Gas Exchange and Transport
Chapter 13: Gas Exchange and Transport
Concentration & Partial Pressure of Respired Gases
The body’s supply of O2 depends on its concentration and pressure in ambient air.
Ambient air pressure at sea level = 760 mmHg
Partial Pressure
Definition: Partial Pressure is the percentage of concentration multiplied by the total pressure of a gas mixture.
Composition of Ambient Air at Sea Level:
Nitrogen (N2):
Concentration: 78.6%
Partial Pressure (PN2): 78.6 ext{%} imes 760 ext{mmHg} = 593 ext{mmHg}
Oxygen (O2):
Concentration: 20.9%
Partial Pressure (PO2): 20.9 ext{%} imes 760 ext{mmHg} = 159 ext{mmHg}
Carbon Dioxide (CO2):
Concentration: 0.03%
Partial Pressure (PCO2): 0.03 ext{%} imes 760 ext{mmHg} = 0.23 ext{mmHg}
Argon (Ar):
Concentration: 0.93%
Partial Pressure (Par): 0.93 ext{%} imes 760 ext{mmHg} = 7 ext{mmHg}
Total: 100% = 760 mmHg
Dalton’s Law
Definition: Total pressure of a gas mixture equals the sum of the partial pressures of all gases in that mixture.
Ambient Air Composition Impacts
In tracheal air:
Water vapor reduces the PO2 in the trachea about 10 mmHg to approximately 149 mmHg.
In alveolar air:
Average Alveolar PO2 = 103 mmHg, with compositions:
O2: 14.5%
CO2: 5.5%
N2: 80%
Movement of Gas in Air & Fluids
Henry’s Law: Gases diffuse from areas of high pressure to low pressure.
Factors Influencing Diffusion Rate into a Fluid:
Pressure differential:
In humans, the pressure difference between alveolar and pulmonary blood gases is critical for gas diffusion across the pulmonary membrane.
Solubility of the gas in the fluid:
CO2 is approximately 25 times more soluble in fluid than O2, which is significant for loading/unloading gases.
Therefore, O2 must be bound to hemoglobin.
Gas Exchange in Lungs & Tissues
Description: Gas exchange occurs passively as established by pressure differentials.
Gas Exchange in the Lungs:
PO2 in alveoli is approximately 100 mmHg.
PO2 in pulmonary capillaries is around 40 mmHg.
Result:
O2 moves into pulmonary capillaries.
PCO2 in pulmonary capillaries is about 46 mmHg, compared to roughly 39 mmHg in alveoli.
Arterial blood leaving the lungs has:
PO2 = 100 mmHg; PCO2 = 40 mmHg.
These values change little even during vigorous exercise; CO2 moves into alveoli.
Gas Exchange in Tissues:
Pressure gradients cause diffusion of O2 into and CO2 out of tissues.
In vigorous exercise:
PO2 in muscles falls toward 0 (from approximately 40 mmHg at rest).
PCO2 may approach 90 mmHg (from 46 mmHg at rest).
O2 diffuses from blood into cells, while CO2 flows from cells into venous blood.
Pulmonary Disease Implications:
Gas transfer capacity may be impaired by:
Thickening of alveolar membranes.
Reduction in alveolar surface area.
Transport of O2 in the Blood
Mechanisms of O2 Transport:
Dissolved in plasma
Combined with hemoglobin (Hb)
Dissolved in Plasma:
For each 1 mmHg pressure, approximately 0.003 mL O2 dissolves into plasma.
This results in about 3 mL of O2 per liter of blood (with a total blood volume of 5 L = 15 mL dissolved O2).
This limited amount would sustain a person for only about 4 seconds.
Dissolved O2 Functions:
Establishes the PO2 of the blood.
Regulates breathing, particularly at altitude.
Governs the loading/unloading of hemoglobin.
Combined with Hemoglobin (Hb):
Each of the four iron (Fe) atoms in hemoglobin can combine with one O2 molecule.
This reaction requires no enzymes, as it is dictated by PO2 in the physical solution.
Each gram of Hb can combine with 1.34 mL O2.
Under normal conditions, each dL of blood contains about 20 mL O2.
This volume is slightly lower in women due to lower Hb levels per dL blood, which helps explain the slightly lower aerobic capacity of women.
