While serving as an instructor (1889–99) at Guy's Hospital, London (M.D., 1890), Starling undertook investigations of lymph secretion that resulted in his clarification of the nature of fluid exchanges between vessels and tissues. Formulating what is known as Starling's hypothesis (1896), he stated that, because the capillary wall may be considered a semipermeable membrane, allowing salt solutions to pass freely through it, the hydrostatic pressure forcing these solutions into tissues is balanced by the osmotic pressure—generated by colloidal (protein) solutions trapped in the capillary—forcing an absorption of fluid from the tissues.

As professor of physiology at University College, London (1899–1923), Starling began a highly profitable collaboration with the British physiologist William Bayliss (Bayliss, Sir William Maddock) that immediately saw their demonstration (1899) of the nervous control of the peristaltic wave (peristalsis), the muscle action responsible for the movement of food through the intestine. In 1902 they isolated a substance that they called secretin, released into the blood from the epithelial cells of the duodenum (between the stomach and small intestine), which in turn stimulates secretion into the intestine of pancreatic digestive juice. Two years later, Starling coined the term hormone to denote such substances released in a restricted part of the body (endocrine gland), carried by the bloodstream to unconnected parts, where, in extremely small quantities, they are capable of profoundly influencing the function of those parts.

After government-sponsored World War I research concerning poison gas defense, Starling developed an isolated heart-lung preparation that enabled him to formulate (1918) his “law of the heart,” stating that the force of muscular contraction of the heart is directly proportional to the extent to which the muscle is stretched.

Studying kidney function, he found (1924) that water, chlorides, bicarbonates, and glucose, lost in the excretory filtrate, are reabsorbed at the lower end of the kidney tubules (glomeruli). His Principles of Human Physiology (1912), continually revised, was a standard international text.