The Respiratory System Part 2



The Respiratory Cycle
A respiratory cycle is a single cycle of inhalation and exhalation. The tidal volume is the amount of air you move into or out of your lungs during a single respiratory cycle.



The Mechanics of Breathing


The Respiratory Muscles


The most important are the diaphragm and the external intercostal muscles .


Muscles Used in Inhalation


Inhalation is an active process involving the contraction of one or more of these muscles:


The contraction of the diaphragm increases the volume of the thoracic cavity by tensing and flattening its floor, and this increase draws air into the lungs. Diaphragmatic contraction is responsible for roughly 75 percent of the air movement in normal breathing at rest.


The external intercostal muscles assist in inhalation by elevating the ribs. This action contributes roughly 25 percent to the volume of air in the lungs.


Accessory muscles, including the sternocleidomastoid, serratus anterior, pectoralis minor, and scalene muscles, can assist the external intercostal muscles in elevating the ribs. These muscles increase the speed and amount of rib movement.


Muscles Used in Exhalation

Exhalation is either passive or active, depending on the level of respiratory activity. When exhalation is active, it may involve one or more of the following muscles:


The internal intercostal and transversus thoracis muscles depress the ribs and reduce the width and depth of the thoracic cavity.


The abdominal muscles, including the external and internal oblique, transversus abdominis, and rectus abdominis muscles, can assist the internal intercostal muscles in exhalation by compressing the abdomen and forcing the diaphragm upward.


Respiratory Rates and Volumes

When you are exercising at peak levels, the amount of air moving into and out of the respiratory tract can be 50 times the amount moved at rest.

Respiratory Rate is the number of breaths you take each minute. The normal respiratory rate of a resting adult ranges from 12 to 18 breaths each minute, roughly one for every four heartbeats. Children breathe more rapidly, at rates of about 18–20 breaths per minute.


The Respiratory Minute Volume is the amount of air moved each minute, symbolized by multiplying the respiratory rate by the tidal volume. This value is called the respiratory minute volume . The tidal volume at rest varies from individual to individual, but it averages around 500 ml per breath. Therefore, the respiratory minute volume at rest, 12 breaths per minute, is approximately 6 liters per minute.


Alveolar Ventilation


The volume of air in the conducting passages is known as the anatomic dead space.


Alveolar ventilation is the amount of air reaching the alveoli each minute. The alveolar ventilation is less than the respiratory minute volume, because some of the air never reaches the alveoli, but remains in the dead space of the lungs.


At rest, alveolar ventilation rates are approximately 4.2 liters per minute The gas arriving in the alveoli is different from that of the surrounding atmosphere, because inhaled air always mixes with "used" air in the conducting passageways (the anatomic dead space) on its way to the exchange surfaces. The air in alveoli thus contains less oxygen and more carbon dioxide than atmospheric air.


Respiratory Performance and Volume Relationships
Only a small proportion of the air in the lungs is exchanged during a single quiet respiratory cycle; the tidal volume can be increased by inhaling more vigorously and exhaling more completely.




Respiratory Volumes and Capacities.

The resting tidal volume is the amount of air you move into or out of your lungs during a single respiratory cycle under resting conditions. The resting tidal volume averages about 500 ml in both males and females.


The expiratory reserve volume (ERV) is the amount of air that you can voluntarily expel after you have completed a normal, quiet respiratory cycle. As an example, if, with maximum use of the accessory muscles, you can expel an additional 1000 ml of air, your expiratory reserve volume is 1000 ml.


The residual volume is the amount of air that remains in your lungs even after a maximal exhalation–typically, about 1200 ml in males and 1100 ml in females.


The minimal volume , a component of the residual volume, is the amount of air that would remain in your lungs if they were allowed to collapse. The minimal volume ranges from 30 to 120 ml, but, unlike other volumes, it cannot be measured in a healthy person. You would have to squeeze out the lungs like a sponge to measure it.


The inspiratory reserve volume (IRV) is the amount of air that you can take in over and above the tidal volume. Inspiratory reserve volumes differ significantly by gender, because, on average, the lungs of males are larger than those of females. The inspiratory reserve volume of males averages 3300 ml, compared with 1900 ml in females.


We can determine respiratory capacities by adding the values of various volumes. Examples include the following:


The inspiratory capacity is the amount of air that you can draw into your lungs after you have completed a quiet respiratory cycle. The inspiratory capacity is the sum of the tidal volume and the inspiratory reserve volume.


The functional residual capacity (FRC) is the amount of air remaining in your lungs after you have completed a quiet respiratory cycle. The FRC is the sum of the expiratory reserve volume and the residual volume.


