Gas Exchange

• Partial pressures of gases

• Composition of lung gases

• Alveolar ventilation

• Diffusion

• Perfusion = blood flow

• Matching of ventilation to perfusion

Partial Pressure of Gas

P =nRT

Vgas equation

Pgas 1 + Pgas 2 + Pgas 3……= Total P

(e.g. 760 mmHg)

partial pressures of gases

Gas Transfer

. Henry’s law:

conc dissolved gas partial pressure of gas x solubility index

• Also:

Dissolved gas in equilibrium with gas combined with other agents e.g. hemoglobin

• Gases in liquids diffuse from area of higher partial pressure to area of lower partial pressure.

PA > PB

O2

CO2

mixed venous blood

venous

40 mmHg 100 mmHg

PAO2

100 mmHg

46 mmHg 40 mmHg

PACO2

40 mmHg Pulmonary vein = arterial

Gas diffusion cont.

Rate of diffusion

pressure difference

surface area

solubility index

membrane thickness molecular weight

Pressure difference: ≈ 60 mmHg for O2

≈ 6 mmHg for CO2

Surface area: large

depends on ventilation:

perfusion matching

Solubility index if: O2 = 1, CO2 = 20

Membrane thickness: small

Respiratory membrane

Fig. is a scanning electron micrograph of mouse lung to show an interalveolar septum.

Red blood cells

A surface view of capillaries in the alveolar wall.

Cross-sectional view of the alveolar wall and their vascular supply.

Diffusing capacity:

= volume of gas that diffuses through respiratory membrane per minute for each mmHg of gas partial pressure difference

with exercise — capillary diameter

~ better matching of ventilation to perfusion (blood flow)

Alveolar Ventilation

Air in alveoli + airways

Dead space = air not involved in gas exchange~ anatomical: air in airways

(in ml ≈ body weight in pounds)

~ alveolar: air in alveoli with blood supply

little gas exchange ventilation wasted

very small normally

V T = VA + VD

tidal volume air to alveoli dead space

500 ml = 350 ml + 150 ml

Minute/total ventilation = VT breaths / min

Alveolar ventilation =

VA = (VT VD ) breaths / min

(500 150 ) 12

≈ 4.2 L / min

Ventilation : Perfusion

Matching

Pulmonary circulation

flow rate in systemic circulation

=flow rate in

pulmonary circulation

blood pressure in pulmonary circulation

mean ≈ 15 mmHg

<<blood pressure in

systemic circulation

mean ≈ 100 mmHg

Even with exercise

Mean P < 30 mmHg

Shunts = blood that returns to L side of heart without being oxygenated

~ anatomical: ≈ 2%

directly to heart from central chest tissues

~ physiologic / pathologic:

blood from areas of lung where

blood flow > airflow

so some blood flow wasted

Factors which tend to disturb

ventilation / perfusion ratio:

• non-uniform pulmonary blood flow

• non-uniform ventilation

• pathological

gravity effects

Effects of Gravityat normal breathing volumes:1. change in volume at lung bases is greater than at apicesi.e. alveolar ventilation > alveolar ventilation

(at bases) (at apices)2. blood flow (perfusion) > blood flow (perfusion)

(at bases) (at apices)

but Overall

Base perfusion > ventilation

Apex ventilation > perfusion

Protective reflexes which match ventilation with perfusion:

1. Hypoxic pulmonary vasoconstriction – NBif blood flow > airflow

2. PCO2 control of airway smooth muscleif airflow > blood flow

PO2 in lung tissuevasoconstriction

blood flow

PCO2 in lung tissue

broncho-constriction air flow

Gas Exchange (summary)

• Alveolar vs total ventilation• Alveolar gas composition (partial pressure)• Gas transfer

partial pressure of gas in liquidrate of gas diffusiondiffusing capacitypulmonary circulationventilation : perfusion matching

dead spaceshuntsprotective reflexes