Elina_Physiology_Alveolar-Blood Gas Exchange

Front 1

Partial pressure P gas in ambient air

Back 1

Pgas '" Fgas X Patm

Parm '" atmospheric pressure; Pgas '" partial pressure of a gas; Fgas :::= concentration of a gas

The POz in ambient air: POz = 0.21 X 760'" 160 mm Hg

Front 2

Partial pressure ofa gas in inspired air

Back 2

Plgas = Fgas (Patrn - H20)

Plgas > partial pressure of inspired gas; PH20 =partial pressure of H20 vapor

Inspired air is defined as air that has been inhaled, warmed to 37'C, and completely humidified.

Example:The P02 of inspired air: PlOl :::: 0.21(760 - 47) = 150mm Hg

Front 3

The partial pressure of H20 is

Back 3

The partial pressure of H20 is dependent only on temperature and at 37'C is 47 mm Hg.

Front 4

Under normal conditions, the P02 and PC02 in the alveolar compartment and pulmonary end
capillary blood are

Back 4

Under normal conditions, the P02 and PC02 in the alveolar compartment and pulmonary end
capillary blood are the same.

Front 5

Alveolar-systemic arterial differences (usually PO2) == A-a. often provides information
about the cause of

Back 5

Alveolar-systemic arterial differences (usually PO2) == A-a. This difference often provides information
about the cause of a hypoxemia.

Front 6

FACTORS AFFECTING ALVEOLAR PCO2

Back 6

two factors affect alveolar PC02-
1- metabolic rate
2- alveolar ventilation

Front 7

PAC02 ~
зависимость от metabolic rate
и alveolar ventilation

Back 7

PAC02 ~ metabolic C02production
alveolar ventilation

Front 8

Alveolar Ventilation

Back 8

There is an inverse relationship between PAC02 and alveolar ventilation

if ventilation increases, PAC02 decreases; if ventilation
decreases, PAC02 increases.

Front 9

Hyperventilation

Back 9

During hyperventilation, there is an inappropriately elevated level of alveolar ventilation, and
PAC02 is depressed.
IfVA is doubled, then PAC02 is decreased by half.
c.g., PAC02 == 40 mm Hg
2 x VA; PAC02 == 20 mm Hg

Front 10

Hypoventilation

Back 10

Hypoventilation
During hypoventilation, there is an inappropriately depressed level of alveolar ventilation, and
PAC02 is elevated.
nYA is halved, then PAC02 is doubled.
<s- PAC02 =40 mm Hg
lf2 VA; PAC02 = 80 mm Hg

Front 11

Metabolic Rate

Back 11

Metabolic Rate
There is a direct relationship between alveolar PC02 and body metabolism. For PAC02 to
remain constant, changes in body metabolism must be matched with equivalent changes in
alveolar ventilation.
IfVA matches metabolism, then PAC02 remains constant


during exercise, if body metabolism doubles, then VA must double if PACOz is to
remain constant.
If body temperature decreases and there is no change in ventilation, PACOz will decrease, and
the individual can be considered to be hyperventilating

Front 12

FACTORS AFFECTING ALVEOLAR POl
The alveolar gas equation
Three important factors can affect PA02;

Back 12

The alveolar gas equation includes all the factors that can affect alveolar PO2

PACOz
PA02 = (Palm - 47)FlO - -R

Patm FIO2 PACO2

Front 13

Patm -important factors can affect PA02;

Back 13

Patm ::;; atmospheric pressure, at sea level 760 mm Hg
An increase in atmospheric pressure will increase alveolar POz> and a decrease
(high altitude) will decrease alveolar pal.

Front 14

FIOz -important factors can affect PA02;

Back 14

FIOz =; fractional concentration of oxygen, room air 0.21
An increase in inspired oxygen concentration will incre.ase alveolar POr

Front 15

PACOz-important factors can affect PA02;

Back 15

PACOz= alveolar pressure of carbon dioxide, normally 40 mm Hg
An increase in alveolar PCOI will decrease alveolar paz, and a decrease will increase
. alveolar POr For most purposes, you can use arterial carbon dioxide (PACOz) in the
calculation.

Front 16

The fourth variable is R

Back 16

The fourth variable is R.It varies from 0.8 to 1.0.


