What you'll learn
- Why your body needs a stable internal environment.
- How temperature affects enzyme-controlled reactions.
- How water balance affects animal cells.
- How the skin and hypothalamus help control body temperature.
The detailed osmoregulation and skin thermoregulation points here are Biology-only content in Edexcel 1BI0. The blood-vessel responses, vasodilation and vasoconstriction, are Higher Tier detail.
Why homeostasis matters
Your cells work best when the conditions around them stay within a narrow range. These conditions include body temperature, water concentration, ion concentration and other factors in the blood and tissue fluid.
A change can come from outside the body, such as moving into a hot room, or from inside the body, such as muscles producing extra heat during exercise.
Homeostasis
Homeostasis is the maintenance of a constant internal environment in response to internal and external change.
“Constant” does not mean perfectly unchanging. It means kept close to a normal level, so cells can keep working properly.
The point of homeostasis
Homeostasis keeps conditions suitable for enzyme-controlled reactions and prevents cells being damaged by water moving in or out too quickly.
Enzymes and temperature
An enzyme is a biological catalyst: it speeds up a chemical reaction in living organisms without being used up. The active site is the part of the enzyme where the substrate, the substance being acted on, fits.
Human enzymes usually work best near normal body temperature, about 37 °C.
If body temperature is too low, particles move more slowly. This means fewer successful collisions between enzymes and substrates, so reactions happen more slowly.
If body temperature is too high, the enzyme’s active site can change shape. This is called denaturation. The substrate no longer fits properly, so the enzyme cannot catalyse the reaction effectively.
Predicting enzyme activity when temperature changes
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At 20 °C, a human enzyme is below its usual optimum temperature, so enzyme and substrate particles move more slowly and collide successfully less often.
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Near 37 °C, the enzyme is close to its optimum, so the reaction rate is likely to be high because the active site shape is suitable and collisions are frequent.
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At a much higher temperature, such as 45 °C, some enzymes may denature, so the active site changes shape and the reaction rate falls.
Denatured does not mean killed
Enzymes are proteins, not living organisms. Saying an enzyme is “killed” is not precise; say its active site changes shape so the substrate no longer fits.
Water balance and animal cells
Osmosis is the movement of water through a partially permeable membrane from a more dilute solution to a more concentrated solution. A partially permeable membrane lets some substances pass through but not others.
Osmoregulation is the control of water concentration in body fluids, such as blood and tissue fluid.
Animal cells are surrounded by a cell membrane but do not have a cell wall. This means they can be damaged if too much water enters or leaves.
- If the fluid around an animal cell is too dilute, water enters by osmosis. The cell swells and may burst.
- If the fluid around an animal cell is too concentrated, water leaves by osmosis. The cell shrinks and may stop working properly.
Predicting water movement in animal cells
A red blood cell is placed in a solution that is more concentrated than its cytoplasm.
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The outside solution contains more dissolved substances, so it has a lower water concentration than the cell cytoplasm.
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Water moves out of the red blood cell by osmosis through the partially permeable cell membrane.
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The red blood cell loses volume and shrinks, which can stop it carrying out its function properly.
Thermoregulation
Thermoregulation is the control of body temperature. In humans, the body aims to stay close to about 37 °C because this is suitable for many enzyme-controlled reactions.
A set point is the normal target level that the body tries to maintain. For body temperature, the set point is roughly 37 °C.
Negative feedback
Negative feedback is a control mechanism where a change is detected and responses act to reverse that change, bringing the condition back towards its normal level.
Thermoregulation uses a control pathway:
- A stimulus is detected, such as body temperature rising above normal.
- Receptors detect the change. Receptors are cells or organs that detect stimuli.
- The coordination centre processes the information. In thermoregulation, this is mainly the hypothalamus, a region of the brain.
- Effectors carry out the response. Effectors are muscles or glands that produce a change.
- The response reverses the original change, returning temperature towards normal.
The diagram below shows thermoregulation as a negative feedback loop, including what happens when you are too hot or too cold.
Tracing a temperature-control response
During exercise in a warm room, your body temperature starts to rise.
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Muscle cells respire faster during exercise, releasing more heat, so body temperature rises above the set point.
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Temperature receptors in the skin and hypothalamus detect the increase, and the hypothalamus coordinates a response.
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Sweat glands produce more sweat and skin blood vessels undergo vasodilation, increasing heat loss so body temperature moves back towards normal.
The skin in thermoregulation
The skin is an organ involved in temperature control. Its two important layers for this topic are the epidermis and the dermis.
The epidermis is the outer layer of the skin. It acts as a protective surface and is where sweat reaches the outside through pores.
The dermis is the layer beneath the epidermis. It contains many structures involved in thermoregulation, including sweat glands, blood vessels, temperature receptors, hair follicles and hair erector muscles.
The diagram below shows the main skin structures you need to link to their functions in temperature control.
Layer check
Most active thermoregulation structures are in the dermis. The epidermis is mainly the outer protective surface where sweat evaporates.
When you are too hot
When body temperature rises above normal, the body increases heat loss.
Sweating
Sweat glands produce sweat, which reaches the skin surface through sweat ducts and pores. When sweat evaporates, it transfers energy from the skin to the surroundings. This cools the body.
Sweat must evaporate
Sweating cools you mainly when the sweat evaporates. If sweat just drips off, less energy is removed from the skin.
Vasodilation
In vasodilation, arterioles supplying capillaries near the skin surface widen. More blood flows close to the surface, so more heat is transferred from the blood to the surroundings.
This helps cool the body and return temperature towards the set point.
When you are too cold
When body temperature falls below normal, the body reduces heat loss and can generate extra heat.
Shivering
Shivering is rapid contraction and relaxation of skeletal muscles. Muscle contraction needs respiration, and respiration releases energy. Some of this energy is transferred as heat, helping warm the body.
Vasoconstriction
In vasoconstriction, arterioles supplying capillaries near the skin surface narrow. Less blood flows close to the surface, so less heat is lost to the surroundings.
Blood vessels do not move deeper
In vasoconstriction, the blood vessels do not physically move away from the skin surface. The amount of blood flowing through surface capillaries is reduced.
Hair erector muscles
Hair erector muscles contract when you are cold, causing hairs to stand up. In many mammals this traps an insulating layer of air. In humans, this response is less effective because we have relatively little body hair, but it can still cause goosebumps.
The hypothalamus
The hypothalamus is the main coordination centre for thermoregulation. It receives information from temperature receptors in the skin and also monitors the temperature of the blood flowing through the brain.
If the blood is too warm or too cold, the hypothalamus sends nerve impulses to effectors such as sweat glands, skeletal muscles and blood vessels in the skin.
The exam chain
For thermoregulation explanations, build the chain: temperature change, receptors detect it, hypothalamus coordinates, effectors respond, temperature returns towards normal.
In the exam
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Use the correct order: stimulus, receptor, coordination centre, effector, response.
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For enzyme questions, link temperature to reaction rate first, then explain denaturation only if the temperature is too high.
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For skin blood vessels, name the Higher Tier terms accurately: vasodilation increases heat loss; vasoconstriction reduces heat loss.
Check yourself
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Why can a very high body temperature reduce enzyme activity?
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What happens to an animal cell if the fluid around it is more concentrated than its cytoplasm?
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Describe how the body responds when body temperature falls below normal, including the role of the hypothalamus.
