From Plant to Pulse: Stephen Hales' Unconventional Path to Discovering Blood Pressure

Stephen Hales, a clergyman, embarked on his scientific journey after reaching the age of 50 and ultimately emerged as a prominent figure in English science during his era. His groundbreaking work encompassed the development of a ventilation system designed for both ships and prisons, as well as significant contributions to the field of plant physiology.

Illustrating the human cardiovascular system, showcasing the heart, arteries, and veins.

In his pursuit of understanding the flow of sap within a vine, Hales found it necessary to stem this flow to prevent harm to the plant. He ingeniously affixed a piece of bladder over the incision, and to his surprise, observed the sap's force causing the bladder to expand. This unexpected discovery led Hales to employ a similar approach for measuring blood pressure, building upon the pioneering work of William Harvey. During the early seventeenth century, Harvey had reported the pulsation of blood as it flowed from a severed artery, as if responding to rhythmic pressure.

Hales initially conducted experiments on horses as his first subjects. He secured a live horse in a supine position to a barn door and inserted a brass tube into its femoral artery, connecting the tube to a nine-foot-long glass tube via a flexible goose windpipe. When he released the arterial ligature, the blood surged to a remarkable height of over eight feet. Subsequently, he delved into studying variables influencing and maintaining blood pressure, including the volume of blood ejected by the heart (cardiac output) and the blood's ability to flow through the tiniest blood vessels (peripheral resistance). Hales estimated cardiac output by calculating the heart rate multiplied by the heart's volume, which he determined using a wax mold. Variations in peripheral resistance were assessed by injecting different substances, such as brandy and saline solutions, into an isolated heart and measuring the output rate. These differences were ascribed to variations in capillary diameters. Hales documented these findings in his 1733 publication titled "Haemastaticks."

While Hales held a profound interest in plants, he applied his observations from the animal kingdom to his study of plants. Furthermore, he recognized intriguing parallels between the plant and animal realms, highlighting the analogous roles of sap in plants and blood in animals.