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Human Physiology, 7/e
Stuart I Fox, Pierce College

Respiratory Physiology

Chapter Summary

The Respiratory System

  1. Alveoli are microscopic thin-walled air sacs that provide an enormous surface area for gas diffusion.
    1. The region of the lungs where gas exchange with the blood occurs is known as the respiratory zone.
    2. The trachea, bronchi, and bronchioles that deliver air to the respiratory zone comprise the conducting zone.
  2. The thoracic cavity is limited by the chest wall and diaphragm.
    1. The structures of the thoracic cavity are covered by thin, wet pleural membranes.
    2. The lungs are covered by a visceral pleura that is normally flush against the parietal pleura that lines the chest wall.
    3. The potential space between the visceral and parietal pleurae is called the intrapleural space.

Physical Aspects of Ventilation

  1. The intrapleural and intrapulmonary pressures vary during ventilation.
    1. The intrapleural pressure is always less than the intrapulmonary pressure.
    2. The intrapulmonary pressure is subatmospheric during inspiration and greater than the atmospheric pressure during expiration.
    3. Pressure changes in the lungs are produced by variations in lung volume, in accordance with the inverse relationship between the volume and pressure of a gas described by Boyle's law.
  2. The mechanics of ventilation are influenced by the physical properties of the lungs.
    1. The compliance of the lungs, or the ease with which they expand, refers specifically to the change in lung volume per change in transpulmonary pressure (the difference between intrapulmonary pressure and intrapleural pressure).
    2. The elasticity of the lungs refers to their tendency to recoil after distension.
    3. The surface tension of the fluid in the alveoli exerts a force directed inward, which acts to resist distension.
  3. On first consideration, it would seem that the surface tension in the alveoli would create a pressure that would cause small alveoli to collapse and empty their air into larger alveoli.
    1. This would occur because the pressure caused by a given amount of surface tension would be greater in smaller alveoli than in large alveoli, as described by the law of La Place.
    2. Surface tension does not normally cause the collapse of alveoli, however, because pulmonary surfactant (a combination of phospholipid and protein) lowers the surface tension sufficiently.
    3. In hyaline membrane disease, the lungs of premature infants collapse because of a lack of surfactant.

Mechanics of Breathing

  1. Inspiration and expiration are accomplished by the contraction and relaxation of striated muscles.
    1. During quiet inspiration, the diaphragm and external intercostal muscles contract and thus increase the volume of the thorax.
    2. During quiet expiration, these muscles relax, and the elastic recoil of the lungs and thorax causes a decrease in thoracic volume.
    3. Forced inspiration and expiration are aided by contraction of the accessory respiratory muscles.
  2. Spirometry aids the diagnosis of a number of pulmonary disorders.
    1. In restrictive disease, such as pulmonary fibrosis, the vital capacity measurement is decreased to below normal.
    2. In obstructive disease, such as asthma and bronchitis, the forced expiratory volume is reduced to below normal because of increased airway resistance to air flow.
  3. Asthma results from bronchoconstriction; emphysema and chronic bronchitis are frequently referred to collectively as chronic obstructive pulmonary disease.

Gas Exchange in the Lungs

  1. According to Dalton's law, the total pressure of a gas mixture is equal to the sum of the pressures that each gas in the mixture would exert independently.
    1. The partial pressure of a gas in a dry gas mixture is thus equal to the total pressure times the percent composition of that gas in the mixture.
    2. Since the total pressure of a gas mixture decreases with altitude above sea level, the partial pressures of the constituent gases likewise decrease with altitude.
    3. When the partial pressure of a gas in a wet gas mixture is calculated, the water vapor pressure must be taken into account.
  2. According to Henry's law, the amount of gas that can be dissolved in a fluid is directly proportional to the partial pressure of that gas in contact with the fluid.
    1. The concentrations of oxygen and carbon dioxide that are dissolved in plasma are proportional to an electric current generated by special electrodes that react with these gases.
    2. Normal arterial blood has a PO2 of 100 mm Hg, indicating a concentration of dissolved oxygen of 0.3 ml per 100 ml of blood; the oxygen contained in red blood cells (about 19.7 mil per 100 ml of blood) does not affect the PO2 measurement.
  3. The PO2 and PCO2 measurements of arterial blood provide information about lung function.
  4. In addition to proper ventilation of the lungs, blood flow (perfusion) in the lungs must adequate and matched to air flow (ventilation) in order for adequate gas exchange to occur.
  5. Abnormally high partial pressures of gases in blood can cause a variety of disorders, including oxygen toxicity, nitrogen narcosis, and decompression sickness.

