Cardiovascular System 2

Chapter 18 Part B: The Cardiovascular System

18.3 Pathway of Blood Through Heart

  • Equal volumes of blood pumped to both pulmonary and systemic circuits.

  • Circuits:

    • Pulmonary circuit:

      • Characteristics: Short, low-pressure circulation.

    • Systemic circuit:

      • Characteristics: Long, high-friction circulation.

  • Ventricular Wall Thickness:

    • Left ventricle walls are 3× thicker than those of the right ventricle.

    • This structural difference allows the left ventricle to pump blood with greater force than the right.

Coronary Circulation

  • Definition: Functional blood supply to the heart muscle itself, which represents the shortest circulation in the body.

  • Blood Supply:

    • Blood is delivered to cardiac tissues when the heart is relaxed.

    • The left ventricle receives the majority of the coronary blood supply.

    • The heart utilizes 1/20th of the body’s total blood flow.

Coronary Arteries

  • Overview of coronary arteries:

    • Arise from the base of the aorta.

    • Supply arterial blood to the heart and encircle it in the coronary sulcus.

    • The branching pattern of coronary arteries varies among individuals.

    • They contain many anastomoses (junctions), which provide additional blood delivery pathways.

  • Left Coronary Artery:

    • Supplies blood to:

      • Interventricular septum.

      • Anterior ventricular walls.

      • Left atrium.

      • Posterior wall of the left ventricle.

    • Major branches include:

      • Anterior interventricular artery

      • Circumflex artery

  • Right Coronary Artery:

    • Supplies blood to:

      • Right atrium.

      • Most of the right ventricle.

    • Major branches include:

      • Right marginal artery

      • Posterior interventricular artery

Coronary Veins

  • Function: Collect blood from capillary beds in the heart muscle.

  • Coronary Sinus:

    • Empties into the right atrium;

    • Formed by the merging of cardiac veins:

      • Great cardiac vein of anterior interventricular sulcus

      • Middle cardiac vein in posterior interventricular sulcus

      • Small cardiac vein from inferior margin.

    • Several anterior cardiac veins empty directly into the right atrium anteriorly.

Clinical – Homeostatic Imbalance 18.3

  • Angina Pectoris:

    • Characterized by thoracic pain caused by a temporary deficiency in blood delivery to the myocardium.

    • Causes weakening of myocardial cells.

  • Myocardial Infarction (Heart Attack):

    • Result of prolonged coronary artery blockage.

    • Leads to areas of cell death that are repaired with noncontractile scar tissue.

18.4 Cardiac Muscle Fibers

  • Cardiac Muscle Characteristics:

    • Striated, short, branched, and interconnected fibers.

    • Typically possess one central nucleus (two at most).

    • Contain large mitochondria constituting 25–35% of cell volume.

  • Structural Features:

    • Organized into sarcomeres containing:

      • Z discs

      • A bands

      • I bands

    • T tubules are wider but less numerous compared to skeletal muscle.

    • The sarcoplasmic reticulum (SR) is simpler than in skeletal muscle, lacking triads.

Intercalated Discs

  • Definition and Function:

    • Special connecting junctions between cardiac cells.

  • Types of Junctions:

    • Desmosomes:

      • Function: Hold cardiac cells together, preventing separation during contraction.

    • Gap Junctions:

      • Function: Permit ions to pass from cell to cell, electrically coupling adjacent cells.

  • Functional Syncytium:

    • Concept whereby cardiac muscle cells function as a single coordinated unit due to intercalated discs.

Endomysium

  • Description: Connective tissue matrix residing within intercellular spaces (between cardiac cells).

  • Includes:

    • Numerous capillaries for blood supply.

    • Connects cardiac muscle to the cardiac skeleton, allowing for structural support during contractions.

Physiology of Skeletal vs. Cardiac Muscle

  • Commonality with Skeletal Muscle:

    • Depolarization Wave: Travels through T tubules causing the sarcoplasmic reticulum (SR) to release calcium ions ( ext{Ca}^{2+}).

    • Excitation-contraction coupling:

    • Process:

      • Ca2+ binds to troponin, which causes myosin engagement with actin.

    • Some cardiac muscle cells are self-excitable.

  • Types of Myocytes:

    • Contractile Cells: Responsible for the contraction of the heart.

    • Pacemaker Cells: Noncontractile cells that spontaneously depolarize to initiate depolarization of the entire heart without needing nervous system stimulation, unlike skeletal muscle fibers.

Heart Contraction Mechanism

  • Synchronization of Contraction: Heart functions as a unit, meaning all parts contract or none do, ensuring effective pumping.

  • Calcium Influx Mechanism:

    • Involves both extracellular fluid and SR calcium release:

    • Depolarization opens slow calcium channels in the sarcolemma.

    • This allows Ca2+ to enter, prompting further release from the SR.

  • Comparison with Skeletal Muscle:

    • Skeletal muscle fibers contract independently without a synchronized mechanism.

Tetanic Contractions

  • Refractory Periods:

    • Cardiac muscles have a longer absolute refractory period compared to skeletal muscle fibers.

    • This period is almost as long as the contraction itself, helping prevent tetanus and allowing relaxation for proper filling of the heart.

Energy Dependence of Cardiac Muscle

  • Aerobic Respiration:

    • The heart relies almost exclusively on aerobic metabolism due to a high number of mitochondria, leading to a greater dependence on ext{O}_2.

  • Comparison with Skeletal Muscle:

    • Skeletal muscle can undergo fermentation in the absence of oxygen.

  • Fuel Sources:

    • Both cardiac and skeletal muscle can utilize various fuel sources; however, cardiac muscle is more adversely affected by oxygen deficiency compared to a deficiency of any one type of fuel source.