Human Physiology: Gas Exchange and Hypoxia

Chapter 13 Lecture Outline: Respiratory Physiology

• Main function: The primary objective of the respiratory system is to supply oxygen to body tissues, which is essential for cellular metabolism, and to facilitate the removal of carbon dioxide, a metabolic waste product, from the bloodstream, thereby maintaining acid-base balance in the body.
• Respiratory processes:
  • Pulmonary ventilation (breathing) entails the movement of air into and out of the lungs.
  • External respiration involves the exchange of gases between the alveoli in the lungs and the blood in the pulmonary capillaries.
  • The transport of respiratory gases in the blood refers to the circulation of oxygen from the lungs to the tissues and carbon dioxide from the tissues back to the lungs.
  • Internal respiration describes the gas exchange that occurs at the tissue level, where oxygen is delivered to cells and carbon dioxide is collected as a waste product.
• Organization of the Respiratory system (Fig. 13-1):
  • The respiratory system is comprised of various structures including the nasal cavity, nostrils, mouth, pharynx, larynx, trachea, bronchi, lungs, alveoli, and the diaphragm, which serves as the primary muscle for inspiration.
• Airways and Blood Vessels (Fig. 13-2, 13-3):
  • The conducting zone includes the trachea, bronchi, bronchioles, and terminal bronchioles, which serve as passageways for air to reach the alveoli.
  • The respiratory zone, where gas exchange occurs, encompasses the respiratory bronchioles, alveolar ducts, and alveolar sacs.
• Airways:
  • The trachea, commonly referred to as the windpipe, consists of three layers, including an epithelial layer known as the mucosa, which contains specialized cells such as goblet cells that secrete mucus and cilia that help to trap and expel foreign debris.
  • Smoking has detrimental effects on the cilia, impairing their function and leading to the accumulation of mucus, which necessitates frequent coughing as a compensatory mechanism.
• Alveoli (Fig. 13-3, 13-4):
  • Alveoli are tiny, hollow sacs resembling clusters of grapes where the crucial process of gas exchange occurs.
  • These structures are lined with type I alveolar cells, which are thin, flat epithelial cells that facilitate the diffusion of gases.
  • Type II alveolar cells are responsible for producing surfactant, a detergent-like substance that reduces alveolar surface tension, thereby enhancing lung compliance and preventing alveolar collapse.
  • The extensive surface area of the alveoli is vital for rapid gas diffusion, enabling efficient oxygen uptake and carbon dioxide elimination.
  • Pores, known as pores of Kohn, connect adjacent alveoli and allow for the equalization of pressure and ventilation among the alveoli.
• Relation of lungs to the thoracic wall (Fig. 13-5):
  • The lungs are encased in a double-layered membrane called the pleura, which includes the visceral pleura attached to the lung surface and the parietal pleura lining the thoracic cavity.
  • The fluid in the pleural space acts as a lubricant, reducing friction during lung expansion and contraction, which is essential for effective breathing.
  • Pleurisy is a condition characterized by inflammation or infection of the pleura, often leading to sharp pain during breathing and potentially impaired respiratory function.
• Steps of Respiration (Fig. 13-6):
  • The process of respiration involves multiple steps: ventilation, which is the bulk flow of air into and out of the lungs; gas exchange in the lungs through diffusion across the alveolar-capillary membrane; transport of gases via the bloodstream; gas exchange at the tissue level where oxygen is delivered and carbon dioxide is collected; and finally, cellular respiration, which utilizes oxygen for energy production and generates carbon dioxide as a byproduct.
• Ventilation and Air Flow:
  • Ventilation is defined as the exchange of air between the atmosphere and the alveoli, a process that is essential for maintaining adequate oxygen levels in the body.
  • The flow of air (F) is described by the equation F = ΔP/R, where flow is directly proportional to the pressure difference (ΔP) between the atmosphere and alveoli and inversely proportional to resistance (R) presented by the airways.
• Ventilation:
  • Boyle's Law states that at a constant temperature, the pressure of a gas is inversely related to its volume; therefore, as the volume of the thoracic cavity increases during inspiration, the pressure decreases, leading to airflow into the lungs.
  • Pressure changes within the thoracic cavity are the primary drivers for gas flow during both inspiration and expiration.
• Pressure Measurements:
  • Pressure within the respiratory system is measured relative to atmospheric pressure, which at sea level is approximately 760 mm Hg or equivalent to 1 atmosphere.
  • Understanding these pressures is crucial for diagnosing respiratory conditions and assessing lung function, as deviations from normal values can indicate underlying issues.
• Intrapulmonary pressure (Palv):
  • This is the pressure found inside the alveoli. During the breathing cycle, intrapulmonary pressure fluctuates, decreasing during inspiration and increasing during expiration.
