Chapter 11. Blood Vessels

A. Introduction

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Image source: Birmingham City University

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Basic Histology of Blood Vessels

The systemic and pulmonary circulations are made up of three basic types of blood vessels: arteries, capillaries, and veins. The definitions of these vessels are related to the direction of blood flow through the vessel with respect to the heart. Arteries convey blood away from the ventricles of the heart and to body tissue. Veins convey blood away from body tissue and to the atria of the heart. Capillaries convey blood from the smallest arteries to the smallest veins.

Almost every generation of arteries and veins have walls composed of three major tissue layers, or tunics.

  • Tunica Intima. The tunica intima is the innermost layer. It consists of endothelium resting on subendothelial connective tissue. The thickness and composition of this subendothelial layer varies depending on the type and size of vessel.
  • Tunica Media. The tunica media is the middle layer. It consists of layers of circularly or obliquely-arranged smooth muscle cells with variable amounts of elastin, collagen, and ground substance interposed.
  • Tunica Adventitia. The tunica adventitia is the outer layer. It is composed primarily of connective tissue consisting primarily of collagen fibers and variable amounts of elastic fibers. The tunica adventitia of large arteries and veins also contains blood vessels (the vasa vasorum) and autonomic nerves (the nervi vascularis).

Comparing Arteries to Veins

Generally, an artery and a vein of similar branching generation run together within the same connective tissue sheath, forming a vascular bundle. Within a vascular bundle, the artery can be distinguished from a vein based on several specific characteristics:

  • The total wall thickness of an artery is usually greater than that of a vein of the same generation,
  • The lumenal diameter of an artery is usually smaller than that of a vein of the same generation,
  • The vein is often collapsed.