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why do veins have valves but arteries do not

Your heart is a strong muscle that can pump blood in spurts to all regions of your body right down to every capillary. The more difficult part is getting it back. Rather than elastic arteries that carry away blood quickly and efficiently on the outbound route, we now have tiny capillaries that need to be cleared into increasingly larger veins to return the blood for cleaning in the liver and re-oxidizing in the lungs before the next departure. Moreover,
blood flows under high pressure in artery (BP is measured as pressure of blood exerted on wall of artery) but in veins the blood pressure is less than 10mm of Hg.

To allow flow of blood back to heart, lumen inside vein is larger in diameter and to prevent backflow, series of valves are there. Your veins have one-way valves in them to keep the blood moving, and in the right direction. You can help in this procedure simply by making movements. Try to avoid staying in one place or position for too long. Blood vessels may be tiny but they cover a lot of ground.

The smallest blood vessels measure only five micrometers. To give you some perspective, a strand of human hair measures about 17 micrometers. But if you took all the blood vessels out of an average child and laid them out in one line, the line would stretch over 60,000 miles. An adults would be closer to 100,000 miles long. There are three kinds of blood vessels:Parteries,Pveins, andPcapillaries. Each of these plays a very specific role in the circulation process.

Arteries carry oxygenated blood away from the heart. Theyre tough on the outside but they contain a smooth interior layer of epithelial cells that allows blood to flow easily. Arteries also contain a strong, muscular middle layer that helps pump blood through the body. Capillaries connect the arteries to veins. The arteries deliver the oxygen-rich blood to thePcapillaries,Pwhere the actual exchange of oxygen and carbon dioxide occurs.

The capillaries then deliver the waste-rich blood to the veinsPfor transport back to the lungs and heart. Veins carry the blood back to the heart. Theyre similar to arteries but not as strong or as thick. Unlike arteries, veins contain valves that ensure blood flows in only one direction. (Arteries dont require valves because pressure from the heart is so strong that blood is only able to flow in one direction. ) Valves also help blood travel back to the heart against the force of gravity.

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