Cardiovascular System: Heart and Blood Vessels

1. Heart Structure and Function

The heart is a muscular organ responsible for circulating blood throughout the body, ensuring oxygen and nutrient delivery to tissues. It consists of four chambers and functions as a double pump for pulmonary and systemic circulation.

• Four Chambers: Two atria (upper, receiving chambers) and two ventricles (lower, pumping chambers).

• Right heart: Pumps blood to the pulmonary circuit (lungs for gas exchange).

• Left heart: Pumps blood to the systemic circuit (oxygenated blood to body tissues).

2. Location and Orientation of the Heart

The heart is located in the mediastinum within the thoracic cavity, posterior to the sternum and between the lungs.

• Apex: Points inferolaterally to the left.

• Base: Directed superiorly and posteriorly.

• Surfaces:

  • Sternocostal surface: Right atrium and ventricle.

  • Diaphragmatic (posterior) surface: Left atrium and ventricle.

3. Pericardium & Pericardial Cavity

The heart is enclosed in a double-walled sac called the pericardium, which protects and anchors the heart while reducing friction during contractions.

• Visceral pericardium (epicardium): Inner layer adhering directly to the heart surface.

• Parietal pericardium + fibrous pericardium: Outer layers forming the pericardial sac, containing lubricating fluid.

4. Heart Wall Layers

The heart wall consists of three layers, each with distinct functions and cellular composition.

• Epicardium: Outer protective layer.

• Myocardium: Muscular middle layer, responsible for contraction (thickest in left ventricle).

• Endocardium: Inner endothelial layer lining the chambers.

• Cardiac muscle cells: Branched, single nucleus, connected by intercalated discs (desmosomes for mechanical strength, gap junctions for electrical connectivity).

5. Fibrous Skeleton

The fibrous skeleton is a framework of connective tissue that stabilizes the position of heart valves, cardiac cells, and coronary vessels.

• Functions in force distribution, preventing overexpansion, and providing electrical isolation between atria and ventricles.

• Each cardiac cell is wrapped in an elastic sheath; muscle layers are wrapped in a fibrous sheet.

• Fibrous sheets encircle the base of the pulmonary trunk and ascending aorta.

• Functions:

  • Stabilizes heart valves and cardiac cells.

  • Supports blood vessels and nerves in the myocardium.

  • Distributes contraction forces.

  • Prevents overexpansion and provides elasticity for recoil after contraction.

6. External Heart Anatomy

Several grooves and sulci mark the external surface of the heart, separating chambers and serving as landmarks for major vessels.

• Coronary sulcus: Separates atria and ventricles.

• Anterior interventricular sulcus: Separates left and right ventricles on the anterior surface.

• Posterior interventricular sulcus: Separates ventricles on the posterior surface.

• Auricles: Expandable extensions of the atria.

7. Internal Heart Anatomy and Chambers

The heart's four chambers are separated by septa and have distinct anatomical features and functions.

• Septa:

  • Interatrial septum: Separates right and left atria.

  • Interventricular septum: Separates right and left ventricles.

• Right Atrium:

  • Receives deoxygenated blood from superior/inferior vena cava and coronary sinus.

  • Contains pectinate muscles and fossa ovalis (remnant of foramen ovale).

  • Auricle expands atrial volume.

• Right Ventricle:

  • Pumps blood to lungs via pulmonary valve → pulmonary trunk → pulmonary arteries.

  • Contains trabeculae carneae and moderator band (transmits conduction to papillary muscles, prevents overexpansion).

• Left Atrium:

  • Receives oxygenated blood from four pulmonary veins.

  • Smooth walls, no pectinate muscles.

• Left Ventricle:

  • Thickest myocardium, generates systemic pressure.

  • Pumps blood through aortic valve → ascending aorta → body.

  • Two papillary muscles anchor valve cusps via chordae tendineae.

  • No moderator band.

8. Heart Valves and Function

Heart valves ensure unidirectional blood flow and prevent backflow during the cardiac cycle.

• AV valves: Tricuspid (right), bicuspid/mitral (left).

• Semilunar valves: Pulmonary (right ventricle to pulmonary trunk), aortic (left ventricle to aorta).

• Valve function:

  • During diastole: AV valves open, semilunar valves closed.

  • During systole: AV valves closed, semilunar valves open.

  • Papillary muscles contract to prevent AV valve prolapse by tension on chordae tendineae.

