ЛЕКЦИЯ № 20. Veins

ЛЕКЦИЯ № 20. Veins

Veins are low-pressure vessels that have larger lumina and thinner walls than arteries. In general, veins have more collagenous connec ti-ve tissue and less muscle and elastic tissue than their arterial coun ter-parts. Although the walls of veins usually exhibit the three layers, they are much less distinct than those of the arter ies. Unlike arteries, veins contain one-way valves composed of exten sions of the intima that prevent reflux of blood away from the heart. Veins can be divided into small veins or venules, medium veins, and large veins.

Venules are the smallest veins, ranging in diameter from approxi mately 15-20 mm (post-capillary venules) up to 1-2 mm (small veins). The walls of the smaller of these are structurally and func tio-nally like those of the capillaries; they consist of an endothelium surrounded by delicate collagen fibers and some pericytes. In those vessels of increased diameter, circularly arranged smooth muscle cells occur surrounding the intima layer, but unlike in the small arteries, these cells are loosely woven and widely spaced. Venules are important in inflammation because their endothelial cells are sensitive to his-tamine released by local mast cells. This causes endothelial cells to contract and separate from each other, exposing a naked basement membrane. Neutrophils stick to the exposed collagen and extravasate (i. e., move out into the connective tissue). Histamine also causes local arterioles to relax, affect ing a rise in venous pressure and increased leaking of fluid. This produces the classic signs of inflammation: redness, heat, and swelling.

Medium veins in the range of 1-9 mm in diameter have a well – developed intima, a media consisting of connective tissue and loosely organized smooth muscle, and an adventitia (usually the thickest layer) composed of collagen bundles, elastic fibers, and smooth muscle cells oriented along the longitudinal axis of the vessel. Venous valves are sheet-like outfoldings of endothelium and underlying connective tissue that form flaps to permit unidi rectional flow of blood.

Large veins, such as the external iliac, hepatic portal, and vena ca-vae, are the major conduits of return toward the heart. The intima is similar to that of medium veins. Although a network of elastic fibers may occur at the boundary between the intima andmedia, a typical internal elastic membrane as seen in arteries is not present. A tunica media may or may not be present. If pre sent, smooth muscle cells are most often circularly arranged. The adventitia is the thickest layer of the wall and consists of elastic fibers and longitudinal bundles of collagen. In the vena cava, this layer also contains well-developed bundles of longitudinally oriented smooth muscle.

New words

vein – вена

low-pressure – низкое давление

to have – иметь

collagenous – коллагеновый

elastic – упругий

although – хотя

walls – стенкы

coun terparts – копии

unlike – в отличие от

exten sions – расширения

intima – интима

to prevent – предотвратить

reflux – рефлюкс

sheet-like – подобный листу

inflammation – воспламенение

because – потому что

longitudinal – продольный

flaps – откидные створки

external – внешний

iliac – подвздошный

hepatic – печеночник

Запомните следующие застывшие словосочетания:

To have a good time From _ morning till night All _ day long

A lot of

Перед названиями университетов артикль не употребляется.

E. g. I shall study at _ St. Petersburg University.

My grandfather graduated from _ Oxford University

Вставьте артикль, где необходимо.

1. Did you have… good time in the country?

2. Oh yes… weather was fine.

3. We were out of… doors from… morning till… night.

4. We played… football,… volley-ball and… other games.

5. We саше… home late at… night and went to… bed at once.

6. Look at… clock! It is… quarter past ten. Go to… bed at once.

7.. sun rises in.east and sets in. west.

8. We spent last summer in. Ukraine.

9. We often went to. wood and gathered lot of… mushrooms.

10. My friend likes to play… chess. He is ready to play… chess all… day long.

11. Willi comes from… very old country on… Nile. It is called… Ethiopia.

12… Ethiopia is… beautiful mountainous country.

13. Nick is… young man. He wants to become… engineer.

14. Very often in… evening his new friends and he walk about… city talking about their native countries.

15. Marina lives in. Ukraine.

16. She lives in… small village near… Kiev.

17. She is… librarian. In summer she has… lot of work work.

18. I want to enter. Moscow University.

19. My Friend lives in… America

20. He studies Indies… history at… Harvard University.

Answer the questions.

1. What are veins?

2. In general do veins have more collagenous connec tive tissue?

3. Do veins have less muscle and elastic tissue than their arterial coun terparts?

4. What valves do veins contain?

5. How veins can be divided?

6. How much the venules are long?

7. Where are venules important?

8. Where do neutrophils stick to?

9. What do contain medium veins?

10. What are the major conduits of return toward the heart?

Make the sentences of your own using the new words (10 sentences).

Find the definite and indefinite articles in the text.