Biomedical Engineering Reference
In-Depth Information
6.11 DISEASE CONDITIONS
6.11.1 Shock/Tissue Necrosis
Shock is a condition that can occur after a massive loss of blood due to 1) a significant
decrease in the blood pressure and 2) the decrease in nutrient exchange (due to blood vol-
ume loss). In response to pressure reductions, there are rapid responses that act to increase
the blood pressure back to resting conditions. Along the carotid artery and the aortic wall
there are pressure sensors termed baro-receptors. When these receptors sense a significant
decrease in blood pressure they instigate a neuronal response that increases cardiac output
by increasing the heart rate to over 150 beats per minute and increases the vascular resis-
tance by causing constriction of the blood vessels. Combined, these responses have the
effect of increasing blood pressure and draw on the venous blood reserve. In the short-
term, hormones can also be released to aid in vasoconstriction and heart rate increase (epi-
nephrine, anti-diuretic hormone, among others). This mechanism typically can work up to
a 25% blood volume loss, without showing signs of a decrease in nutrient exchange within
the microcirculation.
The long-term problem associated with blood loss is the necessary increase of blood
volume to make up the loss. A decrease in capillary pressure will increase the movement
of water back into the capillary along the venous side. Antidiuretic hormone increases the
overall retention of water from the kidneys. Red blood cell maturation rate is increased to
make up for the loss of the cellular component of blood. Typically, with a 25% loss of
blood volume, the water portion can be returned to a normal volume within a few hours.
However, the cellular portion will not return to normal levels for a few days, at best.
The combined response mechanisms act to return nutrient exchange to normal levels.
However, if any of the mentioned mechanisms fail or there is a more significant loss of
blood, it is likely for shock to develop. Shock occurs when the body losses a significant
blood volume, causing the blood pressure to reduce and the exchange of nutrients within
the microcirculation to be reduced. If this condition persists for an extended period of
time, there will be local tissue necrosis (tissue death similar to myocardial infarction) in
regions where the microvascular beds have been shut down to divert the blood flow to
more critical regions (such as the brain). However, with the combination of blood volume
loss and lowered arterial pressure, it is more difficult for the heart to pump blood
throughout the cardiovascular system. The easiest way to remedy this situation is through
a blood transfusion, which will effectively bring the volume of blood back to normal, as
well as bring the cellular volume of blood back to normal.
6.11.2 Edema
Edema is a condition where there is an excessive accumulation of interstitial fluid. This
fluid is not recollected by the capillary networks or the lymphatic system (see Chapter 8).
There are many causes of edema, but they all stem from an imbalance between the hydro-
static pressures and the osmotic pressures within the microvascular beds. The most com-
mon cause for edema is an increase in the arterial blood pressure associated with
hypertension. Due to an increase in the capillary hydrostatic pressure at the arterial side,
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