Quiz 15 — Circulatory and Respiratory Systems

The Guide

One delivery, from the air to a cell

Quiz 12 showed what a cell does with oxygen and sugar: it takes the sugar apart and gets usable energy out, giving off carbon dioxide and water. Quiz 14 showed how the sugar gets into the blood. This quiz is about the other half of that delivery — how oxygen reaches the cell, and how the carbon dioxide gets away again.

Two systems do it together, which is why the test treats them as one topic. Breathing brings air to a place where the blood can reach it. Circulation carries what the blood picked up to every cell in the body, and brings the waste back. Neither is any use without the other.

What the blood is carrying

Blood is a liquid with things floating in it. The liquid is plasma, which is mostly water, and it carries dissolved sugar, salts, and most of the carbon dioxide. Floating in it are three kinds of cell-sized cargo, and each has one job.

Red blood cells carry the oxygen. Each one is packed with hemoglobin, a protein that holds oxygen where there is plenty of it and lets go where there is little. Hemoglobin is also what makes blood red.
White blood cells deal with what does not belong in the body. They are Quiz 18's subject.
Platelets are fragments that pile up at a cut and start the clot that seals it.

Nearly every question about oxygen is a question about red blood cells, and the reason is worth stating plainly: oxygen does not dissolve well in water. Plasma alone could not carry enough of it to keep you alive for a minute. The hemoglobin is what makes the delivery possible at all.

The heart is two pumps side by side

The heart has four chambers. The two on top are the atria, and they receive blood arriving from elsewhere. The two below are the ventricles, and they push blood out. Between and beyond them sit valves, flaps that open one way only, which is why blood does not slide backward between beats. The sound of a heartbeat is those valves closing.

The right side and the left side do different work, and they never mix. The right side takes in blood that has come back from the body, low on oxygen, and sends it to the lungs. The left side takes in blood that has come back from the lungs, full of oxygen, and sends it out to the body. So blood makes two trips, and it passes through the heart between them.

Read the ring in one direction and you have the whole circuit. Blood comes back from the body with its oxygen spent. It enters the right atrium, drops into the right ventricle, and is pushed to the lungs. It comes back full, enters the left atrium, drops into the left ventricle, and is pushed out to the body. Then it starts again.

Why the left ventricle has the thickest wall. The right ventricle pushes blood as far as the lungs, which are next door. The left ventricle pushes blood to your feet, your hands and your brain, and back. It is the same pump doing a much longer job, so its muscle is heavier. When a diagram or a question points at wall thickness, the answer is usually about how far the blood has to be pushed.

Left and right are the body's, not yours. On an anatomical drawing the heart's right side appears on the left of the page, because the drawing shows the person facing you. This trips people up on the test, where the label is the thing to trust rather than the position. The diagram above avoids the problem by labeling every chamber, but you will meet the other kind.

Three kinds of vessel, and one definition worth memorizing

Blood travels in tubes of three kinds, and the words are defined by direction rather than by content.

Arteries: Carry blood away from the heart. They take the full force of each beat, so their walls are thick and springy. What you feel as a pulse is an artery stretching as the wave of pressure goes by.
Capillaries: The smallest vessels, one cell thick, running in networks through every tissue. This thinness is the point: capillaries are the only place where anything crosses between the blood and the cells. Everything else is transport.
Veins: Carry blood back to the heart. By this stage the pressure from the beat is largely spent, so veins have thin walls and valves inside them, and they depend on the squeeze of the muscles around them to keep the blood moving. That is why standing still for hours is harder on the legs than walking.

Now the definition that the test likes to check. An artery is not a vessel carrying oxygen-rich blood. An artery is a vessel carrying blood away from the heart. Almost everywhere, those two descriptions pick out the same vessels — and in one place they do not. The vessel running from the heart to the lungs carries blood that is low in oxygen, and it is still an artery, because it leaves the heart. The vessels bringing blood back from the lungs are veins, and they are the ones full of oxygen. If you hold to away means artery, the exception costs you nothing.

Breathing: getting air to a place the blood can reach

Air comes in through the nose or mouth, where it is warmed and where hairs and mucus catch some of the dust. It goes down the trachea, the windpipe, which divides into two bronchi, one to each lung. Each of those divides again, and again, into narrower and narrower tubes called bronchioles. At the end of the smallest of them are the alveoli: tiny air sacs, hundreds of millions of them, each wrapped in capillaries.

