Pulmonary Structure and Mechanics of Breathing

Pulmonary Structure and Function

• The respiratory system has multiple functions.
• Understand he hierarchical organization of the conducting zone of the respiratory system.
• Understand the hierarchical organization of the respiratory zone of the respiratory system.
• It is important to know how many bronchiolar branchpoints there are.
• There are barriers that oxygen must travel through before reaching the point of utilization.
• It is important know what alveolar cells are secretory.
• Understand the main muscles involved in inspiration.
• Understand the muscles involved in passive expiration.
• Know the muscles involved in active expiration.
• Know what the pleural sac is and what functions it has.

Mechanics of Breathing

• Know the three pressures that determine airflow in the lungs.
• Plural pressure must change to cause airflow into the lungs, understand how this pressure is developed.
• Know the relationship between pleural pressure and pulmonary pressure in normal lungs during inspiration.
• The sequence of events in inspiration should be understood.
• The sequence of events in expiration should be understood.
• The change in pulmonary pressure during normal inspiration and expiration.
• Positive pulmonary pressure is created during expiration.
• Compliance is the measure of the lung's ability to stretch and expand.
• The equation to calculate compliance must be understood.
• Lung compliance is determined by a variety of factors.
• The compliance curve is different for inspiration and expiration due to factors like surface tension and the recruitment of alveoli. -Lung volume will increase by 2L with a transpulmonary pressure change of 5mmHg in one example, compared to another example where lung volume increases by 1.8L under the same transpulmonary pressure change.
• Compliance is determined by dividing the change in volume by the change in pressure (ΔV/ΔP).
• Lungs are most compliant at their relative volume.
• Premature infants often have lungs of low compliance because they lack sufficient surfactant.
• If water was substituted for surfactant, lung compliance would decrease, due to the high surface tension of water.
• Surfactant reduces surface tension in the alveoli, increasing lung compliance.
• Surfactant is produced by Type II alveolar cells (pneumocytes).
• The three factors that determine the work of breathing are compliance, airway resistance, and tissue resistance.
• Compliance is balanced at the end of expiration in the respiratory cycle known as Functional Residual Capacity (FRC).
• Compliance work with fibrosis of the lung reduces compliance.
• Exercise increases lung compliance due to deeper breaths and increased lung volume.
• Resistance is predicted by the equation: Resistance = (Pressure Difference) / Flow.
• Factors that influence airway resistance include airway diameter, lung volume, and bronchoconstriction/bronchodilation.
• Exercise increases resistive work due to increased airflow rates.
• The work of breathing is increased during an asthma attack due to bronchoconstriction and increased mucus production.
• Airway resistance is lowest in the terminal bronchioles and alveoli due to their large combined cross-sectional area.

Lung Volumes

• Obstructive disease: increased airway resistance
• Examples of obstructive diseases: asthma and COPD.
• Restrictive disease: reduced lung compliance
• Examples of restrictive diseases: pulmonary fibrosis and neuromuscular disorders.
• Spirometry is a pulmonary function test used to measure lung volumes and airflow rates.
• With spirometry you can measure force maximal inspiration and expiration.
• Lung volumes: tidal volume (TV), inspiratory reserve volume (IRV), expiratory reserve volume (ERV), and residual volume (RV).
• Lung capacities: inspiratory capacity (IC), functional residual capacity (FRC), vital capacity (VC), and total lung capacity (TLC).
• Tidal Volume (TV) is the volume of air moved during a normal breath, while Total Lung Volume is the entire volume of air the lungs can hold.
• Residual volume exists to keep the alveoli open and prevent lung collapse, it is measured using dilution techniques like helium dilution or body plethysmography.
• An individual's lung volumes are measured through spirometry and compared to predicted normal values based on age, sex, height, and ethnicity.
• Forced Vital Capacity (FVC) is the total volume of air that can be forcibly exhaled after a maximal inspiration usually measured with spirometry.
• FEV1 represents the Forced Expiratory Volume in 1 second, measured with spirometry.
• Abnormally low FEV1 could indicate airway obstruction.
• Abnormality of FEV1 is determined by comparing it to predicted normal values and to the FVC (FEV1/FVC ratio).
• Lung volume variability between individuals and is determined by factors such as age, sex, height, ethnicity, and health status.
• Lung volumes are not necessarily indicative of athletic performance, other factors like cardiovascular fitness and muscle strength also play significant roles.
• Anatomical dead space is the volume of air in the conducting airways where gas exchange does not occur.
• Physiological dead space includes anatomical dead space plus the volume of alveoli that are ventilated but not perfused.
• A quick estimate of individual anatomical dead space is approximately 1 mL per pound of body weight.
• Physiological dead space: caused by conditions that impair blood flow to the lungs.
• If you breathe rapidly at 30 breaths/min with shallow breaths of 250ml when you weigh 250 pounds: ventilation is inadequate due to increased dead space ventilation.
• Spirometry maximal flow-volume loops differ in obstructive and restrictive diseases due to changes in airflow rates and lung volumes.
• Obstructive disease loops show decreased expiratory flow rates, while restrictive disease loops show reduced lung volumes.
• In normal tidal breathing, relationships ensure efficient gas exchange by matching ventilation with perfusion.
• When patients with restrictive disease breath in rapid shallow breaths, it minimizes the work of breathing since they have reduced lung compliance.
• When patients with obstructive disease breath in deep and slow breaths, it maximizes expiratory flow rates and prevent air trapping.

