3.1 - Overview of Blood Vessel Structure and Function

Cardiovascular System

Includes the heart and all blood vessels (arteries, veins, and capillaries) that circulate blood throughout the body

Lymphatic System

Transports lymph (a fluid containing white blood cells and waste products) back into the bloodstream and plays a crucial role in immune defense; often considered part of the circulatory system

Artery

A blood vessel that carries blood away from the heart

Arteriole

The smallest type of artery; further branches into capillaries

Capillary

A tiny vessel where nutrients and wastes are exchanged between blood and tissues

Venule

A small blood vessel formed from capillaries that carries blood to a vein

Vein

A larger blood vessel that returns blood to the heart

Systemic Circuit

Circuit in which systemic arteries provide oxygen-rich blood to body tissues and systemic veins return blood with less oxygen to the heart

Pulmonary Circuit

Circuit in which pulmonary arteries carry oxygen-poor blood to the lungs and pulmonary veins return oxygenated blood to the heart

Lumen

The hollow passageway of a blood vessel through which blood flows

Vasa Vasorum

Small blood vessels within the walls of large vessels (such as the aorta and large veins) that supply blood to nourish the vessel wall cells

Tunica Intima (Tunica Interna)

The most interior layer of a blood vessel wall, composed of epithelial and connective tissue layers directly in contact with blood flow

Endothelium

The specialized simple squamous epithelium lining the tunica intima, continuous throughout the entire vascular system including the heart chambers

Endothelins

Local chemicals released by the endothelium that constrict smooth muscle in the vessel wall to increase blood pressure; overproduction may contribute to hypertension and cardiovascular disease

Basement Membrane (Basal Lamina)

Layer next to the endothelium that binds the endothelium to connective tissue; provides strength while maintaining flexibility and is permeable

Internal Elastic Membrane

A thick, distinct layer of elastic fibers at the boundary between the tunica intima and tunica media in larger arteries; not present in veins

Tunica Media

The substantial middle layer of the vessel wall, generally the thickest layer in arteries, made of smooth muscle and elastic connective tissue

Vasoconstriction

Narrowing of a blood vessel lumen due to smooth muscle contraction in the tunica media, decreasing blood flow and increasing blood pressure in arteries

Vasodilation

Widening of a blood vessel lumen due to smooth muscle relaxation, increasing blood flow and decreasing blood pressure

Nervi Vasorum

Small vascular nerves that run within blood vessel walls and help regulate vasoconstriction and vasodilation

External Elastic Membrane

Structure separating the tunica media from the tunica externa in larger arteries; not usually seen in smaller arteries or in veins

Tunica Externa

The outer tunic of a blood vessel, a substantial sheath of connective tissue composed primarily of collagenous fibers; normally the thickest tunic in veins

Elastic Arteries

Arteries close to the heart with the thickest walls and a high percentage of elastic fibers; vessels larger than 10 mm in diameter; also called conducting arteries

Muscular Arteries

Arteries located farther from the heart with less elastic fiber and more smooth muscle, diameter ranging from 0.1 mm to 10 mm; play a leading role in vasoconstriction; also called distributing arteries

Arteriole (Resistance Vessel)

A very small artery with a lumen averaging 30 micrometers or less, critical for slowing/resisting blood flow and regulating blood pressure

Vascular Tone

The consistent state of partial smooth muscle contraction maintained by blood vessels, similar to skeletal muscle tone

Perfusion

The exchange of gases and other substances between blood in capillaries and surrounding cells/interstitial fluid

Microcirculation

The term used to describe blood flow through capillaries

Continuous Capillaries

The most common type of capillary, found in almost all vascularized tissues, with a complete endothelial lining and tight junctions with small intercellular clefts

Blood-Brain Barrier

Formed by continuous capillaries in the brain with tight junctions and no intercellular clefts, plus a thick basement membrane and astrocyte extensions, preventing movement of nearly all substances into the CNS

Fenestrated Capillaries

Capillaries with pores (fenestrations) in addition to tight junctions, making them permeable to larger molecules; common in the small intestine, kidneys, choroid plexus, and many endocrine glands

Sinusoid Capillaries

The least common, most permeable type of capillary; flattened with extensive intercellular gaps, incomplete basement membranes, and large openings allowing passage of plasma proteins and cells; found in the liver, spleen, bone marrow, lymph nodes, and some endocrine glands

Capillary Bed

A network of intertwined capillary vessels forming a microcirculatory unit within a specific body area, enabling tissue perfusion

Precapillary Sphincters

Circular smooth muscle cells surrounding a capillary at its origin with the arteriole that tightly regulate blood flow into the capillary bed; normally closed

Thoroughfare Channel

Pathway that allows blood to flow directly from arteriole to venous circulation when all precapillary sphincters in a capillary bed are closed, bypassing the capillary bed

Arteriovenous Anastomosis

A connection that allows blood to bypass a capillary bed and lead directly to the venous system

Venule

An extremely small vein, generally 8-100 micrometers in diameter, formed by postcapillary venules joining capillaries exiting a capillary bed

Diapedesis (Emigration)

The process by which white blood cells adhere to the endothelial lining of venules and capillaries and squeeze through adjacent cells to enter tissue fluid

Vein Valves

Structures formed by sections of thickened endothelium reinforced with connective tissue that extend into the lumen of larger veins to promote unidirectional blood flow toward the heart

Skeletal Muscular Pump

Mechanism in which skeletal muscle contraction closes the inferior vein valve and opens the superior valve, pumping blood back toward the heart

Capacitance Vessels

Term for veins, reflecting their high capacitance (capacity to distend and store a high volume of blood even at low pressure)

Venous Reserve

The approximately 21 percent of venous blood held in venous networks within the liver, bone marrow, and integument, which can be redistributed via venoconstriction

Venoconstriction

Constriction of veins triggered by sympathetic stimulation from the vasomotor center in the medulla oblongata, which stiffens the vessel wall and increases pressure to speed blood return to the heart

Vascular Anastomoses

Interconnections of blood vessels that ensure continuous blood supply to body regions, providing resilience if one route is compromised

Collateral Channels

Alternate pathways characteristic of arterial anastomoses, present at joints, abdominal organs, the brain, and the heart

Vascular Shunts

Arteriovenous anastomoses in capillaries that help direct dynamic blood flow to ensure alternate circulation

Percent of Systemic Blood in Veins

Systemic veins contain approximately 64 percent of total blood volume at any given time

Arteries vs Veins: General Appearance

Arteries have thick walls with small, rounded lumens; veins have thin walls with large, often flattened or irregular lumens

Arteries vs Veins: Pressure

Arteries carry blood at high pressure; veins carry blood at low pressure

Arteries vs Veins: Direction of Flow

Arteries conduct blood away from the heart; veins conduct blood toward the heart

Arteries vs Veins: Valves

Valves are absent in arteries but present in veins, most commonly in the limbs and in veins inferior to the heart