The
attention Lavoisir gave to human physiology was the outcome of a
long butlogical progression.His numerous reflections in the domain
of hygiene and public health led him gradually to consider the
factors determining the state of health or illness and to adopt an
approach similar to that of a physiologist.However, when hewrote
down in his laboratory notebook on February 20, 1773 his
celebrated research program on "the air that fixes itself to or is
released from bodies," he was not thinking about these matters.At
the time, his plan was strictly chemical: he wanted to develop a
general theory explaining the five circumstances in which,
according to him, air fixed itself or was released from chemical
bodies: vegetation, respiration, combustion, calcination and
chemical combinations.
Seventeen years later, he was envisaging medical applications for
his research activities.They could, he said, "diminish the mass of
evils that afflict the human species, (...),increase itsenjoyment
and well being.And even if the new routes opened up could prolong
the average life of mankind by only a few years, or even a few
days,then thescientist, too, could aspire to the glorious title of
'benefactor of humanity'." (Lavoisier, Oeuvres, vol. II, p.
703.)
The
Physiology of Respiration
This was a phenomenon that for twenty years Lavoisier kept
encountering at each stage of his research.Already in May 1777, he
had published his "Experiments on Animal Respiration and the
ChangesOccuringWhen Air Passes Through the Lungs," complemented by
a paper on the combustion of candles in atmosphericand "eminently
breathable" air.These ideas were developed in the "Mémoire
sur la chaleur" written with Laplace and published on June 18,
1783.In February 1785 he had read to the RoyalSociety of Medicine
a paper describing"the changes that occur in air in the various
circumstances where people come together in groups." In the first
paper he wrote on animal respiration, presented at the Academy on
November 17, 1790, he presented the synthesis of his findings.
- The purpose of respiration, he said, is not to cool the blood,
as was believed in ancient times.It is rather "a slow combustion
of carbon and hydrogen, similar in every way to that which takes
place in a lamp or lighted candle and, in that respect, breathing
animals are active combustible bodies that are burning and wasting
away."(Lavoisier, Oeuvres, vol. II, p.
691.)
- It is atmospheric air which furnishes the oxygen necessary for
this combustion.The blood provides the fuel and its oxidation in
the lungs explains the change in color.This combustion produces
carbonic gas and water.
- This combustion isalso the source of body heat."Since air
vitalcannot be converted into carbonic acid except by an addition
of carbon, and cannot be converted into water except by an
addition of hydrogen, and the double combination cannot occur
unless theair vitalloses a part of its specific caloric, the
result is that the effect of respiration is to extract from the
blood a portion of carbon and hydrogen and to replace it with a
portion of its specific caloric.During circulation, the caloric is
distributed with the blood through the animal system, and
maintains that almost constant temperature observed in all
breathing animals.(Lavoisier, Oeuvres, vol. II, p. 692.)
- This combustion has an energetic cost: "Since it is the very
substance of the animal, it is the blood which transports the
fuel.If the animal did not habitually replace, through nourishing
themselves,what they losethrough respiration, "the lamp would very
soon run out of oil and the animal would perish, just as the lamp
goes out when it lacks fuel."(Lavoisier, Oeuvres, vol. II, p.
691.)
In this elegant metaphor, F.L.H. Holmes points out , Lavoisier
formulated for the first time an enlarged conception of the
equilibrium necessary between nourishment and the consumption of
nutriments.According to Holmes, "it was no longer a matter just of
the combustion of carbon and hydrogen supplied from the blood; the
substance of the animal itself was consumed in
respiration.Nourishment and respiration were part of one large
process encompassing much of the 'animal economy'."(L. Holmes,
Lavoisier and theChemistry of
Life, Madison-London, University of
Wisconsin Press, 1985, p. 450.)
- The variations in oxygen concentration in the air do not change
the consumption by the organism."It is known that the purer the
air in which combustion occurs, the faster the combustion.Thus,
for example, within a given time, much more charcoal, or any other
fuel, is consumed in vital air than in atmospheric air.It had
always been thought that the same was true for respiration, that
it ought to accelerate in vital air, and then release - either in
the lungs or circulatory system - a greater quantity of
caloric.But the experiment has destroyed all these assumptions
whichwere based only on analogy." (Lavoisier, Oeuvres, vol. II, p.
