This tutorial will take you through the concepts of Digestion and its control.
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This is you. A gut with blood around it and hungry cells around that, just waiting for the food to reach them.
Suppose you eat something you can't digest. It will go in the mouth, but nothing will happen to it... and it will come out the anus... and your cells will remain hungry.
To feed your cells requires several steps.
1. Ingestion - you must eat the food.
These are all pretty much one-way processes, except for ingestion.
Digestion involves the breakdown of large food molecules into small ones. Most of it is done by enzymes in the gut.
Some foods require special pre-treatment like chewing or emulsification, to break large gobs into small ones that the enzymes can surround and break down.
Because different steps are required, the gut is divided into specialized segments to perform these steps one after the other. So the mouth is specialized for chewing, the stomach for storing food, the small intestine for digestion and absorbtion, and the large intestine for absorbtion of fluids and electrolytes.
Because digestion is a series of steps, your body also has to turn on the parts of the gut in order.
When food is about to go in your mouth, your stomach needs to turn on so it will be ready when the food reaches it. When food is in the stomach, the small intestine has to get ready for it. When the food reaches the small intestine, the stomach can turn off and the intestines must get to work.
The different parts of the gut have to talk with one another - to turn each other on and off.
Page 3The first phase of digestion can start even before you've eaten anything! It's called 'cephalic' because it starts in your head - when you see, smell, or even think about food.
This response, though, is dependent on your appetite. You know that at some times, you aren't at all interested in food. What makes you want food at lunchtime, but not right after lunch?
One factor is a hormone called GHRELIN, which is secreted by the empty stomach. You can remember its name because it sounds like the noise your empty stomach makes around lunchtime! GHRELIN stimulates the appetite centers of your brain and makes you feel hungry. It also primes your brain to respond right away to any stimuli related to food (Power and Schulkin, 2008).
What part of the nervous system should your brain activate to get you ready for lunch?
Power, M. L., & Schulkin, J. (2008). Anticipatory physiological regulation in feeding biology. Appetite, 50(2–3), 194–206.
What neurotransmitter will the parasympathetic system use to turn the GI tract on?
Which cranial nerve carries the parasympathetic message to the GI tract?
What kind of receptor will the acetylcholine attach to in the GI tract?
Good work so far!
So during the cephalic stage, before you've even eaten the food, your parasympathetic system has flooded the GI tract with acetylcholine from the vagus nerve. That's attached to the muscarinic receptors on the GI cells.
But what will it make those cells do? For instance, what should your stomach do?
Page 6Here's a summary of the cephalic responses. These are called anticipatory responses. That means they happen before you eat, getting your body ready for the food. If these responses didn't happen, you wouldn't be prepared to process the food you were eating and pass it to the cells.
This flow chart contains a few items we haven't discussed. Along with the vasodilation, increased gastric motility, and secretion of pepsinogen and acid, the stomach also begins to secrete mucus, which protects it from the acid, and a hormone called GASTRIN, which stimulates the cells in the stomach.
In addition, the cephalic response affects your endocrine pancreas and makes it secrete the hormone INSULIN into the blood. This hormone lets your cells pick up glucose from the blood. Now they are ready to pick up the food from your lunch, as soon as you digest it!
You're all ready to sit down and gobble your lunch.
But after you've gobbled it, your brain isn't so excited by food any more, is it? Your brain may even feel a little sick of food. After all, you were hungry because of that GHRELIN from your empty stomach - and now your stomach isn't empty any more, and the GHRELIN levels go down.
So if your brain is sick of food, what will keep your stomach working until that food is digested and sent to your cells?
The Cephalic Phase of digestion turned on your GI tract, using the first line of communication - the parasympathetic system. But now that you're full, the brain isn't so interested any more. The stomach is on its own! So you've entered the next phase of digestion - the GASTRIC phase.
Remember GASTRIN, that hormone the stomach made that excited it to keep secreting? That will keep the stomach running as long as there is food in it.
What would you call this kind of a cycle?
What might happen to someone who secreted too much GASTRIN?
The GASTRIN secretion will keep the stomach working even after the parasympathetic system has reduced its activity.
What's going on inside the stomach? The pepsinogen is reacting with the acid and forming pepsin, a strong protein -digesting enzyme.
Also, the acid is changing any iron you ate into a form that can be absorbed into the blood.
Last, the stomach is secreting a compound called intrinsic factor, which you will need to absorb any Vitamin B12 you ate.
But you are not absorbing food from the stomach. After all, to absorb food it would have to be touching the stomach lining - and your stomach lining is protected by mucus.
The stomach just begins digesting the protein you ate for lunch. Remember, a protein is a chain of many different amino acids, shown here as beads of different colors.
Different enzymes cut between different amino acids. Pepsin will begin the digestion, but these polypeptides will have to meet a lot of other enzymes before they are all cut apart into individual amino acids. That will happen further on, in the small intestine.
