Arteries: Structure, Types, and Function

Arteries are blood vessels that carry blood away from the heart. They form a high-pressure, low-resistance system that distributes oxygenated blood (with the exception of the pulmonary arteries) to the organs and tissues of the body.

Classification of Arteries

Arteries are classified by size and structure into three main types:

Elastic Arteries

Elastic arteries are the largest arteries, located closest to the heart. They serve as conducting vessels and pressure reservoirs.

Structure

Tunica intima:

• Endothelium: Continuous, non-fenestrated
• Subendothelial layer: Thin layer of loose connective tissue
• Internal elastic lamina: Prominent, fenestrated

Tunica media:

• Concentric layers of elastic lamellae (40-70 in the adult aorta)
• Smooth muscle cells between elastic lamellae
• Collagen (types I, III) and elastin
• Proteoglycans (versican, aggrecan)
• No external elastic lamina (merges with adventitia)

Tunica adventitia:

• Thin relative to media
• Collagen and elastic fibers
• Vasa vasorum (in large elastic arteries)
• Nerves (vasomotor)

Elastic Properties

The high elastin content gives elastic arteries their unique properties:

• Compliance (C): Change in volume per change in pressure (C = ΔV/ΔP)
• Distensibility: Ability to stretch under pressure
• Elastic recoil: Return to original shape after stretching

Function during cardiac cycle:

• Systole: Aorta expands, storing kinetic energy as potential energy (Windkessel effect)
• Diastole: Aorta recoils, releasing stored energy, maintaining forward blood flow

This Windkessel effect converts pulsatile flow from the heart into continuous flow through the peripheral circulation.

Age-Related Changes

Muscular Arteries

Muscular arteries distribute blood to specific organs and tissues. They have more smooth muscle relative to elastin compared to elastic arteries.

Structure

Tunica intima:

• Endothelium
• Subendothelial layer
• Internal elastic lamina: Prominent, undulating

Tunica media:

• 10-40 layers of smooth muscle
• Circularly arranged
• Individual muscle cells surrounded by basement membrane
• Collagen and elastic fibers between muscle layers
• Relatively little elastin (0-5 elastic lamellae)
• Vasomotion: Active changes in diameter

External elastic lamina:

• Present in most muscular arteries
• Separates media from adventitia
• More prominent in larger muscular arteries

Tunica adventitia:

• Thick (may be thicker than media in some arteries)
• Collagen, elastic fibers
• Fibroblasts
• Vasa vasorum
• Nerves

Vasomotion

The smooth muscle of muscular arteries actively regulates vessel diameter:

• Vasoconstriction: Smooth muscle contraction reduces diameter
• Vasodilation: Smooth muscle relaxation increases diameter

Regulators of vasomotion:

Autoregulation

Muscular arteries maintain relatively constant blood flow despite changes in perfusion pressure:

Myogenic response:

• Increased pressure: Smooth muscle stretches, contracts (vasoconstriction)
• Decreased pressure: Smooth muscle relaxes (vasodilation)
• Mechanism: Stretch-activated ion channels

Metabolic regulation:

• Increased metabolism: Local metabolites cause vasodilation
• Decreased metabolism: Reduced metabolites cause vasoconstriction
• Key metabolites: Adenosine, CO2, H+, K+, lactate

Small Arteries

Small arteries (0.1-1 mm) serve as the final distributing vessels before the arterioles.

Structure

• Thin tunica intima with prominent internal elastic lamina
• 3-6 layers of smooth muscle in the media
• Thin adventitia
• Minimal vasa vasorum

Function

• Regulate regional blood flow
• Contribute significantly to total peripheral resistance
• Respond to local metabolic demands

Comparison of Artery Types

Mechanical Properties

Wall Stress

Arterial wall stress is described by the Law of Laplace:

σ = P × r / h

Where:

• σ = Wall stress (tension per unit area)
• P = Intraluminal pressure
• r = Internal radius
• h = Wall thickness

Implications:

• Larger radius arteries experience higher wall stress
• Wall thickening (hypertrophy) reduces wall stress
• Hypertension increases wall stress

Stress-Strain Relationship

Arterial walls are non-linear viscoelastic materials:

• At low pressures: Elastin bears the load (stiffness low)
• At high pressures: Collagen recruits and bears the load (stiffness high)
• This protects arteries from overdistension

Pulse Pressure

Pulse pressure = Systolic BP - Diastolic BP

• Normal: 30-50 mmHg
• Increased in: Arterial stiffness, aortic regurgitation, hyperthyroidism
• Decreased in: Aortic stenosis, heart failure, hypovolemia

Clinical Significance

Atherosclerosis

Atherosclerosis preferentially affects muscular arteries:

• Plaque formation: Lipid accumulation, inflammation, fibrous cap
• Site predilection: Branch points, curvatures, areas of low shear stress
• Complications: Stenosis, plaque rupture, thrombosis

Arterial Stiffness

Increased arterial stiffness (arteriosclerosis):

• Reduces Windkessel effect
• Increases pulse pressure
• Increases afterload on the left ventricle
• Causes target organ damage (brain, kidney)
• Associated with aging, hypertension, diabetes

Giant Cell Arteritis

Granulomatous inflammation of large and medium arteries:

• Prefers the temporal artery
• Can involve the aorta
• Associated with polymyalgia rheumatica
• Presents with headache, jaw claudication, vision loss

Takayasu Arteritis

Granulomatous large vessel vasculitis:

• Affects the aorta and its branches
• Causes stenosis, occlusion, aneurysm
• Pulseless disease (upper extremity)
• More common in young Asian women

Aneurysm

Localized dilation of an artery:

• True aneurysm: Involves all three layers (atherosclerotic, cystic medial degeneration)
• False aneurysm (pseudoaneurysm): Breach in arterial wall, contained by adventitia
• Dissection: Tear in intima, blood enters the media

Distribution of Arterial Types in the Body