Answer key · Unit 9

Body systems.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 5

Lesson 1 of 5 · About 25 minutes

Digestion

What happens to a meal: a stomach that empties slowly, the small intestine where food is absorbed, why fat stays longer, what bile does, and what reaches the large intestine.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
A tube through the body
Practice · Choose
Question
Three hours after a mixed meal, much of it has already been absorbed into the body. In which organ is most of the food absorbed?

Correct answer
- Correct The small intestine
Setup, choices and feedback
The Scene at this step
Meal
All three
Time since eating
3 h
Zoom to the gut lining
Whole gut
Choices
Shown in a random order.
- Correct The small intestine
- Wrong The stomach
  Misconception: Thinks most food is digested and absorbed in the stomach.
  Feedback from the catalogue
  The stomach is mostly a store and a mill: it holds the meal, grinds it to a soup and starts on the protein, but almost nothing crosses its wall. Digestion is finished, and the food absorbed, in the small intestine, whose lining is folded into villi with a surface of about thirty square metres.
- Wrong The large intestine
  Misconception: Thinks the large intestine is where the food in a meal is absorbed.
  Feedback from the catalogue
  By the time a meal reaches the large intestine, almost all the food that can be digested has already been absorbed in the small intestine. What arrives is mostly fibre and a few per cent of the food, and the large intestine’s main job is to take back water, so the remains become solid.
- Wrong The liver
  Feedback
  The liver receives the blood that drains the gut and stores sugar from it, but the food does not pass through the liver itself: it is absorbed through the wall of the gut.
Feedback for a right answer
In the small intestine. The dots shrink there, mostly in its first metre or two, because its lining is folded into villi with an enormous surface. In the stomach they only turn to a soup.

Hint
Look for where the dots of food shrink in the Scene.

Worked solution
1. In the stomach food is stored and ground, but almost nothing crosses the wall.
2. In the small intestine the juices of the pancreas and the bile finish the breakdown, and the villi take the small molecules into the blood and the lymph.
3. What reaches the large intestine is mostly fibre and water.
So most food is absorbed in the small intestine. If you chose the stomach, you mixed up where food stays longest with where it is absorbed.
Step 2 of 6
Open this step
The stomach empties slowly
Practice · Reach the target
Question
Meal is fixed at All three. Move Time since eating until half the meal is still in the stomach, and press Check.

Goal
- 0.5 ± 0.03
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
Meal
All threefixed
Time since eating
0 h
Zoom to the gut lining
Whole gut
Read from the Scene at
Share of the meal still in the stomach, 0..1/nodes/gut/outputs/stomach
One setting that reaches it
Time since eating
2 h
Feedback for a right answer
About two hours after eating. The stomach does not empty at once: first it grinds, then its outlet lets the soup on a few millilitres at a time.

Feedback for a wrong answer
Not yet. If more than half is still in the stomach, move the time later; if less, earlier.

Hint
The chart shows what share of the meal is in the stomach at the hour you chose.

Worked solution
1. Start at 0 h: the whole meal is in the stomach.
2. Move Time since eating and read on the chart how much of the meal is still in the stomach.
3. That share falls to a half at about 2 h.
That is the half-emptying time of a mixed meal. In the first hour the stomach lets out little, because it is still grinding the food.
Step 3 of 6
Open this step
Fat stays longer
Practice · Predict, then look
Question
Three hours after a meal of rice, almost all the rice has left the stomach. What will be in the stomach three hours after a meal of butter of the same size?

Correct answer
- Correct Much more: nearly half the butter still there
Setup, choices and feedback
The Scene at this step
Meal
Rice (starch)fixed
Time since eating
3 hfixed
Zoom to the gut lining
Whole gut
Choices
- Correct Much more: nearly half the butter still there
- Wrong About the same: the butter has almost all gone too
  Misconception: Thinks a fatty meal leaves the stomach as fast as a starchy one, or faster, because fat is slippery or light.
  Feedback from the catalogue
  Fat leaves the stomach more slowly than any other food. When it reaches the duodenum, the cells there release hormones that hold the stomach’s outlet back, so the small intestine has time to break it down. In the Scene half a meal of butter leaves the stomach only after about 2.8 hours, and half a meal of rice after about 1.3.
- Wrong Even less, because fat is slippery and slides through
  Misconception: Thinks a fatty meal leaves the stomach as fast as a starchy one, or faster, because fat is slippery or light.
  Feedback from the catalogue
  Fat leaves the stomach more slowly than any other food. When it reaches the duodenum, the cells there release hormones that hold the stomach’s outlet back, so the small intestine has time to break it down. In the Scene half a meal of butter leaves the stomach only after about 2.8 hours, and half a meal of rice after about 1.3.
Once the prediction is locked in, the Scene changes
Meal
Butter (fat)fixed
The Scene then shows: 0.4525 · Share of the meal still in the stomach, 0..1/nodes/gut/outputs/stomach
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Nearly half, 0.45 of the meal, against 0.03 of the rice. Fat reaching the duodenum releases hormones that slow the stomach’s outlet, so the small intestine has time to break it down.

Worked solution
1. A rice meal is half out of the stomach in about 1.3 h, so after 3 h almost nothing is left.
2. A fatty meal is half out only after about 2.8 h.
3. At 3 h only a little more than that half-emptying time has passed, so nearly half the butter is still in the stomach.
If you predicted that fat passes faster, you were thinking of how slippery it is. But the pace is set by the intestine, which controls the stomach’s outlet.
Step 4 of 6
Open this step
Timing a fatty meal
Practice · Calculate
Question
A meal of butter. Move Time since eating and find after how many minutes half the meal has left the stomach.

Correct answer
168 min ± 6 min
Setup, choices and feedback
The Scene at this step
Meal
Butter (fat)fixed
Time since eating
0 h
Zoom to the gut lining
Whole gut
Accepted within
± 6 min
Computed from the Scene at this step’s setting
Time for the stomach to let out half the meal/nodes/gut/outputs/halfEmptied
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 78 min ± 6 min
  Misconception: Thinks a fatty meal leaves the stomach as fast as a starchy one, or faster, because fat is slippery or light.
  Feedback from the catalogue
  Fat leaves the stomach more slowly than any other food. When it reaches the duodenum, the cells there release hormones that hold the stomach’s outlet back, so the small intestine has time to break it down. In the Scene half a meal of butter leaves the stomach only after about 2.8 hours, and half a meal of rice after about 1.3.
Feedback for a right answer
About 2.8 h, that is about 170 min: more than twice as long as a meal of rice.

Hint
Find on the chart the hour at which the share in the stomach falls to a half, and convert to minutes: an hour is 60 minutes.

