Quiz 14 — Digestive and Excretory Systems

Part III starts here

This quiz is about two body systems: the digestive system, which takes food apart so the body can use it, and the excretory system, which carries off the body's waste. It is the first of seven quizzes on the human body. Those seven are about what a body made of cells has to do all day to keep those cells supplied and cleaned up. Seven topics, and they cover the systems: this one, then circulation and breathing, then the nervous and hormone systems, then bones and muscles and skin, then the immune system, then how the body holds itself steady, then food and health.

You have more to compare this against than any other part of the series. You have eaten today, and your body did every step below without being told to.

The Guide

What comes in, and what has to leave

A cell needs sugar and other small molecules delivered to it, and it makes waste that has to be carried off. Quiz 12 showed what happens inside the cell. This quiz covers the two systems at either end of that: the one that takes food apart until the pieces are small enough to reach a cell, and the one that gets rid of what the cells are finished with.

Digestion is breaking food into pieces small enough to cross a membrane

That sentence is the whole reason the digestive system exists. In Quiz 10 you saw that only small molecules cross a cell membrane. A piece of bread cannot. Starch, the long molecule bread is mostly made of, cannot either. So the body takes the bread apart until what is left is small enough to pass through the wall of the intestine and into the blood.

The taking-apart happens two ways, and the test asks you to tell them apart.

	Mechanical breakdown	Chemical breakdown
What it does	Cuts food into smaller pieces	Splits molecules into smaller molecules
Where you meet it	Teeth chewing; the stomach churning	Saliva on starch; acid and enzymes on protein
What changes	The size of the pieces	What the substance is

Chewing a cracker into crumbs is mechanical: it is still cracker, only smaller. Saliva turning the starch in that cracker into sugar is chemical: it is not starch any more, which is why a cracker held in your mouth long enough begins to taste sweet. Try it, and you will have watched a chemical change happen on your own tongue.

Enzymes. The chemical work is done by enzymes — proteins the body makes that speed up a reaction without being used up in it. Each one does a narrow job. The enzyme that works on starch does nothing to protein. Enzymes also work best under particular conditions, chiefly temperature and acidity, and this matters: the enzyme that works in the acid of the stomach stops working when it reaches the small intestine, where conditions are the opposite. That is not a fault. It is how the body keeps each step in its own compartment.

The route, station by station

The digestive tract is one long tube running from the mouth to the anus. Food travels it in one direction, and something different happens at each station.

Mouth: Teeth do the mechanical work. Saliva does the first chemical work, on starch only. What leaves the mouth is a wet ball of food that can be swallowed.
Esophagus: A muscular tube. Rings of muscle squeeze behind the food and push it along, which is why you can swallow lying down, and why an astronaut can eat in orbit. Gravity is not doing this. Nothing is absorbed here; the esophagus is transport.
Stomach: A muscular bag that churns the food and adds acid strong enough to begin taking protein apart. Food stays here a few hours and leaves as a thick liquid. Almost nothing is absorbed through the stomach wall either — the stomach is preparation.
Small intestine: Where nearly everything happens. Enzymes from the pancreas and bile from the liver arrive here and finish the breakdown, and the small molecules that result cross the intestine wall into the blood. The lining is folded, and the folds are covered with millions of tiny projections called villi, so the surface available for absorption is enormous — one long tube with the absorbing area of a room-sized floor.
Large intestine: By now the useful material is gone. What arrives is mostly water, fiber the body cannot digest, and bacteria. The large intestine takes back most of the water and returns it to the body, and forms and stores what remains until it is passed. Bacteria living here feed on the fiber and make a few vitamins as they go.

What the drawing simplifies. The tube is drawn short and neat. The small intestine is about twenty feet long in an adult, coiled into a space the size of a melon, and the large intestine frames it rather than following it in a line. The order of the stations is exactly right, which is what the questions test. The proportions are not.

Food in the tube is not yet inside you. The digestive tract is a passage that runs through the body, open at both ends. A swallowed grape seed travels the whole length and comes out unchanged, having never entered the body at all. Nothing counts as taken in until it crosses the wall of the small intestine and reaches the blood. That is a strange sentence the first time you read it, and it settles a question that comes up later: undigested food and cell waste are two different things, and they leave the body by two different routes.

