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108, BIPN Physiology of Exercise, Lecture 8--Respiration: how it works Page 1 I. The term respiration is used to mean at least three quite different things: A. Movement of air into and out of the respiratory passages and diffusion of the two respiratory gasses O2 and CO2 across the respiratory exchange surface in the lungs, i.e. ventilation. B. Transport of the respiratory gasses between the respiratory exchange surface in the lungs and the cells in the rest of the body, i.e., distribution. This transport is carried out by the blood, so the topic of next week s lectures will be closely related to this meaning of respiration. C. Oxidative metabolism of energy substrates to produce ATP. (This is frequently called cellular respiration. D. Clearly these three meanings are related, but they are different from one another. We ve already discussed the third meaning, and we will now consider the other two. II. The respiratory exchange surface of the human body is made up of an enormous number of tiny air sacs called alveoli. A. Respiratory exchange surfaces have four principle characteristics: 1. They are very thin, which allows O2 and CO2 to diffuse across the surface. 2. They are moist; O2 and CO2 must be dissolved in water in order to cross this diffusion barrier. 3. They have a very rich supply of capillaries (remember from BIPN 100 that the only part of the circulatory system where there is exchange between the blood and the tissues is along the capillaries). 4. They have a very large surface area. B. The alveolar surface in the body of an average human is about the size of one-half of a tennis court. C. Air travels from the nose and mouth through the respiratory passages, which branch repeatedly until each of the tiniest tubes ends in an alveolus. (Figure) 1. Air inside an alveolus is still topologically outside of the body. 2. O2 enters the body when it diffuses across the alveolar wall and into a capillary. III. The periodic behavior that moves air into and out of the respiratory passages is called ventilation. BIPN 108, Physiology of Exercise, Lecture 8--Respiration: how it works Page 2 A. During ventilation, air moves because the thoracic cavity which surrounds the lungs is sealed space, and its volume is increased or decreased by contraction of a set of striated skeletal muscles. 1. The major muscles that participate in ventilation are the diaphragm and muscles that are attached between the ribs (the intercostal muscles). 2. From the ideal gas laws we know that PV = nRT. The space between the lungs and the thoracic wall is sealed, so n, R, and T are constant. Thus, if V increases, P must drop, and if V decreases P must rise. (If your memory of the idea gas laws and of the meaning of the partial pressure of a gas is rusty, this would be a very good time to review those concepts.) 3. As a result, air is sucked into the lungs when the volume of the thoracic cavity increases (during inhalation AKA inspiration), and it is forced out of the lungs during when the volume of the thoracic cavity decreases (during exhalation AKA expiration). B. Careful observation of ventilation has led to several descriptive terms that you should know; they re listed on Figure 9.4 in your text. (Figure) 1. Tidal volume for healthy adults at rest is usually between 0.4 and 1.0 liter/min. 2. At rest, healthy adults have both inspiratory and expiratory reserve volumes. 3. Minute volume refers to the volume of air that enters and leaves the respiratory passages per minute. 4. Minute volume depends on both the rate and the depth of respiratory movements. C. Each inspiration moves new air into the respiratory passages. The air is pulled along the respiratory passages and into the alveoli. 1. However, the air that enters the alveoli has a much different composition than air that entered the nose or mouth. (Figure) 2. Alveolar air has a lower O2 content and a higher CO2 content than inspired air. Why? a. One major reason is that the lungs and respiratory passages do not empty during expiration. Instead air stays in the passages, and this old air mixes with newly inspired air. b. The volume of the respiratory passages is called the anatomical dead space. c. In addition, some alveoli may be poorly ventilated or blood flow in the capillaries of some alveoli may be low, and their volume is called the BIPN 108, Physiology of Exercise, Lecture 8--Respiration: how it works Page 3 physiological dead space. In healthy lungs the physiological dead space is very small, but it can increase up to 50% or more of the total volume in disease states. d. Finally, air becomes saturated (or nearly so) with water vapor as it passes along the respiratory passages, and the addition of water vapor to the gas mixture drops the PO2 of the inspired air. 3. As a result of this mixing AND of gas exchange in the alveoli, expired air has a lower PO2 and a higher PCO2 than inspired air. IV. In alveolar air, both O2 and CO2 (the respiratory gasses) are in the gaseous state, but they diffuse across the alveolar wall as gas molecules dissolved in the thin layer of water that coats the alveolar wall. Once they enter the bloodstream, which carries respiratory gasses between the alveoli and the rest of the cells of the body, the two gasses are treated differently. A. O2 is not very soluble in water. 1. In addition, the solubility of O2 in water decreases with increasing temperature, so for a metabolic homeotherm that regulates body temperature around 37 C (the normal human body temperature), only a little O2 will dissolve in the plasma. 