Chapter 6 of 8

All 8 chapters
  1. 1The nervous system, readunderstood
  2. 2Reflexes, readunderstood
  3. 3The eye, readunderstood
  4. 4Hormones, readunderstood
  5. 5Blood sugar and diabetes, readunderstood
  6. 6Temperature and water balance, readunderstood
  7. 7Hormones and the menstrual cycle, readunderstood
  8. 8Plant responses, readunderstood

S4.5.6 Chapter 6

Temperature and water balance

You sweat when it is hot, shiver when it is cold, and pass dark urine when you have drunk little. On a hot day, which of these cools your body?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how sweating and shivering keep body temperature steady
  • Explain how the kidneys control the water in the body

The inside of your body stays at about 37 °C, whether the air is hot or cold. A healthy body varies by a degree or so between people and through the day. This steady temperature matters because the enzymes in your cells work best near it. Much hotter or colder, they work badly.

A part of the brain called the hypothalamus checks the temperature of the blood that passes through it. Receptors in the skin also send impulses to it. If the temperature drifts, the brain sends impulses to make the body respond. This is negative feedback, as in the control of blood glucose.

Draw it first

When the body is too hot

  1. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled body temperature rises, receptors sense the rise and the brain sends impulses. Then they are labelled the brain sends impulses, sweat glands release sweat and skin vessels widen. Last they are labelled sweat evaporates, heat leaves the body and temperature falls.body temperature risesreceptors sense the risethe brain sends impulses

    1 The body gets too warm. Receptors sense the rise, and the brain sends impulses to the skin.

  2. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled body temperature rises, receptors sense the rise and the brain sends impulses. Then they are labelled the brain sends impulses, sweat glands release sweat and skin vessels widen. Last they are labelled sweat evaporates, heat leaves the body and temperature falls.the brain sends impulsessweat glands release sweatskin vessels widen

    2 The sweat glands release sweat. The blood vessels near the skin widen.

  3. Three pictures of three boxes joined by arrows, one after another. First the boxes are labelled body temperature rises, receptors sense the rise and the brain sends impulses. Then they are labelled the brain sends impulses, sweat glands release sweat and skin vessels widen. Last they are labelled sweat evaporates, heat leaves the body and temperature falls.sweat evaporatesheat leaves the bodytemperature falls

    3 The sweat evaporates and takes heat from the skin. The body loses heat and its temperature falls.

Look at a cut through the skin. It has a sweat gland, a hair with a small muscle, and blood vessels near the surface. When you are hot, the sweat gland releases sweat through a pore onto the skin. The vessels near the skin widen, so more warm blood flows close to the surface and more heat leaves. The hair lies flat.

A cut through the skin when hot. The labels are hair, hair muscle, sweat pore, sweat gland, blood vessels, sweat and when hot. The blood vessels are wide, sweat lies on the surface and the hair lies flat. A long deeper blood vessel runs along the bottom, with a dotted layer of fat under the skin.when hothairhairmusclesweatsweatporebloodvesselssweatgland
The skin when hot. The vessels are wide, sweat lies on the surface and the hair lies flat. The long tube at the bottom is a deeper blood vessel, and the dotted layer is fat.

Why does sweat cool you? Sweat is mostly water. Evaporation is when a liquid turns into a gas at its surface. The particles with most energy escape first, and the ones that are left are cooler. So the skin loses heat. Sweat that only drips off does not cool you. Sweat that evaporates does.

Evaporation takes a lot of heat. About 2.4 kJ is needed to evaporate 1 g of water. Look at the table, then work out the heat taken by a larger amount of sweat.

A table with the columns Sweat and Heat. 1 g of sweat takes 2.4 kJ. 10 g takes 24 kJ.SweatHeat1 g2.4 kJ10 g24 kJ
The heat taken when sweat evaporates.

Worked example

On a hot day, a person sweats 500 g in an hour and all of it evaporates. How much heat does it take from the body?

  1. Each gram that evaporates takes 2.4 kJ.
  2. 500 × 2.4 = 1 200 kJ.

Answer: 1 200 kJ

When the air is very damp, sweat evaporates slowly, so it cools you less. That is why a hot, damp day feels worse than a hot, dry one. A person who is very hot, confused and no longer sweating needs urgent help from a health worker.

