Biomedical Engineering Reference
In-Depth Information
coagulation co-factors. The platelet cell membrane may be the most important structure of
the entire platelets. It contains many glycoproteins which adhere to other platelets, endo-
thelial cells, and proteins during clot formation. Also, the charge of the cell membrane can
accelerate or decelerate coagulation kinetics. The average life span of platelets is 12 days
within circulation, at which time they are removed by the spleen.
Hemostasis is a complex process that culminates in the formation of insoluble fibrin. At
this point, a fibrin mesh forms that is similar in structure to gauze, which acts to seal off
the wound, preventing blood loss. The coagulation cascade is a series of enzymatic reac-
tions, in which an active enzyme cleaves an inactive zymogen into its active form. The
newly activated enzyme cleaves another zymogen into its active form and so on. This pro-
cess continues until thrombin cleaves fibrinogen into fibrin. Platelets are integral in these
coagulation reactions because they provide multiple necessary co-factors for these reac-
tions to occur. Platelets provide a negative phospholipid surface (activated cell mem-
branes, which express negative phospholipids) for coagulation complexes to form on
(coagulation complexes include an activated enzyme and its specific inactive zymogen
and any possible co-factors). Platelets also provide co-factors to accelerate the coagulation
reactions (especially Factor Va and calcium). Without these two critical roles that platelets
play, fibrinogen will not be cleaved to fibrin rapid enough to prevent a major loss of
blood. Lastly, platelets can aggregate with each other and various plasma proteins, form-
ing a temporary mesh until the fibrin mesh has stabilized with Factor XIII. Negative feed-
backs on hemostasis include the removal of platelets that are participating in these
processes and anti-thrombotic proteins that cleave activated zymogens. If the platelet
count is low, coagulation does not proceed as rapid as normal and minor injuries can
cause severe blood loss. This is similar to hemophilia (loss or low concentration of Factor
VIII), which is characterized by prolonged bleeding. It is important to note that shear
stress can alter the platelet physiology significantly and that under disturbed blood flow
conditions (e.g., high shear stresses, recirculation zones, oscillating stresses), platelets may
accelerate cardiovascular disease progression.
The second major component of blood is plasma (Table 5.1). This includes everything
other than cells that is within the blood. The major component of plasma is water. The
remaining components are mostly electrolytes, sugars, urea, phospholipids, cholesterol,
and proteins. The plasma concentration of electrolytes is very similar to that of the intersti-
tial fluid because these compartments are only separated by the very permeable capillary
wall (discussed in Section 3). Therefore, these compartments reach or approach equilib-
rium. Sodium ions are the most abundant positively charged ions in the plasma (although
potassium, calcium, and magnesium are not negligible). Chloride is the most abundant
negatively charged ions (but bicarbonate, phosphate, and sulfate are also present). The
total plasma osmolarity is approximately 300 mOsmolar/L H
2
O. Approximately 2% of this
is accounted for by sugars within the plasma. Cholesterol contributes 4% to 5% of this (in
a normal diet) and phospholipids are approximately 6% to 7% of this. Proteins account for
approximately 1 mOsmolar/L H
2
O of plasma, but account for 7% of the total composition
of plasma. The most crucial and most abundant protein within the blood is albumin,
which accounts for over 60% of the total plasma protein concentration. Albumin has the
primary function of maintaining the osmotic pressure of blood. It acts to balance the mass
transfer across the capillary wall (see Chapter 7). The next major protein contributors to
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