9. Coronary Circulation

The heart muscle (myocardium) receives its blood supply from the coronary arteries, which originate at the base of the ascending aorta.

• Right Coronary Artery (RCA):

  • Passes between right atrium and pulmonary trunk.

  • Atrial branches: Supply right atrium.

  • Right marginal branch: Supplies lateral wall of right ventricle.

  • Posterior interventricular branch: Supplies posterior ventricular walls and septum.

  • Conduction system branches: Supply SA node and AV node.

• Left Coronary Artery (LCA):

  • Circumflex branch

  • Anterior interventricular branch (LAD): Supplies anterior walls of both ventricles and most of the interventricular septum.

• Anastomoses: Connections between branches ensure collateral circulation.

• Coronary veins: Great cardiac vein, middle cardiac vein, small cardiac vein, and others drain into the coronary sinus, which empties into the right atrium.

10. Conducting System & Cardiac Cycle

The heart's conducting system coordinates the sequence of cardiac muscle contraction, ensuring efficient blood flow.

• SA node: Pacemaker (80–100 AP/min), initiates contraction.

• Internodal pathways: Conduct signals to AV node.

• AV node: Delays impulse (~100 ms).

• AV bundle: Conducts to bundle branches → Purkinje fibers → ventricular myocardium.

• Moderator band: Conducts to papillary muscles.

• Cardiac cycle phases:

  • Atrial systole → atrial diastole → ventricular systole (AV valves close, semilunar valves open) → ventricular diastole (semilunar valves close, ventricles fill).

11. Autonomic Regulation of the Heart

Autonomic nervous system controls heart rate and contractility.

• Sympathetic (norepinephrine): Increases heart rate and contraction force.

• Parasympathetic (acetylcholine via CN X): Decreases heart rate and force.

• Medulla oblongata: Cardiac centers regulate autonomic output.

Blood Vessels: Structure and Types

II. Blood Vessel Wall Layers

Blood vessels are composed of three layers, each contributing to vessel function and integrity.

• Tunica intima: Endothelium lining vessel lumen.

• Tunica media: Smooth muscle controlling diameter (vasoconstriction/vasodilation).

• Tunica adventitia: Fibrous connective tissue anchoring vessels; contains vasa vasorum (small vessels supplying larger vessel walls).

III. Arteries vs. Veins

Arteries and veins differ in structure and function, reflecting their roles in circulation.

IV. Artery Types

As blood leaves the heart, it travels through different types of arteries, each with specific structural features and functions.

• Elastic arteries: Large, contain elastic fibers for stretch (e.g., aorta, brachiocephalic, pulmonary trunk, common carotid, subclavian, common iliac).

• Muscular arteries: Medium-sized, thick smooth muscle, regulate blood flow to organs.

• Arterioles: Smallest arteries, control blood flow into capillary beds.

V. Veins and Venous Return

Veins return blood to the heart and contain valves to prevent backflow. Venous return is aided by skeletal muscle contraction and changes in thoracic pressure.

• Venules: Smallest veins, collect blood from capillaries.

• Medium-sized veins: Prominent tunica externa, may contain elastic fibers.

• Large veins: Thick layers, examples include superior and inferior vena cava.

• Valves: Present in veins, especially in limbs, to prevent backflow.

VI. Summary Table: Arteries vs. Veins

VII. Key Terms and Concepts

• Myocardium: Muscular layer of the heart wall.

• Chordae tendineae: Tendinous cords anchoring AV valve cusps to papillary muscles.

• Intercalated discs: Specialized connections between cardiac muscle cells for synchronized contraction.

• SA node: Sinoatrial node, the heart's pacemaker.

• AV node: Atrioventricular node, delays electrical impulse.

• Purkinje fibers: Specialized conducting fibers in the ventricles.

• Vasa vasorum: Small vessels supplying the walls of larger blood vessels.

VIII. Example: Cardiac Cycle Sequence

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

• Ventricular systole: Ventricles contract, AV valves close, semilunar valves open, blood ejected.

• Ventricular diastole: Ventricles relax, semilunar valves close, AV valves open, ventricles fill.

IX. Formulas and Equations

• Blood Pressure Equation:

• Cardiac Output Equation:

X. Additional Info

• Additional info: Some details inferred from standard anatomy and physiology knowledge to clarify fragmented or abbreviated points in the original notes.