HUMAN BIOLOGY

the RespiRatoRy system 187

hoW is breathing controlled?

• Respiratory centers in the brain stem control the rhythmic
  pattern of breathing.


• Brain centers that adjust the rate and depth of breathing
  receive information mainly from sensors that monitor blood
  levels of carbon dioxide.


• These controls contribute to homeostasis by helping to maintain
  proper levels of carbon dioxide, oxygen, and hydrogen ions in
  arterial blood.

takE-homE mEssaGE

aortic bodies Receptors in

the aorta that detect shifts

in blood levels of carbon

dioxide and oxygen.

carotid bodies Recep-

tors in carotid arteries that

detect shifts in blood lev-

els of carbon dioxide and

oxygen.

Overall, the mechanisms that control our breathing
allow gas exchange to match the body’s activity level. For
example, if you start exercising, your skeletal muscles
immediately require more oxygen and begin producing
more CO 2. As you have just read, these changes prompt
the brain’s respiratory center to step up its signals to the
breathing muscles (Figure 10.17).

other controls help match air flow
to blood flow

Controls over air flow also operate in the millions of lung
alveoli. For example, if you get nervous, your heart may
start pumping hard and fast but your lungs may not be
ventilating at a corresponding pace. If too little carbon
dioxide is moving out of the lungs, the rising blood level of
CO 2 makes smooth muscle in the walls of bronchioles relax
and widen, so more air flows through them. On the other
hand, an abnormal decrease in the level of carbon dioxide
in the lungs causes the bronchiole walls to constrict, so
less air flows through them. Shifting oxygen levels have
a similar effect. If you breathe in oxygen faster than it can
enter blood capillaries, the oxygen level rises in parts of
the lungs, capillaries dilate. As more blood flows through
them, it can pick up more oxygen. When less oxygen is
available in the lungs, the vessels constrict and less blood
moves through them.

brain stem
(pons and
medulla)
receptors detect
decreases in pH
of cerebrospinal
fluid due to
rising CO 2
in blood

carotid

bodies

(CO 2 , O 2

receptors)

aortic

bodies (O 2

receptors)

heart

lungs

spinal cord

Figure 10.16 Sensors in arteries and the brain monitor
carbon dioxide, oxygen, and blood pH. (© Cengage Learning)
only minor aspects of breathing
are under conscious control
Reflexes such as swallowing or cough-
ing briefly halt breathing. You also can
deliberately alter your breathing pat-
tern, as when you change your nor-
mal breathing rhythm to talk, laugh,
or sing, or when you hold your breath
underwater. At most, however, you
can only hold your breath for 2 or
3 minutes. As CO 2 builds up and
your blood’s chemistry shifts,
“orders” from the nervous system
force you to take a breath.

Figure 10.17 Breathing patterns change with a person’s

activity level. (© Cengage Learning; photo: © C. Yokochi and J. Rohen, Photographic

Anatomy of the Human Body, 2nd Ed., Igaku-Shoin, Ltd., 1979)

Chemoreceptors

in wall of carotid

arteries and aorta

Diaphragm,

intercostal muscles

Respiratory center

in brain stem

STiMULUS

CO 2 concentration and

acidity rise in the blood

and cerebrospinal fluid.

Tidal volume and rate of breathing change.

RESPonSE

CO 2 concentration

and acidity decline

in the blood and

cerebrospinal fluid.

© iStockphoto.com/lisafx

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