Gas Exchange and Transport

FiO2

Fraction of inspired oxygen

PB

Barometric pressure

PH2O

Partial pressure of water vapor

160 mmHg

Atmospheric O2 (dry, inspired air) at sea level

0.21

What is the fraction of inspired oxygen (FiO2)?

Decreases

As altitude increases, PO2 ___

Warmed, humidified

Inhaled air is ___ and ___ in large airways.

reduces

An increase in PH2O ___ PO2.

Alveolar ventilation

Effective ventilation

Anatomic dead space

Volume of conducting airways

Alveolar dead space

Gas in alveoli not participating in gas exchange

Physiologic dead space

Alveolar plus anatomic dead space

Should approximate anatomic dead space

PH2O

Partial pressure of water

Alveolar oxygen = [fraction of inspired oxygen (barometric pressure - partial pressure of water)] - (alveolar carbon dioxide / respiratory quotient)

Alveolar gas equation

Alveolar CO2 (PACO2)

Determined by rate of CO2 tissue production (Vdot CO2) and alveolar ventilation (Vdot A)

Passive diffusion

Gas exchange across the blood-gas (BG) barrier in the lung is by ___.

Passive diffusion

Passive movement of gas down a partial pressure gradient (difference in PO2, PCO2)

Gas diffusivity, surface area, B-G barrier thickness, driving pressure gradient between alveolar-capillary blood

What influences passive diffusion?

D

Gas diffusivity; solubility

A

Surface area

T

Blood-gas (B-G) barrier thickness

PO2A - PO2cap

Pressure gradient between alveolar-capillary blood

Partial pressure

What is the driving force for diffusion?

Proportional, inversely related

In Fick’s Law, the diffusion constant is ___ to gas solubility and ___ to gas molecular weight.

Huge surface area, very thin diffusion barrier

Why are lungs ideal for diffusion?

Increased surface area of lungs

Why does exercise improve gas exchange?

Carbon dioxide has a much higher solubility

Why does carbon dioxide diffuse 20 times more rapidly than oxygen?

Blood flow across the pulmonary capillaries, uptake of oxygen by hemoglobin, differential between alveolar and capillary partial pressures (change in P)

Perfusion limitations

Other properties of diffusion (barrier thickness, surface area, gas solubility)

Diffusion limitations

Perfusion-limited

The exchange of oxygen across the alveolar blood barrier is primarily ___.

Strenuous exercise (rare alone), pulmonary disease that disrupts blood-gas barrier, reduced alveolar PO2 (reduced change in partial pressure)

Diffusion limitations to oxygen exchange

Hypoxemia

What can significant diffusion limitations cause?

Rarely

How often is CO2 diffusion affected by lung disease because of its very high solubility?

Decreases

As cardiac output increases, capillary transit time ___.

Changes diffusing capacity, decreases available surface area, increases barrier thickness

How does pulmonary disease create diffusion-limitations?

Altitude, hypoventilation; reduced change in partial pressure

How does low alveolar partial pressure of oxygen (PO2) create diffusion limitations?

Pulmonary disease, low alveolar PO2

What can exercise exacerbate?

Tissues, blood

Oxygen diffuses into ___, while carbon dioxide diffuses into ___.

V/Q ratio

Relationship between alveolar ventilation (V) and pulmonary perfusion (Q)

Relationship between alveolar ventilation (V) and pulmonary perfusion (Q)

What is gas exchange strongly influenced by?

Alveolar-arterial oxygen gradient

Also called A-aDO₂ gradient. Indirect measurement of the efficiency of O₂ exchange between the alveoli and pulmonary blood (PAO₂ vs. PaO₂)

No

In healthy animals, does PAO₂ equal PaO₂?

V/Q mismatch

Most common cause of increasing the alveolar-arterial oxygen gradient

4

How many subunits does a mammalian hemoglobin consist of?

Heme, globin

Each mammalian hemoglobin (Hb) contains one ___ and an associated ___.

Increases

Affinity of Hb for O₂ ___ with each bound O₂. (As bound O₂ increases, oxygen affinity of hemoglobin ___.)

SaO2

Oxygen saturation