Tunica externaThe outer layer of tough connective tissue. It anchors the vessel in place. Also called the adventitia.
Tunica mediaThe tunica media: the middle layer of smooth muscle and elastic fibers. It is thickest in arteries, and it narrows or widens the vessel.
Tunica intimaThe inner layer, lined by the smooth endothelium that touches the blood. Also called the interna.
LumenThe lumen: the hollow space in the middle where the blood flows. An artery’s lumen is smaller and rounder than a vein’s.
The artery wall: 6 2
External elastic membraneA band of elastic fibers between the tunica media and tunica externa. Found in larger arteries.
Internal elastic membraneA band of elastic fibers between the tunica intima and tunica media. Found in larger arteries.
Vein and capillary: 4 4
VeinSeen cut across: the same three layers but a much thinner wall. The lumen is wider and often flattened. That is because the blood in it is at low pressure.
ValveFlaps inside many veins that let blood flow only toward the heart. Found especially in the legs.
CapillaryThe smallest blood vessel. It is just wide enough for one red blood cell to squeeze through.
EndotheliumThe endothelium: a single layer of flat cells. It lines every blood vessel, and in a capillary it is almost the whole wall.
How this list was compiled: Read 2026-09-27. OpenStax, Anatomy and Physiology 2e, section 20.1 Structure and Function of Blood Vessels, https://openstax.org/books/anatomy-and-physiology-2e/pages/20-1-structure-and-function-of-blood-vessels : every vessel has a lumen; artery walls are thicker and their lumens smaller and rounder than veins’; from inside out the tunics are the tunica intima (interna), lined with endothelium, the tunica media of smooth muscle and elastic fibres, thickest in arteries, and the tunica externa (adventitia) of connective tissue, often the thickest layer in veins; larger arteries have an internal elastic membrane between intima and media and an external elastic membrane between media and externa; many veins, especially in the limbs, have valves that keep blood flowing toward the heart; a capillary wall is the endothelium with its basement membrane, and the smallest capillaries are barely wide enough for a red blood cell. Licence: own work
The drawing: Drawn for this site on 2026-10-04 by script. The cut faces of the artery and the vein follow Gray's Anatomy, 20th edition (Henry Gray, drawn by Henry Vandyke Carter and others, 1918), fig. 448, Transverse section through a small artery and vein, after Klein and Noble Smith (https://commons.wikimedia.org/wiki/File:Gray448.png), and fig. 450, section of a medium-sized artery (https://commons.wikimedia.org/wiki/File:Gray450.png); both public domain. The valve and the capillary are own work, nothing traced. Licence: own work
How it works
Asked to label the blood vessels using the hints provided? This diagram shows an artery and a vein cut across, a vein valve and a capillary. Start with the 4 main parts.
Read the full guide
Three layers
An artery's wall has three layers round the lumen: the thin tunica intima, lined with endothelium; the thick muscular tunica media; and the tough tunica externa. Larger arteries have an elastic membrane either side of the media.
Veins and capillaries
A vein has the same layers but a thinner wall and a wider lumen, because its blood is at lower pressure. Valves inside keep blood moving toward the heart. A capillary's wall is little more than endothelium.
To tell an artery from a vein on a slide, compare walls: an artery's is thick and round, a vein's thin round a wider lumen.
Questions and answers
What are the three layers of a blood vessel?
From the inside out: the tunica intima, lined with endothelium; the tunica media, of smooth muscle and elastic fibres; and the tunica externa, of connective tissue.
Why are artery walls thicker than vein walls?
Arteries carry blood straight from the heart at high pressure, so they need a thick, muscular tunica media. Veins carry blood back at low pressure.
Why do veins have valves?
Blood in veins is at low pressure, and in the legs it must climb against gravity. Valves let it flow toward the heart and stop it flowing back.