Gas Exchange and Perfusion of Lungs

Gas Exchange by Diffusion

• Diffusion is the movement of gases from areas of high partial pressure to areas of low partial pressure.
• Partial pressure is the pressure exerted by a gas in a mixture of gases and is proportional to the percentage of the gas in the mixture.
• Factors influencing the efficiency of diffusion:
  • Partial pressures of gases
  • Thickness of the alveolar-capillary barrier
  • Surface area of the barrier
• Blood flow is also important for gas exchange.
  • The rate of blood flow through the alveoli determines how much time there is for gas exchange.
  • Perfusion of the alveoli ensures that blood reaches the alveoli for gas exchange.

Perfusion of Lungs

• Pulmonary circulation refers to the flow of blood through the lungs.
• The pulmonary circulation is a low-pressure, high-volume system.
  • It receives the entire cardiac output.
  • Blood pressure in the pulmonary artery is much lower than in the systemic circulation.
• Blood flow distribution in the lungs is not uniform.
  • Gravity influences blood flow distribution, with more blood flow at the bottom of the lungs in an upright position.
  • The muscular tone of pulmonary arterioles also plays a role, with vasoconstriction decreasing blood flow.

Ventilation Terminology

• Dead Space is the volume of air that does not participate in gas exchange.
• Minute ventilation is the total volume of air breathed in per minute.
  • It is calculated by multiplying tidal volume (TV) by respiratory rate (breaths/minute).
• Normal minute ventilation in adults at rest is around 6L/minute.
• Hyperventilation is increased ventilation.
• Hypoventilation is decreased ventilation.
• Tachypnoea is an increased rate of respiration.
• Dyspnoea is a distressful sensation of breathing.

Alveolar Ventilation

• Alveolar ventilation is the volume of air that reaches the alveoli per minute.
  • It is calculated by subtracting dead space volume (VD) from tidal volume (TV) and then multiplying by the respiratory rate.
• Alveolar ventilation is crucial for effective gas exchange.

Control of Respiration

• Carbon dioxide (CO2) is the primary regulator of respiration.
  • CO2 levels influence the activity of the respiratory centers in the medulla.
  • CO2 can pass through the blood-brain barrier, while H+ cannot.
• Hydrogen ion (H+) levels are monitored by peripheral chemoreceptors in the carotid and aortic bodies.
• Oxygen (O2) levels also influence respiration, but only when arterial PO2 is below 60mmHg.

CO2 Transport in Blood

• CO2 is transported in the blood in three forms:
  • Dissolved CO2 (~7%)
  • Carbaminohemoglobin (HbCO2) (~23%)
  • Bicarbonate ions (HCO3-) (~70%)
• The chloride shift helps maintain electrical neutrality in the blood.

Chemoreceptors

• Central chemoreceptors in the medulla respond to changes in CO2 levels.
• Peripheral chemoreceptors in the carotid and aortic bodies respond to changes in O2 and H+ levels.

Summary of Gas Exchange & Perfusion

• Effective gas exchange requires:
  • Adequate ventilation to deliver fresh air to the alveoli
  • Adequate perfusion to bring blood to the alveoli
  • Efficient diffusion across the alveolar-capillary barrier

Explore the mechanisms of gas exchange through diffusion and the role of pulmonary circulation. This quiz covers key concepts like partial pressure, factors influencing diffusion efficiency, and the importance of blood flow in the lungs. Test your understanding of how gases are transported and exchanged in the respiratory system.