Your lactating breast is a tideland of blood and lymph

If there's one thing that's obvious, it's that your milk-making breasts are moist!

• Your milk is 87% water.

• Your breast stroma is 70% water.

• Your breasts' network of blood vessels contain 50% water.

How is the ebb and flow of this water regulated so that your breast doesn't become swollen with fluid? The secret lies in the interactions between your blood and lymph systems. Together, they are intricate tides washing through your breast. The flow of lymph keeps the hardworking tidelands of your mammary connective tissue or stroma fresh and healthy while you make milk.

Interstitial fluid and your lymph

Both your blood and your lymph are specialised kinds of connective tissue which don't contain fibre cells.

Interstitial fluid is a continuously renewing reservoir of fluid embedded in your breasts' connective tissues. Most interstitial fluid comes from the oxygen-fresh, arterial end of your blood capillaries, which nestle in a snug lattice around your milk glands. There is a higher pressure inside your arterial capillaries than in the venous capillaries or venules which run alongside them.

• 90% of the blood stream fluid carried in by the arterial capillaries washes through and is carried out by the venules.

• 10% stays to replenish your interstitial fluid, leaking out from the arterial capillaries into your surrounding breast connective tissue.

Your lymphatic capillaries then drink up the constantly renewing interstitial fluid, also gathering up lymphocyctes, proteins, cellular debris, excess fat, bacteria and viruses - things that are too big to get back into the veins - turning it all into lymph.

The word lymph derives from Lympha, an ancient female Roman deity of fresh water or springs. Your lymph is 96% water, and it is clear like water too. It has the same initial composition as your breasts' interstitial fluid. Your lymph vessels are tree-like networks which empty into your body's great veins then into your heart.

Your breasts are laced with lymphatics, which originate in the lymph capillaries of the connective tissue, and drain through the deep substance of the breast. The same as the rest of the body, there are lymphatics just under the skin running over the surface of your breast. These are valveless and communicate with subdermal lymphatic vessels and merge with the subareolar plexus. Many of the lymphatic vessels sit just under the surface of your skin, above the superficial fascia, eventually draining into lymph nodes, and then into large lymphatic trunks back into the heart.

The lymph drainage of the breast consists of the superficial or cutaneous section, the plexus just under the areola, and the glandular or deep-tissue section. About three quarters of your breasts' lymphatic drainage goes to the armpit or axillary lymph nodes. A variable degree of deep breast tissue, particularly of the medial breast, may collect into lymphatic vessels that perforate the deep fascia to drain into internal mammary nodes.

During lactation, half of the lymphatic vessels threading through the stroma are collapsed at any one time. They are in reserve, waiting to be called upon if needed.

What makes lymph flow through the lymphatic vessels back to the heart?

Lymph moves towards the heart because the larger lymphatic vessels have an automatic waves waves of contraction by the smooth muscle cells in their walls. The lymph can't flow backwards because lymph capillaries have valves to prevent that.

Instead, muscular movement, breathing, and gravity apply mechanical pressures to the lymphatic capillaries. Also, a letdown is a wonderful pressure wave rippling throughout your breasts. This is how your lactating breast continually push lymph back towards the heart from the lymphatic capillaries into the larger lymphatic collector vessels, where muscular walls and one-way valves keep the lymph moving in the right direction.

There is wide variation in the drainage of lymph from the breast. If the lymphatics are dilated due to inflammation or mastitis they can be seen in imaging as thin lines running parallel and perpendicular to the skin.Eventually the lymph travels to one of two large lymphatic ducts just below the neck, where it gets dumped into a large vein and back into the bloodstream. It's safe for the fluid to join the bloodstream again because the toxins have been filtered out in the lymph nodes.

You can't make your lymph flow by massaging it towards your armpits and nodes

You might be advised to massage your breast with very light touch from the nipple up to the axilla to 'help with lymphatic drainage' if your breasts are engorged or you have a mastitis. However, once you understand the working anatomy of your breasts, you'll see this advice (as soothing as it sounds or feels) doesn't really make sense.

For a start, it is impossible to know how much of a swollen area is interstitial fluid and how much is caused by ducts and milk glands engorged with milk.

The problem is that 'reverse' or 'lymphatic' massage in the direction of nipple to armpit- even when done in light circles - risks making problems worse because any external pressure compresses the milk ducts, which are compress with even very light pressure. Lymphatic drainage massage applies pressure backwards towards the milk glands. This might worsen blocked ducts and mastitis, and could increase the amount of interstitial fluid in your breast.

You might hear that this massage should only be the lightest touch. It's true that light touch can be soothing for a woman whose breast is in pain with a mastitis or engorgement, and you lightly stroke your breast yourself with your fingers for this reason, in any direction, if it feels good - but this won't effect how your breast lymph flows, and you don't need to pay someone to do it.

The best effect of this kind of light touch is that it might stimulate letdowns. A letdown is by far the best way to help your lymph move. What your breast needs more than anything if you have an inflammation is to have milk taken out of it, urgently, and to have waves of pressure change running through your breast tissues. Directly breastfeeding your baby, even just for a minute, is your best way of doing this, as often as possible (without pressuring your baby).

Your breast wants to be soothed by the pressure waves of a letdown, which helps your lymph move, and by the release of your milk.

Selected references

Geddes DT, Aljazaf K, Kent JC, Prime DK, Garbin CP, Lai CT, et al. Blood flow characteristics of the human lactating breast. Journal of Human Lactation. 2012.

Zucca-Matthes G, Urban C, Vallejo A. Anatomy of the nipple and breast ducts. Glandular Surgery. 2015;5(1):32-26.

Geddes DT. Ultrasound imaging of the lactating breast: methodology and application. International Breastfeeding Journal. 2009;4:doi:10.1186/1746-4358-1184-1184.

Geddes DT. Ultrasound imaging of the lactating breast: methodology and application. International Breastfeeding Journal. 2009;4:doi:10.1186/1746-4358-1184-1184.

Zucca-Matthes G, Urban C, Vallejo A. Anatomy of the nipple and breast ducts. Glandular Surgery. 2015;5(1):32-26.