Lecture 20: Gaseous Exchange

Overview of Gaseous Exchange

Gaseous exchange is a critical physiological process that occurs between the alveoli of the lungs and the capillaries that surround these air sacs. It allows for the uptake of oxygen (O2) from the air and the removal of carbon dioxide (CO2) from the bloodstream. This exchange is vital for maintaining the body’s metabolic needs and homeostasis.

Mechanism of Breathing

Breathing consists of two key processes: inhalation (inspiration) and exhalation (expiration). Inspiration involves drawing air into the lungs, while expiration is the process of expelling air out of the lungs. The efficiency of these processes is essential for proper gas exchange and delivery of oxygen to tissues.

Control of Breathing

Breathing is controlled by the respiratory control center located in the brain stem, specifically the Medulla Oblongata and the Pons. Various factors influence breathing, including:

• The cerebral cortex, which can initiate voluntary breathing.

• Chemoreceptors that monitor changes in blood pH, partial pressures of oxygen (pO2), and carbon dioxide (pCO2), providing feedback for respiratory adjustments.

Inhalation increases thoracic volume, which decreases intrapulmonary pressure, causing air to flow into the lungs. Conversely, exhalation decreases thoracic volume and increases intrapulmonary pressure, pushing air out of the lungs.

Key Principles of Airflow

Airflow in the respiratory system is governed by principles similar to those of fluid dynamics:

1. The flow of a fluid is directly proportional to the pressure difference between two points.

2. The flow is inversely proportional to resistance, which can be affected by airway diameter and elasticity of the lung tissue.

3. Atmospheric pressure, which is approximately 760 mm Hg at sea level, is the driving force behind respiration.

Boyle’s Law

Boyle’s Law states that, at a constant temperature, the pressure of a gas is inversely related to its volume: P \times V = k . According to this law, as lung volume increases during inspiration, the intrapulmonary pressure decreases, facilitating airflow into the lungs.

Process of Pulmonary Ventilation

Pulmonary ventilation is accomplished through changes in the thoracic cavity and lung volume, primarily due to:

• Diaphragm: A dome-shaped muscle that contracts and flattens during inhalation, which expands the thoracic cavity.

• Intercostal Muscles: External intercostal muscles contract to further increase thoracic volume during inspiration. During forced expiration, internal intercostal muscles may contract to decrease volume more rapidly.

Inspiration and Expiration

• Inspiration: The diaphragm contracts and descends, expanding the thoracic cavity. The external intercostal muscles elevate the ribs, further increasing thoracic volume, allowing for maximal air intake.

• Expiration: When the diaphragm relaxes and moves upward, this elastic recoil leads to the natural decrease of the thoracic volume. In active expiration, internal intercostal muscles contract to force more air out quickly.

Lung Function Tests (LFTs)

Spirometry is a common test to assess pulmonary function by measuring inhalation and exhalation volumes and capacities. Key volumes measured include:

• Tidal Volume (TV): The amount of air inhaled or exhaled during normal respiration.

• Inspiratory Reserve Volume (IRV): The additional air that can be inhaled after a normal inhalation.

• Expiratory Reserve Volume (ERV): The additional air that can be forcibly exhaled after a normal exhalation.

• Residual Volume (RV): The amount of air remaining in the lungs after a forced exhalation.

• Vital Capacity (VC): The maximum amount of air exhaled after maximum inhalation, combined from TV, IRV, and ERV.

• Total Lung Capacity (TLC): The total volume of air the lungs can hold, calculated as VC + RV.

Gaseous Exchange Mechanism

The Respiratory Membrane is a critical component of gaseous exchange, which includes:

• Type I and Type II Alveolar Cells: Type I cells are involved in gas exchange, while Type II cells produce surfactant, reducing surface tension and preventing alveolar collapse.

• Alveolar and Capillary Endothelium: The very thin membranes allow for efficient diffusion of gases down their concentration gradients, facilitating O2 uptake and CO2 release.

Dalton’s Law

According to Dalton’s Law, the total pressure of a mixture of gases is equal to the sum of the partial pressures of each individual gas: P{total} = P{gas1} + P_{gas2}. At sea level, atmospheric pressure is 760 mm Hg, with:

• Oxygen (O2) comprising ~21%, resulting in a partial pressure of approximately 159 mm Hg.

• Nitrogen (N2) making up ~78%, equating to a partial pressure of ~593 mm Hg.

• Carbon Dioxide (CO2) being approximately 0.04%, with a negligible partial pressure of ~0.3 mm Hg.

Factors Influencing Gas Exchange

Several factors can influence gas exchange efficiency, including:

• Changing Partial Pressures: At higher altitudes, atmospheric pressure and, correspondingly, PO2 decrease, impacting oxygen availability.

• Humidity: The presence of water vapor in inspired air reduces the partial pressure of oxygen, consequently affecting gas exchange dynamics.

Alveolar Gas Exchange

Alveoli facilitate rapid gas exchange due to their vast surface area (approximately 70 m² in an adult) and extremely thin walls (5 µm thick). Blood entering the pulmonary capillaries typically has a higher concentration of CO2 than O2, prompting CO2 to diffuse into the alveoli while O2 diffuses into the blood.

Movement of Gases

Gases, specifically O2 and CO2, diffuse across alveolar membranes based on their concentration gradients, moving into or out of red blood cells (erythrocytes). At the tissues, this process reverses, with O2 being unloaded to meet cell metabolic needs while CO2 is loaded into the bloodstream for transport back to the lungs.

Henry’s Law

Henry’s Law states that the concentration of a gas in a liquid is directly proportional to its partial pressure: C = k \times P . Here, C represents gas concentration, k denotes the solubility coefficient, and P signifies the partial pressure of the gas.

Gas Solubility in Blood

Gas solubility in blood is influenced by multiple factors:

• The solubility coefficient of the gas, indicating how well it dissolves in the liquid.

• The temperature of the blood, which can affect solubility.

• The partial pressure of the gas, determining how much gas can be dissolved in the blood at any given time.

These notes provide an extensive overview of the mechanisms behind gaseous exchange, the physiology of respiration, and the fundamental mathematical principles that govern these processes. Understanding these elements is crucial for studying respiratory physiology in depth.