Sense Organs and Homeostatic Mechanisms
Senior 3 · Biology
Sense Organs and Homeostatic Mechanisms
Lesson 4 of 5
25 min
Measure the temperature inside your body early on a cold morning in the hills and again in the afternoon after a football match on a hot pitch, and the two readings will usually differ by less than a degree. The air around you may have changed by 15 °C or more. Your body keeps its inside, the blood and the tissue fluid that bathe every cell, almost constant, because the enzymes in your cells work best within a narrow range. Keeping the internal environment steady is called homeostasis.
Definition
Homeostasis: keeping the internal environment of the body (its temperature, water content, glucose concentration and so on) within narrow limits, whatever happens outside.
Set point: the normal value round which a condition is kept, e.g. a core body temperature of about 37 °C.
Negative feedback: a control system in which a change from the set point causes a response that reverses the change, bringing the condition back to normal. Example: sweating when you are too hot.
Positive feedback: a control system in which a change causes a response that makes the change even bigger. Example: the contractions of the uterus during birth.
Vasodilation: widening of arterioles; vasoconstriction: narrowing of arterioles.
Every homeostatic control has the same parts: receptors detect a change from the set point; a control centre (often the hypothalamus) compares it with the set point; effectors make a response that reverses the change. When conditions return to normal, the receptors stop sending the signal and the response stops. Figure 1 shows this for body temperature.
Positive feedback pushes a condition further away from where it started, so it is used only for events that must be completed quickly and then stop. During birth, the baby's head presses on the cervix; this makes the pituitary release the hormone oxytocin, which makes the uterus contract more strongly, which presses the head harder, which releases more oxytocin, until the baby is born (Unit 12). In blood clotting, platelets at a wound release chemicals that attract more platelets, until the wound is sealed.
Figure 2: simplified; not to scale.
| Function of the skin | How |
|---|---|
| Protection | The cornified layer of dead cells resists damage, water loss and the entry of pathogens; melanin in the Malpighian layer absorbs ultraviolet light |
| Sensitivity | Receptors for touch, pressure, pain, heat and cold |
| Temperature control | Sweating, changes in the blood flow to the skin, hair erector muscles, and the insulating fat layer |
| Excretion | Sweat removes small amounts of urea and salts |
| Making vitamin D | Made in the skin when it is exposed to sunlight |
| Storage | Fat in the subcutaneous layer is an energy store and an insulator |
The hypothalamus is the thermostat. It detects the temperature of the blood flowing through it, and it receives impulses from temperature receptors in the skin. It then sends nerve impulses to the effectors:
| Effector | When too hot | When too cold |
|---|---|---|
| Sweat glands | Secrete more sweat; its evaporation takes heat from the skin | Secrete little sweat |
| Arterioles supplying the skin capillaries | Vasodilation: more blood flows near the surface and loses heat by radiation | Vasoconstriction: less blood flows near the surface, so less heat is lost |
| Skeletal muscles | Relaxed | Shivering: rapid contractions; respiration in the muscles releases heat |
| Hair erector muscles | Relax; hairs lie flat | Contract; hairs stand up (goose bumps). In furry mammals this traps a layer of insulating air; in humans the effect is small |
| Metabolism | Not raised | Adrenaline and thyroxine can raise the metabolic rate, releasing more heat |
Behaviour helps too: in heat we rest in the shade, drink and wear light clothes; in cold we put on layers and keep moving.
Worked example 1: how much heat does sweat remove?
Evaporating 1 g of sweat from the skin takes about 2.4 kJ of heat from the body. A footballer loses 1.5 litres of sweat during a hot match (an illustrative figure). Assume all of it evaporates; 1 litre of sweat has a mass of about 1000 g.
Worked example 2: why newborn babies get cold quickly
Model a baby and an adult as cubes of side 1 cm and 3 cm. Compare their surface area to volume ratios.
Warning
Common mistakes. (1) Writing that capillaries "move up" to the skin surface: they do not move; the arterioles supplying them widen (vasodilation), so more blood flows through them. (2) Writing that sweat itself is cold: sweat cools the skin only when it evaporates. (3) Calling sweating positive feedback: it reverses the change, so it is negative feedback.
Safety: heat illness. Someone who is very hot, confused, dizzy or has stopped sweating after hard exercise in heat may have heat stroke, which is an emergency. Get medical help at once; meanwhile move the person to shade, loosen clothing, cool the skin with water and fanning, and give sips of water only if they are fully awake.
How We Control Our Body Temperature, Cognito. Watch for the thermoregulatory centre in the brain and for what happens when we are too hot and too cold; compare it with Figure 1 and the table.
Tip
Memory hook: "negative negates". Negative feedback negates (cancels) the change; positive feedback adds to it.
Practice: homeostasis and temperature
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2, the tables, the worked examples (Worked example 1 uses an illustrative sweat loss) and the practice questions.
