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Resistance is an accumulation of three different factors: blood viscosity, blood vessel length and vessel radius. [30] Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood.
The tunica intima has the most variation in blood vessels, in terms of their wall thickness and relative size of their lumen. The endothelial cells continuously produce nitric oxide a soluble gas, to the cells of the adjacent smooth muscle layer. This constant synthesis is carried out by the enzyme endothelial nitric oxide synthase (eNOS). [32]
The Moens–Korteweg equation states that PWV is proportional to the square root of the incremental elastic modulus, (E inc), of the vessel wall given constant ratio of wall thickness, h, to vessel radius, r, and blood density, ρ, assuming that the artery wall is isotropic and experiences isovolumetric change with pulse pressure. [5]
P is the blood pressure; t is the wall thickness; r is the inside radius of the cylinder. is the cylinder stress or "hoop stress". For the thin-walled assumption to be valid the vessel must have a wall thickness of no more than about one-tenth (often cited as one twentieth) of its radius.
Intima–media thickness (IMT), also called intimal medial thickness, is a measurement of the thickness of tunica intima and tunica media, the innermost two layers of the wall of an artery. The measurement is usually made by external ultrasound and occasionally by internal, invasive ultrasound catheters .
These lamellae vary in number according to the size of the vessel; the smallest arteries having only a single layer, [2] and those slightly larger three or four layers - up to a maximum of six layers. [3] It is to this coat that the thickness of the wall of the artery is mainly due.
Blood viscosity is a measure of the resistance of blood to flow. It can also be described as the thickness and stickiness of blood. This biophysical property makes it a critical determinant of friction against the vessel walls, the rate of venous return, the work required for the heart to pump blood, and how much oxygen is transported to tissues and organs.
In the largest vessels, the vasa vasorum penetrates the outer (tunica adventitia) layer and middle (tunica media) layer almost to the inner (tunica intima) layer. In smaller vessels it penetrates only the outer layer. In the smallest vessels, the vessels' own circulation nourishes the walls directly and they have no vasa vasorum at all.