Chapter 13: Understanding Concepts
Explain pulmonary elasticity in terms of compliance and elastic recoil.
Compliance refers to how much work is required to stretch or distend the lungs during inspiration. The lower the compliance, the more work must be generated to stretch the lungs. Elastic recoil refers to how easily the lungs rebound after being stretched and is responsible for the return of the lungs to their preinspiratory volume after inspiration.
Describe the components of the respiratory system. What is the site of gas exchange?
Components of the respiratory system include the respiratory airways: nasal passages, pharynx, larynx, trachea, bronchi, and bronchioles, which conduct, clean, cool, and humidify air. At the end of the respiratory airway system are the alveoli, which are dead-end sacs surrounded by capillaries. The alveoli are the sites of gas exchange between blood and air.
Explain why air enters the lungs during inspiration and leaves during expiration.
Air moves into and out of the lungs due to pressure gradients between the lungs and atmosphere. During inspiration the diaphragm and external intercostal muscles contract, increasing the volume and decreasing the pressure within the lungs. Intra-alveolar pressure drops below atmospheric pressure and air enters the lungs down the pressure gradient. The opposite happens during expiration. These muscles relax, decreasing the volume and increasing the intra-alveolar pressure above atmospheric pressure. This forces air out of the lungs as it moves down the pressure gradient.
Why are the lungs normally stretched even during expiration?
This phenomenon occurs because the thoracic wall grows faster than the lungs during development; as a consequence, the lungs are smaller than the thoracic cavity and are stretched as they remain attached to the inner chest wall. So, even during expiration, the lungs remain attached to the thoracic wall and are stretched beyond their actual size.
How does hemoglobin promote the net transfer of O2 from the alveoli to the blood?
Hemoglobin acts as a storage site for oxygen, removing dissolved oxygen from the plasma as soon as it enters the blood from the alveoli. When oxygen binds to hemoglobin it is removed from its dissolved state in the plasma. This maintains the concentration gradient between the alveoli and the blood, so oxygen continues to move into the plasma until hemoglobin is completely saturated. At this point, 98.5 percent of the oxygen is bound to hemoglobin and the remaining 1.5 percent is dissolved in the plasma, measured by PO2.
Why does airway resistance become in important determinant of airflow rates in chronic obstructive pulmonary disease?
In COPD, airway resistance is increased because airway diameter is reduced due to the pathology of the disease. When airway resistance is increased a greater pressure gradient must be created to maintain normal airflow. Patients with COPD must work harder to breath.
Compare atmospheric, intra-alveolar, and intrapleural pressures.
Atmospheric pressure is the pressure exerted by the weight of the air in the atmosphere on objects on the Earth's surface. At sea level it is measured at 760 mm Hg. Intra-alveolar (intrapulmonary) pressure is the pressure within the alveoli. It changes during inspiration and expiration. Intrapleural pressure is the pressure in the pleural sac that is the pressure exerted outside the lungs within the thoracic cavity. Under normal conditions it is always about 4 mm Hg less than intrapulmonary pressure and changes during inspiration and expiration.
Distinguish between cellular and external respiration. List the steps in external respiration.
External respiration involves all of the steps involved in gas exchange and includes the physical act of breathing (ventilation), gas exchange between lungs and blood, transport of gases by blood, and gas exchange between blood and tissues. Internal respiration or cellular respiration involves the metabolic reactions that take place inside the cell to use oxygen to produce ATP.
Why is inspiration normally active and expiration is normally passive?
Inspiration is considered active because it involves the contraction of skeletal muscles to produce the pressure changes associated with inspiration. Expiration is considered passive under resting conditions because the decrease in lung volume occurs when the muscles relax and the naturally elastic lungs recoil to their preinspiratory size.
List the methods of O2 transport and CO2 transport in the blood.
Oxygen is transported in the blood by two means: a small percentage (1.5%) is dissolved in the plasma and98.5 percent is bound to hemoglobin. Carbon dioxide is transported in the blood by three means: 10 percent is dissolved in the plasma, 30 percent is bound to hemoglobin, and 60 percent is converted to bicarbonate.
What determines the partial pressure of a gas in air and in blood?
Partial pressure exerted by each gas in a mixture equals the total pressure exerted on the gas times the fractional (percentage) composition of this gas in the mixture. This holds true for a gas in air or blood.
Compare pulmonary and alveolar ventilation. What is the consequence of anatomic and alveolar dead space?
Pulmonary ventilation is the volume of air breathed in and out in one minute. It is measured by multiplying tidal volume by respiratory rate. Alveolar ventilation is the volume of air exchanged between the atmosphere and the alveoli per minute. It is calculated by taking x respiratory rate (tidal volume - dead space). Anatomic dead space is the volume of air in the airways not utilized in gas exchange. Anatomic dead space is therefore considered when calculating alveolar ventilation because this air does not reach the alveoli. Alveolar dead space is the volume of air in any alveoli not participating in gas exchange due to low blood perfusion. This is normally not an issue in healthy individuals.
State the source and function of pulmonary surfactant.
Surfactant is produced by type II alveolar cells and functions to reduce alveolar surface tension. This property increases compliance, reducing the work required during breathing, and reduces the lungs' tendency to recoil, preventing collapse during each breath.
Explain the Bohr and Haldane effects.
The Bohr effect is the enhanced oxygen release from hemoglobin when carbon dioxide and H+ bind to hemoglobin. The Haldane effect is the increased carrying capacity of carbon dioxide and H+ on hemoglobin when hemoglobin gives up oxygen.
What is the primary factor that determines the percent hemoglobin saturation? What are the significances of the plateau and the steep portions of the O2-Hb dissociation curve?
The PO2 of the blood is the primary factor determining the percentage of saturation of hemoglobin. The PO2 is related to the concentration of oxygen dissolved in the plasma. When blood PO2 increases, oxygen is driven into the RBC to bind to hemoglobin. When blood PO2 decreases, oxygen is released from hemoglobin. The plateau portion of the curve is associated with the PO2 range that exists in the pulmonary capillaries where oxygen is loaded onto hemoglobin. The significance of this portion of the curve is that even if PO2 drops below normal there is little change in the total amount of oxygen transported in the blood. Significant changes do not occur until arterial PO2 drops below 60 mm Hg. The steep portion of the curve, between 0 mm Hg and60 mm Hg, is in the blood range associated with systemic capillaries where oxygen is unloaded to the tissue. The significance of this portion of the curve is that actively metabolizing tissues using more oxygen sees an enhanced oxygen release from hemoglobin. In this portion of the curve a small drop in PO2 leads to large amounts of oxygen made available to the tissue. (REVIEW FIGURE 13-24, on page 473).
Define the various lung volumes and capacities.
Tidal volume (TV)—volume of air entering or leaving lungs during a single breath. Inspiratory reserve volume (IRV)—extra volume of air that can be maximally inspired over and above the typical resting tidal volume. Residual volume (RV)—minimum volume of air remaining in the lungs even after a maximal expiration. Vital capacity (VC)—maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IRV + TV + ERV). Total lung capacity (TLC)—maximum volume of air the lungs can hold (TLC = VC + RV).