Diffusion of Gases

Diffusion of Gases Chapter 16 dS/dT = D A R T  C L (MR)1/2

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

DCO2 >>> DO2 - WHY?

Figure 16.3: Solubility of O2 and CO2 in water Figure 16.3: Solubility of O2 and CO2 in water

Diffusion Capacity of the Lung: DL = VGAS / P1 - P2

= ml / min / mm Hg

Oxygen Transport: 95% carried by hemoglobin 5% dissolved in plasma

Fig 16.4: O2 and CO2 in the Body

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 C (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

Fig 16.6: Transport of O2 Fig 16.8: Hb - O2 Dissociation Curve Fig 16.7: Hb - O2 Loading

Hb-O2 Saturation Curve: Review

 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

Carbonic Anyhydrase Reaction + - CO2 + H2O <--> H2CO3 <--> H + HCO3

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

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 See also Figures 16.12 & 13

Fig 16.12: PO2 --> CO2 Transport Fig 16.11: 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

Gas Transport Summary:

 O2 decreases amount of carbamino, promoting unloading of CO2 (Haldane Effect) +  CO2 as H decreases O2 affintiy and increases unloading of O2 in systemic capillaries (Bohr Effect)

Regulation of Ventilation Medullary Center Respiratory Neurons C P o o l ( V R G ) S p o n t a n e o u s l y - A c t i v e + A P o o l ( D R G ) B P o o l +

Dorsal Respiratory Group (DRG) Inhalation ---> diaphragm and external intercostals Ventral Respiratory Group (VRG) Shuts off DRG and promotes active exhalation Fig 16.15: Brainstem Respiratory Centers Respiratory Input from the Pons Apneustic Center: Prolonged inspiration Pneumotaxic Center: Inhibits Apneustic Center Receives some input from vagal lung stretch receptors (?)

Fig 16.18: Peripheral Chemoreceptors

Inputs to Medulla I n p u t s H i g h e r C e r e b r a l C e n t e r s

C P o o l ( V R G ) - C h e m o r e c e p t o r s : M u s c l e - p H P r o p r i o c e p t o r s - P O 2 + A P o o l - P ( D R G ) C O 2 B P o o l + R e s p i r a t o r y + T r a c k + I r r i t a n t s L u n g S t r e t c h D i a p h r a g m R e c e p t o r s

CNS: Medulla

 Sensitive only to pH (due to PCO2)

Periphery:  Aortic arch  Carotid bodies

 Sensitive to PO2 and pH

 Oxygen only a factor at PO2 < 60 mm Hg

Fig 16.19: Chemoreceptor Control Fig 16.20: Central Chemoreceptors

Increasing Alveolar Ventilation: A Pool Output

Effects of alveolar ventilation on PO2 and PCO2 in the alveoli

O2 Sensing Glomus Cells of the Carotid Bodies O 2 S e n s i n g G l o m u s C e l l s o f t h e C a r o t i d B o d i e s

+ O 2 G a t e d K C h a n n e l w i t h P O 2 = 1 0 0 m m H g

V m = - 7 0 m V

O 2 S e n s i n g G l o m u s C e l l s o f t h e C a r o t i d B o d i e s

+ O 2 G a t e d K C h a n n e l i n l o w P O 2

V m = - 5 0 m V O 2 S e n s i n g G l o m u s C e l l s o f t h e C a r o t i d B o d i e s

I n c r e a s e d F r e q u e n c y o f A P s D o p a m i n e

K + T o V R G A P o o l +

S e n s o r y n e r v e

Fig 16.23 Ventilation - perfusion ratios

Pulmonary Blood Flow  To match perfusion with ventilation:

 Increased alveolar air PCO2 => dilate bronchioles and dilate systemic arterioles

 Decreased PCO2 => constrict bronchioles and constrict systemic arterioles

 Increased PO2 => dilate pulmonary arterioles and constrict systemic arterioles

 Decreased PO2 => constrict pulmonary arterioles and dilate systemic arterioles