The vital capacity is the maximum amount of air that you can move into or out of your lungs in a single respiratory cycle. The vital capacity is the sum of the expiratory reserve, the tidal volume, and the inspiratory reserve and averages around 4800 ml in males and 3400 ml in females.


The total lung capacity is the total volume of your lungs. The sum of the vital capacity and the residual volume, the total lung capacity averages around 6000 ml in males and 4500 ml in females.


Gas Exchange


The actual process of gas exchange occurs between blood and alveolar air across the respiratory membrane. It involves:  (1) the partial pressures of the gases involved and (2) the diffusion of molecules between a gas and a liquid.


Oxygen Transport
Each 100 ml of blood leaving the alveolar capillaries carries away roughly 20 ml of oxygen. Of this amount, only about 0.3 ml (1.5 percent) consists of oxygen molecules in solution. The rest of the oxygen molecules are bound to hemoglobin ( Hb ) .


The percentage of heme units containing bound oxygen at any given moment is called the hemoglobin saturation .



The attachment of the first oxygen molecule makes it easier to bind the second; binding the second promotes binding of the third; and binding of the third enhances binding of the fourth, and so on.


At normal alveolar pressures the hemoglobin saturation is very high (97.5 percent), although complete saturation does not occur until the reaches excessively high levels (about 250 mm Hg).


If the P-O2 increases, the saturation goes up and hemoglobin stores oxygen. If the P-O2 decreases, hemoglobin releases oxygen into its surroundings .


When oxygenated blood arrives in the peripheral capillaries, the blood P-O2 declines rapidly as a result of gas exchange with the interstitial fluid. As the P-O2  falls, hemoglobin gives up its oxygen.

Active tissues consume oxygen at an accelerated rate, so the P-O2 may drop to 15–20 mm Hg. Hemoglobin passing through these capillaries will then go from 97 percent saturation to about 20 percent saturation. This means that as blood circulates through peripheral capillaries, active tissues will receive 3.5 times as much oxygen as will inactive tissues.


Murder or suicide victims who died in their cars inside a closed garage are popular characters for mystery writers. In real life, entire families are killed each winter by leaky furnaces or space heaters. The cause of death is carbon monoxide poisoning . Carbon monoxide competes with oxygen molecules for the binding sites on heme units. Unfortunately, the carbon monoxide usually wins, because at very low partial pressures it has a much stronger affinity for hemoglobin than does oxygen.


The bond formed between CO and heme is extremely durable, so the attachment of a CO molecule essentially makes that heme unit inactive for respiratory purposes. Carbon monoxide will bind to hemoglobin at very low partial pressures. If CO molecules make up just 0.1 percent of the components of inhaled air, enough hemoglobin will be affected that survival will become impossible without medical assistance.


Treatment may include (1) the administration of pure oxygen in a hyperbaric chamber, because at sufficiently high partial pressures, the oxygen molecules will gradually replace CO at the hemoglobin molecules, and, if necessary, (2) the transfusion of compatible red blood cells.


Hemoglobin and pH


When the pH drops (more acidity), the shape of hemoglobin molecules changes;  the oxygen saturation declines. Thus, at a tissue of 40 mm Hg, hemoglobin molecules release 15 percent more oxygen at a pH of 7.2 than they do at a pH of 7.4. This effect of pH on the hemoglobin saturation curve is called the Bohr effect .

Carbon dioxide is the primary compound responsible for the Bohr effect.





The Effects of pH and Temperature on Hemoglobin Saturation.
(a) When the pH drops below normal levels, more oxygen is released; the hemoglobin saturation curve shifts to the right. If the pH increases, less oxygen is released; the curve shifts to the left. (b) When the temperature rises, the saturation curve shifts to the right


Hemoglobin and Temperature


Changes in temperature also affect the slope of the hemoglobin saturation curve . As the temperature rises, hemoglobin releases more oxygen; as the temperature declines, hemoglobin holds oxygen more tightly. For example, active skeletal muscles generate heat, and the heat warms blood that flows through these organs. As the blood warms, the Hb molecules release more oxygen than can be used by the active muscle fibers.



Fetal Hemoglobin
The RBCs of a developing fetus contain fetal hemoglobin . The structure of fetal hemoglobin, which differs from that of adult hemoglobin, gives it a much higher affinity for oxygen. Also, fetal hemoglobin binds more oxygen than does adult hemoglobin




Adaptations to High Altitude


Atmospheric pressure decreases with increasing altitude, and so do the partial pressures of the component gases, including oxygen. People living in Denver or Mexico City function normally with alveolar oxygen pressures in the range of 80–90 mm Hg. At higher elevations, the alveolar partial pressures of oxygen continues to decline. At 3300 meters (10,826 ft), an altitude familiar to many hikers and skiers, the alveolar P-O2 falls to around 60 mm Hg.


