The Digestive and Excretory Systems: breaking down and absorbing the food and
excreting the waste
Introduction:
Besides the fact that it is tasty and gives us something to do, why do we really eat food? Most people
would be able to come up with the energy thing. Our food is our fuel – carbon containing proteins, lipids and
carbs – that provides us with the energy we need to perform all of our cellular activities. Additionally, food
supplies us with the building blocks we need to construct our cells and body parts. It is no coincidence that our
cells are made out of the same proteins, lipids and carbs that we eat. Finally there are certain nutrients that our
cells need that they cannot make and have to get from our diet. Certain amino acids, vitamins and minerals all
serve essential purposes in the cells, but can only be obtained from our diet.
The next question to ask is where does food officially enter your body. This is an easy one, right? The
mouth of course!! (insert annoying game show buzzer here) Officially the mouth is not considered “inside.”
The food is not considered to be inside the body until it gets absorbed into the blood stream. This happens
significantly later than the mouth. We’ll come back to that a little later.
Let’s begin in the mouth. The mouth is a little more exciting that you might think. Here we begin to
chew our food which is a form of mechanical digestion. Basically there are two types of digestion –
mechanical, which includes any type of mashing, crushing or churning and chemical, which involves enzymes
and fluids that actually disrupt chemical bonds in the food. Both start right in mouth. Teeth obviously take care
of the mechanical part and then saliva does the rest. Produced by three pairs of salivary glands, saliva is mostly
water with some mucous and enzymes mixed in to lubricate and begin digesting food. The two main enzymes
we need to worry about are lysozyme and salivary amylase. Lysozyme is there to digest any cellular
components in our food - breakdown the cells so we can access anything inside. It also serves as a protection
against microbial contamination. Salivary amylase is there to begin digesting starch into smaller sugars. Chew
something starchy for long enough and it will start to taste sweet!
Usually before digestion is complete in the mouth we are ready to move on to another mouthful and
have swallowed the bolus of food. (yes, scientists have actually named the ball of chewed up food that we
swallow) The trick is to direct the bolus over the opening to the trachea and down the esophagus into the
stomach. Oxygen in the lungs is good. Chicken Cacciatore in the lungs, not so much so. To do this, we have a
small flap that serves as a traffic diverter (think: one of those railroad switches that changes which track the
train is on). When we swallow, this flap, called the epiglottis, flops over and covers the opening to the trachea.
With the trachea covered, the bolus just slides down into the esophagus.
From here one might assume that gravity takes care of the rest, but in reality, muscles lining the
esophagus actually push the bolus down to the stomach. Kinda like you would squeeze the last bit of toothpaste
out of the tube, the esophagus starts contracting at the top and these contractions follow the tube all the way
down until the food reaches the stomach. These contractions are called peristalsis. So what does this mean?
Yes, you could swallow while standing on your head and the food will still go into your stomach! (now nothing
can hold you back!)
In the stomach we start another round of both chemical and mechanical digestion. The stomach is a
muscular pouch that churns the food together with gastric juices into a liquidy paste called chyme. The gastric
juices are a mixture of a very acidic hydrochloric (HCl) solution mixed together with some enzymes. Notably
this is where we begin digesting any protein in our food into smaller chains of amino acids. The HCl has a very
low pH – usually around 1.5 – 3. Anywhere else in the body, a pH this low would cause some pretty serious
damage, BUT the stomach is specially lined with a mucous layer that protects it. In some cases a bacterial
infection can damage this lining causing the acid to damage the walls of the stomach. This is called an ulcer. I
know what you are thinking - “but ulcers are caused by stress, caffeine and spicy foods!” In reality, these are
the things that aggravate an ulcer, but it is rare for them to cause the ulcer on their own.
Once done with the stomach the chyme is pushed out into the intestines. Now remember what I said
above, a pH that low anywhere else in the body would cause damage. And now I’m saying that all that acidic
chyme is pushed into the intestines. That means, in order to avoid damage, the intestines must greet the chyme
with something to counteract the acid. How do we neutralize an acid? A base, of course. Sodium bicarbonate
(affectionately known as baking soda) is produced by the pancreas and secreted into the intestines bringing the
pH back up to a safe level. The intestines might actually become mildly basic. But this is just one of the many
fluids at work in the intestines.
