Breast milk is the substance produced from the human female mammary glands shortly after childbirth in order to feed newborn infants.
From the sixth month of pregnancy, the mother's mammary glands are fully developed, and the gland cells begin to secrete a substance known as colostrum. This secretion, which is extremely high in protein and low in fat, is ingested by the infant in the first 2-3 days of life. This thin, yellow-ish liquid is high in antibodies, which provides passive immunity to the infant.
After this time, the production of colostrum diminishes, and the mammary glands increase the production of breast milk. Initially, milk produced at the beginning of feeding is watery, thin and sweet. This milk, termed foremilk, is low in fat and high in carbohydrate composition, which quenches the thirst of the infant. As the feed progresses, the foremilk changes in consistency to become much thicker and creamier. This milk, termed hindmilk, is responsible for satisfying hunger.
Milk secreted by the mammary glands is sterile and consists of an abundance of salts, sugars, vitamins, fatty and amino acids, as well as water. It is highly nutritious, providing 750 calories per liter, and, on average, an infant will consume approximately 850 ml a day.
The physiological process in which milk is secreted from the mammary glands is called lactation.
Lactation is another example of a positive feedback system. It is regulated by a homeostatic mechanism through which milk ejection is stimulated by a suckling infant. Milk ejection intensifies and increases through positive feedback until the infant stops suckling.
Stimulus: Mechanical stimulation of the nipple by a suckling infant.
The milk release reflex is also referred to as the 'let-down reflex'. This can also be triggered by factors such as hearing a baby and thinking about breastfeeding.
Receptor: Peripheral mechanoreceptors in the nipple detect suckling.
Control center: Information in the form of electrical signals (nervous impulses) are sent from peripheral mechanoreceptors to the hypothalamus, which in turn relays information in the form of chemical signals (hormones) to the posterior pituitary.
Effector: The hormone oxytocin, which is also responsible for the onset of labor, is released from the posterior pituitary gland and acts as a chemical signal, stimulating the effector cells surrounding the milk-producing glands of the breast.
Response: Cells surrounding the milk-producing glands of the breast contract, triggering milk ejection.
Feedback: The milk production then reinforces the original stimulus and promotes the original milk ejection response, in a positive feedback loop, until the stimulus is removed (the infant stops suckling).
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