Blood Vessels Structure and Function (Part A)

Part 1: Blood Vessel Structure and Function

  • Blood vessels serve as the delivery system of dynamic structures beginning and ending at the heart.

  • Work in conjunction with the lymphatic system to circulate fluids.

    • Types of Blood Vessels:

      • Arteries: Carry blood away from the heart; generally oxygenated (except for pulmonary circulation and umbilical vessels in fetuses).

      • Capillaries: Provide direct contact with tissue cells to serve cellular needs.

      • Veins: Carry blood towards the heart; typically deoxygenated (with exceptions similar to arteries).

The Relationship of Blood Vessels to Each Other and to Lymphatic Vessels

  • Figure 19.1 illustrates the relationship between blood vessels and lymphatic vessels.

    • Structural Hierarchy:

    • Venous System:

      • Large veins (capacitance vessels)

      • Large lymphatic vessels

    • Arterial System:

      • Arteriovenous anastomosis

    • Lymphatic System:

      • Lymphatic capillaries

      • Postcapillary venule

    • Heart: Center for circulation

    • Capillaries: Exchange vessels for nutrients and gases.

19.1 Structure of Blood Vessel Wall

  • Blood vessels consist of a lumen (central, blood-containing space) surrounded by a wall.

  • Walls of all vessels, except for capillaries, contain three layers, or tunics:

    1. Tunica Intima

    2. Tunica Media

    3. Tunica Externa

  • Capillaries: Composed solely of endothelium with a sparse basal lamina.

Tunica Intima

  • Definition: The innermost layer in direct contact with blood.

  • Endothelium: Simple squamous epithelium lining the lumen of all vessels; continuous with the endocardium.

    • Function: Provides a slick surface to reduce friction.

  • Subendothelial Layer: Connective tissue basement membrane found in vessels larger than 1 mm.

Tunica Media

  • Definition: The middle layer composed primarily of smooth muscle and elastin sheets.

  • Innervation: Sympathetic vasomotor nerve fibers control this layer for:

    • Vasoconstriction: Decreased lumen diameter.

    • Vasodilation: Increased lumen diameter.

  • Importance: It is the bulkiest layer, pivotal in maintaining blood flow and blood pressure.

Tunica Externa

  • Definition: The outermost layer of the wall, also known as tunica adventitia.

  • Composition: Predominantly loose collagen fibers for protection, reinforcement, and anchoring to surrounding structures.

  • Infiltrations: Contains nerve fibers and lymphatic vessels.

  • Vasa Vasorum: A network of small blood vessels nourishing the outer layer found in larger vessels.

Generalized Structure of Arteries, Veins, and Capillaries

  • Figure 19.2b depicts the generalized structure of these blood vessels:

    • Arteries:

      • Tunica media (smooth muscle & elastic fibers)

      • External elastic membrane

      • Tunica externa (collagen fibers)

    • Veins:

      • Endothelium

      • Valve

    • Capillaries:

      • Lumen is solely formed by endothelial cells.

19.2 Arteries

  • Classification: Arteries are divided into three groups based on size and function:

    1. Elastic Arteries

    2. Muscular Arteries

    3. Arterioles

Elastic Arteries

  • Characteristics: Thick-walled with large, low-resistance lumens.

  • Examples: Aorta and major branches are known as conducting arteries since they conduct blood from the heart to medium-sized vessels.

  • Composition: Elastin is present in all three tunics, with the majority in the tunica media; substantial smooth muscle exists but is not active in vasoconstriction.

  • Function: Act as pressure reservoirs that expand and recoil with blood ejected from the heart, ensuring continuous blood flow downstream even between heartbeats.

Muscular Arteries

  • Origin: Formed from elastic arteries; known as distributing arteries because they deliver blood to specific body organs.

  • Size Ranges: Their diameters range from the size of a pinky-finger to pencil lead.

  • Structure: Have the thickest tunica media, which contains more smooth muscle than elastic tissue, sandwiched between elastic membranes.

  • Function: Active in vasoconstriction, controlling blood distribution.

Arterioles

  • Definition: Smallest of all arteries; larger arterioles contain all three tunics, while smaller arterioles typically consist of a single layer of smooth muscle surrounding endothelial cells.

  • Function: Control blood flow into capillary beds through vasodilation and vasoconstriction; called resistance arteries since changing diameters alter resistance to blood flow.

  • Pathway: Lead into capillary beds.

19.3 Capillaries

  • Description: Microscopic vessels where the diameter is so small that only a single red blood cell can pass through at a time.

    • Walls: Composed only of thin tunica intima; in smallest vessels, one cell wraps around the entire circumference.

    • Pericytes: Spider-shaped stem cells that stabilize capillary walls, control permeability, and contribute to vessel repair.

  • Function: Facilitate the exchange of gases, nutrients, wastes, and hormones between blood and interstitial fluid.

Types of Capillaries

  • All capillary endothelial cells are joined by tight junctions with intercellular clefts allowing the passage of fluids and small solutes.

