Anatomy & Physiology: The Cardiovascular System – The Heart (Chapter 17 Part A Study Notes)

The Pulmonary and Systemic Circuits

Overview of Heart Function

The heart acts as a dual pump, maintaining two distinct circulatory pathways: the pulmonary and systemic circuits. These circuits ensure the separation and efficient transport of oxygen-poor and oxygen-rich blood throughout the body.

• Right Side of the Heart: Receives oxygen-poor blood from body tissues and pumps it to the lungs via the pulmonary circuit for gas exchange (removal of CO2, uptake of O2).

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

Chambers and Their Roles

• Receiving Chambers:

  • Right Atrium: Receives blood returning from the systemic circuit.

  • Left Atrium: Receives blood returning from the pulmonary circuit.

• Pumping Chambers:

  • Right Ventricle: Pumps blood through the pulmonary circuit.

  • Left Ventricle: Pumps blood through the systemic circuit.

Diagrammatic Representation

Figure 17.1 illustrates the flow of blood through the systemic and pulmonary circuits, highlighting the separation of oxygenated and deoxygenated blood.

Size, Location, and Orientation of the Heart

Physical Characteristics

The heart is a muscular organ approximately the size of a fist and weighs less than 1 pound.

• Location:

  • Situated in the mediastinum between the second rib and fifth intercostal space.

  • Lies on the superior surface of the diaphragm.

  • Two-thirds of the heart is positioned to the left of the midsternal line.

  • Located anterior to the vertebral column and posterior to the sternum.

Orientation Details

• Base: The posterior surface leans toward the right shoulder.

• Apex: Points toward the left hip.

• Apical Impulse: Can be palpated between the fifth and sixth ribs, just below the left nipple.

Visual Aids

Figures 17.2a, 17.2b, and 17.2c show the anatomical position of the heart within the thoracic cavity and its relationship to surrounding structures.

Coverings of the Heart

Pericardium Structure

The heart is enclosed in a double-walled sac called the pericardium, which consists of two main layers:

• Superficial Fibrous Pericardium: Protects, anchors the heart to surrounding structures, and prevents overfilling.

• Deep Serous Pericardium:

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

  • Visceral Layer (Epicardium): Covers the external surface of the heart.

• The two layers are separated by a fluid-filled pericardial cavity, which reduces friction during heart movements.

Clinical Note: Pericarditis

• Pericarditis: Inflammation of the pericardium, causing roughened membrane surfaces and a pericardial friction rub (creaking sound).

• Cardiac Tamponade: Excess fluid accumulation in the pericardial space, compressing the heart and impairing its pumping ability. Treatment involves fluid removal, typically via syringe.

Layers of the Heart Wall

Three Distinct Layers

• Epicardium: The visceral layer of the serous pericardium.

• Myocardium: Composed of circular or spiral bundles of contractile cardiac muscle cells.

  • Cardiac Skeleton: A crisscrossing, interlacing layer of connective tissue that anchors muscle fibers, supports vessels and valves, and limits the spread of electrical impulses.

• Endocardium: The innermost layer, continuous with the endothelial lining of blood vessels. It lines the heart chambers and covers the cardiac skeleton of the valves.

Chambers and Associated Great Vessels

Internal Features

• Four Chambers:

  • Two superior atria

  • Two inferior ventricles

• Interatrial Septum: Separates the atria; contains the fossa ovalis, a remnant of the fetal foramen ovale.

• Interventricular Septum: Separates the ventricles.

Surface Features

• Coronary Sulcus (Atrioventricular Groove): Encircles the junction of the atria and ventricles.

• Anterior Interventricular Sulcus: Marks the anterior position of the interventricular septum.

• Posterior Interventricular Sulcus: Landmark on the posteroinferior surface.

Atria: The Receiving Chambers

• Small, thin-walled chambers with auricles that increase atrial volume.

• Right Atrium:

  • Receives deoxygenated blood from the body via three veins: superior vena cava, inferior vena cava, and coronary sinus.

  • Anterior portion is smooth-walled; posterior portion contains ridges formed by pectinate muscles.

  • Regions separated by the crista terminalis.

• Left Atrium:

  • Receives oxygenated blood from the lungs via four pulmonary veins.

  • Pectinate muscles are found only in the auricles.

Ventricles: The Discharging Chambers

• Make up most of the heart's volume.

• Right Ventricle: Forms most of the anterior surface; pumps blood into the pulmonary trunk.

• Left Ventricle: Forms the posteroinferior surface; pumps blood into the aorta (the largest artery in the body).

• Contain trabeculae carneae (irregular muscle ridges) and papillary muscles (anchor chordae tendineae attached to heart valves).

Heart Valves

Types and Functions

Heart valves ensure unidirectional blood flow and open/close in response to pressure changes.

• Atrioventricular (AV) Valves: Located between atria and ventricles.

  • Tricuspid Valve: Right AV valve, three cusps.

