HUMAN BIOLOGY

the RespiRatoRy system 181

SCIENCE COMES TO LIFE

What is respiration?

• Respiration is the overall exchange of oxygen (O 2 ) and carbon
  dioxide (CO 2 ) between the external environment and body cells.


• Gas exchange in the respiratory system depends on steep
  partial pressure gradients between the outside and inside of
  the body.


• The larger the respiratory surface and the larger the partial
  pressure gradient, the faster gases diffuse.


• The cardiovascular system transports O 2 and CO 2 between the
  lungs and tissues.


• When hemoglobin in red blood cells binds oxygen, it helps
  maintain the pressure gradient that draws air into the lungs.

takE-homE mEssaGE

in environments where there is less oxygen than normal,

such as at high altitude or underwater, the rules of gas

exchange change. For instance, the partial pressure of

oxygen falls the higher you go (Figure 10.8). a person who

isn’t acclimatized to the thinner air at high altitude can

become hypoxic—meaning that tissues are chronically

short of oxygen. above 2,400 meters (about 8,000 feet),

the brain’s respiratory centers trigger hyperventilation—

faster, deeper breathing—to compensate for the oxygen

deficiency. people with heart disease (which impairs blood

pumping) or respiratory problems such as asthma may

experience severe symptoms at high altitudes, such as the

heart pain called angina. such pain indicates that the heart

muscle is receiving too little oxygen.

When you swim or dive, there may be plenty of oxygen

dissolved in the water but the human body has no way to

extract it. (Gills do this for a fish.) people trained to dive

without oxygen tanks can stay submerged only for about

3 minutes.

Deep divers risk nitrogen narcosis or “raptures of the

deep.” this condition develops because water pressure

increases the deeper you go, and at about 45 meters

(150 feet) dangerous amounts of nitrogen gas (N 2 ) start to

become dissolved in tissue fluid and move into cells. in brain

cells the nitrogen interferes with nerve impulses, and the

diver becomes euphoric and drowsy. if a diver ascends from

depth too quickly, the falling pressure causes N 2 to enter

Breathing at high altitude and Underwater

When hemoglobin binds oxygen, it helps

maintain the steep pressure gradient

Gas exchange also gets a boost from the hemoglobin in

red blood cells. Each hemoglobin molecule binds oxygen

molecules in the lungs, where the oxygen concentration is

Figure 10.7 Dense networks of airways and blood vessels

in the lungs support gas exchange there. In the resin cast

shown in this photograph, airways are white and blood vessels

are red.

high. When blood carries red blood cells into tissues where

the oxygen concentration is low, hemoglobin releases oxy-

gen. Thus, by carrying oxygen away from the respiratory

surface, hemoglobin helps maintain the pressure gradient

that helps draw oxygen into the lungs—and into the blood

in lung capillaries. Figure 10.7 gives you an idea of the intri-

cate networks of airways and blood vessels in the lungs.

Later in this chapter you will learn more about the way

oxygen binds to and is released from hemoglobin.

the blood faster than it can be exhaled, so nitrogen bubbles

may form in blood and tissues. the resulting pain (especially

in joints) is called ”the bends” or decompression sickness.

Figure 10.8 High altitudes and underwater environments

present major challenges to breathing. This photograph

shows a climber approaching the summit of Mt. Everest, where

the air contains much less oxygen than air at sea level.

Galen Rowell/Peter Arnold, Inc.

10.3

Martin Dohrn/Royal College of Surgeons/Science Source

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