Chapter 20- Cardiovascular System: Vessels and Circulation.

Arteries

* Arteries progressively branch into smaller vessels as they extend from the heart to the capillaries. There is both a corresponding decrease in lumen diameter and a change in the composition of the tunic wall that includes both a decrease in relative amount of elastic fibers and an increase in the relative amount of smooth muscle. **Arteries may be classified into 3 basic types: elastic arteries, muscular arteries, and arterioles.**

Structure and Function of Blood Vessels.

*Blood vessels are classified into 3 primary types based on function.*

Arteries

ARTERIES convey blood away from the heart to the capillaries.

Arterioles

ARTERIORLES are the smallest arteries, with diameters ranging from 0.3 millimeters to 10 micrometers; these vessels are not named. In general, arteriorles have fewer than 6 layers of smooth muscle in their tunica media. Larger arterioles have all 3 tunics, whereas the smallest arterioles may have a tunica intima surrounded by a single layer of smooth muscle cells. Smooth muscle in the arterioles is slightly contracted (just as your skeletal muscles often are in a partial state of contraction). **This contracted state is called VASOMOTOR TONE and is regulated by the vasomotor center in the brainstem. Vasomotor tone results in vasoconstriction, which allows for varying degrees of change from this slightly contracted state. Blood vessels can be either vasoconstricted to a greater degree to decrease blood flow or vasodilated to allow more blood into an area. **ARTERIORLES have a significant role in regulating systemic blood pressure and blood flow into different areas of the body.

Capillaries

CAPILLARIES (capillaris=relating to hair) are microscopic, relatively porous blood vessels for the exchange of substances between blood and tissues.

Elastic Arteries

ELASTIC ARTERIES are the largest arteries, with diameters ranging from 2.5 to 1 centimeter. They are also called CONDUCTING ARTERIES because they conduct blood- from heart to the smaller muscular arteries. As their name suggests, these arteries have a large proportion of elastic fibers; these are present throughout all 3 tunics, *especially in the tunica media.* The abundant elastic fibers allow the artery to stretch and accommodate the blood when a heart ventricle ejects blood into it during ventricular systole (contraction) and then recoil, which helps propel the blood through the arteries during ventricular distole (relaxation). **The largest arteries close to the heart (e.g. aorta, pulmonary trunk, brachiocephalic, common cartoid, subclavian) and the common iliac arteries are examples of elastic arteries.** Elastic arteries branch into muscular arteries.

Muscular Arteries

MUSCULAR ARTERIES typically have diameters ranging from 1 centimeter to 0.3 millimeters. These medium-sized arteries are also called distributing arteries because they distribute blood to specific body regions and organs. *Muscular arteries have a proportionately thicker tunica media, with multiple layers of smooth muscle cells.* --Unlike in elastic arteries, the elastic fibers in muscular arteries are confined to 2 circumscribed sheets: the INTERNAL ELASTIC LAMINA and EXTERNAL ELASTIC LAMINA.-- The INTERNAL ELASTIC LAMINA seperates the tunica media from the tunica intima... The EXTERNAL ELASTIC LAMINA seperates the tunica media from the tunica externa. **The relatively greater amount of muscle and lesser amount of elastic tissue result in a better ability to vasoconstrict and vasodilate, although with a lessened ability to stretch in comparison to elastic arteries. Most named arteries (e.g. the brachial, anterior tibial, coronary, and inferior nonsenteric arteries) are examples of muscular arteries. Muscular arteries branch into arterioles.

Tunica Externa

The TUNICA EXTERNA (externe=outside) , or tunica adventitia, is the outermost layer of the blood vessel wall. It's composed of areolar connective tissue that contains elastic and collagen fibers. The TUNICA EXTERNA helps anchor the vessels to the other structures. Very large blood vessels require their own blood supply to the tunica externa in the form of a network of small arteries called the VASA VASORUM (vessels of vessels). The Vasa Vasorum extend through the tunica externa.

Tunica Media

The TUNICA MEDIA (medius=middle) is the middle layer of the vessel wall. It's composed predominantly of circulatory arranged layers of smooth muscle cells that are supported by elastic fibers. **Contraction of smooth muscle in the Tunica Media results in VASOCONSTRICTION , or narrowing of the blood vessel lumen. Relaxation of the asmooth muscle causes VASODILATION, or widening of the blood vessel lumen.

Tunica Intima

The innermost layer of a blood vessel wall is the TUNICA INTIMA (intimus= inmost), or tunica interna. It's composed of ENDOETHELIUM (a simple squamous epithelium) that faces the blood vessel lumen and a thin subendothelial layer of areolar connective tissue. The endothelium both provides a smooth surface as the blood moves through the lumen of the blood vessel and releases substances (e.g. nitric oxide, endothelin) to regulate contraction and relation of smooth muscle within the tunica media. **Remember that the endothelium is continuous with the endocaridium, which is the inner lining of the heart.

Lumen

The tunics surround the LUMEN, or inside space of the vessel, through which blood flows.

Veins

VEINS drain blood from the capillaries, transporting it back to the heart.

Tunics

Vessel walls are composed of layers called TUNICS (tunica=coat). **The 3 tunics are the: tunica intima, tunica media, and the tunica externa.**