694.)
- Nitrogen is not a respiratory gas.It enters the lungs and comes
out unchanged.
- A relationship exists between the mechanical work carried out by
a living being and the measurable biochemical phenomena which are
its driving force.Wanting to measure these gaseous exchanges and
the effects that digestion and muscular activity had on them,
Lavoisier and Laplace invented the calorimeter which allowed him
to measure the heat relased by an animal, to compare it to that
released by the combustion of charcoal and thus to determine the
animal's expenditure of energy.Then he measured the oxygen
consumption corresponding to this expenditure and was able to
establish several general notions concerning respiration:
movements and digestion increase its rhythm and the consumption of
air.
Eager to confirm these hypotheses for man, his assistant Seguin
dressed himself in a special suit made of taffeta coated with
elastic gum, through which no air or humidity could penetrate.It
was closed above the head by a strong ligature.A tube stuck around
his lips with putty allowed him to breath.His consumption of
oxygen was measured in different situations.Inactive and on an
empty stomach, at an ambient temperature of 32.5° C., he
consumed 400 millilitersper minute.
Thus was born the notion of an inescapable basic expenditure of
energy, linked to minimal physiological activity, independent of
all exterior stimulation: it was the future basal metabolism. The
consumption increased during digestion as well as with lowered
temperatures and muscular exercise."We have succeeded in observing
two laws of the utmost importance: the first is that the increase
in the number of heartbeats is in quite direct proportion to the
sum of the weights lifted to a determined height; the second is
that the quantity of vital air consumed is in directproportion to
the product of the inspirations throughheartbeats."(Lavoisier,
Oeuvres, vol. II, p. 696.)
Then Lavoisier gives the description of Seguin's efforts during a
quarter of an hour to raise a weight of fifteen poundsto a height
of six hundred fifty feet. "It should be noted," writes Pierre
Dejours, "that in the same paper, theauthors proposed two
biometric laws, perhaps the first to be enunciated: on the one
hand, the correlation between therate at which the
heartbeatsandthe energy consumed duringthe exercise; and on the
other, the correlation between oxygen consumption and the
respiratory and heartbeatrates.The data of modern physiology have
shown these laws to be exact."(P. Dejours, "Lavoisier
physiologiste," inIl y a 200 ans Lavoisier,C.
Demeulenaere-Douyère, ed., Paris, Technique et
Documentation Lavoisier, 1995, p. 14.)
At this time, Lavoisier also expressed one of the first laws of
the physiology ofexertion and invented the modern methods of
objective measurements of specific jobs which are used in
occupationalmedicine: measurement of heartbeatrate, a
procedurestill used universally today, and of the oxygen consumed,
now discarded since the former suffices.It should be noted that
Sequin claimed his contribution to this discovery.
- The locus of this combustion, maintained by respiration,
remained unknown to Lavoisier.Sometimes, he asked himself if the
carbonic gas wasformed "in the lungs or during circulation, by the
combination of oxygen from the air with the carbon from the
blood."At other times, he approached the correct hypothesis: "It
is possible," he wrote, "that a part of this carbonic acid is
formed by digestion, that it is introduced into the circulation
with the chyle and when it reaches the lungs isreleased from the
blood as the oxygen combines with it through a superior affinity."
(Lavoisier, Oeuvres, vol. II, p. 702.)
By the end of his investigation, Lavoisier had identified the
biological regulators the equilibrium of which assures a state of
health: respiration, which brings the combustion agent, oxygen;
nutrition, which furnishes the fuels, hydrogen and carbon; and
thermogenesis, or the production of heat resulting from this
combustion.It remained for him to define its counterweight,
thermolysis, or loss of heat, produced by perspiration.
The
Physiology of Perspiration
Lavoisier assumed that perspiration continuously drew water from
the organism and combined it with the caloric to transform it into
vapor; the loss of the caloric led to a cooling which maintained
the constant temperature of the body."It is not only by the pores
of the skin that this aqueous emanation takes place," Lavoisier
specified."A considerable quantity of humidity is also exhaled by
the lungs at each expiration."(Lavoisier, Oeuvres, vol. II, p.
