KNES 260 Lecture Notes - Lecture 16: Venous Blood, Partial Pressure, Perfusion
27 Feb 2017
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External Respiration
● Exchange of O2 and CO2 across the respiratory membrane
● Influenced by partial pressure gradients, ventilation-perfusion coupling, and structural
characteristics of the respiratory membrane
○ Ventilation perfusion coupling: how breathing is coupled with the movement of
blood to the alveoli
■ Perfusion: blood flow reaching the alveoli
Rate of gas exchange across the alveoli is governed by Fick’s Law of diffusion:
● 21
● k is the diffusion coefficient for the gas
● A is the surface area
● D is the thickness of the barrier (respiratory membranes are 0.5-1m thick)
● The partial pressure gradient of O2 in the lungs is steep
○ 40mmHg in the blood and 104mmHg in the alveoli
● Partial pressure for CO2 is less steep
○ 45 in blood and 40 in alveoli
○ CO2 is 20x more soluble in plasma than oxygen
● Both end up diffusing in equal amounts
Bronchoconstriction/dilation is affected by CO2 only. Vasodilation/constriction is affected by
oxygen.
Internal Respiration: Capillary gas exchange in body tissues
● Partial pressures and diffusion gradients are reversed as compared to external respiration
○ PO2 in tissue is always lower than in the arterial blood
○ Partial pressures in Venous blood are the same as they are in the lungs since that’s
where it’s heading
O2 Transport in Blood
● 1.5% dissolves in plasma
● 98.5% is loosely bound to each Fe of hemoglobin (Hb) in RBC’s
○ Each Hb has 4 Fe’s and can carry 4 O2’s
● 25-30% is unloaded at the tissue during rest
○ More is unloaded during exercise
Rate of loading and unloading O2 is regulated by
● Partial pressures
● Temperature
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Document Summary
Exchange of o2 and co2 across the respiratory membrane. Influenced by partial pressure gradients, ventilation-perfusion coupling, and structural characteristics of the respiratory membrane. Ventilation perfusion coupling: how breathing is coupled with the movement of blood to the alveoli. Rate of gas exchange across the alveoli is governed by fick"s law of diffusion: D is the thickness of the barrier (respiratory membranes are 0. 5-1m thick) K is the diffusion coefficient for the gas. The partial pressure gradient of o2 in the lungs is steep. 40mmhg in the blood and 104mmhg in the alveoli. Partial pressure for co2 is less steep. 45 in blood and 40 in alveoli. Co2 is 20x more soluble in plasma than oxygen. Both end up diffusing in equal amounts. Internal respiration: capillary gas exchange in body tissues. Partial pressures and diffusion gradients are reversed as compared to external respiration. Po2 in tissue is always lower than in the arterial blood.
