Cardiovascular System: Blood Vessels
Blood vessels circulate blood throughout your body helping to transport gases, nutrients, hormones, and waste products to the appropriate organs and tissues. Blood vessels are typically composed of three layers or tunics: the tunica intima (inner layer), the tunica media (middle layer), and the tunica externa (outer layer). The tunica intima is a layer of endothelial cells lining the lumen (opening) of the blood vessel. These cells create a smooth surface for blood flow reducing friction. The tunica intima can contain elastic fibers, which sometimes gives it a wavy appearance (Figure 1, artery). Depending on the slide, erythrocytes from the blood may be visible in the lumen. The tunica media contains layers of smooth muscle cells and elastic fibers. These cells assist in constricting and dilating the blood vessels, changing blood vessel diameter to help regulate blood pressure. The tunica externa (tunica adventitia) is composed of connective tissue which provides structural support for the vessels.
Veins carry blood to the heart, while arteries carry blood away from the heart. Veins are more compliant/collapsed and have valves which prevent backflow of blood to inferior body regions. These valves, however are not typically visible on microscopy. Compared to veins, arteries maintain a more open/rounded structure and have a thicker tunica media with a larger proportion of elastic fibers. Figure 1 contains a micrograph of a large artery and vein.
Figure 1: Artery and vein with and without overlay
Table 1. Layers (tunics) of the blood vessel wall
| Tunics | Description | Function |
|---|---|---|
| Tunica intima | Innermost layer consisting of simple squamous epithelium cells called the endothelium. Nuclei are flattened and cells have very little cytoplasm. | Creates a smooth surface for blood flow. |
| Tunica media | One or more layers of smooth muscles cells. Cells are tapered at the ends and do not have striations. Arteries typically have a thicker tunica media compared to veins. | Responsible for vasoconstriction and vasodilation of blood vessels. |
| Tunica externa (adventitia) | Outermost layer consisting of connective tissue composed primarily of collagenous fibers. | Provides structure and support for the blood vessel. |
Chapter Illustrations by:
Juan Manuel Ramiro Diaz, Ph.D.
Layer of simple squamous epithelium lining the interior surface of the heart and blood vessels
specialized connective tissue fibers that have the ability to stretch and recoil