Conclusion:
65-70 times more O2 is transported this way than through normal dissolution in plasma.
Oxygen Extraction & Carrying Capacity
Volume Percent (vol %): Refers to mL of oxygen extracted from a 100 mL sample of whole blood or packed red blood cells.
Human whole blood carries O2 at 14 vol % at altitude. - Vol % is higher in animals and humans who reside at altitude.
Iron-deficiency Anemia:
Reduces O2 carrying capacity considerably, affecting the ability to maintain even moderate-intensity aerobic exercise.
Hematocrit: Percentage of red blood cells in whole blood.
PO2 & Hb Saturation
Cooperative Binding:
The process by which binding an oxygen molecule to the iron atom in one of the four globin chains of hemoglobin progressively facilitates the binding of subsequent oxygen molecules.
Oxy-hemoglobin Dissociation Curve: Illustrates Hb saturation with oxygen at various PO2 values.
Percent saturation is calculated as:
Oxyhemoglobin Dissociation Curve
Represents the saturation of hemoglobin at various partial pressures of oxygen (PO2).
Key Points:
The curve shows sigmoidal shape due to cooperative binding.
For a given PO2, the percentage saturation of hemoglobin increases as more oxygen is being bound.
PO2 in the Lungs
Normal arterial blood has Hb saturation at approximately 98%.
Increased PO2 does not significantly increase saturation.
In healthy individuals breathing ambient air at sea level, each dL of blood leaving the lungs carries about 20.0 mL of O2:
19.7 mL bound to Hb; 0.3 mL dissolved in plasma.
Hb saturation with O2 changes minimally until PO2 drops to 60 mmHg, providing a buffer against minor fluctuations.
At a PO2 of 60 mmHg, Hb still remains around 90% saturated.
Arteriovenous O2 Difference
The a-vO2 difference indicates the amount of O2 extracted by tissues.
At rest, this averages 4-5 mL O2 per dL of blood.
Functionality:
Maintains a reserve of O2 on Hb for emergency demands.
During exercise, a-vO2 difference increases up to 3 times the resting value as the circulating blood yield its O2 to working muscles.
Bohr Effect
Conditions leading to the Bohr Effect:
Increased PCO2
Increased Temperature
Increased 2,3 DPG
Decreased pH
Description: These conditions cause the oxy-hemoglobin dissociation curve to shift to the right, reducing Hb's efficiency to hold O2 (especially at PO2 range between 20 and 50 mmHg).
The Bohr Effect impacts the unloading of O2 but not the loading under normal alveolar PO2.
2,3 Diphosphoglycerate (2,3 DPG)
Function: Produced during glycolysis in red blood cells, it binds loosely to Hb, reducing its affinity for O2, leading to greater O2 release to tissues.
2,3 DPG increases with intense exercise and may be influenced by altitude training.
Notably, females generally have higher 2,3 DPG levels compared to men at similar activity levels, aiding in compensating for lower Hb.
Myoglobin, Muscle’s O2 Store
Definition: Myoglobin is an iron-containing globular protein located in skeletal and cardiac muscle, which stores O2 intramuscularly.
Unlike Hb, myoglobin contains only one iron atom and readily binds and retains O2 even at lower PO2 levels.
Dissociation Curve: Myoglobin does not exhibit a sigmoidal (S-shaped) curve like hemoglobin, and it does not undergo the Bohr Effect.
CO2 Transport
Mechanisms of CO2 Transport:
Dissolved in plasma: Approximately 5% of CO2 is transported as dissolved CO2 which establishes PCO2 of the blood.
Plasma bicarbonate: Facilitated by carbonic anhydrase, forming bicarbonate ions (HCO3-). Chloride ions move into RBCs to maintain ionic balance (known as the chloride shift). Approximately 60-80% of CO2 exists as plasma bicarbonate. Once in the lungs, H+ and HCO3- convert back into CO2 and H2O.
Combined with Hemoglobin (Hb): CO2 reacts with amino acids in blood proteins, forming carbamino compounds which reverse at decreased plasma PCO2 in lungs and release CO2. The Haldane Effect refers to the reduced affinity of Hb for CO2 when O2 is present, aiding CO2 release in the lungs.