Elastin can be stained with special stains. Elastic fibers in the media allow for the maintenance of blood pressure through the expansion and contraction of the vessel walls. The vasa vasorum, a network of small vessels that supplies the cells of larger vessels, is present in their adventitia and outer part of the media.
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.
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 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. At the level of the electron microscope, three different types of capillaries can be resolved based on the morphology of their endothelial layer:.
Venules are tubes of endothelium. These veins contain a greater amount of smooth muscle than most veins of this size. Also note the outer longitudinal arrangement of smooth muscle in these veins.
Observe the characteristic three layers of the arteries present. Note the large amount of circularly arranged smooth muscle in the media of the muscular arteries. Find the muscular arteries. Internal and external elastic laminae or membranes are stained black and therefore readily observed. Look carefully in the media of the arteries and you should be able to resolve elastic fibers also. In arterioles, the tunica intima consists of a continuous endothelium and a very thin subendothelial layer.
In addition, a thin, fenestrated internal elastic lamina is present in larger arterioles but absent in terminal arterioles. Generally, the tunica media consists of 2 layers of smooth muscle cells. But in the smallest arterioles there is a single layer.
The tunica adventitia is a thin sheath of connective tissue, which is not easily defined. Slide 18 Uterine tube. Find the multitude of vessels in the connective tissue which invests the oviduct. This slide is especially good for observing small arteries , veins , arterioles , venules , and capillaries. Identify the layers of the vessels present. The wall consists of extremely attenuated endothelial cells. In cross section, the lumen of small capillaries may be encircled by a single endothelial cell, while larger capillaries may be made up of portions of 2 or 3 cells.
No smooth muscle is present. Recall that at the EM level, 3 types of capillaries can be distinguished: continuous, fenestrated, and discontinuous. Capillaries and venules are the principal vessels where exchange between the blood vascular space and the interstitium takes place. A surprising amount of exchange of fluid and cells takes place through the wall of the venule. Slide 55 Appendix. In the submucosa and adventitia of the appendix, observe venules , capillaries , and arterioles.
Slide 81 is also useful for small vessels. Veins are best studied along with their corresponding arteries. Veins have the same 3 layers as arteries, but boundaries are indistinct and elastic components are not as well developed in veins. Histologically, veins are usually collapsed because of the thinner, less elastic walls. The thick outermost layer of a vessel tunica adventitia or tunica externa is made of connective tissue.
The middle layer tunica media is thicker and contains more contractile tissue in arteries than in veins. It consists of circularly arranged elastic fibers, connective tissue, and smooth muscle cells.
The inner layer tunica intima is the thinnest layer, comprised of a single layer of endothelium supported by a subendothelial layer. Capillaries consist of a single layer of endothelium and associated connective tissue. Key Terms tunica intima : The innermost layer of a blood vessel. Blood Vessel Function Blood vessels carry nutrients and oxygen throughout the body and aid in gas exchange.
Learning Objectives List the functions of blood vessels. Key Takeaways Key Points Systemic and pulmonary circulatory systems efficiently deliver oxygen to the tissues of the body and remove waste products such as carbon dioxide.
Venous blood except in the pulmonary vein is deoxygenated and returns to the heart to be pumped into the lungs for reoxygenation. Nutrients carried in the blood are released to tissues via the permeable endothelium of blood vessels. Immune cells move throughout the circulatory system and are able to rapidly permeate the walls of blood vessels to attend sites of injury or infection. Blood vessels can increase or decrease blood flow near the surface of the body, either increasing or reducing the amount of heat lost as a means of regulating body temperature.
Key Terms thermoregulation : The maintenance of a constant internal temperature of an organism independent of the temperature of the environment. Licenses and Attributions. CC licensed content, Shared previously.
Capillaries are thin-walled vessels composed of a single endothelial layer. Because of the thin walls of the capillary, the exchange of nutrients and metabolites occurs primarily via diffusion. The arteriolar lumen regulates the flow of blood through the capillaries. Venules are the smallest veins and receive blood from capillaries.
They also play a role in the exchange of oxygen and nutrients for water products. There are post-capillary sphincters located between the capillaries and venules. The venule is very thin-walled and easily prone to rupture with excessive volume. Blood flows from venules into larger veins. Just like the arterial system, three layers make up the vein walls. But unlike the arteries, the venous pressure is low. Veins are thin-walled and are less elastic.
This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance.
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