GCSE · Biology · Edexcel · Spec 1BI0

Skin and thermoregulation

Goosebumps when you're cold. Sweat when you're hot. Two unrelated skin tricks? No: one set of structures, flipped between two settings to hold you near 37 °C.

Biology · Skin

Step down through your skin

A simplified section through skin, top to bottom. The layers are drawn equal in height, so this shows their order, not their real thickness.

↑ Outside of the body

↓ Underneath the skin

Zone 1 of 3

Epidermis

Start at the surface. The epidermis is the top layer of skin cells. Hairs protrude up through it, and every hair is connected to a tiny erector pili muscle. Remember that name: it matters in a minute.

Top layer of skin cells
Hairs protrude through it

Top to bottom: epidermis, then dermis, then fatty tissue.

Watch out: The hairs poke up through the epidermis, but the muscles attached to them and the receptors that sense temperature sit further down, in the dermis.

Biology · Why it matters

How tight is the range?

Your body holds its internal temperature close to 37 °C. How far either side of 37 °C does it let that drift?

Your estimate

5°C

Your body holds its internal temperature close to 37 °C. How far either side of 37 °C does it let that drift? Estimate between 0°C and 10°C.

0°C10°C

One structure, two settings

Body temperature fallsvsBody temperature rises

Read each row left to right: same structure, opposite setting.

Focus

Erector pili muscles

Body temperature falls

Contract

Body temperature rises

Relax

The insight

Same muscles, opposite jobs: one pulls, one lets go.

Hairs

Body temperature falls

Stand upright

Body temperature rises

Lie flat

Arterioles

Body temperature falls

Vasoconstrict: the muscles in the arteriole wall contract and the arteriole narrows

Body temperature rises

Vasodilate: the arteriole widens

Blood flow to the skin surface

Body temperature falls

Reduced

Body temperature rises

Increased

Sweat glands

Body temperature falls

Sweating is the response to a rise in temperature, so look right

Body temperature rises

Secrete sweat, and heat is lost as the sweat evaporates

Heat lost from the skin

Body temperature falls

Less

Body temperature rises

Hairs and heat

Reason it through

Why do your hairs stand up when you get cold?

Link 1 of 4

First link · your turn

Body temperature falls. What do the erector pili muscles do?

Locked — reveal the link above first

Biology · Think it through

Which idea sounds like yours?

It's a hot day and your skin is losing heat fast. Four people explain how.

Which one is closest to what you think right now?

When you get hot, blood vessels move up closer to the skin surface so the heat can escape.When you get cold, the arterioles widen so extra warm blood can reach the skin and warm you up.Sweat cools you down because the sweat radiates heat away from your skin.The arterioles get wider or narrower to change blood flow near the surface, and sweat takes heat away as it evaporates.

How sure are you?

I'm sureI think soI'm guessing

When you get hot, blood vessels move up closer to the skin surface so the heat can escape.

This is a common misconception

It's an easy picture to hold, but the vessels stay put. What changes is the width of the arterioles: they widen (vasodilation), so more blood flows near the skin surface and more heat is lost by radiation.

When you get cold, the arterioles widen so extra warm blood can reach the skin and warm you up.

This is a common misconception

Blood does carry heat, and that's exactly the trap. More blood near the surface means more heat lost by radiation. In the cold the arterioles vasoconstrict, narrowing so less blood flows to the surface.

Sweat cools you down because the sweat radiates heat away from your skin.

This is a common misconception

Radiation is about blood flow near the skin surface. Sweat works another way: when sweat evaporates it takes heat with it, so sweating increases heat loss via evaporation.

The arterioles get wider or narrower to change blood flow near the surface, and sweat takes heat away as it evaporates.

This matches the evidence

That's it. Two separate ways of losing heat: radiation, controlled by how much blood flows near the surface, and evaporation, which depends on sweat.

Arterioles change width, not position. Radiation goes with blood flow near the surface, and sweat cools by evaporation.

Predict, then check

Commit to one before you look.

