HUMAN BIOLOGY

186 ChapteR 10

Controls over Breathing

a respiratory pacemaker in the brain sets

the basic rhythm of breathing

Adults usually take about twelve to fifteen breaths a min-

ute. If you had to remember to inhale and exhale each

time, could you do it, even when you sleep? Luckily for us

all, a respiratory center in the medulla in the brain stem, at

the lower rear of the brain, provides this service. Like the

heart’s SA node, this center contains neurons that fire spon-

taneously. They are the pacemaker for respiration.

As Figure 10.15 suggests, signals from the respiratory

center travel nerve pathways to the diaphragm and chest.

These signals stimulate the rib cage muscles and diaphragm

to contract. As you read in Section 10.4, this causes the rib

cage to expand, and you inhale a breath as air moves into the

lungs. In between nerve impulses, the diaphragm and chest

muscles relax. Elastic recoil returns the rib cage to its unex-

panded state, and you exhale as air in the lungs moves out.

Carbon dioxide is the main trigger for controls

over the rate and depth of breathing

While the respiratory center governs the basic operations

of breathing, other controls determine how rapidly and

deeply the lungs are ventilated. Overall, these controls

monitor three aspects blood chemistry: the levels of

carbon dioxide and oxygen in the bloodstream and the

acidity, or pH, of blood. Sensory receptors that respond to

chemicals are called chemoreceptors. Some of the sensors

are in the brain stem, and others monitor the blood flow-

ing through arteries.

You might guess that the amount of oxygen in blood is

the most important factor in respiratory control systems, but

actually brain stem chemoreceptors are more sensitive to

levels of carbon dioxide. The receptors also detect hydrogen

ions that are produced when dissolved CO 2 leaves the blood-

stream and enters fluid that bathes the medulla. In this fluid

(called cerebrospinal fluid) the drop in pH that goes along

with increasing H^1 indicates that the blood is becoming

more acidic. The brain’s respiratory centers respond to this

signal (Figure 10.16). In short order breathing becomes more

rapid and deeper. Soon the blood level of CO 2 falls—and so

does the blood’s acidity. Notice that this is another example

of a negative feedback loop helping to maintain homeostasis.

The brain also receives information about blood gases

and pH from chemoreceptors in arteries. These receptors

include carotid bodies, where the carotid arteries branch

to the brain, and aortic bodies in artery walls near the

heart. Both types of receptors detect changes in levels of

carbon dioxide and oxygen in the blood. They also detect

changes in blood pH. When there is too little oxygen in the

blood relative to carbon dioxide and hydrogen ions, the

brain responds by increasing the ventilation rate, so more

oxygen can be delivered to tissues.

n The nervous system controls muscle movements that lead

to the normal rhythm of breathing. It also controls how

often and how deeply you breathe.

n Links to pH scale 2.7, Structure and function of skeletal

muscles 6.2, The two circuits of blood flow 7.3

diaphragm

stretch

receptors

in alveoli

of lungs

motor pathways

via spinal cord:

neurons

(pacemaker

for respiration)

phrenic nerve

to diaphragm

intercostal

nerves to

rib muscles

brain stem

(pons and

medulla)

vagus nerve

Figure 10.15 Respiration centers in the brain control the

basic operations of breathing. In quiet breathing, centers

in the brain stem coordinate signals to the diaphragm and

muscles that move the rib cage, triggering inhalation. When

a person breathes deeply or rapidly, another center receives

signals from stretch receptors in the lungs and coordinates

signals for exhalation. (© Cengage Learning)

10.6

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