2. Instead, most of the O2 carried in the bloodstream is bound to a protein molecule, hemoglobin. a. Hemoglobin is a tetramer of monomeric protein sub-units. b. monomer Each is associated with an organic prosthetic group called heme. Heme is a complex ring group and absorbs light in the visible spectrum, which makes it red. c. Each heme binds an iron atom, and O2 binds reversibly to the Fe. d. So each hemoglobin can bind four O2 molecules, and the ease of binding increases as O2 molecules are added sequentially to a single hemoglobin complex. e. This interaction is called cooperativity, and it leads to a sigmoidal oxygen dissociation curve. (Figure) 3. Several physiological variables affect the oxygen affinity of hemoglobin, and some of these variables change during exercise, which can change how hemoglobin either binds or releases O2. B. In contrast, CO2 is very soluble in water. 1. H2O + CO2 <-->H2CO3 <--> H+ + HCO3-. BIPN 108, Physiology of Exercise, Lecture 8--Respiration: how it works Page 4 2. Almost all of the CO2 produced by the tissues of the body is carried to the lungs as HCO3- dissolved in the plasma. 3. Red blood cells facilitate this process because they contain a large amount of the enzyme carbonic anhydrase, which speeds up the reaction between CO2 and H2O or the transformation of HCO3- back into H2O and CO2. V. To effectively support different levels of exertion, the respiratory system must be able to change ventilation, increasing ventilation in response to increased exercise and decreasing ventilation when the person returns to rest. A. O2 and CO2 play important roles in regulating the activity of ventilatory muscles, but the system is complex. (Figure) B. Ventilatory muscles are standard striated skeletal muscles. 1. They contract only when they are driven by cholinergic motor neurons. 2. However, breathing is an unconscious activity; how can that be reconciled with control by motor neurons? C. The control centers for ventilation are located in the brain stem, primarily in the medulla. These centers together make up a central pattern generator, that is, a set of connected neurons that produce repetitive rhythmic output based on their synaptic connections. 1. The rhythmic activity of the respiratory central pattern generator can be modulated by input from a variety of sources. 2. The rhythmic activity of these neurons changes in response to PCO2 in the cerebral spinal fluid (which reflects the PCO2 in the plasma). 3. Neuronal activity from higher centers in the brain are sent to these centers. (E.g., you can choose to hold your breath, at least for awhile.) 4. Neuronal input from chemosensory neurons in the periphery also is integrated by the medullary respiratory centers. D. Peripheral chemosensors that respond to O2 and CO2 (typically sensed as pH) are located in only two places: the carotid bodies and the aortic arch. (Figure) E. At the whole-body level, ventilation is much more sensitive to changes in PCO2 than it is to changes in PO2, although if PO2 gets low enough it will also affect ventilation. (Figure) That is, a small change in PCO2 will change the respiratory rhythm, but it takes a much larger change in PO2 to change the rhythm. The gas exchange surface in the lungs has a huge area because there are en enormous number of alveoli. Ventilation is a periodic behavior in which skeletal muscles contract, driven by motor neurons. Contraction of the muscles changes the volume of the thoracic cavity, producing forces on air. The volume of air inspired and expired varies with the rate and depth of ventilation O2 and CO2 move across the exchange surface in the lungs by diffusion; partial pressure differences provide the driving force. However, alveolar gas is quite different from inspired air. With every breath, new air mixes with air in the residual volume (i.e., in the anatomic and physiological dead space) reducing the PO2 in alveolar air. In addition, air is humidified as it moves toward the alveoli, and the water vapor also contributes to the total air pressure, reducing the PO2. O2 and CO2 are carried between the alveoli and the tissues by the blood. Mechanisms for carrying the two gasses are different. 1) O2 is carried bound to the protein hemoglobin inside of red blood cells. 2) CO2 is carried as HCO3- in the plasma, although the carbonic anhydrase in red blood cells contributes to the process. 3) Notice that O2 remains in the blood when it leaves the tissues, and CO2 remains in the blood when it leaves the lung. The affinity of hemoglobin for oxygen is affected by a number of physiological variables. High affinity favors binding; low affinity favors dissociation. Ventilatory movements are controlled by neurons in brain stem nuclei, which integrate information from a number of sources. Output from these centers causes contraction of striated skeletal muscles that change the volume of the thoracic cavity. Chemoreceptors are located in only a few places in the body: the carotid bodies and the aortic arches. The control of ventilation is much more sensitive to PCO2 in alveolar air (and hence in the blood) than it is to PO2 in alveolar air. The fine print: In these experiments, alveolar PO2 and PCO2 were varied by changing the concentration of O2 and CO2 in inspired air , and the effect on ventilation was measured. In the experiment shown at the top, alveolar PO2 was set at 4 values: 169 mm Hg, 110-mm Hg, 47 mm Hg, or 37 mmHg. Several values of PCO2 were explored (plotted along the X axis). Notice that changing PCO2 by as little as 1 - 5 mm Hg produced a big effect on ventilation. In the experiment shown to the right, the PCO2 was set to one of three values, and the PO2 was set at many different values. Notice how much PO2 had to be changed to cause much of an effect on ventilation. In addition, notice that there is an interaction between PO2 and PCO2 in respiratory regulation.

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