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Chapter 21 Questions
1. Name, in order, all the types of blood vessels, starting at the heart, going into the tissues, and returning to the heart.
The ventricles pump blood from the heart into large, elastic arteries. The elastic arteries gradually turn into muscular arteries, and from there they become arterioles. Blood flows from arterioles into capillaries. From the capillaries, blood flows into the venous system by way of venules. Venules grow larger and become small veins, and from there the small veins grow larger and become medium and or large veins. The large veins eventually bring the blood back to the heart by way of the inferior or superior vena cava.
2. Describe the structure of a capillary.
The capillary wall consists primarily of endothelial cells which rest on a basement membrane. Outside the basement membrane is a delicate layer of loose connective tissue that merges with the connective tissue surrounding the capillary. Pericapillary cells lie between the endothelial cells and the basement membrane.
3. Compare the structure of the three types of capillaries. Explain the ways that materials pass through capillary walls.
Continuous capillaries have walls which have no gaps between the endothelial cells. Continuous capillaries are less permeable to large molecules than other capillary types. Fenestrated capillaries have numerous fenestrates between the endothelial cells. Fenestrate capillaries are in tissues where capillaries are highly permeable. Sinusoidal capillaries have much larger fenestrate than the fenestrate capillaries and gaps may exist between endothelial cells. Substances cross capillary walls by diffusing through the endothelial cells, through fenestrate, or between endothelial cells.
4. Describe the capillary network. Where is the smooth muscle that regulates blood flow into and through the capillary network located? What is the function of a thoroughfare channel?
Arterioles supply blood to each capillary network. Blood then flows through the capillary network and into the venules. Smooth muscle cells called precapillary sphincters are located at the origin of the branches of the metarterioles. Thoroughfare channels connect metarterioles to venules.
5. Contrast the function of capillaries in the skin with the function of capillaries in muscle tissue.
Capillaries in the skin function in thermoregulation, and heat loss results from the flow of a large volume of blood through them. In muscle, however, nutrient and waste product exchange is the major function of the capillaries.


6. Name the three layers of a blood vessel. What kinds of tissue are in each layer?
The tunica intima consists of endothelium, a delicate connective tissue basement membrane, a thin layer of connective tissue called the lamina propria, and a fenestrated layer of elastic fibers called the internal elastic membrane.
The tunica media consists of smooth muscle cells arranged circularly around the blood vessel. The tunica media also contains variable amounts of elastic and collagen fibers, depending on the size of the vessel.
The tunica adventitia is composed of connective tissue, which varies from dense connective tissue near the tunica media to loose connective tissue that merges with the connective tissue surrounding the blood vessel.
7. Compare the amount of elastic fibers and smooth muscle found in each type of artery and vein.
Large elastic arteries have a large amount of elastic tissue and a smaller amount of smooth muscle occurring in their walls. Muscular arteries have relatively thick walls because the tunica media contains 25-40 layers of smooth muscle. Arterioles have no observable elastic membrane and the tunica media consists of one or two layers of circular smooth muscle cells. Venules and small veins have a few smooth muscle cells. Medium and large veins have a few scattered elastic fibers and the tunica media is thin and composed of a thin layer of circularly arranged smooth muscle cells.
8. What is the function of valves in the blood vessels? In which blood vessels are they found?
Valves allow blood to flow toward the heart but not in the opposite direction. Many valves are present in the medium veins, and the number is greater in veins of the lower extremities than in veins of the upper extremities.
9. Define vasa vasorum and arteriovenous anastamosis, and give their function. Define portal veins and name two examples.
Vasa vasorum are blood vessels which penetrate from the exterior of the blood vessel to form a capillary network in the tunica adventitia and the tunica media. These networks provide nutrients to blood vessels. Arteriovenous anastamosis allow blood to flow from arterioles to small veins without passing through capillaries. Portal veins are veins that begin in a primary capillary network, extend some distance, and end in a secondary capillary network. There is no pumping mechanism like the heart between the two capillary networks. There are two systems of portal veins in humans: the hepatic portal system and the hypothalamohypophysial portal system.
10. Describe the innervation of the walls of blood vessels. Which types of vessels have the greatest innervation?
Nerve fibers branch to form plexuses in the tunica adventitia, and nerve terminals containing neurotransmitter vesicles project among smooth muscle cells of the tunica media. Small arteries and arterioles are innervated to a greater extent than other blood vessel types.
11. Describe the changes that occur in the arteries due to aging. In which vessels do the most significant changes occur?
Arteriosclerosis is the hardening of the arteries. It consists of degenerative changes in arteries that make them less elastic. Arteriosclerosis greatly increases resistance to blood flow. Atherosclerosis is the deposition of material in the walls of the arteries to form distinct plaques. It is a common type of arteriosclerosis. The plaques narrow the lumens of blood vessels and make their walls less elastic. The most significant changes occur in the large elastic arteries, such as the aorta, the large arteries that carry blood to the brain, and the coronary arteries.
12. Name the factors associated with premature arteriosclerosis.
There are many risk factors associated with the development of arteriosclerosis. These factors include being at an advanced age, being a male, being a postmenopausal woman, having a family history of arteriosclerosis, smoking cigarettes, having hypertension, having diabetes mellitus, having increased blood LDL and cholesterol levels, being overweight, leading a sedentary lifestyle, and having high blood triglyceride levels.
13. For the vessels of the pulmonary circulation, give their starting point, ending point, and function.
The heart pumps blood from the right ventricle into the pulmonary trunk. This short vessel branches into the right and left pulmonary arteries, one transporting blood to each lung. Within the lungs, gas exchange occurs between air in the lungs and the blood. Two pulmonary veins exit each lung and enter the left atrium.
14. Name the parts of the aorta.
The aorta is usually divided into three parts: the ascending aorta, the aortic arch, and the descending aorta.
15. Name the arteries that branch from the aorta to supply the heart.
The coronary arteries are the only branches of the aorta which supply the heart.
16. Name the arteries that branch from the aorta to supply the head neck.
The brachiocephalic artery branches off to form the right common carotid artery which transports blood to the right side of the head and neck. The left common carotid artery supplies blood to the left side of the head and neck. The common carotid arteries continue superiorly and branch to form the internal and external carotid arteries. The external carotid arteries have several branches that supply the structures of the neck and face. The internal carotid arteries, together with the vertebral arteries, which are branches of the subclavian arteries, supply the brain with blood.