In some cases, they are bound to other compounds, but synthesis does not take place within the cells. The milk of individual first-calf heifers with noninfected mammary glands has a constant amount of sodium, potassium, and lactose. Within a breed, a close inverse relationship exists between the lactose content and the molar sums of potassium and sodium concentrations. These two portions are constant within an individual but vary between animals. The constituents primarily responsible for the osmotic pressure of milk are lactose and sodium and potassium and their associated anions.
Lactose alone is responsible for more than one-third of the osmotic pressure of normal milk.
In addition to the primary secretio of water in milk, a blood plasma transudate, which is rich in sodium and chlorine, contributes to the fluid of milk. This type of secretion appears to be important in the secretion of milk that is associated with advancing lactation, advancing age, and bacterial infection of the udder. Milk secreted during these times has increased percentages of sodium and chlorine and decreased percentages of lactose and potassium. Milk is in osmotic equilibrium with the blood flowing through the udder throughout the period that milk remains in the udder nad not just during its formation. Changes in milk composition occur as water moves in and out of the udder in response to changes in the osmotic pressure of blood.