Chapter 20: Cardiovascular System - Vessels and Circulation

Tunica intima

The innermost tunic of a blood vessel wall, composed of an endothelium of simple squamous epithelium and a subendothelial layer of areolar connective tissue.

Tunica media

The middle layer of a blood vessel wall consisting of circularly arranged smooth muscle cells and elastic fibers; contraction causes vasoconstriction, while relaxation causes vasodilation.

Tunica externa

The outermost layer of a vessel wall composed of areolar connective tissue with elastic and collagen fibers; anchors the vessel and may contain vasa vasorum.

Vasa vasorum

A network of small arteries required to supply the walls of very large blood vessels.

Companion vessels

Arteries and veins that lie next to each other and serve the same body region.

Elastic (conducting) arteries

The largest arteries, with diameters ranging from $2.5\text{ to }1\text{ cm}$, containing abundant elastic fibers that allow them to stretch and recoil to propel blood during diastole (e.g., aorta, pulmonary trunk).

Muscular (distributing) arteries

Medium-sized arteries with diameters from $1\text{ cm to }0.3\text{ mm}$ that distribute blood to specific body regions; characterized by internal and external elastic laminae surrounding a thick tunica media.

Arterioles

The smallest arteries (diameters from $0.3\text{ mm to }10\,\mu\text{m}$) that regulate systemic blood pressure and blood flow into capillaries.

Vasomotor tone

A continuous state of partial smooth muscle contraction in arterioles, maintained by sympathetic motor innervation.

Atheroma

A fibrofatty plaque formed in the tunica intima of elastic and muscular arteries in response to endothelial injury, leading to arterial lumen narrowing.

Aneurysm

A localized, abnormal dilation and thinning of an arterial wall, making it prone to rupture and massive bleeding.

Continuous capillaries

Capillaries with an uninterrupted endothelial lining joined by tight junctions with small gaps called intercellular clefts, allowing only small solutes such as glucose to pass.

Fenestrated capillaries

Capillaries with continuous endothelial cells containing pores (fenestrations) that allow rapid fluid exchange and movement of small plasma proteins (found in the small intestine and kidneys).

Sinusoids

Porous capillaries characterized by large endothelial gaps and an incomplete or absent basement membrane, allowing formed elements and large proteins to pass (found in bone marrow, liver, and spleen).

Metarteriole

A vessel branch of an arteriole that feeds a capillary bed, possessing a proximal section encircled by scattered smooth muscle cells and a distal thoroughfare channel lacking smooth muscle.

Precapillary sphincter

A ring of smooth muscle located at the origin of a true capillary that regulates blood entrance into the capillary bed.

Vasomotion

The cyclical, alternating contraction and relaxation of precapillary sphincters within a capillary bed.

Perfusion

The specific volume of blood delivered to capillaries per unit of time per gram of tissue, typically expressed in $\text{mL/min/g}$.

Arterial anastomosis

The convergence of two or more arteries that supply blood to the same body region, providing alternative collateral pathways.

Arteriovenous anastomosis (shunt)

A direct vascular connection transporting blood from an artery to a vein, entirely bypassing intermediate capillary beds (found in fingers, toes, and ears).

Portal system

A vascular pathway in which blood flows through two consecutive capillary beds connected in sequence by a portal vein before returning to the heart.

Filtration

The bulk movement of fluids and small dissolved solutes out of capillary blood into the interstitial fluid, occurring primarily at the arterial end of a capillary.

Reabsorption

The bulk movement of fluid from the interstitial space back into the blood vessel lumen, occurring predominantly at the venous end of a capillary.

Blood hydrostatic pressure ($HP_b$)

The physical force per unit area exerted by blood against the interior wall of a capillary, which promotes fluid filtration into the interstitial space.

Colloid osmotic pressure (COP)

The osmotic pull on water exerted by non-diffusible proteins (such as albumin) suspended in a solution.

Net filtration pressure (NFP)

The calculated difference between net hydrostatic pressure and net colloid osmotic pressure: $NFP = (HP_b - HP_{if}) - (COP_b - COP_{if})$.

Angiogenesis

The formation and growth of new blood vessels over weeks to months in response to stimulation, such as endurance training, adipose accumulation, or tumor development.

Myogenic response

An intrinsic vascular mechanism where vascular smooth muscle contracts when stretched by increased systemic pressure and relaxes when stretch decreases, maintaining steady