Functions of the
I. Ventilation - Movement of air into
(inspiration) and out of (expiration) the respiratory tract.
A. Contractions of respiratory
muscles change the volume of the thoracic cavity.
Pail Handle Effect increases the thoracic cavity laterally.
a. Contraction of external intercostal muscles is due to stimulation by
b. Ribs lift up and swing out on the sides of the thorax.
Pump Handle Effect increases the volume of the thoracic cavity in the
posterior dimension. As the ribs move up, the sternum moves out.
Diaphragm increases the thoracic cavity in the superior to inferior dimension.
a. Contraction of the diaphragm is due to stimulation by the phrenic
b. Diaphragm pushes down and flattens, enlarging the thoracic cavity.
B. Adhesion between pleural
water lining the insides of the parietal and visceral pleural causes the two
to bond to each other. This is due to the polarity of the water
outer parietal membrane is fused to the inner chest wall.
visceral pleura is fused to the lung's exterior.
4. As the
chest wall moves out due to the actions of the external intercostals and
the lung is stretched out.
ability of the lung to stretch depends on the elasticity of the lung which is
disease and aging.
C. Boyle's Law
- As the volume occupied by a gas increases, the pressure of that
decreases. Mathematically speaking, the pressure
on a contained gas and the volume it
occupies are inversely proportional.
P = 1/V
1. As the lung stretches out the
pressure of air in the intrapulmonic space decreases.
2. When intrapulmonic pressure decreases below atmospheric pressure (760
mm Hg), air
moves into the respiratory tract from
the outside producing inspiration.
3. When the respiratory muscles relax, the thorax becomes smaller.
This causes the
pressure of the air in the
intrapulmonic space to exceed the air pressure outside. This results
in air moving out of the respiratory
tract producing passive expiration.
Tidal Breathing - When we are at rest we use
the external intercostals and the diaphragm to inspire. Expiration is
passive. The muscles of inspiration simply relax.
Forceful Breathing - With exertion or
exercise we use additional muscles for inspiration and expiration.
II. Gas Exchange
External Respiration - Consists of movement of oxygen from
the alveolar space into the blood and movement of carbon dioxide from the blood
into the alveolar space.
Factors Influencing the Diffusion of Gases During
1. Pressures of the gases - Gases will show a
net movement of molecules along a pressure gradient from high to low. In
this regard, each gas will follow Dalton's Law which states that each gas in a
gas mixture will behave independently of the other gases in the mixture.
At the same time, the total pressure of a gas mixture is equal to the sum of the
pressures that each gas would exert independently. For example,
P(total) = P(nitrogen) + P(oxygen) +
P(CO2) + P(water vapor)
pressure of oxygen in the alveolar space will drive this gas into the blood.
The higher pressure of CO2 will cause a net
movement of this gas into the alveolus.
Solubility of the gases in water (Henry's Law) - The amount of gas in the
liquid depends on the solubility of each gas in that liquid. There are a
number of factors that influence the solubility of a gas in a liquid:
a. The chemical reactivity of the gas with water - CO2
dissolves readily in water because it reacts rapidly forming
carbonic acid. Carbonic acid, in turn, dissociates into hydrogen and
CO2 + H2O produces
ionizes to form H+
The solubility of
CO2 in plasma, interstitial and alveolar
fluid compensates for the lack of a steep pressure gradient in permitting the
rapid diffusion of this gas from the blood. Oxygen is much less soluble in
water than CO2. However, the very
high affinity of hemoglobin for oxygen continues to remove the small amount of
dissolved O2 from these fluids. This
insures a continually high gradient and rapid diffusion of this gas.
b. Temperature - As
the temperature of water increases, the solubility of gases decreases.
When you open a warm bottle of soda much more gas bubbles out of solution than
from a cold soda. Since the fluids of our bodies are generally kept at
98.6 degrees F, the influence of temperature on the solubility of blood gases is
c. Partial pressure
- The partial pressure of the gas in the liquid and in the immediate environment
will determine how much gas remains in solution.
