External Respiration in Cells

External Respiration: Gas Exchange in the Lungs

• Purpose of Oxygen Absorption: The primary function of acquiring oxygen is to improve the efficiency of ATP resynthesis, the process of creating energy for cellular functions.
• Cellular Oxygen Consumers: Mitochondria are the primary organelles in cells that consume oxygen.
• ATP Energy Use: The energy stored in ATP is pivotal for various cellular activities including complex substance synthesis, active transport, and mechanical work.
• Respiratory Phases: The correct sequence of respiratory processes is ventilation, gas exchange in lungs, gas transport by blood, biological oxidation, and gas exchange in tissues.
• Inhaled Air Composition: Normal inhaled air contains approximately 20.9% oxygen and 0.03% carbon dioxide.
• Alveolar Air Composition: Alveolar air contains roughly 16% oxygen and 4.5% carbon dioxide.
• Exhaled Air Composition: Exhaled air has approximately 16% oxygen and 4-5% carbon dioxide.
• Gas Exchange Area: The respiratory zone of the lungs, containing alveoli, is the site of gas exchange with the blood.
• Lung Diffusion Capacity: The rate of gas exchange depends on the permeability of the alveolar and capillary membranes.
• Lung Stretch: Lungs are maintained in a stretched state during inspiration and quiet breathing.
• Pleural Pressure: The negative pressure within the pleural cavity (space between the lung and chest wall) is essential for maintaining lung expansion. Elastic recoil force is a major contributor to this.
• Inspiration Mechanism: Inspiration is primarily achieved by diaphragm contraction. Other muscles like external intercostal muscles play a critical role in forced inspiration.
• Expiration Mechanism: Expiration is a combination of elastic recoil and, in forced expiration, the contraction of expiratory muscles.
• Intra-Pulmonary Pressure vs. Atmospheric Pressure: During inspiration, the intrapulmonary pressure is lower than atmospheric pressure. The reverse occurs during expiration.
• Transpulmonary Pressure: The difference between intrapulmonary and intrapleural pressure is called transpulmonary pressure. This pressure difference is critical for lung inflation.
• Zero Transpulmonary Pressure: If the transpulmonary pressure is zero, the lungs will deflate.
• Pleural Cavity Pressure: The reason for the negative intrapleural pressure is the elastic recoil of the lungs and the surface tension of the alveolar fluid.
• Anatomic Dead Space: The conducting zone of the respiratory system, including the trachea, bronchi, and bronchioles, forms the anatomical dead space, which doesn't participate in gas exchange.
• Airway Functions: Respiratory airways warm, humidify, and filter inhaled air, and act as conduits for airflow.
• Surfactant Function: Surfactant reduces surface tension in the alveoli, preventing alveolar collapse during exhalation and facilitating lung expansion.
• Alveolar Air Renewal: Approximately one-seventh of the alveolar air is exchanged with each breath during quiet breathing.
• Continuous Gas Exchange: Gas exchange between air and blood happens constantly during both inhalation and exhalation.
• Effective Ventilation: The most reliable measure of effective ventilation is the assessment of PaO2 and PaCO2, arterial oxygen and carbon dioxide partial pressures.
• Tidal Volume (TV): The volume of air inhaled and exhaled in a quiet breath.
• Minute Ventilation (MV): The total volume of air moved in and out of the lungs per minute (TV x breaths per minute).
• Alveolar Ventilation (VA): The volume of air that reaches the alveoli per minute ((TV - VD) * breaths/min).
• Gas Transport: Carbon dioxide is primarily transported through simple diffusion across alveolar and capillary membranes.
• Alveolar Gas Partial Pressures: The partial pressure of oxygen (PO2) in alveolar air is typically 105-110 mmHg, and the partial pressure of carbon dioxide (PCO2) is about 38-40 mmHg .
• Gas Dissolution: A higher partial pressure of a gas above a liquid than in the liquid will drive the gas into the liquid.