Chapter 17: The Cardiovascular System I – The Heart

Module 17.1: Overview of the Heart

The heart is a muscular organ located in the thoracic cavity, responsible for pumping blood throughout the body. Understanding its position, structure, and function is essential for comprehending cardiovascular physiology.

• Position of the Heart: The heart is situated in the mediastinum, the central compartment of the thoracic cavity, between the lungs. The apex is the pointed end directed inferiorly and to the left, while the base is the broad, superior portion.

• Surface Anatomy: The heart has four chambers: two atria (upper chambers) and two ventricles (lower chambers). Auricles are ear-like extensions of the atria. The atrioventricular sulcus separates the atria from the ventricles, and the interventricular sulcus marks the boundary between the right and left ventricles.

• Double Pump Function: The heart acts as a double pump, with the right side pumping blood to the lungs (pulmonary circuit) and the left side pumping blood to the rest of the body (systemic circuit).

• Blood Vessels: Arteries carry blood away from the heart, veins return blood to the heart, and capillaries are small vessels where exchange occurs.

Example: The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery, while the left ventricle pumps oxygenated blood to the body via the aorta.

Module 17.2: Heart Anatomy and Blood Flow Pathway

This section covers the structural components of the heart and the pathway of blood flow through its chambers and associated vessels.

• Pericardium: The pericardium is a double-walled sac surrounding the heart. It consists of:

  • Fibrous pericardium: Tough outer layer.

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

  • Visceral pericardium (epicardium): Covers the heart surface.

  • Pericardial fluid: Lubricates the space between the layers.

• Heart Wall Layers:

  • Myocardium: Thick, muscular middle layer composed of myocytes (cardiac muscle cells).

  • Endocardium: Inner lining of the heart chambers.

  • Fibrous skeleton: Dense connective tissue supporting the heart valves and muscle attachment.

• Great Vessels:

  • Aorta: Main systemic artery carrying oxygenated blood from the left ventricle.

  • Inferior and Superior Vena Cava: Large veins returning deoxygenated blood to the right atrium.

  • Pulmonary trunk and arteries: Carry deoxygenated blood from the right ventricle to the lungs (pulmonary circuit).

• Heart Chambers: Right and left atria (receive blood), right and left ventricles (pump blood out). Auricles are small, muscular pouches on the atria.

• Heart Valves:

  • Left atrioventricular (bicuspid/mitral) valve

  • Right atrioventricular (tricuspid) valve

  • Pulmonary semilunar valve

  • Aortic semilunar valve

  Function: Ensure unidirectional blood flow and prevent backflow.

• Other Structures: Interatrial septum (separates atria), interventricular septum (separates ventricles), papillary muscles and chordae tendineae (anchor AV valves), trabeculae carneae (muscular ridges in ventricles).

• Blood Flow Pathway: Blood flows through the heart in a specific sequence, passing through the right atrium, right ventricle, pulmonary arteries, lungs, pulmonary veins, left atrium, left ventricle, and aorta.

• Coronary Circulation: Coronary arteries supply oxygenated blood to the heart muscle; cardiac veins drain deoxygenated blood from the heart muscle.

Example: The left atrium receives oxygenated blood from the pulmonary veins and passes it to the left ventricle.

Module 17.3: Cardiac Muscle Tissue Anatomy and Electrophysiology

Cardiac muscle tissue has unique structural and electrical properties that enable the heart to contract rhythmically and efficiently.

• Histology: Cardiac muscle cells (myocytes) are striated, branched, and connected by intercalated discs. They differ from skeletal muscle in their ability to contract automatically.

• Cell Types:

  • Pacemaker cells: Initiate and regulate the heartbeat (autorhythmicity).

  • Contractile cells: Produce the force of contraction.

• Action Potential Phases:

  • Rapid depolarization (influx of Na+ via voltage-gated sodium channels)

  • Plateau phase (influx of Ca2+ via calcium ion channels, balanced by K+ efflux)

  • Repolarization (efflux of K+ via potassium ion channels)

  Importance: The plateau phase prolongs contraction, preventing tetanus.

• Channel Types: Voltage-gated sodium, calcium, potassium, and HCN (hyperpolarization-activated cyclic nucleotide-gated) channels.

• Action Potential Generation: Pacemaker cells generate spontaneous action potentials; contractile cells require stimulation. Skeletal muscle cells do not have autorhythmicity.

• Cardiac Conduction System: Includes the sinoatrial (SA) node, atrioventricular (AV) node, bundle branches, and Purkinje fibers. This system coordinates the heartbeat.

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

  • P wave: Atrial depolarization

  • QRS complex: Ventricular depolarization

  • T wave: Ventricular repolarization

Example: The SA node acts as the heart's natural pacemaker, initiating each heartbeat.

Module 17.4: Mechanical Physiology of the Heart – The Cardiac Cycle

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

• Phases of the Cardiac Cycle:

  • Systole: Contraction phase (chamber empties)

  • Diastole: Relaxation phase (chamber fills)

  Heartbeat: One complete cycle of contraction and relaxation.

• Valve Function: Opening and closing of heart valves are driven by pressure changes in the chambers.

• ECG and Heart Sounds: Specific ECG waves correspond to phases of the cardiac cycle and heart sounds ("lub-dub").

  • First heart sound (S1): Closure of AV valves (after QRS complex)

  • Second heart sound (S2): Closure of semilunar valves (after T wave)

• Pressure and Volume Changes: The left and right ventricles experience similar phases but at different pressures. The aorta receives blood during ventricular systole.

• Key Volumes:

  • End-diastolic volume (EDV): Volume of blood in a ventricle at the end of filling (diastole).

  • End-systolic volume (ESV): Volume of blood remaining after contraction (systole).

Example: During ventricular systole, the pressure in the ventricle rises, causing the semilunar valves to open and blood to be ejected into the aorta or pulmonary artery.

Module 17.5: Cardiac Output and Regulation

Cardiac output is a measure of the heart's efficiency in pumping blood and is regulated by several physiological factors.

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

  • Formula:

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

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

  • Stroke Volume Calculation:

• Regulatory Factors:

  • Preload: Degree of stretch of cardiac muscle before contraction (related to EDV).

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

  • Contractility: Strength of contraction at a given preload.

• Frank-Starling Law: The greater the preload (stretch), the greater the force of contraction, up to a physiological limit.

• Inotropic and Chronotropic Agents:

  • Positive inotropic: Increase contractility (e.g., sympathetic stimulation)

  • Negative inotropic: Decrease contractility

  • Positive chronotropic: Increase heart rate

  • Negative chronotropic: Decrease heart rate

  Effect: These agents alter cardiac output by changing SV and/or HR.

• Predicting Changes: Increases in HR or SV will increase CO; decreases will reduce CO.

Example: During exercise, both heart rate and stroke volume increase, leading to a higher cardiac output.

Table: Summary of Key Cardiac Volumes and Formulas

Additional info: For a more detailed understanding, refer to Figures 17.8, 17.19, and 17.20 in your textbook, which illustrate blood flow, the cardiac cycle, and pressure-volume relationships.