The Circulation of Plasma Phospholipids: Their Transport to Thoracic Duct Lymph*

Two phases of the metabolism of plasma phospholipids have been dealt with recently: (1) their site of formation and (2) their rate of removal from plasma and transfer to various organs. Plasma phospholipids are formed almost exclusively in the liver (1). Although such tissues as kidney and small intestine also synthesize these compounds, they are not available to the plasma (1). The removal of phospholipids from plasma was studied by Zilversmit et al. 5 hours after the introduction of labeled phospholipids into the blood stream of the dog, these workers recovered 76 to 83 per cent of them in plasma, red blood cells, liver, kidney, small intestine, spleen, and muscle (2). As much as 40 to 50 per cent of the injected labeled phospholipids had, however, disappeared from the plasma. Approximately one-third of the phospholipids lost from the plasma was found in the liver. The fate of the plasma phospholipids that enter the liver,’ kidney, small intestine, muscle, etc., remains to be determined. It is shown here that a portion of these phospholipids reaches the lymphatic channels. Radiophospholipids were injected into the blood stream of the dog and their recovery in lymph of the thoracic duct demonstrated. These observations show that a measurable portion of the phospholipids that leave the plasma passes through one or more tissues on its way to the lymph channels and from these is returned to the plasma. The question as to whether phospholipids can pass a capillary membrane has been frequently considered but not settled (3). The demonstration here that labeled plasma phospholipids are rapidly transferred to thoracic duct lymph leaves no further doubt that capillary membranes or sinusoid malls are permeable to phospholipid molecules.