Alveolar Gas Exchange

• Gas exchange is driven by partial pressures and matching of perfusion and ventilation in lung tissues and the body's tissues.

Definitions

• Gas exchange is the passive movement of gases across membranes.
• External respiration involves the flow of air into the lungs and the transfer of oxygen and carbon dioxide through diffusion.
• Internal respiration is capillary gas exchange in body tissues.

Mechanisms of Alveolar Gas Exchange

• Gas exchange is comprised of three components
• The surface area of the alveolo-capillary membrane.
• The partial pressure gradients of the gasses
• The matching of ventilation and perfusio

Gas Exchange Across Alveolo-Capillary Membrane

• Partial pressure gradients refer to the difference between the partial pressure of gas in the alveolar sac and the partial pressure of gas in the blood .
  • This difference is the driving force of gas diffusion across the membrane.
  • The partial pressure gradient can also be called the partial pressure gradient, but it is not the concentration difference.
  • Partial pressure of a gas in the alveolar sac: PA
  • Partial pressure of a gas in the blood: Pa

Movement of Oxygen

• Oxygen diffuses from an area of high partial pressure to an area of low partial pressure.
• PAO2 > PaO2 (Pressure gradient = 60mmHg), moving oxygen into the blood from the alveoli.

Movement of Carbon Dioxide

• Carbon dioxide diffuses from an area of high partial pressure to an area of low partial pressure.
• PACO2 < PaCO2 (Pressure gradient is 5 mmHg) moving carbon dioxide from capillary into alveoli.

Movement of a Gas Between Air and a Liquid (Blood)

• The exchange of carbon dioxide from the blood into the alveoli during inspiration is facilitated.
• The amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in the air above it.
• Gases can be forced to dissolve into a liquid if enough pressure is applied, or leave if the pressure outside is low.
• Decreasing alveolar pressure causes carbon dioxide dissolved in the blood to leave and return to its gaseous state.

Matching of Ventilation and Perfusion

• Ventilation: Gas reaching alveoli
• Perfusion: Blood flow in pulmonary capillaries servicing those alveoli
• Airways: Diameter of bronchioles, controlled by PACO2
• Blood: Diameter of capillaries, controlled by PAO2
• Alveoli with high PAO2 and low PACO2 cause bronchioles to constrict and pulmonary capillaries to dilate.
• Alveoli with low PO2 and high PCO2 cause bronchioles to dilate and pulmonary capillaries constrict, directing blood to alveoli with more O2.

External Respiration Summary

• Mixed venous blood returns to the right heart from tissues.
• Blood is pumped into the pulmonary artery from the right ventricle, and then delivered to the pulmonary capillaries.
• The PCO2 level in the blood is 45 mm Hg, and PO2 is 40 mm Hg, reflecting the tissues' metabolic activity, where PCO2 > PO2 due to cellular activity.
• O2 and CO2 exchange between alveolar air and pulmonary capillary blood.
• Blood leaving the pulmonary capillaries has the same PO2 and PCO2 as alveolar air (PaO2 is 100 mm Hg; PaCO2 is 40 mm Hg, same as PAO2 and PACO2).
• Blood returns to the left heart and is pumped into the aorta.

Internal Respiration

• Partial pressure of oxygen in tissues is low (40 mm Hg) while it is high in the blood (100 mm Hg).
• An oxygen diffusion gradient causes oxygen to diffuse out of the blood and enter the tissue.
• Partial pressure of carbon dioxide is lower in the blood than in the tissue, so carbon dioxide diffuses out of the tissue and into the blood.