The blood vessels are tubular structures throughout the entire body.

Since this tubular system is continuous (without interruption or opening), we sometimes refer to it as a closed system.

Types of Blood Vessels
In general, there are three types of blood vessels--arteries, veins, and capillaries.

We use the following abbreviations:

A. = artery V. = vein
Aa. = arteries Vv. = veins
NAVL = nerve(s), artery(ies), vein(s), lymphatic(s)

Structure of Blood Vessels
In general, blood vessels have a walls composed of three layers as follows:

• Intima - The innermost layer is the intima. The intima is a simple epithelium made up of a single layer of flat epithelial cells.

• Media - The main portion of the wall is the media. It is made up of a combination of FCT and smooth muscle tissue.

• Adventitia - The outer surface of the blood vessel is the adventitia. It is an FCT layer.

Structures of Arteries and Veins
Given an artery and a vein with similar inner diameters, the artery will have a thicker wall than the vein.

This greater thickness is due to the presence of more smooth muscle tissue and the presence of elastic FCT as a significant element.

Capillary Structure
Capillary walls have only one layer--the intima.

Capillary networks (beds) are the exchange areas for the cardiovascular system.

This includes the internal exchange areas between the blood and the individual cells of the body.

Since the capillary wall consists of flat single cells, substances can move readily between the body cells and the blood.