[When questioned on his longevity] First of all, I selected my ancestors very wisely. ... They were long-lived, healthy people. Then, as a chemist, I know how to eat, how to exercise, keep my blood circulating. ... I don't worry. I don't get angry at people. I don't worry about things I can't help. I do what I can to make the world a better place to live, but I don't complain if things aren't right. As a scientist I take the world as I find it.
[About celebrating his 77th birthday by swimming a half mile in 22 minutes] I used swim fins and webbed gloves because a man of intelligence should apply his power efficiently, not just churn the water.

Global nuclear war could have a major impact on climate—manifested by significant surface darkening over many weeks, subfreezing land temperatures persisting for up to several months, large perturbations in global circulation patterns, and dramatic changes in local weather and precipitation rates—a harsh “nuclear winter” in any season. [Co-author with Carl Sagan]

I could clearly see that the blood is divided and flows through tortuous vessels and that it is not poured out into spaces, but is always driven through tubules and distributed by the manifold bendings of the vessels... [F]rom the simplicity Nature employs in all her works, we may conclude... that the network I once believed to be nervous [that is, sinewy] is really a vessel intermingled with the vesicles and sinuses and carrying the mass of blood to them or away from them... though these elude even the keenest sight because of their small size... From these considerations it is highly probable that the question about the mutual union and anastomosis of the vessels can be solved; for if Nature once circulates the blood within vessels and combines their ends in a network, it is probable that they are joined by anastomosis at other times too.

I have heard him [William Harvey] say, that after his Booke of the Circulation of the Blood came-out, that he fell mightily in his Practize, and that 'twas beleeved by the vulgar that he was crack-brained.

I should object to any experimentation which can justly be called painful, for the purpose of elementary instruction ... [but I regret] a condition of the law which permits a boy to troll for pike, or set lines with live frog bait, for idle amusement; and, at the same time, lays the teacher of that boy open to the penalty of fine and imprisonment, if he uses the same animal for the purpose of exhibiting one of the most beautiful and instructive of physiological spectacles, the circulation in the web of the foot. ... [Maybe the frog is] inconvenienced by being wrapped up in a wet rag, and having his toes tied out ... But you must not inflict the least pain on a vertebrated animal for scientific purposes (though you may do a good deal in that way for gain or for sport) without due licence of the Secretary of State for the Home Department, granted under the authority of the Vivisection Act.
... [Yet, in] 1877, two persons may be charged with cruelty to animals. One has impaled a frog, and suffered the creature to writhe about in that condition for hours; the other has pained the animal no more than one of us would be pained by tying strings round his fingers, and keeping him in the position of a hydropathic patient. The first offender says, 'I did it because I find fishing very amusing,' and the magistrate bids him depart in peace; nay, probably wishes him good sport. The second pleads, 'I wanted to impress a scientific truth, with a distinctness attainable in no other way, on the minds of my scholars,' and the magistrate fines him five pounds.
I cannot but think that this is an anomalous and not wholly creditable state of things.

In attempting to discover how much blood passes from the veins into the arteries I made dissections of living animals, opened up arteries in them, and carried out various other investigations. I also considered the symmetry and size of the ventricles of the heart and of the vessels which enter and leave them (since Nature, who does nothing purposelessly, would not purposelessly have given these vessels such relatively large size). I also recalled the elegant and carefully contrived valves and fibres and other structural artistry of the heart; and many other points. I considered rather often and with care all this evidence, and took correspondingly long trying to assess how much blood was transmitted and in how short a time. I also noted that the juice of the ingested food could not supply this amount without our having the veins, on the one hand, completely emptied and the arteries, on the other hand, brought to bursting through excessive inthrust of blood, unless the blood somehow flowed back again from the arteries into the veins and returned to the right ventricle of the heart. In consequence, I began privately to consider that it had a movement, as it were, in a circle.

In man, then, let us take the amount that is extruded by the individual beats, and that cannot return into the heart because of the barrier set in its way by the valves, as half an ounce, or three drachms, or at least one drachm. In half an hour the heart makes over a thousand beats; indeed, in some individuals, and on occasion, two, three, or four thousand. If you multiply the drachms per beat by the number of beats you will see that in half an hour either a thousand times three drachms or times two drachms, or five hundred ounces, or other such proportionate quantity of blood has been passed through the heart into the arteries, that is, in all cases blood in greater amount than can be found in the whole of the body. Similarly in the sheep or the dog. Let us take it that one scruple passes in a single contraction of the heart; then in half an hour a thousand scruples, or three and a half pounds of blood, do so. In a body of this size, as I have found in the sheep, there is often not more than four pounds of blood.
In the above sort of way, by calculating the amount of blood transmitted [at each heart beat] and by making a count of the beats, let us convince ourselves that the whole amount of the blood mass goes through the heart from the veins to the arteries and similarly makes the pulmonary transit.
Even if this may take more than half an hour or an hour or a day for its accomplishment, it does nevertheless show that the beat of the heart is continuously driving through that organ more blood than the ingested food can supply, or all the veins together at any time contain.

