Cardiac Physiology

Chapter 9: Cardiac Physiology
Overview of the Circulatory System

  • Components:

    • Heart:

    • Functions as a pump that imparts pressure to blood, facilitating its flow to tissues.

    • Blood Vessels:

    • Serve as passageways for blood and are essential for nutrient exchange, including arteries (carrying blood away from the heart), veins (carrying blood toward the heart), and capillaries (sites of exchange).

    • Blood:

    • Acts as a transport medium for dissolved nutrients, oxygen (O2), carbon dioxide (CO2), hormones, and waste products throughout the body.

Circulation Phases

  • Pulmonary Circulation:

    • Involves blood flow between the heart (specifically, the right ventricle ejects deoxygenated blood) and the lungs (where gas exchange occurs, oxygenating the blood and removing CO2).

  • Systemic Circulation:

    • Involves blood flow between the heart (specifically, the left ventricle ejects oxygenated blood) and all other body systems and tissues, delivering O2 and nutrients and collecting CO2 and wastes.

Blood Flow through the Heart

  • Path of Blood to Systemic Circulation (Starting with Deoxygenated Blood):

    • Superior Vena Cava: Returns deoxygenated blood from the head and upper limbs to the right atrium.

    • Inferior Vena Cava: Returns deoxygenated blood from the trunk and legs to the right atrium.

    • Right Atrium: Receives deoxygenated blood from both vena cavae and contracts to push blood towards the right ventricle.

    • Right Atrioventricular Valve (Tricuspid Valve): Opens during atrial contraction and ventricular diastole to allow blood flow into the right ventricle, preventing backflow into the right atrium during ventricular systole.

    • Right Ventricle: Fills with deoxygenated blood and then contracts, pumping blood through the pulmonary semilunar valve.

    • Pulmonary Semilunar Valve: Opens during right ventricular systole to allow blood to enter the pulmonary artery, preventing backflow into the right ventricle during ventricular diastole.

    • Pulmonary Arteries: Carry deoxygenated blood from the right ventricle towards the lungs.

    • Lungs: Blood flows through pulmonary capillaries, where it releases CO2 and picks up O2.

    • Pulmonary Veins: Bring oxygenated blood back from the lungs to the left atrium.

    • Left Atrium: Receives oxygen-rich blood from the pulmonary veins and contracts to push blood towards the left ventricle.

    • Left Atrioventricular Valve (Bicuspid or Mitral Valve): Opens during atrial contraction and ventricular diastole to allow blood flow into the left ventricle, preventing backflow into the left atrium during ventricular systole.

    • Left Ventricle: Fills with oxygenated blood and then contracts powerfully, pumping blood out through the aortic semilunar valve.

    • Aortic Semilunar Valve: Opens during left ventricular systole to allow blood to enter the aorta, preventing backflow into the left ventricle during ventricular diastole.

    • Aorta: Distributes oxygenated blood to the rest of the body for systemic circulation.

  • Dual Pump Action:

    • Both ventricles simultaneously pump the same volume of blood with differing pressures due to their distinct roles:

    • Pulmonary circulation operates as a low-pressure (e.g., peak systolic pressure 25 mmHg), low-resistance system because the lungs offer less resistance to blood flow.

    • Systemic circulation operates as a high-pressure (e.g., peak systolic pressure 120 mmHg), high-resistance system, necessary to perfuse all body tissues.

    • Ventricular Thickness:

    • The left ventricle has a considerably thicker and more muscular wall (approximately three times thicker) than the right ventricle to accommodate pumping against much higher systemic pressures.

Heart Valves

  • Functions:

    • Unidirectional Flow: Ensures blood moves in only one direction through the heart chambers and into the great arteries, preventing reflux.

    • Passive Mechanism: Valves open and close solely due to pressure differences across them, not by active muscle contraction.

  • Types of Valves:

    • Atrioventricular (AV) Valves: Located between atria and ventricles, preventing backflow into the atria during ventricular contraction.

    • Right AV valve: Tricuspid Valve (three cusps).

    • Left AV valve: Bicuspid (Mitral) Valve (two cusps).

    • Semilunar Valves: Located at the base of the aorta and pulmonary artery, preventing backflow into the ventricles during ventricular relaxation.

    • Aortic Valve (three cusps).

    • Pulmonary Valve (three cusps).

Heart Wall Structure

  • Layers of Heart Wall (from outer to inner):

    • Epicardium: The outermost layer, also known as the visceral layer of the serous pericardium, providing protective covering and containing coronary vessels.

    • Myocardium:

    • The thickest middle layer, composed of cardiac muscle fibers arranged in complex, interlacing spiral bundles around the heart's circumference. This arrangement allows for a wringing motion that efficiently squeezes blood out of the ventricles.

    • Contraction initiates at the apex of the heart and propagates towards the base (top), effectively squeezing blood upward into the great arteries.

    • Endocardium: The innermost layer, a thin, smooth endothelial lining that covers the heart chambers and valves, minimizing friction with the blood.

Cardiac Muscle Cells

  • Types of Cardiac Muscle Cells:

    • Contractile Cells:

    • Account for approximately 99% of cardiac muscle cells.

