Essence and evolution of the concept
For a long time, it was believed that the body is in static equilibrium. However, the concept of homeostasis proved the opposite: the constancy of the composition and properties of our internal environment is relative and dynamic.
Historically, the formation of this doctrine is associated with two names:
- Claude Bernard was the first to formulate the fundamental principle: for an organism to exist independently of the fluctuations of the external world, its internal environment must remain stable.
- Walter Cannon introduced the very term homeostasis into scientific use.
Today, physiology also uses the synonym homeokinesis, which even more accurately emphasizes the active, kinetic nature of the processes that keep parameters within specified limits.
Key constants of the internal environment
The body's fluid media (blood plasma, lymph, interstitial and intracellular fluid) act as a peculiar reservoir. Both intermediate and end products of metabolism, as well as the results of the work of all organ systems, are concentrated in them.
The homeostatic level is determined by a set of rigid and labile parameters. The most important constants include:
- Circulating blood volume (CBV) and blood mass.
- Blood cell count.
- Blood pressure.
- Osmotic pressure (the basis of water-salt balance).
- Body temperature.
- Acid-base balance (environment reaction, pH).
- Nutrient levels (glucose, amino acids, lipids).
- Gas composition ($O_2$ and $CO_2$ tension).
- Concentration of excretion products (metabolites).
Functional systems and principles of self-regulation
The body does not regulate "everything at once" through a single center. Each individual physiological parameter is maintained at an optimal level by its specific functional system.
According to the self-regulation model, the architecture of homeostasis is as follows:
- Central link: metabolism directly in the tissues. This is the ultimate goal for which all systems work.
- Closed loops: each regulated parameter (whether pH, temperature, or sex hormone levels) has its own feedback loop.
- Cooperative activity: functional systems are not isolated. They work together. If one parameter shifts, it instantly triggers compensatory reactions in other loops to preserve the overall balance and protect tissue metabolism.
Briefly: answers to topical questions
Which homeostasis parameters belong to rigid constants?
According to sources, rigid constants include:
- gas homeostasis parameters: $pO_2$, $pCO_2$, and $pH$;
- blood pH / acid-base balance;
- blood pressure — partially.
pH is characterized as one of the most rigid homeostatic constants; its shift leads to acidosis or alkalosis, and even minor changes in pH disrupt enzyme activity, oxidation-reduction reactions, the sensitivity of cell receptors to biologically active substances, and membrane permeability. For gas homeostasis, it is indicated that with a severe shift, such as in asphyxia, the body cannot exist for more than 2–3 minutes.
Which homeostasis parameters belong to labile constants?
According to sources, labile homeostasis constants include:
- Body temperature: in humans and warm-blooded animals, it is one of the labile constants; deviation from the optimal level is permitted for a certain period of time. Prolonged temperature deviation — hyperthermia or hypothermia — disrupts normal vital processes.
- Nutrient levels in the blood: the nutritional functional system maintains them at an optimal level for metabolism; this parameter is classified as an example of a labile constant of the body.
What links does Anokhin's functional system consist of?
A functional system consists of several key components combined to achieve a result beneficial to the organism:
- Useful result — the adaptive outcome of the system's activity;
- Control apparatus — result receptors and action result acceptor;
- Reverse afferentation — transmission of a signal about the state of the result to the center;
- Management apparatus — the central link (neural center);
- Effector components — internal organs, glands, skeletal muscles that ensure autonomic, hormonal, and behavioral reactions.
Which organs and systems play the main role in maintaining acid-base balance (ABB)?
Sources indicate chemical buffer systems and organ mechanisms of compensation for maintaining acid-base balance. Organ mechanisms work alongside buffer systems and take from several minutes to several hours.
- Blood system / blood buffer systems: hemoglobin is the main blood buffer system; the bicarbonate buffer system is involved in pH regulation.
- Lungs: remove excess $CO_2$; the lungs are listed among the main organs of ABB regulation.
- Kidneys: listed among the main organs of ABB regulation; $H^+$ secretion can increase in the renal tubules.
- Liver: participates in compensating for ABB shifts through blood protein synthesis, ammonia formation, gluconeogenesis using lactate and pyruvate, elimination of non-volatile acids, and excretion of acidic and basic substances with bile.
- Gastrointestinal tract: listed among the main organs of ABB regulation.
Which hormones regulate circulating blood volume and osmotic pressure?
The regulation of circulating blood volume (CBV) and osmotic pressure is carried out by the following hormonal factors:
- Antidiuretic hormone (vasopressin) — enhances facultative water reabsorption in the kidneys, reducing osmotic pressure and diuresis;
- Aldosterone — stimulates sodium and water retention;
- Angiotensin II stimulates thirst and causes vasoconstriction (a component of the renin-angiotensin-aldosterone system);
- Natriuretic hormone (atrial natriuretic peptide) reduces sodium and water reabsorption, increases diuresis, and acts as an aldosterone antagonist.
To remember the main parameters of homeostasis, use the three "S" rule: Setting (pH, osmolarity, temperature), Supply (blood volume, pressure, blood gases, nutrition), and Shipment (waste products).
Class and exam questions
Why is homeostasis called dynamic rather than static?
Because physiological parameters are not fixed permanently. They constantly fluctuate within narrow limits, and self-regulation mechanisms continuously return them to the optimum.
What is the main goal of homeostatic regulation?
Reliable provision of metabolism directly in the body's tissues.
How do functional systems react to a change in one of the constants?
They work in concert. A change in one parameter via feedback principles engages other systems in a compensatory reaction to restore balance.
What else to study on this topic
- Feedback mechanisms in closed regulatory loops
- Acid-base balance (pH) and buffer systems
- Osmotic pressure and water-salt homeostasis
- Temperature homeostasis and thermoregulation
- The role of humoral regulation (hormones) in maintaining balance
Master the entire topic in the "Physiology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Homeostasis" and 4,128 other subject pages
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 114 video lessons on the subject
Free video lessons: PhysiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.