Cardiovascular System Structure and Function

Cardiovascular System Cheatsheet

The cardiovascular system is specialized for transporting blood and lymph throughout the body.

I. Components of the Cardiovascular System

The system is divided into four main parts:

• The Heart: A muscular pump.

• The Macrocirculation: Includes visible arteries and veins.

• The Microcirculation: Comprises capillary networks or capillary beds.

• The Lymph Vascular System.

II. The Heart: Structure and Function

A muscular pump ensuring unidirectional blood flow.

• Location: Lies obliquely, about two-thirds into the left side of the thoracic cavity, within the middle mediastinum.

• Enclosure: Surrounded by the tough fibrous pericardium, containing the great vessels.

• Chambers: Four chambers:

  • Right Atrium & Right Ventricle (pumps blood through low-pressure pulmonary circulation)

  • Left Atrium & Left Ventricle (pumps blood through high-pressure systemic circulation)

• Valves: Guard chamber exits, preventing backflow.

• Septa: Interatrial and interventricular septa separate the right and left sides.

• Circulation Paths:

  • Right Side: Receives deoxygenated blood from the body (via venae cavae) → pumps to lungs for oxygenation (via pulmonary arteries).

  • Left Side: Receives oxygenated blood from the lungs (via pulmonary veins) → pumps to aorta for systemic distribution.

A. Heart Wall Layers

The heart wall consists of three layers:

1. Epicardium:

   • Visceral layer of serous pericardium.

   • Contains blood vessels and nerves supplying the heart.

   • Pericardial cavity (potential space with serous fluid 15-50ml).

   • Cardiac Tamponade: Condition due to excess fluid accumulation, relieved by pericardiocentesis.

2. Myocardium:

   • The principal component of the heart, composed of cardiac muscle.

   • Responsible for contraction.

3. Endocardium:

   • Inner layer of endothelium and subendothelial connective tissue.

   • Contains the conductive system of the heart (e.g., Purkinje fibers).

B. Cardiac Muscle Fibers (Myocardium)

• Arrangement: Cylindrical cells arranged end-to-end; may branch.

• Intercalated Discs:

  • Densely staining cross-bands, highly specialized attachment sites.

  • Components: fascia adherens (thin filament anchor), maculae adherentes (desmosomes, bind cells), and gap junctions (ionic continuity, syncytium behavior).

• Nucleus: Single, centrally located.

• Cytoplasm:

  • Biconical juxtanuclear region rich in mitochondria, Golgi, lipofuscin.

  • Atrial granules in atria contain ANF and BNF (diuretic hormones).

  • Abundant mitochondria and glycogen granules for energy.

• Sarcoplasmic Reticulum (SR): Not as organized as skeletal muscle; forms a diad (SR + T-tubule) at the Z-line.

• T-Tubules: One per sarcomere; larger and more numerous in ventricular muscle. passage is essential for contraction.

• Regeneration: Injury results in fibrous connective tissue replacement (nonfatal MI). Mature cardiac muscle cells were thought unable to divide, but recent studies show potential for replacement/regeneration (e.g., via transplants or cell division).

C. Conduction System

• Formed of modified cardiac muscle cells for initiation and propagation of rhythmic depolarizations.

• Cardiac Skeleton: Fibrous rings around valve orifices and septa.

  • Provides independent attachments for atrial and ventricular myocardium.

  • Acts as an electrical insulator preventing free flow of impulses between atria and ventricles.

• Purkinje Fibers: Found in the subendocardial layer.

  • Contain large amounts of glycogen (pale-staining).

  • Have round, larger nuclei than myocardial cells.

  • Course nerves often found among them.

D. Blood Supply to the Heart

• Coronary Arteries.

• Cardiac Veins: Most drain into the coronary sinus, which then drains into the right atrium.

III. Blood Vessels

Common structural pattern with three layers, except for capillaries.

A. Blood Vessel Wall Layers (Tunics)

1. Tunica Intima: Delimits the vessel wall towards the lumen.

   • Endothelium: Simple squamous epithelium on basal lamina.

