AP Biology The Digestive System

Welcome to Educator.com.0000

We are continuing our discussion of animal physiology with the topic of the digestive system.0002

As chemoheterotrophs, animals need organic molecules from food for both a source of chemical energy and as the building blocks to synthesize materials.0011

In order to obtain energy to make ATP, animals need to break down the chemical bonds in food.0024

In order to do this, an animal needs to ingest the food, then, digest it and finally, absorb it.0032

So, these are the three major steps in the digestive process.0041

There are two general types of digestion: intracellular digestion and extracellular digestion.0046

Intracellular digestion occurs within vacuoles, so vacuoles fuse with lysosomes.0058

And these contain hydrolytic enzymes that can break the bonds of the macromolecules like proteins so that they can be used by the cell.0066

Lysozymes break down the chemical bonds in the nutrients, and in some simple animals, digestion is entirely intracellular like this.0076

For example, sponges digest their food solely by the intracellular route.0089

And when we were talking about another group, not animals, but the protists, we talked also about digestion intracellularly.0097

Extracellular digestion is a lot of what we are going to be talking about today, and this occurs within the digestive tract/GI tract.0105

And the digestive tract consists of a tube, the gastrointestinal tract,0118

with different compartments in it, specialized compartments like the stomach and the small intestine.0123

So, we are going to go on now and first talk about different types of digestive tracts and then,0129

move on to focus in detail on the human digestive system.0134

The simplest type of digestive tract is a gastrovascular cavity.0139

Recall from the discussion on the diversity of life, when we talked about some simple animals like hydra and jellies,0144

the cnidarians, they have a gastrovascular cavity, so gastrovascular cavity, example would be jellies and hydras.0152

A jelly would use its tentacles to capture prey. It would draw that prey, then, into the gastrovascular cavity, which has only a single opening.0162

So, that is something important to know that the gastrovascular cavity has only a single opening.0174

The jelly will capture its prey. The prey will enter the gastrovascular cavity through a single opening.0179

And then, what happens in the gastrovascular cavity is that digestive enzymes are secreted into the cavity.0186

Digestive enzymes, they are secreted into the gastrovascular cavity, and there, digestion takes place.0197

There is no separate circulatory system for animals with the gastrovascular cavity because these nutrients just enter the cavity and then,0212

are broken down and diffuse directly into the cells.0223

Waste products exits through this cavity, as well, and there is only a single opening where nutrients enter and wastes exit.0226

Now, a complete GI tract or an alimentary canal has two openings.0235

One opening is for nutrients to enter. The other opening is for waste to exit.0252

Among the simplest animals with a complete GI tract are annelids, for example, earthworms and rotifers.0259

Of course, more complex animals such as mammals have a complete GI tract.0272

But, this is just talking about the simplest animals that developed a GI tract with two separate openings.0277

And we are going to focus now on the human digestive system.0285

But before I move onto that, I do want to mention that you are going to come across terms with some of the other digestive systems in animals0289

where there will be structures that are not found in the mammalian digestive tract, and one that you should be familiar with is crop.0297

A crop is a reference to a pouch that is found in the GI tract of annelids, insects, birds and some other animals.0305

And this is a place where food can be stored, so it is a pouch for storage of food prior to digestion.0316

We are going to focus now on the human digestive system. Mammalian digestive systems in general are similar, but some of this is particular to humans.0331

So, we are going to go ahead now and talk about an overview of the human digestive system.0341

And just looking and starting up at the top, what we have is the oral cavity, otherwise known as the mouth.0349

After the oral cavity, the food goes into the pharynx, continues on down into the esophagus, the tube that leads to the stomach,0361

enters the stomach, then, goes on to the small intestine, so on through the small intestine, leads to the large intestine here.0384

And then, waste exits the body.0414

In addition to this tract/tube with compartments, the GI tract, there are what is called accessory organs of digestion.0416

These are organs such as the liver, the gall bladder and the pancreas that play a role in digestion, but they are not part of the actual GI tract, of that tube.0426

As we discuss digestion, we are going to focus on three groups of nutrients and how they are broken down.0444

And those three groups are starch, fats and proteins.0449

As we talk about each section of the digestive tract, you should be thinking about what happens to starch at each point,0455

fats and proteins because they each take a little bit of a different path to digestion.0461