Getting air in is muscle work. The diaphragm is a sheet of muscle under the lungs. When it tightens it flattens and pulls down, the space inside the chest gets bigger, the pressure in there drops below the pressure outside, and air moves in to even it out. When the diaphragm relaxes the space shrinks again and air goes out. You are not sucking air in so much as making room for it, and the atmosphere does the rest. The same idea comes back in Part VIII, where gases and pressure are the subject.

The exchange, and the one rule behind it

The wall of an alveolus is one cell thick. The wall of the capillary wrapped around it is one cell thick. So there are two thin layers between the air you just breathed in and the blood, and gases cross them.

The rule is the one from Quiz 10. A substance spreads from where it is crowded to where it is not, and no pump is needed for it. The air in the sac has plenty of oxygen and the arriving blood has little, so oxygen crosses into the blood. That same blood is loaded with carbon dioxide and the air in the sac has little, so carbon dioxide crosses the other way, and you breathe it out.

Then the blood goes to a muscle, and the same rule runs in reverse. A working muscle is using oxygen, so it has little; the arriving blood has plenty; oxygen crosses out of the blood and into the muscle. The muscle is making carbon dioxide, so it has plenty and the blood has little, and the carbon dioxide crosses in. Nothing new is happening at the muscle. It is one rule operating in two places where the conditions are opposite.

Surface matters as much as thinness. If your lungs were two smooth bags, the area available for this exchange would be a few square feet. Divided into hundreds of millions of small sacs, the area comes to something closer to the floor of a classroom, folded up inside your chest. This is the same trick the small intestine uses with its villi, and for the same reason: crossing a boundary is slow, so the body builds as much boundary as it can.

The trip, end to end

Now the question the Map asks can be answered in one pass. Air is drawn into the lungs by the diaphragm and reaches an air sac. Oxygen crosses into the blood in the capillary around it and is picked up by hemoglobin in the red cells. That blood returns to the left side of the heart, is pushed out through arteries, and travels to a capillary running past a working muscle. There the oxygen leaves the blood and enters the muscle cell, which uses it to get energy out of sugar. The carbon dioxide the cell made crosses into the blood, rides back through the veins to the right side of the heart, is pushed to the lungs, crosses into an air sac, and leaves on your next breath out.

The short version

Breathing brings air to sacs one cell thick, wrapped in capillaries one cell thick. Gases cross there, each moving from where there is more of it to where there is less. The heart's right side sends blood to the lungs to be loaded and its left side sends it to the body to be unloaded, and the same crossing rule does both jobs.

Worked Examples

Two questions, worked through

Example 1. Which of these vessels carries blood that is low in oxygen: the aorta, which leaves the left ventricle for the body, or the pulmonary artery, which leaves the right ventricle for the lungs?

Do not sort them by the word artery, because both are arteries and the word only tells you they leave the heart. Sort them by where the blood has been.

Blood in the aorta has just come from the lungs, through the left atrium and left ventricle. It is loaded. Blood in the pulmonary artery has just come back from the body, through the right atrium and right ventricle, and it is on its way to the lungs to be loaded. It has not got there yet.

The pulmonary artery. Follow the ring backward from the vessel and ask where that blood was last. Blood that has just been to the lungs is full; blood that has just been to the body is spent. The name of the vessel tells you the direction, not the contents.

Example 2. When you climb three flights of stairs, your heart rate and your breathing rate both rise. Why both?

Start at the cell, which is where the demand comes from. Leg muscles doing more work run more respiration, so they need more oxygen delivered and they produce more carbon dioxide to be taken away.

Now ask what each system contributes to that. Breathing faster and deeper keeps the air sacs supplied with fresh air, which keeps oxygen crossing in and carbon dioxide crossing out. A faster heart moves the blood around the circuit more times a minute, so each load is delivered and collected sooner.

Because both are steps in one delivery. The lungs load the blood and the heart moves it, so a muscle needing more of everything requires both to speed up. Questions of this shape are answered by naming the cell's need first and then the step each system supplies.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 4

Use the ring diagram from the Guide above: body, right atrium, right ventricle, lungs, left atrium, left ventricle, and back to the body, with the left-hand path drawn in teal and the right-hand path in burgundy.

1.According to the diagram, which chamber pushes blood out to the body?

AThe right atrium
BThe right ventricle
CThe left atrium
DThe left ventricle

2.Blood returning from the body passes through which two chambers, in which order?

ALeft atrium, then left ventricle
BRight atrium, then right ventricle
CRight ventricle, then right atrium
DLeft ventricle, then right atrium

3.The diagram draws the left-hand path in one color and the right-hand path in another. What is the difference between them?