Worked solution
1. Choose Butter (fat) and move Time since eating until half the meal is in the stomach.
2. That happens at about 2.8 h.
3. In minutes: $2.8 \times 60 = 168$ ⁠, about 170 min.
If you got about 78 min, that is the half-emptying time of rice. Fat leaves the stomach slowest of all.
Step 5 of 6
Open this step
On the lining
Practice · Choose
Question
The liver makes bile, and the gallbladder squeezes it into the duodenum when fat arrives. What does bile do to the fat?

Correct answer
- Correct Breaks large drops of fat into tiny droplets, so enzymes have more surface to work on
Setup, choices and feedback
The Scene at this step
Meal
Butter (fat)
Time since eating
4 h
Zoom to the gut lining
Gut lining
Choices
Shown in a random order.
- Correct Breaks large drops of fat into tiny droplets, so enzymes have more surface to work on
- Wrong Cuts the fat molecules into fatty acids and glycerol
  Misconception: Thinks bile is an enzyme that breaks fat molecules down chemically.
  Feedback from the catalogue
  Bile has no enzymes. Bile salts coat large drops of fat and break them into tiny droplets, as washing-up liquid does to greasy plates, so the fat has far more surface. The fat molecules themselves are cut into fatty acids and glycerol by lipase, an enzyme from the pancreas.
- Wrong Carries the fat straight into the blood
  Feedback
  The fat is rebuilt in the cells of the villus and leaves through the lacteal, not the blood. Bile itself stays in the gut, and most bile salts are taken back at its far end and return to the liver.
Feedback for a right answer
Right. Bile salts emulsify the fat: one large drop becomes thousands of tiny droplets, and lipase from the pancreas, the enzyme, cuts them up.

Hint
Think of washing-up liquid in a greasy pan.

Worked solution
1. Fat does not mix with water, and enzymes in water reach only the surface of a drop.
2. Bile coats the fat and breaks it into small droplets: the same fat, far more surface.
3. The chemical bonds are cut by lipase from the pancreas.
If you chose that bile cuts the molecules, you gave it an enzyme’s job. Bile has no enzymes.
Step 6 of 6
Open this step
The next morning
Practice · Calculate
Question
A meal of rice, the next morning. What percentage of the meal has reached the large intestine without being absorbed?

Correct answer
6.363 ± 0.6
Setup, choices and feedback
The Scene at this step
Meal
Rice (starch)fixed
Time since eating
12 hfixed
Zoom to the gut lining
Whole gut
Accepted within
± 0.6
Computed from the Scene at this step’s setting
Share of the meal that has reached the large intestine undigested, 0..1/nodes/gut/outputs/largeIntestine
Wrong answers it recognises
- 94 ± 1
  Misconception: Thinks the large intestine is where the food in a meal is absorbed.
  Feedback from the catalogue
  By the time a meal reaches the large intestine, almost all the food that can be digested has already been absorbed in the small intestine. What arrives is mostly fibre and a few per cent of the food, and the large intestine’s main job is to take back water, so the remains become solid.
Feedback for a right answer
About 6%. About 94% of the rice was absorbed in the small intestine, and only the remainder reaches the large intestine: starch that escaped digestion, plus fibre in real food.

Hint
The chart shows what share of the meal has been absorbed into the body. The rest is what reached the large intestine.

Worked solution
1. By the next morning nothing is left in the stomach or the small intestine.
2. The chart shows that about 94% of the meal was absorbed.
3. What is left has reached the large intestine: $100 - 94 = 6$ ⁠, about 6%.
If you answered 94%, you gave what was absorbed in the small intestine, not what reached the large intestine.

Lesson 2 of 5 · About 25 minutes

Breathing and gas exchange

How air gets into the lungs, how much of it really reaches the air sacs, why a deep breath beats a fast one, and why in thin air the blood carries less oxygen.

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The 7 steps and their answers

Step 1 of 7
Open this step
A breath in
Practice · Choose
Question
With every breath in, air flows into the lungs. What makes it flow in?

Correct answer
- Correct The diaphragm moves down and the rib muscles lift the chest, so the chest and lungs get bigger and the pressure in them drops
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/min
Breath depth
500 mL
Altitude
0 m
Zoom to an air sac
Chest
Choices
Shown in a random order.
- Correct The diaphragm moves down and the rib muscles lift the chest, so the chest and lungs get bigger and the pressure in them drops
- Wrong Muscles in the walls of the lungs inflate them
  Misconception: Thinks the lungs have muscles of their own that pull air in.
  Feedback from the catalogue
  The lungs have no muscles that inflate them. The diaphragm contracts and moves down, and the rib muscles lift the chest, so the chest gets bigger. The lungs cling to its wall and stretch with it, the pressure inside them drops a little below the air outside, and the air is pushed in. In a quiet breath out the muscles simply relax.
- Wrong The windpipe sucks the air in like a pump
  Feedback
  The windpipe is an open tube, held by rings of cartilage so it does not collapse. Air only passes through it.
Feedback for a right answer
Right. The lungs themselves are passive: they stretch with the chest. When the diaphragm relaxes, the elastic lungs spring back and push the air out.

Hint
In the Scene, watch the dome-shaped muscle under the lungs.

Worked solution
1. The diaphragm contracts, flattens and moves down; the rib muscles lift the ribs.
2. The chest’s volume grows, and the lungs, clinging to it, stretch.
3. The pressure in the lungs falls below the air outside, and the air is pushed in.
If you chose the lungs’ muscles: there are none. The force comes from the diaphragm and the rib muscles.
Step 2 of 7
Open this step
Air a minute
Practice · Calculate
Question
Breathing rate is fixed at 15 breaths a minute, and Breath depth at 500 mL. How many litres of air are breathed each minute?

Correct answer
7.5 ± 0.1
Setup, choices and feedback
The Scene at this step
Breathing rate
15 breaths/minfixed
Breath depth
500 mLfixed
Altitude
0 m
Zoom to an air sac
Chest
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Air breathed a minute, L/min/nodes/chest/outputs/minuteVolume
Feedback for a right answer
$15 \times 500 = 7500$ ⁠ millilitres, that is 7.5 L a minute. This is the minute volume.

Hint
Rate times depth, then convert millilitres to litres: a litre is 1000 mL.

Worked solution
1. The givens: 15 breaths a minute, 500 mL a breath.
2. Minute volume = rate × depth = $15 \times 500 = 7500$ ⁠ millilitres a minute.
3. In litres: $7500 \div 1000 = 7.5$ ⁠.
The answer: 7.5 L a minute.
Step 3 of 7
Open this step
The dead space
Practice · Calculate
Question
Breathing at rest: 12 breaths a minute of 500 mL. About 150 mL of each breath only refills the airways. How many litres of air reach the air sacs each minute?

