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.