3. Thickness of
the alveolar-capillary membrane - The alveolar-capillary membrane consists
of the alveolar wall together with the capillary wall. It measures only
0.5 micrometers in thickness. That would be 1/2 of 1/1000 of the thickness
of a dime. As a result, diffusion of O2 and CO2 occurs very rapidly, in
about 0.25 seconds. Pulmonary edema will thicken the membrane and impede
the diffusion of these gases.
4. Surface Area
- The total surface area of the alveolar membranes is enormous, between 70
and 80 square meters. This greatly facilitates diffusion.
Respiratory diseases such as emphysema, tuberculosis, lung cancer and pneumonia
reduce gas exchange by decreasing functional surface area.
Internal respiration is the movement of oxygen from the blood to the tissues by
diffusion and the movement of CO2 from the tissues to the blood by
diffusion. All of the factors influencing external respiration at the
alveoli are at work at the blood tissue interface. The dominant factor
would be the partial pressures of the gases. Higher O2 partial
pressure in the blood drives oxygen into the tissues. Higher CO2
partial pressure in the tissues drives this gas into the blood.
III. Transport of Respiratory Gases in the
Transport of O2 - Most of the O2 is carried in the
blood combined with hemoglobin as oxyhemoglobin. As a result, the factors
which affect hemoglobin's ability to bind to O2 will have the
greatest impact on O2 transport in the blood.
Factors Influencing the Transport of Oxygen
(Refer to the Oxyhemoglobin
Dissociation Curve Interactive Tool )
1. Partial pressure of O2 - directly affects the binding
of oxygen and hemoglobin. The oxygen dissociation curve is a convenient
method of visualizing this relationship. After examining the dissociation
curve, a number of observations can be made.
a. Blood returning to the heart from the tissues has an oxygen partial
pressure of 40mmHg.
However, at this pressure, the hemoglobin (Hb) is still 75%
saturated with oxygen.
b. At an O2 partial pressure of 70mmHg, Hb is 95% saturated.
The significance of this is that at
high altitudes or in cases of cardiopulmonary disease
adequate oxygen transport can still occur.
c. During prolonged vigorous exercise, the blood has the capacity to
increase oxygen unloading
to the muscles and increase O2 pick up at the
alveoli without putting an undue strain on the
2. Partial pressure of CO2 - As the pCO2 in
the blood increases, the affinity of oxygen for Hb is reduced. This can be
seen in the shift of the O2 - Hb dissociation curve to the right
(Bohr shift). The chief consequence of the shift is that the pO2
needed to produce any given saturation of Hb must be greater in the presence of
increased CO2. In addition:
In rapidly metabolizing tissues, there will be an increased unloading of oxygen.
Anything that slows down the movement of CO2 from the blood to the
alveolus will decrease
oxygen loading. This will result in lower levels of
oxygen in the blood leaving the lungs.
3. pH - When
the hydrogen ion concentration in the blood increases and lovers the pH of the
blood below 7.35, the affinity of Hb for oxygen is reduced. Blood pH
levels below 7.35 are mostly associated with increases in pCO2 in the plasma and
CO2 + H2O produces
ionizes to form H+
- Higher than normal body temperature lowers the affinity of Hb for O2.
Actively metabolizing tissues are warmer than tissues at rest. As a
result, an increase in O2 unloading occurs in these more active
Transport of CO2
is carried by the
blood in three forms:
As bicarbonate - 70% of CO2 in the blood . In the plasma, bicarbonate is
formed by the
CO2 + H2O produces
H+ + H2CO3
Inside the RBC, this reaction is speeded up due to the enzyme carbonic
catalyzes the production of carbonic
acid. As a result, the level of bicarbonate inside the RBC
is always higher than in the plasma.
2. As carbaminohemoglobin - 20% of the CO2 in the blood combines with the
amino acids of
the globin portion of the Hb
molecule. CO2 does not compete the O2 for binding to iron.
