Blood Vessels – Tutorial

Introduction to Blood Vessels

Blood vessels are used to transport blood from the heart to tissues all over the body and back to the heart in a continuous loop known as the circulatory system.  Blood vessels are a branched system of pipe-like tubes of various sizes with an inner lumen (chamber) that contains blood as it’s conducted through the body.  Blood is carried from the heart to tissues of the body and back following this typical vessel (vascular) pattern:  heart to arteries to arterioles to capillaries to venules to veins and back to the heart.  There are a few exceptions to this typical blood flow pattern that will not be discussed in this chapter.  The tutorials of this chapter will focus primarily on the wall structure and characteristics of arteries and veins.

The outermost layer of arteries and veins is known as the tunica externa.  The tunica externa is a connective tissue sheath containing collagen and elastic fibers which give flexible support to the vessel wall and anchor it to surrounding tissues.  In veins, the tunica externa also contains a small amount of smooth muscle.

The tunica media is the middle layer of a vessel wall and it consists mainly of circular (concentric) bands of smooth muscle wrapped around the vessel.  The tunica media of large arteries (elastic arteries) also contains a significant amount of elastic fibers that allow the walls of an artery to stretch when the heart contracts and forces a pulse of blood through the lumen of the artery.  Together, the smooth muscle and elastic fibers help support the walls of arteries as they contain high pressure blood moving away from the heart.  The tunica media of veins is much thinner compared to that of arteries and consists almost entirely of smooth muscle cells and scattered collagen fibers.

The tunica intima is the inner-most layer of a vessel wall.  The tunica intima is relatively thin with a simple squamous endothelial lining.  The simple squamous endothelium provides a smooth inner surface which helps reduce friction between the vessel wall and blood as it flows through the lumen.  In large arteries, the tunica intima has a rippled appearance when the vessel is relaxed (not stretched) which allows the tunica intima to stretch along with the other layers of the vessel wall.  In arteries, an internal elastic membrane is located between the tunica intima and the tunica media which allows the two layers to stretch together.

The wall structure of arterioles and venules is the same as that of arteries and veins only arterioles and venules are smaller in size and for practical purposes can be thought of as microscopic arteries and veins.  The structure of capillaries is entirely different than that of arteries, arterioles, venules, and veins.  Capillary walls do not possess a tunica media or tunica externa and instead consist of simple squamous endothelium and a thin basement membrane.  This simple structure makes the walls of capillaries extremely thin and somewhat permeable which allows for the exchange of nutrients, gases, and metabolic wastes between the bloodstream and surrounding tissues.

Tutorial:  Use the image slider below to learn more about the characteristics of arteries and veins. 

Microscopy:  Use the image slider below to learn how to use a microscope to study arteries and veins on a microscope slide.

Tutorial:  Use the hotspot image below to learn more about the structure and characteristics of arteries. 

Tutorial:  Use the hotspot image below to learn more about the structure and characteristics of veins. 

Tutorial:  Use the image slider below to study numerous examples of blood vessel images.