Chapter 18- Gas Exchange and Transport
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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
- localized in
- 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
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