ANSC 438 Home / Beginning / Milk Composition / Mammary Structure
Mammary Development / Mother & Neonate / Lactation / Mastitis

Independent Study

Mammary Macro-structure
Dairy Cow Udder Anatomy


Lymph and Lymphatics - Many molecules of all sizes leave the capillaries but not all return to the venous drainage at the tissue level. Especially the larger molecules like proteins. These, along with cellular metabolites and some secretory products are in the interstitial (extracellular) spaces. If they stayed there, they would disrupt with the normal balance of osmotic pressure in the tissue, upsetting trans-capillary fluid exchange. Excess fluid (called extracellular fluid) would accumulate in the interstitial spaces.

Functions of lymphatics:

  • The extracellular fluids are drained from the tissue and conducted back to the circulatory system via the lymphatic network.
  • Also, the lymphatics contain concentrated areas of leukocytes (particularly lymphocytes and macrophages) in lymph nodes; these leukocytes can mount an immune response to bacteria and foreign material.
  • The lymphatic network serves to transport some things in the body (vitamin K, lipids absorbed in the intestine).

The lymphatic network originates in tissue spaces as very thin, closed endothelial tubes (lymphatic capillaries). These are analogous to blood capillaries, but are much more permeable, with little resistance to passage. They have no basement membrane. Lymph capillaries converge to form larger vessels. Lymph flow is unidirectional from the tissues through lymphatic vessels, eventually dumping lymph into the vena cava.

Lymph is a clear, colorless liquid with a composition similar to blood plasma. Changes in plasma composition will change lymph composition. Protein concentration of lymph is lower than in plasma, 1.5% vs. 6% for plasma. Specific proteins differ, for example albumin is a smaller molecular size than globulins and leaves the capillaries more readily than globulins, so the albumin : globulin ratio is 1.8 in plasma, 2.5 in lymph. Protein concentration in lymph varies inversely with rate of formation. Rate of filtration varies with the tissue. In liver and intestine, lymph has ~5% protein, compared with the extremities where lymph has ~0.5% protein).

Lymph flow rate is usually low. It is influenced primarily by the rate of lymph formation. For example, if blood capillary pressure is increased by arterial vasodilation or venous constriction, the flow rate of lymph increases. Also, the flow rate is affected by compression of lymphatics by contraction of neighboring musculature and by negative intrathoracic pressure (breathing). Valves in the lymph vessels prevent retrograde flow similar to those in veins.

Lymph flow through the mammary gland:

  • goat (lactating) 6.5-35 ml/hr or 150-840 ml/day
  • cow (dry) 14-240 ml/hr or 68-5760 ml/day
  • cow (lactating) 1300 ml/hr or 31,200 ml/day or 31 kg/day
  • 1.6 units lymph leave udder for every unit milk produced.

Edema is the excess accumulation of fluids in tissue spaces. This can retard normal exchange of nutrients and metabolites. Filtration of the extracellular fluid exceeds drainage. Anything that causes increased capillary pressure, such as decreased plasma protein, increased capillary permeability or lymphatic blockage, can result in swelling and congestion of the extravascular compartment.

Udder edema is swelling of the udder. Although it occurs to some degree in most cows at calving time, heifers calving for the first time are especially prone to having udder edema. Fluid accumulates between skin and glandular tissue, as well as in the gland. The skin is usually ~1/4" thick including subcutaneous layers, but during edema it can increase in thickness to 2". Severe edema can strain supportive structures of udder. Udder edema is often caused by an imbalance of hydrostatic and osmotic pressures, increasing fluid flow out of the capillaries into the interstitial spaces. This may occur because of damage to the capillary walls or obstruction of the lymphatic system. Don't know exactly why it happens, but from human medicine, increased salt intake, increased fluid intake, increased environmental temperature and damaged innervation can contribute to edema.

Udder of first calf heifer showing edema.
This heifer had just calved and has substantial udder edema. Note the swelling under the belly.

Mammary Structure