OUTLINE OF REVIEW TOPICS FOR RESPIRATION
Ventilation and Lung Mechanics
1. Air flow between atmosphere and alveoli of lungs is proportional to the difference between atmospheric and alveolar pressures and inversely proportional to airway resistance: Flow = (Patm ? Palv)/R
2. Between breaths, Patm = Palv, no air is flowing, and the dimensions of the lungs and thoracic cage are stable as the result of opposing elastic forces.
a. Lungs are stretched and are attempting to recoil, whereas the chest wall is compressed and attempting to move outward.
b. This creates a subatmospheric intrapleural pressure and hence a transpulmonary pressure that opposes the force of elastic recoil.
3. During inspiration, contractions of diaphragm and inspiratory intercostal muscles increase volume of the thoracic cage.
a. This makes intrapleural pressure more subatmospheric, increases transpulmonary pressure, and causes the lungs to expand.
b. This expansion initially makes alveolar pressure subatmospheric, which creates the pressure difference between the atmosphere and alveoli to drive air flow into lungs.
4. During expiration, the inspiratory muscles cease contracting, allowing the elastic recoil of the chest wall and lungs to return them to their original between-breath size.
This initially compresses the alveolar air, raising alveolar pressure above atmospheric pressure and driving air out of the lungs.
5. Lung compliance is determined by the elastic connective tissue of the lungs and the surface tension of the fluid lining the alveoli. The surface tension is greatly reduced, and compliance increased, by surfactant, produced by cells of the alveoli.
6. Airway resistance determines how much air flows into the lungs at any given pressure difference between atmosphere and alveoli.
a. Major determinant of airway resistance is radii of airways.
b. Airway resistance is greatly increased during an asthma attack because of contraction of airway smooth muscle.
7. The vital capacity is the maximum amount of air that can be exhaled after a maximum inhalation and
a. is the sum of resting tidal volume, inspiratory reserve volume, and the expiratory reserve volume.
b. The air remaining in the lungs is the residual volume.
8. Minute ventilation is the product of tidal volume and respiratory rate.
Alveolar ventilation = (tidal volume ? dead space volume) X (respiratory rate).
Tidal volume = amount of air inspired or expired during each breath.
Dead space = the portion of inspired air that fails to reach areas of gas exchange.
Exchange of Gases in Alveoli and Tissues
1. Exchange of gases in lungs and tissues is by diffusion, as a result of differences in partial pressures. Gases diffuse from a region of higher partial pressure to one of lower partial pressure.
2. In general adequate gas exchange depends on:
a. Thickness of membrane.
b. Surface area of membrane.
c. Solubility of the gas in the substance of the membrane.
d. Pressure difference between the two sides of the membrane.
3. At sea level, atmospheric air has a PO2 of 160 mmHg and a PCO2 near zero.
4. Average values in arterial blood: PO2 is 100 mmHg and PCO2 is 40 mmHg.
5. Hypoventilation
a. exists when there is an increase in the ratio of CO2 production to alveolar ventilation.
b. results in an increase in blood hydrogen ion concentration ([H+]) and a decrease in blood pH. This is called respiratory acidosis.
6. Hyperventilation
a. exists when there is a decrease in the ratio of CO2 production to alveolar ventilation.
b. results in a decrease in blood [H+] and an increase in blood pH. This is called respiratory alkalosis.
Transport of O2 in Blood
1. 98% of O2 is transported bound to hemoglobin and 2% dissolved in blood.
2. At saturation, hemoglobin binds to 4 O2 molecules.
3. The major determinant of the degree to which hemoglobin is saturated with O2 is the blood PO2.
a. Almost 100% saturated at a PO2 of 100 mmHg. The fact that saturation is 90% complete at a PO2 of 60 mmHg permits relatively normal uptake of O2 by the blood even when alveolar PO2 is moderately reduced.
b. Hemoglobin is 75% saturated at the normal systemic venous PO2 of 40 mmHg. Thus only 25% of the O2 has dissociated from hemoglobin and entered the tissues.
Transport of CO2 and Hydrogen Ion in Blood
1. The majority of the CO2 in the blood combines with water to form carbonic acid (H2CO3) (catalyzed by the enzyme carbonic anhydrase), which then dissociates to bicarbonate (HCO3) and H+. Thus the majority of CO2 is carried in the blood as HCO3.
2. H+ generated from carbonic acid is transported in the blood bound to hemoglobin.
Control of Respiration
1. Breathing depends upon cyclical inspiratory muscle excitation by the nerves to the diaphragm and intercostal muscles. This neural activity is triggered by the medullary inspiratory neurons.
2. Inputs to the medullary inspiratory neurons for the involuntary control of ventilation are from
a. peripheral chemoreceptors- the carotid and aortic bodies- and
b. central chemoreceptors.
c. lung stretch receptors.
3. Ventilation is reflexly stimulated by
a. decrease in arterial PO2, mediated by the peripheral chemoreceptors, but only when the decrease is large.
b. even a slight increase in arterial PCO2, mediated via both the peripheral and central chemoreceptors. The stimulus for this reflex is not the increased PCO2 itself, but the concomitant increased [H+] in arterial blood and brain extracellular fluid.
c. an increase in arterial [H+] resulting from causes other than an increase in PCO2 (metabolic acidosis), mediated via the peripheral chemoreceptors. The result of this reflex is to restore [H+] toward normal by lowering PCO2.
4. Ventilation is reflexly inhibited by an increase in arterial PO2, by a decrease in arterial PCO2 or [H+] and by activation of lung stretch receptors.