Correct answer
4.2 ± 0.1
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/minfixed
Breath depth
500 mLfixed
Altitude
0 mfixed
Zoom to an air sac
Chest
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Air reaching the air sacs a minute, L/min/nodes/chest/outputs/alveolarVolume
Wrong answers it recognises
- 6 ± 0.1
  Misconception: Counts all of each breath as reaching the air sacs, forgetting the air that only refills the airways (the dead space).
  Feedback from the catalogue
  Not all the air you breathe reaches the air sacs. About 150 mL of every breath only refills the windpipe and the bronchi, and goes out again without meeting any blood. So the air reaching the air sacs each minute is rate × (depth − 150 mL), not rate × depth. Fast, shallow breathing wastes that share over and over.
Feedback for a right answer
$12 \times (500 - 150) = 4200$ ⁠ millilitres, that is 4.2 L a minute, out of the 6 L breathed.

Hint
From each breath, first take away the part that stays in the airways.

Worked solution
1. Each breath brings $500 - 150 = 350$ ⁠ millilitres to the air sacs.
2. Each minute: $12 \times 350 = 4200$ ⁠ millilitres.
3. In litres: 4.2 L a minute.
If you answered 6 L, you worked out all the air breathed, $12 \times 500$ ⁠, and forgot the dead space.
Step 4 of 7
Open this step
Panting
Practice · Predict, then look
Question
Now pant: 40 shallow breaths a minute of 250 mL each. That is 10 L a minute instead of 6 L. What will happen to the air reaching the air sacs?

Correct answer
- Correct It will not grow: slightly less than 4.2 L will arrive
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/minfixed
Breath depth
500 mLfixed
Altitude
0 mfixed
Zoom to an air sac
Chest
Choices
- Correct It will not grow: slightly less than 4.2 L will arrive
- Wrong It will grow, like the air breathed
  Misconception: Counts all of each breath as reaching the air sacs, forgetting the air that only refills the airways (the dead space).
  Feedback from the catalogue
  Not all the air you breathe reaches the air sacs. About 150 mL of every breath only refills the windpipe and the bronchi, and goes out again without meeting any blood. So the air reaching the air sacs each minute is rate × (depth − 150 mL), not rate × depth. Fast, shallow breathing wastes that share over and over.
Once the prediction is locked in, the Scene changes
Breathing rate
40 breaths/minfixed
Breath depth
250 mLfixed
The Scene then shows: 4 · Air reaching the air sacs a minute, L/min/nodes/chest/outputs/alveolarVolume
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
4.0 L a minute. Of each 250 mL breath only 100 mL reaches the air sacs; the rest just moves back and forth in the windpipe.

Worked solution
1. Each breath brings $250 - 150 = 100$ ⁠ millilitres to the air sacs.
2. Each minute: $40 \times 100 = 4000$ ⁠ millilitres, that is 4.0 L.
3. At rest 4.2 L arrived.
If you predicted more air in the air sacs, you counted all the air breathed. In shallow breaths the dead space eats a large share of each one.
Step 5 of 7
Open this step
Deeper, not faster
Practice · Reach the target
Question
Breathing rate is fixed at 12 breaths a minute. Change Breath depth until 6 L of air a minute reach the air sacs, and press Check.

Goal
- 6 L/min ± 0.05 L/min
  At the start of the step: 4.2 L/min
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/minfixed
Breath depth
500 mL
Altitude
0 mfixed
Zoom to an air sac
Chest
Read from the Scene at
Air reaching the air sacs a minute, L/min/nodes/chest/outputs/alveolarVolume
One setting that reaches it
Breath depth
650 mL
Feedback for a right answer
650 mL: $12 \times (650 - 150) = 6000$ ⁠. A deeper breath sends all the extra to the air sacs, because the dead space stays the same.

Feedback for a wrong answer
Not yet. Work out how much each breath must bring, and add 150 mL.

Hint
Each breath has to bring $6000 \div 12$ ⁠ millilitres to the air sacs, plus the part that stays in the airways.

Worked solution
1. You want 6000 mL a minute in 12 breaths: $6000 \div 12 = 500$ ⁠ millilitres to the air sacs in each breath.
2. Add the dead space: $500 + 150 = 650$ ⁠.
3. Set Breath depth to 650 mL.
If you set 500 mL, you forgot that about 150 mL of every breath never reaches the air sacs.
Step 6 of 7
Open this step
Inside the air sac
Practice · Predict, then look
Question
At rest the blood leaving the air sacs is about 98% loaded with oxygen. What will happen to that loading if you breathe twice as fast, 24 breaths a minute, while sitting still?

Correct answer
- Correct It will barely change
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/min
Breath depth
500 mLfixed
Altitude
0 mfixed
Zoom to an air sac
Air sac
Choices
- Correct It will barely change
- Wrong The blood will carry a lot more oxygen
  Misconception: Thinks breathing faster or deeper at rest loads the blood with much more oxygen.
  Feedback from the catalogue
  At rest at sea level the blood leaving the lungs is already about 97% loaded with oxygen: the haemoglobin is nearly full. Breathing faster can hardly add to it. What it does change is the carbon dioxide: it washes it out of the air sacs, far below its usual level.
- Wrong It will fall, because there is no time to swap gases
  Feedback
  The blood still loads within about a quarter of the capillary, even at this rate. Breathing faster does not shorten the blood’s time in the air sac.
Once the prediction is locked in, the Scene changes
Breathing rate
24 breaths/min
The Scene then shows: 0.9881 · Share of haemoglobin loaded with oxygen in blood leaving the air sacs, 0..1/nodes/chest/outputs/saturation
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
It rises only from 0.977 to 0.988: the haemoglobin was already nearly full. What does change is the carbon dioxide in the air sacs, which falls by about half.

Worked solution
1. At rest the haemoglobin leaving the air sacs is nearly full, about 98%.
2. Nothing can be filled beyond 100%, so faster breathing can add only a little.
3. It does wash the carbon dioxide out of the air sacs.
If you predicted a lot more oxygen, you thought the blood was limited by the air. At rest it is limited by the haemoglobin.
Step 7 of 7
Open this step
Thin air
Practice · Predict, then look
Question
Breathe just as at rest, but in La Paz, 3640 m up, where the air pressure is about two-thirds of that at sea level. How loaded with oxygen will the blood leaving the air sacs be?