We are going to start up at the top with the oral cavity and the esophagus, so we will begin with the mouth.0468

Two things occur in the mouth, two major things: mechanical digestion and chemical digestion, and this is up in the mouth.0474

Mechanical digestion occurs by chewing.0487

Recall that mammals have several different types of teeth, and each of these is specialized to perform a different function.0490

In the front, mammals have sharp incisors and canine teeth.0496

What these do is they cut the food. They tear it.0501

Then, the food is processed by the premolars, those sit a little bit farther back behind the incisors and the canines, and the premolars shred up the food.0504

Finally, the molars in the back grind up the food.0514

The result is that by chewing up food, the surface area of the food is increased, and that allows for more efficient breakdown by digestive enzymes.0518

So, mechanical digestion occurs via the teeth. Chemical digestion occurs via enzymes.0529

And what is found in the mouth is salivary glands secrete amylase, salivary amylase.0536

And amylase hydrolyzes starch, so it breaks down starch. The starch is broken down to form polysaccharides and disaccharides such as maltose.0547

Proteins and fats are not chemically digested at this point. Their chemical digestion does not start until later on in the GI tract.0574

Another function of the mouth, and in particular of the tongue is to shape the food into a ball called a bolus.0584

So, the food gets shaped into this ball shape, and then, it is pushed down. It is swallowed and moved from the oral cavity into the pharynx.0591

And the thing about the pharynx is that it is a common pathway leading both to the airway right here, so this is the trachea, to the trachea.0604

And here, we have the esophagus. Therefore, in order to prevent choking, there is a flap called the epiglottis.0625

Here, you can see the epiglottis is open, so if a person is not in the middle of swallowing, then, the epiglottis can be open.0639

Then, air can go ahead and enter the trachea.0653

However, if somebody is swallowing, then, there would be a risk that food or liquid could enter the trachea and cause choking.0656

When we swallow, what happens is this flap - it is cartilage, it is a flap made of cartilage - of cartilage drops down and covers the trachea.0664

Now, whatever is ingested, food, liquid, whatever is in the mouth, and then, swallowed, it is going to be forced to go into the esophagus rather than0672

going into the trachea because if food or liquid goes into the trachea, obviously, it is going to interfere with your ability to breathe, get oxygen.0681

Even a small amount of food or liquid in the trachea would be a very big concern.0689

Once the food has been swallowed, and it passes into the epiglottis, it is moved into the stomach, and this occurs through peristalsis.0697

The epiglottis is made primarily of smooth muscle.0707

And what peristalsis is, is the rhythmic contractions of the smooth muscle that move food into the esophagus.0711

So, peristalsis is rhythmic contractions of the muscles of the esophagus.0721

At the top of the esophagus is a region of striated muscle, and this region functions as an esophageal sphincter.0725

This one is called the upper esophageal sphincter or UES, and when we are not swallowing, this contracts and closes off the esophagus.0735

When a person does swallow, so during swallowing, then, the epiglottis protects the airway.0755

And the esophageal sphincter relaxes to allow the food to enter the esophagus.0761

Alright, so, we are at the point where food has been processed by the mouth. It has been swallowed.0768

It is being pushed into the stomach via peristalsis.0772

Now, in the stomach, a lot of things occur.0777

For one thing, the stomach churns, and this churning motion helps to mix up the food and also mix digestive enzymes, gastric juices, in with the food.0784

So, to understand the stomach, you need to understand its structure, what it secretes and the functions of the enzymes that it secretes.0795

Food, the bolus, goes down the esophagus.0806

And then, it has to pass through a sphincter at the opening of the stomach called the lower esophageal sphincter.0809

This is also known as the cardiac sphincter.0828

The bolus of food, then, passes through the lower esophageal sphincter and enters the stomach.0842

And the stomach has several functions. It is a muscular compartment, and it functions to store the food temporarily.0847

It functions to break down the food, digest the food and to kill bacteria that are present in the food that we eat.0859

Gastric glands secrete gastric juices, and there are a couple types of cells in the gastric glands; and that is what is discussed up here.0873