706.)
The input/output method and the use of scales were as always much
in evidence.To determine the weight of water lost through
perspiration, it sufficed to determine the total weight loss of an
individual within a given time and then to subtract from it the
amount of water lost by respiration. In the"Premier mémoire
sur la transpiration des animaux"("First Paper on Animal
Perspiration"),heanalyzed the respective roles of cutaneous and
pulmonary perspirationsand respiration: to break down the water
loss connected with each of these three phenomena, the methods
used "although simple in theory, presented extreme difficulties in
practice."(Lavoisier, Oeuvres, vol. II, p. 709.)We are forced to
admit today that they were notsurmounted.The authors lost control
of both the experimental protocol and that of the physiological
theory.The complexity of the hydro-electrolytic exchanges in the
lungs far surpassed what they could imagine.
In 1791, Lavoisier had been successively elected National Treasury
Commissioner, member of the Committee on Weights and Measures,
Treasurer of the Academy of Sciences and member of the Advisory
Board for Arts and Trades.Thisextraordinarily talented man who had
been able to hold ten jobs at once began to be overwhelmed, to the
detriment of his personal research.
In presenting his second paper on perspiration at the Academy on
February 22, 1792, he announced that "the experiments described
belonged entirely to Seguin," his student and assistant.(Quoted by
M. Daumas, Lavoisier théoricienet expérimentateur,
Paris, P.U.F., 1955, p. 65.)Lacking experimental results, and
always more at ease in the domain of political ideas, Seguinhad
concentrated on describing social disparities: "This evenness of
temperature which the wealthy man obtains with so much effort by
combining the production of two hemispheres - Indian silk and
Spanish wool- by using a multitude of men to weave precious
fabrics, is reached by the poor man in a much less complicated
way, one that makes him dependent on no one: nature simply
accelerates his respiration in proportion to his
needs."(Lavoisier, Oeuvres, vol. V, p.
388.)
One can imagine that Lavoisier took a certain distance regarding
his young assistant whose daring theories rather alarmed him.In
eighteen years of study and reflection, he did however, cover the
essential questions: the role of pulmonary ventilation; its
relationship to the circulation of blood and combustion; the
mechanisms of body heat and homeothermia; and the regulation of
the respiratory function and its relationship to perspiration and
nutrition.
The
Animal Machine
Certainly, many questions remainedunanswered.More than a century
would pass before it wasestablished that respiration is intended
to produce the energy necessary for the maintenance and
development of living structures, and that this energy comes from
oxidation - which takes place in the microscopic bodies,
mitochondrions, at the center of cells and tissues - of three
classes of organic substances: carbohydrates, lipids and
proteins.But Lavoisier had made the essential link between
respiration and perspiration, body heat and nutrition,
foreshadowing the modern concept of biological energetics. "The
animal machine is governed principally by three main regulators:
respiration which consumes hydrogen and carbon and providesthe
caloric;perspiration, which increases or decreases, depending on
whether more or less caloric isneeded; and finally, disgestion,
which restores to the blood what it has lost through respiration
and perspiration."(Lavoisier, Oeuvres, vol. II, p.
700.)
Lavoisier had understood that carbon is necessary for combustion
and that it is transported by the blood.A half century later,
Claude Bernard (1813-1878) demonstrated that the real fuels are
the carbohydrates absorbed by the intestines and carried to the
liver by the portal vein.But Lavoisier had in any case understood
that an organism's functions aresubjected to a regulating system
permitting the individual to adapt to all circumstances."Is he
inactive or resting? Then circulation as well as respiration is
slowed down.He uses less air.His lungs exhale less carbon and
hydrogen and consequently he needs less food.Does he have to make
an effort?Respiration accelerates, he consumes more air and loses
more hydrogen and carbon.Consequently, he needs to restore his
strength more often, especially by nutrition."(Lavoisier,
Oeuvres, vol. II, pp. 699-700.)