Correct answer
- Correct Less than 90%
Setup, choices and feedback
The Scene at this step
Breathing rate
12 breaths/minfixed
Breath depth
500 mLfixed
Altitude
0 m
Zoom to an air sac
Air sac
Choices
- Correct Less than 90%
- Wrong Still about 98%, since you breathe the same air
  Feedback
  The air has the same percentage of oxygen, but it is thinner: each breath takes in fewer oxygen molecules, and the pressure pushing it into the blood is smaller.
Once the prediction is locked in, the Scene changes
Altitude
3640 m
The Scene then shows: 0.7713 · Share of haemoglobin loaded with oxygen in blood leaving the air sacs, 0..1/nodes/chest/outputs/saturation
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 77%. The oxygen pressure in the air sacs falls from about 100 mmHg to about 45 mmHg, where haemoglobin’s loading curve is already falling steeply. People who live at altitude make up for it with many more red blood cells.

Worked solution
1. The air has the same share of oxygen, but its pressure is about two-thirds, so the oxygen’s pressure is too.
2. In the air sacs, once it mixes with the air left there, the oxygen pressure falls to about 45 mmHg.
3. At that pressure haemoglobin is only about three-quarters loaded.
That is why people at altitude breathe faster and deeper, and their bodies make more red blood cells.

Lesson 3 of 5 · About 25 minutes

The heart and circulation

Two loops and one heart: where each side of the heart sends the blood, arteries and veins, cardiac output as rate times stroke volume, and what changes as the heart speeds up.

Open the lesson

The 7 steps and their answers

Step 1 of 7
Open this step
Two loops
Practice · Choose
Question
Show the two loops is On. Where does the right ventricle send its blood?

Correct answer
- Correct To the lungs, and from there it returns to the left side of the heart
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/min
Heart
Typical adult
Show the two loops
On
Choices
Shown in a random order.
- Correct To the lungs, and from there it returns to the left side of the heart
- Wrong To the whole body, and from there it returns to the left side of the heart
  Misconception: Thinks blood goes round the body in a single loop, passing through the heart once per round.
  Feedback from the catalogue
  People have two loops. The right side of the heart sends oxygen-poor blood to the lungs, and it comes back rich in oxygen to the left side, which sends it out to the whole body. From there it returns to the right side. So on every full round the blood passes through the heart twice, and both sides pump the same amount each minute.
- Wrong Straight to the left ventricle, through a hole in the wall between them
  Feedback
  In an adult the wall between the ventricles is closed. Blood gets from one side to the other only through the lungs.
Feedback for a right answer
Right. The right side drives the small loop, through the lungs; the left side drives the large loop, through the whole body. On every full round the blood passes through the heart twice.

Hint
Follow the dark blood leaving the right ventricle.

Worked solution
1. Oxygen-poor blood returns from the body to the right atrium, and drops into the right ventricle.
2. The right ventricle pushes it through the pulmonary artery to the lungs, where it picks up oxygen.
3. The pulmonary veins bring it back to the left atrium, and the left ventricle sends it to the whole body.
If you chose the body, you described a single-loop circulation, as in a fish.
Step 2 of 7
Open this step
Arteries and veins
Practice · Choose
Question
The pulmonary artery leads from the right ventricle to the lungs. What blood flows in it?

Correct answer
- Correct Oxygen-poor blood, dark red
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/min
Heart
Typical adult
Show the two loops
On
Choices
Shown in a random order.
- Correct Oxygen-poor blood, dark red
- Wrong Oxygen-rich blood, because it is an artery
  Misconception: Thinks every artery carries oxygen-rich blood and every vein oxygen-poor blood.
  Feedback from the catalogue
  Artery and vein are named by direction, not by oxygen: arteries carry blood away from the heart, veins bring it back. The pulmonary artery carries oxygen-poor blood from the right ventricle to the lungs, and the pulmonary veins carry oxygen-rich blood from the lungs to the left atrium.
- Wrong Blue blood, with no oxygen at all
  Misconception: Thinks blood without oxygen is blue, as the veins under the skin look.
  Feedback from the catalogue
  Blood is always red: bright red when its haemoglobin is loaded with oxygen, dark red, almost maroon, when it has given some away. Even in the veins it is still about three-quarters loaded. Veins look blue because skin sends back more blue light than red from that depth.
Feedback for a right answer
Right. The blood in the right ventricle has just come back from the body, and the pulmonary artery takes it to be loaded. It is the only artery in an adult that carries oxygen-poor blood.

Hint
An artery is named for leaving the heart. Where did the blood in the right ventricle come from?

Worked solution
1. Arteries leave the heart; veins return to it.
2. The right ventricle receives blood back from the body, after it has given some of its oxygen to the tissues.
3. The pulmonary artery takes that blood to the lungs, so it is oxygen-poor.
It is still red, darker, and still about three-quarters loaded: blood is never blue.
Step 3 of 7
Open this step
Litres a minute
Practice · Calculate
Question
A typical adult’s heart beats 70 times a minute, and the left ventricle pushes out about 70 mL each beat. How many litres of blood does it pump each minute?

Correct answer
4.884 ± 3%
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/minfixed
Heart
Typical adultfixed
Show the two loops
Off
Accepted within
± 3%
Computed from the Scene at this step’s setting
Blood each side pumps a minute, L/min/nodes/heart/outputs/cardiacOutput
Wrong answers it recognises
- 70 ± 1
  Misconception: Takes the blood a heart pumps to depend on the heart rate alone, forgetting how much each beat pushes out.
  Feedback from the catalogue
  The blood a heart pumps each minute, the cardiac output, is the heart rate times the stroke volume, the blood each ventricle pushes out per beat. An endurance athlete’s heart pushes out about 100 mL a beat instead of about 70, so it moves the same five litres a minute at only 50 beats.
Feedback for a right answer
$70 \times 70 = 4900$ ⁠ millilitres, about 4.9 L a minute: nearly all the blood in the body, about five litres, passes through the heart every minute.

Hint
Cardiac output is heart rate times stroke volume. Convert millilitres to litres.

Worked solution
1. The givens: 70 beats a minute, about 70 mL a beat.
2. Cardiac output = rate × stroke volume = $70 \times 70 = 4900$ ⁠ millilitres a minute.
3. In litres: about 4.9 L a minute.
If you answered 70, you gave only the heart rate. The blood a heart pumps also depends on how much it pushes out each beat.
Step 4 of 7
Open this step
Once round
Practice · Calculate
Question
The body holds about 5 L of blood, and the heart pumps about 4.9 L a minute. How many seconds does it take for all the blood to pass through the heart once?