Gas Exchange

• At normal atmospheric pressure (760mmHg), if 50% of the air is oxygen, then the partial pressure for oxygen in the air is 380mmHg.
• The gas content of fluid is determined by its partial pressure, solubility, and temperature.
• Carbon dioixide is at the highest concentraion in a cup of water at room temperature, due to its higher solubility compared to oxygen.
• Diffusion rate of a gas in fluid: increasing temperature increases the rate.
• Diffusion rate of a gas in fluid: decreasing pressure difference decreases the diffusion rate.
• Diffusion rate of a gas in fluid: increasing the distance the gas must diffuse decreases the rate.
• The diffusion of gas through a membrane is affected if the membrane becomes thicker.
• Impact on diffusion: thickening reduces the rate of gas exchange.
• The diffusion of gas through a membrane is affected if the pressure difference becomes smaller.
• Impact on diffusion: reduction reduces the driving force for gas exchange.
• The diffusion of gas through a membrane is affected if the surface area decreases.
• Impact on diffusion: decreases the area available for gas exchange.
• Diseases such as pulmonary fibrosis (thickening), emphysema (reduced surface area), and pulmonary edema (increased diffusion distance) affect gas exchange.
• Four factors that cause the difference between PIO2 and PAO2 are: humidification of air in the airways, mixing of inspired air with alveolar air, gas exchange in the alveoli, and alveolar ventilation-perfusion matching.
• Bring PAO2 and PIO2 closer in value by increasing inspired oxygen concentration, improving alveolar ventilation, and optimizing ventilation-perfusion matching.
• At an atmospheric pressure of 600mmHg (PH2O is 4mmHg) on a mountain PIO2 is approximately 118.2 mmHg.
• Assuming a PCO2 of 40mmHg and R of 0.8, PAO2 is approximately 75 mmHg.
• These pressures will decrease the diffusion rate of O2 across the alveolar walls, leading to hypoxemia.
• The PO2 in the atmospheric air is about 160 mmHg, in alveolar air it is about 104 mmHg, in arterial blood it is about 95 mmHg, and in venous blood it is about 40 mmHg.
• These differences are driven by the dilution of inspired air with air already present in the lungs, gas exchange in the alveoli, and oxygen consumption by tissues.
• The PCO2 in the atmospheric air is about 0.3 mmHg, in alveolar air it is about 40 mmHg, in arterial blood it is about 40 mmHg, and in venous blood it is about 46 mmHg.
• Mismatch between alveolar ventilation and capillary perfusion in the upper lung means that some alveoli are ventilated but not adequately perfused.
• Exercise helps correct this mismatch by increasing blood flow to all areas of the lung and improving ventilation-perfusion matching.
• Using the values determined for PA02 in previous question on a mountain, the saturation of hemoglobin would be lower than normal.
• Oxygen breathing apparatus would be helpful because it would increase the PAO2 and improve hemoglobin saturation.
• The P50 value tells you the partial pressure of oxygen at which hemoglobin is 50% saturated.
• Factors that will affect P50 are temperature, pH, PCO2, and 2,3-DPG concentration.
• Temperature: increased temperature shifts curve to the right, decreasing affinity.
• pH: decreased pH shifts curve to the right, decreasing affinity (Bohr effect).
• PCO2: increased PCO2 shifts curve to the right, decreasing affinity (Haldane effect).
• 2,3-DPG concentration increase in 2,3 shifts curve to the right, decreasing affinity.
• Need for multiple factors: ensures oxygen is released from hemoglobin under a variety of physiological conditions.
• The predominate method of moving CO2 through the bloodstream : as bicarbonate ions.
• Blood pH decreases as CO2 increases because CO2 combines with water to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions.
• When blood pH decreases as CO2 decreases, the reaction shifts to remove H+ ions from the blood.
• Hyperventilation (blowing off a lot of CO2 so that PaCO2 drops significantly) increases blood pH, leading to respiratory alkalosis.

Control of Respiration

• The Dorsal Respiratory Group (DRG) is located in the medulla and primarily controls inspiration.
• The Ventral Respiratory Group (VRG) is located in the medulla and controls both inspiration and expiration, especially during active breathing.
• The Pontine Respiratory Group (PRG) is located in the pons and modulates respiratory rhythm and depth.
• If the PRG was destroyed due to trauma, breathing would become irregular and erratic.
• The hypothalamus influences breathing by integrating emotional and thermal stimuli, leading to changes in respiratory rate and depth.
• Motor control centres influence breathing by providing signals to the respiratory muscles, controlling their contraction and relaxation.
• Protective reflexes such as the Hering-Breuer reflex prevent the lungs from overinflating by inhibiting inspiration when lung stretch receptors are activated.
• Central chemoreceptors are located in the medulla and respond to changes in pH and PCO2 in the cerebrospinal fluid.
• Peripheral chemoreceptors are located in the carotid and aortic bodies and respond to changes in PO2, PCO2, and pH in the arterial blood.
• Exercise influences respiratory rate by increasing metabolic demand, leading to increased CO2 production and decreased PO2, which stimulate chemoreceptors.
• If you hyperventilate this will decrease normal breathing rate because it lowers PCO2 levels in the blood, suppressing the drive to breathe.
• If you removed the central chemoreceptors from the system (ie. drug or alcohol overdose): breathing would become irregular or stop because the body would lose its primary mechanism for detecting and responding to changes in blood CO2 levels.