In the cold, the body does the opposite. The vessels near the skin narrow, so less blood flows close to the surface and less heat is lost. The sweat glands stop. The hair stands up and traps a layer of air. In people this does little, and in furry animals it helps a lot.

A cut through the skin when cold. The labels are hair, hair muscle, sweat pore, sweat gland, blood vessels and when cold. The blood vessels are narrow, there is no sweat and the hair stands up. A long deeper blood vessel runs along the bottom, with a dotted layer of fat under the skin.when coldhairhairmusclesweatporebloodvesselssweatgland
The skin when cold. The vessels are narrow, there is no sweat and the hair stands up. The long tube at the bottom is a deeper blood vessel, and the dotted layer is fat.

Shivering is rapid shaking of the muscles. The muscles contract and relax again and again. Their cells use more energy, and respiration in them releases heat. This warms the blood. You also put on warm clothes and move to shelter.

Four boxes joined by arrows that point down. The boxes are labelled body temperature falls, the brain sends impulses, muscles shiver, skin vessels narrow and temperature rises.body temperature fallsthe brain sends impulsesmuscles shiver, skin vesselsnarrowtemperature rises
The body in the cold. As in the hot loop, the last step undoes the first.

Now water. You lose water every day in sweat, in breath and in urine, and you gain it in drink and in food. The body must keep the amount of water steady. The organs that do this are the kidneys. You have two, at the back of the trunk. A tube from each, the ureter, takes urine to the bladder. It leaves the body through the urethra.

The lower trunk from the front, drawn with only the urinary system. The labels are kidneys, ureters, bladder and urethra. A ureter runs from each kidney down to the bladder, and the urethra leaves the bladder.kidneysuretersbladderurethra
The kidneys, ureters, bladder and urethra, from the front, with the other organs left out. The kidneys lie against the back wall of the trunk, partly under the lowest ribs.

Each kidney holds about a million tiny filters called nephrons. Blood comes to the filter. Water, salts, glucose and urea pass out of the blood. Urea is a waste made in the liver from the breakdown of protein. Blood cells and big molecules stay in the blood.

The liquid then runs along a tubule. Here the body takes back what it needs. All the glucose goes back into the blood, and so does most of the water. The rest, with the urea and the extra salts and water, goes down the collecting duct and out as urine.

One nephron, simplified. The labels are blood in, filter, taken back into blood, blood capillary, tubule, collecting duct and urine out. Blood comes in to the filter. Liquid runs along the tubule, where useful things are taken back into the blood in the capillary. What is left goes down the collecting duct and out as urine.blood infiltertakenback intobloodbloodcapillarytubulecollectingducturine out
One nephron. A real nephron has more parts. The loop in the tubule is drawn but not named.

The numbers are large. In a day, the kidneys filter about 180 L of liquid out of the blood. A person makes only about 1.5 L of urine. The rest goes back into the blood.

A table with the columns In a day and Litres. Filtered: 180 L. Urine: 1.5 L.In a dayLitresfiltered180 Lurine1.5 L
Typical amounts for one day. They differ between people.

Worked example

The kidneys filter 180 L in a day and a person makes 1.5 L of urine. What percentage of the filtered water goes back into the blood, to the nearest whole number?

  1. The water taken back is 180 − 1.5 = 178.5 L.
  2. As a fraction of the water filtered, 178.5 ÷ 180 = 0.99.
  3. As a percentage, 0.99 × 100 = 99%.

Answer: 99%

How does the body control how much water leaves? The brain checks the water in the blood. If there is too little, the pituitary gland releases more ADH into the blood. ADH makes the kidney tubules take back more water. Less water is left, so the person makes less urine, and it is more concentrated.

Four boxes joined by arrows that point down. The boxes are labelled too little water in the blood, pituitary releases more ADH, kidneys take back more water and less urine, more concentrated.too little water in the bloodpituitary releases more ADHkidneys take back more waterless urine, more concentrated
When the body is short of water. When there is plenty of water, the pituitary releases less ADH. The kidneys take back less water, so more urine, and a paler one, leaves the body.

You can see the effect. When you drink little, or sweat a lot, your urine is small in amount and dark. When you drink a lot, it is large in amount and pale. Dark urine can mean that you need more water. In untreated diabetes, glucose can also appear in the urine, because the tubule cannot take back so much.

