The most recognized contribution of Servetus to science is the first description of minor circulation of blood, which exposes in his work The first description of blood circulation. It is recognized a great value to, totally deserved, since it was not known at the time the existence of pulmonary circulation of blood. This writing is an excellent lesson in anatomy and physiology of the moment. But to Servetus it was not an exclusive anatomical and physiological matter but much more, as he always expressed. What Servetus really intended was to make a "divine philosophy" that meets "the complete knowledge of the soul and of the spirit". The discovery of the pulmonary circulation of the blood means to him the empirical understanding of the functioning mechanism of “soul” and “human spirit”.
In Latin spiritus, and the Greek pneuma, mean, on the physical plane, air, and also breath or blow, and even what we now understand as wave or vibration. On a philosophical level these terms remain the meaning of “spirit” or “spiritual” in the metaphysical sense. Servetus specifically links the two planes, mixes the concept and proposes it as the central thesis of his theory. At the beginning of The first description of blood circulation Servetus resumes De trinitatis erroribus. Refers invariably air as the spirit of God, in the two planes of the term: "God gives us his spirit when breathes the soul", "God holds the breath of life with his spirit", "gives us the breath", "divinity of God fills the air", "soul penetrates the wind and breath", " ... for the simple fact he breathes the soul to us we can say that God gives us his spirit”, “our soul is like a God’s lamp”, “it is like a spark of the spirit of God...”, “God breathes into Adam's nose the soul as well as a breath of air, so it's up to him” (Isaiah 2; Ps 103).
And furthermore: “God himself holds the breath of life with his spirit and gives breath to people who’s living on earth and spirit to them that walk therein, so that we live in him and move and we are in him (Is 42, Act 17)”, “wind from the four winds and the four breaths breath, called by God, the dead returned to life” (Ez 37), “from breath God takes the souls of men, which are innate life of intake air”, “from the air God takes the soul, and produces both the air and the spark of divinity that fills the air”, “truth is what Orpheus said: the soul goes on the wind and penetrates entirely by breathing, as quoted by Aristotle in De anima". (The first description of blood circulation)
Henceforth, Servetus introduces into his argument a new element, which is organic: blood. Blood will be, as the air, the main idea of his "divine philosophy". Blood is mixed with air in the lungs and delivers the 'spiritus' throughout the body, firstly to brain. He mentioned before blood and heart at the end of Declaration on Jesus Christ, when he quotes Jeremiah and says that the spirit of God "prints" into the hearts of men with "inside ink" (blood) the knowledge of Christ. "What soul has some elemental substance, Ezekiel tells, what have something of substance of blood, God tells. I’ll explain this in more detail, so you'll understand (addressing the reader) that the substance of the created spirit of Christ is essentially linked to the substance of the holy Spirit. I'm calling spirit to air, as holy language has no special term to designate the air. Moreover, this fact makes us understand that in air there is some divine breath that fills the spirit of the Lord." (The first description of blood circulation)
Servetus, once again and said so expressly, if there was any doubt about what he calls spirit, initiates, from this paragraph, his lesson in anatomy and physiology of the soul. First he distinguishes three bodily "spirits". The first two come to constitute the soul (then he conceives the soul as organic). They belong to body element on which acts external universal spirit or air, which is blood. These are the “natural spirit”, which corresponds to the venous blood, and the “vital spirit”, which is blood of the heart and arteries, once is mixed with the air that has passed through the lungs. He calls them both as “blood spirits”. The third bodily "spirit", which he calls “animal spirit”, corresponds to activity of the brain, "a ray of light that acts on brain and nerves". Servetus clearly identifies brain activity with mental activity. Brain or mental activity is fed (and so is the soul that resides in the blood) by the universal spirit or air, which is the spirit of God. At the three bodily spirits, therefore, there is energy of universal spirit-air-God.
Soul is blood; its bodily matter is blood. And when blood passes through the lungs and combines with intake air collects power of God's spirit (becomes “vital spirit”). This “vital spirit” is blood on its bodily nature and air on its “spirirual” nature.
Servetus explains in the following paragraphs as blood, driven from the right ventricle of the heart, "throughout a long circuit through the lungs" is combined with the intake air, and this oxygenated blood returns to the left ventricle of the heart and, from this, is distributed throughout the body.
Blood mixed with air (“vital spirit”) is transfused to entire body throughout arteries. But the more subtle one, which has more air (by some natural mechanism that Servetus does not specify but it seems that points to the simple physical fact that the lighter material rises, in contrast to the heavier, which goes down) directs to the upper parts, to the brain. In the reticular plexus at the base of the brain, the blood would be subtilized again, so continue up through "a very thin vessels or capillaries arteries, located in the choroid plexus, which contain the mind itself".
Servetus then states that "sensitivity" does not concern specifically to the "soft matter" of nerves and brain, but to subtle contribution of “vital spirit” through very thin blood vessels and membranous filaments extending to "the origin of nerves". The spirit, the blood with more air ratio, lighter and subtle, constantly tends to go towards the "membranous filaments" of nerves. "The sensitivity of the nerves is not in soft matter, nor in the brain. Every nerve ending in membranous filaments endowed with exquisite sensibility, so that the spirit tends constantly toward them. Thus, from these little vessels of meninges or choroid, as from a source, it spreads like lightning the bright animal spirits through the nerves to the eyes and other sensory organs. And by the same paths, but in reverse, are sent from outside to the same source light images of sensed things, penetrating inward as through a light environment." Thus, the seat of the mind or rational soul is not properly the soft mass of the brain, which is cold and insensitive, but rather the blood vessels "that are united to and provide power to sensory nerves".