Detailed Study Notes on Blood Vessels Anatomy

Fundamentals of Anatomy & Physiology

Chapter 21: Blood Vessel Anatomy

Introduction to Blood Vessels & Circulation

  • BLOOD VESSELS

    • Instrumental in overall cardiovascular (CV) regulation.

    • Functions:

    • Transport gases, nutrients, hormones, and wastes to and from cells, tissues, and organs.

    • Vasoconstrict and vasodilate to control blood pressure.

      • Regulate blood flow to tissues.

    • Endothelial cells secrete various chemicals impacting:

      • Blood clotting: thrombosis and fibrinolysis.

      • Inflammatory response: signaling white blood cells (WBCs).

      • Formation of new blood vessels (angiogenesis).

Classification of Blood Vessels

  • Classified by size and histological organization with 5 general classes differing in size, structure, and functional properties:

    1. Resilient: To withstand changes in pressure.

    2. Flexible: To move with underlying tissues and organs.

    3. Layered walls: Provide vessels with considerable strength.

Basic Classes of Blood Vessels

  1. ARTERIES

    • Carry blood away from the heart.

    • Branch repeatedly and decrease in diameter as they enter peripheral tissues.

  2. ARTERIOLES

    • Smallest branches of arteries.

  3. CAPILLARIES

    • Smallest blood vessels; site of diffusion between blood and interstitial fluid.

  4. VENULES

    • Collect blood from capillaries; unite to form veins.

  5. VEINS

    • Carry blood back to the heart.

General Structure of Blood Vessels

  • Largest Blood Vessels:

    • Pulmonary Trunk: Originates from the right ventricle, carries blood to pulmonary circulation.

    • Aorta: Originates from the left ventricle, carries blood to systemic circulation.

  • Smallest Blood Vessels: CAPILLARIES

    • Vital functions of the CV system depend on events at the capillary level.

    • Chemicals and gases diffuse rapidly across capillary walls due to small diameter and thin walls.

Structure of Blood Vessel Walls

  • Composed of three tunic layers:

    1. Tunica Intima

    2. Tunica Media

    3. Tunica Externa

Tunica Intima

  • Inner layer includes:

    • Endothelial cell lining with basal lamina.

    • Subendothelial connective tissue layer with variable numbers of elastic fibers.

    • In arteries: additional layer of elastic fibers known as internal elastic membrane or lamina.

    • Functional Properties:

      • Distensibility: Ability to stretch under pressure.

      • Elasticity: Ability to recoil to original size after stretching.

Tunica Media

  • Middle layer consisting of concentric sheets of smooth muscle and loose connective tissue.

    • Smooth muscle encircles endothelial cells lining the lumen.

    • Collagen fibers bind tunica media to tunica intima and tunica externa.

    • Thickest layer in small arteries.

    • Contains an external elastic membrane separating it from tunica externa.

Autonomic Control of Blood Vessels

  • Sympathetic Nervous System innervates smooth muscle cells of the tunica media through vasomotor nerves.

    • Vasoconstriction: Narrowing the vessel diameter due to smooth muscle contraction.

    • Vasodilation: Increased vessel diameter due to smooth muscle relaxation.

Tunica Externa

  • Outer layer made of connective tissue sheath.

    • Contains fibers anchoring and stabilizing the blood vessel.

    • In arteries:

    • Contains collagen fibers and scattered bands of elastic fibers.

    • In veins:

    • Contains elastic fibers and smooth muscle cells.

    • Vasa vasorum: Small arteries and veins within the walls of larger vessels supplying nutrients to smoother muscle and fibroblasts of tunica media and tunica externa, which are too far from lumen for nutrient diffusion.

Differences Between Arteries & Veins

  • Vessel Walls: Arteries have thicker walls; tunica media contains smooth muscle and elastic fibers to resist arterial pressure generated by the heart.

  • Vessel Lumen:

    • Elastic fibers in arterial walls recoil, constricting the lumen.

    • Veins are often collapsed or distorted.

  • Vessel Lining:

    • Arterial endothelial lining cannot contract with arterial constriction, resulting in folds.

  • Valves in Veins:

    • Internal structures preventing backflow of blood toward capillaries, indicated by slight distention.

Structure & Function of Arteries

  • ARTERIES:

    • Have thicker, muscular walls with elastic and contractile properties.