The parietal cells maintain the acidic environment of the stomach, so what happens is hydrochloric acid is secreted from the parietal cells.0890

And the function of hydrochloric acid is to kill bacteria that might be present in the food.0902

The second major type of cells are chief cells, and chief cells secrete pepsinogen. This is a precursor to pepsin.0910

The acid, HCl, converts pepsinogen to pepsin.0918

Once some pepsins have been made, that pepsin can go on and convert even more pepsinogen to pepsin.0923

What is pepsin? It is a protease.0930

I talked about amylase starting the digestion of starch up in the mouth, and once the food enters the stomach, the chemical digestion of protein begins.0932

And it begins with pepsin hydrolyzing the proteins to polypeptides.0941

There are also mucus cells present in the stomach, and as you can imagine, their job is to secrete mucus.0949

Now, why would we need to have mucus in the stomach? Well, this mucus layer protects the lining of the stomach.0957

Because of the HCl secreted by the parietal cells, the pH in the stomach is very low. It is typically between 2 and 4.0964

And if there was no protection, the acid would just eat away at the stomach lining.0974

And in fact, when there is a breach in the mucosal layer, an ulcer forms.0979

If there is a part of the lining where it is not protected by mucus, then,0985

that would be a risk factor to form an ulcer or essentially a sore in the lining of the stomach.0989

Something interesting that was discovered maybe a decade or so ago or maybe even longer is that a bacteria plays are role in ulcers.0996

So, an ulcer is in part an infectious disease.1004

A type of bacteria called Helicobacter pylori or H. pylori, infection with H. pylori contributes to ulcer formation.1008

And so, antibiotics are actually one treatment for an ulcer.1019

Now, since the environment in the stomach is acidic, pepsin actually works best in an acidic environment.1025

And this is in contrast to most of the enzymes in the body.1034

Most of the enzymes in the human body work best at a pH of around 7.4. However, pepsin is an exception.1036

So, it is present in the stomach, and it is active at this higher or at this lower pH.1046

We have gone through the major functions of the stomach here, and it temporarily stores food.1054

And the food is passed slowly into the small intestine after digestion.1060

Food is digested chemically with pepsin, and bacteria are killed.1065

Once this food has been processed, it has been churned, it has been mixed with the pepsin, the pepsin has broken down a lot of the proteins,1072

it becomes a semi-liquid substance called a chyme, so this is digested food; and it is in a semi-liquid form.1080

And it is this digested food that is going to leave the stomach.1089

There is a second sphincter at the end of the stomach by the exit called the pyloric sphincter.1093

Chyme will exit through the pyloric sphincter and then, enter the small intestine.1104

That is the next section of the GI tract that we are going to talk about.1109

Now, in order to understand what goes on in the small intestine,1115

you have to be familiar with the pancreas because enzymes from the pancreas are secreted into the small intestine.1118

The small intestine is the major site of digestion. That is where much of the chemical digestion takes place.1123

And while the small intestine makes some of its own digestive enzymes, many of them are made by the pancreas.1130

The pancreas is an accessory organ of digestion, and we are going to talk about the various substances secreted by the pancreas.1136

Now, the pancreas is both an endocrine gland and an exocrine gland.1143

Endocrine glands secrete hormones directly into the blood stream. For example, the thyroid gland is an endocrine gland.1149

Well, the pancreas secretes insulin and glucagon directly into the bloodstream, and they are hormones.1159

So, we are not going to talk about that function now.1166

We are actually going to talk about the endocrine function of the pancreas when we discuss the endocrine system.1169

Right now, we are going to focus on the exocrine function of the pancreas.1175

Exocrine organs have ducts, and this means that substances are secreted through ducts.1179

So, there is a pancreatic duct, and the digestive enzymes are secreted into the small intestine through this duct.1192

We will start with the enzymes, and then, we will also talk about bicarbonate, which is another substance secreted by the pancreas.1206

Multiple enzymes are secreted. Let's start with amylase.1213

So, we talked about salivary amylase. Pancreatic amylase has the same function as salivary amylase.1216

It digests starch, and just like with salivary amylase, starch is broken down into polysaccharides and disaccharides.1222

Not all of the starches are digested in the mouth. Some of them make it down into the stomach still not completely digested.1237