  • Ultimately, intrapulmonary pressure equalizes with atmospheric pressure at the end of each breath cycle to maintain balance within the respiratory system.
• Intrapleural pressure (Pip):
  • The intrapleural pressure refers to the pressure within the pleural cavity, which is always maintained at a level slightly less than that of the intrapulmonary pressure to ensure the lungs remain inflated and adhere to the thoracic wall.
  • This negative pressure is critical for normal lung function, as it prevents lung collapse.
• Transpulmonary pressure (Ptp):
  • Transpulmonary pressure is defined as the difference between intrapulmonary pressure and intrapleural pressure. It is a key factor in determining the state of lung expansion.
  • If Ptp is high, the lungs expand more easily, whereas a low Ptp indicates increased difficulty in lung inflation.
• Inspiration (Fig. 13-7, 13-12):
  • During inspiration, the intercostal muscles between the ribs and the diaphragm contract, leading to an increase in the volume of the thoracic cavity.
  • This expansion results in a decrease in intrapleural pressure and an increase in transpulmonary pressure, creating a pressure gradient that causes air to flow into the alveoli.
  • The resulting drop in alveolar pressure relative to atmospheric pressure is what drives inhalation.
• Expiration (Fig. 13-15):
  • Expiration is primarily a passive process during which the respiratory muscles relax, leading to a decrease in the volume of the thoracic cavity.
  • This results in an increase in intrapleural pressure, which in turn leads to a decrease in transpulmonary pressure.
  • The increase in alveolar pressure pushes air out of the lungs and into the atmosphere, facilitating exhalation.
• Lung Compliance:
  • Lung compliance measures the ease with which the lungs can expand; it is the inverse of stiffness.
  • A greater compliance indicates that the lungs can be expanded more effortlessly, thereby allowing for more effective ventilation and gas exchange.
  • Lung compliance is influenced by the stretchability of lung tissue and the surface tension in the alveoli, with surfactant playing a crucial role in modulating these factors. (Fig. 13-16)
• Lung Compliance and Surfactant:
  • Type II alveolar cells are specialized for the production of surfactant, which is essential for maintaining alveolar stability and compliance.
  • By reducing surface tension within the alveoli, surfactant prevents lung collapse and ensures that the lungs can expand easily during inhalation.
  • Surfactant is particularly important for premature infants, as they may suffer from Respiratory Distress Syndrome due to insufficient surfactant production, leading to compromised lung function.
• Airway Resistance:
  • Airway resistance is typically low under normal conditions but can be influenced by the diameter of the airways; the radii of the airways can change due to various physical, neural, and chemical factors.
  • Medium-sized bronchi demonstrate the greatest resistance to airflow, which can affect overall respiratory efficiency.
• Asthma:
  • Asthma is a chronic condition characterized by the constriction of airway smooth muscles, leading to increased airway resistance and difficulty in breathing.
  • This condition is typically associated with underlying inflammation due to various triggers, such as allergies or viral infections, and can result in bronchial hyper-responsiveness, where the airways react excessively to certain stimuli, including physical activity.
• Asthma Treatment: Effective management involves reducing airway inflammation as well as overcoming acute bronchoconstriction using bronchodilators. These medications work by either mimicking the actions of beta-2 adrenergic receptors or blocking muscarinic receptors to promote relaxation of airway smooth muscles.
• Chronic Obstructive Pulmonary Disease (COPD):
  • COPD is a progressive lung disease that encompasses two distinct forms: Emphysema and Chronic Bronchitis, both of which lead to decreased airflow and reduced oxygenation.
  • Emphysema is characterized by the destruction of alveolar walls, reducing the surface area available for gas exchange, while chronic bronchitis results from ongoing inflammation and mucus accumulation that narrows the airways.
• Lung Volumes and Capacities (Fig. 13-18):
  • Various lung volumes and capacities are critical measurements that influence overall respiratory function and help assess lung health in clinical contexts.
• Alveolar Ventilation (Fig. 13-19):
  • Alveolar ventilation is determined by subtracting the volume of air in the conducting airways, which does not participate in gas exchange, from the tidal volume, thereby giving a measure of the effective ventilation utilized for gas exchange.
• Gas Exchange in Alveoli and Tissues (Fig. 13-20):
  • The process of gas exchange involves the movement of oxygen from the alveoli into the blood and the movement of carbon dioxide from the blood into the alveoli for exhalation.
• Partial Pressures of Gases (Fig. 13-21):
  • The concept of partial pressures is critical in understanding gas exchange, as differences in partial pressures drive the diffusion of gases in the lungs and tissues, enabling efficient respiratory processes.
• Alveolar Gas Pressures (Table 13-6): Provides valuable reference values for understanding the specific partial pressures of oxygen and carbon dioxide in the alveoli, which are essential for assessing gas exchange efficiency.
