BASIC CONCEPTS IN HOMEOSTASIS

BASIC CONCEPTS IN HOMEOSTASIS

In the eighteenth and nineteenth centuries, a series of eminent
scientists laid the foundations of our understanding of homeosta-
sis and the response to injury. The classical concepts of homeosta-
sis and the response to injury are:
• ‘The stability of the “milieu intérieur” is the primary condition
for freedom and independence of existence.’ (Claude Bernard)
i.e. body systems act to maintain internal constancy
• ‘Homeostasis: the co-ordinated physiological process which
maintains most of the steady states of the organism.’ (Walter
Cannon)
i.e. complex homeostatic responses involving the brain,
nerves, heart, lungs, kidneys and spleen work to maintain
body constancy
• ‘There is a circumstance attending accidental injury which
does not belong to the disease, namely that the injury done,
has in all cases a tendency to produce both the deposition and
means of cure.’ (John Hunter)
i.e. responses to injury are, in general, beneficial to the host
and allow healing/survival
In essence, the concept evolved that the constancy of the ‘milieu
intérieur’ allowed for the independence of organisms, that com-
plex homeostatic responses sought to maintain this constancy,
and that within this range of responses were the elements of heal-
ing and repair. These ideas pertained to normal physiology and
mild/moderate injury. In the modern era, such concepts do not
account for disease evolution following major injury/sepsis or the
injured patient who would have died but for artificial organ sup-
port. Such patients exemplify less of the classical homeostatic
control system (signal detector–processor–effector regulated by a
negative feedback loop) and more of the ‘open loop’ system,
whereby only with medical/surgical resolution of the primary
abnormality is a return to classical homeostasis possible. Current
understanding of such events as the response to major
sepsis/injury relies on chaos theory and the use of a structured
network knowledge-base approach.
As a consequence of modern understanding of the metabolic
response to injury, elective surgical practice seeks to reduce the
need for a homeostatic response by minimising the primary insult
(minimal access surgery and ‘stress-free’ perioperative care). In
emergency surgery, where the presence of tissue trauma/sepsis/
hypovolaemia often compounds the primary problem, there is a
requirement to augment artificially homeostatic responses (resus-
citation) and to close the ‘open’ loop by intervening to resolve the
primary insult (e.g. surgical treatment of major abdominal sepsis)
and provide organ support (critical care) while the patient comes
back to a situation in which homeostasis can achieve a return to
normality