Cardiac Anatomy and Physiology: Structure, Function, and Regulation of the Heart

Module 17.1 Overview of the Heart

Introduction to Heart Anatomy

The heart is a muscular organ located in the thoracic cavity, responsible for pumping blood throughout the body. Understanding its structure and function is fundamental to the study of cardiovascular physiology.

• Location of the Heart: The heart is situated in the mediastinum, the central compartment of the thoracic cavity, between the lungs, posterior to the sternum, and superior to the diaphragm. Its base is directed upward and to the right, while the apex points downward and to the left.

• Basic Surface Anatomy: The heart consists of four chambers: two atria (right and left) and two ventricles (right and left). Major vessels include arteries (carry blood away from the heart), veins (return blood to the heart), and capillaries (sites of exchange).

• Double Pump Function: The heart acts as a double pump: the right side pumps deoxygenated blood to the lungs (pulmonary circuit), and the left side pumps oxygenated blood to the systemic circulation. This separation is vital for efficient oxygenation of blood.

• Major Vessels: Key vessels include the aorta, pulmonary arteries and veins, superior and inferior vena cava, and coronary arteries and veins.

• Other Structures: The heart is surrounded by the pericardium and contains valves (atrioventricular and semilunar) to ensure unidirectional blood flow.

Additional info: The pericardium consists of fibrous and serous layers, providing protection and reducing friction.

Module 17.2 Heart Anatomy and Blood Flow Pathway

Pericardium, Heart Wall, and Blood Flow

The heart's structure includes protective coverings, specialized tissues, and a precise pathway for blood flow, ensuring efficient circulation.

• Pericardium: The pericardium is a double-walled sac surrounding the heart, consisting of:

  • Fibrous pericardium (outer layer)

  • Serous pericardium (parietal and visceral layers)

  • Pericardial cavity (contains lubricating fluid)

• Heart Wall Layers:

  • Epicardium: Outer layer (visceral pericardium)

  • Myocardium: Thick, muscular middle layer responsible for contraction

  • Endocardium: Inner endothelial lining

• Chambers and Valves:

  • Right atrium, right ventricle, left atrium, left ventricle

  • Atrioventricular valves: tricuspid (right), bicuspid/mitral (left)

  • Semilunar valves: pulmonary (right), aortic (left)

• Blood Flow Pathway:

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

  2. Passes through tricuspid valve to right ventricle

  3. Pumped through pulmonary valve into pulmonary trunk and arteries to lungs

  4. Oxygenated blood returns via pulmonary veins to left atrium

  5. Passes through mitral valve to left ventricle

  6. Pumped through aortic valve into aorta and systemic circulation

• Coronary Circulation: Supplies blood to the heart muscle itself via coronary arteries and veins.

Example: The left ventricle has the thickest myocardium, as it must generate high pressure to pump blood throughout the systemic circuit.

Module 17.3 Cardiac Muscle Tissue Anatomy and Electrophysiology

Structure and Function of Cardiac Muscle

Cardiac muscle tissue is specialized for continuous, rhythmic contraction and is distinct from skeletal and smooth muscle.

• Cardiac Muscle Features:

  • Striated, branched cells connected by intercalated discs

  • Contains gap junctions for electrical coupling and desmosomes for mechanical strength

  • Involuntary control, autorhythmicity (can generate its own action potentials)

• Comparison to Skeletal Muscle:

  • Both are striated, but cardiac muscle is involuntary and has unique intercalated discs

  • Skeletal muscle is voluntary and multinucleated

• Electrophysiology:

  • Cardiac action potentials involve rapid depolarization (Na+ influx), plateau phase (Ca2+ influx), and repolarization (K+ efflux)

  • Pacemaker cells in the sinoatrial (SA) node initiate the heartbeat

• Action Potential Phases:

  1. Rapid depolarization

  2. Plateau phase

  3. Repolarization

Example: The plateau phase in cardiac muscle action potentials prevents tetanus, ensuring rhythmic contractions.

Additional info: The cardiac conduction system includes the SA node, AV node, bundle of His, bundle branches, and Purkinje fibers.

Module 17.4 Mechanical Physiology of the Heart: The Cardiac Cycle

Phases and Events of the Cardiac Cycle

The cardiac cycle describes the sequence of mechanical and electrical events that occur during one heartbeat, ensuring coordinated blood flow.

• Phases of the Cardiac Cycle:

  1. Atrial systole: Atria contract, pushing blood into ventricles

  2. Ventricular systole: Ventricles contract, ejecting blood into pulmonary trunk and aorta

  3. Diastole: Chambers relax and fill with blood

• Heart Sounds: "Lub" (S1) is closure of AV valves; "Dub" (S2) is closure of semilunar valves

• Electrocardiogram (ECG) Waves:

  • P wave: atrial depolarization

  • QRS complex: ventricular depolarization

  • T wave: ventricular repolarization

• Pressure Changes: Pressure in the heart chambers and major vessels changes throughout the cycle, driving blood flow and valve operation.

Example: During ventricular systole, the left ventricle generates enough pressure to open the aortic valve and propel blood into the systemic circulation.

Module 17.5 Cardiac Output and Regulation

Measuring and Regulating Cardiac Output

Cardiac output is a key measure of heart function, reflecting the volume of blood pumped by each ventricle per minute. It is regulated by intrinsic and extrinsic mechanisms.

• Cardiac Output (CO): Defined as the product of heart rate (HR) and stroke volume (SV): $CO = HR \times SV$

• Stroke Volume (SV): The amount of blood ejected by a ventricle in one contraction. Calculated as: $SV = EDV - ESV$ where EDV = end-diastolic volume, ESV = end-systolic volume

• Factors Affecting Cardiac Output:

  • Preload (venous return)

  • Afterload (resistance in arteries)

  • Contractility (strength of contraction)

  • Heart rate (autonomic nervous system influence)

• Regulation:

  • Intrinsic: Frank-Starling law (increased stretch leads to increased force of contraction)

  • Extrinsic: Sympathetic and parasympathetic nervous system, hormones (e.g., epinephrine)

Example: During exercise, sympathetic stimulation increases heart rate and contractility, raising cardiac output to meet metabolic demands.

Summary Table: Key Cardiac Parameters

Parameter	Definition	Formula
Cardiac Output (CO)	Volume of blood pumped per minute	$CO = HR \times SV$
Stroke Volume (SV)	Volume of blood ejected per beat	$SV = EDV - ESV$
End-Diastolic Volume (EDV)	Volume in ventricle at end of filling	Measured in mL
End-Systolic Volume (ESV)	Volume in ventricle after contraction	Measured in mL