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The function of the mammary gland providing milk nourishment to the newborn mammal. This process is under the control of the endocrine and nervous systems. It involves transformation of an inactive duct system to a lobuloalveolar glandular structure during pregnancy, cellular production of the components of milk (galacto-poiesis), secretion into the ducts, and ejection under the stimulus of milking or suckling.
Lactation makes demands on the maternal regulation of calcium metabolism. Resorption of bone increases in lactating rats and women, and there is a marked increase in the absorption of calcium from the intestine. The elevated need for calcium results in an increased role for parathyroid hormone, calcitonin, and vitamin D in the regulation of the absorption and utilization of calcium. In humans a concomitant phenomenon frequently associated with lactation is amenorrhea. Consequently in some societies prolonged nursing is used as a birth control technique. See Mammary gland, Milk
the process of the formation, accumulation, and periodic release of milk in man and mammals.
Lactation begins after parturition. Only a mammary gland that has gone through particular stages of development in the course of pregnancy is capable of secreting milk. The milk forms in the secretory epithelial cells of the alveoli of the mammary gland from component parts of the blood. There are four stages of lactation: the absorption of the precursors of milk from the blood by the mammary gland, the synthesis of the component parts of the milk in the secretory cells of the gland, the formation and accumulation of the synthesized products in the cytoplasm of the secretory cells, and the secretion of the milk into the cavities of the alveoli and other spaces of the mammary gland, such as the cisternae. The milk proteins are synthesized from amino acids; the milk sugar, or lactose, from glucose; and the butterfat, from the neutral fat and free fatty acids of the blood. The hypothalamus and the pituitary gland play a fundamental role in the regulation of lactation. Lactogenic hormone is secreted in large quantities during pregnancy. A hormone of the posterior pituitary lobe, oxytocin, also participates in the process of milk production.
In man. On the first day after parturition, only a few drops of colostrum can be drawn from the mammary glands. Subsequent increases in secretion proceed variously: in some cases, the quantity of milk increases gradually: in others, the influx of milk occurs suddenly and vigorously. In some women (mainly primaparas) the milk appears late; secretion begins only by the fifth or sixth day, or even as late as the beginning of the second week, developing slowly by the third week. The secretion increases gradually and continuously from the moment of influx, reaching a maximum between the tenth and 20th week and remaining at that level until the end of lactation. The amount of breast milk may fluctuate in the same woman on different days and at different hours of the same day (after sleep there is more; after a sleepless night, at the end of the day, or after strenuous work, there is less), but, in general, the amount is approximately the same for every 24-hour period. If the mammary gland has a high lactation capacity and the woman is nursing two or more children, the daily amount may reach 2,000–3,000 g.
A fully balanced diet for the mother is necessary for successful lactation. A nursing mother should consume a daily average of 110–130 g of protein, 100–130 g of fat, 450–500 g of carbohydrates, 2 mg of vitamin A, 3.5 mg of riboflavin, 25 mg of nicotinic acid, 4 mg of pyridoxine, and 100–120 mg of ascorbic acid. She should take an increased quantity of fluids—to 1.5–2l, depending on the amount of milk she is producing. Milk (but not more than 0.5 l) and milk products (for example, cottage cheese) and meat, fish, and eggs are obligatory components of the nursing woman’s diet.
The most frequent disturbance of lactation is hypogalactia, or insufficient lactation. A complete absence of milk secretion, called agalactia, is very rare and occurs as a result of hormonal disturbances. An involuntary discharge of milk is sometimes observed, at nursing time or independent of nursing, that arises not from an abundance of milk but as a functional disturbance of mammary activity in women with heightened nervous excitability; this requires no special treatment but generally fortifying measures. Decreased lactation is observed with all chronic wasting diseases in the mother (diabetes, nephritis, cancer), with acute illnesses accompanied by high temperatures and loss of appetite (for example, mastitis), with chronic infections (especially tuberculosis), and with diarrhea. Medicines taken by the nursing mother only slightly reflect on lactation; however, quantities may decrease under the influence of laxatives, diuretics, camphor, and atropine.
In animals. Lactation increases in animals during the early period after parturition and then gradually decreases. Cows produce the greatest amount of milk at the end of the first month and during the second month after calving. In some animals (cows, mares, goats), lactation decreases with a new pregnancy. In order to obtain higher milk yields during the next lactation, the cow is not milked for 1.5–2 months before calving (the dry period). This should be begun even earlier with mares and goats. The period from the beginning of lactation (after parturition) until the end of milk production is called the lactation period. In cows, this period averages 300 days; in goats, eight to ten months; in sheep, four to five months; in mares, nine months or more; and in sows, 60 to 70 days. The duration of the lactation period and the amount of milk obtained depend on the species, breed, feeding schedule, feed content, time of onset of the new pregnancy, and length of the dry period. Disturbances of lactation are observed with mastitides, involuntary milk discharge, and milking difficulties.
REFERENCESEspe, D. Sekretsiia moloka. Moscow, 1950. (Bibliography; translated from English.)
Emel’ianov, A. S. Laktatsionnaia deiatel’nost’ korov i upravlenie eiu. Vologda, 1953.
Grachev, I. I. Reflektornaia reguliatsiia laktatsii. Leningrad, 1964.
Azimov, G. I. Kak obrazuetsia moloko [2nd ed.] Moscow, 1965.
Neirogormonal’naia reguliatsiia laktatsii. Moscow-Leningrad .