  • Types of Capillaries:

    1. Continuous Capillaries:

      • Most common type, abundant in skin, muscles, lungs, and central nervous system (CNS).

      • Unique continuous capillaries of the brain form the blood-brain barrier, completely enclosed with tight junctions and lacking intercellular clefts.

    2. Fenestrated Capillaries:

      • Found in areas of active filtration (kidneys), absorption (intestines), or hormone secretion.

      • Endothelial cells possess Swiss cheese-like pores (fenestrations) that enhance permeability.

      • Fenestrations are typically covered with a thin glycoprotein diaphragm.

    3. Sinusoidal Capillaries:

      • Contain fewer tight junctions, larger intercellular clefts, and incomplete basement membranes.

      • Located in the liver, bone marrow, spleen, and adrenal medulla.

      • Allow sluggish blood flow for modification of larger molecules and blood cells passing between blood and tissue; contain macrophages in lining to capture and eradicate foreign invaders.

Capillary Beds

  • Definition: Networks of interwoven capillaries situated between arterioles and venules, involved in microcirculation—the flow of blood from arteriole to venule.

  • Structure:

    • Terminal Arterioles: Branch off arterioles and further divide into multiple capillaries making up the capillary bed.

    • Exchange: Oxygen exchange, as well as nutrients and waste between blood and tissues, occurs at the capillary level.

  • Postcapillary Venule: Capillaries drain into this vessel.

Flow through a Capillary Bed

  • Control of Flow: Blood flow through the capillary bed is controlled by the diameter of the terminal arterioles and upstream arterioles.

    • Local chemical conditions and arteriolar vasomotor nerve fibers regulate blood flow entering the capillary bed.

    • Vasodilation: Arterioles and terminal arterioles dilate to meet blood needs.

    • Vasoconstriction: Constriction occurs to shunt blood away from the capillary bed when it is not needed.

  • Microcirculation: flow of blood through the bed from arteriole to venule

  • Terminal arteriole: a branch off an arteriole that further branches into 10 to 20 capillaries (exchange vessels) that form a capillary bed.

  • Capillaries then drain into postcapillary venule

Special Capillary Features in Serous Membranes

  • Vascular Shunt: A channel directly connecting an arteriole to a venule, bypassing true capillaries, consisting of metarteriole and thoroughfare channel.

  • Precapillary Sphincter: A cuff of smooth muscle surrounding each true capillary branching from a metarteriole; functions as a valve, regulating blood flow into the capillary bed, controlled by local chemical conditions (not innervated).

19.4 Veins

  • Definition: Veins carry blood toward the heart. Formation initiated when capillary beds unite in postcapillary venules, merging into progressively larger veins.

  • Venules: Formed from capillary unions; consist of endothelium and few pericytes, allowing fluids and white blood cells to enter tissues due to their porosity. Larger venules include one or two layers of smooth muscle.

  • Formation: As venules converge, they develop all tunics but with thinner walls and larger lumens compared to arteries; the tunica media is thinner while the tunica externa is significantly thicker, composed of collagen fibers and elastic networks.

  • Functionality: Due to large lumens and thin walls, veins serve well as storage vessels, termed capacitance vessels (blood reservoirs), capable of holding up to 65% of the total blood supply.

Relative Proportion of Blood Volume Throughout the Cardiovascular System

  • Distribution:

    • Systemic Veins and Venules: 60%

    • Pulmonary Blood Vessels: 12%

    • Heart: 8%

    • Capillaries: 5%

    • Systemic Arteries and Arterioles: 15%

Adaptations Ensuring Return of Blood to Heart

  • Blood Pressure: Typically lower in veins than in arteries, prompting adaptations to facilitate venous return.

  • Large-Diameter Lumen: Provides minimal resistance to blood flow.

  • Other Adaptations:

    • Venous Valves: Prevent backflow of blood; prevalent in limb veins.

    • Venous Sinuses: Flattened veins with very thin walls, formed solely of endothelium; example: coronary sinus of the heart and dural sinuses of the brain.

Clinical - Homeostatic Imbalance 19.1: Varicose Veins

  • Definition: Enlarged and painful veins resulting from incompetent (leaky) valves.

  • Contributing Factors: Includes hereditary predispositions and conditions hindering venous return.

    • Example: Prolonged periods of standing, obesity, or pregnancy cause blood pooling in lower limbs, weakening valves, affecting over 15% of adults.

19.5 Anastomoses

  • Definition: Vascular anastomoses refer to interconnections of blood vessels.

    • Arterial Anastomoses: Provide alternative pathways (collateral channels) to ensure continuous blood flow in case of arterial blockage; frequently occur in joints, abdominal organs, brain, and heart, but not present in the retina, kidneys, or spleen.

    • Arteriovenous Anastomoses: Shunts within capillaries, e.g., the metarteriole–thoroughfare channel.

    • Venous Anastomoses: Abundant in veins ensuring occluded veins rarely obstruct blood flow.