This is the chromatic scale in the key of C.
C = root
C# = flattened ninth
D = ninth
Eb = minor third (or flattened third)
E = third
F = fourth (referred to as suspended fourth when used on its own over a triad)
F# = flattened fifth
G = fifth
G# = augmented fifth
A = sixth
Bb = flattened seventh
B = seventh (or natural or major seventh)
When soloing and, as it were, in "free-fall", most improvisors will use pretty well all of these intervals over simple chords. It is highly unlikely that anyone would think "ah-ha!I see an opportunity for a natural seventh note coming up". They are more likely to be thinking about the construction of the phrase, and what sounds satisfying to them in the round.
Some tunes actually start on the natural seventh, i.e. When it's sleepy time down south, Time on my hands, and many more.