moi

Question 1

Although our tidelike breathing seems so beautifully simple, its control is fairly complex. We will cover only the most basic aspects of the respiratory controls.

Neural Regulation: Setting the Basic Rhythm
The activity of the respiratory muscles, the diaphragm and external intercostals, is regulated by nerve impulses transmitted from the brain by the phrenic nerves and intercostal nerves. Neural centers that control respiratory rhythm and depth are located mainly in the medulla oblongata and pons (Figure 13.12). Much remains to be discovered about neural regulation of breathing. A brief summary of what we know follows.

Question 2

The medulla oblongata contains two respiratory centers. The first, the ventral respiratory group (VRG), contains both inspiratory and expiratory neurons that alternately send impulses to control the rhythm of breathing. The inspiratory neurons stimulate the diaphragm and external intercostal muscles via the phrenic and intercostal nerves, respectively, during quiet breathing. Impulses from the expiratory neurons stop the stimulation of the diaphragm and external intercostal muscles, allowing passive exhalation to occur. Impulses from the VRG maintain a normal quiet breathing rate of 12 to 15 respirations/minute, a rate called eupnea (ūp-ne′ah).
The other medullary center, the dorsal respiratory group (DRG), integrates sensory information from chemoreceptors and peripheral stretch receptors. The DRG communicates this information to the VRG to help modify breathing rhythms.
The pons respiratory centers, which also communicate with the VRG, help to smooth the transitions (modify timing) between inhalation and exhalation during activities such as singing, sleeping or exercising.

The bronchioles and alveoli have stretch receptors that respond to extreme overinflation (which might damage the lungs) by initiating protective reflexes. In the case of overinflation, the vagus nerves send impulses from the stretch receptors to the medulla oblongata; soon thereafter, inspiration ends and expiration occurs. This is one example of DRG integration during respiratory control.
During exercise, we breathe more vigorously and deeply because the brain centers send more impulses to the respiratory muscles. This respiratory pattern is called hyperpnea (hy-perp′ne-ah). After strenuous exercise, expiration becomes active, and the abdominal muscles and any other muscles capable of depressing the ribs are used to aid expiration.

Homeostatic Imbalance 13.10
If the medullary centers are completely suppressed (as with an overdose of sleeping pills, morphine, or alcohol), respiration stops completely, and without intervention death occurs.

Nonneural Factors Influencing Respiratory Rate and Depth

Physical Factors
Although the medulla oblongata’s respiratory centers set the basic rhythm of breathing, physical factors such as talking, coughing, and exercising can modify both the rate and depth of breathing. We have already examined some of these factors in our discussion of nonrespiratory air movements. Increased body temperature also causes an increase in the rate of breathing.

Volition (Conscious Control)
We all have consciously controlled our breathing pattern at one time or another. During singing and swallowing, breath control is extremely important, and many of us have held our breath for short periods to swim underwater. However, voluntary control of breathing is limited, and the respiratory centers will simply ignore messages from the cortex (our wishes) when the oxygen supply in the blood is getting low or blood pH is falling. All you need do to prove this is to try to talk normally or to hold your breath after running at breakneck speed for a few minutes. It simply cannot be done. Many toddlers try to manipulate their parents by holding their breath “to death.” Even though this threat causes many parents to become anxious, they need not worry because the involuntary controls take over and normal respiration begins again.

Emotional Factors
Emotional factors also modify the rate and depth of breathing. Have you ever watched a horror movie with bated (held) breath or been so scared by what you saw that you were nearly panting? Have you ever touched something cold and clammy and gasped? All of these result from reflexes initiated by emotional stimuli acting through centers in the hypothalamus.

Chemical Factors
Although many factors can modify respiratory rate and depth, the most important factors are chemical—the levels of carbon dioxide and oxygen in the blood (see Figure 13.12). An increased level of carbon dioxide and a decreased blood pH are the most important stimuli leading to an increase in the rate and depth of breathing. An increase in the carbon dioxide level can cause a decreased blood pH because high results in more carbonic acid, which lowers blood pH. However, a low blood pH could also result from metabolic activities independent of breathing. Changes in the carbon dioxide concentration or ion concentration (which affects pH) in brain tissue seem to act directly on the centers in the medulla oblongata by influencing the pH of local tissues in the brain stem (see Figure 13.12).
Conversely, changes in oxygen concentration in the blood are detected by peripheral chemoreceptor regions in the aorta (aortic body in the aortic arch) and in the fork of the common carotid artery (the carotid body). These, in turn, send impulses to the medulla oblongata when the blood oxygen level is dropping. When oxygen levels are low, these same chemoreceptors are also able to detect high carbon dioxide levels. Although every cell in the body must have oxygen to live, it is the body’s need to rid itself of carbon dioxide that is the most important stimulus for breathing. A decrease in the oxygen level becomes an important stimulus only when the level is dangerously low.