So, they are broken down further here in the small intestine.1247

In addition, there are several proteases that are secreted by the pancreas into the small intestine.1254

And there are multiple different ones, and they have slightly different functions; but they all break down proteins.1262

And we talked about pepsin. Pepsin breaks down protein into polypeptides.1269

And here, we also have secreted from the pancreas, trypsin and chymotrypsin.1274

And what these do is they attack polypeptides at certain sequences, so they recognize certain sequences in a polypeptide.1287

And they break them down to smaller polypeptides.1295

The digestion of starches is continued here in the small intestine. The digestion of proteins is continued in the small intestine.1303

And the digestion of fats, we will talk about in a moment.1313

Now, another protease or actually peptidase is carboxypeptidase, which is produced by the pancreas.1316

And it breaks these peptides all the way down into amino acids.1328

So, as nutrients make their way through the GI tract, they are broken down into smaller and smaller components.1336

Now, we have not talked about fats yet because digestion of fats does not get started until the small intestine except for mechanical digestion.1341

Digestion of starches began in the mouth, continued on in the small intestine.1350

Digestion of proteins began in the stomach, continued in the small intestine.1355

Digestion of fats is just getting going now, and there are lipases that are secreted by the pancreas that hydrolyze fats to glycerol and fatty acids.1361

So, the breakdown of fats is initiated in the small intestine.1383

The liver also plays a role in the digestion of nutrients in the small intestine. The liver has many functions.1388

We are only going to focus now on the function that is related to digestion.1395

But, as we go through the different organ systems, I will mention the liver in different contexts.1400

For example, the liver produces clotting factors. The liver functions in the storage of sugar.1406

Here, though, what we are talking about is the fact that the liver produces bile.1412

Bile is a substance that is stored in the gall bladder, which is associated with the liver. The function of bile is to emulsify fat.1417

What do we mean by emulsify? Well, what bile does is it disperses fats into smaller particles.1431

So, there might a fat globule, and the bile is essentially a detergent that breaks that fat globule up into smaller particles.1438

What this does is it increases their...so, if you have this glob of fat, and then, bile acts on it; and it breaks it up into smaller particles,1450

this increases the surface area of the fats.1460

So, instead of one big glob, there are many small particles so that lipases can attack and break down the fats, as we just discussed.1465

We have talked about some of the accessory organs of digestion and the role they play in digestion in the small intestine.1478

Now, I am going to talk about the structure of the small intestine and enzymes produced by the small intestine, itself.1486

First of all, the small intestine is over 20 feet long, and it has three sections: the duodenum or duodenum, jejunum and ileum.1494

In the duodenum, what occurs is digestion.1504

The digestive enzymes I talked about from the pancreas as well as bile, these are all secreted here into the duodenum where digestion takes place.1506

The pancreas also secretes bicarbonate, so when chyme enters the small intestine, when it enters the duodenum, the chyme is acidic.1519

It has got the acidic gastric juices in it, and it enters. It is acidic.1530

That is neutralized by bicarbonates.1536

So, in addition to the pancreatic enzymes,1538

the pancreas secretes bicarbonate so that the pH in the small intestine is higher than the pH in the stomach so bicarbonate from the pancreas.1541

The jejunum and the ileum are not major sites of digestion. They are sites of absorption.1555

So, remember the three steps to digestion is ingestion, so the intake of food; digestion, the breakdown of food; and then, finally, absorption.1561

The digested nutrients need to be absorbed. They need to enter the animal's bloodstream.1570

The structure of the pancreas is very closely related to its functions.1576

In order to carry out the digestive and absorption functions, the small intestine needs to have a large surface area.1582

And if you look at the lining of the small intestine, and you took a close look; and you used a microscope and everything,1589

what you would see is small fingerlike projections called villi, and that is what this is. This is a villus.1595

Then, if you further looked closely at what these folds are constructed of, you would see more folding within the surface,1605

so folding upon folding, and these are microvilli.1621

So, we have villi, and then, within the villi, there are microvilli. Here is a close-up of the cells of the microvillus.1626

And what it shows here is what is called a brush border that extends into the lumen or the cavity of the small intestine.1641

