Pulmonary Structure and Function

Pulmonary Structure and Function

• The respiratory system is responsible for gas exchange, regulating blood pH, vocalization, protection, and blood pressure regulation.
• The conducting zone includes the nose, pharynx, larynx, trachea, bronchi, and bronchioles, which warm, humidify, and filter air.
• The respiratory zone includes the respiratory bronchioles, alveolar ducts, and alveoli, where gas exchange occurs.
• There are approximately 23 bronchiolar branchpoints in the human respiratory system.
• Oxygen must pass through the alveolar epithelium, interstitial fluid, and capillary endothelium to reach the point of utilization.
• Type II alveolar cells are secretory, producing surfactant to reduce surface tension.
• Main muscles involved in inspiration include the diaphragm, external intercostals, sternocleidomastoid, scalenes, and alae nasi.
• Passive expiration primarily involves the relaxation of inspiratory muscles.
• Active expiration utilizes the internal intercostals, abdominal muscles, and quadratus lumborum.
• The pleural sac is a double-layered membrane surrounding each lung, creating a potential space filled with pleural fluid, reduces friction, and creates pressure gradient

Mechanics of Breathing

• Airflow in the lungs depends on alveolar pressure, atmospheric pressure, and intrapleural pressure.
• Intrapleural pressure needs to decrease to create airflow in the lungs
• During normal inspiration, pleural pressure decreases, leading to an increase in pulmonary volume and a subsequent decrease in pulmonary pressure.
• The sequence of events in inspiration begins with diaphragm and external intercostal muscle contraction, leading to increased thoracic volume, decreased intrapleural pressure, increased alveolar volume, decreased alveolar pressure, and airflow into the lungs.
• The opposite occurs during expiration, starting with relaxation of the diaphragm and external intercostals.
• Positive pulmonary pressure during expiration is created by contracting expiratory muscles, which decreases thoracic volume and increases alveolar pressure above atmospheric pressure.
• Lung compliance is the measure of the lung's ability to stretch and expand, calculated as the change in volume divided by the change in pressure.
• Lung compliance is determined by the elasticity of lung tissue and the surface tension within the alveoli.
• The compliance curve differs because the pressure required to inflate the lungs at low volumes is higher during inspiration.
• The more compliant lung is Will's, as his lung volume increases more (2L) than Chris's (1.8L) under the same transpulmonary pressure change (5 mmHg).
• Lungs are most compliant at their functional residual capacity (FRC).
• Premature infants have lungs with low compliance due to a lack of surfactant.
• Replacing surfactant with water would greatly reduce lung compliance due to increased surface tension.
• Surfactant is a substance composed of phospholipids and proteins, produced by Type II alveolar cells in the lungs.
• Factors determining the work of breathing include compliance, tissue resistance, and airway resistance.
• Compliance is balanced at the end of normal tidal expiration and at the beginning of inspiration.
• Fibrosis of the lung decreases lung compliance, thus it requires more work to inflate the lungs.
• Exercise increases lung compliance by increasing the depth of breathing and utilizing more alveoli.
• Airway resistance is predicted by Poiseuille's Law and influenced by the radius of the airways, the length of the airways, and the viscosity of the air.
• Exercise increases resistive work due to increased airflow rates and turbulence in the airways.
• The work of breathing increases during an asthma attack due to bronchoconstriction, inflammation, and mucus production, all of which increase airway resistance.
• Airway resistance is lowest in the terminal bronchioles and alveoli because of their large total cross-sectional area.

Lung Volumes

• Obstructive diseases (e.g., asthma, COPD) are characterized by increased airway resistance, leading to reduced airflow.
• Restrictive diseases (e.g., pulmonary fibrosis, neuromuscular disorders) involve reduced lung compliance, limiting lung expansion.
• Spirometry is a pulmonary function test that measures the volume and speed of air that a person can inhale and exhale; done by breathing into a mouthpiece connected to a recording device.
• Spirometry recordings include lung volumes (tidal volume, inspiratory reserve volume, expiratory reserve volume, residual volume) and lung capacities (inspiratory capacity, functional residual capacity, vital capacity, total lung capacity).
• Tidal volume is the volume of air inhaled or exhaled during normal breathing (average ~500ml), and total lung volume is the total amount of air the lungs can hold (average is 6L).
• Residual volume exists to prevent alveolar collapse and maintain gas exchange even after maximal exhalation; measured via nitrogen washout, helium dilution, or body plethysmography.
• Comparing an individual's lung volumes to predicted normal values based on age, sex, height, and ethnicity determines if lung volumes are normal.
• FVC (Forced Vital Capacity) is measured by having the individual take a deep breath and exhale as forcefully and completely as possible into a spirometer.
• FEV1 (Forced Expiratory Volume in 1 second) measures the volume of air exhaled during the first second of a forced exhalation.
• A low FEV1 suggests airway obstruction (e.g., asthma, COPD).
• FEV1 is abnormal if it is significantly lower than the predicted value for the individuals age, sex, height, and ethnicity, typically defined as below the lower limit of normal for the patient.
• Variability in lung volumes is determined by factors such as age, sex, height, body composition, and respiratory muscle strength.
• Predicting athletic performance based solely on lung volume is unreliable because other factors such as cardiovascular fitness, muscle efficiency, and technique are also important.
• Anatomical dead space includes the volume of the conducting airways where no gas exchange occurs, while physiological dead space includes anatomical dead space plus any alveolar dead space where gas exchange is impaired.
• A quick estimate of anatomical dead space is approximately 1 mL per pound of body weight.
• Physiological dead space is caused by ventilation without perfusion or perfusion without ventilation in the alveoli.
• Rapid, shallow breathing with a tidal volume of 250 mL at 30 breaths/min in a 250-pound individual can lead to inadequate alveolar ventilation, as a significant portion of each breath remains in the dead space.
• Maximal flow-volume loops differ between normal, obstructive, and restrictive diseases due to variations in airflow rates and lung volumes.
• These relationships impact normal tidal breathing by affecting the efficiency of gas exchange and the work of breathing.
• Patients with restrictive disease should breathe in rapid shallow breaths to minimize the work of breathing and maximize ventilation.
• Patients with obstructive disease should breathe in deep slow breaths to maximize expiratory time and minimize airway collapse.

