A Slice of Pizza Going Down

So what digestion’s all about

is pulling your food’s goodies out.

To get them out, though, food must go

through several organs in a row.

This pathway that the food moves through

includes the mouth; the stomach, too;

and both the small and large intestines.

All have key roles in digestion.

Plus off the path, two organs sit

that aid digestion quite a bit:

The pancreas makes enzymes, while

the liver makes a juice called bile.

Let’s see what all these organs do

as what you eat gets pushed on through.

This section has the inside scoop:

from pizza pie to what we poop!

(a) Pizza in the Mouth: Chewing and Swallowing

The mouth’s the place where you begin

to break down food that you take in.

Not just with teeth, which grind and chew,

but salivary enzymes, too.

The pizza’s breakdown starts at once.

You take a bite, and then you munch.

And as you grind and as you crush,

you mix saliva with the mush.

Saliva’s filled with enzymes that

begin digesting carbs and fat.

One enzyme working in this space

is starch-digesting amylase.

Now, every single starch contains

hundreds of glucoses in chains,

and what the amylase will do

is turn those chains to pairs of two.

(Called “maltose,” these are just the prep

for breakdown in a later step,

when maltose will be split again

so only glucoses remain.)

So amylase splits carbs in food

that you’ve already mushed and chewed.

And now that mushed-up food must go

into the stomach, down below.

In seconds, not too long at all,

the food you’re chewing forms a ball:

a mushy bolus, soft and round,

that gets pushed back and swallowed down.

It moves into the throat, then must

pass into the esophagus.

The bolus makes this tube react

so muscles in its walls contract.

These peristaltic waves begin

to push upon the lump within

and slowly move the bolus south

into the stomach’s waiting mouth.

(b) The Stomach at Work: Churning and Splitting

The stomach opens just enough

to let in all the swallowed stuff.

It breaks some down, and it may store

the rest for several hours or more.

The stomach’s walls are strong; it must

crush foods like chewy pizza crust.

The stomach churns until, in time,

the crust turns into liquid: chyme.

All of that chyme gets sloshed around

while enzymes chemically break down

the nutrients, so bit by bit

the bonds that make them whole are split.

One enzyme, lipase, starts on fat;

another’s pepsin, something that

breaks proteins. But we’re not quite through!

There’s still some breakdown left to do.

The stomach’s push and grind is slow,

so gradually, the chyme will go

as little squirts of gooey soup

into the small intestine’s loop.

(c) The Small Intestine: A Major Role in Breakdown

Inside the small intestine’s space,

a lot of final steps take place.

New enzymes split most of the rest

of chyme remaining to digest.

What’s left when all this breakdown’s through

are fatty acids; sugars, too;

and also in the mix one finds

amino acids, many kinds.

COOL FACT ABOUT THE STOMACH

• A male king penguin can store a whole fish in his stomach for three weeks without digesting it! Our stomachs hold food for only around three hours, and we break down whatever’s inside. But the male king penguin is holding that fish for his soon-to-be-hatched chick, so he can’t digest it. While his female partner is out fishing at sea, the sacrificing dad cares for his unborn chick: he incubates a single egg on his large flat feet while living off his body fat. Then, if the chick hatches and Mom hasn’t returned yet, Dad can chuck up some well-preserved fish to feed his hungry offspring.

COOL FACT ABOUT THE SMALL INTESTINE

• Giraffes have very long small intestines: they can be stretched to reach 180 feet. That’s more than half the length of a football field! A cow’s small intestine is quite long too; it can stretch to 150 feet. The reason for these extra-long organs is that cows and giraffes are herbivores, or plant-eating animals. They need very, very long intestines to help digest and absorb tough cellulose in the cell walls of the plants they eat. By these standards, the human small intestine is short, at only twenty feet long.

Plus, vitamins like C and A,

and minerals you need each day.

These nutrients are now so small

they’ll pass through the intestine’s wall.

Now since we’ve covered quite a lot,

it might seem like we’re done; we’re not.

It’s critical that we discuss

the liver and the pancreas.

