Arteries

Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Each of these types of arteries contain 3 primary layers: the tunica intima, tunica media, and tunica adventitia. In particular, the tunica media contains smooth muscle cells (allowing for vasoconstriction), and in the larger vessels, a significant amount of elastin (allowing these vessels to stretch and recoil in response to changes in pressure during systole and diastole).

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Overview

Definition

Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body.

General characteristics

All arteries have the following characteristics:

  • Thicker walls with smaller lumens than veins 
  • Appear round on histology
  • Are resistance vessels: 
    • Create most of the resistance in the circulation
    • High-pressure system
    • Can regulate vascular resistance and blood flow velocity by contracting or relaxing the smooth muscle within their walls 
  • Vessels for the transport of flowing blood: The blood passes through arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the smallest (the arterioles) before entering the capillary beds.

Layers of the Vessel Wall

All arteries have the same basic structure and are made up of three primary layers: the tunica intima, tunica media, and tunica adventitia (also known as tunica externa).

Structure of an artery wall

Structure of an artery wall

Image: “Structure of an artery wall” by Phil Schatz. License: CC BY 4.0
Cross section of artery and vein

Cross section of artery and vein

Image: “Types of Arteries and Arterioles” by Phil Schatz. License: CC BY 4.0, edited by Lecturio.

Tunica intima

  • Made up of:
    • A single layer of endothelial cells (simple, squamous epithelial cells)
    • Small amounts of fibrous connective tissue
  • Functions:
    • Acts as a selectively permeable barrier
    • Secretes vasoactive substances
    • Provides a smooth lining to the blood vessel, preventing aggregation of platelets and/or RBCs when intact
  • Endothelium appears wavy owing to constriction of the smooth muscle.
  • Vascularized by direct diffusion from the lumen
  • In capillaries: This is the only layer present in the vessel wall.
  • In larger vessels: There is an internal elastic membrane separating the tunica intima from the tunica media.

Tunica media

  • Made up of: 
    • Smooth muscle
    • Elastic fibers (e.g., elastin)
    • Collagen
  • Functions:
    • Smooth muscle allows the vessels to adjust blood flow through vasoconstriction and vasodilation.
    • Elastin in larger vessels allows for the vessel to recoil.
    • Collagen provides strength and structure.
  • Usually the thickest layer (can vary in size)
  • In larger vessels: There is an external elastic membrane separating the tunica media from the tunica adventitia.

Tunica adventitia

  • Also called the tunica externa
  • Made up of connective tissue: 
    • Collagenous and elastic fibers
    • Merges with connective tissue surrounding neighboring vessels, nerves, and organs
  • Functions:
    • Strengthens the vessel wall
    • Anchors the vessel
  • Innervated by tiny nerves known as nervi vasorum
  • Vascularized by tiny vessels known as vasa vasorum

Types of Arteries

There are 3 primary types of arteries, based on their overall size, function, and composition (known as segmental differentiation). The arteries generally exist on a continuum, with gradual changes in vessel morphology moving down the arterial tree.

The 3 primary types of arteries are:

  • Large elastic arteries (also known as conducting arteries)
  • Medium muscular arteries (also known as distributing arteries)
  • Small arteries and arterioles
Types of Arteries and Arterioles

3 primary types of arteries

Image: “Types of Arteries and Arterioles” by Phil Schatz. License: CC BY 4.0
Intraluminal pressures of different vessels

Intraluminal pressures of different vessels

Image by Lecturio.

Large/elastic arteries

This group includes the largest arteries in the body; all large/elastic arteries are close to the heart. The aorta is the largest artery in the body.

Large/elastic arteries include: 

  • Aorta
  • Pulmonary trunk and arteries
  • Brachiocephalic trunk
  • Common carotid arteries
  • Subclavian arteries
  • Common iliac arteries

Characteristics:

  • Large lumens
  • Tunica intima includes a subendothelial layer containing additional smooth muscle.
  • Many elastic fiber networks within the media
    • Elastic fibers have fenestrations: 
      • Small gaps, or “windows,” within the elastic fibers
      • Allow the diffusion of nutrients throughout the wall
    • Elastic fibers are the dominant tissue type within the aorta tunica media.
    • Allow these vessels to recoil
  • Recoil: Vessels can stretch and return to their original shape after receiving the stroke volume of blood ejected by the left ventricle during systole.
  • Effects of stretching and recoil on blood pressure:
    • Minimizes changes in blood pressure downstream during systole
    • Maintains blood pressure during diastole
  • Windkessel function:
    • During systole, the vessel walls stretch, enabling blood that is ejected during systole to be momentarily “stored.”
    • During diastole, the vessel walls recoil, and this “retracting force” moves the stored blood forward to the periphery.

