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Homeostasis

Q: Which homeostatic variables are classified as plastic constants?

Plastic constants include: Core body temperature: in humans and warm-blooded animals, this is a plastic constant, allowing temporary deviations from the optimum. Prolonged deviation (hyperthermia or hypothermia) disrupts normal vital processes.Blood nutrient levels: the nutritional functional system maintains these at levels optimal for metabolism; this parameter serves as an example of a plastic constant.

For medical students2 min readUpdated 2026-10-10

Homeostasis is the dynamic constancy of the body's internal environment, maintained by integrated physiological mechanisms. The ultimate goal of this continuous process is to create optimal conditions for cellular metabolism.

DiscoveryClaude Bernard demonstrated that the stability of the internal environment is the condition for an independent life.

TermThe term "homeostasis" was first proposed by the physiologist Walter Cannon.

FluidsBalance is maintained across blood, lymph, intracellular, and interstitial fluids.

Central GoalAll regulatory mechanisms are directed toward ensuring adequate tissue metabolism.

Concept and Evolution

For a long time, it was believed that the organism existed in static equilibrium. However, the concept of homeostasis proved the opposite: the composition and properties of our internal environment are relative and dynamic.

Historically, this theory was shaped by two key figures:

Claude Bernard first formulated the fundamental principle: for an organism to live independently of external fluctuations, its internal environment must remain stable.
Walter Cannon coined the term homeostasis.

Today, physiology also uses the synonym homeokinesis, which even more accurately emphasizes the active, kinetic nature of the processes keeping parameters within defined limits.

Key Constants of the Internal Environment

The body's fluid compartments (blood plasma, lymph, interstitial fluid, and intracellular fluid) act as a physiological reservoir. They concentrate both intermediate and final products of metabolism, as well as the output of all organ systems.

Homeostatic status is defined by a set of rigid and plastic variables. The most critical constants include:

Circulating blood volume (CBV) and total blood mass.
Blood cell counts.
Blood pressure.
Osmotic pressure (the foundation of water-electrolyte balance).
Core body temperature.
Acid-base balance (pH).
Nutrient levels (glucose, amino acids, lipids).
Blood gas content ($O_2$ and $CO_2$ partial pressures).
Concentration of metabolic waste products.

Functional Systems and Self-Regulation

The body does not regulate "everything at once" through a single center. Each physiological parameter is maintained at its optimal level by a specific functional system.

According to the self-regulation model, the architecture of homeostasis includes:

Central link: direct tissue metabolism. This is the ultimate objective served by all physiological systems.
Closed loops: each regulated parameter (whether pH, temperature, or sex hormone levels) has its own feedback loop.
Coordinated activity: functional systems are not isolated; they work cooperatively. If one parameter shifts, it immediately triggers compensatory reactions in other loops to preserve overall balance and protect tissue metabolism.

Mnemonic

To remember the main parameters of homeostasis, use the three "S" rule: State/Environment (pH, osmolarity, temperature), Supply (CBV, pressure, gases, nutrients), and Sink/Waste (metabolic clearance).

Frequently asked questions

Which homeostatic variables are classified as rigid constants?

Rigid constants include:

Blood gas parameters: $pO_2$, $pO_2$, and pH;
Blood pH / acid-base balance;
Blood pressure (partially).

pH is one of the most strictly defended homeostatic constants; shifts lead to acidosis or alkalosis, and even minor pH changes disrupt enzyme activity, redox reactions, cell receptor sensitivity to signaling molecules, and membrane permeability. Regarding blood gas homeostasis, severe disruption (such as asphyxia) is incompatible with life for more than 2–3 minutes.

Which homeostatic variables are classified as plastic constants?

Plastic constants include:

Core body temperature: in humans and warm-blooded animals, this is a plastic constant, allowing temporary deviations from the optimum. Prolonged deviation (hyperthermia or hypothermia) disrupts normal vital processes.
Blood nutrient levels: the nutritional functional system maintains these at levels optimal for metabolism; this parameter serves as an example of a plastic constant.

What components comprise an Anokhin functional system?

A functional system consists of several key components integrated to achieve a useful adaptive result:

Useful adaptive result — the adaptive outcome of the system's activity;
Control apparatus — result receptors and the action result acceptor;
Afferent feedback — transmission of signals regarding the state of the result back to the center;
Control apparatus — the central integration link (neural center);
Effector components — internal organs, glands, and skeletal muscles executing autonomic, hormonal, and behavioral responses.

Which organs and systems play the primary role in maintaining acid-base balance (ABB)?

Acid-base balance is maintained by chemical buffer systems and organ-level compensatory mechanisms. Organ mechanisms work alongside buffer systems and operate over minutes to hours.

Blood system / buffer systems: hemoglobin is the primary blood buffer system; the bicarbonate buffer system regulates pH.
Lungs: eliminate excess $CO_2$; primary organs of ABB regulation.
Kidneys: primary organs of ABB regulation; can enhance $H^+$ secretion in the renal tubules.
Liver: participates in ABB compensation via plasma protein synthesis, ammonia production, gluconeogenesis utilizing lactate and pyruvate, elimination of non-volatile acids, and excretion of acidic and basic substances into bile.
Gastrointestinal tract: also contributes to ABB regulation.

Which hormones regulate circulating blood volume and osmotic pressure?

Circulating blood volume (CBV) and osmotic pressure are regulated by the following hormonal factors:

Antidiuretic hormone (vasopressin) — enhances facultative water reabsorption in the kidneys, decreasing osmotic pressure and urine output;
Aldosterone — stimulates sodium and water retention;
Angiotensin II — stimulates thirst and vasoconstriction (part of the renin-angiotensin-aldosterone system);
Natriuretic peptide (atrial natriuretic peptide) — decreases sodium and water reabsorption, increases diuresis, and acts as an antagonist to aldosterone.

Why is homeostasis called dynamic rather than static?

Because physiological parameters are not fixed rigidly. They constantly fluctuate within narrow ranges, while self-regulatory mechanisms continuously drive them back toward the optimum.

What is the primary goal of homeostatic regulation?

To reliably support metabolism directly within the body's tissues.

How do functional systems respond to changes in a single constant?

They operate cooperatively. A change in one parameter via feedback loops engages other systems in a compensatory reaction to restore balance.

Go deeper

Feedback mechanisms in closed regulatory loops
Acid-base balance (pH) and buffer systems
Osmotic pressure and water-electrolyte homeostasis
Temperature homeostasis and thermoregulation
The role of humoral regulation (hormones) in maintaining balance