  We study flows of smooth vector fields $X$ over invariant surfaces $M$ which
are levels of rational first integrals. It leads us to study constrained
systems, that is, systems with impasses. We identify a subset $\mathcal{I}
\subset M$ which we call "pseudo-impasse" set and analyze the flow of X by
points of $\mathcal{I}$. Systems well known in the literature exemplify our
results: Lorenz, Chen, Falkner-Skan and Fisher-Kolmogorov. We also study
1-parameter families of integrable systems and unfolding of minimal sets. Our
main tool is the geometric singular perturbation theory.