Ppeople live and work at altitudes this high or higher. Important physiological adjustments include an increased respiratory rate, an increased heart rate, and an elevated hematocrit. Thus, even though the hemoglobin is not fully saturated, the bloodstream holds more of it, and the round–trip between the lungs and the peripheral tissues takes less time. These responses represent an excellent example of the functional interplay between the respiratory and cardiovascular systems. However, most such adaptations take days to weeks to appear. As a result, athletes planning to compete in events held at high altitude must begin training under such conditions well in advance.


Not everyone can tolerate high–altitude conditions. Roughly 20 percent of people who ascend to 2600 meters (8530 ft) or higher experience mountain sickness , or altitude sickness . Symptoms may include headache, disorientation, and fatal pulmonary or cerebral edema.


Carbon Dioxide Transport
Carbon dioxide is generated by aerobic metabolism in peripheral tissues. After entering the bloodstream, a CO2 molecule is (1) converted to a molecule of carbonic acid, (2) bound to the protein portion of hemoglobin molecules within red blood cells, or (3) dissolved in plasma. All three are completely reversible reactions.

Carbonic Acid Formation
Most of the carbon dioxide absorbed by blood (roughly 70 percent of the total) is transported as molecules of carbonic acid. Carbon dioxide is converted to carbonic acid through the activity of the enzyme carbonic anhydrase in RBCs. The carbonic acid molecules immediately dissociate into a hydrogen ion and a bicarbonate ion.


Most of the hydrogen ions bind to hemoglobin molecules. The Hb molecules thus function as buffers, tying up the released hydrogen ions before the ions leave the RBCs and affect the plasma pH.


The bicarbonate ions move into the surrounding plasma with the aid of a countertransport mechanism that exchanges intracellular bicarbonate ions for extracellular chloride ions. This exchange, which trades one anion for another, does not require ATP. The result is a mass movement of chloride ions into the RBCs, an event known as the chloride shift .


Hemoglobin Binding
Roughly 23 percent of the carbon dioxide carried by your blood will be bound to the globular protein portions of the Hb molecules inside RBCs. These molecules are attached to exposed amino groups of the Hb molecules. The resulting compound is called carbaminohemoglobin


Plasma Transport
Plasma becomes saturated with carbon dioxide quite rapidly, and only about 7 percent of the carbon dioxide absorbed by peripheral capillaries is transported in the form of dissolved gas molecules.



Control of Respiration


The activities of the respiratory centers are coordinated with changes in cardiovascular function, such as fluctuations in blood pressure and cardiac output.


Local Regulation of Gas Transport and Alveolar Function


Obviously, if a peripheral tissue becomes more active, the interstitial P-O2 falls and the P-CO2  rises.


Local factors coordinate (1) lung perfusion , or blood flow to the alveoli, with (2) alveolar ventilation , or airflow, over a wide range of conditions and activity levels.


As blood flows toward the alveolar capillaries, it is directed toward lobules in which the P-CO2  is relatively high.


This movement occurs because alveolar capillaries constrict when the local P-CO2  is low.


By directing blood flow to alveoli with low O2 levels and improving airflow to alveoli with high CO2 levels, local adjustments improve the efficiency of gas transport.  These are local controls.


The Respiratory Centers of the Brain  are three pairs of nuclei in the reticular formation of the medulla oblongata and pons. The motor neurons in the spinal cord are generally controlled by respiratory reflexes , but they can also be controlled voluntarily through commands delivered by the corticospinal pathway.

Respiratory Centers in the Medulla Oblongata


Each center can be subdivided into a dorsal respiratory group (DRG) and a ventral respiratory group (VRG) . The DRG's inspiratory center contains neurons that control lower motor neurons innervating the external intercostal muscles and the diaphragm. The DRG functions in every respiratory cycle, whether quiet or forced.


The VRG functions only during forced breathing.


There is reciprocal inhibition between the neurons involved with inhalation and exhalation. When the inspiratory neurons are active, the expiratory neurons are inhibited, and vice versa. The pattern of interaction between these groups differs between quiet breathing and forced breathing. During quiet breathing  Activity in the DRG increases over a period of about 2 seconds, providing stimulation to the inspiratory muscles. Over this period, inhalation occurs.


After 2 seconds, the DRG neurons become inactive. They remain quiet for the next 3 seconds and allow the inspiratory muscles to relax. Over this period, passive exhalation occurs.  So, you have about one breath per 5 seconds at rest.