Regulation of Breathing

  1. The rhythmicity center in the medulla oblongata directly controls the muscles of respiration.
    1. Activity of the inspiratory and expiratory neurons varies in a reciprocal way to produce an automatic breathing cycle.
    2. Activity in the medulla is influenced by the apneustic and pneumotaxic centers in the pons, as well as by sensory feedback information.
    3. Conscious breathing involves direct control by the cerebral cortex via corticospinal tracts.
  2. Breathing is affected by chemoreceptors sensitive to the PO2, pH, and PCO2 of the blood.
    1. The PCO2 of the blood and consequent changes in pH are usually of greater importance than the blood PO2 in the regulation of breathing.
    2. Central chemoreceptors in the medulla oblongata are sensitive to changes in blood PCO2 because the resultant changes in the pH of the cerebrospinal fluid.
    3. The peripheral chemoreceptors in the aortic and carotid bodies are sensitive to changes in blood PCO2 indirectly, because of consequent changes in blood pH.
  3. Decreases in blood PO2 directly stimulate breathing only when the blood PO2 is lower than 50 mm Hg. A drop in PO2 also stimulates breathing indirectly, by making the chemoreceptors more sensitive to changes in PCO2 and pH.
  4. At tidal volumes of 1 L or more, inspiration is inhibited by stretch receptors in the lungs (the Hering, Breuer reflex). A similar reflex may act to inhibit expiration.

Hemoglobin and Oxygen Transport

  1. Hemoglobin is composed of two alpha and two beta polypeptide chains and four heme groups, each containing a central atom of iron.
    1. When the iron is in the reduced form and not attached to oxygen, the hemoglobin is called deoxyhemoglobin, or reduced hemoglobin; when it is attached to oxygen, it is called oxyhemoglobin.
    2. If the iron is attached to carbon monoxide, the hemoglobin is called carboxyhemoglobin. When the iron is in an oxidized state and unable to transport any gas, the hemoglobin is called methemoglobin.
    3. Deoxyhemoglobin combines with oxygen in the lungs (the loading reaction) and breaks its bonds with oxygen in the tissue capillaries (the unloading reaction). The extent of each reaction is determined by the PO2 and the affinity of hemoglobin for oxygen.
  2. A graph of percent oxyhemoglobin is saturation at different values of PO2 is called an oxyhemoglobin dissociation curve.
    1. At rest, the difference between arterial and venous oxyhemoglobin saturations indicates that about 22% of the oxyhemoglobin unloads its oxygen to the tissues.
    2. During exercise, the venous PO2 and percent oxyhemoglobin saturation are decreased, indicating a higher percent of the oxyhemoglobin unloaded its oxygen to the tissues.
  3. The pH and temperature of the blood influence the affinity of hemoglobin for oxygen and thus the extent of loading and unloading.
    1. A fall in pH decreases the affinity of hemoglobin for oxygen, and a rise in pH increases the affinity. This is called the Bohr effect.
    2. A rise in temperature decreases the affinity of hemoglobin for oxygen.
    3. When the affinity is decreased, the oxyhemoglobin dissociation curve is shifted to the right. This indicates a greater percentage unloading of oxygen to the tissues.
  4. The affinity of hemoglobin for oxygen is also decreased by an organic molecule in the red blood cells called 2,3-diphosphoglyceric acid (2,3-DPG).
    1. Since oxyhemoglobin inhibits 2,3-DPG production, 2,3-DPG concentrations will be higher when anemia or low PO2 (as in high altitude) causes a decrease in oxyhemoglobin.
    2. If a person is anemic, the lowered hemoglobin concentration is partially compensated for because a higher percent of the oxyhemoglobin will unload its oxygen as a result of the effect of 2,3-DPG.
    3. Fetal hemoglobin cannot bind to 2,3-DPG, and thus it has a higher affinity for oxygen than the mother's hemoglobin. This facilitates the transfer of oxygen to the fetus.
  5. Inherited defects in the amino acid composition of hemoglobin are responsible for such diseases as sickle-cell anemia and thalassemia.
  6. Striated muscles have myoglobin, a pigment related to hemoglobin, which can combine with oxygen and deliver it to the muscle cell mitochondria at low PO2values.