AHow much oxygen the blood is carrying
BWhether the vessels are arteries or veins
CHow fast the blood is moving
DWhich side of the body the blood is on

4.A valve between the right atrium and the right ventricle fails to close properly. Based on the diagram, where would blood go that should not?

AForward into the lungs before the ventricle squeezes
BAcross into the left side of the heart
CBackward into the right atrium when the ventricle squeezes
DNowhere; the valves do not affect the direction of flow

Table — questions 5 to 7

Oxygen and carbon dioxide measured in blood at four points in the circuit, in units of gas per 100 milliliters of blood.

Where the blood was measured	Oxygen	Carbon dioxide
Arriving at the lungs	14	52
Leaving the lungs	20	48
Arriving at a working muscle	20	48
Leaving that working muscle	9	56

5.Between which two points does the blood gain oxygen?

ABetween arriving at the muscle and leaving it
BBetween arriving at the lungs and leaving them
CBetween leaving the lungs and arriving at the muscle
DThe table does not show oxygen rising anywhere.

6.At the working muscle, oxygen falls from 20 to 9 and carbon dioxide rises from 48 to 56. What accounts for both changes?

AThe muscle filters the blood as it passes.
BOxygen is converted into carbon dioxide inside the vessel.
CThe blood slows down, so it carries less of both gases.
DThe muscle cells used oxygen and produced carbon dioxide.

7.A student says the table shows the lungs making oxygen. What is wrong with that reading?

ANothing; the oxygen figure rises, so the lungs must produce it.
BThe rise is too small to mean anything.
CThe oxygen came from the air; the blood picked it up rather than the lungs producing it.
DThe table measures carbon dioxide only, so it says nothing about oxygen.

Passage — questions 8 to 10

A storm takes the power out for three days. A family runs a gasoline generator in the attached garage, with the garage door raised about a foot for air, and keeps the door to the kitchen closed. By the second evening two of them have headaches and feel dizzy and sick, and a child is unusually sleepy. They go outside for air and begin to feel better within the hour.

A gasoline engine gives off carbon monoxide, a gas with no color and no smell. Carbon monoxide attaches to hemoglobin in the red blood cells, in the place oxygen would occupy, and it holds on far more tightly than oxygen does. Air in the garage was later found to contain a normal amount of oxygen.

8.Which step of the delivery does carbon monoxide interfere with?

ADrawing air into the lungs
BCarrying oxygen in the blood
CPumping blood out of the heart
DRemoving urea from the blood

9.A neighbor says the air in the garage held a normal amount of oxygen, so the air cannot have been what made them ill. How should that be judged?

AIt is wrong, because oxygen in the air is no use if the blood cannot carry it to the cells.
BIt is right, because illness from air always means too little oxygen in it.
CIt is right, because carbon monoxide affects the lungs and not the blood.
DIt cannot be judged without knowing how long the generator ran.

10.Which conclusion does the passage best support?

AThe generator was safe because the garage door was partly open.
BThe family would have smelled the danger if the gas had reached a harmful level.
CAn engine of this kind belongs outdoors and well away from doors and windows, since the gas gives no warning.
DHeadaches and sleepiness are the ordinary result of losing power for several days.

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10 Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. D — the left ventricle. Follow the burgundy path down the right-hand side of the ring: lungs, left atrium, left ventricle, body. The ventricle is the chamber that pushes; the atrium above it receives. B is the other pushing chamber, and it pushes to the lungs rather than to the body. Atrium and ventricle are the two jobs, receive and push; left and right are the two destinations, body and lungs. Every chamber question is answered by naming one from each pair.

2. B — right atrium, then right ventricle. Blood coming back from the body is low on oxygen, so it enters the teal path. Atria receive, ventricles push, so the atrium always comes first. C has the right side of the heart but the wrong order, which is the commonest slip on this item. Blood cannot be pushed out of a chamber it has not yet arrived in.

3. A — how much oxygen the blood is carrying. The diagram says so under the drawing, and the labels agree: the teal path runs from the body to the lungs, which is the stretch where oxygen has been spent and not yet replaced. B is the answer people expect, because textbook drawings often color arteries and veins differently too. Here it does not work: the vessel from the heart to the lungs is an artery and it is on the teal path, while the vessels from the lungs to the heart are veins and they are on the burgundy path. The color is about the contents, not the direction.

4. C — backward into the right atrium when the ventricle squeezes. A valve is a flap that opens one way. The one between an atrium and a ventricle lets blood down and stops it coming back up. If it does not close, some of the blood the ventricle is squeezing goes back the way it came instead of onward. B imagines blood crossing from one side of the heart to the other. The two sides are separated by a solid wall, and no valve joins them. D denies that valves matter, but one-way flow is exactly what they exist for.

