Physiology 5th Ed.

Chapter 5. Respiratory Physiology

 

The function of the respiratory system is the exchange of oxygen and carbon dioxide between the environment and the cells of the body. Fresh air is brought into the lungs during the inspiratory phase of the breathing cycle, oxygen and carbon dioxide are exchanged between inspired air and pulmonary capillary blood, and the air is then expired.

STRUCTURE OF THE RESPIRATORY SYSTEM

LUNG VOLUMES AND CAPACITIES

MECHANICS OF BREATHING

GAS EXCHANGE

OXYGEN TRANSPORT IN BLOOD

CARBON DIOXIDE TRANSPORT IN BLOOD

VENTILATION/PERFUSION RELATIONSHIPS

CONTROL OF BREATHING

INTEGRATIVE FUNCTIONS

HYPOXEMIA AND HYPOXIA

SUMMARY

ent Lung volumes and capacities are measured with a spirometer (except for those volumes and capacities that include the residual volume).

ent Dead space is the volume of the airways and lungs that does not participate in gas exchange. Anatomic dead space is the volume of conducting airways. Physiologic dead space includes the anatomic dead space plus those regions of the respiratory zone that do not participate in gas exchange.

ent The alveolar ventilation equation expresses the inverse relationship between image and alveolar ventilation. The alveolar gas equation extends this relationship to predict image.

ent In quiet breathing, respiratory muscles (diaphragm) are used only for inspiration; expiration is passive.

ent Compliance of the lungs and the chest wall is measured as the slope of the pressure-volume relationship. As a result of their elastic forces, the chest wall tends to spring out and the lungs tend to collapse. At FRC, these two forces are exactly balanced and intrapleural pressure is negative. Compliance of the lungs increases in emphysema and with aging. Compliance decreases in fibrosis and when pulmonary surfactant is absent.

ent Surfactant, a mixture of phospholipids produced by type II alveolar cells, reduces surface tension so that the alveoli can remain inflated despite their small radii. Neonatal respiratory distress syndrome occurs when surfactant is absent.

ent Airflow into and out of the lungs is driven by the pressure gradient between the atmosphere and the alveoli and is inversely proportional to the resistance of the airways. Stimulation of β2-adrenergic receptors dilates the airways, and stimulation of cholinergic muscarinic receptors constricts the airways.

ent Diffusion of O2 and CO2 across the alveolar/pulmonary capillary barrier is governed by Fick’s law and driven by the partial pressure difference of the gas. Mixed venous blood enters the pulmonary capillaries and is “arterialized” as O2 is added to it and CO2 is removed from it. Blood leaving the pulmonary capillaries will become systemic arterial blood.

ent Diffusion-limited gas exchange is illustrated by CO and by O2 in fibrosis or strenuous exercise. Perfusion-limited gas exchange is illustrated by N2O, CO2, and O2 under normal conditions.

ent O2 is transported in blood in dissolved form and bound to hemoglobin. One molecule of hemoglobin can bind four molecules of O2. The sigmoidal shape of the O2-hemoglobin dissociation curve reflects increased affinity for each successive molecule of O2 that is bound. Shifts to the right of the O2-hemoglobin dissociation curve are associated with decreased affinity, increased P50, and increased unloading of O2 in the tissues. Shifts to the left are associated with increased affinity, decreased P50, and decreased unloading of O2 in the tissues. CO decreases the O2-binding capacity of hemoglobin and causes a shift to the left.

ent CO2 is transported in blood in dissolved form, as carbaminohemoglobin, and as HCO3. HCO3 is produced in red blood cells from CO2 and H2O, catalyzed by carbonic anhydrase. HCO3 is transported in the plasma to the lungs where the reactions occur in reverse to regenerate CO2, which then is expired.

ent Pulmonary blood flow is the cardiac output of the right heart, and it is equal to the cardiac output of the left heart. Pulmonary blood flow is regulated primarily by image, with alveolar hypoxia producing vasoconstriction.

ent Pulmonary blood flow is unevenly distributed in the lungs of a person who is standing: Blood flow is lowest at the apex of the lung and highest at the base. Ventilation is similarly distributed, although regional variations in ventilatory rates are not as great as for blood flow. Thus, image is highest at the apex of the lung and lowest at the base, with an average value of 0.8. Where image is highest, image is highest and image is lowest.

ent image defects impair gas exchange. If ventilation is decreased relative to perfusion, then image and image will approach their values in mixed venous blood. If perfusion is decreased relative to ventilation, then image and imagewill approach their values in inspired air.

ent Breathing is controlled by the medullary respiratory center, which receives sensory information from central chemoreceptors in the brain stem, from peripheral chemoreceptors in the carotid and aortic bodies, and from mechanoreceptors in the lungs and joints. Central chemoreceptors are sensitive primarily to changes in the pH of CSF, with decreases in pH causing hyperventilation. Peripheral chemoreceptors are sensitive primarily to O2, with hypoxemia causing hyperventilation.