A table with the columns Drink, ADH and Urine. Drink little: more ADH, small, dark. Drink a lot: less ADH, large, pale.DrinkADHUrinelittlemoresmall, darka lotlesslarge, pale
The control of water in one table.

Try it

Why sweat cools the skin

You need: a bowl of water at room temperature, a towel, a clock or a way of counting a minute.

Be safe: Use water at room temperature only. Never use hot water.

  1. Dip the fingers and back of one hand in the water. Shake off the drops. Keep the other hand dry.
  2. Hold both hands up in the air for one minute. Do not touch anything with them.
  3. Compare how the two hands feel. Does one feel cooler?
  4. Now blow gently on the wet hand, then on the dry hand. Compare again. Dry your hand on the towel.

What you will see: The wet hand feels cooler. When you blow on it, it feels cooler still, because moving air carries the water vapour away and the water evaporates faster. This is how sweat cools your skin. In very damp air the water evaporates slowly, so the cooling is smaller.

If you cannot try it, this is what happens.
A table with the columns Hand and Feels. Wet hand: cooler. Dry hand: same.HandFeelswetcoolerdrysame

A wrong idea many people have

When you are hot, the blood vessels move closer to the skin to lose heat.

Why it feels right: Textbook pictures often show the vessels near the skin, and the skin looks redder when we are hot.

The test that breaks it: Blood vessels are fixed in place, and they cannot move. What changes is their width. When you are hot, the vessels near the skin widen, and more blood flows through them.

What is true: The vessels widen when you are hot and narrow when you are cold. They do not move. More blood near the surface means more heat leaves.

Key words

evaporation
a liquid turning into a gas at its surface, which takes heat from what is left.
shivering
rapid shaking of the muscles that releases heat.
kidneys
two organs that filter the blood and make urine.
nephrons
the tiny filters in the kidneys.
urea
a waste made in the liver from the breakdown of protein, which leaves in urine.
ADH
a hormone from the pituitary gland that makes the kidneys take back more water.

Back to your guess

The answer is C: Sweat evaporates from the skin and takes heat away from the body.

Sweat takes heat from the skin when it evaporates. The blood vessels near the skin also widen, so that more heat can leave. Narrow vessels and raised hairs happen in the cold.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Why does the body keep its temperature steady, near the same value all the time?

    Tap your answer.

    1. Not this one. The body makes heat all the time, in respiration. What changes is how much heat it keeps or loses. Try another.

    2. Yes, that is right. Enzymes speed up the reactions in cells. They work best near the body temperature. Much hotter or colder, they work badly.

    3. Not this one. Blood flows at many temperatures. It is the enzymes in the cells that work best in a small range. Go to S4.1.5. Try another.

    See the answer

    The enzymes in the cells work best near that temperature.

    Enzymes speed up the reactions in cells. They work best near the body temperature. Much hotter or colder, they work badly.

    • If you chose A: The body makes heat all the time, in respiration. What changes is how much heat it keeps or loses.
    • If you chose C: Blood flows at many temperatures. It is the enzymes in the cells that work best in a small range. Go to S4.1.5.

  2. Lucía is hot after running. Which describes what happens in her skin?

    Tap your answer.

    1. Not this one. That is what happens in the cold. When you are hot, the vessels widen. Try another.

    2. Not this one. Shivering happens when you are cold. A hot body sweats. Try another.

    3. Not this one. Vessels cannot move, and sweating does not stop when you are hot. The vessels widen and the sweat glands release sweat. Try another.

    4. Yes, that is right. Wide vessels bring more warm blood near the surface, so more heat leaves. Sweat evaporates and takes more heat with it.

    See the answer

    The blood vessels near the skin widen and the sweat glands release sweat.

    Wide vessels bring more warm blood near the surface, so more heat leaves. Sweat evaporates and takes more heat with it.

    • If you chose A: That is what happens in the cold. When you are hot, the vessels widen.
    • If you chose B: Shivering happens when you are cold. A hot body sweats.
    • If you chose C: Vessels cannot move, and sweating does not stop when you are hot. The vessels widen and the sweat glands release sweat.

  3. Why does a person shiver in the cold?

    Tap your answer.

    1. Not this one. There is no sweat in the cold. The sweat glands stop. Try another.

    2. Not this one. In the cold the skin vessels narrow, and shivering does not widen them. Try another.

    3. Yes, that is right. Shivering is rapid contracting and relaxing of the muscles. Respiration in the muscle cells releases heat, which warms the blood.

