Inside The Body
Internal
noun, の adjective
The body's inside means inside the body or internal.
体内 refers specifically to internal body stuff, so you'll often see it in medical and scientific contexts.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
どうしても水は体内から出る。
Water comes out of the body no matter what.
体内にウィルスが入ったようだ。
It seems that a virus has entered the body.
体内のアルコール分が不足してる気がします。
I feel like the alcohol content in the body is lacking.