Diffusion of Gases

dS/dT = D A R T DC

L (MR)^1/2

• D = diffusion coefficient which includes solubility of molecule in aqueous solution
• A = surface area for diffusion
• DC = concentration gradient
• L = path length or thickness of membrane
• MR = molecular radius of gas molecule

Pulmonary Blood Flow

To match perfusion with ventilation:

• Increased alveolar air P_CO2 => dilate bronchioles and dilate systemic arterioles
• Decreased P_CO2 => constrict bronchioles and constrict systemic arterioles
• Increased P_O2 => dilate pulmonary arterioles and constrict systemic arterioles
• Decreased P_O2 => constrict pulmonary arterioles and dilate systemic arterioles

Respiratory Considerations

• Surface area of capillaries in lung and tissues
• Thin membrane for diffusion
• MR for CO₂ and O₂ is very similar
• Solubility differs for CO₂ and O₂
• Gases are warmed as they enter lungs

Respiratory Considerations

DC (in mm Hg)

Gas Air Alveolar Air Arterial Venous

O₂ 160 100 100 40

CO₂ 0.25 40 40 46

H₂O 0.0 47

N₂ 600 573

Terminology

• Hypoxia
• Hypercapnia

Hb-O₂ Saturation Curve

• Shift to the right decreases affinity, increases P₅₀, and increases unloading of O₂
• Caused by acidity (Bohr effect), increased temperature, and elevated 2,3 - DPG
• Increased by Epi, thyroid hormones, prolonged hypoxia, etc.

CO₂ in the blood

• 7% as dissolved CO₂
• 23% as carbamino compounds on Hb
• Protein-NH₃⁺ + CO₂ <---> Protein -NH₂COOH
• 70% as HCO₃^- via carbonic anhydrase

Haldane Effect: in the Lungs O₂ promotes the unloading of CO₂

• As Hb binds O₂ , Hb becomes a stronger acid, i.e., it gives up an H⁺
• This released H⁺ binds to HCO₃^- --> HCO₃^- + H⁺ ---> H₂CO₃ ---> CO₂ + H₂O
• The CO₂ is then released or blown off in the exhaled air

In the tissues the Bohr effect causes the increased release of O₂

• As CO₂ increases, H⁺ is formed and some is buffered by binding to Hb
• This decreases Hb affinity for O₂ and promotes the unloading of O₂in the tissues

Chloride Shift: RBCs in the systemic capillaries

Chloride Shift

• RBC [Cl^-] increases in systemic capillaries
• RBC volume and blood hematocrit (Hct) increases in systemic capillaries
• Venous Hct is 3% greater than arterial Hct

Chloride Shift Reversed: RBCs in the Lungs

Regulation of Ventilation

Medullary Center Respiratory Neurons

• Dorsal Respiratory Group (DRG)

  Inhalation ---> diaphragm and external intercostals

• Ventral Respiratory Group (VRG)

  Shuts off DRG and promotes active exhalation

   

Medullary Centers

Respiratory Input from the Pons

• Apneustic Center
• Prolonged inspiration
• Pneumotaxic Center
• Inhibits Apneustic Center
• Receives some input from vagal lung stretch receptors (?)

Inputs

P_CO2 (pH receptors)

• CNS: Medulla
• Periphery:Aortic Arch and Carotid Bodies

Increasing Alveolar Ventilation: A Pool Output

Effects of alveolar ventilation on P_O2 and P_CO2 in the alveoli

O₂ Sensing Glomus Cells of the Carotid Bodies