Blood vessels

Blood vessels are an integral component of the circulatory system. The five types of blood vessels are (in order of circulation): arteries, arterioles, capillaries, venules, and veins. The primary function of large blood vessels (i.e., arteries and veins) is the transport of blood to and from the heart, whereas smaller blood vessels (e.g., capillaries) enable substance exchange between the cells and blood. Arteries carry oxygen-rich blood from the heart to the periphery. Once they reach an organ or limb, they branch and divide into arterioles and eventually into numerous small capillaries, forming a capillary bed that enables the delivery of oxygen and nutrients to the surrounding tissues. Distal to the capillary bed, postcapillary venules join together to form veins, which deliver oxygen-poor blood back to the heart. Both arteries and veins are composed of the same three layers of tissue: the tunica intima, tunica media, and tunica adventitia. Arteries contain significantly more smooth muscle than veins (especially in the tunica media) whereas veins contain valves in the tunica intima. Capillaries are entirely composed of an endothelial layer with or without a basement membrane. There are three different types of capillaries (continuous, fenestrated, and sinusoidal), all of which vary in terms of permeability and function. Blood vessels are referred to collectively as the vascular system and, together with the heart, make up the circulatory system or cardiovascular system.

A separate article on vascular physiology addresses the topics of hemodynamics, blood pressure regulation, and capillary exchange.

Consist of arteries and veins, which have the three main microscopic wall layers.

Large blood vessel layers

Arteries

• Function: transportation of arterial (oxygenated) blood from the heart to the periphery
  • Exceptions:
    • Pulmonary artery
    • Umbilical arteries of fetal circulation
• Course: artery → arteriole → capillary
• Types of arteries: blood vessel wall components vary according to the location and function

Veins

• Function: transportation of venous (deoxygenated) blood from the periphery to the heart
  • Exceptions:
    • Pulmonary veins
    • Umbilical veins of fetal circulation
• Course: capillary → venule → vein
• Characteristic feature: Veins are known as capacitance vessels because they are capable of storing a significantly larger volume of blood than arteries due to their large lumen and high compliance.
• Structure: see “blood vessel layers” above

Differences between arteries and veins

References:^[1][2][3]

Microcirculation

• Description: Blood flow through the small blood vessels (arterioles, capillaries, and venules) that is responsible for substance exchange between blood and tissue.
• Course: arteriole → metarteriole → capillary → venule

Arterioles

• Function
  • Regulate blood flow into capillaries by constriction or dilation (controlled by the sympathetic nervous system)
  • Arterioles and small arteries are also called resistance vessels because they provide ∼ 50% of the total peripheral resistance.
• Structure
  • Composed of 2–3 wall layers
  • Tunica media usually only consists of 1–2 layers of smooth muscle cells
  • The smallest arterioles do not have a tunica externa

Metarteriole

• Function: a short microvessel that connects arterioles with capillaries
• Structure
  • Tunica intima
  • A single smooth muscle cell that forms a ring around the microvessel

Capillaries

• Function: exchange of gases and nutrients with tissue (density of distribution of capillaries varies depending on the region/organ)
  • Capillary bed: site of substance capillary fluid exchange between blood and tissue in accordance with Starling forces
• Characteristic features
  • Large total cross-sectional area
  • Low blood flow velocity
  • Short diffusion path composed of a single layer of endothelial cells and a basal lamina
• Structure (from inner to outer layer)
  • Endothelial cells (single layer)
  • Pericytes
  • Basement membrane
    • Not all capillaries have a continuous basement membrane.
  • The exact structure of capillaries depends on the required substance exchange of the surrounding tissue

Endothelium makes up part of the blood-brain barrier. The dense tight junctions between endothelial cells prevent paracellular transport through the brain capillaries.

Venules

• Function: collect blood from capillaries to transfer to veins
• Structure: depends on the type of venule
  • Postcapillary venules
    • Are located directly following a capillary bed
    • Structure resembles that of capillaries (allowing high permeability and substance exchange)
  • Collecting venules :
    • Precede the collecting veins
    • Structure resembles that of larger blood vessels (i.e., with three layers)
  • High endothelial venules possess special surface molecules for leukocyte recognition, which are the sites of leukocyte extravasation into or from lymphatic organs

References:^[1][3]

• Cardiovascular examination
• Atherosclerosis
• Hypertension
• Hypotension
• Peripheral arterial disease
• Coronary heart disease
• Chronic venous disease
• Edema
• Cystic medial necrosis
• Aneurysm
• Aortic dissection
• Heart failure
• Shock