External and Internal Respiration
A) External respiration or Pulmonary Gas Exchange is the diffusion of O2 from air in the alveoli of the lungs to blood in the pulmonary capillaries and the diffusion of CO2 in the opposite direction. External Respiration converts deoxygenated blood coming from the right side of the heart to oxygenated blood that returns to the left side of the heart. B) Internal Respiration or Systemic Gas Exchange is the exchange of O2 and CO2 between systemic capillaries and tissue cells.
External Respiration occurs in the lungs only. Internal Respiration occurs throughout the body.
The rate of pulmonary and systemic gas exchange depends on several factors:
1) Partial Pressure Differences of Gases : Alveolar PO2 must be higher than blood PO2 for oxygen to diffuse from alveolar air into blood. These pressures change during exercise. For example, PO2 is lower in the tissues due to the contraction of muscles using high amounts of O2. This creates a bigger gradient therefore the O2 will diffuse faster from blood into the tissues. At increasing altitudes, total atmospheric pressure decreases and as a result, the partial pressure of O2 does as well. The P O2 of inhaled air decreases with increasing altitude. Alveolar P O2 decreases correspondingly and O2 diffuses into the blood more slowly possibly causing high altitude sickness. 2) Surface Area Available for Gas Exchange : Increased surface area equals increased rate of gas exchange. As discussed, the alveoli are about 2450 m2. 3) Diffusion Distance : Since the respiratory membrane is generally thin, diffusion occurs at a rapid rate. Pathologies such as pulmonary edema may cause a buildup of interstitial fluid and slow the rate of diffusion. 4) Molecular Weight and Solubility of the Gases : O2 has a lower MW than CO2. However, the solubility of CO2 in some portions of the respiratory membrane is ~ 24x greater than O2. Taking both factors into account, net outward CO2 diffusion occurs 20x more rapidly than inward O2 diffusion.
Transport of Oxygen and Carbon Dioxide
A) Oxygen Transport
O2 does NOT dissolve easily in water, so only ~ 1.5% of O2 is dissolved in blood plasma (mostly water). 98.5% is bound to Hemoglobin in RBCs. Hemoglobin bound with O2 is called Oxyhemoglobin . Each hemoglobin molecule is capable of binding 4 O2 molecules. The Percent Saturation of Hemoglobin expresses the average saturation of hemoglobin with O2.
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