The perineal artery, a continuation of the internal pudendal artery supplies a small amount of blood to the caudal portion of both halves of the udder. Arteries become smaller to form arterioles which then become capillaries to provide nutrients to the alveoli. Blood flows through the capillaries to venules which form veins which drain the blood away from the udder toward the heart.
Veins converge to form a venous circle before leaving the mammary gland.
While the cow is standing, most of the blood returns to the heart via the abdominal veins, however, blood may return preferentially through other veins while the cow is lying down. Blood flow in high producing cows can approach 800 litres per hour. Approximately 500 volumes of blood is required to produce 1 volume of milk. Lymph originates as a filtrate of blood serum and contains no red blood cells and only half the amount of protein. The lymph is then transferred to larger lymph vessels to the thoracic lymph duct which returns the lymph to the blood via the anterior vena cava.