Hi Nano!
.....you wrote"My project is to create the inside of a blood artery.
Is it the case that I need to turn off the sky and terrain so to have
a "blank canvas" so to speak?
If so how do I do this?"
Edit : There are 2 ways of doing this.
either like Stem showed you here or like this.....
Ok, here's what you can do.....
1. You delete the ground plane and just press the "Delete" key as Stem instructed in his post.
2. You go to the Sky Edit options and click on the little arrow.
3. A window will open with several sky options, choose backdrops.

Now from there, you can choose any colour you want for the backdrops.
....this will make the sky or any celestial atmosphere go away. Notice that
you never get a complete transparent background, you always need to choose
a backdrop colour.