Lymphatics in the diaphragm form a specialised system draining fluid from the peritoneal cavity and returning it to the vascular system. Fluid enters subperitoneal lymphatic lacunae, between muscle fibres of the diaphragm, the lacunae being separated from the peritoneal cavity by a barrier comprising, successively, lymphatic endothelium, a layer of collagenous fibres, a thin fenestrated layer of elastic tissue, and the peritoneal mesothelium. To reach the lacunae, peritoneal fluid passes through stomata located between cuboidal mesothelial cells of the lacunar roof. Whilst the distribution of mesothelial stomata and subjacent lymphatic lacunae varies in different species, stomata appear to be exclusive to the diaphragm and may serve as the main drainage channels for absorption from the peritoneal cavity. Clinically, they may provide escape for tumour cells, pathogens and toxins from the peritoneal cavity. They could provide access for blood transfusions, for intraperitoneal chemotherapy to treat malignancies, and for peritoneal dialysis in treating chronic renal failure. From the lacunae, fluid traverses the diaphragm via intrinsic lymphatics to reach collecting lymphatics beneath the diaphragmatic pleura. Both intrinsic and collecting lymphatics contain valves. The collecting lymphatics drain principally into retrosternal (parasternal) lymphatic trunks that carry lymph to the great veins after it filters through mediastinal lymph nodes.