Correct answer
61.43 s ± 5%
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/minfixed
Heart
Typical adultfixed
Show the two loops
On
Accepted within
± 5%
Computed from the Scene at this step’s setting
Time for all 5 L of blood to pass through the heart once/nodes/heart/outputs/lapTime
Any unit of the same quantity is accepted; it is converted before checking.
Feedback for a right answer
About 61 s: roughly a minute. At full effort, with the heart pumping 17 L a minute, the round takes less than twenty seconds.

Hint
Divide the amount of blood by the flow, which gives minutes. Then convert to seconds.

Worked solution
1. Time = amount ÷ flow: $5 \div 4.88 \approx 1.02$ ⁠ minutes.
2. In seconds: $1.02 \times 60 \approx 61$ ⁠.
3. The answer: about 61 s.
If you got about 1 s, you gave minutes instead of seconds.
Step 5 of 7
Open this step
An athlete's heart
Practice · Predict, then look
Question
An endurance athlete’s heart pumps almost the same 5 L a minute at rest, but at only 50 beats. What will the board show for its stroke volume?

Correct answer
- Correct Bigger: about 100 mL a beat
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/minfixed
Heart
Typical adultfixed
Show the two loops
Off
Choices
- Correct Bigger: about 100 mL a beat
- Wrong The same, about 70 mL
  Misconception: Takes the blood a heart pumps to depend on the heart rate alone, forgetting how much each beat pushes out.
  Feedback from the catalogue
  The blood a heart pumps each minute, the cardiac output, is the heart rate times the stroke volume, the blood each ventricle pushes out per beat. An endurance athlete’s heart pushes out about 100 mL a beat instead of about 70, so it moves the same five litres a minute at only 50 beats.
- Wrong Smaller, because the heart is resting
  Misconception: Takes the blood a heart pumps to depend on the heart rate alone, forgetting how much each beat pushes out.
  Feedback from the catalogue
  The blood a heart pumps each minute, the cardiac output, is the heart rate times the stroke volume, the blood each ventricle pushes out per beat. An endurance athlete’s heart pushes out about 100 mL a beat instead of about 70, so it moves the same five litres a minute at only 50 beats.
Once the prediction is locked in, the Scene changes
Heart rate
50 beats/minfixed
Heart
Endurance athletefixed
The Scene then shows: 99.99 · Blood each ventricle pushes out a beat, mL/nodes/heart/outputs/strokeVolume
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 100 mL: $50 \times 100 = 5000$ ⁠. Endurance training enlarges the left ventricle, so every beat pushes out more blood and the heart can beat slowly.

Worked solution
1. Cardiac output = rate × stroke volume.
2. The same output, 5000 mL a minute, at 50 beats: the stroke volume is $5000 \div 50 = 100$ ⁠ millilitres.
3. That is more than a typical heart’s 70 mL.
If you predicted the same volume, you thought the output depended on the rate alone. Then fewer beats would pump less blood.
Step 6 of 7
Open this step
In exercise
Practice · Reach the target
Question
Heart is fixed at Typical adult. At rest it pumps about 5 L a minute. Change Heart rate until it pumps 10 L a minute, and press Check.

Goal
- 10 L/min ± 0.2 L/min
  At the start of the step: 4.884 L/min
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/min
Heart
Typical adultfixed
Show the two loops
Off
Read from the Scene at
Blood each side pumps a minute, L/min/nodes/heart/outputs/cardiacOutput
One setting that reaches it
Heart rate
118 beats/min
Feedback for a right answer
About 120 beats a minute, not 140. In exercise the heart both fills more and empties better, and the stroke volume rises to about 84 mL.

Feedback for a wrong answer
Not yet. Read the output at the foot of the chart, and change the rate.

Hint
In exercise the stroke volume grows too, so you will not need to double the rate.

Worked solution
1. At rest: 70 beats × about 70 mL = about 4.9 L a minute.
2. If the stroke volume stayed fixed, the rate would have to double, to 140.
3. But in exercise the stroke volume grows, so 120 beats already do it: $120 \times 84 \approx 10\,000$ ⁠ millilitres, that is 10 L.
The body raises its output two ways at once: faster, and harder.
Step 7 of 7
Open this step
Time to fill
Practice · Predict, then look
Question
At rest the ventricles spend about two-thirds of each beat relaxed, filling. What will happen to that share at 180 beats a minute?

Correct answer
- Correct It will fall below half the beat
Setup, choices and feedback
The Scene at this step
Heart rate
70 beats/min
Heart
Typical adultfixed
Show the two loops
Off
Choices
- Correct It will fall below half the beat
- Wrong It will stay two-thirds: everything just runs faster
  Feedback
  The squeeze itself shortens only a little as the heart speeds up, so most of the time saved comes out of the relaxed part.
Once the prediction is locked in, the Scene changes
Heart rate
180 beats/min
The Scene then shows: 0.4284 · Share of each beat the ventricles spend relaxed, 0..1/nodes/heart/outputs/relaxedShare
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
It falls to 0.43 of the beat. The squeeze shortens a little, and the relaxed part a lot. That is why heart rate has a ceiling: above it the ventricles cannot fill in time.

Worked solution
1. At 70 beats a beat lasts $60 \div 70 \approx 0.86$ ⁠ seconds, and at 180 only $60 \div 180 \approx 0.33$ ⁠ seconds.
2. The squeeze shortens only a little, so most of the cut comes out of the relaxed part.
3. The relaxed share falls from about two-thirds to less than half.
When there is no time to fill, the stroke volume stops growing.

Lesson 4 of 5 · About 20 minutes

The systems work together

One chain from the air to the muscle: how blood is redirected in exercise, why the brain always gets the same, what changes after a meal, and which systems work harder in a run.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
One chain
Practice · Choose
Question
The lungs take oxygen into the body, but the leg muscles are far from them. Which system brings the oxygen to the muscles? Light it up in the Scene.

Correct answer
- Correct Circulation
Setup, choices and feedback
The Scene at this step
Light up a system
All
Exercise effort
0 %
Time since eating
360 min
Peel back the layers
Organs
Choices
Shown in a random order.
- Correct Circulation
- Wrong Breathing
  Feedback
  The breathing system brings the air only as far as the air sacs. From there the oxygen crosses into the blood, and the blood carries it.
- Wrong Nerves
  Feedback
  Nerves carry signals, not substances. They do speed up the heart and breathing when you start to move.
Feedback for a right answer
Circulation. The blood loads oxygen in the air sacs, the heart pushes it through the arteries, and it unloads the oxygen in the capillaries inside the muscle. Each system does one part of the job.

Hint
Which system reaches every organ in the body?