It is usually not recognized that for every injurious or parasitic microbe there are dozens of beneficial ones. Without the latter, there would be no bread to eat nor wine to drink, no fertile soils and no potable waters, no clothing and no sanitation. One can visualize no form of higher life without the existence of the microbes. They are the universal scavengers. They keep in constant circulation the chemical elements which are so essential to the continuation of plant and animal life.

Nature … is, as it were, a continual circulation. Water is rais'd in Vapour into the Air by one Quality and precipitated down in drops by another, the Rivers run into the Sea, and the Sea again supplies them.

Perhaps I may without immodesty lay claim to the appellation of Mathematical Adam, as I believe that I have given more names (passed into general circulation) of the creatures of the mathematical reason than all the other mathematicians of the age combined.

Science has thus, most unexpectedly, placed in our hands a new power of great but unknown energy. It does not wake the winds from their caverns; nor give wings to water by the urgency of heat; nor drive to exhaustion the muscular power of animals; nor operate by complicated mechanism; nor summon any other form of gravitating force, but, by the simplest means—the mere contact of metallic surfaces of small extent, with feeble chemical agents, a power everywhere diffused through nature, but generally concealed from our senses, is mysteriously evolved, and by circulation in insulated wires, it is still more mysteriously augmented, a thousand and a thousand fold, until it breaks forth with incredible energy.

The Chinese, who aspire to be thought an enlightened nation, to this day are ignorant of the circulation of the blood; and even in England the man who made that noble discovery lost all his practice in the consequence of his ingenuity; and Hume informs us that no physician in the United Kingdom who had attained the age of forty ever submitted to become a convert to Harvey’s theory, but went on preferring numpsimus to sumpsimus to the day of his death.

The specific character of the greater part of the toxins which are known to us (I need only instance such toxins as those of tetanus and diphtheria) would suggest that the substances produced for effecting the correlation of organs within the body, through the intermediation of the blood stream, might also belong to this class, since here also specificity of action must be a distinguishing characteristic. These chemical messengers, however, or 'hormones' (from όρμάω, I excite or arouse), as we might call them, have to be carried from the organ where they are produced to the organ which they affect by means of the blood stream and the continually recurring physiological needs of the organism must determine their repeated production and circulation through the body.

There is no generation from an egg in the Mineral Kingdom. Hence no vascular circulation of the humours as in the remaining Natural Kingdoms.

This organ deserves to be styled the starting point of life and the sun of our microcosm just as much as the sun deserves to be styled the heart of the world. For it is by the heart's vigorous beat that the blood is moved, perfected, activated, and protected from injury and coagulation. The heart is the tutelary deity of the body, the basis of life, the source of all things, carrying out its function of nourishing, warming, and activating body as a whole. But we shall more fittingly speak of these matters when we consider the final cause of this kind of movement.

Though Hippocrates understood not the Circulation of the Blood, yet by accurately observing the Effects of the Disease, which he looked upon as an unknown Entity, and by remarking the Endeavours of Nature, by which the Disease tended to either Health or Recovery, did from thence deduce a proper Method of Cure, namely by assisting the salutary Endeavours of Nature, and by resisting those of the Disease; and thus Hippocrates, ignorant of the Causes, cured Disease as well as ourselves, stocked with so many Discoveries.

We have reason not to be afraid of the machine, for there is always constructive change, the enemy of machines, making them change to fit new conditions.
We suffer not from overproduction but from undercirculation. You have heard of technocracy. I wish I had those fellows for my competitors. I'd like to take the automobile it is said they predicted could be made now that would last fifty years. Even if never used, this automobile would not be worth anything except to a junkman in ten years, because of the changes in men's tastes and ideas. This desire for change is an inherent quality in human nature, so that the present generation must not try to crystallize the needs of the future ones.
We have been measuring too much in terms of the dollar. What we should do is think in terms of useful materials—things that will be of value to us in our daily life.

Worry affects circulation, the heart, the glands, the whole nervous system, and profoundly affects the heart. I have never known a man who died from overwork, but many who died from doubt.

“True is it, my incorporate friends,” quoth he, “That I receive the general food at first, Which you do live upon; and fit it is, Because I am the storehouse and the shop Of the whole body. But, if you do remember, I send it through the rivers of your blood, Even to the court, the heart, to th’ seat o’ th’ brain; And, through the cranks and offices of man, The strongest nerves and small inferior veins From me receive that natural competency Whereby they live. And though that all at once”— You, good friends, this says the belly, mark me.