Circulation: Venules, Veins & Venous Return

Vocabulary flashcards covering structural features of venules and veins, functional differences between veins and arteries, and key mechanisms that regulate venous pressure and return.

Venule

Microscopic vessel that first collects blood from capillary beds; walls mostly tunica intima with little or no tunica media.

Vein

Low-pressure vessel with thin walls, larger lumens, and a present but thin tunica media; returns blood toward the heart.

Tunica Intima

Innermost vessel layer composed of endothelium and underlying connective tissue.

Tunica Media

Middle layer of blood-vessel wall made of smooth muscle; much thinner in veins than in arteries.

Tunica Adventitia (Externa)

Outermost connective-tissue layer of a blood vessel; thinner in veins than in arteries.

Lumen

Central open space within a vessel through which blood flows; wider in veins than in comparable arteries.

Compliance

Ability of a vessel to stretch in response to a given change in internal (distending) pressure; high in veins, low in arteries.

Pressure Reservoir

Functional description of elastic arteries, which store pressure during systole and maintain it during diastole due to low compliance.

Volume Reservoir

Functional description of veins, which store the majority (>50 %) of blood volume at rest due to high compliance.

Central Venous Pressure (CVP)

Blood pressure in the large central veins (superior & inferior vena cava); primary upstream pressure driving venous return.

Venous Return

Volume of blood per minute flowing back to the heart; driven by the gradient between CVP and right-atrial pressure.

Venous Valve

One-way flap in medium and large veins that prevents backflow and aids unidirectional venous return.

Skeletal Muscle Pump

Compression of limb veins by contracting skeletal muscles, raising local venous pressure and propelling blood toward the heart.

Respiratory Pump

Changes in thoracic and abdominal pressures during breathing that enhance venous return, especially from abdominal veins.

Venomotor Tone

Degree of contraction of venous smooth muscle; increased by sympathetic activity, causing venoconstriction and reduced compliance to boost venous return.

Boyle’s Law (in vessels)

Principle that decreasing the volume of a closed space (e.g., vein compressed by muscle) raises the pressure of the contained fluid.

Alpha-Adrenergic Receptor

Sympathetic receptor subtype on venous (and arterial) smooth muscle that mediates vasoconstriction when activated.

Vasoconstriction

Narrowing of a blood vessel’s lumen due to smooth-muscle contraction, raising intravascular pressure.

Cardiac Output (CO)

Volume of blood ejected by each ventricle per minute; increases require mobilization of blood from venous reservoirs.

Blood Volume

Total quantity of blood in the circulatory system; influences both arterial and venous pressures, regulated slowly by the kidneys.

Intravascular Volume

Synonym for blood volume within a vessel; larger increases are needed in veins than arteries to raise pressure because of higher compliance.

Smooth Muscle (vascular)

Muscle tissue in vessel walls that controls diameter; sparse in venules, thin in veins, thick in arteries.

Elastic Artery

Large artery with abundant elastic fibers; low compliance makes it a pressure reservoir.

Erythrocyte

Red blood cell; often seen trapped in histological sections of vessels.

Artifact (histology)

Distortion or feature produced during tissue processing, e.g., collapsed vein lumen in photomicrographs.

Endothelium

Simple squamous epithelial lining of all blood vessels, forming part of the tunica intima.

Distending Pressure

Internal pressure exerted by blood that tends to expand a vessel wall.

Vena Cavae

Superior and inferior large systemic veins that empty into the right atrium; key components of CVP.

Low Compliance vs. High Compliance

Comparative terms: arteries have low compliance (stiff), veins have high compliance (stretchy).