HUMAN BIOLOGY

the RespiRatoRy system 183

expiration Exhalation of

a breath.

inspiration Inhalation of

a breath.

respiratory cycle The in/

out pattern of breathing.

tidal volume Volume of

air entering and leaving

the lungs during a normal

breath.

vital capacity Maximum

volume of air that can move

out of the lungs after the

deepest possible inhalation.

Pneumothorax, or a “collapsed lung,” is caused by

injury or illness that allows air to enter the pleural cavity.

The lungs can’t expand normally and breathing becomes

difficult and painful.

how much air is in a “breath”?

About 500 milliliters (two cupfuls) of air enters or leaves

your lungs in a normal breath. This volume of air is

called tidal volume. You can increase the amount of air

you inhale or exhale, however. In addition to air taken in

as part of the tidal volume, a person can forcibly inhale

roughly 3,100 milliliters of air, called the inspiratory reserve

volume. By forcibly exhaling, you can expel an additional

expiratory reserve volume of about 1,200 milliliters of air.

Vital capacity is the maximum vol-

ume of air that can move out of the

lungs after you inhale as deeply

as possible. It is about 4,800 milli-

liters for a healthy young man and

about 3,800 milliliters for a healthy

young woman. As a practical matter,

people rarely take in more than half

their vital capacity, even when they

breathe deeply during strenuous

exercise. At the end of your deepest

exhalation, your lungs still are not

completely emptied of air; another

roughly 1,200 milliliters of residual

volume remains (Figure 10.10).

How much of the 500 milliliters of inspired air is

available for gas exchange? Between breaths, about

150 milliliters of exhaled “dead” air remains in the air-

ways. Thus only about 350 (500 2 150) milliliters of fresh

air actually reaches the alveoli each time you inhale. An

adult typically breathes at least twelve times per minute.

This rate of ventilation supplies the alveoli with 4,200

(350 3 12) milliliters of fresh air every 60 seconds—about

the volume of soda pop in four 1-liter bottles.

When food “goes down the wrong way” and enters the

trachea (instead of the esophagus), it’s impossible to inhale

or exhale normally. A choking person can suffocate in

just a few minutes. The Heimlich maneuver can dislodge

material from the trachea by elevating the diaphragm

(Figure 10.11). This reduces the chest volume, forcing air up

the trachea. With luck, the air will rush out with enough

force to eject the item.

0

1,000

2,000

3,000

4,000

5,000

Lung volume (milliliters)

6,000

Time

inspiratory

reserve

volume

expiratory

reserve volume

tidal

volume

vital

capacity

total

lung

capacity

residual

volume

Figure 10.10 Animated! Lung volume changes during quiet
breathing and during forced inspiration and expiration. In this
graph you can see “spikes” above and below the normal tidal
volume. (© Cengage Learning)

Figure 10.11 Animated! The Heimlich maneuver is an
emergency procedure designed to rescue a choking adult.
(Top: © Cengage Learning 2015; Bottom: From Starr/ Taggart/Evers/Starr, Biology,
12E. © 2009 Cengage Learning)
What is the respiratory cycle?

• The in/out air movements of breathing make up the
  respiratory cycle.


• Breathing air movements occur when lung volume changes as
  the chest cavity expands and shrinks.


• These changes alter the pressure gradients between the lungs
  and outside air.

takE-homE mEssaGE

You may recall from Chapter 2 that the hydrogen bonds
between water molecules prevent them from being easily
pulled apart. This cohesiveness of water molecules in the
fluid in the pleural sac also helps your lungs hug the chest
wall, in much the same way that two wet panes of glass
resist being pulled apart.

a Back blows. Have the person lean forward. Use the heel of

your hand to strike between the shoulder blades.

B Abdominal thrusts. Stand behind the person and place one

fist below the rib cage, just above the navel, with your thumb

facing inward. Cover the fist with your other hand and thrust

inward and upward with both fists.

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).