Blood Vessel Structure and Function: Arteries, Veins, and Capillaries

Arteries

Carry blood away from the heart.

Veins

Carry blood back to the heart.

Capillaries

Connect the smallest arteries to the smallest veins.

Lumen in a blood vessel

The central blood-containing space.

Three tunics of arteries and veins

Tunica interna (intima), tunica media, and tunica externa (adventitia).

Tunica interna (intima)

Acts as a selectively permeable barrier and secretes chemicals that stimulate dilation or constriction.

Epithelium in the tunica interna

Simple squamous epithelium.

Tunica media

Consists of smooth muscle, collagen, and elastic tissue; controls vasoconstriction and vasodilation.

Tunica externa (adventitia)

Anchors the vessel and provides passage for small nerves and lymphatic vessels.

Conducting (elastic or large) arteries

The biggest arteries such as the aorta, common carotid, subclavian, pulmonary trunk, and common iliac arteries.

Elastic tissue in conducting arteries

Allows arteries to expand during systole and recoil during diastole, maintaining pressure and blood flow.

Vasoconstriction

Lumen diameter decreases as the muscle contracts.

Vasodilation

Lumen diameter increases as the muscle relaxes.

Regulation of tunica media

The sympathetic nervous system.

Significance of tunica externa

Protects and reinforces vessels.

Role of endothelium in blood vessels

It normally repels blood cells and platelets.

Primary function of capillaries

To directly serve tissue cells.

General route of blood circulation

Blood travels from the heart through arteries to capillaries and returns via veins.

Role of vasa vasorum

Small vessels that supply blood to the outer part of larger vessels.

Blood pressure during systole

It increases as the heart contracts.

Blood pressure during diastole

It decreases as the heart relaxes.

Importance of the aorta

It is the largest artery in the body, distributing oxygenated blood to systemic circulation.

Role of collagen fibers in blood vessels

They protect and reinforce the vessels.

Primary function of smooth muscle in the tunica media

Controls vessel diameter, affecting blood flow and pressure.

Distributing arteries

Also known as muscular or medium arteries.

Resistance arteries

Small arteries that regulate blood flow to specific organs.

Arterioles

The smallest arteries that distribute blood to specific organs.

Muscular arteries

Arteries that have a thicker tunica media relative to their lumen and very little tunica externa.

Smooth muscle layers in muscular arteries

Constitute three-fourths of the wall thickness in muscular arteries.

Metarterioles

Short vessels linking arterioles to capillaries with a precapillary sphincter.

Precapillary sphincter

Controls blood entering the capillary bed.

Aneurysm

A weak point in an artery or heart wall forming a bulging sac that may rupture.

Common sites for aneurysms

Abdominal aorta, renal arteries, and the arterial circle at the base of the brain.

Arterial sense organs

Sensory structures in vessel walls that monitor blood pressure and chemistry.

Baroreceptors

Sensory structures that monitor blood pressure.

Carotid sinuses

Baroreceptors located in the carotid arteries.

Chemoreceptors

Sensory structures that detect changes in blood chemistry.

Types of capillaries

Continuous, fenestrated, and sinusoidal.

Continuous capillaries

Most common in most tissues.

Key feature of continuous capillaries

Endothelial cells have tight junctions forming a continuous tube.

Small solutes in continuous capillaries

Pass through intercellular clefts between endothelial cells.

Blood-brain barrier

A type of continuous capillary without intercellular clefts; least permeable.

Fenestrated capillaries

Capillaries with pores allowing increased permeability.

Scarce capillaries

Found in tendons, ligaments, epithelial tissue, and the cornea/lens of the eye.

Composition of capillaries

Endothelium and basal lamina.

Location of fenestrated capillaries

Found in kidneys and small intestine.

Sinusoids

Discontinuous capillaries in the liver, bone marrow, and spleen with wide gaps.

Function of capillary beds

Networks controlling blood flow through constriction of upstream arterioles.

Significance of precapillary sphincters

They control blood flow within capillary beds.

Differences between veins and arteries

Veins have thinner walls, less muscle, larger lumens, and can collapse when empty.

Venous sinuses

Veins with thin walls and large lumens that cannot vasoconstrict.

Diameter range of medium veins

Up to 10 mm.

Structure of large veins

Diameter greater than 10 mm, thin tunica media, thick tunica externa.

One-way valves in veins

Formed by the tunica interna to prevent backflow.

Causes of varicose veins

Valve failure leading to blood pooling and vessel distension.

Promotion of varicose vein development

Heredity, obesity, and pregnancy.

Hemorrhoids

Varicose veins of the anal canal.