The concept of homeostasis was first introduced by French physiologist Claude Bernard in the late 1800s.
His reasoning was that for life to persist, living organisms must keep certain internal milieu within relatively stable and narrow limits (Bernard, 1878).
(Note: In this context, “milieu” refers to the internal environment of an organism, not its social environment.)
Video 1 Claude Bernard – Unsung Heroes of Science 2022 Hertford College, Oxford
The term homeostasis was introduced by American physiologist Walter Cannon in 1929, about fifty years after Claude Bernard’s work.
Like Bernard, Cannon used the term to describe processes in living systems.
The word homeostasis is derived from Greek:
“homeo” meaning “similar” (distinct from “homo,” which means “same”), and
“stasis” meaning “a stable state” or “standing still”, indicating a condition that remains relatively constant.
Why use “homeo”
Unlike human-designed systems—like thermostats—that tend to operate with fixed set points, natural systems (those shaped by evolution) typically function within flexible, acceptable ranges.
For example, variables like:
- Hydration levels
- Blood glucose and sodium concentrations
- Body temperature
…don’t stay at one exact value but fluctuate within healthy limits.
Homeostasis is recognized as a fundamental concept in physiology. It refers to the self-regulating mechanisms that enable an organism to maintain internal balance while adapting to shifts in the external environment.
Rather than being fixed or unchanging, homeostasis is a dynamic process—constantly adjusting internal conditions to help the organism cope with external pressures and survive.
The key homeostatic processes in the human body
These are the systems and mechanisms that help maintain stable internal conditions essential for survival and function:
| Homeostasis | Stimulus | Variable | Sensor (Receptor) | Control Centre | Effector |
| Thermoregulation | Change in body temperature | Core body temperature | Thermoreceptors (skin & hypothalamus) | Hypothalamus | Sweat glands, blood vessels, muscles (shivering) |
| Water Balance | Blood osmolarity change (dehydration) | Blood osmolarity | Osmoreceptors (hypothalamus) | Hypothalamus | Kidneys (water reabsorption), thirst center |
| Blood Glucose Regulation | Blood glucose level change | Blood glucose concentration | Pancreatic beta and alpha cells | Pancreas | Insulin & glucagon secretion affecting liver and cells |
| Blood Gas Balance | Changes in CO₂/O₂ levels, pH | pO₂, pCO₂, blood pH | Chemoreceptors (carotid/aortic bodies, brainstem) | Medulla oblongata | Respiratory muscles (rate and depth of breathing) |
| Blood pH Regulation | Blood pH deviation | Blood pH | Chemoreceptors (brainstem, carotid bodies) | Brainstem, kidneys | Lungs (CO₂ removal), kidneys (H⁺/HCO₃⁻ balance) |
| Blood Pressure Regulation | Blood pressure changes | Blood pressure | Baroreceptors (carotid sinus, aortic arch) | Medulla oblongata | Heart, blood vessels, kidneys (via RAAS) |
| Electrolyte Balance | Changes in electrolyte concentration | Na⁺, K⁺, Ca²⁺ levels | Electrolyte sensors (kidneys, adrenal glands) | Kidneys, adrenal cortex | Kidneys (reabsorption/secretion), hormone release |
| Heart Rate Regulation | Change in blood pressure or O₂ demand | Heart rate, cardiac output | Baroreceptors, chemoreceptors | Medulla oblongata | Heart (SA node), blood vessels |
| Metabolic Rate Regulation | Changes in metabolic demand | Thyroid hormone levels | Hypothalamus, thyroid gland | Hypothalamus, thyroid gland | Various tissues, increasing/decreasing metabolism |
| Calcium Homeostasis | Blood calcium level changes | Serum Ca²⁺ concentration | Parathyroid calcium sensors | Parathyroid glands | Bones, kidneys, intestines (via PTH, calcitonin) |
| Iron Homeostasis | Iron levels in blood or stores | Serum iron levels | Liver (hepcidin regulation) | Liver | Intestines (iron absorption), bone marrow (production) |
| Hormonal Homeostasis | Fluctuations in hormone levels | Hormone concentrations | Endocrine gland receptors | Hypothalamus, pituitary gland | Various endocrine glands |
| Immune Homeostasis | Presence of pathogens or immune signals | Immune activity levels | Immune cells (macrophages, lymphocytes) | Lymphoid organs | Cytokine secretion, immune cell activation |
| Body Fluid Volume | Changes in blood volume or pressure | Blood/plasma volume | Baroreceptors, osmoreceptors | Hypothalamus, kidneys | Kidneys, thirst center |
Thyroid hormone (TH) plays a key role in maintaining the body’s energy balance. In response to environmental factors like food intake or temperature changes, as well as hormonal signals such as leptin, pathways in the hypothalamus regulate both the activity of the sympathetic nervous system and the secretion of thyroid hormone via the hypothalamic-pituitary-thyroid (HPT) axis.
Together, TH and sympathetic nervous system output influence multiple organs to control energy metabolism, thereby managing overall energy homeostasis in the body. Specifically, thyroid hormone signalling and sympathetic stimulation:
- Promote adaptive thermogenesis in brown adipose tissue (BAT),
- Regulate cardiovascular functions including blood pressure and heart rate,
- Influence glucose balance through effects on pancreatic β-cells,
- Control thyroid hormone clearance and glucose production in the liver,
- And affect other tissues such as white adipose tissue (WAT) and skeletal muscle.
Figure 1.1 Body energy balance (McAninch and Bianco, 2014)
These homeostatic processes are highly interdependent. The human body functions as an integrated system, so changes in one variable often affect others. Here’s how they connect:
Examples of connections between homeostatic processes:
- Thermoregulation and Metabolic Rate:
When body temperature drops, metabolism can increase to generate more heat. Thyroid hormones regulate metabolism, linking these two. - Water Balance and Blood Pressure:
Water retention by the kidneys increases blood volume, which raises blood pressure. The RAAS system controls both fluid balance and vascular resistance. - Blood Glucose and Hormonal Homeostasis:
Insulin and glucagon regulate blood sugar, but these hormones are influenced by overall endocrine system status, including stress hormones like cortisol. - Blood pH and Blood Gas Balance:
CO₂ levels affect blood pH directly since CO₂ reacts with water to form carbonic acid. Breathing adjusts CO₂, thus controlling pH. - Electrolyte Balance and Heart Rate:
Electrolytes like potassium and calcium are crucial for proper cardiac muscle function, affecting heart rhythm and rate. - Immune Homeostasis and Hormonal Homeostasis:
Stress hormones (cortisol) can suppress immune responses, linking immune balance with endocrine regulation.
Homeostatic mechanisms form a complex network, with feedback loops and cross-talk among systems to maintain overall internal stability. The body constantly integrates signals from many sensors and adjusts multiple effectors simultaneously to keep everything in balance.
References:
Hoermann, R., Midgley, J.E.M., Larisch, R. and Dietrich, J.W. (2015). Homeostatic Control of the Thyroid–Pituitary Axis: Perspectives for Diagnosis and Treatment. Frontiers in Endocrinology, [online] 6. doi:https://doi.org/10.3389/fendo.2015.00177.
McAninch, E.A. and Bianco, A.C. (2014). Thyroid hormone signaling in energy homeostasis and energy metabolism. Annals of the New York Academy of Sciences, 1311(1), pp.77–87. doi:https://doi.org/10.1111/nyas.12374.
NT Contributor (2006). Homeostasis – Part 1: anatomy and physiology | Nursing Times. [online] Nursing Times. Available at: https://www.nursingtimes.net/respiratory/homeostasis-part-1-anatomy-and-physiology-04-04-2006/.