Section 3

The Respiratory System

• External respiration is the exchange of gases between the atmosphere and the blood. Internal respiration is the exchange of gases between the blood and the cells of the body.

• The lungs are the site of gas exchange between the atmosphere and the blood.

• Air enters through the mouth or nose, passes through the pharynx, larynx, trachea, bronchi and bronchioles and into alveoli. A network of capillaries surrounds each alveolus. All exchange of gases in the lungs occurs at the alveoli.

• Most oxygen is carried attached to hemoglobin. Some carbon dioxide is carried bound to hemoglobin. A small amount is dissolved in plasma. Most carbon dioxide is carried as bicarbonate ions.

• During inspiration, the diaphragm and rib muscles contract, the thoracic cavity expands, and air is pulled into the lungs. During expiration, the diaphragm and rib muscles relax, the thoracic cavity contracts, and air is forced out of the lungs.

• The rate of breathing is controlled by nerve centers in the brain that monitor the level of carbon dioxide in the blood.

Vocabulary respiratory system (p. 946) external respiration (p. 946) internal respiration (p. 946) lung (p. 946)

pharynx (p. 947) epiglottis (p. 947) trachea (p. 947) larynx (p. 947)

bronchus (p. 947) bronchiole (p. 947) alveolus (p. 947) inspiration (p. 949)

diaphragm (p. 949) expiration (p. 950)

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