I hesitated to reply to this question. The mammary gland and the process of lactation are so complex that any summary of a couple of hundred words must necessarily be vague. I don't know exactly what you want to know and at what level, so you may have to come back, either for more detail or for greater simplicity. Here goes...

 During lactation the milk ducts start from little "dead ends", called alveoli. The alveoli are lined with the cells that secrete the milk as a fluid. The bases of those cells are connected to a fine membrane in close contact with the walls of capillary blood vessels. The raw materials that the secretory cells need for making the milk are carried towards the breast in the blood, and passed across the cell membranes into the secretory cells of the alveolus.

 Milk includes certain salts, lactose (milk sugar), and water. These have to be combined in various ways to maintain the right osmotic balance. Disease conditions such as mastitis can cause a greater release of salts, which in osmotic compensation causes a smaller release of lactose into the blood.

Milk also includes various proteins, both particular to milk, such as lactalbumin, and other proteins either more or less randomly or functionally included, such as antibodies from the mother' s immune system.  The proteins either get absorbed directly from the blood, or, if they are milk proteins in particular, the glandular cell puts them together from amino acids. They pass through the cell wall into the lumen, the hollow of the alveolus, by a process called exocytosis, the standard channel for secreting proteins through cell walls.

 Then there are the fats and fat-soluble materials, such as certain vitamins.

 All the raw materials from the blood enter through the bases of the cells. Most of the materials undergo various degrees of change and metabolism by the glandular cells as theypass the products on towards the inner wall of the alveolus. Fats for example get collected into large (relative to the size of the cells) droplets and as the droplets get secreted into the duct, they get coated with a layer of cell wall material consisting largely of proteins and lipids, as they pass through the cell wall. All the fat in fresh cream is in the form of those little coated droplets. It keeps the fat from clotting into large buttery lumps that could cause trouble.

The blood contains practically no lactose; and the milk contains hardly any other sugar; this suits the babies' digestive systems better than flooding them with glucose and fructose. However this requirement leaves the cell with the problem of making the lactose. It must form each molecule of lactose from a molecule of glucose combined with a molecule of galactose. There is not enough galactose in blood to support full-scale lactation either, so the cell must make up the difference. It takes a molecule of glucose and modifies it chemically into galactose which it then fastens onto another glucose molecule, making lactose. That lactose it then secretes into the lumen of the alveolus, from which the milk moves down during lactation.

 As you can guess, the final mix takes place in the alveoli and the ducts.

 By this time I am beginning to lose count of what I have still to tell you, so I am signing off at this point. Let us know if we can tell you more.

 All the best,

 Jon