Chapter 18- Gas Exchange and Transport
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Chapter 18- Gas Exchange and Transport
Gas Exchange Transport
Gas Exchange and Transport
We breathe atmospheric O2 to exchange with cellular CO2
1. Alveoli exchange O2
2. Blood transports O2
3. Cells exchange O2
4. Cells exchange CO2
5. Blood transports CO2
6. Alveoli exchange CO2
What controls diffusion of gases in the body?
Diffusion rate is directly proportional to:
1. Surface area for exchange
2. Partial pressure gradient of gases
• Diffusion rate is inversely related to:
1. Membrane thickness
2. Diffusion distance
Bulk flow of Oxygen
Bulk flow is the net flow of gas
between lung and tissue
• PO2 is higher in alveoli than in blood
– O2 diffuses from alveoli to blood
• PO2 is higher in blood than in tissues
• PCO2 is higher in tissues than in the
– CO2 diffuses from tissues to blood
• PCO2 is higher in blood than in the
Diffusion and Solubility of Gases
• Driving force for bulk flow:
PO2 (alveoli) > PO2 (tissue)
• Bulk flow means both solute
and water are transported.
• Hence bulk flow of O2 has three components
1. O2 diffuses from alveolus to pulmonary blood
2. Circulation carries O2 and blood from lung to tissues
3. O2 diffuses from capillaries into tissues
Bulk flow of Carbon Dioxide
• Net flow of CO2 from tissue to lung
• Driving force for bulk flow
: PCO2 (tissues) > PCO2 (alveoli)
• Bulk flow of CO2 has three components
1. CO2 diffuses from tissues to capillary blood
2. Circulation carries CO2 and blood from tissues to lung
3. CO2 diffuses from pulmonary blood into alveoli
• This is bulk flow because circulation
carries CO2 and water
What happens when alveolar PO2 decreases?
• Decrease PO2 of alveoli reduce the gradient for diffusion
• Impairs delivery of O2 to tissues
• Possible causes:
1. Low PO2 at high altitudes
2. Increased airway resistance
What else could impair alveolar diffusion?
• Delivery of O2 to blood in the capillary should be faster than the flow of blood through the capillary
• Slowing diffusion can decrease PO2 in plasma below the maximum
How can alveolar diffusion be slowed?
1. Alveolar destruction reduces surface area (emphysema)
2. Thickened alveolar membrane (fibrosis) increases distance
3. Fluid in alveolus reduces rate (pulmonary edema)
Gas Transport in the Blood
• Low O2 solubility makes hemoglobin necessary.
• Tissues need 250 ml O2/min
• O2 dissolved in plasma can be delivered at 15 ml O2/min
• If O2 is bound to hemoglobin, delivery can be as high as 1000 ml O2/min
How does O2 binding to hemoglobin work?
Hemoglobin has 4 polypeptide chains (adults have 2α and 2β chains)
• Each chain binds one O2
How is CO2 transported from tissue to lungs?
• CO2 is more soluble in plasma than O2
• Three mechanisms operate to carry CO2 in venous blood:
1. 7% is carried in plasma; 93% diffuses into RBCs
2. 70% is converted to bicarbonate
3. 23% is bound to hemoglobin
What determines the direction of the carbonic anhydrase reaction?
• Mass action!
• High CO2 in the tissues drives the reaction toward bicarbonate formation
• Low CO2 in the alveoli drives the reaction toward CO2 formation
Is breathing spontaneous?
Yes, conscious control of breathing is not required
Does the autonomic nervous system control breathing?
• No, rate and depth of breathing are under somatic motor control
• Ventilation muscles are skeletal; they must be stimulated by neurons
How does CNS control breathing?
• No input is required from upper parts of the brain.
• Brains stem (BS) centers control involuntary breathing
• Pacemaker cells in BS produce rhythmic pattern of breathing
• BS integrates sensory and higher CNS inputs
• Efferent signals from BS travel separate
somatic motor pathways for inspiration and expiration
• Inspiration motor neurons synapse with muscles of inspiration
• Expiration motor neurons synapse with muscles of expiration
What sensory information is received?
• CO2, O2 and pH influence ventilation
• High PCO2 is detected by chemoreceptors
1. Periphery in arteries
2. Central in medulla
• Stimulation increased depth and frequency of ventilation
• Low PO2 occurs at high altitude
• Low PO2 and PCO2 occurs in COPD