Occurs by bulk flow of gasses and by diffusion of
gasses through tissue
H6.1 Define partial pressure
Dalton’s Law of partial pressure states that the
total pressure exerted by a mixture of gasses is
the sum of the pressures exerted independently
by each gas in a mixture
 Furthermore, the pressure exerted by each gas, its
partial pressure, is directly proportional to its
percentage in the total gas mixture.
 Atmospheric pressure is approximately 760 mm Hg
(760 torrs) at sea level and since air is a mixture of
gases, we can look at it more closely in terms of the
gases that make it up
 Partial pressures are calculated by multiplying the
fractional composition of each gas in the air by the
atmospheric pressure
 For example:
PN2 = 760 x 79.02% = 600.6 mm Hg
PO2 = 760 x 20.95% = 159.2 mm Hg
PCO2 = 760 x 0.03% = 0.2 mm Hg
***different texts give slightly different partial pressures
If there is a drop in air pressure, such as is seen at high
altitudes, there is also a drop in the partial pressure of
oxygen (pO2)
Although the air at higher altitudes still contains 20.95%
oxygen, the atmospheric pressure is less influenced by
gravity, and results in a drop in ALL partial pressure values
in proportion to the decrease in atmospheric pressure
The saturation of oxygen is then lowered and thus, the
amount of oxygen that is transported; the body compensates
over time by increasing the number of red blood cells (We
will talk about this again at a later date)
H6.2 Explain the oxygen dissociation curves of
adult and fetal hemoglobin and myoglobin
• Molecular oxygen is carried in blood in two
ways; bound to hemoglobin within rbcs and
dissolved in plasma (only about 1.5%)
• 98.5% of oxygen sent out from the lungs to
tissues is carried in a loose chemical combination
with hemoglobin
• Each hemoglobin molecule can combine with 4
oxygen molecules; the process is rapid and
reversible
• Attachment of the first oxygen molecule
changes the shape of Hgb allowing for faster
attachment of the next 2 oxygen molecules and
even faster attachment of the 4th, therefore,
affinity of Hgb for oxygen changes with its
state of saturation
 The hemoglobin-oxygen combination is called oxyhemoglobin
and is written HbO2
 Hemoglobin that has released oxygen is called reduced
hemoglobin or deoxyhemoglobin and is written HHb
 Loading and unloading of oxygen can be indicated by a single
reversible equation:
lungs
HHb + O2
↔
HbO2 + H+
Tissues
Materno-fetal respiratory gas exchange – showing the saturation of Hgb at various partial
pressure of O2
Fig 1.4 Oxygen dissociation curves for human maternal and fetal blood, indicating the
physiologic range of PO2 and O2 for mother and fetus. (Modified from Towell ME: Fetal
respiratory physiology in Perinatal Medicine.1976 Edited by JW Goodwin, GW Chance:
Longman; Toronto, Canada.)
Although fetal partial pressure of oxygen is much lower, the saturation is relatively higher than in
the adult. This is because fetal haemoglobin (75% - 80% of the haemoglobin at birth) has a
greater affinity for oxygen than adult haemoglobin. The fetal oxy-haemoglobin dissociation curve
is displaced to the left (see fig 1.4).
Important shifts of the dissociation curves take place in the placenta. The maternal blood gains
CO2, the pH falls and the curve shifts to the right releasing additional oxygen. On the fetal side of
the placenta CO2 is lost, the pH rises and the curve shifts to the left allowing additional oxygen
uptake (double Bohr effect).- the effect of pH on Hgb affinity for O2
*** Increased pH  increased O2 uptake
Decreased pH  oxygen unloading

Oxygen Dissociation Curves
The graph illustrates what percent of the proteins
present have bound O2 at a particular PO2
Figure 4·45 Oxygen-binding curves of
myoglobin and hemoglobin.
(from Horton et al 1996 "Principles of
Biochemistry", Prentice Hall)
• The fraction of each protein
saturated with oxygen, Y, is
plotted against the partial
pressure of oxygen, pO2.