The excretory system: getting rid of what the cells make

Cells at work produce waste. Respiration in Quiz 12 gave off carbon dioxide. Breaking down more protein than the body needs leaves a nitrogen waste called urea, which is poisonous if it builds up. Cells also produce excess water and salt. All of it ends up in the blood, and all of it has to come out. Excretion is the removal of waste that the body's own cells produced.

Waste	Where it comes from	How it leaves
Carbon dioxide	Cellular respiration	The lungs, when you breathe out
Urea	The liver, from surplus protein	The kidneys, in urine
Excess water and salt	Food, drink, and cell activity	The kidneys, and the skin as sweat

So four organs do excretory work: the lungs, the kidneys, the skin, and the liver, which does not remove waste itself but makes urea and hands it to the blood for the kidneys to collect. The large intestine is not on this list, and that is the distinction worth holding on to. Passing undigested food is not excretion. That material was never taken into the body, so removing it is not the same job.

How a kidney works: filter everything small, then take back what you need

You have two kidneys, each about the size of a fist, and blood runs through them continuously. At first this looks wasteful, but there is a reason for it, and the two steps below are what almost every kidney question on the test is about.

Step one is not fussy. The kidney does not pick out the waste; it pushes out everything below a certain size, useful and useless together. Proteins and blood cells are too large to go through, so they stay in the blood. Everything else — water, salt, sugar, urea — leaves it.

Step two is where the sorting happens. The body takes back what it still wants: nearly all of the water, all of the glucose, and as much salt as it needs that day. Whatever is not taken back continues on and leaves as urine.

Sorting by taking back rather than by picking out has one large advantage. The kidney does not have to recognize every possible harmful substance in order to remove it. Anything the body has no use for is simply not reclaimed, and out it goes. That is why the same organ can clear both a waste your cells make every hour and a drug you took for the first time this morning.

Why doctors test urine. Urine is a record of what the body decided not to keep. Glucose is normally taken back completely, so healthy urine has none in it. Glucose in the urine usually means there was more in the blood than step two could reclaim, which is one of the standard signs of diabetes. Protein in the urine means something else: protein should never have got through step one at all, so finding it there suggests the filter itself is damaged. Neither result is a diagnosis by itself. Both are reasons to look further.

The two systems meet at the blood

Put them together and the shape is simple. The digestive system loads the blood — small molecules cross the intestine wall and travel to the cells that need them. The excretory system unloads it — the kidneys, lungs and skin pull out what the cells are finished with. Everything in between rides in the blood, which is Quiz 15.

The short version

Food is broken down until the pieces are small enough to cross into the blood, and that crossing happens in the small intestine. Waste made by the cells is taken out of the blood by the kidneys, lungs and skin — the kidneys by filtering out everything small and then taking back what the body still wants.

Worked Examples

Two questions, worked through

Example 1. An enzyme is tested at several acidity levels. Where in the digestive tract does it most likely work?

pH of the test	2	4	7	9
Protein broken down (mg)	48	11	1	0

Read the table before reasoning about the body. The enzyme does its most work at pH 2 and almost none by pH 7. A low pH number means strongly acidic. So this enzyme needs an acidic setting, and the row also tells you what it acts on: protein.

Now go to the tract. The mouth is close to neutral. The small intestine is not acidic. The stomach is the acidic station, and the stomach is where protein breakdown begins.

The stomach. When a question gives you enzyme activity against pH or temperature, find the peak first, then ask which part of the body has those conditions. The peak is the answer to a question about location.

Example 2. A patient's urine contains glucose. A friend says this proves the kidneys are failing. Is that a fair conclusion?

Start with what should happen. Glucose is filtered out of the blood in step one and then taken back completely in step two, so it should not appear in urine at all. Its presence means step two did not reclaim all of it. There are two ways that could come about: the kidney's reclaiming is damaged, or there was more glucose in the blood than any healthy kidney could take back.

The second is far more common, and it has a name — diabetes. The evidence given does not tell you which of the two is happening.

No. The result shows glucose was not all reclaimed. It does not show why, and the more common cause is high blood sugar rather than kidney failure. This is the Quiz 5 habit applied to the body: say exactly what the evidence supports, and no more.

