Human Physiology, 14th edition (2016)

552 Chapter 16

Oxygen Toxicity

Although breathing 100% oxygen at one or two atmospheres

pressure can be safely tolerated for a few hours, higher par-

tial oxygen pressures can be very dangerous. Oxygen tox-

icity may develop rapidly when the P^ O 2 rises above about

2.5 atmospheres. This is apparently caused by the oxidation

of enzymes and other destructive changes that can damage the

nervous system and lead to coma and death. For these reasons,

deep-sea divers commonly use gas mixtures in which oxygen

is diluted with inert gases such as nitrogen (as in ordinary air)

or helium.

Nitrogen Narcosis

Although at sea level nitrogen is physiologically inert, larger

amounts of dissolved nitrogen under hyperbaric (high-

pressure) conditions have deleterious effects, possibly caused

by the increased amounts of nitrogen dissolved in plasma mem-

branes at the high partial pressures. Nitrogen narcosis resem-

bles alcohol intoxication; depending on the depth of the dive,

the diver may experience what Jacques Cousteau termed “rap-

ture of the deep.” Dizziness and extreme drowsiness are other

narcotizing effects.

oxygen in a localized area of a lung, perhaps because of

pneumonia or an embolus. This response can be maladap-

tive when the hypoxia is chronic and occurs throughout the

lungs, producing widespread pulmonary vasoconstriction

that elevates pulmonary artery pressure, which can lead to

right heart failure. By contrast, in systemic arteries hypoxia

causes decreased Ca^2 1 , relaxation of vascular smooth mus-

cles, and vasodilation.

Constriction of the pulmonary arterioles where the alveo-

lar P^ O 2 is low and their dilation where the alveolar P^ O 2 is high

helps to match ventilation to perfusion (the term perfusion

refers to blood flow). If this did not occur, blood from poorly

ventilated alveoli would mix with blood from well-ventilated

alveoli, and the blood leaving the lungs would have a lowered

P^ O 2 as a result of this dilution effect.

Dilution of the P^ O 2 of pulmonary vein blood actually does

occur to some degree, despite these regulatory mechanisms.

When a person stands upright, the force of gravity causes a

greater blood flow to the base of the lungs than to the apex

(top). Ventilation likewise increases from apex to base, because

there is less lung tissue in the apex and less expansion of alve-

oli during inspiration. However, the increase in ventilation

from apex to base is not proportionate to the increase in blood

perfusion. The ventilation/perfusion ratio at the apex is high

(0.24 L air divided by 0.07 L blood per minute gives a ratio

of 3.4/1.0), while at the base of the lungs it is low (0.82 L air

divided by 1.29 L blood per minute gives a ratio of 0.6/1.0).

This is illustrated in figure 16.23.

Functionally, the alveoli at the apex of the lungs are over-

ventilated (or underperfused) and they are larger than alveoli at

the base. This mismatch of ventilation/perfusion ratios is nor-

mal and is largely responsible for the 5 mmHg difference in P^ O 2

between alveolar air and arterial blood. Abnormally large mis-

matches of ventilation/perfusion ratios can occur in cases of

pneumonia, pulmonary emboli, edema, and other pulmonary

disorders. In chronic pulmonary disorders such as emphysema,

chronic bronchitis, and cystic fibrosis, widespread alveolar

hypoxia and consequent vasoconstriction of pulmonary arte-

rioles can produce pulmonary hypertension. This can eventu-

ally lead to right heart (ventricle) failure, a condition called cor

pulmonale.

Disorders Caused by High Partial

Pressures of Gases

The total atmospheric pressure increases by one atmosphere

(760 mmHg) for every 10 m (33 ft) below sea level. If a diver

descends 10 meters below sea level, therefore, the partial pres-

sures and amounts of dissolved gases in the plasma will be

twice those values at sea level. At 20 meters, they are three

times, and at 30 meters they are four times the values at sea

level. The increased amounts of nitrogen and oxygen dissolved

in the blood plasma under these conditions can have serious

effects on the body.

Figure 16.23 Lung ventilation/perfusion ratios. The

ventilation, blood flow, and ventilation/perfusion ratios are

indicated for the apex and base of the lungs. The ratios

indicate that the apex is relatively overventilated and the base

underventilated in relation to their blood flows. As a result of such

uneven matching of ventilation to perfusion, the blood leaving the

lungs has a P^ O 2 that is slightly lower (by about 5 mmHg) than the

P^ O 2 of alveolar air.

Base

Apex

Blood flow

(L/min)

Ratio

0.24 0.07 3.40

0.82 1.29 0.63

Ventilation

(L/min)

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