GAS
EXCHANGE
• Gaseous exchange refers to the exchange of
gases, namely Oxygen and Carbon Dioxide and
relies on a process called diffusion.
• Diffusion is the movement of gases from an
area of high pressure to low. The difference
between the high and low pressure is called the
diffusion gradient
• The bigger the diffusion gradient the greater
the diffusion and gaseous exchange that takes
place.
DEFINITIONS
Partial Pressure:
“Air is made up of several gases, each of these
gases make up air as a whole, individually they
exert part (partial) of total air pressure”
Partial pressure =
Barometric pressure x Fractional pressure
e.g. pO2 = 760 x 0.21 = 159.6 mmHg
Work out the partial pressures for:
1. Co2 (carbon dioxide)
2. N2 (Nitrogen)
3. H2o (Water)
The partial pressures of the 4
gases add up to 760mm Hg.
Dalton’s Law; in a mixture of
gases, the total pressure equals
the sum of the partial pressures
exerted by each gas.
Oxygen Transport
• O2 is transported by the blood either,
– Combined with haemoglobin (Hb) in the red blood cells
(>98%) or,
– Dissolved in the blood plasma (<2%).
Oxygen Transport
• The resting body requires 250ml of O2 per
minute.
• We have four to six billion haemoglobin
containing red blood cells.
• The haemoglobin allows nearly 70 times more
O2 than dissolved in plasma.
The Site of Gas Exchange
• External Respiration
• Pp of O2 higher in alveoli than
pulmonary capillaries so O2 diffuses
to blood
• Pp of CO2 diffuses from pulmonary
capillaries into alveoli
•Internal respiration
•Pp of CO2 in muscle tissues is higher
than blood so diffuses into capillaries
•Pp of O2 higher in blood than muscle
tissues so diffuses from capillaries into
muscle tissue
Factors Influencing External Respiration
Efficient external respiration
depends on 3 main factors
1) Surface area and structure
of the respiratory
membrane
2) Partial Pressure gradients
3) Matching alveolar airflow
to pulmonary capillary
blood flow.
Gas exchange at the lungs
There is imbalance between gases in the alveoli
and the blood that causes a pressure gradient
which results in a movement of gases across the
respiratory membrane.
Movement is 2 way – oxygen from alveoli to
blood and carbon dioxide from blood to alveoli
The diffusion gradient
As oxygen is moved around the body to the
muscle tissues for use, the partial pressure it
exerts is greatly reduced (high to low pressure)
(po2)
Oxygen in atmosphere air - 159 mmHg
Oxygen in the Alveoli
- 105 mmHG
Oxygen in the arteries
- 100 mmHg
Oxygen in the body cells - 40 mmHg
Oxygen back to lungs - 40mmHg
This is mirrored in the sense that as the muscle
cells produce carbon dioxide where co2 pressure
is at its highest it travels to the lungs where
pressure expels it out of the body when we
breath
With altitude there is a decrease in atmospheric
pressure but the percentage of gases within air
remains the same
At sea level:
Resting ppO2 in the arteries is 100mmHg
At the muscle tissue at sea is 40mmHg
At altitude of 8000 feet:
Resting ppO2 in the arteries falls to 60mmHg
At the muscle tissue at altitude is still 40mmHg
Pressure gradient falls from 60 to 20 therefore
oxygen movement is greatly reduced
Endurance training
Long distance runners etc may train at high
altitude based on the principle that the body
will create more red blood cells and
haemoglobin to adapt to oxygen shortage at
muscles
Although recent research suggests living at
altitude and training at sea level maybe more
advantageous
Haemoglobin
Haemoglobin molecules
can transport up to four
O2’s
When 4 O2’s are bound to
haemoglobin, it is 100%
saturated, with fewer O2’s it is
partially saturated.
Co-operative binding:
haemoglobin’s affinity
for O2 increases as its
saturation increases.
Oxygen binding occurs in
response to the high PO2
in the lungs
Lets Now Look at Haemoglobin Saturation
• Haemoglobin saturation is the amount of
oxygen bound by each molecule of
haemoglobin
• Each molecule of haemoglobin can carry four
molecules of O2.
• When oxygen binds to haemoglobin, it forms
OXYHAEMOGLOBIN;
• Haemoglobin that is not bound to oxygen is
referred to as DEOXYHAEMOGLOBIN.
Haemoglobin Saturation
• The binding of O2 to haemoglobin
depends on the PO2 in the blood and the
bonding strength, or affinity, between
haemoglobin and oxygen.
• The graph on the following page shows
an oxygen dissociation curve, which
reveals the amount of haemoglobin
saturation at different PO2 values.
The Oxygen Disassociation Curve
In the lungs the partial
Haemoglobin saturation is determined
pressure is approximately
by the partial pressure of oxygen.
When
100mm
Hg at this Partial
haemoglobin
these values are graphed theyPressure
produce
has a high affinity to 02
the Oxygen Disassociation and
Curve
is 98% saturated.
In the tissues of other organs
a typical PO2 is 40 mmHg
here haemoglobin has a
lower affinity for O2 and
releases some but not all of
its O2 to the tissues. When
haemoglobin leaves the
tissues it is still 75%
saturated.
Haemoglobin Saturation at High Values
Lungs at sea level: PO2
of 100mmHg
haemoglobin is 98%
SATURATED
When the PO2 in the lungs
declines below typical sea
level values, haemoglobin
still has a high affinity for
O2 and remains almost
fully saturated.
Lungs at high
elevations: PO2 of
80mmHg,
haemoglobin 95
% saturated
Even though PO2
differs by 20 mmHg
there is almost no
difference in
haemoglobin
saturation.
Haemoglobin Saturation at Low Values
What effects effective dissociation
between oxygen and haemoglobin?
1.
2.
3.
4.
Fall in the pO2 within the muscle
Increase in blood and muscle temperature
Increase in pCO2 within the muscle
Fall in pH due to lactic acid production
Factors Altering Haemoglobin Saturation
(Exercise)
The Bohr Shift
Gas exchange at the muscles
This is very similar to exchange at the lungs, high
to low pressure gradient.
At the muscles the oxygen attaches to myoglobin
rather than haemoglobin and is taken
to the mitochondria within the cell to be used
(Slow twitch muscle fibres have higher amounts of
myoglobin)