• Most arteries carry oxygenated blood from the heart to the rest of the body. As the oxygen is released into the tissues, the blood slowly turns a darker shade of red and returns to the heart in veins. 

• Most arteries will be a pink to reddish color, while veins are a blue or purple color. 

• Arteries are thick and muscular. Veins are thin and flabby. 

• From dissection of an animal body, you can see where oxygen is going from the heart by following the arteries. You can see where carbon dioxide is leaving by following the veins back to the heart (See Story Time about how the circulation paths were discovered).

What you can NOT see (without a microscope) is how arteries are connected to veins.

Carbon dioxide moves from high to low areas of concentration. The same is true for oxygen.

The heart has an artery that carries blood to the lungs to pick up oxygen, and release carbon dioxide; this is the only artery in the body that does not have oxygen-rich blood. When blood from the heart reaches the lungs, the carbon dioxide leaves the blood much like the carbon dioxide that leaves a coke bottle when it is opened.

A device that doctors use to measure oxygen in the blood is an oxymeter.  An oxymeter is clipped on a finger or ear lobe and uses light beams to determine how much oxygen is attached to the hemoglobin in the blood cells. This reflects the amount of oxygen getting into the blood stream. Some oxymeters also provide sensors for carbon dioxide levels to help make sure its levels are not toxic.  A more accurate measure of testing the body's oxygen levels is to test the blood directly by taking a small blood sample from an artery.

A pulse oxymeter

How do we know about gas exchange in body tissues?

Scientists measure oxygen and carbon dioxide in various heart chambers and blood vessels. They notice that the big changes in both oxygen and carbon dioxide occur in the small capillaries, because the gas composition is very different between blood entering a capillary bed and blood leaving it. How do the gases get from the blood to the cells in the body?  

• As the blood gets closer to its destination, blood vessels get smaller and thinner, until they get so small and thin that gases and other molecules can freely move in and out of the blood stream. 

• The idea is similar to diffusion, which we learned about in the Cells Are Us lesson.  What this means is that if there is more oxygen in the blood, some will move into the cells that need oxygen. 
• The cell is always using up oxygen and needs more from the blood. We know this because we can observe the color change from red to maroon of blood as oxygen is released from the blood to the tissues.

As the cell uses oxygen, it produces carbon dioxide. The carbon dioxide diffuses from the cell into the blood. How do we know this? Scientists have come up with clever ways of measuring the concentrations  of gases at these locations. 

• For example, to measure the concentration of carbon dioxide, scientists measure the acidity of the blood (carbon dioxide in blood converts to an acid compound.) 
• To measure acidity, scientists measure the acidity of the solution with a pH meter.  
• The higher the concentration of carbon dioxide, the more acidic the blood is, and the more toxic it is for the cells. 
• The enzymes, that make cells work, function best when the cells are not acidic. The further a cell gets from its "optimal" acidity level, the less effective its enzymes become and the more likely the cell is to die.  

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