Excretion and Homeostasis
Senior 5 · Biology
Excretion and Homeostasis
Lesson 3 of 4
25 min
Before sunrise in the highlands near the Virunga volcanoes, a farmer walking to her field shivers and her fingers go pale. Later in the week, harvesting beans under the afternoon sun in the lowlands of Bugesera, she sweats and her face flushes. In both places her core body temperature stays within a fraction of a degree of 37 °C. Enzymes work best within a narrow range of temperature, and the brain is especially sensitive, so mammals and birds control their temperature closely. This lesson explains how.
Definition
Endotherm: an animal that keeps its body temperature fairly constant mainly by heat from its own metabolism, such as mammals and birds.
Ectotherm: an animal whose body temperature depends mainly on heat from its surroundings, such as reptiles, amphibians, fish and insects; it controls its temperature mainly by behaviour.
Thermoregulation: the control of body temperature.
Vasodilation: the widening of arterioles when the muscle in their walls relaxes. Vasoconstriction: their narrowing when the muscle contracts.
Hypothalamus: the part of the brain, below the cerebrum, that contains the thermoregulatory centre.
Heat is gained from respiration in the body, especially in the liver and muscles, and from the surroundings. It is exchanged with the surroundings in four ways:
An ectotherm's body temperature follows its surroundings unless it moves (Figure 5). Lizards bask on sunny rocks in the morning, turn their bodies broadside to the sun to absorb more radiation, press against warm surfaces, and retreat to shade or burrows at midday. Ectotherms need far less food than endotherms of the same size, because they do not burn food to stay warm, but they are sluggish when cold.
The thermoregulatory centre in the hypothalamus receives information from two sets of receptors:
It then sends impulses along nerves, and causes hormones to be released, to bring about the responses in the table: a negative feedback loop.
| Effector | When too hot | When too cold |
|---|---|---|
| Arterioles in the skin | Vasodilation: more blood flows through surface capillaries, so more heat is lost by radiation | Vasoconstriction: blood bypasses the surface through shunt vessels, so less heat is lost |
| Sweat glands | Secrete more sweat, which evaporates and cools the skin | Secrete little sweat |
| Hair erector muscles | Relax: hairs lie flat | Contract: hairs stand up and trap a layer of insulating air (in humans, only "goose pimples") |
| Skeletal muscles | Less activity | Shivering: rapid involuntary contractions release heat from respiration |
| Metabolic rate | Lowered | Raised by adrenaline and, over weeks, by thyroxine |
| Behaviour | Seek shade, wear light clothing, reduce activity | Seek warmth, curl up, wear more clothing |
Note: in vasodilation it is the arterioles that widen; the capillaries do not move up or down in the skin.
In an infection, chemicals called pyrogens reset the hypothalamus to a higher set point, so a person feels cold and shivers while their temperature rises: a fever. If the body cannot lose heat fast enough, as in hard work in hot, humid weather, the core temperature can rise dangerously: heat stroke. In cold, wet, windy conditions, it can fall dangerously: hypothermia.
Worked example 1: how much heat does sweating remove?
Evaporating water from the skin absorbs about per gram. During an hour's hard running, an athlete loses 0.80 kg of sweat by evaporation. Calculate the heat removed, and compare it with a resting heat output of about 360 kJ per hour.
Note: sweat that drips off without evaporating removes almost no heat. This is why humid weather, which slows evaporation, makes heat stroke more likely.
Worked example 2: why newborn babies chill quickly
Illustrative values: a newborn baby has a mass of 3.5 kg and a surface area of about ; an adult has a mass of 70 kg and a surface area of about . Compare their surface area per kilogram.
Meaning: each kilogram of a baby has about 2.4 times as much skin to lose heat through, and a baby cannot shiver well. Babies make heat in brown fat, a tissue rich in mitochondria that releases energy as heat instead of making ATP. They must still be kept warm: the World Health Organization recommends kangaroo mother care, continuous skin-to-skin contact with the mother, for small and preterm babies (WHO recommendations for care of the preterm or low-birth-weight infant, 2022).
Warning
Common mistakes and safety
FuseSchool: "Temperature Regulation Of The Human Body". Watch for the role of the hypothalamus and for the responses to heat and cold, including vasodilation and vasoconstriction; compare them with the table and Figure 4.
Tip
Exam technique: in vasodilation answers, examiners look for three linked ideas: "arterioles in the skin dilate → more blood flows through the capillaries near the surface → more heat lost by radiation." Never write that blood vessels "move".
Practice: regulation of body temperature
Key points
Sources and credits
Figures 4 and 5, the table, the worked examples and the practice questions are by Ganzaa. The values in Figure 5 and both worked examples are illustrative.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
