Right side
包 supplies the sound (bāo → pào, regular b/p drift plus tone shift).
包 itself depicts a fetus wrapped inside a womb — the original sense was 'wrapping, enclosing.' That image fits beautifully here: a
泡 is a thin film of water wrapped around a pocket of air. Same phonetic in
抱 hug,
跑 run,
饱 full.