And there are enzymes produced by these epithelial cells, by these cells lining the small intestine and1653

some digestion that are associated with this brush border, and some digestion takes place here, as well.1659

So, while it is very important to have the pancreatic enzymes being secreted into the small intestine,1665

some enzymes are actually produced by the small intestine, itself, by the epithelial cells of the small intestine.1670

So, in the duodenum, there are production of certain enzymes.1678

What happens in the duodenum is chemical digestion, and this is due to pancreatic enzymes.1683

And the pancreatic enzymes I mentioned were amylases, proteases and lipases. Bile also enters the duodenum and emulsifies fats.1695

In addition, in the duodenum, so chemical digestion due to pancreatic enzymes and intestinal enzymes,1708

enzymes that are produced within the small intestinal cells.1717

So, let's talk more about these intestinal enzymes- proteins, first of all.1724

So, we talked about proteins being broken down by pepsin in the stomach and then, further broken down by peptidases in the small intestine.1737

In addition, the intestines produce dipeptidase, carboxypeptidase and aminopeptidase.1747

The pancreatic enzymes break down the proteins into small polypeptides sometimes even into amino acids.1772

But, these additional intestinal enzymes ensure that proteins are broken all the way1781

down to the amino acid level because peptides cannot be absorbed into our bloodstream.1786

What has to happen is the proteins need to be digested all the way down from an intact protein to a1794

polypeptide to a smaller polypeptide, all the way down into the amino acid level to be ready for absorption.1800

Starches: starches are digested by amylase in the mouth, and amylase secreted by the pancreas into polysaccharides or even disaccharides.1808

But, in order to be absorbed, starches need to be broken all the way down into monosaccharides.1823

And if we just counted on the amylase, we would be left with a bunch of sugar that are disaccharides that we could not absorb into our bloodstream.1828

So, what happens is disaccharidases produced by the small intestine break down disaccharides into monosaccharides.1836

We take disaccharides and break them down into monosaccharides.1850

This is easier to understand if you know some of the examples.1854

For example, lactase and it ends in -ase. It is an enzyme.1857

What lactase does is it breaks lactose down, which is a disaccharide, into its monosaccharide form, and the same with maltase. It breaks down maltose.1862

And in fact, many people, when they reach adulthood, do not produce sufficient lactase to breakdown dairy products.1881

And that is why some adults cannot tolerate dairy products.1889

They will get upset stomach, GI symptoms, diarrhea, because the lactose sugars are not being digested in the small intestine.1894

Then, the glucose, the monosaccharides are not being absorbed. Instead, the lactose just pass on into the large intestine where the bacteria live.1904

And the bacteria have all these great nutrients, these sugars. They eat them and produce gas, so lactase insufficiency leads to lactose intolerance.1914

Now, what is happened then, in the small intestine, in the duodenum, is that they chyme has been neutralized by the carbonate secreted by the pancreas.1931

Pancreatic and intestinal enzymes have broken down starches to their monosaccharide form.1943

They have broken down proteins to their amino acid form, so what happens with fats at this point is the bile from the pancreas has emulsified the fats.1951

And the pancreatic lipases produced in the pancreas secreted into the small intestine have broken down these fat particles into glycerol and fatty acids.1971

And we are going to leave them at that point until we talk about absorption.1987

Now, the jejunum and the ileum are sites of absorption.1992

And I am going to talk about those right after we just sum up what we have talked about with the digestive enzymes.1998

So, we are going to leave it at the nutrients have been digested in the duodenum, and they are about to enter the jejunum,2004

and sum up the various digestive enzymes that you should be familiar with.2011

This table shows you where the enzyme is produced, the enzyme name and then, the action, what does it digest.2016

Site of production: mouth, specifically the salivary glands. The enzyme produced by the salivary glands, the enzyme in saliva is amylase.2024

Amylase digests starch into polysaccharides and into disaccharides.2037

So, here in the mouth, starches are already being broken down. You move into the stomach, and pepsin is produced.2044

This is a protease, and it hydrolyzes proteins into polypeptides.2052

The pancreas is a site of production. However, pancreatic enzymes are secreted into the small intestine.2057

Major categories here, amylase, like salivary amylase, digest starch into polysaccharides and disaccharides.2065