R =respiratory exchange ratio = CO2produced mLmin.\ 02 consumed mL\min =normally 0.8

Example: person breathing room air at sea level
PA0 2
"" (760 - 47) 0.21 - 40/O.8 =100 mm Hg

Front 17

The Effect of PAC02 on PA02

Back 17

PI02 = P inspired 02 i.e., the PO2 in the conducting airways during inspiration
Because PACO2 affects alveolar PO2, hyperventilation and hypoventilation also affect PA02 •

Front 18

Hyperventilation (e.g., PACOz = 20 mm Hg)

Back 18

Hyperventilation (e.g., PACOz = 20 mm Hg)
PAO2 =PIO2 - PACO2 (assume R = I)
normal = 150 - 40 = 110 mm Hg
hyperventilation -= 150- 20 = 130 mm Hg

Front 19

Hypoventilation (e.g., PAC02 = 80 mm Hg)

Back 19

Hypoventilation (e.g., PAC02 = 80 mm Hg)
normal = ISO - 40 = 110 mm Hg
hypoventilation = 150 - 80 = 70 mm Hg

Front 20

Simple diffusion is

Back 20

Simple diffusion is the process of gas exchange between the alveolar compartment and pulmonary
capillary blood.

Front 21

FICK LAW OF DIFFUSION

Back 21

Vgas is the rate of gas diffusion

Vgas= A X D X(P1-P2)
T

Front 22

factors affect the rate of diffusion.

Back 22

Two structural factors and two gas factors affect the rate of diffusion.

Front 23

Structural Features That Affect the Rate of Diffusion

Back 23

A = surface area for exchange, 1.decrease in emphysema, increase in exercise
T = thickness of the membranes between alveolar gas and capillary blood, i in fibrosis and
many other restrictive diseases


a loss of surface area and/or
an increase in the thickness of the membrane system between the alveolar air and the pulmonary
capillary blood.----the rate of oxygen and carbon dioxide diffusion decreases

Front 24

Factors That Are Specific to Each Gas Present

Back 24

D(diffusion constant) =main factor is solubility
The only clinically significant feature of D is solubility. The more soluble the gas, the faster it
will diffuse across the membranes. CO2 is the most soluble gas with which we will be dealing.
The great solubility of CO2 is the main reason why it diffuses faster across the alveolar membranes
than 02'

Gradient across the membrane
(PI - P2 ) : This is the gas partial pressure difference across the alveolar membrane. The greater
the partial pressure difference, the greater the rate of diffusion.

Front 25

gradient for 02 is:

Back 25

100 - 40 = 60 mm Hg


An increase in the P02 gradient across the lung membranes will compensate for a structural
problem. If supplemental 02 is administered, alveolar P02 will increase, along with the gradient.
This increased gradient will return the rate of O2 diffusion toward normal. The greater the
structural problem, the greater the gradient necessary for a normal rate of 02 diffusion.

Front 26

The gradient for CO2 is

Back 26

The gradient for CO2 is
47 - 40 = 7 mm Hg.
Even though the gradient for CO2 is less than for 02' CO2 still diffuses faster because of its
greater solubility

Front 27

DIFFUSION CAPACITY OF THE LUNG (OLCO)
There are two terms to describe the dynamics of the transfer of individual substances between
the interstitium and the capillary:

Back 27

• If the substance equilibrates between the capillary and the interstitium, it is said to be
in a perfusion-limited situation.
• If the substance does not equilibrate between the capillary and the interstitium, it is
said to be in a diffusion-limited situation

Front 28

DIFFUSION CAPACITY OF THE LUNG (OLCO)

Carbon monoxide is

Back 28

Carbon monoxide is a unique gas in that it can never equilibrate between the alveolar air and
the capillary blood. In other words, it is always in a diffusion-limited situation.

Front 29

partial pressure Carbon monoxide in the plasma is

Back 29

the amount dissolved in the plasma
is zero (therefore, partial pressure in the plasma is zero).

Front 30

VCO=

Back 30

VCO = A X D X PACO
T

Front 31

In a young individual with normal lung surface area (A) and thickness (T), a I-mm Hg gradient
of carbon monoxide will produce an uptake of

Back 31

In a young individual with normal lung surface area (A) and thickness (T), a I-mm Hg gradient
of carbon monoxide will produce an uptake of 25 mLlmin.

Front 32

DL (rate of CO diffusion) decreases in

Back 32

DL (rate of CO diffusion) decreases in emphysema and fibrosis

Front 33

DL (rate of CO diffusion) increases

Back 33

increases during exercise.