When you exercise, your muscles act something like electric motors. Your muscles take in a source of energy and they use it to generate force. An electric motor uses electricity to supply its energy. Your muscles are biochemical motors, and they use a chemical called adenosine triphosphate (ATP) for their energy source. During the process of "burning" ATP as fuel, your muscles need three things. They need oxygen, because it's needed to produce ATP. They need to get rid of metabolic wastes (carbon dioxide, lactic acid). And, they need to get rid of heat. Working muscles generate a lot of heat.

In order to continue exercising, your muscles must continuously make ATP. To make this happen, your body must supply oxygen to the muscles and get rid of the waste products and heat. The tougher the exercise, the greater the needs are of the working muscle. If these needs are not met, then you will become exhausted and you won't be able to keep going. To meet these needs, the body has an incredibly detailed plan involving the heart, blood vessels, nervous system, lungs and even the skin.

After just two minutes of exercise, the body responds to supply working muscles with oxygen. As you exercise, the blood vessels in your muscles dilate so that more blood can flow - it's like the difference between a fire hose and a garden hose. Obviously, more water can flow through the fire hose because it's larger. The blood carries the much-needed oxygen to the muscles. While this is going on, a pretty cool diversion happens. Blood that would have gone to the stomach or the kidneys goes instead to the muscles.

It's up to your lungs and the rest of your respiratory system to provide more oxygen for all the blood that's headed to the working muscles. When you exercise, you breathe harder and faster. This is because sympathetic nerves stimulate the respiratory muscles to make you breathe faster. Byproducts from muscles, like carbon dioxide and lactic acid, signal the respiratory centers in your brain, which, in turn, stimulate the respiratory muscles, telling them to work harder. And, slightly higher blood pressure, caused by the increased force of each heartbeat, opens blood flow to more air sacs in the lungs, allowing more oxygen to enter the blood.

So, now your body has increased the flow of oxygen-rich blood to your muscles, but your muscles still need to get the oxygen out of the blood. An exchange of oxygen and carbon dioxide is the key here. A protein called hemoglobin (it's found in red blood cells) handles this job. Hemoglobin can bind oxygen and/or carbon dioxide; the amount of oxygen bound to hemoglobin is determined by the oxygen concentration and carbon dioxide concentration. Normally, hemoglobin works like this: Hemoglobin in red blood cells entering the lungs has carbon dioxide bound to it. In the lungs, oxygen concentration is high and carbon dioxide concentration is low. Hemoglobin binds oxygen and releases carbon dioxide. Hemoglobin is transported through the heart and blood vessels to the muscle. In muscle, the carbon dioxide concentration is high and the oxygen concentration is low. Hemoglobin releases oxygen and binds carbon dioxide. The hemoglobin is then transported back to the lungs and the cycle repeats. As you exercise, the metabolic activity is high so more acids (like lactic acid) are produced. This reduces the attraction between oxygen and hemoglobin and causes the hemoglobin to release more oxygen than usual. This increases the oxygen delivered to the muscle.

Your exercising body is using energy and producing waste, such as lactic acid and carbon dioxide. To keep exercising, your body needs to dump these metabolic wastes. Our bodies are pretty efficient machines. All that extra blood that is flowing to the muscles and bringing more oxygen can do double duty - it can also take the wastes away. For example, the hemoglobin in the blood will carry away the carbon dioxide.

OK, extra oxygen - check. Dumping waste products like lactic acid and carbon dioxide - check. But what about getting rid of all the extra heat? It's actually pretty simple. Your body heats up when you exercise. Your skin feels hotter to the touch and may look flushed, and you sweat. Obviously, these things signal how much heat your body is giving off, but they're also doing something else - cooling your body off.

You see, sweat is like your very own air conditioner. As sweat evaporates from the skin, it is removing heat and cooling the body. It's also removing fluid from your body, so it's very important to replace those lost fluids. That's why your coach or parents are always telling you to drink plenty of water and/or sports drinks when you're playing sports or exercising.

Make two bar graphs (one for each person) using the information you found. Compare the graphs, noting how they are different.