  We compare the isoperimetric profiles of $S^2 \times \re^3$ and of $S^3
\times \re^2$ with that of a round 5-sphere (of appropriate radius). Then we
use this comparison to obtain lower bounds for the Yamabe constants of $S^2
\times \re^3$ and $S^3 \times \re^2$. Explicitly we show that $Y(S^3 \times
\re^2, [g_0^3 +dx^2]) > (3 /4) Y(S^5)$ and $Y(S^2 \times \re^3, [g_0^2 +dx^2])
> 0.63 Y(S^5)$. We also obtain explicit lower bounds in higher dimensions and
for products of Euclidean space with a closed manifold of positive Ricci
curvature. The techniques are a more general version of those used by the same
authors in previous work and the results are a complement to the work developed
by B. Ammann, M. Dahl and E. Humbert to obtain explicit gap theorems for the
Yamabe invariants in low dimensions.
