Biomedical Engineering Reference
In-Depth Information
Tunica adventitia
(fibroblasts, collagen,
nerves)
Tunica media
(smooth muscle cells,
collagen, elastin)
Tunica intima
(endothelial cells,
basal lamina)
Lumen
FIGURE 17.3
Blood vessels, except capillaries, are composed of three layers: the tunica intima, tunica
media, and tunica adventitia. Capillaries lack the tunicae media and adventitia. The major components of
each layer are given.
fi bers (tunica adventitia). These layers give extra support and strength to large blood vessels, pre-
venting rupture.
The arteries transport oxygen-rich blood from the heart to all tissues. The only exception is the
pulmonary artery that comes from the heart and leads oxygen-poor blood to the lungs for oxygen-
ation, and release of carbon dioxide. The blood pressure in the arteries is high, is reduced in the
capillary bed, and is low in the veins. The veins transport the blood back to the heart. Veins contain
valves to avoid retrograde fl ow. The surface area of the capillaries is large, especially in the lungs
(effi cient exchange of oxygen and CO
2
) and the organs and tissues (nutrient and gas exchange).
Because of this, capillaries only consist of one layer of endothelial cells on a thin basal lamina. This
morphology enables effi cient and easy exchange of gases and nutrients.
17.4 BLOOD- CONTACTING DEVICES
Blood-contacting devices are medical devices that are intended to function in direct contact with
blood. These devices include vascular prosthesis, heart valves, intravenous catheters, blood bags,
ventricular assist devise, guide wires, intravascular stents, hemodialysis fi lters, tubing for heart-lung
machines, etc. To avoid loss of function, the surfaces of these devices should display no coagulation,
infl ammation, or hemolysis.
1,16
-
18
The interaction of the synthetic surface with plasma proteins and the blood cells and platelets
determines blood compatibility. The chemical composition and topography of the synthetic surface
are of vital importance for blood compatibility. Additionally, some devices perform a mechani-
cal task, like heart valves and blood-pumps. Hemolysis, or lysis of blood cells, by these devices is
mostly dependent on the design of the moving parts.
The principal problem of all these devices is that they are more or less thrombogenic, mean-
ing that they cause biomaterial-induced thrombus (blood clot) formation. To prevent failure of the
device, often systemic anticoagulation drug therapy is used. The downside of this therapy is that it
is chronic and increases the chance of hemorrhage for the patient. Therefore, biomedical engineers
have designed a variety of surface modifi cations that aim to prevent thrombus formation on the
surface. Some of these modifi cation strategies will be discussed in this chapter. However, it should
be kept in mind that in order to design successful blood-contacting surfaces, it is essential to under-
stand the events that occur at the interface between blood and a synthetic surface.