ent During exercise, the ventilation rate and cardiac output increase to match the body’s needs for O2 so that mean values for image and image do not change. The O2-hemoglobin dissociation curve shifts to the right as a result of increased tissue PCO2, increased temperature, and decreased tissue pH.

ent At high altitude, hypoxemia results from the decreased PO2 of inspired air. Adaptive responses to hypoxemia include hyperventilation, respiratory alkalosis, pulmonary vasoconstriction, polycythemia, increased 2,3-DPG production, and a right shift of the O2-hemoglobin dissociation curve.

ent Hypoxemia, or decreased image, is caused by high altitude, hypoventilation, diffusion defects, image defects, and right-to-left shunts. Hypoxia, or decreased O2 delivery to tissues, is caused by decreased cardiac output or decreased O2 content of blood.


Challenge Yourself

Answer each question with a word, phrase, sentence, or numerical solution. When a list of possible answers is supplied with the question, one, more than one, or none of the choices may be correct. Correct answers are provided at the end of the book.

1 If tidal volume is 500 mL, inspiratory reserve volume is 3 L, and vital capacity is 5 L, what is expiratory reserve volume?

2 What are the units of FEV1?

3 Room air is a mixture of O2 and N2 saturated with H2O vapor. If barometric pressure is 740 mm Hg and the fractional concentration of O2 is 21%, what is the partial pressure of N2?

4 A person at sea level breathes a mixture containing 0.1% carbon monoxide (CO). The uptake of CO was measured using the single breath method as 28 mL/minute. What is the lung diffusing capacity for CO (DLCO)?

5 Which of the following increase(s) hemoglobin P50: increased H+ concentration, increased pH, increased image, increased 2,3-diphosphoglycerate (DPG) concentration?

6 Which of the following decrease(s) the O2-binding capacity of hemoglobin: decreasing hemoglobin concentration, decreasing imageto 60 mm Hg, increasing arterial PO2 to 120 mm Hg, left-shift of the O2-hemoglobin dissociation curve?

7 If the ventilation/perfusion (image) ratio of a lung region decreases, how will the PO2 and PCO2 in the blood in that region change?

8 In perfusion-limited O2 exchange, is the PO2 at the end of the pulmonary capillary closer to imageor image?

9 Which of the following is/are higher at the base of the lung than at the apex: blood flow, image, ventilation, PO2, PCO2?

10 Which cause(s) of hypoxemia is/are associated with increased A − a gradient: high altitude, hypoventilation, breathing 10% O2, imagedefects, fibrosis, right-to-left shunt?

11 What is the largest lung volume or capacity that can be inspired above FRC?

12 Which of the following is/are decreased in both restrictive and obstructive lung diseases: vital capacity, FEV1, FEV1/FVC?

13 If tidal volume = 450 mL, breaths/minute = 14/minute,image= 45 mm Hg, image= 55 mm Hg, image= 100 mm Hg, image= 25 mm Hg, and cardiac output = 5 L/minute, what is alveolar ventilation?

14 In persons with emphysema, to balance the collapsing force on the lungs with the expanding force on the chest wall, does functional residual capacity (FRC) increase, decrease, or remain unchanged?

15 Which of the following pairs of pressures would cause the structure to collapse: alveolar pressure = +5 cm H2O and intrapleural pressure = −5 cm H2O; airway pressure = 0 and intrapleural pressure = −5 cm H2O; airway pressure = +15 cm H2O and intrapleural pressure = +20 cm H2O?

16 Which cause of hypoxia is corrected best with supplemental O2: anemia, decreased cardiac output, high altitude, right-to-left shunt?

17 Upon ascent to high altitude, what is the correct sequence of these events: hyperventilation, decreasedimage, decreased image, decreased image, decreased image, increased pH?

18 Where am I? For each item in the following list, give its correct location in the respiratory system. (The location may be anatomic, a graph or portion of a graph, an equation, or a concept.)

FEV1

image = 0

PA > Pa

Afterload of right ventricle

γ chains

P50

Slope of pressure-volume curve

Normally, pressure lower than PB

DL

image <60 mm Hg stimulates breathing

19 In perfusion-limited gas exchange, PO2 at the end of the pulmonary capillary is: equal to mixed venous PO2, greater than alveolar PO2, less than alveolar PO2, or equal to systemic arterial PO2?

20 In persons with restrictive lung disease, to balance the collapsing force on the lungs with the expanding force on the chest wall, does functional residual capacity (FRC) increase, decrease, or remain unchanged?


SELECTED READINGS

Slonim NB, Hamilton LH: Respiratory Physiology, 5th ed. St Louis, Mosby, 1987.

West JB: Pulmonary Pathophysiology, 5th ed. Baltimore, Lippincott, Williams & Wilkins, 1998.

West JB: Respiratory Physiology—the Essentials, 6th ed. Baltimore, Williams & Wilkins, 2000.