5 Steps to a 5 AP Biology, 2014-2015 Edition

180 ❯ STEP 4. Review the Knowledge You Need to Score High

because it is moving down its concentration gradient. Once there, it travels with the blood

to the rest of the body.

Before we move on to the digestive system, we should discuss the mechanism by which

breathing actually occurs. The rib cage and the diaphragm play important roles in the

breathing process. Inhalation causes the volume of the thoracic cavity to increase. As a

result, the air pressure in the chest falls below that of the atmosphere, and air flows into the

body. This is accompanied by a contraction of the rib-cage muscles and the diaphragm,

allowing for the increase in thoracic volume. After the air exchange occurs, the muscles

relax, causing the diaphragm to move up against the lungs, reducing the thoracic volume.

This causes the pressure in the lungs to exceed that of the atmosphere—driving the air con-

taining CO 2 out of the body.

nasal cavity

nostril

pharynx

epiglottis

glottis

larynx

trachea

bronchus

bronchiole

lung

diaphragm

pulmonary venule

pulmonary arteriole

alveolus

capillary network

Figure 15.2 The human lungs, with close-up view of alveoli, bronchi, and

bronchioles.(FromBiology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990,

1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc.

Reproduced with permission of The McGraw-Hill Companies.)