    • Elasticity: Change diameter passively in response to blood pressure.

    • Contractility: Actively change diameter, controlled primarily by the sympathetic division of the ANS, pivotal in hemostasis.

  • Vasoconstriction: Contraction of smooth muscle cells controlled by the ANS, decreasing lumen size.

  • Vasodilation: Relaxation of arterial smooth muscle, enlarging the lumen.

  • Effects of Peripheral Vasoconstriction/Vasodilation:

    1. Affects afterload on the heart (pressure during systole).

    2. Influences peripheral blood pressure.

    3. Impacts capillary blood flow.

  • Flow Pathway: Blood flows from the heart through:

    • Elastic arteries → Muscular arteries → Arterioles.

Arterial Types

  1. Elastic Arteries (Conducting Arteries)

    • Carry large volumes of blood under high pressure; up to 2.5 cm in diameter (e.g., pulmonary trunk, aorta).

    • Walls are resilient with many elastic fibers to tolerate high-pressure changes of the cardiac cycle.

  2. Muscular Arteries (Distributing Arteries)

    • Medium-sized, characterized by a thick, well-developed tunica media, distributing and controlling blood flow and pressure to skeletal muscle and internal organs.

    • Act as pressure points in bleeding control.

  3. Arterioles

    • Small diameter with poorly defined tunica externa and thin or incomplete tunica media, primarily regulating blood flow into capillaries.

Structure & Function of Capillaries

  • CAPILLARIES: Smallest vessels, with thin walls, typically organized in capillary beds throughout tissues.

    • Primary site for exchange functions of the CV System; allow materials to diffuse between blood and interstitial fluid.

    • Capillary exchange is rapid due to:

    • Thin walls and short diffusion distances.

    • Exchanges occur via:

    • Two types of diffusion and transcytosis (endocytosis-exocytosis).

Capillary Structure

  • Comprise an endothelial cell tube inside a thin basement membrane, without tunica media or tunica externa.

  • Diameter comparably sized to red blood cells.

  • Pericytes: Cells around some capillaries controlling blood flow.

  • Types of Capillaries:

    1. Continuous Capillaries: Complete endothelial lining; permit diffusion of water and small solutes while preventing loss of blood cells and plasma proteins.

    2. Fenestrated Capillaries: Possess pores, permitting rapid exchange of water and larger solutes.

    3. Sinusoids: Have gaps between endothelial cells, allowing free exchange of H₂O and large plasma proteins, located in the liver and spleen.

Capillary Beds

  • Capillary Plexus: An interconnected network; precapillary sphincters control entrance to capillaries.

  • Facilitates blood flow through various capillary connections linking arterioles to venules.

Structure & Function of Veins

  • VEINS: Collect blood from capillaries, returning it to the heart; serve as blood reservoirs.

    • Larger in diameter than corresponding arteries with thinner walls due to lower blood pressure.

    • Classification: By size - few elastic fibers and less smooth muscle compared to arteries.

Types of Veins

  1. Venules: Very small; collect blood from capillary beds.

  2. Medium-Sized Veins: Comparable to muscular arteries.

  3. Large Veins: Contain all three layers with a notably thick tunica externa and thin tunica media (e.g., superior & inferior vena cava).

Capacitance in Veins

  • Capacitance: Relationship between blood vessel volume and blood pressure.

    • Veins accommodate larger blood volume changes due to thinner walls and less smooth muscle.

Venous Valves

  • Prevent backward blood flow due to low blood pressure in veins and venules, especially important against gravity.

  • Contraction and movement facilitate venous return, counteracting blood pooling conditions such as varicose veins and hemorrhoids.

Blood Distribution in the CV System

  • Total blood volume is unevenly distributed:

    • Heart, arteries, and capillaries: 30-35%.

    • Venous system: 55-65% (1/3 circulating within large venous networks like liver and skin).

Summary Table of Vessels

  • Types of Arteries and Veins:

    • Elastic arteries: Large arteries with extensive elastic laminae; conduct blood at high pressure.

    • Muscular arteries: Thick-walled arteries with well-developed tunica media; regulate blood flow.

    • Arterioles: Control blood flow to capillary beds.

    • Venules: Thin-walled vessels draining capillary beds.

    • Veins: Large lumen, thin-walled vessels with valves returning blood to the heart.