Lactose pulls water into the milk by osmotic pressure along the
concentration gradient.  To put that into English, the lactose pulls
water in in the body's attempt to equalize the electrochemical
properties of the fluid inside and outside of the milk ducts.

We can use the flow of water between the blood and interstitial fluids
as an example to understand this process:
One molecule of salt in the blood stream will pull in one molecule of
water from the surrounding tissues.
One molecule of albumin (the major blood protein) will pull in something
like 18 molecules of water.  This is why people who have protein energy
malnutrition get swollen - they are not taking in enough protein to make
albumin, so nothing is making water come back into the bloodstream from
the body tissues, and water gets "stuck" there.  This is not the only
cause of edema, but it is the cogent one in toxemia/eclampsia.

Lactose has a high "osmotic pressure", like albumin does.  This means it
attracts lots of water per molecule, though I'm not sure how much
exactly.  Therefore, lactose has the primary responsibility for milk
volume....the more lactose the breast makes, the more water is pulled
into the milk, and the more volume the mom has.

There is also an inverse relationship between lactose and sodium in the
milk.  When lactose production falls rapidly, pores between milk gland
cells open up (we think to protect the breast during involution), and
more sodium can enter the milk.  This happens during mastitis too, as
the metabolism of the breast falls during the inflammation.

The above is a simplification, but I hope it still retains sufficient
validity to be useful.  If not, I'm sure someone will say so!  Nicely,
of course! ;-}
--
Catherine Watson Genna, IBCLC  New York City  mailto:[log in to unmask]

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