Vak opens a set of five organs that, taken together, cover the entire range of bodily action Samkhya considers fundamental enough to warrant its own tattva: speaking (vak), grasping (pani), moving (pada), excreting (payu), and procreating (upastha). The list is notable for what it includes without embarrassment; excretion and reproduction sit as equal members alongside speech and locomotion, none ranked as more or less dignified than the others, because the classification is functional rather than evaluative, five distinct kinds of effect a body has on the world, not five achievements to be admired in descending order.
All five Karmendriyas share their origin, the sattvic aspect of ahamkara, with manas and the five Jnanendriyas, forming the complete set of eleven organs that Samkhya treats as intermediate between the ego that claims experience and the objects, gross and subtle, that experience is of. The Karmendriyas specifically supply the output side of that intermediate apparatus: once buddhi has rendered a judgment and ahamkara has claimed it, one or more of the five active organs carries out whatever response follows.
Unlike the Jnanendriyas, the Karmendriyas have no fixed one-to-one correspondence with the five tanmatras; their classification is by TYPE OF ACTION rather than by a single perceptual quality, which is why vak, pani, pada, payu and upastha are grouped by function, speaking, grasping, moving, excreting, procreating, rather than by any elemental chain analogous to the shabda-to-akasha sequence the sense organs follow.