1. 3.6 – The Respiratory System
Respiratory system The organ system that is made up of the nose, mouth, trachea, bronchi, and lungs; the system that provides oxygen for the body and allows carbon dioxide to leave the body

2. The respiratory system exchanges gases between the body and the environment. Oxygen diffuses into the body, and carbon dioxide diffuses out.
See (Figure 1, pg 91) Respiratory system and Circulatory system

3. Structural Features See Figure 2, pg 91.
The main parts of the human respiratory system are the nose, pharynx, mouth, trachea, bronchi, lungs, and diaphragm.
Many of the epithelial cells that line the trachea and bronchi produce mucus and have cilia (hairlike projections).
Cilia help move mucus and filter out foreign material that might enter the respiratory system.
The trachea is supported by rings of cartilage. This keeps the trachea open and allows the air to flow freely.
The trachea separates into two branches called bronchi.
Each bronchi branch again and again, ending in tiny sacs called alveoli.

4. Gas Exchange
Breathing brings air into and out of the lungs so that gas exchange can occur.
Gas exchange takes place in the alveoli, which are surrounded by capillary networks containing blood.
See Figure 3, pg 92
The alveoli have very thin walls to allow for the diffusion of oxygen and carbon dioxide between the alveoli and the bloodstream.
The circulatory system delivers oxygen to the cells and removes carbon dioxide from the cells.

5. Breathing
Breathing involves bringing air into the lungs (inhalation) and pushing the air out (exhalation).
See Figure 4, pg 93
The muscles between the ribs and the diaphragm (large sheet of muscle under the lungs) increase or decrease the volume of the lungs.
As the volume of the lungs changes, the pressure inside them also changes allowing air to flow into and out of the alveoli.

6. Control of Breathing
Breathing is controlled by a part of the brain that detects the levels of carbon dioxide in the blood.
As the levels of carbon dioxide increases, the brain sends a signal to the diaphragm and muscles between the ribs to increase the breathing rate.
This eventually lowers the levels of carbon dioxide in the blood and increases the levels of oxygen in the blood.
Many diseases such as tuberculosis, cancers, and SARS affect the respiratory system.
The respiratory system in fish includes gills, which obtain oxygen from the surrounding water and get rid of carbon dioxide.