18.1 The Heart: Structure and Chambers

Overview of Heart Chambers and Circulation

The heart is a muscular organ responsible for pumping blood through the pulmonary and systemic circuits. It consists of four chambers and associated valves that ensure unidirectional blood flow.

• Right Side of the Heart: Receives oxygen-poor blood from the body and pumps it to the lungs for oxygenation.

• Left Side of the Heart: Receives oxygen-rich blood from the lungs and pumps it to the rest of the body.

• Chambers: Two receiving chambers (right and left atria) and two pumping chambers (right and left ventricles).

Size, Location, and Orientation

• Size: The heart is about the size of a fist and weighs 250-300 grams.

• Location: Found in the mediastinum, two-thirds left of the midline.

• Orientation: The base is directed toward the right shoulder; the apex points toward the left hip.

Coverings of the Heart

• Pericardium: A double-walled sac that encloses the heart.

• Fibrous Pericardium: Protects, anchors, and prevents overfilling.

• Serous Pericardium: Contains parietal and visceral layers, with pericardial fluid between them to reduce friction.

Layers of the Heart Wall

• Epicardium: Visceral layer of the serous pericardium.

• Myocardium: Cardiac muscle forming the bulk of the heart.

• Endocardium: Thin layer lining the heart chambers and covering the valves.

Chambers and Associated Great Vessels

• Interatrial Septum: Separates the atria.

• Interventricular Septum: Separates the ventricles.

• Right Atrium: Receives blood from the superior and inferior vena cava and coronary sinus.

• Left Atrium: Receives blood from the pulmonary veins.

• Right Ventricle: Pumps blood into the pulmonary trunk.

• Left Ventricle: Pumps blood into the aorta.

18.2 Heart Valves and Blood Flow

Heart Valves

Valves ensure unidirectional blood flow through the heart chambers.

• Atrioventricular (AV) Valves: Tricuspid (right) and mitral (left) valves prevent backflow into the atria.

• Semilunar (SL) Valves: Pulmonary and aortic valves prevent backflow into the ventricles.

• Chordae Tendineae: Anchor AV valves to papillary muscles, preventing valve inversion.

Blood Flow Sequence

• Blood flows from atria to ventricles, then to either the lungs or the rest of the body.

• Right side pumps to pulmonary circuit; left side pumps to systemic circuit.

• Equal volumes are pumped by both sides, but the left ventricle works harder due to higher resistance.

Coronary Circulation

The heart receives its own blood supply via the coronary arteries and veins.

• Coronary Arteries: Branch from the aorta and supply oxygenated blood to the myocardium.

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

18.4 Cardiac Muscle Anatomy and Physiology

Microscopic Anatomy

Cardiac muscle cells are striated, branched, and interconnected, forming a functional syncytium.

• Intercalated Discs: Specialized junctions that connect cardiac muscle fibers, allowing rapid transmission of electrical impulses.

• Cardiac Muscle Fibers: Short, branched, and contain many mitochondria for energy production.

• Endomysium: Loose connective tissue matrix supporting the muscle cells.

Comparison to Skeletal Muscle

• Cardiac muscle cells are self-excitable and can initiate their own depolarization.

• All cells contract as a unit due to gap junctions.

• Cardiac muscle has a longer refractory period than skeletal muscle, preventing tetanic contractions.

• Cardiac muscle relies more on aerobic metabolism and has more mitochondria than skeletal muscle.

18.5 Pacemaker Cells and the Intrinsic Conduction System

Setting the Heart's Rhythm

The heart's rhythm is set by pacemaker cells in the intrinsic conduction system, which coordinate contraction.

• Sinoatrial (SA) Node: Primary pacemaker, located in the right atrium.

• Atrioventricular (AV) Node: Delays impulse, allowing atria to contract before ventricles.

• Bundle of His, Bundle Branches, Purkinje Fibers: Conduct impulses through the ventricles.

Electrocardiography (ECG)

• ECG records electrical activity of the heart.

• P wave: Atrial depolarization.

• QRS complex: Ventricular depolarization.

• T wave: Ventricular repolarization.

18.6 Cardiac Cycle and Heart Sounds

Phases of the Cardiac Cycle

The cardiac cycle consists of systole (contraction) and diastole (relaxation) of the heart chambers.

• Systole: Contraction phase, blood is ejected from the chambers.

• Diastole: Relaxation phase, chambers fill with blood.

Heart Sounds

• First Heart Sound (S1): Closure of AV valves.

• Second Heart Sound (S2): Closure of semilunar valves.

• Murmurs: Abnormal heart sounds due to turbulent blood flow.

18.7 Stroke Volume and Cardiac Output

Definitions and Formulas

• Cardiac Output (CO): Amount of blood pumped by each ventricle per minute.

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

• Heart Rate (HR): Number of beats per minute.

Formula:

Average CO = 70 mL/beat × 75 beats/min = 5250 mL/min

Regulation of Stroke Volume

• Preload: Degree of stretch of cardiac muscle before contraction.

• Contractility: Strength of contraction at a given muscle length.

• Afterload: Pressure the ventricles must overcome to eject blood.

Regulation of Heart Rate

• Sympathetic Stimulation: Increases heart rate and contractility.

• Parasympathetic Stimulation: Decreases heart rate.

• Hormones: Epinephrine and thyroxine increase heart rate.

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

Homeostatic Imbalances

• Congestive Heart Failure: Occurs when the heart is unable to pump efficiently, leading to inadequate tissue perfusion.

• Pulmonary Congestion: Left side failure; blood backs up in the lungs.

• Peripheral Congestion: Right side failure; blood pools in body organs.

Developmental Aspects and Heart Function Throughout Life

Before Birth

• The heart begins as a pair of endothelial tubes that fuse to form a single heart tube with four bulges representing the chambers.

Heart Function Throughout Life

• Exercise: Enlarges and strengthens the heart.

• Sclerosis: Thickening of valve flaps over time due to constant pressure.

• Decline in Cardiac Reserve: Occurs with age due to reduced sympathetic stimulation.

• Fibrosis: May occur in the conduction system, leading to arrhythmias.

• Atherosclerosis: Fatty deposits in vessel walls, increasing risk of heart disease.

Example:

During exercise, sympathetic stimulation increases heart rate and contractility, allowing the heart to meet increased oxygen demands.

Additional info: The notes have been expanded with definitions, examples, and academic context for clarity and completeness.