    • Responsible for the mechanical work of pumping blood through direct contraction and relaxation.

    • Autorhythmic (Pacemaker) Cells:

    • Represent a small percentage (about 1%) of muscle cells, found primarily in the SA and AV nodes.

    • Possess the unique ability to spontaneously initiate and conduct action potentials throughout the heart, setting the heart rate.

    • Lack a stable resting membrane potential; instead, they exhibit a gradual depolarization known as the pacemaker potential.

Pacemaker Potential in Autorhythmic Cells (located primarily in SA and AV nodes)

  • Ionic Channels Contributing to Spontaneous Depolarization:

    • Funny Na+ Channels (If):

    • These are voltage-gated Na+ channels that open when the membrane hyperpolarizes at the end of the previous action potential (around -60 ext{ mV}).

    • Named ‘funny’ because they activate at more negative membrane potentials compared to typical voltage-gated channels, allowing a slow influx of Na+ that initiates depolarization.

    • K+ Channels:

    • Open during repolarization of the preceding action potential, but slowly close during the pacemaker potential, gradually decreasing K+ outflow and contributing to depolarization.

    • T-type Ca2+ Channels (Transient-type):

    • Briefly open as the membrane potential approaches the threshold (-50 ext{ to } -40 ext{ mV}), allowing a small influx of Ca2+ that helps depolarize the membrane to the firing threshold for an action potential.

    • L-type Ca2+ Channels (Long-lasting-type):

    • These are the primary channels responsible for the rapid depolarization phase of the action potential in autorhythmic cells. They open at threshold, allowing a large influx of Ca2+.

Excitable Cells and Signal Pathway (Cardiac Conduction System)

  • Pathway of Electrical Impulse:

    • Nodal Cells:

    • SA Node (Sinoatrial Node): Located in the right atrial wall, it acts as the primary pacemaker of the heart, generating impulses at the highest rate (typically 60-100 beats/min).

    • Signal spreads rapidly through the atrial myocardium from the SA node via interatrial and internodal pathways through gap junctions, causing atrial contraction.

    • AV Node (Atrioventricular Node): Located in the interatrial septum, it receives the impulse from the atria. It provides a crucial nodal delay (approximately 100 ms) to the impulse, allowing time for atrial contraction to complete and ventricles to fill before ventricular contraction begins.

    • Bundle of His (Atrioventricular Bundle): A short tract of conducting fibers that emerges from the AV node and penetrates the fibrous skeleton separating atria and ventricles. It is the only electrical connection between atria and ventricles.

    • This bundle then branches into the right and left bundle branches, which extend down along the interventricular septum to the apex of the heart.

    • Purkinje Fibers: Specialized conductive cells that rapidly distribute the impulse throughout the ventricular myocardium, ensuring synchronized ventricular contraction from the apex upwards.

  • Contraction Coordination:

    • Due to the AV nodal delay, atrial contraction completes before ventricular contraction initiates, ensuring efficient ventricular filling.

    • The rapid and widespread distribution of the impulse by Purkinje fibers ensures that all cardiac muscle cells in the ventricles contract as a nearly synchronized unit, maximizing pumping efficiency.

Cardiac Action Potential (in contractile cardiac muscle cells)

  • Phases of Cardiac Action Potential (longer than skeletal muscle or neuron APs):

    • Resting Membrane Potential: Approximately -90 ext{ mV}, which is more stable than pacemaker cells, primarily sustained by leaky K+ channels.

    • Phase 0: Depolarization: Initiated by the rapid opening of voltage-gated Na+ channels when an impulse arrives from adjacent cells, causing a swift influx of Na+ and a sharp rise in membrane potential to about +20 ext{ mV}.

    • Phase 1: Small Repolarization: Occurs very briefly due to the inactivation of Na+ channels and the opening of transient voltage-gated K+ channels (K+ outflow).

    • Phase 2: Plateau Phase:

    • This is a prolonged phase caused by two simultaneous events: Ca2+ influx through slow-opening L-type Ca2+ channels (opening around -40 ext{ mV}) and a decrease in K+ permeability (closing of some K+ channels).

    • This plateau lasts approximately 200-250 ms, prolonging the action potential and, consequently, the duration of cardiac muscle contraction, critical for efficient blood ejection.

    • Phase 3: Repolarization: Involves the inactivation of L-type Ca2+ channels and the opening of delayed rectifier K+ channels (more K+ outflow), leading to a rapid return of the membrane potential to its resting state.

Calcium-Induced Calcium Release (EC Coupling in Cardiac Muscle)

  • Mechanism of Contraction Initiation:

    • T-Tubules: The sarcolemma of cardiac muscle invaginates deeply into T-tubules, which contain modified DHP receptor L-type Ca2+ channels.

    • When an action potential propagates along the sarcolemma and into the T-tubules, these voltage-gated L-type Ca2+ channels open, allowing a small but crucial influx of Ca2+ from the extracellular fluid (ECF) into the cytosol.

    • Ryanodine Ca2+ Release Channels (RyR): The incoming ECF Ca2+ acts as a