Trypsin, chymotrypsin and carboxypeptidase all digest proteins.2073

Trypsin and chymotrypsin digest proteins into polypeptides, whereas, carboxypeptidase breaks polypeptides all the way down to the amino acid level.2079

Right here, we have had starch, got started. Here, we have protein, starch again.2092

This is all protein, and this is all occurring in the small intestine and finally, now, fats.2107

Well, after being emulsified or dispersed by bile in the small intestine,2117

lipases attack the fats and break them down into monoglycerides or glycerol and fatty acids, so here, we have fats.2123

So, starch starts digestion up in the mouth. Proteins start digestion in the stomach, and fat digestion really gets started in the small intestine.2132

And just to mention at least briefly about DNA and RNA, DNA and RNA also need to be broken down.2143

And what happens is this actually begins in the small intestine where DNA and RNA are broken down.2153

I actually should say "DNA and RNA are broken down into nucleotides", and this is DNA nucleic acids.2161

DNA and RNA, their digestion begins in the small intestine.2173

Now, also in the small intestine are enzymes produced by intestinal cells. These are disaccharidases, which continue on the digestion of starch.2178

Now, we have left starch just based on pancreatic enzymes and oral cavity enzymes in the disaccharide form.2192

But, to be absorbed, the starch needs to be broken down all the way into monosaccharide form.2199

So, I am going to box in the form that can be absorbed by the intestinal cells.2205

Proteins: protein digestion started in the stomach. It is continuing on with these pancreatic enzymes and with small intestine enzymes.2214

And dipeptidases: carboxypeptidase and aminopeptidase are going to finish out the job of breaking down peptides into amino acids,2224

which can, then, be absorbed into the blood stream.2233

Nucleotidases and nucleosidases finish out the breakdown of DNA and RNA, of nucleic acids.2237

The nucleotides are further broken down into nucleosides, which are, then, broken down into nitrogenous bases and sugars by nucleosidases.2248

Meanwhile, the fats just stay in this form of monoglycerides and fatty acids.2260

And we will talk about what happens to those when they are taken up by the intestinal cells.2265

So, picking up in the latter part of the small intestine where digestion takes place, and these two segments are the jejunum and the ileum.2271

The small intestine has that very large surface area that we talked about with the villi and2290

the microvilli that increase its surface area greatly so that there is a large absorptive surface.2295

Now, you might recall in that picture, within that I showed you of a villus, within it were some blood vessels.2303

These capillaries within the villi take up the nutrients.2318

Here is the small intestine, and there is all these nutrients in it; and then, the cells here in the intestinal wall will take up the nutrients.2327

The nutrients will enter the capillaries within the villus, within these fingerlike projections called the villi.2336

The nutrients will be taken up, enter the capillary and then, enter the bloodstream.2344

These nutrients... well, actually the blood in the capillaries leaves the intestines and enters the hepatic portal circulation.2351

Nutrients are absorbed and enter capillaries, a capillary network within the walls of the intestine that transport nutrients.2362

And then, there are capillaries within intestinal walls, and then, these capillaries drain or enter into the hepatic portal circulation.2380

And it tells you it is hepatic meaning liver, and that circulation leads to the liver. This way, there is some regulation.2398

It is not just a bunch of nutrients at which it actually could include toxins, drugs, alcohol,2406

anything that has been ingested that should not just be dumped into the bloodstream.2412

There needs to be some sort of control, and the control is that these nutrients actually go to the liver first.2416

The liver is a very important site of detoxification, so the liver can detoxify drugs or alcohol. It can also pick up glucose for storage.2422

So, there is some regulation of nutrients entering the body because absorption in the jejunum2433

and ileum results in entry of nutrients into capillary that, then, go through the liver.2437

The blood goes through the liver, so it is important to know what form the nutrients are in when they are absorbed.2444

Recall that starches, by the time they have gotten through the early part of the small intestine, are in the form of monosaccharides.2453

And these monosaccharides are picked up by these capillaries and make their way to the liver.2463

Proteins are all the way in the form of amino acids.2470

They have been broken down into amino acids, and they are absorbed again via these capillaries.2474

Fats are a little more complicated.2480

Now, remember that when we left off with the duodenum, that the fats had been broken down into monoglycerides and fatty acids.2493