   • Subendothelial connective tissue.

   • Internal Elastic Lamina: Delimits intima from media.

2. Tunica Media:

   • Layer of circumferential smooth muscle fibers, elastic lamellae, collagen, and reticular fibers.

   • Proportion of elements varies by vessel type.

   • External Elastic Lamina: Delimits media from adventitia.

3. Tunica Adventitia: Outermost layer, blends with surrounding connective tissue.

   • Mainly connective tissue fibers (collagen, elastic), smooth muscle cells.

   • Contains vasa vasorum (blood supply to outer 2/3) and nervi vasorum (vasomotor innervation).

B. Endothelial Cells

Simple squamous epithelium, continuous layer.

• Morphology: Very flat, elongated, spindle-shaped cells with a centrally located nucleus.

• Junctions:

  • Intercellular junctions crucial for integrity (occluding/tight junctions with JAM, gap junctions, desmosomes).

  • Cell-to-extracellular matrix junctions (hemidesmosomes, focal adhesions).

• Cytoplasm: Relatively simple, few organelles.

  • Weibel-Palade bodies: Contain von Willebrand factor VIII and P-selectin (identification of endothelium-derived tumors).

  • Pinocytotic vesicles: Involved in material passage across cells.

• Surface Molecules: Express adhesion molecules and receptors (LDL, insulin, histamine).

• Functional Roles: Once considered simple lining cells, now known to be active participants in blood-tissue interactions, crucial for vascular health and disease.

C. Endothelial Cell Functions

Maintain structural and functional integrity of blood vessels.

1. Selective Permeability Barrier: Regulates passage of substances.

   • Mechanisms: Simple diffusion (, ), active transport (glucose, amino acids), pinocytosis (small molecules, water), receptor-mediated endocytosis (LDL), fenestration (large molecules).

   • Movement related to size and charge of molecules.

2. Immune Response Modulation:

   • Possess surface molecules (e.g., selectins, ICAM 1&2) that attract leukocytes.

3. Nonthrombogenic Barrier: Between blood and subendothelial tissue.

   • Produce anticoagulants (thrombomodulin) and antithrombogenic substances (prostacyclin, tissue plasminogen activator).

   • Upon damage, release prothrombogenic agents (factor VII, von Willebrand's factor) to promote clotting.

4. Synthesis Function: Secrete various substances:

   • Collagen (II, IV, V), laminin.

   • Endothelin, ACE (vasoconstrictors, raise blood pressure).

   • Nitric Oxide (): Endothelium-derived relaxing factor (vasodilation, modulates blood flow/resistance).

   • Growth factors (CSFs, FGF) and inhibitors.

5. Lipoprotein Modification: Oxidation of LDL and VLDL contributes to atheromatous plaques.

D. Endothelial Cells in Arterial Disease (Atherosclerosis)

Crucial role in pathogenesis, especially atherosclerosis.

• Injury: Physical abrasion or toxic insult (e.g., nicotine) → dysfunctional endothelial cells.

• Increased Permeability: Allows large molecules (like LDL cholesterol) to escape.

• Monocyte Migration: Circulating monocytes adhere, migrate to tunica intima, and differentiate into macrophages.

• Foam Cell Formation: Macrophages phagocytose oxidized LDLs, transforming into foam cells.

• Fatty Streak: Early lesion formed by foam cells and T lymphocytes.

• Fibrofatty Plaque: Endothelial cells release PDGF and other growth factors → stimulate smooth muscle cell migration from tunica media to intima, proliferation, and collagen synthesis, forming a capsule around the lipid core.

• Atheromatous Plaque: Foam cells (from macrophages and vascular smooth muscle cells) accumulate LDLs; smooth muscle cells produce extracellular matrix → increases tunica intima thickness.

• Plaque Rupture: Progression involves lipid accumulation, enzymatic activity, apoptosis of smooth muscle cells, and loss of endothelium integrity, leading to plaque rupture, platelet attachment, and thrombosis (heart attacks, stroke).

