7. Case problems with pictures in topic "Phуsiology of respiration"

Good afternoon!

Case problem 7.1. What consequences would result if inflammation caused a buildup of fluid in the alveoli and interstitial spaces?

Figure 7.1. (a) Cross section through an area of the respiratory zone. There are 18 alveoli in this figure, only four of which are labeled. Two often share a common wall. (b) Schematic enlargement of a portion of an alveolar wall. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.1. The rate of diffusion of gases between the air and the capillaries may be decreased due to the increased resistance to diffusion.

Case problem 7.2. How can a collapsed lung be re-expanded in a patient with a pneumothorax? (Hint: What changes in Pip and Ptp would be needed to re-expand the lung?)

Figure 7.2. Pneumothorax. The lung collapses as air enters from the pleural cavity either from inside the lung or from the atmosphere through the thoracic wall. The combination of lung elastic recoil and surface tension causes collapse of the lung when pleural and airway pressures equalize. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.2. A tube is placed through the chest wall into the now enlarged pleural space. (The original hole causing the pneumothorax would need to be repaired first.) Suction is then applied to the chest tube. The negative pressure decreases Pip below Patm and thereby increases Ptp, which results in re-expansion of the lung.

Case problem 7.3. How do the changes in Ptp between each step (–) explain whether the volume of the lung is increasing or decreasing?

Figure 7.3. Summary of alveolar (Palv), intrapleural (Pip), and transpulmonary (Ptp) pressure changes and airflow during a typical respiratory cycle. At the end of expiration, Palv is equal to Patm and there is no airflow. At mid-inspiration, the chest wall is expanding, lowering Pip and making Ptp more positive. This expands the lung, making Palv negative, and results in an inward airflow. At end of inspiration, the chest wall is no longer expanding but has yet to start passive recoil. Because lung size is not changing and the airway is open to the atmosphere, Palv is equal to Patm and there is no airflow. As the respiratory muscles relax, the lungs and chest wall start to passively collapse due to elastic recoil. At midexpiration, the lung is collapsing, thus compressing alveolar gas. As a result, Palv is positive relative to Patm and airflow is outward. The cycle starts over again at the end of expiration. Notice that throughout a typical respiratory cycle with a normal tidal volume, Pip is negative relative to Patm. In the graph on the left, the difference between Palv and Pip (Palv − Pip) at any point along the curves is equivalent to Ptp. For clarity, the chest-wall elastic recoil is not shown. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.3.

Case problem 7.4. Premature infants with inadequate surfactant have decreased lung compliance (respiratory distress syndrome of the newborn). If surfactant is not available to administer for therapy, what would you suggest could be done to inflate the lung?

Figure 7.4. A graphic representation of lung compliance. Changes in lung volume and transpulmonary pressure are measured as a subject takes progressively larger breaths. When compliance is lower than normal (the lung is stiffer), there is a lesser increase in lung volume for any given increase in transpulmonary pressure. When compliance is increased, as in emphysema, small decreases in Ptp allow the lung to collapse. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.4. Anything that increases Ptp during inspiration will, theoretically, increase lung volume. This can be done with positive airway pressure generated by mechanical ventilation, which will increase Palv. This approach can work but also increases the risk of pneumothorax by inducing air leaks from the lung into the intrapleural space.

Case problem 7.5. What would be the effect of breathing through a plastic tube with a length of 20 cm and diameter of 4 cm? (Hint: Use the formula for the volume of a perfect cylinder.)

Figure 7.5. Effects of anatomical dead space on alveolar ventilation. Anatomical dead space is the volume of the conducting airways. Of a 500 ml tidal volume breath, 350 ml enters the airway involved in gas exchange. The remaining 150 ml remains in the conducting airways and does not participate in gas exchange. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.5. The anatomical dead space would be increased by about 251 mL (or 251 cm3). (The volume of the tube can be approximated as that of a perfect cylinder [π r2h = 3.1416 × 22 × 20].) This large increase in anatomical dead space would decrease alveolar ventilation, and tidal volume would have to be increased in compensation. (There would also be an increase in airway resistance.)

Case problem 7.6. How does this figure illustrate the general principle of physiology described in Chapter 1 that physiological processes require the transfer and balance of matter and energy?

Figure 7.6. Summary of typical oxygen and carbon dioxide exchanges between atmosphere, lungs, blood, and tissues during 1 min in a resting individual. Note that the values in this figure for oxygen and carbon dioxide in blood are not the values per liter of blood but, rather, the amounts transported per minute in the cardiac output (5 l in this example). The volume of oxygen in 1 l of arterial blood is 200 ml O2/l of blood – that is, 1000 mL O2/5 l of blood. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.6. The cells require oxygen for cellular respiration and, in turn, produce carbon dioxide as a toxic metabolic waste product. To support the net uptake of oxygen and net removal of carbon dioxide, oxygen must be transferred from the atmosphere to all of the cells and organs of the body while carbon dioxide must be transferred from the cells to the atmosphere. This requires a highly efficient transport process that involves diffusion of oxygen and carbon dioxide in opposite directions in the lungs and the cells, and bulk flow of blood carrying oxygen and carbon dioxide around the circulatory system from the lungs to the cells and then back to the lungs. These processes result in a net gain of oxygen (250 ml/per min at rest) from the atmosphere for consumption in the cells, and the net loss of carbon dioxide (200 ml/min at rest) from the cells to the atmosphere.