What happens is the intestinal epithelial cells take up the monoglycerides and fatty acids.2501

And there, these reform into triglycerides, so monoglycerides and fatty acids are taken up by the intestinal cells.2510

But, unlike, say, amino acids, let's say that just go into the capillaries and leave,2523

what happens is in these cells here, it is not just like amino acids pass through, go to the capillaries, leave.2527

Actually, inside these cells, some processes take place.2536

And what happens is the monoglycerides and fatty acids are taken up by intestinal cells and reconstituted or reformed into triglycerides.2543

So, they are reefed inside these cells, reformed in the triglycerides. Then, couple things are added to the triglycerides.2561

Cholesterol plus phosphates, phospholipids - excuse me, phospholipids, I meant to say - are added, and these form what is called chylomicrons.2570

So, fat is broken down into monoglycerides and fatty acids.2598

It is here in the intestine. These are picked up by the intestinal cells, reformed into triglycerides and then, packaged as chylomicrons.2603

These chylomicrons do not enter the capillaries. Instead, they enter what this is right here, which is a different type of vessel called a lacteal.2613

Chylomicrons enter the lacteals. What is a lacteal?2626

It is essentially a lymphatic capillary, so lacteals are lymphatic vessels.2631

So, instead of going straight in the capillaries and then, going into the hepatic portal circulation,2640

fats, in the form of chylomicrons, end up in the lymphatic circulation.2645

However, up near the neck, the lymphatic circulation joins the general circulation. It enters veins.2649

Eventually, these lacteals join the systemic circulation.2657

Therefore, the fats that have been digested are going to end up in the regular circulation, but they just take a little bit of different route.2669

They will end up in the venous circulation.2677

Now, we have gone from the point of ingesting food in the mouth,2681

digesting it in the stomach and small intestine and then, absorbing the nutrients in the small intestine.2685

Finally, the food enters the large intestine.2691

Although the large intestine has a greater diameter than the small intestine, it is not nearly as long.2696

And it consists of three parts: the colon, the cecum and the rectum.2702

The appendix is an outpouching of the cecum, and it is a vestigial organ. We have talked about that earlier on when we talked about evolution.2709

As far as we know, the appendix does not really serve a function in humans. It can cause problems, though.2721

It can become infected, impacted and result in appendicitis.2727

So, the three major parts: colon, cecum and rectum. The major functions of the large intestine are the absorption of water and salts.2733

Once the nutrients have been absorbed, there is waste products left, and there is also a lot of water left and salts.2746

And without the function of a large intestine, people would become very dehydrated because all that water would be lost.2754

By the time the waste finish their process through the large intestine, the waste is mostly solid.2763

The water has been reabsorbed back into the bloodstream.2771

Another function is the absorption of salts, the absorption of vitamins.2777

And in fact, in the large intestine, bacteria live. One of these bacteria is E. coli, and E. coli produce vitamin K.2783

They produce vitamin K in the large intestine, and that is absorbed and enters the circulation.2794

So, water, salts and vitamin K are all produced or retrieved here in the large intestine. Then, the waste exits the body via the rectum.2801

The digestive system is regulated by several hormones.2818

Without this regulation, there would be a lot of wasted energy because gastric juices would be produced when they were not needed.2823

And the pancreas would secrete enzymes when there is no nutrients around to digest.2829

So, this system of hormones triggers the various steps in the digestive pathway as they are needed.2833

Major hormones: gastrin, it is produced in the stomach. Its target organ is also the stomach.2841

And gastrin stimulates the production of the gastric juices that we talked about containing pepsin and acid.2848

And the release is triggered by the presence of food in the stomach.2857

So, you eat, food enters the stomach. That triggers the release of gastrin, which, then, stimulates the production of gastric juices.2863

Secretin acts on the pancreas and stimulates the production of pancreatic enzymes and bicarbonate into the small intestine.2870

Cholecystokinin, often just known as CCK, acts on several areas: the pancreas, the gall bladder and the stomach.2881

In the pancreas, it stimulates the release of pancreatic enzymes. It stimulates the release of bile from the gall bladder.2889

And it also slows down the stomach so that the food will remain in the stomach and get fully digested before moving on.2897

