Biomedical Engineering Reference
In-Depth Information
smooth muscle cells. As discussed in Chapter 4, atherosclerosis is the deposition of lipids
within the arterial wall. Monocytes that enter the vessel wall engulf the lipids, transform
into foam cells, and attach themselves to the vessel wall. Foam cells can release cytokines
and growth factors which aid in plaque formation.
Risk factors for arteriosclerosis include a diet high in cholesterol, high blood pressure,
cigarette smoking, diabetes, obesity, being elderly, and being male (possibly from increase
iron levels or decreased estrogen levels). Most of these risk factors can be addressed by
having the patient make a lifestyle change. However, if the disease progresses too far, it is
common for medical doctors to perform a balloon angioplasty to flatten the plaque and to
use stenting to prevent restenosis, or bypass surgery to redirect flow.
When a blood vessel that supplies the brain with blood becomes atherosclerotic, there is
a large possibility for stroke to develop. A stroke occurs when the blood supply to the
brain is interrupted. To prevent an interruption in brain blood supply, the brain vascular
system has a unique structure called the Circle of Willis. The Circle of Willis receives
blood supply from the carotid arteries and the vertebral arteries, and then it feeds all of
the major brain blood vessels with blood. Therefore, a major blockage prior to or within
the Circle of Willis will more than likely not affect the blood supply to the brain because
of this redundant system. However, a severe block distal to the Circle of Willis can cause a
decrease in blood flow to particular sections of the brain. A brain that does not receive any
blood for 5 to 10 seconds will instigate unconsciousness. After 4 to 5 minutes without
blood, there will be some permanent neurological damage to the brain.
High blood pressure (or hypertension) is a risk factor for many cardiovascular diseases.
Patients with hypertension tend to have a mean arterial pressure that is greater than
110 mmHg (the normal mean arterial pressure during one cardiac cycle is approximately
90 mmHg). Typically, this relates to a diastolic pressure greater than 90 mmHg and a sys-
tolic pressure greater than 140 mmHg. In severe cases, the systolic pressure can approach
200 mmHg, with the diastolic pressure approaching 150 mmHg. Any increases in blood
pressure can lead to a higher chance of death predominantly from 1) an increased work
conducted by the heart, 2) hemorrhaging, or 3) vessel rupture. An increased heart work
can easily lead to heart failure because the cardiac muscle will use an excessive amount of
energy to maintain the work and require a larger percentage of the blood supply. At some
time, when the cardiac tissue exhausts all of the energy, the heart will not be able to con-
tract anymore. Hemorrhaging associated with high blood pressure is common in the kid-
neys. This will lead to kidney failure because of the blood that is being pooled within the
kidney. Kidney failure will always lead to death. Blood vessel rupture is also a serious
problem if it occurs in the brain or in the cardiac tissue. As we have previously discussed,
this can lead to stroke or cardiac ischemia.
A common cause for high blood pressure is excessive salt in the diet. With excess salt
in the body, the osmotic balance of the blood and the extracellular fluid is shifted. To
counter this, a person will typically increase the volume of fluids that they drink and pro-
duce more anti-diuretic hormone (see Chapter 12), which both act to increase the volume
of fluid within the extracellular space. With an increase in extracellular fluid, the external
pressure acting on the blood vessels increases; that is, there is a preference for the blood
vessels to collapse. Therefore, to maintain the same flow throughout the body, the mean
arterial pressure must be increased.
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