***MILK is ALWAYS ISOSMOTIC with BLOOD PLASMA ***

• Cell membranes act to restrict diffusion of water.
• Cell membranes are semipermeable, that is they permit water to move across, but not many solutes move freely across cell membranes.
• Osmosis - the spontaneous flow of water across a membrane, down a concentration gradient of water.
• Osmotic pressure - the pressure required to prevent osmotic flow (osmosis of water into a given solution).
• Isosmotic - When two solutions with the same osmotic pressure are isosmotic.

Remember - Lactose is the major osmole in milk.

Synthesis of lactose in the Golgi results in an impermeant small molecule that is formed effectively at an extracytoplasmic site. Water is osmotically drawn into the Golgi/secretory vesicles to try to dilute the lactose. Lactose accounts for ~50% of the osmotic pressure of milk (the rest is citrate, ions, proteins, etc.). This flow of water probably induces an electrical potential difference across the apical membrane when the secretory vesicles fuse with the apical membrane and release their contents. The electrical potential causes Na+ and K+ to preferentially return to the intracellular (cytoplasmic) fluid, even against a concentration gradient. The apical membrane is impermeant to Ca, citrate and phosphate - these do not flow back into the cell.

Ducts are impermeable to the major constituents of milk. There is no resorption of water in the ducts. Ducts act only as a conduit for moving milk from alveoli to the teat and the large ducts for passive storage of milk between milk removals. THEREFORE, the ionic composition of milk is determined in the secretory alveolar cells.

The concentration of lactose in milk is inversely related to the molar sum of K+ and Na+ in milk. Lactose synthesis keeps milk K+ and Na+ low.

Species differences in lactose : ion ratios may be due to differences in the rates of lactose synthesis and in water permeability across cell membranes.

Blood and milk are isosmotic (6.6 atmospheres), but are not in chemical equilibrium:

Milk has:

90 x as much sugar as blood

13 x as much Ca as blood

10 x as much Pi as blood

9 x as much fat as blood

5 x as much K as blood

1/2 as much protein as blood

1/7 as much Na as blood

Salts and Water in Milk

(see Peaker, 1978, Vol. 4, p. 437, in Lactation, B. L. Larson ed.; Holt, 1981, J. Dairy Sci. 64:1958)

Salts in milk exist as ions, ion complexes in solution, ion complexes with proteins, and as dispersed Ca-P in casein micelles.

• The major cations are: K+,Na+, Ca++, Mg++
• These are balanced by anions: Cl-, Pi, citrate also by sugar-phosphates, nucleotide-phosphates, carbonate, sialyloligosaccharides, proteins and other minor anions.

Most of these have functions in the cell and their concentrations in milk reflect intracellular levels.

• Na, K, Cl are very diffusable.
• Ca, Mg, Pi and citrate only partly diffusible.
• Most of the Ca and Mg are in the casein micelles. Some Ca is bound to the phosphates on the casein, while some is part of the dicalcium-phosphate complex that is part of the micelle.
• Mg, phosphate, and citrate are also part of the micelle.

Remember, water is a major component of milk of all species. Cow milk is ~87% water.