Medium muscular arteries

  • Primary role: distribution of blood to the organs
  • Also known as distributing arteries
  • Tunica media has a predominance of smooth muscle:
    • Average 25–40 layers of smooth muscle (approximately ¾ of the wall thickness) 
    • Significant ability to vasoconstrict and vasodilate, resulting in significant ability to regulate blood flow
  • These arteries are more distant from the heart than large arteries.
  • Most “named” blood vessels, which are not large are medium muscular arteries (e.g., brachial, femoral, and splenic arteries).

Small arteries and arterioles

  • These vessels are usually not named.
  • Most of these vessels can be observed only under magnification.
  • Also known as resistance vessels
  • Relatively little elastic tissue
  • The tunica media is thick in proportion to its lumen relative to other arteries.
  • Small arteries: 
    • Have up to 25 layers of smooth muscle
    • Small arteries narrow toward the capillary network and become arterioles.
  • Arterioles:
    • Last vessels prior to entering the capillary beds
    • Primary point at which the body controls relative blood flow to the capillary beds of various organs 
    • Only 1–3 layers of smooth muscle
  • Metarterioles:
    • Short vessels linking arterioles and capillaries
    • Have precapillary sphincters: individual smooth muscle cells encircling the entrance to a capillary, regulating blood flow to the capillary beds

Clinical Relevance

Vasculitides

Vasculitides are a group of autoimmune conditions characterized by immune-mediated blood vessel inflammation and wall damage. Loss of vessel integrity can lead to bleeding, as well as to downstream ischemia and necrosis. These processes can be primary or secondary, and they tend to affect only vessels of a specific type or in a specific location.

Table: Classification of vasculitides
Category of vasculitisExamples of conditions in each category
Large-vessel vasculitis
  • Takayasu arteritis
  • Giant cell arteritis
Medium-vessel vasculitis
  • Polyarteritis nodosa
  • Kawasaki disease
  • Thromboangiitis obliterans (Buerger’s disease)
Small-vessel vasculitis
  • ANCA-associated vasculitis:
    • Microscopic polyangiitis
    • Granulomatosis with polyangiitis
    • Eosinophilic granulomatosis with polyangiitis (Churg–Strauss syndrome)
  • Immune-complex vasculitis:
    • Anti–glomerular basement membrane disease (Goodpasture syndrome)
    • Cryoglobulinemic vasculitis
    • IgA vasculitis (IgAV) (Henoch–Schönlein purpura)
Variable-vessel vasculitis
  • Behçet syndrome
  • Cogan syndrome
Single-organ vasculitis
  • Primary CNS vasculitis
  • Cutaneous leukocytoclastic angiitis
  • Isolated aortitis

Diseases involving abnormalities of the arterial walls (structure and/or function)

  • Hypertension: elevated blood pressure, defined as > 130 mm Hg systolic or > 80 mm Hg diastolic: Hypertension can result from either an increase in cardiac output or from systemic vascular resistance. Vascular resistance is largely determined by the relative constriction and/or dilation of the arterial walls. Hypertension is a risk factor for many secondary diseases, such as stroke, congestive heart failure, myocardial infarction, and chronic kidney disease. 
  • Atherosclerosis: common form of arterial disease in which lipid deposition forms a plaque within large, medium, and small blood vessels throughout the body: Endothelial injury triggers deposition of lipids, release of inflammatory molecules, smooth muscle hyperplasia, and the increased formation of connective tissue in the region, ultimately giving rise to the atherosclerotic plaques. These plaques lead to narrowing of the vessel lumen, loss of elasticity, and reduced or obstructed blood flow. Ruptured plaques can become embolic, leading to acute end-organ damage.
  • Arteriosclerosis: condition involving hardening of the vessels and loss of their intrinsic elasticity: Arteriosclerosis is most commonly associated with hypertension and diabetes mellitus. 
  • Aortic aneurysm: abnormal dilation in the aorta resulting from a thinning and/or weakening of the aortic wall, causing it to bulge out: Aortic aneurysms may occur in the thoracic cavity (thoracic aortic aneurysms that affect primarily the ascending aorta) or in the abdominal cavity (abdominal aortic aneurysms). Rupture of an aortic aneurysm is a life-threatening emergency.

References

  1. Taylor, A.M., Bordoni, B. (2021). Histology, blood vascular system. In StatPearls. Retrieved April 26, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK553217/ 
  2. Saladin, K.S., Miller, L. (2004). Anatomy and Physiology, 3rd ed., pp. 749–751). 
  3. Moore, K.L., Dalley, A.F. (2006). Clinically Oriented Anatomy, 5th ed., pp. 39–41).

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