• Myoglobin is an oxygen binding
pigment in muscle
• When hemoglobin carrying
oxygen reaches the muscles, the
oxygen is taken over and stored
by myoglobin
• To be able to do this, myoglobin
must have a higher affinity for
oxygen than hemoglobin (at
equal pO2) but still be able to
release oxygen when the muscles
need it (during exercise)
H6.3 Describe how carbon dioxide is carried by the
blood, including the action of carbonic anhydrase,
the chloride shift, and buffering by plasma proteins
• Systemic capillaries deliver oxygen to the tissues and remove
carbon dioxide
• About 8% of CO2 in the blood is dissolved in plasma with
another 20% bound to hemoglobin (carbaminohemoglobin)
• Because carbon dioxide binds to the protein portion of
hemoglobin and not to the heme irons, it does not compete
with oxygen
• It does, however, cause hemoglobin’s shape to change and
thus, lowers its affinity for oxygen
 The remaining 72% of carbon dioxide diffuses into rbcs,
where the enzyme carbonic anhydrase catalyzes the
combination of carbon dioxide with water to form
carbonic acid (H2CO3) (unstable)
 Carbonic acid dissociates into bicarbonate (HCO3-) and
hydrogen (H+) ions
 The hydrogen ion binds to deoxyhemoglobin, and the
bicarbonate moves out of the rbc and into the plasma via a
transporter that exchanges one chloride ion for a
bicarbonate (the chloride shift)
 This reaction removes large
amounts of carbon dioxide
from the plasma and
facilitates the diffusion of
additional carbon dioxide
into the plasma from
surrounding tissues
 Formation of carbonic acid is
also important in maintaining
the acid-base balance of the
blood
 The bicarbonate acts as the
major buffer of the blood
plasma
• Carbon dioxide is carried in the
lungs in these forms
•The lower pCO2 of the air inside
the alveoli causes the carbonic
anhydrase reaction to reverse
direction, converting carbonic acid
into water and carbon dioxide
• The carbon dioxide diffuses out
of the rbcs and into the alveoli,
and leaves the body with the next
exhalation
H6.4 Explain the role of the Bohr shift in the supply
of oxygen to respiring tissues
• The Bohr effect is the release of oxygen by
hemoglobin molecules in response to the
surrounding elevated levels of carbon dioxide
• The Hgb-O2 bond is weakened leading to increased
oxygen unloading where it is needed
• When carbonic acid dissociates into bicarbonate
and hydrogen ions, pH is lowered
• This lowered pH reduces hemoglobin’s affinity for
oxygen, and oxygen is released more readily
 The effect of the lowered pH causes a shift of the
oxyhemoglobin dissociation curve to the right
 Increasing temperature has a similar effect on
hemoglobin’s affinity for oxygen
 Because skeletal muscles produce carbon dioxide
more rapidly during exercise and active muscles
produce heat, the blood unloads a high percentage of
the oxygen that it carries to the tissues during exercise
(See Fig. 44.33)
Figure 4·49 Bohr effect. Lowering the pH
decreases the affinity of hemoglobin for
oxygen.
(from Horton et al 1996 “Principles of
Biochemistry”, Prentice Hall)
 The more energy the cells need, the more cellular
respiration takes place and the more carbon dioxide is
produced
 The increased carbon dioxide concentration will reduce
the saturation of hemoglobin and release oxygen to the
cells
H.6.5 Explain how and why ventilation rate varies
with exercise
• During exercise, more oxygen is used and more carbon
dioxide is produced
• Each breath is initiated by neurons in the respiratory control
center of the brain stem which sends impulses to the
diaphragm and intercostal muscles, stimulating them to
contract which expands the chest cavity resulting in
inspiration
• Although the breathing cycle is driven by neurons in the brain
stem, the neurons must be able to respond to changes in blood
pO2 and pCO2 in order to maintain homeostasis
• A rise in pCO2 causes an increase in carbonic acid
which dissociates to bicarbonate and hydrogen
ions causing a drop in blood pH
• This fall in pH, stimulates chemoreceptors in the
aortic arch (aortic bodies) and at the bifurcation
point of the common carotid arteries (carotid
bodies) to send impulses to the respiratory center
in the brain stem (medulla), which in turn,
responds by increasing the breathing rate
• Chemoreceptors in the brain do not respond to
blood pH, but do respond after a brief delay, to
increased carbon dioxide levels in the CSF
H6.6 Outline the possible causes of asthma and its
effect on the gas exchange system
• Asthma is a chronic lung condition that is characterized by
inflammation of the air passages in the lungs.
• The condition presents with episodes of coughing, difficulty
breathing, wheezing, and chest tightness, alone or in
combination; often these episodes are accompanied by a sense
of panic
• Although hard to pin down, it is believed that asthma is
caused by bronchospasms triggered by factors such as cold
air, exercise, pollens, or other allergens, and viral illness
• It has also been found that the bronchospasms have little
effect on air flow through the lungs, and that inflammation
of the bronchi usually occurs before the bronchospasms
and that the inflammation is probably the result of an
immune response
• However, once airways are thickened with
inflammatory exudate, the effect of
bronchospasm is magnified dramatically,
reducing air flow
• Many asthma sufferers live in industrial areas
H6.7 Explain the problem of gas exchange at high
altitudes and the way the body acclimatizes
• When you travel quickly from sea level to elevations
above 8000 ft., where air density and pO2 levels are
lower, your body responds with symptoms of acute
mountain sickness which include headaches,
shortness of breath, nausea, and dizziness; can lead
to pulmonary and cerebral edema and may be fatal
• When you move on a long term basis from sea
level to the mountains, your body begins to make
respiratory and hematopoietic adjustments in a
process called acclimatization (stabilization
occurs within 2-3 days)
• When blood oxygen tension declines, the kidneys
intervene by accelerating the production of
erythropoietin, which stimulates bone marrow
production of rbcs
• Peripheral chemoreceptors are stimulated and the
brain attempts to restore gas exchange to previous
levels by increasing the ventilation rate
• People living permanently at high altitude have greater
lung surface area and larger vital capacity* than those
living at sea level, and more red blood cells as well as a
larger chest size
*vital capacity: the volume of air that can be expelled from
the lungs by forcible expiration after the deepest inspiration;
total exchangeable air