(d) The Pancreas and the Liver: Two Organs That Can’t Be Left Out

Digesting food does not pass through

these organs, yet they have roles, too.

Assisting food breakdown is one;

then processing when breakdown’s done.

First, both these organs will produce

a critical digestive juice.

And then they each deliver this

into your small intestine’s mix.

The pancreas produces eight

more enzymes that participate

in the last stages of digestion

of chyme in your small intestine.

The liver’s bile is needed too,

as small intestines do not do

too well digesting fat without

a squirt of bile to help them out.

But livers still have work to do

once all of this digestion’s through,

because the liver is the thing

that does some final processing.

The nutrients formed by digestion

move out from the small intestine

to the blood, and then they pass

directly through the liver’s mass.

The liver stores what cells don’t need

right now—but later, it can feed

out nutrients, so that they go

to cells when their supplies get low.

COOL FACTS ABOUT THE LIVER

• In addition to its other functions, the liver also detoxifies blood, turning harmful chemicals into less harmful ones. For example, ammonia, which in large amounts can damage the tissues of the brain, is something we naturally form when bacteria break down proteins in the large intestine. Normally, the small amounts we make are picked up in the blood and end up in the liver. The liver turns ammonia into harmless urea, a compound that we easily get rid of in our urine and sweat. The liver also breaks down medicines we take so they don’t stay in our system for too long.
• Alaskan natives and travelers to the Arctic are often warned to stay away from polar bear livers: eating one can be deadly. The liver stores lots of nutrients for later use, including some kinds of vitamins, like A, E, and D. Polar bears live in the freezing Arctic, where they don’t get to eat vitamin A very often, so they store large amounts of it in their livers. This helps keep them alive, but it’s way too much for humans to consume all at once. Though you’re not likely to ever eat anything from a polar bear, it’s useful to know that we need only tiny amounts of vitamin A every day—too much can be poisonous.

• Most fats don’t mix with, or emulsify, in water. If you’d like to see this for yourself, try this experiment. Using a jar with a lid, pour in some water and some oil, then shake it up. The oil drops will break up and mix with the water for a little while—this means you’ve moved the oil around. But since it can’t mix permanently with the water, it won’t stay put. If you let the mixture sit, the oil will move out of the water and end up on the top, until you shake it up again.

(e) The Large Intestine: Getting Rid of Waste

Digesting food takes lots of time.

Still, we can’t break down all that chyme;

so wastes remaining from digestion

pass next to the large intestine.

This large organ is the place

where chyme is turned to solid waste.

But it turns out this chyme contains

some nutrients in its remains.

Now, you don’t have the enzymes to

break down these last bits coming through.

But luckily there is a fix.

Bacteria take care of this!

We have bacteria that live

in large intestines, and they give

help breaking down wastes to provide

us with the nutrients inside.

Then you make feces from the rest:

the parts of food you can’t digest.

And that’s what you eliminate

each time you go to defecate.

(f) Summing It Up

So now you’ve got an overview

of what your pizza must go through

inside that long digestive tract.

No pizza bit is left intact.

The next time you eat pizza, just

remember: sauce and fat and crust

transform inside of you to give

you nutrients you need to live!

COOL FACTS ABOUT THE LARGE INTESTINE

• Rabbits eat their own feces. They do this in order to fully digest the cellulose in the plants they feed on. Their large intestines produce two kinds of feces: a soft feces (called “cecotropes”), which are eaten, and later, a hard feces (called “fecal pellets”), which they leave behind.
• Humans are omnivores. We eat both plants and meat. We can’t, however, digest the tough cellulose walls of plants with our own enzymes; and the bacteria in the large intestine digest only a small amount of the cellulose we eat. So we just eliminate most of the cellulose in our feces.
• There are actually trillions of bacteria that live in the large intestine. And there are thousands of different species! Most of these are helpful bacteria that break down some of the proteins and carbohydrates that our digestive system has been unable to digest on its own. As a result, sugars and important vitamins that we need are released. The good bacteria also help fight off bad bacteria that can make us sick if there are too many of them in our body.