Worked solution
1. The breathing system brings air to the air sacs.
2. The oxygen diffuses into the blood in the capillaries, and binds to haemoglobin.
3. Circulation carries that blood to the muscles and brings carbon dioxide back.
Breathing and blood work as a chain: without either one the oxygen does not arrive.
Step 2 of 6
Open this step
Blood goes where it is needed
Practice · Predict, then look
Question
At rest the brain gets about 0.75 L of blood a minute. What will happen to that when Exercise effort rises to 50% and the heart pumps two and a half times as much?

Correct answer
- Correct It will stay almost the same
Setup, choices and feedback
The Scene at this step
Light up a system
Circulation
Exercise effort
0 %
Time since eating
360 minfixed
Peel back the layers
Organs
Choices
- Correct It will stay almost the same
- Wrong It will grow two and a half times, like all the blood
  Misconception: Thinks that in exercise every organ gets more blood, in proportion to the heart's output.
  Feedback from the catalogue
  In exercise the heart pumps more, but the blood is also redirected. Vessels in the working muscles open, and those in the gut and kidneys narrow, so these get less than at rest. The brain gets almost the same all the time, about three-quarters of a litre a minute, asleep or running.
- Wrong It will fall sharply, to feed the muscles
  Feedback
  The gut and kidneys do give up blood, but the brain is protected: its vessels keep the flow almost constant.
Once the prediction is locked in, the Scene changes
Exercise effort
50 %
The Scene then shows: 0.85 · Blood to the brain, L/min/nodes/person/outputs/brainFlow
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
0.85 L a minute, a small rise. Most of the extra goes to the muscles, which now get more than half the blood.

Worked solution
1. In exercise the heart’s output rises from about 5 L to about 12.5 L a minute.
2. Vessels in the working muscles open, and those in the gut and kidneys narrow.
3. The brain keeps an almost constant flow, about 0.75–0.85 L a minute.
If you predicted that every organ gets more, you assumed the blood is always shared in the same proportions. It is sent where it is needed.
Step 3 of 6
Open this step
The muscles' share
Practice · Calculate
Question
At 50% Exercise effort the heart pumps about 12.5 L a minute, and the muscles get about 7.2 L of it. What percentage of the blood do the muscles get?

Correct answer
57.73 ± 1.5
Setup, choices and feedback
The Scene at this step
Light up a system
Muscles
Exercise effort
50 %fixed
Time since eating
360 minfixed
Peel back the layers
Muscles
Accepted within
± 1.5
Computed from the Scene at this step’s setting
Share of the blood going to the muscles, 0..1/nodes/person/outputs/muscleShare
Wrong answers it recognises
- 20 ± 1.5
  Misconception: Thinks that in exercise every organ gets more blood, in proportion to the heart's output.
  Feedback from the catalogue
  In exercise the heart pumps more, but the blood is also redirected. Vessels in the working muscles open, and those in the gut and kidneys narrow, so these get less than at rest. The brain gets almost the same all the time, about three-quarters of a litre a minute, asleep or running.
Feedback for a right answer
$\dfrac{7.2}{12.5} \times 100 \approx 58$ ⁠, about 58%. At rest the muscles get only about 20%.

Hint
Part over whole, times 100: $\dfrac{7.2}{12.5} \times 100$ ⁠.

Worked solution
1. The part: 7.2 L a minute to the muscles; the whole: 12.5 L a minute from the heart.
2. Percentage = part ÷ whole × 100 = $7.2 \div 12.5 \times 100 \approx 58$ ⁠.
3. The answer: about 58%.
If you answered 20%, you gave the muscles’ share at rest, as if the blood were always shared in the same proportions.
Step 4 of 6
Open this step
Half the blood
Practice · Reach the target
Question
Change Exercise effort until the muscles get half of all the blood the heart pumps, and press Check.

Goal
- 0.5 ± 0.02
  At the start of the step: 0.1987
Setup, choices and feedback
The Scene at this step
Light up a system
Muscles
Exercise effort
0 %
Time since eating
360 minfixed
Peel back the layers
Muscles
Read from the Scene at
Share of the blood going to the muscles, 0..1/nodes/person/outputs/muscleShare
One setting that reaches it
Light up a system
All
Exercise effort
34 %
Feedback for a right answer
At about 37% effort, a very brisk walk. The heart then pumps about 10.5 L a minute, and about 5.3 L of it goes to the muscles.

Feedback for a wrong answer
Not yet. If the muscles get less than half, raise the effort; if more, lower it.

Hint
On the chart, the muscles’ segment should be half the length of the whole bar.

Worked solution
1. At rest the muscles get about 20% of the blood.
2. As the effort rises, the heart pumps more and the muscles take a growing share.
3. Their share passes a half at about 37% effort.
At full effort they get more than 80%.
Step 5 of 6
Open this step
After a meal
Practice · Predict, then look
Question
Sitting at rest. What will happen to the blood flowing to the gut and the liver half an hour after a meal?

Correct answer
- Correct It will grow
Setup, choices and feedback
The Scene at this step
Light up a system
Digestion
Exercise effort
0 %fixed
Time since eating
360 min
Peel back the layers
Organs
Choices
- Correct It will grow
- Wrong It will not change: the gut works anyway
  Feedback
  A working gut needs more blood, to fuel its muscles and glands and to carry away what it absorbs.
- Wrong It will shrink, because the meal fills the gut
  Feedback
  The opposite: after a meal the gut’s vessels open, and it gets more.
Once the prediction is locked in, the Scene changes
Time since eating
30 min
The Scene then shows: 2.137 · Blood to the gut and liver, L/min/nodes/person/outputs/gutFlow
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
From about 1.3 L to about 2.1 L a minute, and the heart speeds up a little to supply it. Digestion, circulation and the liver work together: the blood leaving the gut takes the sugar and amino acids straight to the liver.

Worked solution
1. After a meal the gut’s muscles and glands work hard.
2. Its vessels open, and the flow to it and the liver rises from about 1.3 L to about 2.1 L a minute.
3. The heart raises its output a little, from about 5.0 L to about 5.8 L.
It is also why hard exercise right after a big meal feels heavy: the gut and the muscles compete for the same blood.
Step 6 of 6
Open this step
All at once
Practice · Choose
Question
At 75% Exercise effort, which systems work harder than at rest? Choose all of them. Light each one up in the Scene and check on the chart.