Case problem 7.7. What is the effect of strenuous exercise on PO2 at the end of a capillary in a normal region of the lung? In a region of the lung with diffusion limitation due to disease?

Figure 7.7. Equilibration of blood PO2 with an alveolus with a PO2 of 105 mmHg along the length of a pulmonary capillary. Note that in an abnormal alveolar-diffusion barrier (diseased), the blood is not fully oxygenated Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.7. The increase in cardiac output with exercise greatly increases pulmonary blood flow and decreases the amount of time erythrocytes are exposed to increased oxygen from the alveoli. In a normal region of the lung, there is a large safety factor such that a large increase in blood flow still allows normal oxygen uptake. However, even small increases in the rate of capillary blood flow in a diseased portion of the lung will decrease oxygen uptake due to a loss of this safety factor.

Case problem 7.8. How would you treat carbon monoxide poisoning? Look back at Equation 13.8 and realize that CO can be displaced from hemoglobin binding if the dissolved O2 concentration (PO2) is increased enough. That is, CO and O2 compete for the same binding sites on hemoglobin albeit with CO having a 210 times higher affinity.

Figure 7.8. Effects of DPG concentration, temperature, acidity, the presence of fetal hemoglobin and carbon monoxide and anemia on the relationship between PO2 and hemoglobin saturation or O2 content. The temperature of normal blood, of course, never diverges from 37°C as much as shown in the figure, but the principle is still the same when the changes are within the physiological range. High acidity and low acidity can be caused by high PCO2 and low PCO2, respectively. Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin, allowing an adequate oxygen content from oxygen diffusion from the maternal to fetal blood in the placenta. Carbon monoxide occupies O2 binding sites on the hemoglobin molecule and reduces the oxygen-carrying capacity of the blood (notice the change in Y-axis label) as well as shifting the curve to the left. Anemia is a reduction in the hemoglobin concentration in the blood. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.8. The most important treatment is to displace CO from hemoglobin with O2. Although CO binds to hemoglobin more avidly than O2, it can dissociate from the hemoglobin molecule if forced off by increasing dissolved O2. This is initially done by giving the patient 100% O2 to breathe to increase the inspired and arterial PO2 as high as possible while ensuring that alveolar ventilation is maintained to rid the body of the carbon monoxide. If the CO poisoning is severe, the patient must be transported to a facility that can give hyperbaric O2 therapy. By increasing the barometric pressure, the inspired and arterial PO2 can be dramatically increased. Ultimately, the success of the therapy depends on the length of time the patient has been exposed to high CO, the magnitude of the CO poisoning, and the rapidity of the reduction in CO binding to hemoglobin in order to restore O2 delivery to the tissues.

Case problem 7.9. Several decades ago, removal of the carotid bodies was tried as a treatment for asthma. It was thought that it would reduce shortness of breath and airway hyperreactivity. What would be the effect of bilateral carotid body removal on someone taking a trip to the top of a mountain (an altitude of 3000 meters)?

Figure 7.9. Location of the carotid and aortic bodies. Note that each carotid body is quite close to a carotid sinus, the major arterial baroreceptor. Both right and left common carotid bifurcations contain a carotid sinus and a carotid body. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.9. The ventilatory response to the hypoxia of altitude would be greatly diminished, and it is likely that the person would be extremely hypoxemic as a result. Carotid body removal did not help in the treatment of asthma, and this approach was abandoned.

Case problem 7.10. How does this figure illustrate the general principle of physiology described in Chapter 1 that homeostasis is essential for health and survival?

Figure 7.10. Sequence of events by which a low arterial PO2 causes hyperventilation, which maintains alveolar (and, hence, arterial) PO2 at a value higher than would exist if the ventilation had remained unchanged. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.10. An adequate supply of oxygen to all cells is required for normal organ function, and maintenance of oxygen delivery in the face of decreased oxygen uptake in the lung is an important homeostatic reflex. The most common cause of a decrease in the inspired PO2 is temporary or permanent habitation at altitude, where the atmospheric pressure and therefore the PO2 of the air is lower than at sea level. Without compensation for the lower inspired PO2, arterial blood PO2 could decrease to life-threatening levels. All homeostatic processes in the body depend on a continual input of energy derived from heat or ATP; synthesis of ATP requires oxygen. The arterial chemoreceptors can detect a decrease in arterial PO2 that results from ascent to high altitude and reflexively increase alveolar ventilation to enhance oxygen uptake from the air into the pulmonary capillaries for delivery to the rest of the body. The inability to adequately increase alveolar ventilation at altitude can result in harmful consequences leading to organ damage and even death.

Case problem 7.11. The existence of chemoreceptors in the pulmonary artery has been suggested. Hypothesise a function for peripheral chemoreceptors located on and sensing the PO2 and PCO2 of the blood in the pulmonary artery.

Figure 7.11. Summary of factors that stimulate ventilation during exercise. Note: “?” indicates a theoretical input. Eric P. Widmaier Human Physiology. The Mechanisms of Body Function, New NY: McGraw-Hill Education. 2019. https://lccn.loc.gov/2017048599

Hint for answer 7.11. These receptors may facilitate the increase in alveolar ventilation that occurs during exercise because pulmonary artery PO2 will decrease and pulmonary artery PCO2 will increase. This would match the increase in tissue metabolism to the increase in alveolar ventilation.

Good luck in your studies!