Gas Exchange and Respiratory Mechanics

• The lecture reviews the gross structure of the lungs and pleurae.
• This lecture Explains the functional importance of the intrapleural space's partial vacuum.
• This lecture Relates Boyle's law to inspiration and expiration, and explains the roles of respiratory muscles and lung elasticity in air flow.
• This presentation lists physical factors influencing pulmonary ventilation.

Dalton's Law of Partial Pressures

• Dalton's law states that the total pressure exerted by a gas mixture is the sum of the partial pressures exerted by each gas.
• Total atmospheric pressure is 760 mm Hg.
• Nitrogen is 78.6% of air, with a partial pressure (P_N2) of 597 mm Hg, calculated as 0.786 * 760 mm Hg.
• Oxygen is 20.9% of air, with a partial pressure (P_O2) of 159 mm Hg, calculated as 0.209 * 760 mm Hg.
• Other gases, including water and carbon dioxide, make up 0.5% of air, with a combined partial pressure of 3.8 mm Hg, calculated as 0.005 * 760 mm Hg.

Henry's Law

• Henry's law states that the amount of gas that dissolves in a liquid is proportional to its partial pressure when the gas mixture is in contact with the liquid.
• The amount of each gas that dissolves depends on gas solubility and the temperature of the liquid.
• Some gases dissolve more easily than others.
• Carbon dioxide is 24 times more soluble in water than oxygen.
• Oxygen is two times more soluble in water than nitrogen.
• Solubility decreases as the temperature of the liquid rises.

External Respiration

• Alveolar gas composition differs from atmospheric gas due to gas exchange in the lungs, humidification of air, and mixing of alveolar gas with each breath.
• A steep partial pressure gradient exists between air in the alveoli and gases in pulmonary artery blood.
• Oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses out of the blood and into the alveoli.

Alveolar Gas Exchange

• The respiratory membrane is normally very thin and presents a huge surface area for gas exchange.
• Ventilation-perfusion coupling matches alveolar ventilation (air reaching the alveoli) with pulmonary blood perfusion (blood flow reaching the alveoli).
• Ventilation and perfusion are balanced to ensure oxygen and carbon dioxide levels match physiological demands.
• Oxygen partial pressure controls perfusion by changing arteriolar diameter.
• Arterioles constrict in areas with low oxygen partial pressure and dilate in well-ventilated areas to optimize oxygen uptake.
• Carbon dioxide partial pressure controls ventilation by changing bronchiolar diameter.
• Bronchioles dilate in areas with high alveolar carbon dioxide and constrict in areas with low carbon dioxide to increase ventilation.

Internal Respiration

• Internal respiration is capillary gas exchange in body tissues.
• Oxygen and carbon dioxide diffusion gradients are reversed compared to external respiration, but the factors promoting gas exchange are the same.
• The partial pressure of oxygen is always lower in tissues than in blood, causing oxygen to diffuse into the tissues.
• A similar but less dramatic gradient exists for carbon dioxide in the reverse direction.
• Oxygen is poorly soluble in the blood; only 1.5% is dissolved in plasma, while the remaining 98.5% is carried on hemoglobin.
• Up to four oxygen molecules (O₂) can bind reversibly to a hemoglobin molecule, with one oxygen on each iron ion (Fe2+).
• Hemoglobin's affinity for oxygen changes with each successive oxygen molecule bound or released, making oxygen loading and unloading very efficient.
• The greater the difference between the amount of oxygen in tissue and the amount of oxygen in blood, the more oxygen will detach from haemoglobin and diffuse into the tissue.
• At high oxygen partial pressure, large changes in available oxygen in the air (e.g., at high altitude) have little effect on oxygen saturation because the O₂ saturation curve is relatively flat.
• At low Po₂ (e.g., in muscles during exercise), hemoglobin easily releases oxygen to body tissues.
• Hemoglobin unloads little oxygen at higher plasma partial pressures of oxygen.
• During vigorous exercise, when plasma partial pressure of oxygen falls dramatically, much more oxygen can be unloaded.
• Factors such as higher temperature, higher carbon dioxide partial pressure, lower (acidic) blood pH, and increased production of bisphosphoglyceric acid (BPG) encourage hemoglobin to release oxygen more easily in metabolically active tissues.

Hypoxia

• Hypoxia is inadequate oxygen delivery to tissues.
• Anemic hypoxia results from too few red blood cells or abnormal/too little hemoglobin.
• Ischemic hypoxia is caused by impaired or blocked circulation.
• Histotoxic hypoxia occurs when cells are unable to use oxygen, such as from metabolic poisons like cyanide or arsenic.
• Hypoxemic hypoxia: results from insufficient oxygen reaching the blood, which may be caused by abnormal ventilation.
• Carbon monoxide poisoning is often caused by fire and results in hemoglobin having a 200x greater affinity for carbon monoxide than oxygen.

Carbon Dioxide Transport

• Carbon dioxide is transported in the blood in three ways:
  • 7-10% dissolved in plasma
  • 20% carried on hemoglobin, bound to globins
  • 70% exists as bicarbonate ions (HCO3-), buffering blood pH.
• The carbonic acid-bicarbonate buffer system forms when CO₂ combines with water & dissociates into carbonic acid & bicarbonate ions to release or absorb H+.
• Slow, shallow breathing increases CO₂ in the blood, decreasing pH, whereas rapid, deep breathing decreases CO₂ in the blood, increasing pH.

Explore the principles governing gas behavior and respiration. Understand Dalton's and Henry's laws. Learn about partial pressures, gas solubility, and the mechanics of breathing and internal respiration.