The Quiz

Ten questions

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

Diagram — questions 1 to 4

Use the diagram of the digestive tract from the Guide above: mouth, esophagus, stomach, small intestine, large intestine, with the liver, gallbladder and pancreas beside the tube.

1.At which station do nutrients cross into the blood?

AThe esophagus
BThe stomach
CThe small intestine
DThe large intestine

2.According to the diagram, what happens to most of the water that reaches the large intestine?

AIt is broken down by enzymes.
BIt is taken back into the body.
CIt is sent to the pancreas.
DIt passes out unchanged.

3.The diagram places the pancreas beside the tube rather than on it. What does that show?

AFood does not pass through the pancreas; it sends enzymes in.
BThe pancreas has no part in digestion.
CFood reaches the pancreas after the large intestine.
DThe pancreas stores food between meals.

4.A patient cannot swallow and is fed through a tube that delivers liquid food directly to the stomach. Which station is bypassed?

AThe small intestine only
BThe large intestine only
CThe stomach and everything after it
DThe mouth and the esophagus

Table — questions 5 to 7

Amounts of four substances measured at three points in a healthy person's kidney, in grams per 100 milliliters of fluid.

Substance	In the blood arriving	In the filtered fluid	In the urine
Protein	8.00	0.00	0.00
Glucose	0.10	0.10	0.00
Urea	0.03	0.03	2.00
Salt	0.72	0.72	1.50

5.Which substance is filtered out of the blood and then taken back completely?

AProtein
BGlucose
CUrea
DSalt

6.Protein is present in the arriving blood but absent from the filtered fluid. What best explains this?

AProtein is destroyed by the kidney as it passes.
BProtein is taken back faster than it is filtered.
CProtein molecules are too large to pass the filter.
DProtein is converted into urea inside the kidney.

7.Urea measures 0.03 in the filtered fluid and 2.00 in the urine. What accounts for the increase?

AWater is taken back while the urea is left behind, so what remains is concentrated.
BThe kidney manufactures additional urea as the fluid passes through.
CUrea from the large intestine is added at the end.
DThe measurement is an error, since nothing can rise after filtering.

Passage — questions 8 to 10

Two electricians spend a July day pulling cable in an unfinished building with no air conditioning. Both sweat heavily. One drinks water steadily through the shift. The other has two cups of coffee in the morning and little else.

By late afternoon the first man is passing pale urine several times. The second has gone once, and what he passed was dark and scant. He says he must have a kidney problem and that he will see a doctor if it happens again on a hot day.

8.Why is the first man's urine pale and plentiful?

AHis kidneys are filtering less than the other man's.
BDrinking water stops the body from producing urea.
CSweating has removed the salt that would darken it.
DHis body has water to spare, so less of it is taken back.

9.What is the most likely reason the second man's urine is dark and scant?

AHis kidneys have stopped filtering his blood.
BHe is short of water, so his kidneys are taking back as much as they can.
CHis liver has stopped making urea.
DSweat and urine cannot both leave the body on the same day.

10.How should his conclusion about a kidney problem be judged?

AIt is well supported, since dark urine is a sign of kidney disease.
BIt cannot be judged, because urine tells you nothing about the body.
CIt is not supported here, because what he describes is the expected response to losing water.
DIt is not supported, because only a blood test can show anything about the kidneys.

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. C — the small intestine. The diagram says it at station 4, and the reason is the villi: the folded lining gives an enormous surface for small molecules to cross into the blood. B is the common answer because the stomach is where people picture digestion happening. The stomach prepares food and absorbs almost nothing. Preparing and absorbing are two different jobs at two different stations.

2. B — it is taken back into the body. By the time material reaches the large intestine the nutrients are gone. What is left is water, fiber and bacteria, and reclaiming that water is the large intestine's main job. D treats the large intestine as a corridor. If it worked that way you would lose a great deal of water with every meal. This is also why an illness that hurries material through the large intestine is dangerous: the water never gets reclaimed.

3. A — food does not pass through it; it sends enzymes in. The tube runs mouth to anus in one line. The liver, gallbladder and pancreas sit off to the side and deliver into it. They are part of digestion without being stations on the route. B reads "not on the tube" as "not involved." The pancreas supplies most of the enzymes that finish the job in the small intestine. Being off the route is a fact about position, not about importance.

