2. BLOOD VESSELS

3. WHAT ARE BLOOD VESSELS? Blood vessels are intricate networks of hollow tubes that transport blood throughout the entire body.

4. Blood vessels carry blood to every part of your body. At each tissue or organ in your body, blood makes an exchange—it "drops off" oxygen and nutrients and "picks up" waste products (toxins and carbon dioxide). After the exchange, blood returns to your heart. This exchange happens in the capillaries.

5. CAPILLARIES Capillaries are small vessels, whose walls are thin enough to allow the diffusion of nutrients, oxygen and carbon dioxide across them. They are "where the action is" in gas and nutrient exchange: with a few exceptions, no cell of the body is very far from one, because access to the blood is an absolute requirement: cell death would result from lack of oxygen or the loss of nutrient and waste transport.

6. Capillaries are by far the most numerous class of vessels, though they're so small they can only be appreciated in microscopic sections, they're just about the same size as the red blood cells that flow through them! CAPILLARY WALL RED BLOOD CELL

7. Capillaries connect arteries and veins.

8. ARTERIES Arteries pump blood away from the heart. Oxygenated blood is carried away from the heart to all structures of the body by arteries, which are elastic tubes with thick walls. Because they carry blood with oxygen, arteries appear red. Blood flows through arteries with great force. This is why the walls of arteries are thick and flexible. The thicker walls help protect the arteries against damage from the high pressure.

9. Thick wall made of smooth muscle, because they are under very high pressure because of the heart. Elastic outside allows flexibility.

10. ARTERIOLES Arterioles feed the blood into capillaries. Arteries become smaller and smaller as they get further away from the heart, finally becoming arterioles (small arteries).

11. ARTERIOLES Arteries become smaller and smaller as they get further away from the heart, finally becoming arterioles (small arteries). Arterioles feed the blood into capillaries. ARTERIOLE ARTERIAL CAPILLARY

12. VENULES Capillaries pick up the blood from the tissues and return it to the venules (small veins). Venules attach to veins which brings the blood back to the heart.

13. VENULES VENOUS CAPILLARIES

14. VEINS Veins carry the oxygen-poor blood back to your heart. Veins are hollow tubes, similar to arteries, with thinner, less elastic walls, which transport deoxygenated blood back to the heart. Because they carry blood without oxygen, veins appear blue. The walls of veins are much thinner than artery walls—they don't have to be as thick because blood flows through veins at a lower pressure.

15. Since veins are on the post-capillary side of the circulatory loop, operating at much lower pressures, there's less need for burst resistance. Thin walls are also important because much of the pressure that drives blood through veins is generated not by the heart, but by contraction of the muscles in the region of the vein. This "squishes" the blood back through the vein. Because they have low pressures, some veins have venous valves in them to prevent back flow. This is especially true of medium sized veins in the extremities, as they have to lift blood against gravity.

16. Veins have thinner muscle walls than arteries. Because veins are under less pressure, they have valves to prevent back flow.

17. If an animal is held totally immobile for a long period of time, the return of the blood to the heart is diminished, because contraction of the muscles of the limbs no longer pushes blood back to the heart. Since the heart can only put out what it takes in, the end result of decreased venous return is decreased arterial output. At some point the output declines to the point where the brain is no longer receiving sufficient blood, and the animal loses consciousness. This happens surprisingly often. EXAMPLE: Soldiers on parade, when forced to remain at the position of "attention" for long periods, will from time to time keel over and quite literally "drop out" of the ranks as they faint.

18. Veins are much easier to get to as you can readily see them on most animals. Also blood runs faster in the arteries and arteries are set much deeper. Blood for certain tests must be drawn from arteries (usually the one in the wrists) an example is arterial blood gases. WHY DO WE DRAW BLOOD SAMPLES FROM VEINS?

19. Difference in size of the muscle wall of arteries and veins.

20. BLOOD CIRCULATION PROCESS http://health.howstuffworks.com/adam- 200084.htm