Additionally the liver produces a substance called bile (that then gets stored in the gall bladder). Again,
most people refer to that gross yellow stomach fluid as bile (which we generally only see during a rough
stomach virus) but that is actually chyme. Bile is a dark fluid that acts as the body’s emulsifier, or detergent.
Even when eating healthily, some of our diet should be made of fat and let’s face it, most of our body is water.
What do we know about fat and water? We should know that fat is nonpolar and water is polar and therefore
they don’t mix. So how would we digest fats if we can’t get them to mix with the enzymes dissolved in water?
The same way we get water to mix with grease when cleaning dishes (which I’m sure you all do for your
parents all the time because you are loving and considerate children…..right?) We add a detergent that brings
water and fats together. Bile is just like that detergent (with fewer bubbles). Bile will not break any bonds in
the lipid molecules; it is not an enzyme. It does, however, allow the lipids to mix with the enzymes. Lipid
digesting enzymes, along with proteases (digest proteins) and disaccharidases (digest disaccharides) are all
produced in the pancreas and released into the small intestines. At this point, any macromolecules that we can
digest, have been digested. But remember what we said, food has not entered our body until it is absorbed into
the blood stream. That comes next.
If we examine the structure of the small intestine we’ll notice that it, too, is a long muscular tube. Like
the esophagus, peristalsis is at work moving food down the small intestines as well. If we cut the small
intestines open, we’ll notice that the insides are lined with tiny finger like projections called villi, each of which
is lined with even smaller projections called microvilli. The purpose of all these tiny fingers is much like the
purpose of the many alveoli in the lungs. The more surface area we have, the more we can absorb in a short
period of time. Now that we’ve gone through the trouble of breaking all the big molecules in our food into
monomers capable of diffusion, it would be a shame to have them pass through without getting absorbed. This
is the point of the many villi and microvilli – to increase surface area to maximize absorption of the food into
the blood stream. Digested food will then hitch a ride with the blood through the body to all the cells. Usually
one of the first stops is the liver, which acts as a toxin filter. The liver often bears the brunt of a lot of
“unnatural” or harmful chemicals that we ingest, but it helps limit the damage done to other organs.
Not everything can be absorbed by the small intestines, however. For starters, sometimes we just eat too
much for the intestines to absorb it all. This excess will continue traveling into the large intestines, a.k.a. colon.
In fact, if an individual is pregnant, and therefore needs all the nutrients they can get, the body has a way of
slowing down the passage of food through the digestive tract to maximize absorption. (It seems that evolution
has thought of everything! Go Darwin!). Additionally there are some things that we eat that we cannot digest
no matter how much time we have. Fiber (cellulose from the cells of plants) may be good for us, but not
because we can break it down. In fact, our bodies do not produce the enzymes needed to break the polymer
cellulose into its monomer, glucose. Even though we would be able to benefit from the glucose, it just can’t be
done. So fiber keeps traveling along the intestines into the large intestines.
The large intestines are basically the last step in the process. The only things that make it here are the
excess and indigestible portions of our food. Here we reabsorb some of the water added to the food to make
digestion easier (no sense in wasting water) which in turn solidifies the remaining waste. This is the point at
which fiber helps us as well. Fiber is pretty good at holding on to water allowing the solid waste to remain soft
and flow more smoothly out of the body. Not to get too graphic, but the only role of fiber is to keep wastes
from sitting in the body where chemicals may do harm. From the large intestines the wastes pass to the rectum
and leave through the anus (stop giggling.)
So that takes care of the solid wastes as well as the digested foods. All that is left to deal with is the
liquid waste. Urine is primarily made up of excess water and metabolic wastes. Unlike solid waste, metabolic
wastes are actually created through chemical processes in the body – namely metabolism. A large component
of urine is a nitrogen containing compound called urea. Urea is a product of protein digestion. While most
proteins are digested into amino acids which are immediately reused, some protein is digested for its energy
containing purposes. As we know, the cell respiration pathway really only cares about carbons, oxygens and
hydrogens, making the nitrogen portion of protein pretty useless and useless means we can excrete it.
The trick with this kind of waste is that it is produced by the cells and released into the blood stream, so
it isn’t going to be a simple matter of pushing it through the intestines. Instead these wastes flow with the blood
through the kidneys. The kidneys are like two big brita filters in the body. All blood passes through them at
some point and the kidneys “clean” the blood, turning the wastes into urine. Basically the cleaning process
happens in the tiny little structures that make up each kidney called the nephrons. Having millions of nephrons
brings us back to the idea of increased surface area! More nephrons, the more blood we can clean faster.