Correct answer
- Correct Circulation
- Correct Breathing
- Correct Muscles
- Correct Skin
Setup, choices and feedback
The Scene at this step
Light up a system
All
Exercise effort
75 %
Time since eating
360 min
Peel back the layers
Organs
Choices
Shown in a random order. More than one choice is correct: every one must be selected.
- Correct Circulation
- Correct Breathing
- Correct Muscles
- Correct Skin
- Wrong Digestion
  Misconception: Thinks that in exercise every organ gets more blood, in proportion to the heart's output.
  Feedback from the catalogue
  In exercise the heart pumps more, but the blood is also redirected. Vessels in the working muscles open, and those in the gut and kidneys narrow, so these get less than at rest. The brain gets almost the same all the time, about three-quarters of a litre a minute, asleep or running.
- Wrong Kidneys
  Misconception: Thinks that in exercise every organ gets more blood, in proportion to the heart's output.
  Feedback from the catalogue
  In exercise the heart pumps more, but the blood is also redirected. Vessels in the working muscles open, and those in the gut and kidneys narrow, so these get less than at rest. The brain gets almost the same all the time, about three-quarters of a litre a minute, asleep or running.
Feedback for a right answer
The heart, breathing, the muscles and the skin. The skin gets four times the blood, to shed the heat the muscles make. Digestion and the kidneys get less, and catch up after the exercise.

Hint
Check on the chart which organs get more blood and which get less.

Worked solution
1. Circulation: the output rises from about 5 L to about 16 L a minute.
2. Breathing: from about 6 L to about 60 L of air a minute.
3. Muscles: they make the movement, and get about 70% of the blood.
4. Skin: its flow rises from about 0.4 L to about 1.7 L a minute, to shed heat.
5. The gut and the kidneys get less blood.
If you chose digestion or the kidneys, you assumed every organ gets more blood in exercise.

Lesson 5 of 5 · About 25 minutes

Muscles and levers

Muscles only pull, so they work in pairs; the biceps pulls eight times the weight because its tendon sits close to the elbow; and inside the muscle, actin and myosin filaments slide past each other and pull hardest in the middle.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
Muscles only pull
Practice · Predict, then look
Question
With the hand holding a kettlebell, the biceps glows: it pulls the forearm up. Predict which muscle works when Task switches to Push a cable down.

Correct answer
- Correct The triceps, behind the arm, pulls
Setup, choices and feedback
The Scene at this step
Load
5 kg
Elbow angle
90 °
Task
Hold a weight
Zoom into the muscle
Armfixed
Choices
- Correct The triceps, behind the arm, pulls
- Wrong The biceps, which now pushes the forearm down
  Misconception: Thinks a muscle can push as well as pull.
  Feedback from the catalogue
  A muscle only pulls: it can shorten, but it cannot lengthen itself with force. To push something, the body uses a muscle that pulls on the other side of the joint. That is why muscles work in antagonistic pairs: the biceps bends the elbow and the triceps straightens it. When one pulls, the other relaxes and is stretched.
- Wrong No muscle: the cable pulls up, and the arm only has to relax
  Feedback
  If the arm relaxes, the cable pulls the hand up. To hold it, something must pull against it.
Once the prediction is locked in, the Scene changes
Task
Push a cable down
The Scene then shows: 2 · 1 the biceps (and the other elbow benders) pull, 2 the triceps/nodes/arm/outputs/working
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The triceps. Its tendon wraps the point of the elbow, behind the joint, and when it pulls it straightens the elbow and pushes the hand down. The biceps relaxes. A muscle cannot push, so every joint needs a pair that pulls both ways.

Worked solution
1. The cable pulls the hand up, so it tries to bend the elbow.
2. To hold still, something must straighten the elbow.
3. The biceps, in front, can only bend it. Only the triceps, behind, straightens it.
4. So the glow moves to the triceps.
Step 2 of 6
Open this step
A losing lever
Practice · Calculate
Question
The hand holds 8 kg with the elbow at a right angle. The kettlebell is 33 cm from the elbow, the tendon 4.53 cm from it, and the forearm itself adds a moment of 2.35 N·m. How hard does the biceps pull? ( $g = 9.81$ ⁠ m/s²)

Correct answer
623.3 N ± 3%
Setup, choices and feedback
The Scene at this step
Load
8 kgfixed
Elbow angle
90 °fixed
Task
Hold a weightfixed
Zoom into the muscle
Arm
Accepted within
± 3%
Computed from the Scene at this step’s setting
The working muscle's pull along its tendon/nodes/arm/outputs/muscleForce
Any unit of the same quantity is accepted; it is converted before checking.
Wrong answers it recognises
- 78.48 N ± 5%
  Misconception: Thinks a muscle holding a weight pulls with about the weight's own force.
  Feedback from the catalogue
  The weight and the muscle are not the same distance from the elbow. What balances is their moments, force times distance: the weight hangs 33 cm from the elbow, and the tendon joins only about 4.5 cm from it. So the muscle must pull about $33 / 4.5$ ⁠ times, seven times or more, the weight.
Feedback for a right answer
About 620 N, roughly eight times the kettlebell’s weight, 78 N. Like lifting a sack of over 60 kg, just to hold 8 kg still. And the joint itself is squeezed: the upper-arm bone presses on the forearm with the difference, about 530 N.

Hint
First work out the moment the kettlebell makes about the elbow, add the forearm’s, and divide by the tendon’s distance.

Worked solution
1. The kettlebell’s weight: $8 \times 9.81 = 78.5$ ⁠ N.
2. Its moment: $78.5 \times 0.33 = 25.9$ ⁠ N·m; with the forearm, $25.9 + 2.35 = 28.3$ ⁠ N·m.
3. The muscle’s moment equals it: $F \times 0.0453 = 28.3$ ⁠.
4. $F = 28.3 / 0.0453 \approx 623$ ⁠ N.
If you answered about $78$ ⁠ N, you gave the kettlebell’s weight: you forgot the tendon is seven times closer to the elbow than the hand.
Step 3 of 6
Open this step
Why so close?
Practice · Choose
Question
Why does the biceps tendon join so close to the elbow, even though that makes the muscle pull eight times the weight?

