Step outside on a January morning at −25 °C, or run a 10 km race in July, and the temperature deep inside your body barely moves from about 37 °C. Your blood pH stays between 7.35 and 7.45 whether you just ate a pizza or sprinted up the stairs. That constancy is not an accident, and it is not passive. Your cells can only work within narrow limits: enzymes denature if they get too hot, proteins change shape if pH drifts, and cells swell or shrink if the water balance around them changes. Homeostasis is the set of processes that sense changes in the body's internal environment and correct them before cells are harmed. Every system in this unit (nervous, endocrine, excretory) is part of that correcting machinery.
Your cells are not bathed by the outside world; they are bathed by extracellular fluid (the interstitial fluid around cells, plus blood plasma). The French physiologist Claude Bernard called this the milieu intérieur, and the American physiologist Walter Cannon later named its regulation homeostasis (Greek for "staying the same"). The word is slightly misleading: the variables are not fixed. Core temperature (about 36.5–37.5 °C), blood glucose (about 4–7 mmol/L) and blood pressure rise and fall all day, but they fluctuate around a set point within a normal range. This is a dynamic equilibrium: constant on average, yet maintained by continual adjustments that cost energy. No single system does this alone. The nervous system senses change and responds in milliseconds (sweating, shivering, breathing rate). The endocrine system responds more slowly, but its effects last longer (thyroxine for metabolic rate, ADH and aldosterone for water and salt). The excretory system is an effector for both, since the kidneys carry out the orders that set water, salt and pH. The hypothalamus links them: it is part of the brain, yet it controls the pituitary gland and so much of the endocrine system.
The feedback loop
Every homeostatic control system, whether it regulates temperature or water, has the same five parts. A stimulus is a change in a variable (body temperature rises). A receptor (sensor) detects it (thermoreceptors in the skin and hypothalamus). The control centre (integrator), usually the brain or an endocrine gland, compares the input to the set point and decides on a response. An effector (a muscle or gland) carries out the response (sweat glands secrete sweat). The response changes the variable, and that change is sensed again, closing the loop. Information travels from receptor to control centre along sensory nerves or as a change in blood chemistry, and from control centre to effector along motor nerves or as a hormone.
- The response opposes (reverses) the original change
- Brings the variable back toward the set point
- Stabilizing: used for almost all homeostasis
- Runs continuously; it has no natural end
- Examples: body temperature, blood glucose, blood pH, water balance, thyroxine levels
- The response amplifies (reinforces) the original change
- Pushes the variable further from where it started
- Destabilizing: used only to finish a process quickly
- Must be stopped by an outside event (an end point)
- Examples: oxytocin during childbirth, blood clotting, the LH surge before ovulation
Childbirth is the classic positive-feedback loop. The baby's head presses on the cervix; stretch receptors send nerve signals to the hypothalamus; the posterior pituitary releases oxytocin; oxytocin makes the uterine muscle contract harder; harder contractions push the head further against the cervix, which releases even more oxytocin. The loop keeps escalating until its end point, the birth of the baby, removes the stretch stimulus. Blood clotting works the same way: platelets that stick to a damaged vessel release chemicals that attract more platelets, and the clotting-factor cascade activates more of itself, until the clot seals the wound. Positive feedback is useful only because it has an end point. A positive loop with no end point would drive a variable to an extreme and is dangerous: for example, a very high fever can raise metabolic rate, which produces more heat, which raises temperature further.
Thermoregulation
Humans are endotherms: we generate most of our body heat from metabolism and regulate it internally. The control centre is the hypothalamus, a small region at the base of the brain that acts as the body's thermostat. It receives input from thermoreceptors in the skin (which give early warning of the outside temperature) and from its own receptors that monitor the temperature of the blood flowing through it (core temperature). It then drives effectors through the nervous system (fast) and the endocrine system (slower but longer-lasting). A fever is not a failure of this system: in response to infection, the hypothalamus deliberately raises its set point, so you shiver and feel cold even as your temperature climbs, and you sweat once the set point drops back.
| Too hot (core above set point) | Too cold (core below set point) |
|---|---|
| Vasodilation: arterioles supplying the skin widen, so more warm blood flows near the surface and heat radiates away. Skin looks flushed. | Vasoconstriction: arterioles supplying the skin narrow, so warm blood is kept in the core. Skin looks pale; fingers and toes get cold first. |
| Sweating: sweat glands secrete water onto the skin; evaporation absorbs heat (about 2.4 kJ per gram of water evaporated). | Shivering: rapid involuntary skeletal-muscle contractions. Muscle contraction is inefficient, so most of the ATP energy is released as heat. |
| Hair muscles relax, so hairs lie flat. | Piloerection: arrector pili muscles pull hairs upright (goosebumps). In furry mammals this traps an insulating layer of air; in humans it is a nearly useless leftover. |
| Metabolic rate decreases slightly; less thyroxine released. | Metabolic rate increases: hypothalamus → TRH → pituitary → TSH → thyroid releases more thyroxine, which raises cellular respiration and heat production over hours to days. Adrenaline gives a short-term boost. |
| Behaviour: remove clothing, seek shade, drink cold fluids, reduce activity. | Behaviour: add layers, curl up (reduces surface area), seek shelter, move around. |
| When it fails, hyperthermia: core temperature rises above normal because heat cannot be lost fast enough, for example during hard exercise in hot, humid air where sweat cannot evaporate. Heat exhaustion can progress to heat stroke (core above about 40 °C): the control system itself fails, sweating may stop and proteins begin to denature. A medical emergency. | When it fails, hypothermia: core temperature falls below about 35 °C because heat loss outpaces heat production (cold water removes heat far faster than air at the same temperature). As it worsens, shivering stops, enzymes slow, the heart becomes unstable and thinking becomes confused. |
Blood pH and the bicarbonate buffer
Cells constantly produce acid. Cellular respiration releases CO₂, which reacts with water in the blood, and hard exercise adds lactic acid. Yet blood pH stays between 7.35 and 7.45. Below 7.35 is acidosis and above 7.45 is alkalosis; both disturb enzyme and nerve function. Three lines of defence work together. The first is the bicarbonate buffer system, which reacts within seconds:
- Buffer (seconds): if extra enters the blood, bicarbonate combines with it to make carbonic acid, shifting the equilibrium to the left, so rises far less than it otherwise would. If is removed, carbonic acid dissociates to replace it.
- Lungs (minutes): chemoreceptors in the medulla oblongata and in the carotid and aortic bodies detect rising and . The breathing centre speeds up and deepens breathing, so more is exhaled. Removing pulls the whole equilibrium to the left and removes .
- Kidneys (hours to days): kidney tubule cells secrete into the urine and reabsorb (and make new) into the blood. This is the only route that actually removes non-volatile acid from the body, and the only one that can restore the bicarbonate the buffer has used up.
- Set up: line one half of the chamber with damp paper towel and the other with dry paper towel. Keep light, temperature, surface texture and chamber shape identical on both sides (controlled variables).
- Control trial: first run the test with both sides dry (or both damp). The animals should split roughly 50 : 50. If they do not, something else (a light source, a draft, a smell) is biasing them and must be fixed.
- Run: place 10 isopods in the centre, cover the chamber, wait a short settling time, then count the number on each side every minute for 10 minutes (the dependent variable).
- Replicate: repeat with at least three groups, and rotate the chamber 180° between trials so any hidden bias in the room is spread across both treatments.
- Analyse: calculate the mean percentage on the damp side and compare it with the 50% expected by chance. A consistently large difference supports the hypothesis.
- Ethics: handle animals gently with a soft brush, keep trials short and return them to a suitable habitat afterwards.