Elucidation of the structural basis of endothelial barrier function and the study of transcapillary fluid exchange dynamics are areas of active research. There has been significant enhancement in our understanding of the ultrastructural basis of endothelial barrier function. The role of glycocalyx has received special attention. Experimental evidence has called for a revision in the classic Starling principle of transcapillary exchange. The glycocalyx model provides a potential structural mechanism for the revised Starling principle. This knowledge can provide the framework for understanding the volume expansion effect of fluid therapy and the physiological basis of fluid therapy.