Breast milk is reabsorbed by the mother’s body when it is not removed by the infant or through expression. This complex fluid is derived from the mother’s blood supply, providing complete nutrition for the infant. Reabsorption is a fundamental physiological mechanism that manages milk volume and regulates supply. The inability to regularly empty the breast leads to this reabsorption, which signals the mammary gland to slow down or halt new milk production.
Milk Storage Within the Breast
Milk production and storage occur in the tiny, balloon-like structures within the breast called alveoli. Each alveolus is lined by specialized milk-secreting cells, known as lactocytes, which synthesize milk from the mother’s bloodstream. The milk is then released into the hollow center, or lumen, of the alveolus.
These alveoli cluster into lobules connected to a network of ducts that transport the milk toward the nipple. The mammary gland is designed to produce and hold milk until the infant triggers the milk-ejection reflex. When the breast is full, the milk fills the alveolar lumen, causing tissues to distend and raising the pressure inside the gland.
Milk is stored in thousands of these microscopic sacs and connecting ducts, not a single large chamber. Its presence creates a hydrostatic pressure that opposes the continuous production process. This pressure that builds up inside the alveoli when milk is not removed is the initial physical trigger for reabsorption.
The Physiological Mechanism of Reabsorption
The physical mechanism by which milk components move back into the mother’s circulation relies on the structure of the mammary secretory epithelium. The lactocytes that line the alveoli are normally held together by tight junctions, forming a strong barrier between the milk and the surrounding tissue fluid.
When the breast is full and the pressure within the alveoli increases, these tight junctions separate slightly, opening the paracellular pathway. This pathway allows for the passive transport of smaller components from the milk back into the mother’s interstitial fluid, and subsequently into the bloodstream and lymphatic system. Water and electrolytes, such as sodium and chloride, move across this pathway most easily.
The reabsorption of water, driven by osmotic forces, helps reduce the volume and pressure within the engorged breast. While larger milk components like lactose and many proteins are too big to pass through the tight junctions, some smaller proteins and fats can be reabsorbed through other transcellular routes.
Regulating Milk Supply and Weaning
The reabsorption of milk components is intrinsically linked to the chemical regulation of milk synthesis, which controls the overall supply. A whey protein found naturally in breast milk, called the Feedback Inhibitor of Lactation (FIL), plays a significant role in this regulation. When milk is not removed, the concentration of FIL increases within the alveolar lumen.
The accumulation of FIL acts as a local signal, telling the lactocytes to slow down new milk production. This is a form of autocrine control, meaning the milk regulates its own production based on the removal rate. The higher the concentration of FIL, the stronger the signal to inhibit further synthesis.
This mechanism is relevant during weaning or when managing engorgement. If milk removal is abruptly stopped, the combination of rising pressure and high FIL concentration quickly signals the body to reduce the supply. During a gradual weaning process, the slow removal of milk allows the FIL mechanism to slowly down-regulate production, helping the mammary gland adjust over time.