The cleaning process requires three steps: Filtration, reabsorption and secretion. In filtration,
everything that is small enough to diffuse through the walls of the capillaries is pushed out of the blood into the
tubules of the nephrons. EVERYTHING: the good, the bad and the ugly. Pretty much the only things that are
left in the blood are the blood cells and proteins which are both too big to diffuse out of the capillaries. During
the reabsorption phase, the nephron tubules are surrounded by capillaries and the “good stuff” (glucose,
electrolytes, amino acids, etc) is reabsorbed back into the blood, leaving only wastes and excess in the
nephrons. Finally the secretion phase is all about absorbing and secreting H+ ions and water in an attempt to
balance the pH. Once these three activities are done pretty much all that is left in the nephrons is the urine and
the blood is freshly cleaned.
Urine will travel from each tiny nephron into a larger tube called a ureter. A ureter carries the urine
from each kidney and funnels it into the bladder, a muscular pouch that stores urine until it gets full. Once full,
urine is expelled from the body through the urethra, which, yes, both males and females have. Now all the
wastes have been eliminated, all the good stuff has been returned to the body and we’re all set to carry out the
rest of our cellular processes.
1. Food provides us with three basic “things”. What are they?
Energy, building blocks to construct cell and body parts, nutrients like essential aa, vitamins and
minerals
2. Food doesn’t officially enter your “body” until it enters the blood stream. Where is that?
The villi of the small intestine
3. Name 3 places that you have body “parts or components” that have large surface area
Small intestine (villi), nephrons, alveoli, capillaries
4. Which parts of the digestive system would disaccharides “touch” or pass through on their way to becoming
monosaccharides?
Mouth, esophagus, stomach, SI
5. Does bile breakdown lipid?
No, it will not chemically break it apart, it does separate the triglycerides from each other so that the
lipid can “mix” with milk (remember that triglycerides are NOT monomers!)
6. Where are two places that mechanical digestion takes place?
Mostly in the mouth, but in the stomach as well (churning, a kind of contracting)
7. A bacteria called H. pylori is responsible for damaging the lining of your stomach (or intestines) and causing an
ulcer. What would be a possible treatment to get rid of the bacteria?
antibiotic
8.
Where is stomach chyme neutralized? What does the neutralizing?
Small intestine, sodium bicarbonate
9. What does the liver have to do with digestion and absorption (two jobs)
It produces bile (to emulsify for digestion), and it actually acts a a filter for the food (once it gets
into the blood), to filter out toxins. It can also convert excess glucose to glycogen (the animal
“starch”)
10. What is the role of fiber in the diet?
It is not absorbed, It basically moves through the system to the large intestine. There it absorbs
some water , keeping your stool “soft” (otherwise you would get constipated.
11. How is the role of the large intestine different than that of the small intestine?
The SI breaks down dimers and triglycerides to monomers and fatty acids, The villi are the
absorption site in the SI
12. What is the “every day name” (proper name) for metabolic waste? Urine
13. How is urea formed? From the breakdown of protein in the cells
14. How is metabolic waste different from “the other waste”?
Urine contains urea, water and salts. These are transported by the blood to the kidneys, where the bloo is
filtered (the urea, water and salts are removed through the ureters, bladder, and urethra
15. What is the functional unit of the kidney? What is its job?
Nephron. Filtering
16. What are the three steps of the cleaning process?
Filtration, reabsorption, secretion
17. During filtration, what leaves the blood stream (what forms the filtrate)
Salts, urea, water, glucose amino acids
18. During reabsorption, what is returned to the blood stream?
Glucose and aminl acids
19. What is the purpose of secretion?
To adjust the pH of the urine (other pH adjustments in the body occur with the releast of carbonic
acid in the blood stream, and sodium bicarbonate (in the sI).
20. What is the pathway by which urine leaves the kidney and exits the body?
Kidney ureter- bladder, urethra out to toilet (or a diaper)
21. What is the difference between a ureter and a urethra? Do both genders have these?
Ureters are found between the kidneys and the bladder, urethras are found after the bladder. Yes!
22. SUMMARY: Name 4 structures in the body that have high surface area. Identify what they are
absorbing/filtering .
(see number 3. Nephrons filter urea, salts and water; alveoli exchange CO2 and O2, villi absorb food
from the SI into the body. Capillaries carry ALL these things.