Human Physiology, 14th edition (2016)

Respiratory Physiology 533

16.1 THE RESPIRATORY SYSTEM

The respiratory system is divided into a respiratory zone,

which is the site of gas exchange between air and blood,

and a conducting zone. The exchange of gases between

air and blood occurs across the walls of respiratory alveoli,

which permit rapid rates of gas diffusion.

air to blood. Carbon dioxide, conversely, moves from the blood

to the air within the lungs by diffusing down its concentration

gradient. As a result of this gas exchange, the inspired air con-

tains more oxygen and less carbon dioxide than the expired air.

More importantly, blood leaving the lungs (in the pulmonary

veins) has a higher oxygen and a lower carbon dioxide concen-

tration than the blood delivered to the lungs in the pulmonary

arteries. This is because the lungs function to bring the blood

into gaseous equilibrium with the air.

Gas exchange between the air and blood occurs entirely by

diffusion through lung tissue. This diffusion occurs very rap-

idly because of the large surface area within the lungs and the

very small diffusion distance between blood and air.

Structure of the Respiratory System

Gas exchange in the lungs occurs across an estimated 300

million tiny (about 100  m m in diameter) air sacs known as

alveoli. Their enormous number provides a large surface area

(60 to 80 square meters, or about 760 square feet) for diffu-

sion of gases. The diffusion rate between the alveolar air and

capillary blood also depends on the distance separating them.

The thickness of the average alveolar cell and capillary endo-

thelial cells is about 0.15  m m each, forming an extremely thin

air-blood distance of only about 0.3  m m.

There are two types of alveolar cells, designated type I

alveolar cells and type II alveolar cells ( fig. 16.1 ). The type I

alveolar cells comprise 95% to 97% of the total surface area

Peter, a smoker with asthma, takes two inhalers—one for

his periodic attacks and a different one for a month when

required. He passed out one day while working in his

closed garage with his car running, but fortunately was

found by his wife in time and was treated with hyperbaric

oxygen. Later that year, he was the victim of a mugging

in which he was stabbed in the chest. The severe pain

dissipated when a physician put a needle into his chest

and pulled out air with a syringe. To celebrate his recov-

ery, Peter and his wife went on vacation in the Sierra

Mountains at an altitude of 9,000 feet. He developed a

severe headache and nausea and was brought to the

hospital, where a pulse oximeter measured normal blood

oxygenation, but a pulmonary function test revealed an

abnormally low FEV 1.

Some of the new terms and concepts you will

encounter include:

• Pneumothorax, obstructive disorders, and FEV 1


• Hyperbaric oxygen treatment, carboxyhemoglobin,
  and acute mountain sickness

Clinical Investigation

LEARNING OUTCOMES

After studying this section, you should be able to:

1. Describe the structures and functions of the
   conducting and respiratory zones of the lungs.


2. Describe the location and significance of the pleural
   membranes.

The term respiration includes three separate but related func-
tions: (1) ventilation (breathing); (2) gas exchange, which
occurs between the air and blood in the lungs and between the
blood and other tissues of the body; and (3) oxygen utilization
by the tissues in the energy-liberating reactions of cell respira-
tion. Ventilation and the exchange of gases (oxygen and car-
bon dioxide) between the air and blood are collectively called
external respiration. Gas exchange between the blood and other
tissues and oxygen utilization by the tissues are collectively
known as internal respiration.
Ventilation is the mechanical process that moves air into
and out of the lungs. Because the oxygen concentration of air
is higher in the lungs than in the blood, oxygen diffuses from

Figure 16.1 The relationship between lung alveoli

and pulmonary capillaries. Notice that alveolar walls are quite

narrow and lined with type I and type II alveolar cells. Pulmonary

macrophages can phagocytose particles that enter the lungs.

Type I alveolar cell Type II alveolar cell

Alveolus

White blood cell

Red blood cell

Macrophage

Capillary

Fluid with surfactant