5. B — between arriving at the lungs and leaving them. Read the oxygen column: 14 arriving, 20 leaving. That is the only rise in the column, and it is the loading step. C names two rows where oxygen is the same, 20 and 20, which is itself worth noticing: nothing is gained or lost while the blood is simply traveling. Gains and losses happen at the capillaries, in the lungs and in the tissues, and nowhere in between.

6. D — the muscle cells used oxygen and produced carbon dioxide. This is Quiz 12 showing up in the numbers. A working cell takes in oxygen and gives off carbon dioxide, so blood leaving that muscle has less of the one and more of the other. B sounds mechanical and reasonable and is not what happens: the oxygen is not turned into carbon dioxide in the vessel. It is used inside the cell, and the carbon dioxide comes from the sugar being taken apart. When a question offers you a change that happens in the blood and a change that happens in the cell, ask which one has a reason behind it.

7. C — the oxygen came from the air. A number rising tells you something arrived; it does not tell you where from. The air sacs were full of freshly breathed air, and the blood took oxygen out of it. A treats the rise as its own explanation, which is the habit Quiz 5 warned about. B dismisses a change that is in fact large — the oxygen figure went up by nearly half. Neither reads the table wrongly; both stop reading too soon.

8. B — carrying oxygen in the blood. The passage says carbon monoxide attaches to hemoglobin where oxygen would sit, and holds on tightly. Hemoglobin is the carrier. Breathing was working, the heart was pumping, and the step in the middle had been taken out. A puts the fault in the lungs, which is the natural guess when a gas is involved. The lungs were doing their job. This is why the delivery is worth thinking of as a chain: air in, gases across, carried in the blood, across again at the tissue. Any one link can fail while the others work.

9. A — wrong, because oxygen in the air is no use if the blood cannot carry it. The neighbor is checking the first link and concluding about the whole chain. Plenty of oxygen reached the air sacs and crossed into the blood. There was simply nowhere for much of it to ride, because carbon monoxide was already attached to the red blood cells, in the places where oxygen would ride. B states a rule that sounds sensible and is not true, and the passage is built to test exactly that. Air can harm you by what it adds as well as by what it lacks.

10. C — an engine of this kind belongs outdoors, well away from doors and windows. Everything in the passage points there: the gas has no smell to warn anyone, the raised door was not enough, and the people recovered once they were in fresh air. A treats the open door as a safeguard, and it was not. This one is worth carrying out of the quiz, because storms and outages come around every year. A generator runs outside, well clear of doors, windows and vents — a garage with the door up is still indoors. Headache, dizziness and sleepiness in more than one person at once is the sign to get everyone out into the air first and ask questions afterward. A carbon monoxide alarm costs little and is the only warning this gas gives.

Your Score

What the number means

8 to 10	Solid. Check topic 15 on your map, and go on to Quiz 16.
6 or 7	Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewer	Worth another pass. Draw the ring from memory: body, right atrium, right ventricle, lungs, left atrium, left ventricle, body. Then say out loud where the oxygen gets on and where it gets off.

Misses on 1 to 4 are the circuit, and drawing it beats rereading it. Misses on 5 to 7 are usually about reading a column rather than a row: find the one number that changes, then ask what happened at that place. Misses on 8 to 10 are about the chain — air in, across at the lungs, carried in the blood, across again at the tissue — and being able to name which link a problem sits in. That last skill is what the test is really after here, and it is the one that carries into Quiz 16.

How anyone worked this out

For fourteen centuries European physicians followed Galen, who taught that blood was made in the liver from food, sent out to the body, and used up there like fuel. Nothing went in a circle. Blood was thought to seep from the right side of the heart to the left through pores in the wall between them, which no one had ever seen.

In 1628 William Harvey published a short book that settled it with arithmetic. He estimated how much blood the heart pushes out with each beat, multiplied by the beats in an hour, and got a quantity far greater than the weight of the whole body. No one eats that much. The blood therefore cannot be made fresh and consumed; the same blood must be going around and coming back. He backed the calculation with the valves in the veins, which allow blood to move toward the heart and not away, and with a tourniquet on his own arm.

Harvey was not the first to see part of it. In the 1240s, the Arab physician Ibn al-Nafis had already argued that no blood passes through the wall between the heart's two sides and that it must instead travel to the lungs and return. His account sat in Arabic manuscripts, largely unread in Europe, and was not widely known there until the twentieth century.