3. As a dissolved gas - 10% of CO2 in the blood is found in a dissolved
state as a gas.
the Transport of CO2
1. pCO2 - The
loading and unloading of CO2 from and to the blood is primarily affected by the
pCO2. The movement of CO2 along a concentration gradient is facilitated by
the high solubility of CO2 in water which is the chief constituent of plasma,
interstitial fluid and intracellular fluid.
formation - The largest amount of CO2 in the blood is carried in the form of
bicarbonate. As CO2 enters the blood bicarbonate ions form. This
formation occurs rapidly in the RBC due to the presence of the enzyme carbonic
anhydrase. In the plasma this formation occurs relatively slowly.
At the tissues, as bicarbonate ions form in the RBC, hydrogen ions are
generated. The H+ ions combine
with Hb in the RBC. The binding of H+ with Hb weakens the bond
between O2 and hemoglobin. This leads to the unloading of O2 to the
tissues (figure A).
Due to the overabundance of
bicarbonate in the RBC, bicarbonate diffuses to the plasma. In order to
balance the loss of negative charges from the RBC, chloride (Cl-)
diffuses into the RBC. This is called the chloride shift (figure A).
The unloading of oxygen to the tissues supplies more hemoglobin that can bind to
hydrogen ions. This additional formation of reduced hemoglobin
(Hb + H+)
leads to the uptake of more CO2 from the tissues.
This phenomenon of O2 unloading leading to CO2 loading
in the blood near the tissues is called the Haldane
d. At the
alveolus (figure B), the diffusion of CO2 from the blood leads to a reduction of
H+ inside the RBC. This frees up hemoglobin to bind with
incoming O2. In effect, the unloading of CO2 into
the alveolus, favors the loading of O2 into the blood. In
addition, the binding of oxygen with Hb releases H+ inside the RBC.
This will favor the unloading of CO2 into the alveoli (Haldane)
is the main
oxygen transport protein
during the last seven months of development in the
. Functionally, fetal hemoglobin
differs most from adult hemoglobin
that it is able to bind oxygen with greater affinity than the
adult form, giving the developing fetus better access to oxygen
from the mother's bloodstream.
newborns, fetal hemoglobin is nearly completely replaced by
adult hemoglobin by approximately the twelfth week of postnatal
life. In adults, fetal hemoglobin
production can be reactivated pharmacologically, which is useful
in the treatment of such diseases as sickle
After the first 10 to 12 weeks of development,
the fetus' primary form of hemoglobin switches
from embryonic hemoglobin
to fetal hemoglobin. At birth, fetal hemoglobin
comprises 50-95% of the child's hemoglobin.
These levels decline after six months as adult
hemoglobin synthesis is activated while fetal
hemoglobin synthesis is deactivated. Soon after,
adult hemoglobin takes over as the
predominant form of hemoglobin in normal
of the Respiratory System in Regulation of the
pH of the Extracellular Fluids
Body cells release CO2
to the extracellular fluids (ECF)
such as the blood, interstitial fluid and the
cerebrospinal fluid. As CO2
enters the ECF, 70% of it reacts with water as
seen in the formula below:
As the CO2
enters the blood, this reaction is driven to the
right causing the accumulation of H+
and bicarbonate ions. If this accumulation
becomes excessive (hypercapnia) the resulting
condition of acidosis would prevent the blood
from carrying sufficient oxygen to keep ;the
brain functioning. The
solution to this problem is a readjustment in
pulmonary ventilation to eliminate the excess CO2
more rapidly. This adjustment
is called hyperventilation.
CO2 passes into the
cerebrospinal fluid from the blood, In the
CSF, the CO2
reacts with water producing an elevation
in the concentration of H+.
Special receptors in the medulla oblongata are
stimulated by the rise in H+.
These receptors stimulate the neurons of the
Dorsal Respiratory Group to depolarize more
rapidly increasing the breathing rate. As
is blown off, the reaction seen above is driven
to the left. This ultimately reduces the H+
concentration in the blood and in the CSF.
If too much CO2
is released during
hyperventilation, the pH of the blood and CSF
may rise as high as 7.45 a condition called
alkalosis. At this pH, hemoglobin will not
release sufficient O2
to the brain. The corrective
response to alkalosis is hypoventilation.
The scarcity of H+
in the CSF will lead to a decrease in the
respiratory rate allowing the CO2
to accumulate bringing up the H+
concentration and lowering the pH to the optimal