    See the answer

    The muscles contract quickly and release heat.

    Shivering is rapid contracting and relaxing of the muscles. Respiration in the muscle cells releases heat, which warms the blood.

    • If you chose A: There is no sweat in the cold. The sweat glands stop.
    • If you chose B: In the cold the skin vessels narrow, and shivering does not widen them.

  4. How does sweat cool the body?

    Tap your answer.

    1. Yes, that is right. When sweat evaporates, the particles with most energy leave the skin. This takes heat from it, and the skin cools.

    2. Not this one. Sweat is released at about body temperature. It cools because it evaporates. Try another.

    3. Not this one. Sweat does not block heat. It cools the skin as it evaporates. Try another.

    See the answer

    It evaporates and takes heat from the skin.

    When sweat evaporates, the particles with most energy leave the skin. This takes heat from it, and the skin cools.

    • If you chose B: Sweat is released at about body temperature. It cools because it evaporates.
    • If you chose C: Sweat does not block heat. It cools the skin as it evaporates.

  5. What do the kidneys do with the blood that reaches them?

    Tap your answer.

    1. Not this one. The kidneys do not store blood. The blood flows through them and returns. Try another.

    2. Not this one. Most of the filtered water goes back into the blood. Only a small part leaves as urine. Try another.

    3. Not this one. It is the other way round. Urea is a waste and leaves in the urine. The water the body needs is taken back. Try another.

    4. Yes, that is right. The nephron filters the blood. The tubule takes back all the glucose and as much water and salt as the body needs. What is left leaves as urine.

    See the answer

    They filter it, take back what the body needs and pass the rest out as urine.

    The nephron filters the blood. The tubule takes back all the glucose and as much water and salt as the body needs. What is left leaves as urine.

    • If you chose A: The kidneys do not store blood. The blood flows through them and returns.
    • If you chose B: Most of the filtered water goes back into the blood. Only a small part leaves as urine.
    • If you chose C: It is the other way round. Urea is a waste and leaves in the urine. The water the body needs is taken back.

  6. Kofi sweats a lot on a hot day and drinks little. What happens to the ADH in his blood and to his urine?

    Tap your answer.

    1. Not this one. That happens when you drink a lot. When water is short, there is more ADH, not less. Try another.

    2. Yes, that is right. Too little water in the blood makes the pituitary release more ADH. The kidneys take back more water. Less water is left, so the urine is small in amount and concentrated.

    3. Not this one. More ADH makes the kidney take back more water, so there is less urine, not more. Try another.

    See the answer

    There is more ADH, and his urine is small in amount and dark.

    Too little water in the blood makes the pituitary release more ADH. The kidneys take back more water. Less water is left, so the urine is small in amount and concentrated.

    • If you chose A: That happens when you drink a lot. When water is short, there is more ADH, not less.
    • If you chose C: More ADH makes the kidney take back more water, so there is less urine, not more.

  7. From an earlier chapter

    Which gland releases ADH into the blood?

    Tap your answer.

    1. Not this one. The adrenal glands release adrenaline. Go to S4.5.4. Try another.

    2. Yes, that is right. The pituitary gland under the brain releases ADH. It makes the kidneys take back more water.

    3. Not this one. The pancreas releases insulin and glucagon. Go to S4.5.4. Try another.

    See the answer

    The pituitary gland

    The pituitary gland under the brain releases ADH. It makes the kidneys take back more water.

    • If you chose A: The adrenal glands release adrenaline. Go to S4.5.4.
    • If you chose C: The pancreas releases insulin and glucagon. Go to S4.5.4.

  8. A person sweats 300 g and all of it evaporates. Each gram takes 2.4 kJ of heat from the body. How much heat is taken, in kJ?

    See the answer

    720 kJ

    300 × 2.4 = 720 kJ.

  9. In a day the kidneys filter 170 L of liquid and a person makes 1.5 L of urine. How many litres go back into the blood?

    See the answer

    168.5 L

    170 − 1.5 = 168.5 L go back into the blood.

Keep it

Copy this into your exercise book.

Hot: sweat evaporates, skin vessels widen. Cold: vessels narrow, shivering. Kidneys filter blood, take back what is needed. ADH from the pituitary makes them take back more water.

Draw: A nephron with blood in, the filter, the tubule, water taken back and urine out.

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