The new generations of physicians are, to a large extent, unaware of the complex philosophical and biological concepts that created the bases of modern medicine. Building on the Hellenistic tradition of the four humors and their qualities, Galen (AD 129 to c. 216) provided a persuasive scheme of the structure and function of the cardiorespiratory system, which lasted, without serious contest, for 1,300 yr. Galen combined teleological concepts with careful clinical observation to defend a coherent and integrated system in which the fire-heart-flaming at the center of the body-interacts with lungs' air-pneuma to create life. Remarkably, however, he achieved these goals, despite failing to grasp the concept of systemic and pulmonary blood circulations, understand the source and destiny of venous and arterial blood, recognize the lung as the organ responsible for gas exchange, comprehend the actual events taking place in the left ventricle, and identify the source of internal heat. In this article, we outline the alternative theories Galen put forward to explain these complex phenomena. We then discuss how the final consequences of Galen's flawed anatomical and physiological conceptions do not differ substantially from those obtained if one applies modern concepts. Recognition of this state of affairs may explain why the ancient practitioner could achieve relative success, without harming the patient, to understand and treat a multitude of symptoms and illnesses.
Keywords: Galen; blood; heart; history; lungs; oxygen.