Moving air in and out of lungs is powered by muscles. Diffusion of oxygen occurs at a very small scale (less than 1 mm). Larger movements of oxygen require muscles to work. The diaphragm contracts to create this movement. Contraction of the diaphragm (Figure 4-8) creates suction (or negative pressure) in the chest cavity around the lungs. This creates a gradient causing air to flow into the lungs. When the diaphragm relaxes, the elastic chest cavity squeezes on the lungs causing positive pressure and air is exhaled. This is another example of flow-down gradients, this time with pressure gradients.

Stop & Think #8: Use flow-down gradients to describe how you would blow out a candle.

Diagram of the movement of the diaphragm and lungs during inhalation and expiration.

FIGURE 4-9. Inhalation and expiration. Contraction of the diaphragm moves it down causing air to fill the lungs (inhalation). Relaxation of the diaphragm allows it to move upward causing air to be released (expiration). ("Inhalation" and "Expiration" by LadyofHats is in the Public Domain, CC0)

Air flowing into the lungs can meet resistance. When you breathe hard, you probably breathe through your mouth instead of your nose. There is less resistance to flow since the mouth can open wider than the nostrils. This allows air to flow in and out faster. The bronchioles inside the lungs can also dilate or constrict. When they are constricted, this causes increased resistance to airflow. During an asthma attack, for example, bronchioles are severely constricted. Medication can be inhaled that relaxes these muscles to decrease resistance and allowing the person to breathe freely.

Learning objectives

Explain how breathing involves flow-down gradients of pressure. Include how resistance to flow affects air movement.

Stop & Think Questions Explained

8. You need to compress the chest cavity to increase pressure as much as possible over air pressure. Pressure is created by relaxing the diaphragm, as well as contracting muscles in the abdomen and between ribs. At the same time, open the mouth slightly to allow air out.