Blood—the Circulatory Computer Tape

BLOOD-THE CIRCULATORY COMPUTER TAPE IRVINE H. PAGE, M.D.* Those of us brought up at the turn of the century were taught that blood is composed of such things as red blood cells, leukocytes, plasma proteins, gases, electrolytes, and, after a fatty meal, considerable amounts of fat. In general, there were certain limits above or below which these substances did not go. They might even be interrelated, such as the pH, the CO2 tension, and the buffering capacity of the plasma proteins; but these were mostly the concern of physical chemists or academic physiologists. Recently, we have come to realize that blood is composed of, and carries, a vast number of disparate substances, some of them in extremely minute amounts. A potent drug enters the blood stream and is carried by a special transport mechanism to its site of action. Minute amounts of vitamins, an array of nutrients, electrolytes, and gases must all be treated in a specific fashion. Let me take the circulation, or perfusion of tissues, as my special example. Even when the nervous system is destroyed, or blocked chemically, the body is able to distribute blood equitably according to need. Most organs can, in part, control their own rate of perfusion by increasing, or decreasing, their vascular resistance. Thus, they can accept more or less blood by means of altering the caliber of their vessels, provided arterial pressure remains the same. But if each organ acted singly rather than in concert, the cardiac output and arterial pressure would be hard put to maintain constancy. To maintain equilibrium there must be a system of signals and responses that allows the cardiovascular system to act as a whole. The nervous system helps in this holistic endeavor but, as I have said, its action is not critical; the body can regulate blood distribution adequately in the nervous system's absence. This means, then, that the chemical agents in the blood must themselves act in an integrated * Research Division, Cleveland Clinic Foundation, Cleveland, Ohio 44106. Perspectives in Biology and Medicine · Winter 1972 | 219 fashion to convey the necessary signals and put them into effect. A number of such hemic "communication chemicals" have been discovered , such as angiotensin, serotonin, histamine, bradykinin, prostaglandins , carbon dioxide, sodium ions, catecholamines, pituitary polypeptides. It is inconceivable that potent vasoactive substances would circulate in a disorderly, uncontrolled state. They must all act relatedly to achieve by chemical intercommunication the intricate and changing pattern of the organs' need for blood. It is these substances dissolved in blood and extracellular fluid that make up the environment in which smooth muscle of the heart and of the blood vessels lives and functions. The blood stream is the line of communication of all organs from which arise the signals of their need for blood. On this interconnecting band must be printed the patterns and directions for the responses of the blood vessels and the heart. Blood is indeed the "tape of the computer." Without such a system of chemical intercommunication and an integrating nervous system, circulatory responses could be chaos. Too long we have looked upon the blood as a crude solvent and transport system that brings and carries away a variety of nutrient and waste substances. We have not thought of it as carrying coded messages to integrate and serve the intricate demands of the circulation ; this has been reserved to the neural communcation system. We must not expect to understand the blood as a communication system when we take a single sample of blood and analyze it with the rough methods now available. The subtlety of this communicating system, I fear, has been largely overlooked. We have a long way to go before we overcome even the gross disrespect mirrored by the order, "Do a blood chemistry." The remarkable thing about bodies, women, and minds is that no matter how hard you work to understand them, they are always one step ahead of you. "Do a blood chemistry," indeed! You might just as well "do a woman," or "do a mind"! The chemical content of the blood stream, I suggest, constitutes part of the coded directions controlling integration of the perfusion of tissues. We will only learn to...