Gas Exchange
Page
1. Introduction
• Oxygen
and carbon dioxide diffuse between the alveoli and pulmonary capillaries in the
lungs, and between the systemic capillaries and cells throughout the body.
• The
diffusion of these gases, moving in opposite directions, is called gas exchange.
Page
2. Goals
• To apply gas law relationships - between partial pressure,
solubility, and concentration - to gas exchange.
• To
explore the factors which affect external and internal respiration.
Page
3. Dalton's Law of Partial
Pressures
• Fill out
this chart as you work through this page:
• In order
to understand gas exchange, we must first understand the air we breathe.
The atmosphere is a mixture of gases, including oxygen, carbon dioxide,
nitrogen, and water.
• The combined pressure of these gases equals atmospheric
pressure.
• At sea
level, atmospheric pressure is 760 mm Hg, which means that the atmosphere pushes
a column of mercury to a height of 760 millimeters.
Each gas within the atmosphere is responsible for part of that pressure
in proportion to its percentage in the atmosphere.
• Oxygen
comprises 20.9% of the atmosphere. The
pressure exerted by oxygen is 20.9%
of the total pressure of 760 millimeters of mercury, which equals
159 millimeters of mercury. This
value is known as the partial pressure of oxygen, and is written as
"P" with the subscript "O2".
• Notice
that the partial pressures of the four gases add up to 760 millimeters of
mercury, the total atmospheric pressure. This
demonstrates Dalton's Law of Partial Pressures, which states that in a mixture
of gases, the total pressure equals the sum of the partial pressures exerted by
each gas. The partial pressure of
each gas is directly proportional to its percentage in the total gas mixture.
Page
4.
Effect of High Altitude on Partial Pressures
• Fill out
this chart as you work through this page:
• Atmospheric
pressure decreases with increasing altitude. For example, on the top of Mt.
Whitney, atmospheric pressure drops to approximately 440 millimeters of mercury.
•
Oxygen still makes up 20.9% of the atmosphere, but the PO2
is 20.9% of 440 millimeters of mercury, or about 92 millimeters of mercury.
Compare that to the PO2 at sea level of 159 millimeters of mercury. Lower
atmospheric pressure means fewer gas molecules, and therefore fewer oxygen
molecules, are available. That
explains why you may gasp for breath at high altitudes.
• As
you can see, at high altitudes the partial pressures of all gases are lower than
at sea level.
Page 5. Henry's Law
• Within the lungs, oxygen and carbon dioxide diffuse between
the air in the alveoli and the blood, that is between a gas and a liquid.
• This movement is governed by Henry's Law, which states that
the amount of gas which dissolves in a liquid is proportional to:
1. the
partial pressure of the gas
2. the
solubility of the gas
• In this
container, the oxygen in the air is at equilibrium with the oxygen in the
liquid. At equilibrium, the
pressure of the oxygen in the air is the same as in the liquid, with the gas
molecules diffusing at the same rate in both directions.
• If you
increase the pressure in the container more oxygen molecules dissolve in the
liquid, moving from a region of high pressure to a region of low pressure.
Diffusion continues until a new equilibrium is reached.
This is what happens when oxygen moves from the alveoli into the blood.
•
Now let's look at the diffusion of carbon dioxide. Although both gases are at the same pressure, far more carbon
dioxide dissolves in the liquid than oxygen.
This occurs because carbon dioxide is much more soluble than oxygen. As
stated in Henry's Law, the amount of oxygen and carbon dioxide which dissolves
is proportional to the partial pressure and the solubility of each gas.
Page 6. Sites of Gas
Exchange
Sites of gas
exchange in the body:
• External Respiration.
• Blood that is low in oxygen is pumped from the right side
of the heart, through the pulmonary arteries to the lungs.
• External
respiration occurs within the lungs, as carbon dioxide diffuses from the
pulmonary capillaries into the alveoli, and oxygen diffuses from the alveoli
into the pulmonary capillaries.
•
Oxygen-rich blood leaves the lungs and is transported through the pulmonary
veins to the left side of the heart.
• Internal Respiration.
• From
there it is pumped through the systemic circuit to tissues throughout the body.
• Internal
respiration occurs within tissues, as oxygen diffuses from the systemic
capillaries into the cells, and carbon dioxide diffuses from the cells into the
systemic capillaries.
Page 7. Factors
Influencing External Respiration
•
Efficient external
respiration depends on three main factors:
1.
The surface area and structure of the respiratory membrane. The 300 million alveoli, covered with a dense network of pulmonary
capillaries, provide an enormous surface area for efficient gas exchange. In
addition, the thinness of the respiratory membrane increases efficiency.
2. The
partial pressure gradients between the alveoli and capillaries.
3. Efficient
gas exchange requires matching alveolar airflow to pulmonary capillary blood
flow.
Page 8. External
Respiration: Partial Pressures
•
Let's see how partial pressure gradients affect gas exchange between the alveoli
and the pulmonary capillaries.
•
Notice that the partial pressures in the alveoli differ from those in the
atmosphere. This difference is caused by a combination of several factors:
1.
Humidification of inhaled air. As
it travels through the respiratory passageways to the alveoli, air is
humidified, picking up water molecules. This greatly increases the partial
pressure of water.
2.
Gas exchange between the alveoli and pulmonary capillaries.
A continuous exchange of oxygen and carbon dioxide occurs between the
alveoli and pulmonary capillaries, changing the partial pressures of both gases.
Oxygen diffuses out of the alveoli into the pulmonary capillaries and carbon
dioxide diffuses from the pulmonary capillaries into the alveoli.
