The Oxygen Cascade I

Question 1

3 important things happen to the air

we breathe in:

Answer 1

O2% decrease
CO2% increase
H2O% increase

Question 2

O2 Cascade: a succession of stages

Answer 2

Inspired air:
PiO2 » 150 mm Hg
PiCO2 » 0.3 mm Hg
Alveolar air:
PAO2 » 105 mm Hg 
PACO2 » 40 mm Hg 
Arterial blood:
PaO2 » 100 mm Hg
PaCO2 » 40 mm Hg 
Mixed venous:
PvO2 » 40 mm Hg 
PvCO2 » 45 mm Hg

Question 3

Dalton’s Law of Partial Pressures

Answer 3

The TOTAL pressure of a gas is simply the
sum of the individual partial pressures (Pi) of
each constituent gas:

Question 4

• ______air contains virtually no CO2. Therefore, all

______CO2 must come from metabolism.

Answer 4

Inspired

alveolar

Question 5

• However, ____depends not only on how fast O2 is

utilized, but also on the kind of fuel metabolised.

Answer 5

VCO2

Question 6

• For ________(i.e. at very high work rates):
1 molecule of CO2 is produced for every
molecule of O2 consumed.

Answer 6

carbohydrate

Question 7

• Respiratory exchange ratio (R) =

Answer 7

VCO2/VO2
In steady state R = Respiratory Quotient
(RQ, measured at the tissue/blood compartment.)

Question 8

VE is equal to VI only if

Answer 8

R is equal to 1 (i.e. carbohydrate)

Question 9

Oxygen uptake can be expressed as

Answer 9

oxygen

consumption per kilogram of body weight.

Question 10

Oxygen “uptake” Is the most relevant measure of the

Answer 10

cardiorespiratory system.

Question 11

• Maximal oxygen uptake (VO2 max), or maximal

aerobic capacity, is the maximum rate of

Answer 11

oxygen consumption possible by an individual.

Question 12

Fick Principle

Answer 12

Oxygen uptake into pulmonary circulation is dependent on the

aterio-venous O2 difference and cardiac output (Q).

Question 13

The coupling between pulmonary convective
mechanisms and metabolic demand can be expressed
by

Answer 13

the alveolar gas equations.

Question 14

Alveolar Gas Equation
• Clinically it is useful to know both the ___ gradient and
the amount of__________

Answer 14

“A-a”

“right-to-left shunt”.

Question 15

• PAO2 therefore needs to be known, but this is difficult to ______.
• Instead, the ________________ are commonly
used.

Answer 15

measure

Alveolar Gas Equation

Question 16

The Perfect Lung PO2

Answer 16

PO2 atm steps down alveoli, contant for artery. Steps down to mitochondria

Question 17

The realistic Lung oxygen cascade

Answer 17

PO2 atm steps down upper airway to alveoli, step down to cap. step down to artery. Steps down to tissue and mitochondria

Question 18

If PACO2 is known, then___________

can be used to calculate PAO2.

Answer 18

the alveolar gas equation

Question 19

For a well ventilated lung, PACO2 approx =

Answer 19

PaCO2.

Question 20

Alveolar Ventilation

eqn

Answer 20

VA (dot)= fR x (VT – VD)

Question 21

At rest, changes in ____gases are small, since

VT/FRC small.

Answer 21

alveolar

Question 22

How do these alter PACO2?
Hyperventilation:
Hypoventilation:

Answer 22

decr. PACO2 , and therefore
¯PaCO2
incr ­PACO2 , and therefore
­PaCO2

Question 23

Drops in the realistic Lung oxygen cascade (alveoli, cap, artery) due to

Answer 23

Ventilation & Perfusion inequalities or shunt

Question 24

Ventilation in alveoli is matched to Perfusion through pulmonary capillaries

Answer 24

Perfusion

Question 25

Hypoxemia

definition

Answer 25

Abnormally low levels of oxygen (partial pressure,

content or % saturation) in arterial blood.

Question 26

Hypoxemia

Common causes:

Answer 26

Hypoventilation (voluntary or pathological)
Diffusion limitation
Reduced PIO2
V/Q mismatch
Right to left shunt

Question 27

A Shunt refers to

Answer 27

blood which enters
the arterial system
without going
through the
ventilated areas of
the lung.

Question 28

Very Large Shunt
Normally only a _____
% of total cardiac output
(Qs/QT)

Answer 28

very small

Question 29

Very Large Shunt Most commonly arises from

Answer 29

congenital heart

abnormalities.

Question 30

Venous-to-arterial (rightto-left) circulatory shunts

may result in severe?

Answer 30

hypoxaemia

Question 31

Blood Flow (perfusion):
When standing, gravitational effects mean that blood
flow

Answer 31

decreases from the base (bottom) to the apex

(top) of lungs.

Question 32

Ventilation-Perfusion Ratio
V(dot)/Q ratio takes into account _______variations in V(dot)A and
capillary perfusion.

Answer 32

regional

eg. effects of gravity

Question 33

Ventilation:
Similarly, when standing, gravitational effects mean
that ventilation

Answer 33

decreases from the base to the apex
of the lung, but to a much lesser extent than the
affect on blood flow.
ie V/Q is lower at the base of the lung.

Question 34

Pulmonary Hypoxic Vasoconstriction

Answer 34

Decreased tissue PO2 around underventilated alveoli constricts their arterioles, diverting blood to better ventilated alveoli

Question 35

At the apex, low arterial pressure in the pulmonary
circulation tends to collapse the small vessels,
____ blood flow.

Answer 35

incr­___Resistance

decr

Question 36

At base of lungs, higher pressure distends vessels,

____­blood flow.

Answer 36

decr___Resistance

incr