The Oxygen Cascade I Flashcards
3 important things happen to the air
we breathe in:
O2% decrease
CO2% increase
H2O% increase
O2 Cascade: a succession of stages
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
Dalton’s Law of Partial Pressures
The TOTAL pressure of a gas is simply the
sum of the individual partial pressures (Pi) of
each constituent gas:
• ______air contains virtually no CO2. Therefore, all
______CO2 must come from metabolism.
Inspired
alveolar
• However, ____depends not only on how fast O2 is
utilized, but also on the kind of fuel metabolised.
VCO2
• For ________(i.e. at very high work rates):
1 molecule of CO2 is produced for every
molecule of O2 consumed.
carbohydrate
• Respiratory exchange ratio (R) =
VCO2/VO2
In steady state R = Respiratory Quotient
(RQ, measured at the tissue/blood compartment.)
VE is equal to VI only if
R is equal to 1 (i.e. carbohydrate)
Oxygen uptake can be expressed as
oxygen
consumption per kilogram of body weight.
Oxygen “uptake” Is the most relevant measure of the
cardiorespiratory system.
• Maximal oxygen uptake (VO2 max), or maximal
aerobic capacity, is the maximum rate of
oxygen consumption possible by an individual.
Fick Principle
Oxygen uptake into pulmonary circulation is dependent on the
aterio-venous O2 difference and cardiac output (Q).
The coupling between pulmonary convective
mechanisms and metabolic demand can be expressed
by
the alveolar gas equations.
Alveolar Gas Equation
• Clinically it is useful to know both the ___ gradient and
the amount of__________
“A-a”
“right-to-left shunt”.
• PAO2 therefore needs to be known, but this is difficult to ______.
• Instead, the ________________ are commonly
used.
measure
Alveolar Gas Equation
The Perfect Lung PO2
PO2 atm steps down alveoli, contant for artery. Steps down to mitochondria
The realistic Lung oxygen cascade
PO2 atm steps down upper airway to alveoli, step down to cap. step down to artery. Steps down to tissue and mitochondria
If PACO2 is known, then___________
can be used to calculate PAO2.
the alveolar gas equation
For a well ventilated lung, PACO2 approx =
PaCO2.
Alveolar Ventilation
eqn
VA (dot)= fR x (VT – VD)
At rest, changes in ____gases are small, since
VT/FRC small.
alveolar
How do these alter PACO2?
Hyperventilation:
Hypoventilation:
decr. PACO2 , and therefore
¯PaCO2
incr PACO2 , and therefore
PaCO2
Drops in the realistic Lung oxygen cascade (alveoli, cap, artery) due to
Ventilation & Perfusion inequalities or shunt
Ventilation in alveoli is matched to Perfusion through pulmonary capillaries
Perfusion
Hypoxemia
definition
Abnormally low levels of oxygen (partial pressure,
content or % saturation) in arterial blood.
Hypoxemia
Common causes:
Hypoventilation (voluntary or pathological) Diffusion limitation Reduced PIO2 V/Q mismatch Right to left shunt
A Shunt refers to
blood which enters the arterial system without going through the ventilated areas of the lung.
Very Large Shunt
Normally only a _____
% of total cardiac output
(Qs/QT)
very small
Very Large Shunt Most commonly arises from
congenital heart
abnormalities.
Venous-to-arterial (rightto-left) circulatory shunts
may result in severe?
hypoxaemia
Blood Flow (perfusion):
When standing, gravitational effects mean that blood
flow
decreases from the base (bottom) to the apex
(top) of lungs.
Ventilation-Perfusion Ratio
V(dot)/Q ratio takes into account _______variations in V(dot)A and
capillary perfusion.
regional
eg. effects of gravity
Ventilation:
Similarly, when standing, gravitational effects mean
that ventilation
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.
Pulmonary Hypoxic Vasoconstriction
Decreased tissue PO2 around underventilated alveoli constricts their arterioles, diverting blood to better ventilated alveoli
At the apex, low arterial pressure in the pulmonary
circulation tends to collapse the small vessels,
____ blood flow.
incr___Resistance
decr
At base of lungs, higher pressure distends vessels,
____blood flow.
decr___Resistance
incr
