Breastfeeding

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Abnormal heart sounds
Normal heart sounds
Action potentials in myocytes
Action potentials in pacemaker cells
Baroreceptors
Blood pressure, blood flow, and resistance
Cardiac conduction velocity
Cardiac cycle
Cardiac excitation-contraction coupling
Cardiovascular system anatomy and physiology
Cerebral circulation
Changes in pressure-volume loops
Chemoreceptors
Compliance of blood vessels
Coronary circulation
ECG basics
ECG axis
ECG intervals
ECG rate and rhythm
ECG QRS transition
ECG normal sinus rhythm
ECG cardiac infarction and ischemia
ECG cardiac hypertrophy and enlargement
Electrical conduction in the heart
Excitability and refractory periods
Frank-Starling relationship
Laminar flow and Reynolds number
Lymphatic system anatomy and physiology
Microcirculation and Starling forces
Pressure-volume loops
Pressures in the cardiovascular system
Renin-angiotensin-aldosterone system
Resistance to blood flow
Stroke volume, ejection fraction, and cardiac output
Cellular structure and function
Selective permeability of the cell membrane
Cell-cell junctions
Osmosis
Cell signaling pathways
Cytoskeleton and intracellular motility
Cell membrane
Extracellular matrix
Endocytosis and exocytosis
Resting membrane potential
Nuclear structure
Atrophy, aplasia, and hypoplasia
Hair, skin and nails
Skin anatomy and physiology
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Parathyroid hormone
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Insulin
Synthesis of adrenocortical hormones
Cortisol
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Growth hormone and somatostatin
Adrenocorticotropic hormone
Endocrine system anatomy and physiology
Androgens and antiandrogens
Gastrointestinal system anatomy and physiology
Anatomy and physiology of the teeth
Enteric nervous system
Hunger and satiety
Esophageal motility
Chewing and swallowing
Gastric motility
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Bile secretion and enterohepatic circulation
Liver anatomy and physiology
Carbohydrates and sugars
Proteins
Prebiotics and probiotics
Hydration
Fats and lipids
Blood components
Platelet plug formation (primary hemostasis)
Coagulation (secondary hemostasis)
Role of Vitamin K in coagulation
Clot retraction and fibrinolysis
Blood groups and transfusions
Introduction to the immune system
Vaccinations
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B-cell development
T-cell development
Cytokines
Antibody classes
B-cell activation and differentiation
Somatic hypermutation and affinity maturation
T-cell activation
VDJ rearrangement
MHC class I and MHC class II molecules
Cell-mediated immunity of CD4 cells
Cell-mediated immunity of natural killer and CD8 cells
Contracting the immune response and peripheral tolerance
B- and T-cell memory
Skeletal system anatomy and physiology
Cartilage structure and growth
Bone remodeling and repair
Fibrous, cartilage, and synovial joints
Muscular system anatomy and physiology
Muscle contraction
Slow twitch and fast twitch muscle fibers
Sliding filament model of muscle contraction
Neuromuscular junction and motor unit
Nervous system anatomy and physiology
Anatomy and physiology of the eye
Anatomy and physiology of the ear
Neuron action potential
Sympathetic nervous system
Parasympathetic nervous system
Adrenergic receptors
Cholinergic receptors
Pyramidal and extrapyramidal tracts
Basal ganglia: Direct and indirect pathway of movement
Cerebellum
Somatosensory receptors
Optic pathways and visual fields
Vestibular transduction
Olfactory transduction and pathways
Taste and the tongue
Vestibulo-ocular reflex and nystagmus
Auditory transduction and pathways
Photoreception
Somatosensory pathways
Cranial nerves
Brachial plexus
Muscle spindles and golgi tendon organs
Renal system anatomy and physiology
Body fluid compartments
Movement of water between body compartments
Renal clearance
Kidney countercurrent multiplication
Antidiuretic hormone
Osmoregulation
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Proximal convoluted tubule
Distal convoluted tubule
Urea recycling
Tubular secretion of PAH
Tubular reabsorption of glucose
Physiologic pH and buffers
Buffering and Henderson-Hasselbalch equation
The role of the kidney in acid-base balance
Plasma anion gap
Acid-base map and compensatory mechanisms
Metabolic acidosis
Metabolic alkalosis
Respiratory acidosis
Respiratory alkalosis
Phosphate, calcium and magnesium homeostasis
Loop of Henle
Anatomy and physiology of the female reproductive system
Estrogen and progesterone
Oxytocin and prolactin
Menstrual cycle
Pregnancy
Stages of labor
Breastfeeding
Menopause
Anatomy and physiology of the male reproductive system
Testosterone
Puberty and Tanner staging
Respiratory system anatomy and physiology
Lung volumes and capacities
Ventilation
Alveolar surface tension and surfactant
Anatomic and physiologic dead space
Alveolar gas equation
Hypoxia
Oxygen binding capacity and oxygen content
Oxygen-hemoglobin dissociation curve
Erythropoietin
Carbon dioxide transport in blood
Regulation of pulmonary blood flow
Zones of pulmonary blood flow
Pulmonary shunts
Ventilation-perfusion ratios and V/Q mismatch