• Gas Exchange in Alveoli and Blood (Fig. 13-23):
  • Gas exchange in the alveoli is primarily facilitated by diffusion, with oxygen passing into the blood due to a higher partial pressure compared to that of carbon dioxide, which diffuses from the blood into the alveoli.
• Matching of Ventilation and Blood Flow (Fig. 13-24):
  • Optimal gas exchange requires coordination between ventilation (airflow) and perfusion (blood flow) to the alveoli, ensuring that areas of high ventilation correspond with areas of high blood flow.
• Oxygen Transport in Blood (Fig. 13-25):
  • Once oxygen diffuses into the blood from the alveoli, it primarily binds to hemoglobin in red blood cells, which facilitates efficient transport to tissues across the body.
• Effect of PO2 on Hemoglobin Saturation (Fig. 13-26):
  • The saturation of hemoglobin with oxygen is highly influenced by the partial pressure of oxygen (PO2); a higher PO2 typically leads to a greater saturation of hemoglobin.
• Oxygen Movement in Lungs and Tissues (Fig. 13-28):
  • The interchange of oxygen between the lungs and tissues is driven by concentration gradients established by circulation and cellular respiration demands.
• Effects of Blood Factors on Hemoglobin Saturation (Fig. 13-29):
  • Certain physiological factors, such as pH, temperature, and levels of 2,3-bisphosphoglycerate (2,3-BPG) can alter hemoglobin's affinity for oxygen, thus impacting oxygen delivery to cells.
• Carbon Dioxide Transport in Blood (Fig. 13-30):
  • Carbon dioxide is transported in the blood through three main forms: dissolved in plasma, as bicarbonate ions, and bound to hemoglobin, which collectively allows for effective removal of CO2 from tissues.
• Transport of Hydrogen Ions (Fig. 13-31):
  • Hydrogen ions produced during metabolic processes are also transported in blood, primarily in association with proteins or as free ions, influencing acid-base balance in the body.
• Neural Generation of Rhythmical Breathing (Fig. 13-32):
  • Breathing is controlled by the respiratory centers in the brainstem, which generate rhythmic patterns of inspiration and expiration based on the body's metabolic needs.
• Baroreceptors (Fig. 13-33):
  • Baroreceptors are sensory receptors that monitor blood pressure and play a role in homeostasis, influencing cardiovascular and respiratory function.
• Hyperventilation (Fig. 13-35):
  • Hyperventilation occurs when there is an increase in the rate and depth of breathing, which can lead to decreased levels of carbon dioxide (PCO2) in the blood along with increased oxygen levels (PO2).
• Reflexively Induced Hyperventilation (Fig. 13-39):
  • This occurs as a compensatory response to elevated hydrogen ion concentration (increased acidity), wherein ventilation rates increase to reduce carbon dioxide levels and restore pH balance.
• Control of Ventilation (Fig. 13-40): The control mechanisms for ventilation involve complex interactions between neural pathways, chemical signals, and feedback from peripheral sensors, which appropriately adjust breathing patterns based on metabolic and environmental conditions.
• Control of Ventilation During Exercise (Fig. 13-43): Ventilation increases during physical activity to meet the higher oxygen demands of muscles, which is managed by both neural and hormonal responses. The body adapts quickly to changes in activity levels, optimizing oxygen delivery and carbon dioxide removal.
• Sleep Apnea (Fig. 13-44):
  • Sleep apnea is a sleep disorder characterized by intermittent blockage of the upper airway, leading to temporary cessation of breathing, decreased oxygen intake, and elevated carbon dioxide levels, which can have long-term health consequences if not managed properly.
• Other Ventilatory Responses:
  • Additional ventilatory responses include protective reflexes such as coughing and sneezing, voluntary control over breathing during activities like speaking or singing, and reflexes mediating responses from J receptors, which respond to pulmonary congestion and irritants.
• Hypoxia:
  • Hypoxia refers to a condition in which there is inadequate oxygen delivery to body tissues, which can lead to cellular dysfunction and organ damage.
  • There are four primary types of hypoxia: anemic hypoxia due to insufficient hemoglobin, ischemic hypoxia resultant from impaired blood flow, histotoxic hypoxia due to the inability of cells to utilize oxygen (as seen in cyanide poisoning), and hypoxemic hypoxia arising from low arterial oxygen pressure, commonly observed at high altitudes.
• Carbon Monoxide Poisoning:
  • Carbon monoxide poisoning is a leading cause of accidental death, especially in confined spaces during fires, due to inhalation of CO gas.
  • Carbon monoxide binds to hemoglobin with a much higher affinity than oxygen, forming carboxyhemoglobin and leading to reduced oxygen transport in the blood. Symptoms may include characteristic cherry-red discoloration of the skin, confusion, headache, and respiratory distress.