Correct answer
- Correct Each centimetre the muscle shortens moves the hand about seven: the hand moves far and fast
Setup, choices and feedback
The Scene at this step
Load
8 kg
Elbow angle
90 °
Task
Hold a weight
Zoom into the muscle
Arm
Choices
Shown in a random order.
- Correct Each centimetre the muscle shortens moves the hand about seven: the hand moves far and fast
- Wrong That way the muscle needs to pull less
  Misconception: Thinks a muscle holding a weight pulls with about the weight's own force.
  Feedback from the catalogue
  The weight and the muscle are not the same distance from the elbow. What balances is their moments, force times distance: the weight hangs 33 cm from the elbow, and the tendon joins only about 4.5 cm from it. So the muscle must pull about $33 / 4.5$ ⁠ times, seven times or more, the weight.
- Wrong No reason: it is just poor design
  Feedback
  There is a cost, but there is a gain too: a short, fairly slow muscle swings the hand through a wide arc.
- Wrong So the elbow is squeezed less
  Feedback
  The reverse: the harder the muscle pulls, the harder the upper-arm bone presses on the forearm.
Feedback for a right answer
A lever trades force for movement. A muscle can shorten by only about a third of its length, and not very fast; a tendon close to the joint turns that small shortening into a hand movement seven times bigger and faster, to throw, strike or reach. It also keeps the arm slim.

Worked solution
1. The ratio of the distances: $33 / 4.53 \approx 7.3$ ⁠.
2. Force: the muscle pulls $7.3$ ⁠ times the weight and more.
3. Movement: when the muscle shortens by 1 cm, the hand moves about 7 cm in the same time, so $7$ ⁠ times as fast.
4. The body prefers range and speed to force: a third-class lever, like tweezers.
Step 4 of 6
Open this step
The limit
Practice · Reach the target
Question
Elbow angle is locked at 90°, a right angle. Find the heaviest Load the biceps can still hold, and press Check.

Goal
- 0.975 ± 0.025
  At the start of the step: 0.6076
Setup, choices and feedback
The Scene at this step
Load
10 kg
Elbow angle
90 °fixed
Task
Hold a weightfixed
Zoom into the muscle
Arm
Read from the Scene at
The pull as a share of the strongest, over 1 when it cannot hold/nodes/arm/outputs/effort
One setting that reaches it
Load
16.5 kg
Feedback for a right answer
16.5 kg. The muscle pulls about 1,230 N here, nearly all the 1,260 N it can manage at this angle. Half a kilogram more, and the arm would give way. The muscle’s arrow is seven and a half times the kettlebell’s.

Feedback for a wrong answer
Not yet. If the arm still holds comfortably, add weight; if it gives way, take off half a kilogram.

Hint
Raise the load half a kilogram at a time until the amber line on the chart crosses the blue one, then step back once.

Worked solution
1. The hardest the muscle can pull at a right angle: about 1,260 N.
2. The largest moment: $1260 \times 0.0453 \approx 57.2$ ⁠ N·m.
3. Less the forearm, $57.2 - 2.35 = 54.8$ ⁠ N·m, so a weight of up to $54.8 / 0.33 \approx 166$ ⁠ N.
4. $166 / 9.81 \approx 16.9$ ⁠ kg; on the slider, in half-kilogram steps, 16.5 kg.
Step 5 of 6
Open this step
Sliding filaments
Practice · Choose
Question
Drag Elbow angle from 160°, nearly straight, to 45° and watch the sarcomere. What happens as the biceps shortens?

Correct answer
- Correct The actin and myosin filaments slide past each other, and the Z discs draw closer
Setup, choices and feedback
The Scene at this step
Load
5 kg
Elbow angle
90 °
Task
Hold a weight
Zoom into the muscle
Filaments
Choices
Shown in a random order.
- Correct The actin and myosin filaments slide past each other, and the Z discs draw closer
- Wrong The myosin and actin filaments shorten
  Misconception: Thinks a muscle shortens because its protein filaments themselves get shorter.
  Feedback from the catalogue
  The filaments do not change length. The myosin heads drag the actin filaments towards the middle of the sarcomere, the filaments slide past each other, and the Z discs draw closer. Each sarcomere shortens a little, and thousands of sarcomeres in a row shorten the whole muscle.
- Wrong Some of the sarcomeres disappear
  Feedback
  The number of sarcomeres in a fibre is fixed; each one shortens a little.
- Wrong The sarcomere folds up like an accordion
  Feedback
  The filaments stay straight and parallel. They only overlap more.
Feedback for a right answer
They slide. The myosin heads grab the actin and drag it towards the middle, cycle after cycle, each paid for with one molecule of ATP. Each filament keeps its length; only the overlap grows. This is the sliding filament theory, from $1954$ ⁠.

Worked solution
1. Watch one myosin filament: its length does not change as the elbow bends.
2. The actin filaments do not change either.
3. What changes is the distance between the Z discs: the actin slides inwards, and the overlap grows.
4. The sarcomere shortens without any filament shortening.
Step 6 of 6
Open this step
Strongest in the middle
Practice · Predict, then look
Question
At a right angle, 90°, the biceps only just holds 16 kg. As the arm straightens, the kettlebell hangs closer under the elbow. Predict what happens when Elbow angle goes to 160°, nearly straight.

Correct answer
- Correct The arm gives way: the muscle is too stretched to pull hard enough
Setup, choices and feedback
The Scene at this step
Load
16 kgfixed
Elbow angle
90 °
Task
Hold a weightfixed
Zoom into the muscle
Arm
Choices
- Correct The arm gives way: the muscle is too stretched to pull hard enough
- Wrong It gets easier: the kettlebell has a shorter lever arm
  Feedback
  The kettlebell’s lever arm does shrink, but so does the muscle’s, almost in step. The pull needed hardly changes.
- Wrong Nothing changes: same weight, same muscle
  No feedback of its own; the learner sees the standard message
  Not yet. Look again and try once more.
Once the prediction is locked in, the Scene changes
Elbow angle
160 °
The Scene then shows: 5 · 1 light load, 2 the biceps holds a weight, 3 the triceps holds a cable, 4 near the limit (70% of the strongest pull or more), 5 too heavy/nodes/arm/outputs/regime
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The arm gives way. The pull needed has hardly changed, about 1,170 N, but with the arm nearly straight the sarcomeres are stretched to about 3 µm, and only about three-quarters of the myosin heads reach the actin. The muscle can pull only about 980 N. A muscle is strongest in the middle of its range.

Worked solution
1. As the arm straightens, both the kettlebell’s lever arm and the muscle’s shrink, almost in step: the pull needed stays near 1,200 N.
2. But the muscle stretches, and the actin slides out from between the myosin filaments.
3. Less overlap, fewer heads pulling: the strongest pull falls from 1,260 N to about 980 N.
4. The pull needed is more than the muscle can give: the arm gives way.

Across the unit · 15 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

Digestion

A tube through the body

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

The stomach empties slowly

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Fat stays longer

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Timing a fatty meal

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

On the lining

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

The next morning

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Breathing and gas exchange

A breath in

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

Air a minute

Question

Correct answer

The Scene at this step

Hint

Worked solution

The dead space

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Panting

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Worked solution

Deeper, not faster

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Inside the air sac

Question