Transcript

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Breast milk is pretty amazing; it has all of the nutrients that a baby needs in the first six months of life. The benefits for the baby are impressive - they include lower rates of allergies, ear and lung infections, obesity, and sudden infant death, as well as healthier weight gain, and other long-term outcomes. That’s compared to infants given cow-milk formula. Moms can benefit from breastfeeding, too. It reduces uterine bleeding, burns calories, and decreases the risk of breast, ovarian, and uterine cancer, as well as osteoporosis, arthritis, type II diabetes, and heart disease. Finally, breastfeeding is free and offers mothers and babies a valuable opportunity to bond from the very first skin-to-skin contact—which should start minutes after birth.

To understand breastfeeding, let’s start with the breasts themselves. Breast tissue develops during puberty, and is made up of adipose or fat tissue, as well as glandular tissue that makes the milk, and lactiferous ducts which serve as passageways which guide the milk to the nipple.

Zooming in on the glandular tissue, there is the alveolus, which is a modified sweat gland made up of alveolar cells which actually make the breast milk. Wrapping around the alveolus are special myoepithelial cells that squeeze down and push the milk out of the alveolus, down the lactiferous ducts, and out one of the pores on the nipple, at which point it enters the baby’s mouth. When the breasts are full of milk they can get heavy, and there are suspensory ligaments called Cooper’s ligaments which help to hold them up against the chest wall.

During pregnancy the placenta releases human placental lactogen and progesterone, and the anterior pituitary gland releases prolactin, and all three of these hormones stimulate the growth of more glandular tissue and prepare the alveolar cells to produce milk. However, even though the breasts are capable of making milk by mid-pregnancy, the high levels of progesterone associated with pregnancy prevent milk letdown. So during pregnancy, the breasts don’t release milk, except for some occasional leakages from the nipples. Overall, the breasts enlarge, the area around the nipple, called the areola, begins to darken, and the areolar glands, also called Montgomery glands, which look like bumps on the areola, start to produce lipoid fluid which moisturizes the nipple.

Once the baby’s delivered, though, the placenta, or afterbirth is also delivered, so placental progesterone disappears, and milk begins to flow. Initially, though, the breasts don’t actually make milk, they make colostrum, which is a yellowish fluid that’s rich in immune cells and antibodies, but low in fat.

Colostrum coats the baby’s gastrointestinal tract and has a laxative effect, which helps the baby pass the first stool which is called meconium. Within a few days after delivery, the breasts start producing milk which, relative to colostrum, has a much higher fat content.

In fact, the amount of fat in the milk also varies during a feeding session. When milk is sitting in the breast, fat globules stick to the alveolar walls, rather than moving into the lactiferous ducts. So when a baby begins feeding and drinks the milk that was in the lactiferous ducts first, that milk has a relatively low fat content. The process of feeding, though, increases the milk flow, and those fat globules get swept into the lactiferous ducts, causing the fat content of the milk to steadily increase as the feeding session continues.

Breast milk also contains lactose, vitamins, micronutrients, and various proteins, like casein and maternal antibodies. Most importantly it contains secretory IgA, which supplements the baby’s gastrointestinal immune system. The amount of vitamin D in the breastmilk is typically insufficient for bone health, and this is because of the modern, mostly indoor life of newborns, so often supplemental vitamin D is needed.

Now, milk letdown is a conditioned reflex, and it usually starts with a baby latching and sucking on the breast. A good latch is one in which the baby’s mouth is wide open, covering the areola with the lips flanged out, the nipple up against the roof of the mouth, and the baby’s tongue up against the bottom of the areola. Mechanoreceptors in the nipple sense this stimulation and send a signal via intercostal nerves to the dorsal root ganglion, then via the spinal cord to the hypothalamus.

When the hypothalamus gets that signal two things happen: first, the hypothalamus blocks prolactin inhibiting neurons from releasing dopamine; which allows lactotrophic cells in the anterior pituitary to make prolactin. Second, the hypothalamus stimulates a group of hypothalamic paraventricular cells to produce oxytocin, which is then sent down the pituitary stalk to the posterior pituitary, where it’s secreted. Now, prolactin stimulates alveolar cell milk production, and oxytocin stimulates the myoepithelial cells to contract which pushes that milk into the ducts, a process called milk let-down.

Interestingly, sometimes when a baby cries, the sound triggers a signal in mom’s brain and is sent to the hypothalamus to initiate the letdown reflex as well.

Sources

  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Avoidance of bottles during the establishment of breast feeds in preterm infants" Cochrane Database of Systematic Reviews (2016)
  6. "Breast milk alkylglycerols sustain beige adipocytes through adipose tissue macrophages" Journal of Clinical Investigation (2019)
  7. "The functional biology of human milk oligosaccharides" Early Human Development (2015)