Biomedical Engineering 403

Hemodynamics

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7
Hemodynamics

Blood vessels exposed to 2 kinds of stress:
        Tensile stress
        Shearing stress
both can affect the functional integrity of the endothelium cells.

Laminar flow--The attraction between the fluid and the vessel keeps the
molecules of liquid in contact with the wall    from moving.  Blood inside
slides past and blood in the center of the vessel moves fastest.  Thus,
there is     little resistance from the interaction of the fluid with the
vessel wall, but only the friction between concentric      fluid layers.
This means that viscosity is the rate-limiting factor.

Blood consists of 
erythrocytes--most numerous; 33% of cell weight is hemoglobin; tend to
aggregate
leukocytes--large; carry antibodies
platelets --very small, but enormous in number
in plasma.

The viscosity of whole blood is more than twice that of plasma.

Hematocrit is the fraction of blood volume that is occupied by red cells
(usually around 45%).  Thus, hematrocrit       affects blood viscosity.
In conditions where hematocrit level is elevated (e.g., dehydration,
disorders        characterized by high hematocrit), the increased
viscosity of the blood increases cardiac work.  Patients with  anemia may
have increased blood flow to tissues due to reduced viscosity.

Optimal hematocrit for tissue oxygenation is a delicate balance:
        A high hematocrit increases oxygen carrying capacity of the blood,
        but also raises viscosity, which reduces flow.

Cells tend to migrate toward the center of the vessel, which affects the
apparent viscosity.  Thus, the layers near the         vessels walls have
viscosities similar to that of plasma (since they contain only plasma).  
The faster the flow, the more cells are are pushed toward the center,
thus, the lower the viscosity.  This factor       becomes important in
cardiovascular shock, in which cardiac output may drop enough to slow
speed,       therefore increasing the apparent viscosity of blood.  Heart
must work harder.  The flexibility of the red cells        also reduces
apparent blood viscosity.

The movement of cells toward the center of the vessel also means that
cells move faster than plasma.

Plasma skimming

Turbulent flow--in turbulent flow, fluid does not remain confined to
specific laminae, but mixing occurs.  Larger       diameter, high
velocity, and low viscosity predispose to turbulence.  Because of large
diameter and high        velocity, the aorta is especially succeptible to
tubulence; particularly in cases of aortic valve stenosis.  Also
sometimes, pulmonary artery and inferior vena cava.

Irregularities in vessel walls also produce turbulence. Turbulence can 
        cause damage to vessel walls 
        makes heart work harder
        clots more likely to form in turbulent blood flow.


Pulsatile flow--
The relationship between pressure and flow is more complicated in the
arterial system, since it is not a steady flow as         discussed above
and the elastic nature of arteries.  The pulse pressure depends on:
                Volume and speed of ventricular ejection
                Rate of runoff through the peripheral vessels
                Distensibility of the artery wall
Despite pulsatile nature, flow is still usually laminar.

None of above applies to capillaries, in which flow is very slow and cells
squeeze through single file.  Diameter of    many capillaries is less than
that of a red blood cell, Deformability of RBCs is critical to capillary
flow.  Conditions in which RBCs are inflexible (sickle cell anemia,
spherocytosis) impair capillary flow.
Flow in capillaries is very slow because of the small cross-section of the
vessels.  Allows ample time for exchange.
The total cross-sectional area of the capillaries is 700-800 times that of
the aorta, so despite the small caliber of   individual capillaries, the
enormous number of parallel vessels in any capillary bed, means relatively
low      resistance through the bed as a whole.

Narrowing of vessels (as in the case of atherosclerotic plaque) 
        decreases conduction velocity
        causes turbulence 
        reduces elasticity, which increases BP.

No!
Please don't make me go to any of those pages!
I want to go somewhere completely different!