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

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

• Semilunar (SL) Valves: Located between ventricles and major arteries.

  • Pulmonary Semilunar Valve: Between right ventricle and pulmonary trunk.

  • Aortic Semilunar Valve: Between left ventricle and aorta.

  • Each SL valve has three half-moon-shaped cusps.

• No valves between major veins and atria; backflow is prevented by inertia and compression during heart contraction.

Clinical Note: Valve Disorders

• Incompetent Valve: Blood backflows, causing the heart to repump the same blood.

• Valvular Stenosis: Stiff valve flaps constrict the opening, requiring increased force for blood pumping.

• Defective valves may be replaced with mechanical, animal, or cadaver valves.

Pathway of Blood Through the Heart

Right Side (Pulmonary Circuit)

1. Blood enters the right atrium from the superior vena cava, inferior vena cava, and coronary sinus.

2. Passes through the tricuspid valve into the right ventricle.

3. Exits via the pulmonary semilunar valve into the pulmonary trunk and arteries, then to the lungs.

Left Side (Systemic Circuit)

1. Oxygenated blood returns from the lungs via four pulmonary veins to the left atrium.

2. Passes through the mitral valve into the left ventricle.

3. Exits via the aortic semilunar valve into the aorta and systemic circulation.

Comparative Features

• Equal volumes of blood are pumped to both circuits.

• Pulmonary circuit: short, low-pressure.

• Systemic circuit: long, high-friction; left ventricle walls are three times thicker and pump with greater pressure.

Coronary Circulation

Arterial Supply

• Coronary arteries arise from the base of the aorta and encircle the heart in the coronary sulcus.

• Left coronary artery branches: anterior interventricular artery and circumflex artery.

• Right coronary artery branches: right marginal artery and posterior interventricular artery.

• Arteries contain anastomoses (junctions) for additional blood delivery routes, but cannot fully compensate for occlusion.

• Left ventricle receives most of the coronary blood supply.

Venous Drainage

• Cardiac veins collect blood from capillary beds.

• Coronary sinus empties into the right atrium; formed by merging cardiac veins:

  • Great cardiac vein (anterior interventricular sulcus)

  • Middle cardiac vein (posterior interventricular sulcus)

  • Small cardiac vein (inferior margin)

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

Clinical Note: Coronary Disorders

• Angina Pectoris: Thoracic pain due to transient deficiency in blood delivery to myocardium; cells are weakened.

• Myocardial Infarction (Heart Attack): Prolonged coronary blockage leads to cell death, replaced by noncontractile scar tissue.

Microscopic Anatomy of Cardiac Muscle

Cellular Structure

• Cardiac muscle cells are striated, short, branched, and interconnected, typically with one central nucleus.

• Contain numerous large mitochondria (25–35% of cell volume) for resistance to fatigue.

• Sarcomeres present with Z discs, A bands, and I bands.

• T tubules are wider but less numerous, entering only once at the Z disc.

• Sarcoplasmic reticulum (SR) is simpler than in skeletal muscle; no triads.

Intercalated Discs

• Connecting junctions between cardiac cells containing:

  • Desmosomes: Hold cells together, prevent separation during contraction.

  • Gap Junctions: Allow ions to pass, electrically couple adjacent cells, enabling the heart to function as a functional syncytium (single coordinated unit).

Connective Tissue Matrix

• Intercellular space contains endomysium with numerous capillaries, connecting muscle to the cardiac skeleton.

Comparison: Skeletal vs. Cardiac Muscle Physiology

Similarities

• Contraction is preceded by a depolarizing action potential.

• Depolarization travels down T tubules, causing SR to release Ca2+.

• Excitation-contraction coupling occurs as Ca2+ binds to troponin, causing filaments to slide.

Differences

• Some cardiac muscle cells are self-excitable (pacemaker cells), initiating depolarization without nervous system stimulation.

• Heart contracts as a unit (functional syncytium), unlike skeletal muscle which contracts independently.

• Influx of Ca2+ from extracellular fluid triggers Ca2+ release from SR; skeletal muscle does not use extracellular Ca2+.

• Cardiac muscle fibers have a longer absolute refractory period, preventing tetanic contractions and allowing efficient pumping.

• Cardiac muscle relies almost exclusively on aerobic respiration due to abundant mitochondria; cannot function without oxygen.

Table: Key Differences Between Skeletal and Cardiac Muscle

Feature	Skeletal Muscle	Cardiac Muscle
Cell Shape	Long, cylindrical, multinucleate	Short, branched, one or two nuclei
Intercalated Discs	Absent	Present
Pacemaker Cells	Absent	Present
Contraction	Independent fibers	Functional syncytium
Source of Ca2+	Sarcoplasmic reticulum only	Sarcoplasmic reticulum and extracellular fluid
Refractory Period	Short	Long (prevents tetanus)
Energy Requirement	Can use anaerobic respiration	Requires aerobic respiration

Additional info: Table entries inferred and expanded for clarity and completeness.