veins (from the Latin vena),in the circulatory system,are blood vessels that carry blood towards the heart.
Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart.
Veins differ from arteries in structure and function; for example, arteries are more muscular than veins, veins are often closer to the skin and contain valves to help keep blood flowing toward the heart, while arteries carry blood away from the heart.
Blood that flows up to the brain faces the same problem.
If the blood is having a hard time climbing up, you will feel light-headed and possibly even faint.
Fainting is your brain's natural request for more oxygen-rich blood.
When you faint, your head comes down to the same level as your heart, making it easy for the blood to quickly reach the brain.
Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated blood to the heart.
Veins differ from arteries in structure and function; for example, arteries are more muscular than veins, veins are often closer to the skin and contain valves to help keep blood flowing toward the heart, while arteries carry blood away from the heart.
The vein valves are necessary to keep blood flowing toward the heart, but they are also necessary to allow blood to flow against the force of gravity.
For example, blood that is returning to the heart from the foot has to be able to flow up the leg. Generally, the force of gravity would discourage that from happening. The vein valves, however, provide footholds for the blood as it climbs its way up.
For example, blood that is returning to the heart from the foot has to be able to flow up the leg. Generally, the force of gravity would discourage that from happening. The vein valves, however, provide footholds for the blood as it climbs its way up.
Blood that flows up to the brain faces the same problem.
If the blood is having a hard time climbing up, you will feel light-headed and possibly even faint.
Fainting is your brain's natural request for more oxygen-rich blood.
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When you faint, your head comes down to the same level as your heart, making it easy for the blood to quickly reach the brain.
Because it lacks oxygen, the waste-rich blood that flows through the veins has a deep red color, almost like maroon. Because the walls of the veins are rather thin, the waste-rich blood is visible through the skin on some parts of the body. Look at your wrist, or hands, or ankles. You can probably see your veins carrying your blood back to your heart. Your skin refracts light, though, so that deep red color actually appears a little blue from outside the skin.
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