Download Exchange of Oxygen and Carbon Dioxide

Exchange of Oxygen and Carbon Dioxide
External Respiration
Air containing oxygen enters the alveoli by the process of ventilation
(described in a later section). The partial pressure of oxygen in the
alveoli is slightly lower than the partial pressure of oxygen found in the
atmosphere. Air taken into the lungs (tidal volume minus the volume of
the conduction zone) mixes with air that is already in the lungs
(functional residual volume). ). Because gas exchange is constantly
occurring (even between breaths, when we hold our breath, etc.), the air
making up the functional residual volume has had some oxygen
removed from it and some carbon dioxide added to it. When a new tidal
volume of air is inhaled, the portion of this air that enters the alveoli
mixes with the functional residual volume and effectively lowers the
fraction of oxygen in this alveolar air.
Inhaled air at sea level typically has a partial pressure of oxygen near
160 mmHg. In the alveoli, the partial pressure of oxygen would vary
with our ventilation pattern, but typically equilibrates at about 105
mmHg. It is the difference between alveolar oxygen partial pressure and
the plasma oxygen partial pressure that drives external respiration
across the alveolar membrane. Blood coming through the pulmonary
arterial circulation is lower in oxygen, with a typical partial pressure of
40 mmHg. The differential concentration gradient for oxygen to move
from the alveolar air into the capillary blood starts at about 65 mmHg
(105 mmHg in the alveoli minus 40 mmHg in blood), providing enough
of a difference in partial pressures for oxygen to diffuse from the aveoli
into the capillary. Diffusion will occur along the length of the pulmonary
capillary until the partial pressures come into equilibrium near 105
mmHg.
EXAMPLE
Altitude Sickness
Altitude sickness, also called acute mountain sickness, can strike people
climbing to elevations above 8,000 feet (although it typically occurs only at
altitudes much higher than this). At elevations high above sea level, there is the
same percentage of oxygen (21%), but much less atmospheric pressure. This
lowers the partial pressure of the oxygen being inhaled so less oxygen enters the
body. If the body doesn’t adapt well, a person can experience altitude sickness
ranging from mild to severe forms. Mild to moderate altitude sickness can cause
nausea, vomiting, tachycardia, shortness of breath with exercise, or difficulty
sleeping. Mild to moderate cases usually resolve themselves when the person
descends to a lower altitude. However, severe cases are another matter. They
can result in cyanosis, pulmonary congestion, confusion and stupor, a cough with
or without blood, a gray or very pale complexion, the inability to walk a straight
line, if able to walk at all, and shortness of breath when at rest. These cases
require immediate evacuation to lower altitudes. Without treatment, severe
altitude sickness may result in death due to pulmonary complications or brain
swelling. The good news is that altitude sickness can be prevented. Individuals
who climb to extremely high altitudes, like Mount Everest, should do so slowly to
allow their bodies to become acclimated to the atmospheric differences.
The partial pressure of oxygen in the system circulation is slightly lower.
As blood enters the left atrium of the heart, a small amount of the
pulmonary blood mixes with the bronchial blood that has nourished the
lung tissue, lowering the partial pressure so that the actual
concentration of oxygen is about 100 mmHg, which is a typical value
measured in a sample of arterial blood.
The functioning of the pressure gradient for carbon dioxide works in
reverse of that for oxygen, with the carbon dioxide partial pressure being
higher in the pulmonary arterial blood than in the alveoli. Even with a
much smaller gradient for carbon dioxide than for oxygen, nearly as
much carbon dioxide diffuses from the blood to the alveoli as oxygen
diffuses from the alveoli to the blood because of the much higher
solubility of carbon dioxide in the plasma.
External respiration: pulmonary
PO2 PCO2
Pulmonary arteries leading to capillaries 40
45
Alveoli
105
40
Pulmonary veins
100
40
Internal Respiration
Internal respiration occurs between the blood and systemic tissues of
the body. The systemic arteries carry essentially the same concentration
of oxygen and carbon dioxide as the pulmonary veins. The tissues are
continually using oxygen, and the partial pressure of oxygen in active
cells remains below 40 mmHg. Oxygen circulating in the systemic
arterial blood readily diffuses across the membranes of the blood vessels
into the tissues, replenishing the supply of oxygen in the cells. The final
concentration of oxygen and carbon dioxide in the systemic veins is
essentially the same as it is in the pulmonary arteries.
Just as the tissues are continually using up the oxygen, they are
continually producing carbon dioxide. The partial pressure of carbon
dioxide in tissues is always greater than 45 mmHg. This accounts for the
diffusion of carbon dioxide into the systemic capillaries, raising the
pressure to 45 mmHg. The systemic veins carry essentially the same
concentration of oxygen and carbon dioxide as the pulmonary arteries.
Internal respiration: tissues
PO2
PCO2
Systemic arteries leading to capillaries
100
40
Metabolically active tissues
< 40
> 45
Systemic veins
40
45