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When the infant suckles, sensory nerve fibers in the areola trigger a neuroendocrine reflex that results in milk secretion from lactocytes into the alveoli. The posterior pituitary releases oxytocin, which stimulates myoepithelial cells to squeeze milk from the alveoli so it can drain into the lactiferous ducts, collect in the lactiferous sinuses, and discharge through the nipple pores. It takes less than 1 minute from the time when an infant begins suckling (the latent period) until milk is secreted (the let-down). [link] summarizes the positive feedback loop of the let-down reflex    .

Let-down reflex

This figure shows the process of let down reflex, the process in which the brain receives sensory impulses to release the hormones necessary for producing and discharging milk to the suckling newborn.
A positive feedback loop ensures continued milk production as long as the infant continues to breastfeed.

The prolactin-mediated synthesis of milk changes with time. Frequent milk removal by breastfeeding (or pumping) will maintain high circulating prolactin levels for several months. However, even with continued breastfeeding, baseline prolactin will decrease over time to its pre-pregnancy level. In addition to prolactin and oxytocin, growth hormone, cortisol, parathyroid hormone, and insulin contribute to lactation, in part by facilitating the transport of maternal amino acids, fatty acids, glucose, and calcium to breast milk.

Changes in the composition of breast milk

In the final weeks of pregnancy, the alveoli swell with colostrum    , a thick, yellowish substance that is high in protein but contains less fat and glucose than mature breast milk ( [link] ). Before childbirth, some women experience leakage of colostrum from the nipples. In contrast, mature breast milk does not leak during pregnancy and is not secreted until several days after childbirth.

*Cow’s milk should never be given to an infant. Its composition is not suitable and its proteins are difficult for the infant to digest.
Compositions of Human Colostrum, Mature Breast Milk, and Cow’s Milk (g/L)
Human colostrum Human breast milk Cow’s milk*
Total protein 23 11 31
Immunoglobulins 19 0.1 1
Fat 30 45 38
Lactose 57 71 47
Calcium 0.5 0.3 1.4
Phosphorus 0.16 0.14 0.90
Sodium 0.50 0.15 0.41

Colostrum is secreted during the first 48–72 hours postpartum. Only a small volume of colostrum is produced—approximately 3 ounces in a 24-hour period—but it is sufficient for the newborn in the first few days of life. Colostrum is rich with immunoglobulins, which confer gastrointestinal, and also likely systemic, immunity as the newborn adjusts to a nonsterile environment.

After about the third postpartum day, the mother secretes transitional milk that represents an intermediate between mature milk and colostrum. This is followed by mature milk from approximately postpartum day 10 (see [link] ). As you can see in the accompanying table, cow’s milk is not a substitute for breast milk. It contains less lactose, less fat, and more protein and minerals. Moreover, the proteins in cow’s milk are difficult for an infant’s immature digestive system to metabolize and absorb.

The first few weeks of breastfeeding may involve leakage, soreness, and periods of milk engorgement as the relationship between milk supply and infant demand becomes established. Once this period is complete, the mother will produce approximately 1.5 liters of milk per day for a single infant, and more if she has twins or triplets. As the infant goes through growth spurts, the milk supply constantly adjusts to accommodate changes in demand. A woman can continue to lactate for years, but once breastfeeding is stopped for approximately 1 week, any remaining milk will be reabsorbed; in most cases, no more will be produced, even if suckling or pumping is resumed.

Mature milk changes from the beginning to the end of a feeding. The early milk, called foremilk    , is watery, translucent, and rich in lactose and protein. Its purpose is to quench the infant’s thirst. Hindmilk is delivered toward the end of a feeding. It is opaque, creamy, and rich in fat, and serves to satisfy the infant’s appetite.

During the first days of a newborn’s life, it is important for meconium to be cleared from the intestines and for bilirubin to be kept low in the circulation. Recall that bilirubin, a product of erythrocyte breakdown, is processed by the liver and secreted in bile. It enters the gastrointestinal tract and exits the body in the stool. Breast milk has laxative properties that help expel meconium from the intestines and clear bilirubin through the excretion of bile. A high concentration of bilirubin in the blood causes jaundice. Some degree of jaundice is normal in newborns, but a high level of bilirubin—which is neurotoxic—can cause brain damage. Newborns, who do not yet have a fully functional blood–brain barrier, are highly vulnerable to the bilirubin circulating in the blood. Indeed, hyperbilirubinemia, a high level of circulating bilirubin, is the most common condition requiring medical attention in newborns. Newborns with hyperbilirubinemia are treated with phototherapy because UV light helps to break down the bilirubin quickly.