Gas Exchange

• If air is 50% oxygen at normal atmospheric pressure (760 mmHg), the partial pressure for oxygen is 380 mmHg.
• The gas content of a fluid is determined by the partial pressure of the gas, the solubility of the gas in the fluid, and temperature.
• CO2 is at a higher concentration in a cup of water at room temperature compared to O2.
• The diffusion rate of a gas in a fluid increases with an increase in temperature but decreases with a decrease in pressure difference or an increase in diffusion distance.
• The diffusion of gas through a membrane is affected if membrane thickness increases, pressure difference decreases, or surface area decreases.
• Diseases impacting diffusion include pulmonary fibrosis (thickening), emphysema (decreased surface area), and pneumonia (both).
• Four factors causing a difference between PIO2 and PAO2 include: humidification of inspired air, incomplete alveolar ventilation, and V/Q mismatch.
• Bringing PIO2 and PAO2 closer can be achieved by increasing inspired oxygen concentration and improving alveolar ventilation.
• The PO2 in atmospheric air is about 160 mmHg, in alveolar air around 104 mmHg, in arterial blood around 95 mmHg, and in venous blood around 40 mmHg; differences because of gas exchange and mixing with deoxygenated blood.
• PCO2 is lowest in atmospheric air (0.3 mmHg), higher in alveolar air (40 mmHg), slightly higher in arterial blood (40 mmHg), and highest in venous blood (46 mmHg); differences because of gas exchange and metabolism.
• Mismatch between alveolar ventilation and capillary perfusion in the upper lung occurs because gravity causes less perfusion relative to ventilation.
• Exercise reduces the V/Q mismatch by increasing blood flow to the upper lung regions.
• A low PAO2 on a mountain reduces hemoglobin saturation, and breathing supplemental oxygen would be helpful to increase PAO2 and improve hemoglobin saturation.
• The P50 value indicates the partial pressure of oxygen at which hemoglobin is 50% saturated.
• Factors affecting P50 include pH, temperature, PCO2, and 2,3-DPG; a right shift decreases affinity, while a left shift increases affinity.
• Multiple factors inhibit oxygen association with hemoglobin to allow oxygen to be released more readily to tissues during periods of increased metabolic demand.
• The predominant method of moving CO2 through the bloodstream is as bicarbonate ions (HCO3-).
• Blood pH decreases as CO2 increases because CO2 combines with water to form carbonic acid, which dissociates into hydrogen ions (H+) and bicarbonate ions.
• Blood pH increases as CO2 decreases because the equilibrium shifts away from carbonic acid formation, reducing the concentration of hydrogen ions (H+).
• Hyperventilation, which blows off CO2, increases blood pH due to decreased PaCO2.

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.
• The Pontine Respiratory Group (PRG) in the pons modulates respiratory rhythm; destruction can lead to irregular breathing patterns.
• The hypothalamus influences breathing by integrating emotional and thermal stimuli.
• Motor control centers influence breathing by controlling the activity of respiratory muscles, and adjusting the depth and rate.
• Hering-Breuer reflexes prevent lung overinflation by inhibiting inspiration in response to lung stretch.
• Central chemoreceptors in the medulla respond to changes in pH and PCO2 of the cerebrospinal fluid, while peripheral chemoreceptors in the carotid and aortic bodies respond to changes in PO2, PCO2, and pH of the blood.
• Exercise increases respiratory rate by stimulating chemoreceptors, proprioceptors, and mechanoreceptors.
• Hyperventilation decreases breathing rate by decreasing PCO2 levels in the blood, which reduces the drive to breathe.
• Removing central chemoreceptors would impair the response to increased PCO2, which reduces the drive to breathe following drug or alcohol overdose.