So, these are the major hormones that you should be familiar with.2906

Let's go ahead and do some examples now reviewing the digestive system.2911

Example one: how does the structure of an intestinal cell relate to its function?2915

The function of intestinal cells is, they have a digestive function. They produce digestive enzymes, and they also have an absorption function.2923

And these require a large surface area.2933

And recall that if you look at the lining of the intestine, what you are going to see is that it is lined by these foldings in the intestinal wall called villi.2939

And then, if you looked even closer, you would see, when you actually took one of these and did a close-up,2957

you would see that there is foldings called microvilli, and they are the cells all along here, so there are the villi.2965

And within those, the microvilli, so fingerlike projections called villi with further folds called microvilli that greatly increase the surface area.2976

In addition, associated with these are capillaries for absorption of amino acids and sugars.2985

There are also lacteals in here that absorb chylomicrons that absorb fats.2995

So, here is a very good example of structure and function being closely related.3003

What does role does bile play in the digestion of fats?3008

Remember that bile is produced by the liver, and it is stored in the gall bladder; and bile emulsifies fats.3011

This occurs in the small intestine, and emulsification means that the fats are dispersed into smaller particles.3021

The result is this is going to increase the surface area available to the lipases.3040

As I said, instead of having one big fat glob, there are many small ones.3050

And it is much easier, then, for the lipases to attack each of these than to attack this and not even be able to get up the substance in the center.3055

Example two: match the following terms to their descriptions.3067

No. 1, lipase: we just talked about lipases, a peptidase produced in the pancreas.3071

Well, this name tells you what it does. It is an enzyme with the lipids or fats, stores bile produced in the liver- that is not correct.3078

Site of production of hydrochloric acid in the stomach or an enzyme produced in the pancreas that hydrolyzes lipids.3089

So, E or D - excuse me - D is correct.3098

Parietal cells: a peptidase produced in the pancreas- that is incorrect.3103

Stores bile produced by the liver- not correct. Parietal cells are the site of production of HCl.3109

Gall bladder: gall bladder, remember, stores bile produced by the liver.3120

And finally, we have trypsin. Trypsin is one of the peptidases produced by the pancreas secreted into the small intestine.3127

Describe the digestion and absorption of starch beginning with its ingestion.3139

So, the starch is going to be ingested into the mouth, and in the mouth, there will be two types of digestion: 1. mechanical.3144

The teeth will break up the food including the food that is a starch and thereby increase its surface area.3157

And this occurs via the teeth, and it increases the surface area so that chemical digestion can take place more efficiently.3166

Chemical digestion in the mouth is via salivary amylase. This is secreted by the salivary glands via ducts into the mouth.3176

Amylase breaks the starch into polysaccharides and disaccharides.3187

The starch, then, enters the stomach where it is churned up and is sterilized by hydrochloric acid, but no major digestion of starch takes place there.3206

The next big step is in the small intestine. In the duodenum in the small intestine, additional chemical digestion takes place.3220

There are amylases secreted by the pancreas, which also break down starches into polysaccharides and disaccharides.3230

There are also disaccharidases, and these are produced by intestinal cells, and they break down these disaccharides such as maltose into monosaccharides.3240

Finally, in the latter parts of the small intestine, the jejunum and the ileum, the monosaccharides are absorbed.3263

They enter the capillaries and then, enter the hepatic portal system and then, go to the liver, so capillaries and then, eventually, to the liver.3278

Example four: what are two functions of the large intestine?3294

The large intestine is a site of absorption, so one function is the absorption of water. This is the major function.3299

A second function is the absorption of minerals, salts in the large intestine, so absorption is one function, two, waste storage.3308

Waste is stored prior to removal, and finally, it is a site of vitamin K production.3326

The E. coli in the large intestine produces vitamin K- site of vitamin K production.3331

Why is that the stomach lining damaged by the acid present in gastric fluids?3338

The pH in the stomach is about 2 to 4, which is a pretty low pH, and what protects it is mucus.3342

The mucosal glands/cells in the stomach produce mucus. This mucosal layer lines the stomach and prevents damage.3350

A breach in this layer can result in an ulcer.3361

That concludes this discussion of the digestive system at Educator.com3366