Homeostatic Imbalance 13.11
In people who retain carbon dioxide, such as people with emphysema, chronic bronchitis, or other chronic lung diseases, the brain no longer recognizes an increased level of carbon dioxide as important. In such cases, a dropping oxygen level becomes the respiratory stimulus. This interesting fact explains why such patients are always given a low level of oxygen; it helps to maintain the prevailing stimulus for breathing (a low oxygen level).

The respiratory system in healthy individuals has homeostatic mechanisms. As carbon dioxide or other sources of acids begin to accumulate in the blood and tissues, and pH starts to drop, you begin to breathe more deeply and more rapidly. Hyperventilation is an increase in the rate and depth of breathing that exceeds the body’s need to remove carbon dioxide. In other words, during hyperventilation, we exhale more than we should, resulting in elevated blood pH (there is less carbonic acid).
By contrast, when blood starts to become slightly alkaline, or basic, breathing slows and becomes shallow. Slower breathing allows carbon dioxide to accumulate in the blood and brings the blood pH back into the normal range.
Indeed, control of breathing during rest is aimed primarily at regulating the hydrogen ion concentration in the brain. Hypoventilation (extremely slow or shallow breathing) or hyperventilation can dramatically change the amount of carbonic acid in the blood. Carbonic acid increases dramatically during hypoventilation and decreases substantially during hyperventilation. In both situations, the buffering ability of the blood is likely to be overwhelmed; the result is acidosis or alkalosis.

Homeostatic Imbalance 13.12
Hyperventilation, often brought on by anxiety attacks, frequently leads to brief periods of apnea (ap′ne-ah), cessation of breathing, until the carbon dioxide builds up in the blood again. If breathing stops for an extended time, cyanosis may occur as a result of insufficient oxygen in the blood. In addition, the hyperventilating person may get dizzy and faint because the resulting alkalosis causes cerebral blood vessels to constrict. Such attacks can be prevented by having the hyperventilating person breathe into a paper bag. Because exhaled air contains more carbon dioxide than atmospheric air, it upsets the normal diffusion gradient that causes carbon dioxide to be unloaded from the blood and leave the body. As a result, the carbon dioxide (and thus carbonic acid) level begins to rise in the blood, ending alkalosis and returning blood pH to normal.The medulla oblongata contains two respiratory centers. The first, the ventral respiratory group (VRG), contains both inspiratory and expiratory neurons that alternately send impulses to control the rhythm of breathing. The inspiratory neurons stimulate the diaphragm and external intercostal muscles via the phrenic and intercostal nerves, respectively, during quiet breathing. Impulses from the expiratory neurons stop the stimulation of the diaphragm and external intercostal muscles, allowing passive exhalation to occur. Impulses from the VRG maintain a normal quiet breathing rate of 12 to 15 respirations/minute, a rate called eupnea (ūp-ne′ah).
The other medullary center, the dorsal respiratory group (DRG), integrates sensory information from chemoreceptors and peripheral stretch receptors. The DRG communicates this information to the VRG to help modify breathing rhythms.
The pons respiratory centers, which also communicate with the VRG, help to smooth the transitions (modify timing) between inhalation and exhalation during activities such as singing, sleeping or exercising.

The bronchioles and alveoli have stretch receptors that respond to extreme overinflation (which might damage the lungs) by initiating protective reflexes. In the case of overinflation, the vagus nerves send impulses from the stretch receptors to the medulla oblongata; soon thereafter, inspiration ends and expiration occurs. This is one example of DRG integration during respiratory control.
During exercise, we breathe more vigorously and deeply because the brain centers send more impulses to the respiratory muscles. This respiratory pattern is called hyperpnea (hy-perp′ne-ah). After strenuous exercise, expiration becomes active, and the abdominal muscles and any other muscles capable of depressing the ribs are used to aid expiration.

Homeostatic Imbalance 13.10
If the medullary centers are completely suppressed (as with an overdose of sleeping pills, morphine, or alcohol), respiration stops completely, and without intervention death occurs.

Nonneural Factors Influencing Respiratory Rate and Depth

Physical Factors
Although the medulla oblongata’s respiratory centers set the basic rhythm of breathing, physical factors such as talking, coughing, and exercising can modify both the rate and depth of breathing. We have already examined some of these factors in our discussion of nonrespiratory air movements. Increased body temperature also causes an increase in the rate of breathing.