During forced breathing the level of activity in the DRG increases, it stimulates neurons of the VRG that activate the accessory muscles involved in inhalation.

At the end of each inhalation, active exhalation occurs as the neurons of the expiratory center stimulate the appropriate accessory muscles



Basic Regulatory Patterns of Respiration.
(a) Quiet breathing.                                        (b) Forced breathing.


Central nervous system stimulants, such as cocaine, amphetamines, ecstacy, or even caffeine, increase your respiratory rate by facilitating the respiratory centers. These actions can be opposed by CNS depressants, such as ethyl alcohol, barbiturates or opiates. A mixture of these stimulants and depressants is often fatal.


The Apneustic and Pneumotaxic Centers
The apneustic centers and the pneumotaxic centers of the pons are paired nuclei that adjust the output of the respiratory rhythmicity centers. Their activities regulate the respiratory rate and the depth of respiration in response to sensory stimuli or input from other centers in the brain.


Each apneustic center provides continuous stimulation to the DRG on that side of the brain stem. During quiet breathing, stimulation from the apneustic center helps increase the intensity of inhalation over the next 2 seconds. Under normal conditions, after 2 seconds the apneustic center is inhibited by signals from the pneumotaxic center on that side. During forced breathing, the apneustic centers also respond to sensory input from the vagus nerves regarding the amount of lung inflation.

The pneumotaxic centers inhibit the apneustic centers and promote passive or active exhalation. Centers in the hypothalamus and cerebrum can alter the activity of the pneumotaxic centers, as well as the respiratory rate and depth. However, essentially normal respiratory cycles continue even if the brain stem superior to the pons has been severely damaged. If the inhibitory output of the pneumotaxic centers is cut off by a stroke or other damage to the brain stem, and if sensory innervation from the lungs is eliminated by cutting the vagus nerves, the person inhales to maximum capacity and maintains that state for 10–20 seconds at a time. Intervening exhalations are brief, and little pulmonary ventilation occurs.



Pathways for conscious control over respiratory muscles are not shown.



Respiratory Reflexes
The activities of the respiratory centers are modified by sensory information from several sources:

1. Chemoreceptors sensitive to the pH, or of the blood or cerebrospinal fluid.

2. Changes in blood pressure in the aortic or carotid sinuses.

3. Stretch receptors that respond to changes in the volume of the lungs.

4. Irritating physical or chemical stimuli in the nasal cavity, larynx, or bronchial tree.

5. Other sensations, including pain, changes in body temperature, and abnormal visceral sensations.

Information from these receptors alters the pattern of respiration. The induced changes have been called respiratory reflexes .


Sudden infant death syndrome (SIDS) , also known as crib death , kills an estimated 10,000 infants each year in the United States alone (the world-wide birth rate is one every 7 seconds). Most crib deaths occur between midnight and 9:00 A.M., in the late fall or winter, and involve infants two to four months old.


The age at the time of death corresponds with a period when the pacemaker complex and respiratory centers are establishing connections with other portions of the brain. It has recently been proposed that SIDS results from a problem in the interconnection process that disrupts the reflexive respiratory pattern.


The Chemoreceptor Reflexes
The respiratory centers are strongly influenced by chemoreceptor inputs from cranial nerves IX and X and from receptors that monitor the composition of the cerebrospinal fluid (CSF):


Chemoreceptors are located on the ventrolateral surface of the medulla oblongata in a region known as the chemosensitive area . The neurons in that area respond only to the and pH of the CSF and are often called central chemoreceptors .


The Baroreceptor Reflexes
We described the effects of carotid and aortic baroreceptor stimulation on systemic blood pressure in Chapter 21 . The output from these baroreceptors also affects the respiratory centers. When blood pressure falls, the respiratory rate increases; when blood pressure rises, the respiratory rate declines. This adjustment results from the stimulation or inhibition of the respiratory centers by sensory fibers in the glossopharyngeal (IX) and vagus (X) nerves.


Protective Reflexes operate when you are exposed to toxic vapors, chemical irritants, or mechanical stimulation of the respiratory tract. The receptors involved are located in the epithelium of the respiratory tract. Examples of protective reflexes include sneezing, coughing, and laryngeal spasms.

