Histology Fact Sheet: Blood Vessels

Blood vessels are the tubes or channels that blood carried through in order to reach the various tissues throughout the body.

There are two systems of vessels eminating from the heart. The pulmonary vessels, transport blood from the right ventricle to the lungs for oxygenation and then returns it to the left atrium. The systemic vessels transports blood from the left ventricle to the body tissues for oxygen delivery.  The systemic vessls then returns the blood to the right atrium.

Blood flow in the vessels typically follows this pattern: artery - arteriole - capillary - post capillary venule -vein. 

Capillaries, arterioles, and postcapillary venules make up the microvascular bed of a tissue.

The tissue lining a blood vessel is called endothelium.  Endothelium is simple squamous epithelium that lines the vascular system.

Classification of Blood Vessels

Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins.

Arteries

Arteries carry blood away from the heart.

Histology hint from Sarah Bellham: Learn this definition of artery - "arteries carry blood away from the heart".  Do not consider oxygenation when classifying a vessel as an artery or vein. While most arteries carry oxygenated blood, there are exceptions, and it will trip you up.

Pulmonary arteries transport deoxygenated blood from the right ventricle to the lungs. The systemic arteries transport oxygenated blood from the left ventricle to almost all the body tissues.

Blood leaves the ventricles of the heart and goes into large elastic arteries.  The arteries branch repeatedly. The branching leads to smaller and smaller arteries. Small microscopic arteries called arterioles. The arterioles are vital in controlling blood flow into the tissue capillaries.

Histology of Arteries

An artery consists of three layers.

Tunica Intima/Tunica Interna

The innermost layer is called the tunica intima or tunica interna.  This is simple squamous epithelium (endothelium). It is on a connective tissue basement membrane with elastic fibers.

Tunica Media

The middle layer is called the tunica media.  The tunica media is made of smooth muscle. The tunica media is usually the thickest layer. The tunica media provides support for the vessel.  It also changes the vessel diameter to control blood flow and blood pressure.

Tunica Externa/Tunica Adventitia

The outermost layer is called the tunica externa or tunica adventitia. This layer attaches the vessel to the neighboring tissue. This layer is connective tissue with varying quantites of elastic fibers and collagen fibers. The connective tissue in this layer is very dense where it is next to the tunica media.  It becomes loose connective tissue near the outer portion of the vessel. 

In large vessels, the tunica adventitia contains vasa vasorum (blood vessels) and nervi vascularis (nerves). Vasa vasorum are literally the blood vessels of the blood vessels. These are the vessels which supply the vessel wall. Nervi vascularis are nerves of the blood vessels. These are the nerves which supply the vessel wall.

Classification of Arteries

Elastic Arteries

Elastic arteries are the arteries leaving the heart and the major branches. For example, the aorta is an elastic artery. In an elastic artery, the tunica media is the thickest layer.

Muscular Arteries

Most of the named arteries are muscular arteries (with the exception of the aorta and the major branches off the aorta). The dividing line between elastic arteries and muscular arteries is not clear cut. However, a pronounced internal elastic membrane and external elastic membrane are distinguishing characteristics of muscular arteries.

Histology hint from Sarah Bellham: Elastic arteries also have an internal elastic membrane. However, there is so much elastic material in the tunica intima of an elastic artery, that a single, discrete internal elastic membrane is not visible on a routine histology slide.

Arterioles

Arterioles are small branches of arteries with only one or two layers of smooth muscle in the tunica media. Smooth muscle cells in the arterioles where they branch to form capillaries regulate blood flow from the arterioles into the capillaries.

A metarteriole is a an intermediate between an arteriole and capillary.

Histology of Capillaries

Capillaries are the smallest and most abundant blood vessels.

The exchange of gases between the blood and the tissues occurs at the capillaries. The capillaries are very thin walled in order to easily allow for this exchange of gases.

The distribution of the capillaries depends on the metabolic activity and oxygen demand of the body tissues. Tissues such as skeletal muscle, liver, and kidney have extensive capillary networks because they are metabolically active and require an abundant supply of oxygen and nutrients. Other tissues, such as connective tissue, have a less abundant supply of capillaries. The epidermis of the skin and the lens and cornea of the eye completely lack a capillary network.

Classification of Capillaries

Continuous Capillaries

Pinocytotic vesicles are used to transport things across the epithelium in continuous capillaries.

Fenestrated Capillaries

Fenestrated capillaries have pores or fenestrae.

Sinuosoidal Capillaries/Sinuosoids

Sinusoidal capillaries (sinusoids) are wide, leaky capillaries. Sinuosids are found in the liver, spleen, and bone marrow.

Veins

Veins carry blood back to the heart.  After blood passes through the capillaries, it goes to venules. From the venules, blood flows into progressively larger and larger veins until it is returned to the heart.

In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs.

Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells.

Histology of Veins

The walls of veins have the same three layers as the arteries. However, when examining the histology, the tunics in veins are not as clearly marked off as are the tunics in arteries. Although all the three layers are present, there is less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries.  This is because blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time.

The diameter of the lumen of a vein is larger than the diameter of the lumen of an artery.

Veins have valves whereas arteries do not have valves. Medium and large veins have venous valves which help keep the blood flowing toward the heart. Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity.

Classification of Veins

Large Veins

Medium Veins

Venules

Venules are small branches of veins.

Arteriovenous Anastomoses

An arteriovenous anastomoses (AV anastomoses or AV shunt) is a direct connection between arteries and veins, bypassing the capillary bed.

Portal System

The normal flow of blood in the vasculature is: artery - arteriole - capillary - post capillary venule -vein. However, exceptions to this pattern of blood flow exist.

A venous portal system is when a vein is inbetween two capillary beds. An example of this is the hepatic portal system. Another example of a venous portal system is seen in the brain between the hypothalamus and pituitary.

An arterial portal system is when an arteriole is between two capillary beds. This is seen in the kidney.

Sources:

Histology: A Text and Atlas
Michael H. Ross/Edward J. Reith

http://www.training.seer.cancer.gov; funded by the U.S. National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) Program, via contract number N01-CN-67006, with Emory University, Atlanta SEER Cancer Registry, Atlanta, Georgia, U.S.A.