Blood vessels

Last updated: March 2, 2022

Summarytoggle arrow icon

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.

Large blood vesselstoggle arrow icon

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

Large blood vessel layers

Layer (from the lumen outward)

Sublayers Components Function
Tunica intima (intima)
  • Subendothelial layer
  • Internal elastic lamina
Tunica media (media)
  • Muscular layer
  • External elastic lamina
  • Vessel elasticity
Tunica adventitia (externa/adventitia)
  • No sublayers
  • Attaches vessel to neighboring structures
  • Contains vessel-specific blood supply (vasa vasorum), nerves, and lymph drainage in large vessels


Elastic arteries Muscular arteries
Location Vessels close to the heart (aorta , pulmonary trunk, and their large branches) Vessels farther from the heart (e.g., brachial artery, femoral artery)
Components of the tunica media

Primarily elastic fibers

Primarily smooth muscle


Absorb energy of left ventricular contraction of the heart during systole → dampen pulsatile blood flow to reduce the difference in blood pressure between systole and diastole

Regulate blood flow through the lumen by changing vascular resistance (i.e., resistance vessels)


Differences between arteries and veins

Characteristics Arteries Veins
Blood pressure High Low
Diameter of the lumen Relatively small Relatively large
Features of the layers Intima
  • No valves
  • Forms valves to prevent retrograde blood flow
Adventitia Relatively thin Thick compared to other layers


Small blood vesselstoggle arrow icon





Type of capillary Location Structure Transport properties

Continuous capillaries

  • Endothelium forms nearly complete tube with only irregular fenestrations

Fenestrated capillaries

  • Endothelium contains fenestrations
  • Fenestrations are spanned by a diaphragm with a negative charge

Sinusoidal capillaries (discontinuous)

  • Passage of macromolecules

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


  • 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


Referencestoggle arrow icon

  1. NIH SEER Training Modules - Classification & Structure of Blood Vessels. Updated: January 1, 2019. Accessed: April 22, 2019.
  2. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences ; 2016
  3. Leslie P. Gartner, James L. Hiatt. Color Textbook of Histology. Grune & Stratton Inc. ; 2006

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