Volition (Conscious Control)
We all have consciously controlled our breathing pattern at one time or another. During singing and swallowing, breath control is extremely important, and many of us have held our breath for short periods to swim underwater. However, voluntary control of breathing is limited, and the respiratory centers will simply ignore messages from the cortex (our wishes) when the oxygen supply in the blood is getting low or blood pH is falling. All you need do to prove this is to try to talk normally or to hold your breath after running at breakneck speed for a few minutes. It simply cannot be done. Many toddlers try to manipulate their parents by holding their breath “to death.” Even though this threat causes many parents to become anxious, they need not worry because the involuntary controls take over and normal respiration begins again.

Emotional Factors
Emotional factors also modify the rate and depth of breathing. Have you ever watched a horror movie with bated (held) breath or been so scared by what you saw that you were nearly panting? Have you ever touched something cold and clammy and gasped? All of these result from reflexes initiated by emotional stimuli acting through centers in the hypothalamus.

Chemical Factors
Although many factors can modify respiratory rate and depth, the most important factors are chemical—the levels of carbon dioxide and oxygen in the blood (see Figure 13.12). An increased level of carbon dioxide and a decreased blood pH are the most important stimuli leading to an increase in the rate and depth of breathing. An increase in the carbon dioxide level can cause a decreased blood pH because high results in more carbonic acid, which lowers blood pH. However, a low blood pH could also result from metabolic activities independent of breathing. Changes in the carbon dioxide concentration or ion concentration (which affects pH) in brain tissue seem to act directly on the centers in the medulla oblongata by influencing the pH of local tissues in the brain stem (see Figure 13.12).
Conversely, changes in oxygen concentration in the blood are detected by peripheral chemoreceptor regions in the aorta (aortic body in the aortic arch) and in the fork of the common carotid artery (the carotid body). These, in turn, send impulses to the medulla oblongata when the blood oxygen level is dropping. When oxygen levels are low, these same chemoreceptors are also able to detect high carbon dioxide levels. Although every cell in the body must have oxygen to live, it is the body’s need to rid itself of carbon dioxide that is the most important stimulus for breathing. A decrease in the oxygen level becomes an important stimulus only when the level is dangerously low.

Homeostatic Imbalance 13.11
In people who retain carbon dioxide, such as people with emphysema, chronic bronchitis, or other chronic lung diseases, the brain no longer recognizes an increased level of carbon dioxide as important. In such cases, a dropping oxygen level becomes the respiratory stimulus. This interesting fact explains why such patients are always given a low level of oxygen; it helps to maintain the prevailing stimulus for breathing (a low oxygen level).

The respiratory system in healthy individuals has homeostatic mechanisms. As carbon dioxide or other sources of acids begin to accumulate in the blood and tissues, and pH starts to drop, you begin to breathe more deeply and more rapidly. Hyperventilation is an increase in the rate and depth of breathing that exceeds the body’s need to remove carbon dioxide. In other words, during hyperventilation, we exhale more than we should, resulting in elevated blood pH (there is less carbonic acid).
By contrast, when blood starts to become slightly alkaline, or basic, breathing slows and becomes shallow. Slower breathing allows carbon dioxide to accumulate in the blood and brings the blood pH back into the normal range.
Indeed, control of breathing during rest is aimed primarily at regulating the hydrogen ion concentration in the brain. Hypoventilation (extremely slow or shallow breathing) or hyperventilation can dramatically change the amount of carbonic acid in the blood. Carbonic acid increases dramatically during hypoventilation and decreases substantially during hyperventilation. In both situations, the buffering ability of the blood is likely to be overwhelmed; the result is acidosis or alkalosis.

Homeostatic Imbalance 13.12
Hyperventilation, often brought on by anxiety attacks, frequently leads to brief periods of apnea (ap′ne-ah), cessation of breathing, until the carbon dioxide builds up in the blood again. If breathing stops for an extended time, cyanosis may occur as a result of insufficient oxygen in the blood. In addition, the hyperventilating person may get dizzy and faint because the resulting alkalosis causes cerebral blood vessels to constrict. Such attacks can be prevented by having the hyperventilating person breathe into a paper bag. Because exhaled air contains more carbon dioxide than atmospheric air, it upsets the normal diffusion gradient that causes carbon dioxide to be unloaded from the blood and leave the body. As a result, the carbon dioxide (and thus carbonic acid) level begins to rise in the blood, ending alkalosis and returning blood pH to normal.