Chapter review

The lactating mother supplies all the hydration and nutrients that a growing infant needs for the first 4–6 months of life. During pregnancy, the body prepares for lactation by stimulating the growth and development of branching lactiferous ducts and alveoli lined with milk-secreting lactocytes, and by creating colostrum. These functions are attributable to the actions of several hormones, including prolactin. Following childbirth, suckling triggers oxytocin release, which stimulates myoepithelial cells to squeeze milk from alveoli. Breast milk then drains toward the nipple pores to be consumed by the infant. Colostrum, the milk produced in the first postpartum days, provides immunoglobulins that increase the newborn’s immune defenses. Colostrum, transitional milk, and mature breast milk are ideally suited to each stage of the newborn’s development, and breastfeeding helps the newborn’s digestive system expel meconium and clear bilirubin. Mature milk changes from the beginning to the end of a feeding. Foremilk quenches the infant’s thirst, whereas hindmilk satisfies the infant’s appetite.

Questions & Answers

vocal cords
MT Reply
musah was admitted to your ward yesterday with traumatic amputation of the right thump . he complained this morning that he has not passed enough urine. Questions. 1) will you consider his complain to be pathological or physiological?. 2) in less than three sentences, justify your opinion. 3) Apply your understanding of the renin- angiotensin-aldosterone system to explain the factors and mechanisms accounting for his complains.
Ellen Reply
the mineral salt which break up a large portion of bone tissue is
Aurora Reply
calcium
Calcium
Sawda
please I need help. musah was admitted to your ward yesterday with traumatic amputation of the right thump. he complained this morning that he has not passed enough urine. Questions. 1). will you consider his complain to be pathological or physiological?.
Ellen
2). In less than three sentences, justify your opinion.
Ellen
Physiological
Ruth
cholesterol and triglycerides in simplest terms are fats (lipid) found in blood
Roy Reply
calcium is important for bone density and development. it also helps for safe / normal blood clotting, blood circulation to and from the heart, and muscle movement
Roy
what is muscle tissue
Rifat Reply
the muscle tissue is one of the 4 basic tissues in our body and is responsible for movement of our body
What Is difference between Ovary and Ovum
Sawda
ovary is female reproductive organ while ovum is a female gamete formed from ovary
Ubaidah
Thank You
Sawda
What Is Cholesterol and Triglycerides
Sawda
the mineral salt which break up a large portion of bone tissue
Aurora
in simple way muscle tissue is responsible for movement of our body.
Wellington
Hi Be Home Be safe , how are things doing hope all is well
Natarajan Reply
3 longitudinal bands of smooth muscles found in large intestines
Jamia
what's is sutures
Nimeshka Reply
what would I like to know
Roy Reply
anything u can tell me
Roy
anatomy mins
Manish
when two or more bones meet.
Joseph
Joints
Anita
I am interested in learning but it is a little threatening corona virus covid 19
Samnang Reply
I don't know about Corona virus
Vicky
what would you like to know?
Mbasa
what is a peripheral protien
Ayesha Reply
actually its located in between the lipid layer, it does not specify if it's closer to the inside or the outside of the cell
Justin
It is protein found in lipid bilayer but found attached with Cytoplasm aspect
Jamal
what are the collection of blood.?
sunshine Reply
Effect of exercise on different body systems?
Rania Reply
what is ambroylogy
kashif Reply
embryology..is the biological studing of embryos
Ava
I know biological study but embryology mean any pic, example?
kashif
https://upload.wikimedia.org/wikipedia/commons/b/b6/Comparative_embryology_of_the_vertebrates%3B_with_2057_drawings_and_photos._grouped_as_380_illus_%281953%29_%2820482505100%29.jpg
Ava
I like to learn about medical and more
Samnang
what is the function of the blood
Yolanda Reply
Transporting of oxygen,fighting against germs, forms clotting ,distribution of nutrients and minerals through out the body ,
Nimco
Transportation of gases such as oxygen and water blance and carrei metabolites to the exit organ and Acid base equilibrium and clotting blood and Immune
Jamal
What are Gross and microscopicAnatomy
Waiswa Reply
study of the internal structures of a human being
Niyi
gross anatomy is the study of body parts that can be seen with our naked eyes while micro anatomy involves the study of body parts that cannot be seen with our naked eyes but with the aid of a microscope
Oppong
gross means examination of specimen or tissue with bare (unaided ) eye while microscopic means examination of same with the help of microscope
Jamia

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Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
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