PHTY208 Lecture Notes - Lecture 11: Respiratory Failure, Perfusion, Hemoglobin
Arterial Blood Gases:
What is a blood gas?!
-Dissolved gases include nitrogen, carbon dioxide, oxygen, water vapour and other gases!
-Each gas in a mixture gases exerts its own pressure as if no other gases were present!
-Pressure of a specific gas is partial pressure P!
Gas exchange:!
-Atmospheric pressure (760mmHg) !
-Each gas diffuses across a permeable membrane from the area where its partial pressure is
greater to the area where its partial pressure is less!
-The greater the difference, the faster the rate of diffusion!
Henry’s Law:!
-Quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas
and its solubility!
-Higher partial pressure of a gas over a liquid and higher solubility, more of the gas will stay in
solution!
-Much more CO2 is dissolved in blood than O2 because CO2 is 24 times more soluble!
-Even though the air we breathe is mostlyN2, very little dissolves in blood due to low solubility!
Partial pressure of Oxygen:!
-Alveolar Po2= 105mmHg!
-Pulmonary capillary Po2 = 40mmHg!
-Oxygen diffuses into capillary blood!
-Po2 of pulmonary veins = 100mmHg!
Partial pressure of Carbon dioxide:!
-deoxygenated blood PCo2 = 45mmHg!
-Alveolar air PCo2 = 40mmHg!
-Oxygenated blood of pulmonary veins has PCo2 of 40mmHg!
Rate of gas exchange:!
-Rate of gas exchange based on several forces!
-Partial pressure difference of the gases!
-Surface area available for gas exchange!
-Molecular weight and solubility of the gases!
-Diffusion distance!
Carbon dioxide transport:!
-Each 100mL of blood contains 53mL ofCO2!
-Transported in 3 ways:!
-Dissolved Co2 (7%)!
-Bound to amino acids (23%)!
-Most Co2 transported in blood plasma (70%)!
CO2 + H2O- H2CO3 - H + HCO3!
-HCO3 accumulates inside RBCs as they pick up carbon dioxide!
-Some diffuses out into plasma!
-To balance the loss of negative ions, chloride (Cl) moves into RBCs from plasma!
-Reverse happens in lungs- Cl moves out as moves back into RBCs!
Blood gas sampling:!
-An arterial blood gas sample provides key information about a patients respiratory and
metabolic systems!
-Collection of blood involves an arterial puncture usually to the radial artery, very invasive, rapid
generation of results, most critical care units have blood gas analyser!
Arterial blood gases:!
-pH- measure of acid/base balance and ventilation!
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Document Summary
Dissolved gases include nitrogen, carbon dioxide, oxygen, water vapour and other gases. Each gas in a mixture gases exerts its own pressure as if no other gases were present. Pressure of a speci c gas is partial pressure p. Each gas di uses across a permeable membrane from the area where its partial pressure is greater to the area where its partial pressure is less. The greater the di erence, the faster the rate of di usion. Quantity of a gas that will dissolve in a liquid is proportional to the partial pressures of the gas and its solubility. Higher partial pressure of a gas over a liquid and higher solubility, more of the gas will stay in solution. Much more co2 is dissolved in blood than o2 because co2 is 24 times more soluble. Even though the air we breathe is mostlyn2, very little dissolves in blood due to low solubility.
