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11.33: Processes of Breathing

Difficulty Level: At Grade Created by: CK-12
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Where does oxygen go in your body?

Once you take in a breath of air, the oxygen doesn't just stay there in your lungs. It has a lot of traveling to do! Oxygen has to reach each one of your cells. How do you think the oxygen is moved?

The Journey of a Breath of Air

Breathing is only part of the process of bringing oxygen to where it is needed in the body. After oxygen enters the lungs, what happens?

  1. The oxygen enters the blood stream from the alveoli , tiny sacs in the lungs where gas exchange takes place ( Figure below ).
  2. The oxygen-rich blood returns to the heart.
  3. Oxygen-rich blood is then pumped through the aorta , the large artery that receives blood directly from the heart.
  4. From the aorta, oxygen-rich blood travels to the smaller arteries and, finally, to the capillaries , the smallest type of blood vessel.
  5. The oxygen molecules move, by diffusion, out of the capillaries and into the body cells.
  6. While oxygen moves from the capillaries and into body cells, carbon dioxide moves from the cells into the capillaries.
  7. Carbon dioxide is brought, through the blood, back to the heart and then to the lungs. Then it is released into the air during exhalation.

Gas exchange is the movement of oxygen into the blood and carbon dioxide out of the blood.


  • alveoli : Tiny sacs in the lungs where gas exchange takes place.
  • aorta : Largest artery; receives blood directly from the heart.
  • capillary : Smallest type of blood vessel that connects very small arteries and veins.


  • Oxygen enters the lungs, then passes through the alveoli and into the blood. The oxygen is carried around the body in blood vessels.
  • Carbon dioxide moves into the blood capillaries and is brought to the lungs to be released into the air during exhalation.


Use the resource below to answer the questions that follow.

  1. What causes oxygen to enter the blood? Where does this occur?
  2. What causes carbon dioxide to exit the blood? Where does this occur?
  3. How does hemoglobin carry oxygen through the blood?
  4. How does oxygen enter the tissues of the body? Where does this occur?


  1. How does oxygen enter the blood stream?
  2. What is the name of the waste gas that is released during exhalation?

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Difficulty Level:

At Grade


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Nov 04, 2014
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