Page 380 - Anatomy and Physiology of Farm Animals, 8th Edition

The Respiratory System / 365

Within the lungs, a unique type of Gas Transport in Blood
vascular mechanism functions at the
VetBooks.ir level of small arterial blood vessels to Both oxygen and carbon dioxide dissolve

balance blood flow and airflow. This
mechanism, local hypoxic (low oxygen) in plasma, and the partial pressures of each
are a measure of the amount dissolved.
vasoconstriction, produces local vaso- However, the quantity of each gas that is
constriction in response to low levels of transported as a dissolved gas is very small
alveolar oxygen (such as with poor alve- compared with the amounts of each trans-
olar ventilation). The vasoconstriction ported in other forms in the blood. Only
reduces blood flow into the area of poor 1.5% of the total oxygen and 7% of the
ventilation and shunts blood into better‐ carbon dioxide are dissolved.
ventilated areas of the lungs. It is not Most oxygen in the blood (98.5%) is found
clear how low levels of alveolar oxygen chemically bound to hemoglobin in erythro-
are detected and what vasoactive agent cytes (see Chapter 15). Figure 19‐15A illus-
or agents are responsible for the trates the relation between the partial
vasoconstriction. pressure of oxygen and the percent satura-
The hypoxic vasoconstriction mech- tion of hemoglobin by oxygen. At partial
anism operates well on a local basis to pressures of oxygen normally found in alve-
redirect blood flow into different areas olar air (about 100 mmHg), hemoglobin is
of the lungs. However, when both lungs almost completely saturated with oxygen
are exposed to low oxygen levels, such (i.e., hemoglobin molecules cannot bind any
as at high altitudes, the mechanism additional oxygen). Normally, the hemo-
produces a general increase in vascular globin in erythrocytes is almost completely
resistance throughout both lungs. saturated with oxygen as blood passes
Pulmonary hypertension (high pulmo- through the pulmonary capillaries. At par-
nary circulation blood pressure) tial pressures of oxygen typically found in
results, and the right side of the heart venous blood (40 mmHg) a great deal of
must work harder to pump blood hemoglobin still has oxygen bound to it
through the lungs. Right heart failure (Fig. 19‐15A).
with peripheral edema can result if the Several factors affect the ability of
right heart cannot compensate for the hemoglobin to bind chemically with
increased resistance. A syndrome with oxygen. An increase in temperature, a
pulmonary hypertension, right heart reduction in pH, or an increase in the con-
failure, and peripheral edema resulting centration of carbon dioxide reduces the
from exposure to high altitude is rec- ability of hemoglobin to bind oxygen.
ognized in cattle as brisket disease, in These factors alter the relation between
which edema collects in the pendulous hemoglobin saturation and the partial
brisket. pressure of oxygen so that the saturation
is less for any given partial pressure.
Gas Exchange in the Tissues. Cells in Figure 19‐15B illustrates the effect of low-
peripheral tissues consume oxygen and ering pH on hemoglobin saturation, and
produce carbon dioxide during normal the effects of increases in temperature or
metabolism. This maintains relatively carbon dioxide are similar. High tempera-
low oxygen and high carbon dioxide ture, low pH, and high carbon dioxide
concentrations (partial pressures) in the occur in tissues with high metabolic rates
extracellular fluid around capillaries. As (e.g., exercising skeletal muscle); the effects
arterial blood enters capillaries, partial of these factors on the relationship between
pressure gradients promote the diffusion hemoglobin and oxygen is that more oxy-
of oxygen out of the blood to the interstitial gen is liberated from hemoglobin and
fluid and carbon dioxide from the delivered to the metabolizing cells when
interstitial fluid into the blood (Fig. 19‐14). blood passes through such areas.