Respiratory Physiology 57716.5 Regulation of Breathing 553
A. The rhythmicity center in the medulla oblongata directly
controls the muscles of respiration.
- Activity of the inspiratory and expiratory neurons varies
in a reciprocal way to produce an automatic breathing
cycle. - Activity in the medulla is influenced by the apneustic
and pneumotaxic centers in the pons, as well as by
sensory feedback information. - Conscious breathing involves direct control by the
cerebral cortex via corticospinal tracts.
B. Breathing is affected by chemoreceptors sensitive to the P^ O 2 ,
pH, and P^ O 2 of the blood. - The P^ CO 2 of the blood and consequent changes in pH are
usually of greater importance than the blood P^ O 2 in the
regulation of breathing. - Central chemoreceptors in the medulla oblongata
are sensitive to changes in blood P^ CO 2 because of the
resultant changes in the pH of cerebrospinal fluid. - The peripheral chemoreceptors in the aortic and carotid
bodies are sensitive to changes in blood P^ CO 2 indirectly,
because of consequent changes in blood pH.
C. Decreases in blood P^ O 2 directly stimulate breathing only when
the blood P^ O 2 is lower than 50 mmHg. A drop in P^ O 2 also stimu-
lates breathing indirectly, by making the chemoreceptors more
sensitive to changes in P^ CO 2 and pH.
D. At tidal volumes of 1 L or more, inspiration is inhibited by
stretch receptors in the lungs (the Hering-Breuer reflex). A
similar reflex may act to inhibit expiration.
16.6 Hemoglobin and Oxygen Transport 559
A. Hemoglobin is composed of two alpha and two beta
polypeptide chains and four heme groups each containing a
central atom of iron.
- When the iron is in the reduced form and not attached to
oxygen, the hemoglobin is called deoxyhemoglobin, or
reduced hemoglobin; when it is attached to oxygen, it is
called oxyhemoglobin. - If the iron is attached to carbon monoxide, the
hemoglobin is called carboxyhemoglobin. When the
iron is in an oxidized state and unable to transport any
gas, the hemoglobin is called methemoglobin. - Deoxyhemoglobin combines with oxygen in the lungs
(the loading reaction) and breaks its bonds with oxygen
in the tissue capillaries (the unloading reaction). The
extent of each reaction is determined by the P^ O 2 and the
affinity of hemoglobin for oxygen.
B. A graph of percent oxyhemoglobin saturation at different
values of P^ O 2 is called an oxyhemoglobin dissociation curve. - At rest, the difference between arterial and venous
oxyhemoglobin saturations indicates that about 22%
of the oxyhemoglobin unloads its oxygen to the
tissues. - During exercise, the venous P^ O 2 and percent
oxyhemoglobin saturation are decreased, indicating that
a higher percentage of the oxyhemoglobin has unloaded
its oxygen to the tissues.
C. The pH and temperature of the blood influence the affinity
of hemoglobin for oxygen, and thus the extent of loading
and unloading.
1. A fall in pH decreases the affinity of hemoglobin for
oxygen, and a rise in pH increases the affinity. This is
called the Bohr effect.
2. A rise in temperature decreases the affinity of
hemoglobin for oxygen.
3. When the affinity is decreased, the oxyhemoglobin
dissociation curve is shifted to the right. This indicates a
greater unloading percentage of oxygen to the tissues.
D. The affinity of hemoglobin for oxygen is also decreased by
an organic molecule in the red blood cells called
2,3-diphosphoglyceric acid (2,3-DPG).
1. Because oxyhemoglobin inhibits 2,3-DPG production,
2,3-DPG concentrations will be higher when anemia
or low P^ O 2 (as in high altitude) cause a decrease in
oxyhemoglobin.
2. If a person is anemic, the lowered hemoglobin
concentration is partially compensated for because a
higher percentage of the oxyhemoglobin will unload its
oxygen as a result of the effect of 2,3-DPG.
3. Fetal hemoglobin cannot bind to 2,3-DPG, and thus it has
a higher affinity for oxygen than the mother’s hemoglobin.
This facilitates the transfer of oxygen to the fetus.
E. Inherited defects in the amino acid composition of
hemoglobin are responsible for such diseases as sickle-cell
anemia and thalassemia.
F. Striated muscles contain myoglobin, a pigment related to
hemoglobin that can combine with oxygen and deliver it to
the muscle cell mitochondria at low P^ O 2 values.16.7 Carbon Dioxide Transport 565
A. Red blood cells contain an enzyme called carbonic
anhydrase that catalyzes the reversible reaction whereby
carbon dioxide and water are used to form carbonic acid.
1. This reaction is favored by the high P^ O 2 in the tissue
capillaries, and as a result, carbon dioxide produced by the
tissues is converted into carbonic acid in the red blood cells.
2. Carbonic acid then ionizes to form H^1 and HCO 32
(bicarbonate).
3. Because much of the H^1 is buffered by hemoglobin, but
more bicarbonate is free to diffuse outward, an electrical
gradient is established that draws Cl^2 into the red blood
cells. This is called the chloride shift.
4. A reverse chloride shift occurs in the lungs. In this
process, the low P^ CO 2 favors the conversion of carbonic
acid to carbon dioxide, which can be exhaled.
B. By adjusting the blood concentration of carbon dioxide,
and thus of carbonic acid, the process of ventilation helps to
maintain proper acid-base balance of the blood.
1. Normal arterial blood pH is 7.40. A pH below 7.35 is
termed acidosis; a pH above 7.45 is termed alkalosis.
2. Hyperventilation causes respiratory alkalosis, and
hypoventilation causes respiratory acidosis.
3. Metabolic acidosis stimulates hyperventilation, which can
cause a respiratory alkalosis as a partial compensation.