Sneezing is triggered by an irritation of the wall of your nasal cavity. Coughing is triggered by an irritation of your larynx, trachea, or bronchi. Both reflexes involve apnea, a period in which respiration is suspended. They are usually followed by a forceful expulsion of air intended to remove the offending stimulus. The glottis is forcibly closed while the lungs are still relatively full. The abdominal and internal intercostal muscles then contract suddenly, creating pressures that will blast air out of your respiratory passageways when the glottis reopens. Air leaving the larynx can travel at 160 kph (99 mph), carrying mucus, foreign particles, and irritating gases out of the respiratory tract via the nose or mouth.

Laryngeal spasms result from the entry of chemical irritants, foreign objects, or fluids into the area around the glottis. This reflex generally closes your airway temporarily. A very strong stimulus, such as a toxic gas, could close the glottis so powerfully that you could lose consciousness and die without taking another breath. Fine chicken bones or fish bones that pierce the laryngeal walls can also stimulate laryngeal spasms, swelling, or both, restricting the airway.


Other Sensations That Affect Respiratory Function
Several other sensory stimuli can affect the activities of the respiratory centers. Examples include the following:


Sudden pain or immersion in cold water can produce a temporary apnea.


Chronic pain stimulates the sympathetic division of the autonomic nervous system, leading to an increase in the respiratory rate.


Both fever and an increase in body temperature due to exertion or overheating cause an increase in the respiratory rate. A reduction in body temperature leads to a decrease in the respiratory rate.


Curiously, stretching the anal sphincter stimulates the respiratory centers and increases the rate of respiration. Although this reflex is occasionally used to stimulate respiration in an emergency, it is not clear which pathways are involved. Enough said about that!


Voluntary Control of Respiration
Activity of your cerebral cortex has an indirect effect on your respiratory centers, as the following examples show:


Conscious thought processes tied to strong emotions, such as rage or fear, affect the respiratory rate by stimulating centers in the hypothalamus.


Emotional states can affect respiration through the activation of the sympathetic or parasympathetic division of the autonomic nervous system. Sympathetic activation causes bronchodilation and increases the respiratory rate; parasympathetic stimulation has the opposite effect.


An anticipation of strenuous exercise can trigger an automatic increase in the respiratory rate, along with increased cardiac output, by sympathetic stimulation. 


You cannot kill yourself by holding your breath "till you turn blue." Once the rises to critical levels, you will be forced to take a breath.


Changes in the Respiratory System at Birth


Before delivery, pulmonary arterial resistance is high, because the pulmonary vessels are collapsed. The rib cage is compressed, and the lungs and conducting passageways contain only small amounts of fluid and no air. During delivery, the lungs are compressed further, and as the placental connection is lost, blood oxygen levels fall and carbon dioxide levels climb rapidly. At birth, the newborn infant takes a truly heroic first breath through powerful contractions of the diaphragmatic and external intercostal muscles.


The changes in blood flow that occur lead to the closure of the foramen ovale , an interatrial connection, and the ductus arteriosus , the fetal connection between the pulmonary trunk and the aorta.


Subsequent breaths complete the inflation of the alveoli.


Aging and The Respiratory System


Many factors interact to reduce the efficiency of the respiratory system in elderly individuals. Three examples are particularly noteworthy:


As one's age increases, elastic tissue deteriorates throughout the body, reducing the compliance of the lungs and lowering their vital capacity.


Chest movements are restricted by arthritic changes in the rib articulations and by decreased flexibility at the costal cartilages. The stiffening and reduction in chest movement effectively limit the respiratory minute volume. This restriction contributes to the reduction in exercise performance and capabilities with increasing age.


Some degree of emphysema is normal in individuals over age 50. However, the extent varies widely with the lifetime exposure to cigarette smoke and other respiratory irritants.




Emphysema - Lung Cancer


Emphysema is a chronic, progressive condition characterized by shortness of breath and an inability to tolerate physical exertion. The underlying problem is the destruction of alveolar surfaces and inadequate surface area for oxygen and carbon dioxide exchange. In essence, respiratory bronchioles and alveoli are functionally eliminated.


Emphysema has been linked to the inhalation of air that contains fine particulate matter or toxic vapors, such as those in cigarette smoke.


An estimated 66 percent of adult males and 25 percent of adult females have detectable areas of emphysema in their lungs.


Lung cancer is an aggressive class of malignancies originating in the bronchial passageways or alveoli. These cancers affect the epithelial cells that line conducting passageways, mucous glands, or alveoli. Symptoms generally do not appear until the condition has progressed to the point at which the tumor masses are restricting airflow or compressing adjacent structures


Deaths from lung cancer were rare at the turn of the 20th century, but 29,000 such deaths occurred in 1956, 105,000 in 1978, and 154,900 in 2002 in the United States. This rise coincides with an increased rate of smoking in the population.


Lung cancer is increasing markedly among women, but declining among men.