About 1/5 of the air you breath is in oxygen.  Your cells have to have oxygen to do their work.  If cells don't have oxygen, they will die - some in 3 to 5 minutes.  That's why you can't hold your breath for a long time.  Your body makes you breath to keep your cells alive.
        When you inhale, or breath in, your respiratory system  brings air that has oxygen into your body.  The cells in your body use the oxygen.  As your cells work, they make carbon dioxide.  Carbon dioxide leaves your body as a waste when you exhale, or breath out.
        Your respiratory system includes your nose, your lungs, and the tubes that connect them.  Air enters your body through your nose, which has the job of getting air ready for your lungs. Air that is very dry, dirty, or cold could could harm your lungs.  Your nose moistens, cleans, and warms the air you inhale.

     When you inhale, air comes into your body.  You already know that air moves through your nose, throat, and trachea.  Your trachea divides into 2 bronchial tubes, each of which goes into a lung.  Inside your lungs, your bronchial tubes divide into smaller and smaller tubes.  The smallest tubes go to groups of tiny pouches that are called air sacs.
        Inhaled air, which has lots of oxygen, enters the air sacs.  At this moment, the blood in the vessels around the air sacs has a lot of carbon dioxide, which the blood picked up from body cells.  The blood contains little oxygen.
        An exchange of gases quickly takes place.  Oxygen goes from the air sacs into the blood vessels.  The blood blood now has oxygen to take to the body cells.  At the same time the oxygen goes out of the air sacs, carbon dioxide goes from the blood vessels into the air sacs.  The carbon dioxide leave your body when you exhale.

   You now know about the path that air takes into and out of your lungs.  You might wonder, though, what makes the air move.  How do you inhale and exhale?  The answer involves muscles.
        Your soft, spongy lungs don't have any muscles.  The muscles that allow you to breath aren't inside your lungs.  Your diaphragm is a big sheet of muscle below your lungs.  You also have muscles in-between your ribs.  They work with your diaphragm to help you breath.
        Suppose you want to blow up a beach ball.  First, you have to take a deep breath.  For this to happen, your diaphragm contracts, or tightens.  As it contracts, it moves down and flattens.  The muscles in-between your ribs also contract, pulling your ribs up and out. These movements make the space inside your chest bigger.  Air rushes into your lungs to fill in the extra space.
        When you are ready to exhale into the beach ball your diaphragm relaxes.  It moves up taking its normal domelike shape.  The muscles in-between your ribs also relax.  Your ribs in and down.  With these movements, the space inside your chest gets smaller.  Air is forced out of your lungs.
 
 

        You have learned that oxygen moves from your lungs into your blood.  In your blood, the oxygen is carried by red blood cells.  There cells float in plasma.  Red blood cells are bright red when they are are carrying oxygen.
        Blood vessels carry oxygen-rich blood to your heart, from your lungs.  Your heart pumps the blood into a big blood vessel called an artery.  This big artery divides into smaller arteries.  The smaller arteries eventually divide into capillaries.  Many kilometers of capillaries carry blood in your body.  Almost every body cell is near a capillary.
        Capillaries are so thin that red blood cells have to move through them single file.  As red blood cells move through a capillary, they release their oxygen to body cells that aren't inside the capillary.
        After red blood cells release their oxygen, they become  dark, dull, and purplish.  Now the blood flows from the capillaries into veins.  Veins take the blood to your heart, which pumps the blood to your lungs.  There the red blood cells pick up more oxygen from the air sacs.  Almost all your blood goes through your lungs about once a minute.