The Physiology of Nutrition
Trying to reach a better understanding of the physiology of
nutrition, on July 28, 1792, Lavoisier proposed that the Academy
award a prize for research on the subject: 5000 livres would be
given to the best study on animalization, that is, the organic
synthesis of animal tissues after the absorption of food.He had
guessed that the liver plays an essential role in this process."It
is known that the liver occupies considerable space in animals'
bodies;that a part of the abdominal vascular system is destined to
this viscera; that the blood is disposed there in a particular way
for the secretion of bile; thatall other functions depend on the
constant and regular flow of this substance; that the liver exists
in all animal orders, including insects and worms; that it is
often accompanied by a gall bladder, depending on the nature
ofthese beings; that there is an essentialrelationship between the
spleen, the pancreas and the liver.Here then , forthe first time
in years,is data that anatomy has offered for the speculation of
physiologists. (...)It is easy to foresee that other than the
secretion of bile, or rather through the secretion of bile, an
organ as important in size, connections and vascular structures as
the liver fills a whole system of functions, the extent of which
science has not yet grasped."(Lavoisier, Oeuvres, vol. VI, p.
35.)
Lavoisier recalled that the initialphases of the physiology of
digestion were already well known: the successive actions of
saliva in the mouth, gastric juices in the stomach,and of bile
andpancreatic juices which convertnourishment into chyle, of which
"one part is absorbed in the blood to compensate for the losses
that are constantly taking place through respiration and
perspiration," and"all the material that Nature cannot used is
rejected in the form of excrement."(Lavoisier, Oeuvres, vol. VI, p.
34.)
But many unknowns remained: the anatomiesof the liver and gall
bladder; the chemical composition of bileand the hepatic
parenchyma in various animal species; the difference between the
composition of the blood in the portal vein and that in other
regions of the body; the definitions of the functions of the liver
and bile and their relationship to other organs; the main
illnesses of the liver and the biliary tract in both human beings
and animals.In his commentary, Lavoisier underlined the
importancehe attributed to the hepatic function in anabolism.In a
most original way, he expressed his confidence in the capacity of
chemistry to explore the mechanisms of life."By proposing this
subject and emphasizing all its difficulties, the Academy realizes
that it will require chemical research basedin particular on the
recently acquired analytical means of chemistry.It feels and hopes
that this work will oblige those who undertake it to determine the
nature of the blood in the portal vein, and to compare it with the
arterial and venous blood in other regions.(...)It is time to take
up the complicated questions presented by the phenomena ofthe
animal system , and it is from the joint efforts of physics,
anatomy and chemistry that we can hope to find
answers."(Lavoisier, Oeuvres, vol. VI, pp.
35-36.)
Along with the chemical revolution, Lavoisier hadalso beguna
revolution in biology.Claude Bernard, in turn, would show that the
phenomena which occur in the living body, conform to laws as
precise and stable as those in inanimatesubstances.But he would
bear witness to the importance ofthe contributions of Lavoisier
whom he considered to be, after WilliamHarvey (1578-1657), the
most important figure in modern biology.
"Lavoisier," he wrote, "had already clearly shown that the
physico-chemical phenomena of living beings are sustained by the
same causes as those of mineral bodies.He showed thatbreathing
animalsand calcinedmetalsabsorb from the air the sameactive and
vital principle, oxygen,and that the absence of this respirable
air stops calcination as well as respiration.In another
experiment, Lavoisier and Laplace demonstrated that oxygen, in
penetrating living beings, engenders the organic heat that
sustains them, by a real combustion similar to the combustion of
fire boxes.The ancient myth of life compared to a flame that
blazes and isthen extinguished was no longer a simple metaphor but
a scientific reality.The same chemical conditions feed the fire in
inorganic nature and life in organic nature. If, starting from the
fact pointed out by Lavoisier, we have now come down to the
experimental analysis of vital functions, we shall see that in all
tissues and organs, it is oxygen that is always both the
stimulator of physico-chemical phenomena and the condition of the
functional activity of organized matter.Oxygen penetrates animals
bythe respiratory surface, and circulation spreadslife in all the
organs and organic elements by distributing the oxygen dissolved
in the arterial blood."(Claude Bernard, "Le problème de la
physiologie générale," in La science expérimentale, Paris, Baillière, 1878, pp. 122-23.)