Principles of Gas Exchange

partial pressure of O2?

• total pressure x the fraction of O2 present =

  • 760 * .21 = 160 mmHg

Inspired air is humified, water vapor has partial pressure, 100% humified air has partial pressure of 47 mmHg

• 760-47 * .21 = 150 mmHG partial pressure of inspired air

Effects of high alitdue on partial pressure

• Decreases PO2: why?

  • decrease in total pressure Pbar at high altitiude

  • PO2 = 380-47 * .21 = 70 mmhg

Henrys law for breathing

• linear relationship between partial pressure of O2 and amount dissolved (this is not true for PO2 and amt of O2 bounded to Hb)

• P02 = 100 mmHg = 0.3 ml O2/ 100 ml blood

• PO2 = 200 mmHg = 0.6 ml O2 /100 ml blood

Fick’s law: diffusion of gases (simple diffusion): ways to increase?

• increase membrane (surface area)

• Increase amount dissolved

• Increase partial pressure difference (conc. gradient)

• Decrease membrane thickness

Diffusing Capacity:

amount of gas the lungs can transfer into blood

• perfusion limited: diffusing capacity is limited by blood flow

  • need to increase blood flwo to lungs to increase amt of O2 delivered to blood → humans

• diffusion limited: diffusing capacity is limited by rate of diffusion

  • need to increase rate of diffusion to increase the amount of O2 delivered to the blood

explain why we are perfusion limited with O2

• if you increase rate of diffusion, no change in O2 in blood as it would reach equilibrium

  • increases O2 both ways

safety factor of O2: disease conditions?

• O2 in alveoli come to equilibrium 1/3 of way thru capillary, so even if velocity increases (excercise) the O2 in blood will come to equilibrium with alveoli

• in disease, if the O2 does not come to equilibrium with alevoli, the diffusion capacity of O2 would be diffusion limited

pulmonary ciruclation: pressures

• low pressure system compared to systemic

• Pressure in pulmonary is so low that there is no filtration

  • important bec if filtration → fluid in alveoli (pulomonary edema)

congestive left heart failure

1. blood would build up in pulmonary circualtion

2. pulmonary capillary pressure increases

3. filtration from from pulomary capillaries

4. pulmonary edema

5. Poor gas exchange

MAP =

CO * TRP

Regulation of pulmonary vascualr resistance: effect of CO

• excercise → increases CO2 → increase MAP → decrease in TPR so that there is no change in MAP

  • recruitment: some capillaries are closed, more are recruited to open

  • distension: increase radius of each capillary

3 benefits of recruitment and distension

• Decreased resistance causes decreased pressure and prevents filtration

• Decreased velocity of blood flow so more time for gas exchange

• Increases SA for gas exchange

effect of ANS on pulmonary vascular resistace

• not an important regulator of pulmonary vascular resistance (blood flow)

• but does regulate airflow: PSNS: airway constriction, SNS: airway dilation

effect of hypoxia

• match air flow and blood flow

• decreased airlfow → vasoconstriction → blood flow

going to high altitude

• decreased PO2 in all airways

• gen vasoconsriction thruout lungs

• increased resistance to blood flow through lungs

• increased BP → pulmonary edema

• increased work to pump blood

PO2 in air = 160 mmHg, PO2 in alveoli = ? why?

105 mmHg

• mixed with dead space air

• Mixing with residual volume air

• AIr is humified (water vapor added)

PO2 in blood in capillaries under alveoli is 105 mmHG, what is PO2 of blood leaving lungs?

100 mmHG: shunting

• some blood going thru lungs does not go past alveoli, the PO2 in this blood stays at 40, when this mixed it decreases

cells use O2, so Po2 at cells goes to?

<40 mmHg

cells make CO2 so CO@ diffuses from cells to blood down conc gradient