As the stomach churns away, some of the fluid inside it moves down through the pyloric sphincter into the small intestine. This starts the third part of the digestive process, the INTESTINAL phase.
The stomach really hasn't done a lot. It digested some of the protein. It didn't do much at all to the carbohydrates or fats you had for lunch.
The small intestines need to deal with those right away. They need a lot of digestive enzymes, ASAP. Also, they need to get rid of all that stomach acid before it eats holes in them!
The first part of the small intestine is the duodenum. When food enters the duodenum, it sends out for help with digesting the stuff. Where can the duodenum get the help it needs for this big job? From the accessory digestive organs.
To tell these organs it needs help, the duodenum secretes hormones.
Hormones secreted by the duodenum:
SECRETIN - stimulates the pancreas to send the duodenum some bicarbonate (HCO3-), an antacid. The bicarbonate will neutralize the stomach acids before they can eat holes in the duodenum lining.
CHOLECYSTOKININ (CCK) - stimulates the gall bladder to contract and squirt bile into the duodenum, to help with fat digestion. It will also stimulate the pancreas to release digestive enzymes into the duodenum. And it will inhibit the stomach, helping it to turn off now that its job is done.
GIP - causes the endocrine pancreas to release even more INSULIN into the blood. After all, that food is getting closer to being absorbed! The cells need to be ready to pick it up.
A man was unable to make CHOLECYSTOKININ. He lost weight without trying to diet at all (Högenauer et al., 2001).
If you were to look in this man's duodenum, what do you think you would find?
If you did an analysis of the man's stool, what do you think you would find?
Högenauer, C., Meyer, R. L., Netto, G. J., Bell, D., Little, K. H., Ferries, L., … Fordtran, J. S. (2001). Malabsorption due to cholecystokinin deficiency in a patient with autoimmune polyglandular syndrome type I. The New England Journal of Medicine; Boston, 344(4), 270–274.
Remember our initial model of the GI tract:
The man without CHOLECYSTOKININ is unable to make his pancreas release the digestive enzymes needed to break down fats and proteins in the small intestines. He also can't make his gall bladder release the bile needed to disperse the fats into little globules so the enzymes could work on them. As a result, these foods are not DIGESTED - that is, they are not broken into small enough molecules to be ABSORBED into the blood. This failure to absorb food from the GI tract into the blood is called MALABSORBTION.
The man is losing weight because his cells aren't getting enough food; the undigested food is going out in his stools. He complains of terrible watery diarrhea, as well! What could be causing his watery diarrhea?
As food moves down the small intestines, it mixes with the bile and pancreatic enzymes, and the food molecules are slowly broken down into smaller molecules. Cells on the intestinal lining trap these molecules and finish digesting them; then they pass the molecules to the blood or lymph, and they are carried away until they eventually reach your cells.
Just like your duodenum, the later parts of the small intestine (the jejunum and ileum) can secrete hormones. They secrete GLP-1, which helps to inhibit the stomach, and the ileum secretes POLYPEPTIDE Y (PYY), which reduces appetite.
When the food has reached the end of your small intestine, you've digested and absorbed most of the nutrients!
Here's another case: a woman has celiac disease, which destroys the cells lining her small intestine. She complains of weight loss, diarrhea, and terrible bloating, gas, and cramping.
You can explain the weight loss and diarrhea by now, but what about the gas?
To understand this, we have to look at the last part of the intestines. When the small intestines have done all they can, the food is passed to the large intestine.
The large intestine contains an incredible amount of bacteria.
There are around 500-1000 different species in there (DiBaise et al., 2008); and about 60% of your stools' dry weight is just bacteria! (Guarner and Malagelada, 2003)
They survive by digesting the stuff you couldn't digest in your stomach and small intestine - and when they do that, they produce gas. This is why you get gas after you eat something (like beans) that you aren't good at digesting; the bacteria get more food.
If something interfered with your ability to digest or absorb food in your small intestines, all that good stuff would end up in the large intestine. The bacteria would go to town, and produce loads of gas!
Our large intestine is not just a home for bacteria, though. What would happen to someone who couldn't absorb electrolytes (ions) in the large intestine?
And with this, we have reached the end of the digestive tract!
You should be able to add additional information from your book to this simple picture of digestion. For instance, you should be able to tell where the brush border enzymes fit in, and name the ducts from the pancreas and the gall bladder. You shoud be able to tell where bile is made and define emulsification. And you should be able to explain what fats and carbohydrates are broken down into, and how they are transported into the blood.
DiBaise, JK, H Zhang, MD Crowell, R Krajralnik-Brown, GA Decker, and BE Rittmann, 2008. Gut microbiota and its possible relationship with obesity. Mayo Clin Proc. 2008 Apr;83(4):460-9.
Copyright Patricia S. Bowne 2017 firstname.lastname@example.org
Information for this activity was drawn from Fox, S.I., 2013. Human Physiology, 13th Ed. McGraw-Hill.