Biomedical Engineering Reference
In-Depth Information
Lymphatic white cells, the B and T lymphocytes, are derived from lymphoid stem cells that
are located in the lymphatic system, especially in the lymph nodes. These cell types represent the
adaptive part of the immune system, catalyzing a specifi c response to each new invader, generating
specifi c antibodies (B cells), and also priming cells to attack and remove the invader (T cells).
Furthermore, some additional white cells are present in the circulation like auxiliary cells, mast
cells, and natural killer cells. These cells only represent a small fraction of the white blood cells and
are not directly involved in the contact with synthetic surfaces.
17.2.3 P
LATELETS
In the bone marrow, the megakaryocyte produces a large number of anuclear cellular fragments,
called platelets that are important for blood coagulation and vessel wall repair.
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6
These platelets have
a half life of about 8-10 days, and inactive platelets will be removed in the spleen by macrophages.
The platelets are mainly involved in the coagulation response, but also have a role in the attachment
of leukocytes onto synthetic surfaces. The number of platelets in the blood can vary between 150
and 400
10
6
/mL) will
result in serious bleeding complications, requiring transfusion to restore platelet levels.
10
6
/mL, but only a severe reduction of approximately 90% (below 20
×
×
17.2.4 P
LASMA
Blood plasma is the liquid part of the blood in which all cells are suspended. A large array of ions,
sugars, fatty acids, amino acids, proteins, and other soluble molecules are present in plasma.
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The pH and osmolarity have to be kept constant to avoid damage to blood cells and tissues. The
pH is kept constant at 7.4 by making use of the buffering capacity of carbonate ions. As a conse-
quence, hyperventilation causes the pH of blood to drop as a result of an increased concentration
of dissolved CO
2
and a shift in the balance of the carbonate ions. Eventually, this may result in
fainting. Osmolarity is largely controlled by action of the kidneys that actively regulate the concen-
trations of ions. Increased levels of ions in the blood can lead to hypertension, which on the long
run can damage the heart and vasculature. Blood sugar levels are also tightly controlled, with loss
of control mechanisms resulting in diabetes. The effects of diabetes are both short-term (fainting,
unconsciousness, and even coma) and long-term as for instance the diabetic foot that occurs as a
result of impaired blood fl ow in the leg.
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In this way an increased sugar level in the blood causes
damage to the blood vessels, resulting in limb loss and in the worst case even death. Additionally,
blood-contacting devices have a higher risk of failing in diabetic patients. The fatty acids and lipids
in the blood are in general bound to transporter proteins. The low-density lipoproteins (LDL) and
high-density lipoproteins (HDL) have long been used as an indicator for the risk of cardiovascular
disease in humans. High LDL and low HDL have been associated with an increased risk in athero-
sclerosis, myocardial infarction, and stroke.
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An extensive array of different proteins is present in plasma.
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The immune system (IgG, IgM,
IgA), complement system (C3, C4, C5, C9, C1q), nutrient transport (apolipoproteins, transferrin),
and coagulation (prothrombin, antithrombin (AT), factor XII, prekallikrein, plasminogen) are fully
dependent on plasma proteins. Most of these proteins are released in the blood system by organs
like the liver.
17.3 BLOOD VESSELS
Blood transports nutrients, waste products, gases, and (excess) heat throughout the body. Cells from
the immune system can reach each tissue by moving through the blood vessels, and leaving the
circulation when necessary. There are three major forms of blood vessels: (i) arteries, (ii) veins, and
(iii) capillaries.
2
The anatomy of vessels is given in Figure 17.3. Capillaries and small postcapil-
lary venules consist of only an endothelial layer with a basal lamina (tunica intima). The arteries
and veins contain extra layers with smooth muscle cells (tunica media) and fi broblasts and collagen