On a very hot day, a hiker has lost a lot of water through sweating and hasn't drunk anything since. Her body temperature is still high. What do you predict her sweat glands do?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

Your body keeps its internal temperature at 37 °C, within a very small range either side.
That's because cells, enzymes and cellular processes work most efficiently in this narrow range.
Temperature receptors in the skin detect changes in the external temperature, which can push your internal temperature around.
Skin has three layers: the epidermis on top, the dermis below it, then fatty tissue that insulates.
Have a goMaya is certain the fatty tissue is the top layer of skin because 'fat sits on the outside'. Which layer is actually on top, and where does the fatty tissue sit?
The epidermis is on top. The fatty tissue is the bottom layer, underneath the dermis.
The order is epidermis, dermis, fatty tissue, and the fatty tissue sits at the bottom to insulate.
Hairs poke up through the epidermis and connect to erector pili muscles; the dermis holds the receptors and sweat glands.
Have a goThree things: a hair, a temperature receptor, a sweat gland. Two live in the dermis. Which one pokes up through the epidermis?
The hair. The receptors and the sweat glands are in the dermis.
Hairs protrude through the epidermis, while the dermis is where the receptors and sweat glands are found.
The skin also has arterioles (small arteries) and venules (small veins).
When body temperature falls, erector pili muscles contract, hairs stand upright and trap insulating air, so less heat is lost.
When body temperature falls, arterioles vasoconstrict (narrow), so less blood reaches the skin surface and less heat is lost by radiation.
When body temperature rises, erector pili muscles relax and hairs lie flat, so no air layer is trapped and heat is lost faster.
Have a goYou notice someone's arm hairs are lying flat, with no air layer trapped. Is their body temperature rising or falling, and is heat leaving faster or slower?
Rising, and heat is leaving faster.
Flat hairs can't trap insulating air, so heat is lost faster. Upright hairs go with a falling temperature.
When body temperature rises, arterioles vasodilate (widen), giving more heat loss by radiation, and sweat evaporating takes heat away.

The big picture

Your skin keeps your internal temperature close to 37 °C by changing three things: the hairs, the arterioles and the sweat glands. When you get colder, they cut heat loss. When you get hotter, they increase it.

Key points

1Internal body temperature is held at 37 °C, plus or minus half a degree, because cells, enzymes and cellular processes work most efficiently in that narrow range.

2Skin layers from the top: epidermis, dermis, fatty tissue (which insulates).

3Erector pili muscles, arterioles and sweat glands change heat loss, and each does the opposite when temperature rises instead of falls.

4Vasoconstriction is the narrowing of blood vessels; vasodilation is the widening. The vessels widen or narrow, they don't move.

5Sweat glands secrete sweat when temperature rises, and heat is lost by evaporation as the sweat evaporates.

6A person who has lost a lot of water and not replaced it may stop sweating, because they have no more water to lose to produce sweat.

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Worked example

Problem

Priya is jogging on a hot afternoon and her body temperature is rising. Talk through what her skin does, structure by structure, to help her lose heat.

⚠ Watch out

Saying blood vessels move closer to or away from the skin surface, or that sweat cools you by radiating heat. Arterioles widen or narrow, and sweat cools by evaporation.

🧠

Memory hook

Cold means keep it in: hairs up, arterioles narrow. Hot means let it out: hairs flat, arterioles wide, sweat on.

✓

Check yourself

Cover the page and sketch three stacked boxes for the layers of skin. Then place these in the right box: hairs poking through, temperature receptors, sweat glands. Which box holds the insulation?

Flashcards

(14)

What internal body temperature does the body maintain?

37 °C, within a very small range of plus or minus half a degree.

Why must internal body temperature stay in such a narrow range?

Cells, enzymes and cellular processes work most efficiently within it.

Epidermis

The top layer of skin cells. Hairs protrude through it.

Dermis

The middle layers of skin, below the epidermis. It contains the temperature receptors and sweat glands.

What does the layer of fatty tissue under the dermis do?

It provides insulation.

What are the hairs connected to?

Erector pili muscles.

What do the temperature receptors in the skin detect?

Changes in the external temperature.

Arteriole versus venule

An arteriole is a small artery. A venule is a small vein.

Body temperature falls: what happens to the hairs, and why does it help?

Erector pili muscles contract and hairs stand upright, trapping an insulating layer of air against the skin, so less heat is lost.

Vasoconstriction

The narrowing of blood vessels, reducing blood flow through them. When body temperature falls, it cuts blood flow to the skin surface and so reduces heat loss by radiation.

Vasodilation

The widening of blood vessels, increasing blood flow through them. When body temperature rises, more blood flows to the skin surface and more heat is lost by radiation.

Why do flat hairs mean faster heat loss?

Relaxed erector pili muscles let hairs lie flat, so they cannot trap an insulating layer of air.

How does sweating increase heat loss?

Sweat glands secrete sweat, and when the sweat evaporates it takes heat with it: heat loss by evaporation.

Why might a person who has lost a lot of water stop sweating?

They have no more water to lose to produce sweat.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Step down through your skin

How tight is the range?

Which idea sounds like yours?

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