IV. Arteries

Deliver blood to capillaries.

A. Classification of Arteries

1. Large (Elastic) Arteries (Conducting arteries): E.g., aorta, pulmonary artery.

   • Intima: Endothelium, subendothelial layer, inconspicuous internal elastic lamina.

   • Media: Numerous elastic lamellae (elastin) with fenestrations; vascular smooth muscle cells synthesize ECM.

   • Adventitia: Less than half the thickness of media; collagen, elastic fibers, vasa vasorum, nervi vasorum.

2. Muscular Arteries (Distributing arteries).

   • Intima: Thinner, prominent internal elastic lamina.

   • Media: Predominantly vascular smooth muscle fibers (spiral arrangement); less elastic material; NO fibroblasts.

   • Adventitia: Relatively thick; fibroblasts, collagen, elastic fibers, outer elastic lamina.

3. Arterioles (Resistance vessels): Diameter .

   • Smallest arteries associated with capillary networks.

   • Regulate blood flow into capillary beds (flow regulators).

   • Intima: Thin, with internal elastic lamina.

   • Media: 1-8 layers of smooth muscle cells (1-2 in arterioles).

   • Adventitia: Scant.

   • Precapillary sphincters: Slight thickening of smooth muscle at origin of capillary bed to control blood flow.

V. Capillaries

Smallest blood vessels (4-15 ), form networks for exchange.

A. Capillary Wall Organization

• Endothelium: Simple squamous epithelium and basal membrane (BM).

• Pericytes: Undifferentiated cells surrounding the capillary, can differentiate into endothelial and smooth muscle cells. Provide vascular support and stability. Involved in vascular diseases.

B. Types of Capillaries

Based on endothelial features:

1. Continuous Capillaries:

   • Uninterrupted endothelium on continuous basal lamina.

   • Tight junctions between endothelial cells restrict passage.

   • Found in: connective tissue, cardiac/skeletal/smooth muscles, skin, lungs, CNS.

2. Fenestrated Capillaries:

   • Endothelial cells have numerous fenestrations (circular openings), providing channels across the wall.

   • Continuous basal lamina across fenestrations acts as a selective filter.

   • Fenestration extent can vary with physiological need.

   • Found in: Areas with intense metabolic activity (kidney, gallbladder, pancreas, intestinal tract, endocrine glands).

3. Discontinuous Capillaries (Sinusoids):

   • Large, irregularly shaped vessels.

   • Fenestrated endothelial cells may not form a complete layer.

   • Wide, irregular intercellular gaps allow passage of blood plasma proteins.

   • Discontinuous or rudimental basal lamina.

   • Found in: liver, spleen, bone marrow.

VI. Veins

Originate from capillary networks, coalescing into venules.

• Walls thinner than arteries, larger diameter.

• Tunica Intima: Very thin.

• Elastic Laminae: Internal and external elastic laminae absent or very thin.

• Tunica Media: Appears thinner than tunica adventitia; layers tend to blend.

A. Venules

Smallest venous vessels.

• Postcapillary Venules: Endothelial lining + BM + pericytes.

  • Principal site of action for vasoactive agents (histamine, serotonin).

  • In lymphoid tissues, High Endothelial Venules (HEV) with cuboidal/columnar endothelial cells facilitate lymphocyte migration.

• Small Muscular Venules: Distinguished by presence of a tunica media.

VII. Microcirculation

Exchange of gases, fluids, nutrients, and metabolic residues between blood and tissues, mainly at the capillary level.

• Control: Amount of blood through capillary bed controlled by arterioles and precapillary sphincters.

• Pathway: Arteriole → capillaries → post-capillary venule → small muscular venule → small vein.

• Arteriovenous (AV) Anastomoses / Shunts: Direct routes between arteries and venules that bypass capillaries.

  • Common in fingertips, nose, lips, erectile tissue.

  • Arteriole of AV shunt has a relatively thick smooth muscle layer.