Regulation of fluid flow through the mammary gland of dairy cows and its effect on milk production: a systematic review.

Dairy milk consists of more than 85% water. Therefore, understanding the regulation of fluid absorption in the mammary gland is relevant to improving milk production. In recent decades, studies using different approaches, including blood flow, transmembrane fluid flow, tight junction, fluid flow of the paracellular pathway and functional mammary epithelial cell state, have been conducted aiming to investigate how mammary gland fluid absorption is regulated. However, the relationship between regulation mechanisms of fluid flow and milk production has not been studied systematically. The present review summarizes a series of key milk yield regulatory factors mediated by whole-mammary fluid flow, including milk, mammary blood flow, blood/tissue fluid-cell fluid flow and cell-alveolus fluid flow. Whole-mammary fluid flow regulates milk production by altering transporter activity, ion channels, local microcirculation-related factors, driving force of fluid transport (osmotic pressure or electrochemical gradient), cellular connection state and a cell volume sensitive mechanism. In addition, whole-mammary fluid flow plays important roles in milk synthesis and secretion. Knowledge gained from fluid flow-mediated regulatory mechanisms of the dairy mammary gland will lead to a fundamental understanding of lactation biology and will be beneficial for the improvement of dairy productivity. © 2017 Society of Chemical Industry.