I have been doing some rudimentary research into the basement membrane and found the it contains collagen fibers which are known for their elasticity. Does this mean that the basement membrane can expand in capillaries?
It's a good question. Though there is hardly some evidence to answer your question. However I will share what all I found. So in capillary basement membrane, mostly collagen type 1 and 4 are present. Now collagen type 4 aids in elongation of the new capillaries-
in the ex vivo rat aortic model of angiogenesis using a type I collagen matrix, type IV collagen promoted new vessel elongation and survival in a dose-dependent manner compared with controls (no type IV collagen), which grew until day 9 and then regressed during the second and third week of culture (treatment with 3, 30, 300 g/ml of type IV collagen in 43, 57, 119% increase in microvascular length) . In all then, type IV collagen seems important for stability of blood vessel formation.
The collagen in capillary basement membrane impart mechanical stability to the vessel.
Also elastin is a component of the basement membrane. Reference
Now though I couldn't find that it's the elastin of basement membrane only, but-
Comprising 50% of the dry weight of major arteries, elastin is the most prominent ECM protein in arteries . In vessels, elastic lamina alternate with rings of smooth muscle forming a strong, flexible part of the arterial wall [43, 44].
Here is the article