Carbon Dioxide Transport and Acid-Base Balance

  1. Red blood cells contain an enzyme called carbonic anhydrase, which catalyzes the reversible reaction whereby carbon dioxide and water are used to form carbonic acid.
    1. This reaction is favored by the high PCO2 in the tissue capillaries, and as a result, carbon dioxide produced by the tissues is converted into carbonic acid in the red blood cells.
    2. Carbonic acid then ionizes to form H+ and HCO3- (bicarbonate).
    3. Since much of the H+ is buffered by hemoglobin, but more bicarbonate is free to diffuse outward, an electrical gradient is established that draws Cl- into the red blood cells. This is called the chloride shift.
    4. A reverse chloride shift occurs in the lungs. In this process, the low PCO2 favors the conversion of carbonic acid to carbon dioxide, which can be exhaled.
  2. By adjusting the blood concentration of carbon dioxide and thus of carbonic acid, the process of ventilation helps to maintain proper acid-base balance of the blood.
    1. Normal arterial blood pH is 7.40; a pH less than 7.35 is termed acidosis, and a pH greater than 7.45 is termed alkalosis.
    2. Hyperventilation causes respiratory alkalosis, and hypoventilation causes respiratory acidosis.
    3. Metabolic acidosis stimulates hyperventilation, which can cause a respiratory alkalosis as a partial compensation.

Effect of Exercise and High Altitude on Respiratory Function

  1. During exercise there is increased ventilation, or hyperpnea, which is matched to the increased metabolic rate so that the arterial blood PCO2 remains normal.
    1. This hyperpnea may be caused by proprioceptor information, cerebral input, and/or changes in arterial PCO2 and pH.
    2. During heavy exercise the anaerobic threshold may be reached at about 55% of the maximal oxygen uptake. At this point, lactic acid is released into the blood by the muscles.
    3. Endurance training enables muscles to utilize oxygen more effectively, so that greater levels of exercise can be performed before the anaerobic threshold is reached.
  2. Acclimatization to a high altitude involves changes that help to deliver oxygen more effectively to the tissues, despite reduced arterial PO2.
    1. Hyperventilation occurs in response to the low PO2.
    2. The red blood cells produce more 2,3-DPG, which lowers the affinity of hemoglobin for oxygen and improves the unloading reaction.
    3. The kidneys produce the hormone erythropoietin, which stimulates the bone marrow to increase its production of red blood cells, so that more oxygen can be carried by the blood at given values of PO2.

After studying this chapter, students should be able to . . .

  1. describe the functions of the respiratory system, distinguish between the conducting and respiratory zone structures, and discuss the significance of the thoracic membranes.
  2. explain how the intrapulmonary and intrapleural pressures vary during ventilation and relate these pressure changes to Boyle?s law.
  3. define the terms compliance and elasticity and explain how these lung properties affect ventilation.
  4. discuss the significance of surface tension in lung mechanics, explain how the law of La Place applies to lung function, and describe the role of pulmonary surfactant.
  5. explain how inspiration and expiration are accomplished in unforced breathing and describe the accessory respiratory muscles used in forced breathing.
  6. define the various lung volumes and capacities that can be measured by spirometry and explain how obstructive diseases may be detected by the FEV test.
  7. describe the nature of asthma, bronchitis, emphysema, and pulmonary fibrosis.
  8. explain Dalton?s law and illustrate how the partial pressure of a gas in a mixture of gases is calculated.
  9. explain Henry?s law, describe how blood PO2and PCO2are measured, and discuss the clinical significance of these measurements.
  10. describe the roles of the medulla oblongata, pons, and cerebral cortex in the regulation of breathing.
  11. explain why the PCO2 and pH of blood, rather than its oxygen content serve as the primary stimuli in the control of breathing.
  12. explain how the chemoreceptors in the medulla oblongata and the peripheral chemoreceptors in the aortic and carotid bodies respond to changes in PCO2, pH, and PO2.
  13. describe the Hering, Breuer reflex and discuss its significance.
  14. describe the different forms of hemoglobin and discuss the significance of these different forms.
  15. describe the loading and unloading reactions and explain how the extent of these reactions is influenced by the PO2 and affinity of hemoglobin for oxygen.
  16. describe the oxyhemoglobin dissociation curve, discuss the significance of its shape, and demonstrate how this curve is used to derive the unloading percentage for oxygen.
  17. explain how oxygen transport is influenced by changes in blood pH and temperature, and explain the effect and physiological significance of 2,3-DPG on oxygen transport.
  18. list the different forms in which carbon dioxide is carried by the blood and explain the chloride shift in the tissues and the reverse chloride shift in the lungs.
  19. explain how carbon dioxide affects blood pH and how hypoventilation and hyperventilation affect acid-base balance.
  20. describe the hyperpnea of exercise and explain how the anaerobic threshold is affected by endurance training.
  21. explain the respiratory adjustments to life at a high altitude.