The respiratory mechanism
is a way of controlling how much carbon dioxide is in the blood. Elevating arterial
CO2 or decreasing pH increases the rhythmic stimulation of the respiratory muscles
by the medulla.
- Normally control of CO2
is based on the rate and depth of ventilation.
- Ventilation is however
not synonymous with oxygenation. Ventilation refers only to air moving in
and out of the lungs.
- If everything else
is working the way it should, then oxygenation happens.
- However, as we learned
in the shunt section, proper ventilation
does not ensure oxygenation.
ventilation will increase oxygenation
CO2 is a by-product
of metabolism. As long as the cells are functioning, they will produce CO2.
- CO2 has several
forms in the blood.
- Like oxygen, some
is dissolved directly into the plasma. The PaCO2 is the measurement
of the partial pressure of carbon dioxide dissolved in the plasma. It
is measured in mm Hg (millimeters of mercury).
- The rest is inside
the red blood cells on a hemoglobin molecule.
with water, carbon dioxide becomes carbonic acid (H2CO3):
+ H2O <--> H2CO3
CO2's affinity for
hemoglobin is much less than oxygen's.
- So, when both are available,
hemoglobin will accept oxygen rather than CO2.
- In the oxygen rich environment
of the alveoli, hemoglobin carries oxygen.
- Oxygenated blood then
travels through the body.
There is less oxygen and
more CO2 near the cells.
- Remember that CO2
in solution is an acid.
- Therefore, in the
capillaries, the blood is more acidic.
- The lower pH changes
the affinity of oxygen for hemoglobin (see Oxyhemoglobin Dissociation
curve) and oxygen is released to the tissues.
- In this oxygen depleted
environment, the CO2 is now able to ride the hemoglobin back to the
lungs and out of the body.
HERE TO VIEW ANIMATED DESCRIPTION OF THIS PROCESS!
As we learned above, acid
lowers the pH.
- So, a high PaCO2
means a lower pH.
- Notice I said "lower"
- CO2 will "move"
the pH, but other factors may still cause an alkalosis.
PaCO2 raises the acidity of blood.
- Respiratory mechanism
(depth and rate of breathing) controls CO2
- CO2 in solution
is an acid.
- Higher PaCO2 causes
acidosis (lower pH), or neutralizes alkalosis.
- Lower PaCO2 causes
alkalosis (raises pH.), or neutralizes acidosis.