refractory periods, preventing tetanic contractions.

Intrinsic Cardiac Conduction System

The heart's electrical system initiates and distributes impulses, coordinating contraction:

• Sinoatrial (SA) node: Pacemaker; initiates impulses (60–100 bpm).

• Atrioventricular (AV) node: Delays impulse, allowing atrial contraction before ventricular contraction (40–60 bpm if acting as pacemaker).

• Bundle of His, bundle branches, Purkinje fibers: Distribute impulses through ventricles (Purkinje: 30–40 bpm if acting as pacemaker).

Clinical terms:

• Arrhythmias: Irregular heart rhythms.

• Fibrillation: Rapid, uncoordinated contractions; can be fatal if not corrected.

• Heart block: Impaired conduction between atria and ventricles; may require artificial pacemaker.

Electrophysiology of the Heart

Action Potentials

• Cardiac muscle action potential: Longer duration than skeletal muscle, with a plateau phase due to calcium influx.

• Prevents tetanus: Ensures rhythmic contraction and relaxation.

Electrocardiogram (ECG/EKG)

An ECG records the electrical activity of the heart:

• P wave: Atrial depolarization (precedes atrial contraction).

• QRS complex: Ventricular depolarization (precedes ventricular contraction).

• T wave: Ventricular repolarization (precedes ventricular relaxation).

Depolarization always precedes contraction; repolarization precedes relaxation.

Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

• Ventricular filling: Blood flows from atria to ventricles (AV valves open).

• Isovolumetric contraction: Ventricles contract with all valves closed, increasing pressure.

• Ventricular ejection: Semilunar valves open; blood is pumped into arteries.

• Isovolumetric relaxation: Ventricles relax; all valves closed.

Heart Sounds

• "Lub" (S1): Closure of AV valves at the beginning of ventricular systole.

• "Dub" (S2): Closure of semilunar valves at the beginning of ventricular diastole.

Cardiac Output and Regulation

Cardiac Output (CO)

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

• Formula:

• HR: Heart rate (beats per minute)

• SV: Stroke volume (ml per beat)

Cardiac reserve: The difference between resting and maximal cardiac output.

Regulation of Cardiac Output

• Preload: Degree of stretch of cardiac muscle cells before contraction (related to venous return).

• Frank-Starling Law: The heart increases its force of contraction in response to increased venous return (greater stretch leads to stronger contraction).

• Contractility: Strength of contraction at a given muscle length; increased by sympathetic stimulation and certain hormones.

• Afterload: Pressure the ventricles must overcome to eject blood (mainly determined by arterial blood pressure).

Factors Affecting Heart Function

• Autonomic nervous system: Sympathetic stimulation increases heart rate and contractility; parasympathetic decreases heart rate.

• Hormones: Epinephrine, norepinephrine, and thyroid hormone increase heart rate and contractility.

• Electrolytes: Potassium and calcium levels affect electrical activity and contraction strength.

• Body temperature: Increased temperature raises heart rate; decreased temperature lowers it.

Homeostatic Imbalances

• Ischemia: Reduced blood flow to heart tissue.

• Hypoxia: Reduced oxygen supply.

• Angina pectoris: Chest pain due to myocardial ischemia.

• Myocardial infarction (MI): Heart attack; prolonged coronary blockage causing cell death.

• Congestive heart failure (CHF): Progressive weakening of the heart, leading to inadequate circulation.

Key Terms Table

Additional info: These notes are based on standard college-level Anatomy & Physiology content and expand on the provided slides and text for clarity and completeness.