a. Brain Influences (Cortical Influences). The brain can send a message to
the respiratory center allowing humans to voluntarily control breathing and even stop
breathing for a short time. The fact that we can voluntarily stop breathing is a protection
because in this way we can keep water or irritating gases from entering our lungs.
However, when CO2 builds up to a certain level in our blood, impulses are sent to the
inspiratory muscles, causing us to breathe whether we want to or not. It is not possible
to kill yourself by holding your breath.
b. Medullary Rhythmicity Area. This area controls the basic rhythm of
respiration. The respiratory center in the medulla (an inner layer of an organ) adjusts
the alveolar ventilation almost exactly to the demands of the body. The result is that
there is hardly any change of oxygen and carbon dioxide in arterial blood. The
respiratory center of the medulla or its immediate vicinity is the primary site of action of
carbon dioxide and hydrogen ions. Reduced oxygen concentrations in the blood
stimulate chemoreceptors (a receptor sensitive to chemical changes in the blood
stream). Impulses are transmitted to the respiratory center to increase ventilation (the
volume of air exchanged in one minute) when these receptors are stimulated. Normally,
the increase in ventilation prevents a rise in carbon dioxide to the point that the
respiratory center would be stimulated.
1-15. FACTORS THAT CAN DECREASE OXYGEN TRANSPORT TO TISSUES
a. Decreased Levels of Hb. Hb is reduced hemoglobin (hemoglobin that has
not combined with oxygen). Since oxygen does not dissolve well in water, very little
oxygen can be carried by water through the body. Hb (hemoglobin) combines with
oxygen and carries about 97 percent of the needed oxygen to various parts of the body.
If there is less Hb in the body to combine with and carry oxygen around, there will
necessarily be less oxygen transported to tissues of the body.
b. Cardiac Failure. The cardiovascular system transports gases in the blood
between the lungs and the cells. The heart pumps blood carrying oxygen through the
body. The cells of the body cannot survive long if they are starved for blood carrying
oxygen. If oxygen is withheld from the cells of the brain for 5 to 6 minutes, a person can
suffer severe and permanent brain injury or death.
c. Decreased Rate of Respiration. In order for a human to survive, there must
be a continuous supply of oxygen to the blood as well as a continuous removal of
carbon dioxide from the body. Respiration, the inhaling of oxygen and expelling of
carbon dioxide, takes care of the oxygen and carbon dioxide levels in the body. If the
respiration rate is lowered, an individual may have enough oxygen but too much carbon
dioxide in the body.