1. The Respiratory System
The Respiratory System functions to move oxygen from the outside environment into the body. It also removes carbon dioxide and water from the body.

2. Why the Body Needs Oxygen
The energy-releasing chemical reactions that take place every day inside your cells require oxygen.
Respiration is the chemical reactions inside a cell.
Breathing is the movement of air in and out of your lungs.

3. The Respiratory System Helpers
Respiration could not take place without your circulatory and digestive systems.
The digestive system absorbs glucose from food.
The circulatory system carries both oxygen from your lungs and glucose from food to your cells.

4. The Air You Breathe
The oxygen your body needs comes from the atmosphere.
The atmosphere is made up of:
21% oxygen
78% nitrogen
1% carbon dioxide, helium, and other gases
Your body doesn’t use most of the air that you breathe into your lungs. When you exhale, most of the air goes back into the atmosphere.

5. The Path of Air
As air travels from the outside environment to the lungs, it passes through the following organs:
Nose
Pharynx
Trachea
Bronchi
The structures of these organs enable them to warm and moisten air and to remove harmful materials.

6. The Nose Air enters the body through two nostrils.
The lining of the nose is coated with cilia and mucus, which trap particles and warm and moisten the air.
The whip-like motion of the cilia sweeps the mucus into the throat, where you swallow it.
Other times particles and bacteria may act as an irritant and cause you to sneeze.

7. The Pharynx
Air moves from the nose downward into the throat, or pharynx.
Part of the pharynx is also a passageway for food.

8. The Trachea The trachea leads from the pharynx toward the lungs.
The walls of the trachea are made up of rings of cartilage which protect the trachea and keep it from collapsing.
The trachea is lined with cilia and mucus that sweep upward, moving mucus toward the pharynx.
If the trachea is irritated you cough.

9. The Bronchi and Lungs
Air moves from the trachea into the right and left bronchi.
One bronchus leads to each lung. Part of each bronchus is outside the lung and part is inside.
After it reaches the lungs, air moves through smaller and smaller bronchi until it reaches the alveoli.
In the alveoli, oxygen passes into the blood and carbon dioxide passes out of the blood.

10. Structure and Function in Gas Exchange
After air enters an alveolus, oxygen passes through the wall of the alveolus and then through the capillary wall into the blood.
Carbon dioxide and water pass from the blood into the alveoli.

11. How Gas Exchange Occurs
As blood travels through a capillary that is wrapped around an alveolus, it has a lot of carbon dioxide and very little oxygen.
Oxygen gradually attaches to the hemoglobin in red blood cells. At the same time it is getting rid of the carbon dioxide.
At the end of the journey around the alveolus, the blood is rich in oxygen and poor in carbon dioxide.

12. The Function of a Large Surface Area
Your lungs can absorb a large amount of oxygen because of the large surface area of the alveoli.
An adult’s lungs contain about 300 million alveoli.

13. How You Breathe
In an average day, you may breathe more than 20, 000 times.
Your brain, which directs your breathing rate, responds to internal stimuli that indicate how much oxygen your body needs.

14. Muscles for Breathing Breathing is controlled by muscles.
The lungs are surrounded by the ribs, which have muscles attached to them.
At the base of the lungs is the diaphragm, a large dome shaped muscle that plays an important role in breathing.

15. The Process of Breathing
When you inhale, the rib muscles contract, lifting the chest wall upward and outward.
At the same time, the diaphragm contracts and moves downward.
The combined action of these muscles makes the chest cavity larger, providing extra space for the lungs to expand. There is now room for more air.
When you exhale the rib muscles and diaphragm relax, and the chest cavity becomes smaller.
This decrease in size squeezes air out of the lungs.

16. The Process of Breathing

17. How You Speak
The larynx, or voice box, is located in the top part of the trachea, underneath the epiglottis.
Two vocal cords, which are folds of connective tissue that produce your voice, stretch across the openings of the larynx.

18. How the Vocal Chords Work
The vocal cords have a slit-like opening between them.
When you speak, muscles make the vocal cords contract, narrowing the opening.
Air from the lungs rushes through this opening.
The movement of the vocal cords makes the air molecules vibrate. The vibration creates a sound – your voice.

19. High and Low Tones
The length of the vocal cords affects whether you produce low or high tones.
When the vocal cords contract and shorten, you speak in a higher voice. When they relax and become longer, you speak in a lower voice.
The length of vocal cords changes during a person’s lifetime.