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Figure

The medullary rhythmicity and pneumotaxic areas of the respiratory center control breathing.

U Where is the respiratory center located?

^9 Describe how the respiratory center maintains a normal breathing pattern.

^9 Explain how the breathing pattern may be changed.

Factors Affecting Breathing

In a mixture of gases such as air, each gas accounts for a portion of the total pressure the mixture produces. The amount of pressure each gas contributes is called the partial pressure of that gas and is proportional to its concentration. For example, because air is 21% oxygen, oxygen accounts for 21% of the atmospheric pressure (21% of 760 Hg), or 160 mm Hg. Thus, the partial pressure of oxygen, symbolized PO2, in atmospheric air is 160 mm Hg. Similarly, the partial pressure of carbon dioxide (Pco2) in air is 0.3 mm Hg.

Gas molecules from the air may enter, or dissolve, in liquid. This is what happens when carbon dioxide is added to a carbonated beverage, or when inspired gases dissolve in the blood in the alveolar capillaries. Although the calculation of the concentration of a dissolved gas is a bit complicated, it turns out that the partial pressure of a gas dissolved in a liquid is by definition equal to the partial pressure of that gas in the air the liquid has equilibrated with. Thus, instead of concentrations of oxygen and carbon dioxide in the body fluids, we will refer to PO2 and PCO2.

A number of factors influence breathing rate and depth. These include PO2 and PCO2 in body fluids, the degree to which lung tissues are stretched, and emotional state. For example, central chemoreceptors are found in chemosensitive areas within the respiratory center, located in the ventral portion of the medulla oblongata near the origins of the vagus nerves. These chemoreceptors are very sensitive to changes in PCO2 and blood pH. If PCO2 or the hydrogen ion concentration rises, the central chemoreceptors signal the respiratory center, and breathing rate increases.

The similarity of the effects of carbon dioxide and hydrogen ions is a consequence of the fact that carbon dioxide combines with water in the cerebrospinal fluid to form carbonic acid (H2CO3):

The carbonic acid thus formed soon ionizes, releasing hydrogen ions (H+) and bicarbonate ions (HCO3-):

It is the presence of hydrogen ions rather than the carbon dioxide that influences the central chemoreceptors. In any event, breathing rate and tidal volume increase when a person inhales air rich in carbon dioxide or when body cells produce excess carbon dioxide or hydrogen ions. These changes increase alveolar ventilation. As a result, more carbon dioxide is exhaled, and the blood PCO2 and hydrogen ion concentration return toward normal.

Sensory nerve (branch of vagus nerve)

Aorta

Heart

Aorta

Heart

Medulla oblongata

Sensory nerve (branch of glossopharyngeal nerve)

Carotid bodies

Common carotid artery

Aortic bodies

Decreased blood oxygen concentration stimulates peripheral chemoreceptors in the carotid and aortic bodies.

Medulla oblongata

Sensory nerve (branch of glossopharyngeal nerve)

Carotid bodies

Common carotid artery

Aortic bodies ure I9.3G

Figure

Decreased blood oxygen concentration stimulates peripheral chemoreceptors in the carotid and aortic bodies.

Adding carbon dioxide to air can stimulate the rate and depth of breathing. Ordinary air is about 0.04% carbon dioxide. If a patient inhales air containing 4% carbon dioxide, breathing rate usually doubles.

Low blood PO2 has little direct effect on the central chemoreceptors associated with the respiratory center. Instead, changes in the blood PO2 are primarily sensed by peripheral chemoreceptors in specialized structures called the carotid bodies and aortic bodies, which are located in the walls of the carotid sinuses and aortic arch (fig. 19.30). When decreased PO2 stimulates these peripheral receptors, impulses are transmitted to the respiratory center, and the breathing rate and tidal volume increase, thus increasing alveolar ventilation. This mechanism is usually not triggered until the PO2 reaches a very low level; thus, oxygen seems to play only a minor role in the control of normal respiration.

The peripheral chemoreceptors of the carotid and aortic bodies are also stimulated by changes in the blood PCo2 and pH. However, CO2 and hydrogen ions have a much greater effect on the central chemoreceptors of the respiratory center than they do on the carotid and aortic bodies.

An exception to normal respiratory control may occur in patients who have chronic obstructive pulmonary diseases (COPD), such as asthma, bronchitis, and emphysema. These patients gradually adapt to high concentrations of carbon dioxide, and for them, low oxygen concentrations may serve as a necessary respiratory stimulus. When such a patient is placed on 100% oxygen, the low arterial Po2 may be corrected, the stimulus removed, and breathing may stop.

An inflation reflex (Hering-Breuer reflex) helps regulate the depth of breathing. This reflex occurs when stretch receptors in the visceral pleura, bronchioles, and alveoli are stimulated as lung tissues are stretched. The sensory impulses of the reflex travel via the vagus nerves to the pneumotaxic area of the respiratory center and shorten the duration of inspiratory movements. This action prevents overinflation of the lungs during forceful breathing (fig. 19.31).

Emotional upset or strong sensory stimulation may alter the normal breathing pattern. Gasping and rapid breathing are familiar responses to fear, anger, shock, excitement, horror, surprise, sexual stimulation, or even the chill of stepping into a cold shower. Because control of the respiratory muscles is voluntary, we can alter breathing pattern consciously, or even stop it altogether for a short time. During childbirth, for example, women often concentrate on controlling their breathing, which distracts them from the pain.

If a person decides to stop breathing, the blood concentrations of carbon dioxide and hydrogen ions begin to rise, and the concentration of oxygen falls. These changes (primarily the increased CO2) stimulate the respiratory center, and soon the need to inhale overpowers the desire to hold the breath—much to the relief of parents when young children threaten to hold their breaths until they turn blue! On the other hand, a person can increase the breath-holding time by breathing rapidly and deeply in advance. (This could be dangerous, see box on following page.) This action, termed hyperventilation (hi"per-ven"ti-la'shun), lowers the blood carbon dioxide concentration below normal. Following hyperventilation, it takes longer than usual for the carbon dioxide concentration to reach the level needed to override the conscious effort of breath holding.

Table 19.6 discusses factors affecting breathing. Clinical Application 19.4 focuses on one influence on breathing—exercise.

Respiratory center

Sensory pathway-

Vagus nerve

Respiratory center

Sensory pathway-

Vagus nerve

Phrenic nerve

Stretch receptors

Spinal cord

Motor pathways

Lung

Intercostal Nerves

External intercostal muscles

Intercostal nerve

Diaphragm

Phrenic nerve

Stretch receptors

Lung

Spinal cord

Motor pathways

External intercostal muscles

Intercostal nerve

Diaphragm

Figure 19.31

In the process of inspiration, motor impulses travel from the respiratory center to the diaphragm and external intercostal muscles, which contract and cause the lungs to expand. This expansion stimulates stretch receptors in the lungs to send inhibiting impulses to the respiratory center, thus preventing overinflation.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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