3.
Mixing of new and old air. Since
the alveoli do not completely empty between breaths, the air in the alveoli is a
mixture of new air and air remaining from previous breaths.
•
Label this diagram:
Page 9. External
Respiration: Loading O2
• Let's
first look at the loading of oxygen into
the blood. Each alveolus is
surrounded by a network of capillaries. This
diagram shows just one alveolus and one capillary.
• The PO2 of the alveolar air is 104 mm Hg.
At rest, the oxygen-poor blood entering the pulmonary capillaries has a PO2
of 40 mm Hg.
•
As blood flows past the alveolus, the PO2
increases.
•
Notice that there is a net diffusion of oxygen along its partial pressure
gradient, from the alveolus into the blood, until equilibrium is reached.
The PO2
of the oxygen-rich blood has increased to 104 mm Hg.
•
As indicated in the graph, equilibrium is reached rapidly, within the first
third of the pulmonary capillary.
•
Label this diagram:
Page 10. External
Respiration: Unloading CO2
• Now
let's look at the unloading of carbon dioxide
from the blood into the alveolus.
•
The PCO2 of
the alveolar air is 40 millimeters of mercury. At rest, the PCO2 of the blood entering the pulmonary capillaries is 45
millimeters of mercury.
•
As blood flows past the alveolus, the PCO2
decreases. Carbon dioxide diffuses
along its partial pressure gradient, from the blood into the alveolus, until
equilibrium is reached. The PCO2 of
the blood has decreases to 40 millimeters of mercury.
•
Equilibrium is reached rapidly, within the first four tenths of the pulmonary
capillary.
•
Label this diagram:
Page 11. External Respiration O2 and CO2 Exchange
• Loading
oxygen and unloading carbon dioxide occur simultaneously.
As you inhale, you replenish oxygen, and as you exhale, you eliminate
carbon dioxide.
•
Notice how much
smaller carbon dioxide's partial pressure gradient is than oxygen's. As Henry's law states, the number of molecules which dissolve
in a liquid is proportional to both the partial pressure and the gas solubility.
Since carbon dioxide is very soluble in blood, a large number of
molecules diffuse along this small partial pressure gradient.
Oxygen, which is less soluble, requires a much larger concentration
gradient to provide adequate oxygen to the body.
• The third factor in external respiration is
ventilation-perfusion coupling, which facilitates efficient gas exchange.
It does this by maintaining alveolar airflow that is proportional to the
pulmonary capillary blood flow.
• When airflow through a bronchiole is restricted, as when
blocked by mucus, the resulting low PO2
causes the local arterioles to vasoconstrict. This response redirects the blood to other alveoli which
have a higher airflow, and therefore have more oxygen available to be picked up
by the blood.
•
In regions with high airflow compared to their blood supply, the resulting high
PO2
causes the local arterioles to vasodilate.
This brings more blood to the alveoli, allowing the blood to pick up the
abundant oxygen.
Page 13. Ventilation-Perfusion
Coupling: Effect of PCO2
• We've
seen that during ventilation-perfusion coupling, the arterioles
respond to changes in PO2.
The bronchioles, on the other hand, respond to changes in PCO2.
• When
airflow through a bronchiole is lower than normal, the PCO2 rises.
The bronchioles respond by dilating, thereby eliminating the excess
carbon dioxide from the alveoli.
• When
airflow through a bronchiole is high compared to its blood supply, the PCO2 drops. The
bronchioles then constrict, reducing
the airflow so it is proportional to the
local blood flow.
Page 14. Predict the
Effect of PO2
and PCO2
• Assume that ventilation to an alveolar sac is low, due to a
small tumor growing in the bronchiole. The PO2
decreases because oxygen is not replenished, and the PCO2
increases, because the carbon dioxide is
not eliminated. See if you can predict the response of the arterioles and
bronchioles.
• The low
PO2 causes the arterioles to constrict, and the high PCO2 causes the bronchioles to dilate. The airflow and blood
flow are now in the proper proportions for optimum gas exchange. Notice that
both the arterioles and bronchioles respond simultaneously.
Page 15. Internal
Respiration
• During internal respiration:
• Oxygen
diffuses from the systemic capillaries into the cells.
• Carbon
dioxide diffuses from the cells into the systemic capillaries.
• Factors affecting the exchange of oxygen and carbon dioxide
during internal respiration:
1. The
available surface area, which varies in different tissues throughout the body.
2. Gases
diffuse along their partial pressure gradients.
3. The rate
of blood flow in a specific tissue.
• Blood
flow in a tissue varies for many reasons, including the tissue's metabolic rate.
Recall that during metabolism, cells use oxygen and produce carbon dioxide.
• Label this diagram as you go through this page:
• The exchange of oxygen and carbon dioxide
during internal respiration:
•
In relatively inactive organs, the tissue cells have a PO2
of 40 millimeters of
mercury, and a PCO2
of 45 millimeters of mercury.
•
As the blood enters the systemic capillaries, it has a PO2
of 100 millimeters of
mercury, and a PCO2
of 40 millimeters of mercury.
•
Notice that the PO2 of blood entering the systemic capillaries is lower than
the alveolar PO2
of 104 millimeters of mercury. This
small decrease is due primarily to imperfect ventilation-perfusion coupling in
the lungs.
•
Gas exchange continues until equilibrium is reached.
At equilibrium, the blood in the systemic capillaries has a PO2
of 40 millimeters of
mercury, and a PCO2
of 45 millimeters of mercury.
•
The oxygen-poor blood now returns, through the systemic veins, to the right side
of the heart.