Somatic receptors are found in various areas such as the intercostal muscles, rib joints, accessory muscles of respiration, and tendons. They are responsible for detecting changes in the length and tension of the respiratory muscles. Although they do not directly control respiration, they provide information about lung volume and play a role in ending inspiration. These receptors are particularly important in individuals with increased airway resistance and decreased pulmonary compliance as they can enhance muscle force within a single breath. Additionally, somatic receptors help minimize chest wall distortion during inspiration in newborns, who have highly compliant rib cages.

The ability to engage in exercise relies on the cardiac and respiratory systems' capacity to increase oxygen delivery to tissues and remove carbon dioxide from the body. Ventilation immediately increases as soon as exercise begins, and this rise in minute ventilation closely matches the increase in oxygen consumption and carbon dioxide production that accompany exercise (see Fig. 25.7). At low to moderate levels, ventilation is directly related to both carbon dioxide production and oxygen consumption (see Fig. 25.7). During maximal exercise, a physically fit individual can achieve an oxygen consumption of 4 L/minute with a minute ventilation volume of 120 L/minute, which is nearly 15 times higher than the resting level.

Exercise is noteworthy because it does not cause significant changes in blood gases, except at maximal exertion. Changes in arterial carbon dioxide and oxygen levels are minimal during exercise. Arterial pH remains within the normal range during moderate exercise. However, during strenuous exercise, arterial pH begins to decrease as lactic acid is released from muscles during anaerobic metabolism. This decrease in arterial pH stimulates ventilation disproportionately to the level of exercise. The point at which sustained metabolic (lactic) acidosis begins is referred to as the anaerobic threshold (see Fig. 25.7).

Abnormalities in the control of breathing can occur due to primary or secondary reasons. During sleep, approximately one third of healthy individuals experience brief episodes of apnea or hypoventilation, which do not significantly affect arterial oxygen or carbon dioxide levels. These apnea episodes usually last less than 10 seconds and occur during the lighter stages of slow-wave and rapid eye movement (REM) sleep. In sleep apnea syndromes, the duration of apnea is abnormally prolonged and leads to changes in arterial oxygen and carbon dioxide levels. There are two main categories of sleep apnea (see Fig. 25.8). The first is obstructive sleep apnea (OSA), which is the most common type. It occurs when the upper airway, usually the hypopharynx, closes during inspiration. This obstruction of the airway causes a cessation of airflow, more severe than snoring.

The second sleep apnea syndrome is known as central sleep apnea. This type of apnea occurs when the ventilatory drive to breathe is impaired due to dysfunction in the central respiratory centers.

The clinical histories of individuals with OSA are often similar. Usually, a spouse reports that the affected individual snores. The snoring gradually becomes louder and then stops abruptly while the individual continues to make vigorous respiratory efforts (see Fig. 25.8). The individual then awakens, falls back to sleep, and the cycle repeats throughout the night. Individuals with OSA wake up when arterial hypoxemia and hypercapnia stimulate both peripheral and central chemoreceptors. Breathing is temporarily restored before the next apneic event occurs. Individuals with OSA can experience hundreds of these events each night, disrupting their sleep. Complications of OSA include sleep deprivation, polycythemia, right-sided cardiac failure (corpulmonale), and pulmonary hypertension secondary to recurrent hypoxic events. OSA is common in individuals with obesity and those with excessive compliance of the hypopharynx, upper airway edema, and structural abnormalities of the upper airway.
Somatic receptors are also located in the intercostal muscles, rib joints, accessory muscles of respiration, and tendons, and they respond to changes in the length and tension of the respiratory muscles. Although they do not directly control respiration, they do provide information about lung volume and play a role in terminating inspiration. They are especially important in individuals with increased airway resistance and decreased pulmonary compliance because they can augment muscle force within the same breath. Somatic receptors also help minimize the chest wall distortion during inspiration in newborns, who have very compliant rib cages.
The ability to exercise depends on the capacity of the cardiac and respiratory systems to increase delivery of O2 to tissues and remove CO2 from the body. Ventilation increases immediately when exercise begins, and this increase in minute ventilation closely matches the increases in O2 consumption and CO2 production that accompany exercise (
Fig. 25.7 ). Ventilation is linearly related to both CO2 production and O2 consumption at low to moderate levels (see
Fig. 25.7 ). During maximal exercise, a physically fit individual can achieve an O2 consumption of 4 L/minute with a minute ventilation volume of 120 L/minute, which is almost 15 times the resting level.
Exercise is remarkable because of the lack of significant changes in blood gases. Except at maximal exertion, changes in PaCO2 and PaO2 are minimal during exercise. Arterial pH remains within normal values during moderate exercise. During strenuous exercise, arterial pH begins to fall as lactic acid is liberated from muscles during anaerobic metabolism. This decrease in arterial pH stimulates ventilation that is out of proportion to the level of exercise. The level of exercise at which sustained metabolic (lactic) acidosis begins is called the anaerobic threshold (see
Fig. 25.7
Abnormalities in the Control of Breathing
Changes in the ventilatory pattern can occur for both primary and secondary reasons. During sleep, approximately one third of normal individuals have brief episodes of apnea or hypoventilation that have no significant effects on PaO2 or PaCO2. The apnea usually lasts less than 10 seconds, and it occurs in the lighter stages of slow-wave and rapid eye movement (REM) sleep. In sleep apnea syndromes, the duration of apnea is abnormally prolonged, and it changes PaO2 and PaCO2. There are two major categories of sleep apnea (
Fig. 25.8 ). The first, obstructive sleep apnea (OSA), is the most common of the sleep apnea syndromes, and it occurs when the upper airway (generally the hypopharynx) closes during inspiration. Although the process is similar to what happens during snoring, it is more severe, inasmuch as it obstructs the airway and causes cessation of airflow.
The second sleep apnea syndrome is central sleep apnea. This variant of apnea occurs when the ventilatory drive to
TheclinicalhistoriesofindividualswithOSAareverysimilar.Aspouseusuallyreportsthattheaffectedindividualsnores.Thesnoringbecomeslouderandlouderandthenstopswhiletheindividualcontinuestomakevigorousrespiratoryefforts(see
Fig.25.8 ).Theindividualthenawakens,fallsbacktosleep,andcontinuesthesameprocessrepetitivelythroughoutthenight.IndividualswithOSAawakenwhenthearterialhypoxemiaandhypercapniastimulatebothperipheralandcentralchemoreceptors.Respirationisrestoredbrieflybeforethenextapneiceventoccurs.IndividualswithOSAcanhavehundredsoftheseeventseachnightthatinterruptsleep.ComplicationsofOSAincludesleepdeprivation,polycythemia,right-sidedcardiacfailure(corpulmonale),andpulmonaryhypertensionsecondarytotherecurrent,hypoxicevents.OSAiscommoninindividualswithobesityandinthosewithexcessivecomplianceofthehypopharynx,upperairwayedema,andstructuralabnormalitiesoftheupperairway.