Blood Vessels Lab

Introduction

The vascular system has a common histological organization that appears with particular clarity at the level of the large vessels. The tissue components (endothelium, smooth muscle, elastic elements and connective tissue) that form the vascular walls are arranged in concentric layers. Under the influence of local functional factors, this layered structure has undergone segmental differentiations that characterize each type of vessel. For descriptive purposes the concentric layers have been classified as three tunics as follows (from the lumen outward):

• The tunica intima comprises the endothelium, the adjacent basement membrane, the subendothelial connective tissue, and the internal elastic lamina; in smaller vessels, pericytes appear between layers of the split basement membrane of the endothelium.
• The tunica media is composed of smooth muscle cells, elastic lamellae including the external elastic lamina and collagen fibers.
• The tunica adventitia contains connective tissue, a few cells, macrophages, mast cells, fibroblasts, and the nerves and vessels that supply the vascular wall.

Elastic Artery

Elastic arteries are primarily represented by the large vessels emerging from the heart ventricles, such as the aorta and the pulmonary artery. This slide shows the three layers of the wall of the aorta. The tunica intima is thin and composed of endothelial cells and their underlying supporting tissue. The tunica media is the largest portion of the wall and is composed of elastic fibers, smooth muscle and collagenous tissue. Note the presence of large numbers of elastic fibers. Elastic fibers in the media allow for the maintenance of blood pressure through the expansion and contraction of the vessel walls. Finally, the tunica adventitia is the outermost component of the arterial wall. It contains mostly connective tissue and a few small blood vessels called vasa vasorum that support the cells that make up the arterial wall.

Muscular Artery

Muscular arteries follow the elastic arteries. Through the controlled contraction of their walls, these arteries distribute the blood to different parts of the body according to regional needs. The media of muscular arteries is composed of many concentric layers of smooth muscle cells arranged in a low angle helix, interspersed with less frequent and sometimes discontinuous elastic lamellae. The internal elastica marks the conventional boundary between intima and media. The external elastica separates the media from the adventitia. The media and adventitia of muscular arteries are approximately equal in thickness. The tunica media of muscular arteries contains fewer elastic fibers and more smooth muscle cells than elastic arteries.

Small Muscular Artery

As muscular arteries branch and decrease in size, the number of layers of smooth muscle cells in the tunica media decreases. Also, the internal and external elastic laminae become much less prominent.

Arterioles

Arterioles contain an internal elastic lamina and one or two layers of smooth muscle cells. There is no external elastic lamina, and the adventitia consists of a thin layer of collagen and isolated elastic fibers. By the contraction of their muscle fibers, the arterioles generate the "peripheral resistance" that reduces the blood pressure at the periphery, and thereby protects the capillaries and venules.

Capillaries

Capillaries are vessels of small diameter (4 to 10 microns) whose wall is reduced to an attenuated endothelium surrounded by a basement membrane, a few pericytes, and connective tissue. The narrow capillary lumen allows passage of red blood cells in single file.

Continuous Capillary

At the level of the electron microscope, three different types of capillaries can be resolved based on the morphology of their endothelial layer: continuous, fenestrated and discontinuous. Endothelial cells in continuous capillaries completely enclose the lumen of the blood vessel. The only gaps are the junctions between adjacent endothelial cells where small molecules can diffuse between the bloodstream and surrounding tissue. Continuous capillaries are prominent in adipose and muscle tissue and in the brain.

Fenestrated Capillary

Capillaries with a fenestrated endothelium have gaps between endothelial cells, but the basement membrane is still continuous. These capillaries are far more permeable than those with continuous endothelial linings and are found in the renal glomeruli, endocrine glands, intestinal villi, and exocrine pancreas.

Venules

Veins of small and medium size are characterized by a thin media containing only a few layers of smooth muscle cells. These vessels have a much thicker adventitia composed of collagen and occasionally some longitudinal smooth muscle fibers.Venules are tubes of endothelium. Small venules (up to 40-50 microns in diameter) are surrounded by pericytes, contractile cells with long, branching processes that are involved in the control of blood as it flows through the microvasculature. Large venules (50-100 microns in diameter) are surrounded by one or two layers of smooth muscle cells. Beyond the pericytes and the smooth muscle cells is a thin layer of connective tissue.This image of a venule shows several of its characteristic features. Identify its endothelium and narrow layer of smooth muscle cells. The largest wall component of the venule is usually the adventitia composed of connective tissue.

Artery and Venule

Compare the structure of the venule to that of the small artery. The lumens of the vessels are similar in size but the artery has a thicker medial layer with more smooth muscle. Veins, with thinner walls, are more compliant and capable of holding more blood. Consequently, arteries tend to maintain their shape better than veins in histological sections.

Vena Cava

This image shows the wall of the vena cava, which is the largest vein in the body. Note the relatively thin media compared to the aorta. The media of vena cava contains many fewer elastic fibers than the media of a large elastic artery like the aorta.

Lab Quiz

1. Why the large number of elastic lamina in this structure?

Answer: Maintain diastolic pressure
2. Name the scalloped structure.

Answer: Internal elastic lamina.
3. Name the type of vessel.

Answer: Muscular artery
4. Identify A, B, C, and D.

Answer: A = tunica intima, B = tunica media, C = tunica adventitia, D = internal elastic lamina