Mechanical pressures are the engine room of breastfeeding and lactation

It helps to understand how mechanical pressures work

Mechanical pressures occur everywhere in the natural world. Mechanobiology is a new field of science which investigates the effects of the physical forces which act on living tissues, either from within or from the outside.

The powerful effects of the physical forces which act upon our body and our baby have been previously overlooked in the science of human lactation. This is not surprising, since our society's relationship with the bodies of women still remains problematic.

Yet mechanical pressures are fundamental to lactation and breastfeeding when things are going well. It's especially important to understand the way mechanical pressures work when problems come up.

Ways in which mechanical pressures affect breastfeeding and your milk flow

Mechanical pressures acts in many different ways when you're making milk. Here are some that are important to know about.

Milk ejection or letdown

• Specialised cells wrap around your milk glands and contract when oxytocin surges through your blood stream in response to your baby's suckling. This causes the milk-making cells (or lactocytes) inside your milk glands to squash down and take on a random, scrunched-up shape. This in turn places pressure or mechanical stress upon the milk that is inside the little gland, so that the milk flows out into the duct.

• Specialised cells also wrap around your milk ducts. Again, they contract when oxytocin surgers through your blood stream in response to your baby's suckling. However, in response, the duct actually dilates or grows wider, and also shortens. This decreases pressure in the duct, so that milk flows easily out of the milk gland into the duct, and then out of your nipple.

The vacuum generated by baby's sucking

• Your directly breastfeeding baby applies a mechanical force, which is the vacuum. This vacuum or negative pressure acts upon the nipple and breast tissue which is drawn up inside your baby's mouth, causing the tissues to expand.

• Milk travels along pressure gradients, from areas of high pressure to areas of low pressure. The mechanical pressure of the vacuum (or the negative pressure of suckling) and the positive pressure of milk ejection in the milk duct system transfer milk from the breast, working in tandem. Sometimes you'll notice that your breasts leak milk in the absence of suckling, due to the positive mechanical pressure of milk ejection. In the absence of suckling some backflow of milk into emptier ducts occurs with milk ejection, following pressure gradients from high pressure to lower pressure.

Nipple and breast tissue drag

• When the mechanical force of nipple and breast tissue drag pulls in a different direction to the mechanical force of the vacuum, your baby may fuss a lot, or back arch and pull off the breast. You can find out about this here.

• Your directly breastfeeding baby also applies the mechanical force of the vacuum onto the skin of your nipple. This pressure stretches the outer layer of your nipple skin. You can read about this here.

  • If there is nipple and breast tissue drag, this stretching force may become so focussed on one small area at the tip of the nipple (or sometimes at the base of the nipple) that the epidermis breaks apart, resulting in a crack or ulcer.

  • If there is nipple and breast tissue drag, the mechanical force of suckling might also bend the nipple or place pressure on the tissues inside your nipple, resulting in deep tissue bruising.

Breast inflammation (such as mastitis or engorgement)

• If there isn't enough milk removed from the inside of a milk gland, high volumes of milk press back on the lactocytes, stretching and thinning them. This growing pressure also stretches the tight junctions between the lactocytes, placing mechanical stress or strain upon them. If the backpressure from the milk is high enough, the milk gland might break apart. You can find out about this here. If enough alveoli glands stretch tight or break in one area of the breast, the resulting inflammation might become a painful, red lump, usually referred to as a mastitis. You can find out about this here.

• The pressure or tension inside the breast increases

  • If there is increased blood flow in the capillaries

  • More fluid than usual leaking out of the capillaries

  • Bruising (which is breaking of the capillaries so they leak some blood) of the deep tissues.

• The rising pressure of this tension acts upon the milk ducts, which are easily compressed, and may occlude them. This worsens the cycle of rising milk volume and backpressure inside the alveolar gland, which triggers more blood flow into the area and more fluid than usual leaking out, in a cycle of inflammation.

Externally applied pressure

• Mechanical pressure might be experienced inside the breast from externally applied forces.

  • Mechanical forces such as a kick by the baby or massage may result in deep breast tissue bruising, resulting in raised tension or pressure inside your breast, which in turn might compress the milk ducts, resulting in inflammation.

  • Prolonged application of mechanical pressures upon the breast result in prolonged ductal compression, which results in backpressure of milk, triggering inflammation. This prolonged pressure could come from an ill-fitting garment or bra or sleeping with your arm pressed against your breast or from using a breast shell or silverette, to give just a few examples.

• Fit and hold problems are a form of externally applied mechanical pressure which results in nipple and breast tissue drag, as discussed above.