Info

Function

Air Movement

Mechanism

Function

Air Movement

Mechanism

Coughing

Deep breath is taken, glottis is closed, and air is forced against the closure; suddenly the glottis is opened, and a blast of air passes upward

Clears lower respiratory passages

Sneezing

Same as coughing, except air moving upward is directed into the nasal cavity by depressing the uvula

Clears upper respiratory passages

Laughing

Deep breath is released in a series of short expirations

Expresses happiness

Crying

Same as laughing

Expresses sadness

Hiccuping

Diaphragm contracts spasmodically while glottis is closed

No useful function known

Yawning

Deep breath is taken

Ventilates a larger proportion of the alveoli and aids oxygenation of the blood

Speech

Air is forced through the larynx, causing vocal cords to vibrate; words are formed by lips, tongue, and soft palate

Vocal communication

Control of Breathing

Normal breathing is a rhythmic, involuntary act that continues when a person is unconscious.

Respiratory Center

Groups of neurons in the brain stem comprise the respiratory center, which controls breathing. This center periodically initiates impulses that travel on cranial and spinal nerves to breathing muscles, causing inspiration and expiration. The respiratory center also adjusts the rate and depth of breathing to meet cellular needs for supply of oxygen and removal of carbon dioxide, even during strenuous physical exercise.

The components of the respiratory center are widely scattered throughout the pons and medulla oblongata. However, two areas of the respiratory center are of special interest. They are the rhythmicity area of the medulla and the pneumotaxic area of the pons (fig. 19.28).

The medullary rhythmicity area includes two groups of neurons that extend throughout the length of the medulla oblongata. They are called the dorsal respiratory group and the ventral respiratory group.

The dorsal respiratory group is responsible for the basic rhythm of breathing. The neurons of this group emit bursts of impulses that signal the diaphragm and other inspiratory muscles to contract. The impulses of each burst begin weakly, strengthen for about two seconds, and cease abruptly. The breathing muscles that contract in response to the impulses cause the volume of air entering the lungs to increase steadily. The neurons remain inactive while expiration occurs passively, and then they emit another burst of inspiratory impulses so that the inspiration-expiration cycle repeats.

The ventral respiratory group is quiescent during normal breathing. However, when more forceful breathing is necessary, the neurons in this group generate impulses that increase inspiratory movement. Other neurons in the group activate the muscles associated with forceful expiration.

A condition called sleep apnea is responsible for some cases of sudden infant death and for snoring. Babies who have difficulty breathing just after birth are often sent home with monitoring devices, which sound an alarm when the child stops breathing, alerting parents to resuscitate the infant. The position in which the baby sleeps seems to affect the risk of sleep apnea—sleeping on the back or side is safest during the first year of life.

Adults with sleep apnea may cease breathing for ten to twenty seconds, hundreds of times a night. Bed-mates may be aware of the problem because the frequent cessation in breathing causes snoring. The greatest danger of adult sleep apnea is the fatigue, headache, depression, and drowsiness that follows during waking hours.

Sleep apnea is diagnosed in a sleep lab, which monitors breathing during slumber. One treatment for sleep apnea is nasal continuous positive airway pressure. A device is strapped onto the nose at night that regulates the amount of air sent into and out of the respiratory system. Much simpler and more comfortable are inexpensive tape devices that hold the nostrils open.

The neurons in the pneumotaxic area continuously transmit impulses to the dorsal respiratory group and regulate the duration of inspiratory bursts originating from the dorsal group. In this way the pneumotaxic neurons control the rate of breathing. More specifically, when the pneumotaxic signals are strong, the inspiratory bursts are shorter, and the rate of breathing increases (tachypnea); when the pneumotaxic signals are weak, the inspiratory bursts are longer, and the rate of breathing decreases (bradypnea) (fig. 19.29).

Respiratory Disorders that Decrease Ventilation

Injuries to the respiratory center or to spinal nerve tracts that transmit motor impulses may paralyze breathing muscles. Paralysis may also be due to a disease, such as poliomyelitis, that affects parts of the central nervous system and injures motor neurons. The consequences of such paralysis depend on which muscles are affected. Sometimes, other muscles, by increasing their responses, can compensate for functional losses of a paralyzed muscle. Otherwise, mechanical ventilation is necessary.

comes increasingly difficult, and it produces a characteristic wheezing sound as air moves through narrowed passages.

A person with asthma usually finds it harder to force air out of the lungs than to bring it in. This is because inspiration utilizes powerful breathing muscles, and, as they contract, the lungs expand, opening the air passages. Expiration, on the other hand, is a passive process due to elastic recoil of stretched tissues. Expiration also compresses the tissues and constricts the bronchioles, further impairing air movement through the narrowed air passages.

Emphysema is a progressive, degenerative disease that destroys many

Bronchial asthma is usually an allergic reaction to foreign antigens in the respiratory tract, such as plant pollen that enters with inhaled air. Cells of the larger airways secrete copious amounts of mucus, which traps allergens. Ciliated columnar epithelial cells move the mucus up and out of the bronchi, then up and out of the trachea. Thus, upper respiratory structures remain relatively clear. However, in the lower respiratory areas, mucus drainage plus ede-matous secretions accumulate because fewer cells are ciliated. The allergens and secretions irritate smooth muscles, stimulating bron-chioconstrictions. Breathing be alveolar walls. As a result, clusters of small air sacs merge into larger chambers, which decreases the total surface area of the alveolar walls. At the same time, the alveolar walls lose their elasticity, and the capillary networks associated with the alveoli become less abundant (fig. 19C).

Because of the loss of tissue elasticity, a person with emphysema finds it increasingly difficult to force air out of the lungs. Abnormal muscular efforts are required to compensate for the lack of elastic recoil that normally contributes to expiration. About 3 percent of the 2 million people in the United States who have emphysema inherit the condition; the majority of the other cases are due to smoking or other respiratory irritants.

An experimental treatment for severe emphysema is lung volume reduction surgery. As its name suggests, the procedure reduces lung volume, which opens collapsed airways and eases breathing. So far, it seems to noticeably improve lung function (as measured by distance walked in 6 minutes) and quality of life. ■

Lung Volume Reduction Surgery Procedure

Figure 19C

(a) Normal lung tissue (100x). (b) As emphysema develops, alveoli merge, forming larger chambers (100x).

Figure 19C

(a) Normal lung tissue (100x). (b) As emphysema develops, alveoli merge, forming larger chambers (100x).

Information Blood Vessels

Midbrain

Pneumotaxic area Pons

Medulla oblongata Ventral respiratory group Dorsal respiratory group

Medullary rhythmicity area

Figure

Midbrain

Pneumotaxic area Pons

Medulla oblongata Ventral respiratory group Dorsal respiratory group

Medullary rhythmicity area

Sleep Apnea

Sleep Apnea

Have You Been Told Over And Over Again That You Snore A Lot, But You Choose To Ignore It? Have you been experiencing lack of sleep at night and find yourself waking up in the wee hours of the morning to find yourself gasping for air?

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