Sunday, January 15, 2006

Why Are Arteries and Veins Different Colors?

If you've ever stared at the blood vessels (veins and arteries) in your wrist, then you've probably noticed that your arteries look purple and your veins look blue. However, this is an illusion of nature; arteries and veins are actually both whitish in color! This illusion is due to the way that different wavelengths of light pass through our skin.

White light is composed of the different colors of visible light (red through violet). Each of the colors has its own set of wavelengths, with red having the longest wavelengths and violet having the shortest wavelengths.

Long wavelengths penetrate our skin more easily than shorter wavelengths do. So, red light can penetrate deeply enough into our skin that it reaches our blood vessels, where it is absorbed.

In contrast, blue and violet wavelengths are so short that they can not penetrate our skin very well. This means that they are reflected back at our eyes before they have a chance to be absorbed.

Therefore, when we look at blood vessels under our skin, we see the blue and violet light that is being reflected back at us, so our veins and arteries appear to be blue and purple even though our blood is not actually blue!

But why aren't veins and arteries the exact same color as each other? The answer revolves around the fact that arteries carry oxygenated blood (blood that contains a lot of oxygen) away from the heart, while veins return the deoxygenated blood (blood with less oxygen) from the tissues to the heart. [An easy way to remember this difference is to think arteries = away.]

When we inhale air, oxygen passes from our lungs into our deoxygenated blood. This addition of oxygen means that the blood is now oxygenated. The heart then pumps the oxygenated blood through the arteries to the rest of the body, where some of the oxygen is then used by our tissues (which make up our bodily organs). The leftover (deoxygenated) blood is then returned to the heart by our veins.

Oxygen changes the color of blood in such a way that oxygenated blood is a very bright red color, while deoxygenated blood is a darker red (not blue as some people believe). Now it should make sense why arteries are a bit redder than veins!

Little Lion Experiment:

Besides color, another difference between arteries in veins is that arteries pulsate (expand and contract) much more than veins do as blood flows through them. The slight changes in the shape and size of our veins is so slight that you can't even feel it.

In contrast, you can feel the pulsing of your arteries just by pressing your finger gently over an artery. This is called "finding your pulse." The two easiest places to find your pulse are on the groove of the underside of your wrist and on your neck right below you ear.

A person's pulse reflects how often blood is pumped between their heart and their blood vessels. In other words, it shows how hard their heart is working to get enough oxygen to their body.

To see how your pulse changes with the changing demands that you place on your body, place a watch or clock with a second hand in front of you. Sit still for 5 minutes, then press your index finger gently over your wrist or one side of your neck. Record how many times your feel a pulse in 12 seconds. Then multiply this number by 5 to get the number of pulses in one minute (5 x 12 seconds = 60 seconds = 1 minute). This number is called your resting pulse.

Next, if you are physically fit to do so, jog in place for a minute, do 30 jumping jacks, or do some other safe form of exercise for about 1 minute. Immediately after that, sit down and record how many times your feel a pulse in 12 seconds. Then multiply this number by 5 to get the number of pulses in one minute.

Compare your resting pulse to your pulse just after your exercised to see how much harder your heart has to work when you move around. This difference shows that physical activity requires extra oxygen since your tissues need oxygen in order to make energy.