609

CHAPTER

36

Gas Transport & pH

in the Lung

OBJECTIVES

After studying this chapter, you should be able to:

■

Describe the manner in which O

2

flows “downhill” from the lungs to the tissues

and CO

2

flows “downhill” from the tissues to the lungs.

■

Describe the reactions of O

2

with hemoglobin and the oxygen–hemoglobin disso-

ciation curve.

■

List the important factors affecting the affinity of hemoglobin for O

2

and the phys-

iologic significance of each.

■

List the reactions that increase the amount of CO

2

in the blood, and draw the CO

2

dissociation curve for arterial and venous blood.

■

List the principal buffers in blood and, using the Henderson–Hasselbalch equation,

describe what is unique about the bicarbonate buffer system.

■

Define alkalosis and acidosis and outline respiratory and renal compensatory

mechanisms in response to alkalosis and acidosis.

■

Define hypoxia and describe its four principal forms.

■

List and explain the effects of carbon monoxide on the body.

■

Describe the effects of hypercapnia and hypocapnia, and give examples of condi-

tions that can cause them.

INTRODUCTION

The partial pressure gradients for O
2
and CO
2
, plotted in

graphic form in Figure 36–1, emphasize that they are the key

to gas movement and that O
2
“flows downhill” from the air

through the alveoli and blood into the tissues, whereas CO
2
“flows downhill” from the tissues to the alveoli. However, the

amount of both of these gases transported to and from the tis-

sues would be grossly inadequate if it were not that about 99%

of the O
2
that dissolves in the blood combines with the O
2

carrying protein hemoglobin and that about 94.5% of the CO

2

that dissolves enters into a series of reversible chemical reac-

tions that convert it into other compounds. Thus, the pres-

ence of hemoglobin increases the O

2

-carrying capacity of the

blood 70-fold, and the reactions of CO

2

increase the blood

CO

2

content 17-fold. In this chapter, physiologic details that

underlie O

2

and CO

2

movement under various conditions are

discussed.

OXYGEN TRANSPORT

OXYGEN DELIVERY TO THE TISSUES

The O
2
delivery system in the body consists of the lungs and
the cardiovascular system. O
2
delivery to a particular tissue

depends on the amount of O

2

entering the lungs, the adequa-

cy of pulmonary gas exchange, the blood flow to the tissue,

and the capacity of the blood to carry O

2

. The blood flow de-
pends on the degree of constriction of the vascular bed in the
tissue and the cardiac output. The amount of O
2
in the blood
is determined by the amount of dissolved O
2
, the amount of