cardiovascular

Lecture Objectives
By the end of this lecture, students are expected to:
1. Summarize the structures of muscular arteries and veins differ.
2. identify the three layers of the vein.
3. identify the different types of capillaries, and how their structure is related to their function.
4. identify the three layers of the artery.
5. Compare the histological features of conducting and distributing arteries.
6. Compare the histological features of large arteries and large veins in tunica adventitia .
7. Summarize the functional and histological structure of capillary bed.
8. Outline the histological structure of arteriole and venules.

Elastic Arteries
Elastic arteries are the aorta, the pulmonary artery, and their largest branches; these large vessels are also called conducting arteries because their major role is to carry blood to smaller arteries, the most prominent feature of elastic arteries is the thick media in which elastic lamellae, each about 10 ?m thick, alternate with layers of smooth muscle fibers. The adult aorta has about 50 elastic lamellae (more if the individual is hypertensive).
• Tunica intima is made up of an epithelium, which is a single layer of flattened epithelial cells, together with a supporting layer of elastin rich collagen. It is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death .The internal elastic lamina is not easily discerned because it is similar to the elastic laminae of the next layer.
• Tunica media is broad and elastic with concentric fenestrated sheets of elastin, and collagen and only relatively few smooth muscle fibers.
• Tunica adventitia has small vasa vasorum as the large arteries need their own blood supply.



Muscular Arteries
The muscular arteries distribute blood to the organs and help regulate blood pressure by contracting or relaxing, these arteries distribute blood to various parts of the body, These included arteries such as the femoral and coronary arteries, the walls of these arteries have lots of smooth muscle, which means that they are able to contract or relax (dilate) to change the amount of blood delivered, as needed.
Comparing these arteries to the elastic arteries, the sheet of elastin is now much reduced, and found at the border between the tunica intima and tunica media in a layer called the internal elastic layer (IEL) which can be seen very clearly. Less well defined is the external elastic layer (EEL), between the tunica media and tunica adventitia. There is a well defined circular layer of smooth muscle in the tunica media.
• The tunica intima has an endothelium of flattened endothelial cells. The tunica media is primarily a layer of smooth muscle, with some elastin an collagen. muscle layer, and is sandwiched between the IEL and EEL.
• The Tunica Adventitia is very broad, and mostly contains collagen and elastin.

Arteriole.
Arterioles almost always branch to form anastomosing networks or beds of capillaries that surround the parenchymal cells of the organ. Smooth muscle fibers act as sphincters closing arterioles and producing periodic blood flow into capillaries . Acting as “resistance vessels,” muscle tone usually keeps arterioles partially closed In certain tissues and organs arterioles deviate from this simple path to accommodate various specialized functions, for example, thermoregulation by the skin involves arterioles that can bypass capillary networks and connect directly to venules.
The media and adventitia are thicker in these arteriovenous shunts (or arteriovenous anastomoses) and richly innervated by sympathetic and parasympathetic nerve fibers. The autonomic fibers control the degree of vasoconstriction at the shunts, regulating blood flow through the capillary beds. High capillary blood flow in the skin allows more heat dissipation from the body, while reduced capillary blood flow conserves hea