4. D — the mouth and the esophagus. The order is fixed. If food is delivered to the stomach, the only stations skipped are the ones before it, and everything after the stomach happens as usual. C moves in the wrong direction along the tube. Questions like this are answered by finding the entry point and asking what lies upstream of it, which is a good reason to be able to draw the tube from memory.

5. B — glucose. Read across the glucose row: 0.10 in the blood, 0.10 in the filtered fluid, 0.00 in the urine. It went through the filter and then every bit of it came back. A also ends at 0.00 in the urine, but the protein row never has anything in the filtered fluid to take back — it was never filtered out in the first place. The two rows end the same way for opposite reasons, and reading only the last column cannot tell them apart.

6. C — the molecules are too large to pass the filter. Step one sorts by size and nothing else. Proteins and blood cells are above the limit, so they stay in the blood, which is why 8.00 becomes 0.00 at the filter and stays there. B is a reasonable-sounding mechanism that the table rules out. If protein were being taken back, some would have to appear in the filtered fluid first. There is none. When a table gives you a middle column, use it — that column is what separates the two steps.

7. A — water is taken back and the urea is left behind. The measurement is an amount per 100 milliliters, so it rises when the fluid it is dissolved in shrinks. Nearly all the water is reclaimed in step two, and the same urea in much less water reads as a much larger number. B has the kidney making urea, which is the liver's job. The kidney collects and concentrates; it does not manufacture. And D refuses the number rather than explaining it — a concentration can certainly rise without anything being added, and noticing that these are amounts per volume is the whole of this question.

8. D — his body has water to spare, so less is taken back. Step two returns as much water as the body needs that day. With plenty coming in, less needs reclaiming, so more passes on. The urine is pale because the same waste is dissolved in more water. A gets the step wrong. Filtering runs at much the same rate in both men; what differs is how much water is taken back afterward. Nearly every question about pale or dark urine is about step two.

9. B — he is short of water, so his kidneys are reclaiming as much as they can. He has been sweating all day and drinking almost nothing. Holding on to water is exactly what the body should do, and dark, scant urine is the visible result. A jumps to failure. A kidney that had stopped working would not produce concentrated urine; concentrating it is work, and the darkness is evidence that the kidneys are doing their job under pressure rather than failing at it.

10. C — not supported, because this is the expected response to losing water. A conclusion has to be better than the ordinary explanation to be worth drawing. Here the ordinary explanation covers everything he observed: heat, sweat, almost no water, dark urine. D overcorrects. Urine is genuinely informative — it is how glucose and protein get found, as the Guide described. The problem is not that urine says nothing. It is that this particular sign, on this particular day, has a plainer cause. Worth saying plainly, since it is his health and not only a test question: what he describes is dehydration, and the answer to it is water during the shift. If dark, scant urine shows up on a day when he has been drinking normally, that is the day the doctor becomes the right call.

Your Score

What the number means

8 to 10	Solid. Check topic 14 on your map, and go on to Quiz 15.
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 tube from memory, five stations in order, one phrase under each. Then draw the kidney's two steps. Those two drawings carry most of the quiz.

Misses on 1 to 4 are the route, and the drawing fixes those faster than the reading does. Misses on 5 to 7 are usually about the middle column of the table — the fluid after filtering and before taking back — and that column is where the two steps can be told apart. Misses on 8 to 10 are worth the most attention, because they are not really about kidneys. They are about how far a piece of evidence lets you go, which is the habit Quiz 5 was for and which the test asks about in every part of the series.

Where the stomach was first watched

In 1822 a fur trader named Alexis St. Martin was shot in the side at a trading post on Mackinac Island. He survived, but the wound healed into an opening that led directly into his stomach and never closed. The army surgeon who treated him, William Beaumont, realized he could look in.

Over the following years Beaumont lowered pieces of food on silk threads into the opening and pulled them out at intervals to see what had happened to them. He drew off stomach fluid and tested what it could dissolve outside the body. He established that digestion in the stomach is chemical work done by an acid fluid, not the grinding or the cooking that physicians had assumed.

The arrangement was not a partnership between equals. St. Martin was a poor man who was at various points employed as Beaumont's servant and bound by contract to submit to the experiments, and he left more than once. He outlived Beaumont by twenty-seven years, and his family, who had reason to distrust doctors by then, delayed his burial and set the grave deep so that no one would dig him up for study.