Professor Dave again, let’s digest some food. We just completed our understanding of how
we take in oxygen from the atmosphere and distribute it around the body, but our cells
need more than just oxygen. We need nutrients to sustain our bodily functions,
which means we need to eat food. And that food needs to get broken down into
tiny components that cells can use for energy production, so how does this work? Let’s look at the digestive system now. The digestive system can be split up into
two parts. The alimentary canal, also known as the gastrointestinal
tract, or GI tract, is essentially one long continuous tube that starts at your mouth,
where food goes in, and winds all the way down the body to end at the anus, where some
of the food comes back out. Along the way, much of the food is broken
down into molecular fragments that can be absorbed through the lining of this tract
and into the blood that surrounds. The rest of the digestive system is made up
of accessory digestive organs like the teeth, tongue, gallbladder, salivary glands, liver,
and pancreas, which are not part of the GI tract, but have a considerable role in digestion. All of these components work together to produce
a sequence of six actions. First, ingestion. This is the basic act of eating, meaning putting
food into your mouth. Next is propulsion. This begins when you swallow the food, and
continues with involuntary peristalsis, which is part of the autonomic nervous system. These actions push food along the GI tract,
down the pharynx, and esophagus, and down into the stomach, eventually continuing through
the intestines and out the anus. This happens with the help of an action called
segmentation, or successive local constrictions in the GI tract. The third is mechanical breakdown, which begins
by using teeth and saliva, and continues with further breakdown by digestive juices in the
stomach, a very acidic environment due to the presence of gastric acid, which contains
hydrochloric acid. Next is the part that is more formally referred
to as digestion. This is when enzymes go and perform highly
directed chemical reactions to break down all the polymers in the food. They break proteins down into individual amino
acids, polysaccharides into individual monosaccharides, and so forth, which is categorically different
than the earlier mechnical methods of breakdown. Next, once everything is broken down as much
as it can be, absorption will occur, where all of these nutrients pass through the lining
of the small and large intestines into blood and lymph on the other side. Finally, anything that is not absorbed by
the body will be dealt with during defecation, where it is expunged from the body in the
form of feces. All of this is aided by specific chemical
stimuli that activate certain reflexes, some of which are mediated by the central nervous system. Let’s start out by taking a closer look
at some of the organs of this system. Most of these sit in the abdominopelvic cavity. The peritoneum is the membrane of this cavity,
and it is divided into the visceral peritoneum, covering the surfaces of these organs, and
the parietal peritoneum, lining the body wall, with the peritoneal cavity in between, containing
a serous fluid that lubricates the organs for ease of mobility. Blood is supplied to these organs via the
splanchnic circulation. Now let’s get a closer look at the GI tract
as a whole, to see the basic structure that surrounds the lumen of the tract. The innermost layer is the mucosa, or mucous membrane. This secretes digestive enzymes and hormones
into the tract, and also absorbs digested food into the blood. It is made of a single columnar epithelium,
followed by a lamina propria made of loose areolar connective tissue, and then the muscularis
mucosae, made of smooth muscle. Beyond this is the submucosa, which is made
of areolar connective tissue, and full of blood vessels, lymphatic vessels, lymphoid
follicles, and nerve fibers, and we can see these entering through a mesentery. Digested food just has to get to this layer
in order for nutrients to spread all over the body. Surrounding this layer is the muscularis externa,
which is the muscular layer that performs segmentation and peristalsis, which help the
food along the tract as we discussed earlier. It is made of an inner circular layer and
an outer longitudinal layer. And lastly, we continue to find the serosa. This is areolar connective tissue covered
with a single layer of squamous epithelial cells that form a mesothelium. We should also note the intrinsic nerve plexuses,
which are the submucosal nerve plexus in the submucosa and the myenteric nerve plexus in
the muscularis externa. These allow for communication all along the
GI tract and regulate digestive system activity. Now that we have the basics down regarding
the GI tract, let’s look at the accessory organs. We already talked a bit about the mouth, or
the oral cavity, when discussing the sense of taste, so here we will just highlight the
features relevant to digestion. The walls of the mouth are lined with a thick
stratified squamous epithelium, and the oral mucosa produces antimicrobial peptides called
defensins, since this region will need a lot of protection from the elements. We can also see the hard and soft palate,
the uvula, and the palatine tonsils. The tongue is made of skeletal muscle fibers,
and is secured to the floor of the mouth by the lingual frenulum. Salivary glands produce saliva which cleans
the mouth, moistens and dissolves food, and contains enzymes that begin breaking down
certain foods. Teeth also help this process during mastication,
or chewing, which grinds food down into smaller pieces. The growth and development of teeth requires
its own tutorial, so for now, let’s continue down to the pharynx. Food will move through the oropharynx and
laryngopharynx, just like air does when we breathe, but with the larynx covered by the
epiglottis, the food will then move into the esophagus. This is a muscular tube that joins the stomach
at the cardial orifice within the abdominal cavity. The esophagus wall has the basic structure
that we described earlier, although at this junction with the stomach, stratified squamous
epithelium that is abrasion-resistant will change into simple columnar epithelium, which
is ideal for secretion. In the stomach, food enters through the cardia
and is converted into a paste called chyme. We can also see the dome-shaped fundus, the
body, and longitudinal folds called rugae. Zooming in, we can see gastric pits, which
lead into tubular gastric glands, which is where the gastric juice is produced. This generates a very acidic environment,
with a pH between 1.5 and 3.5, that is necessary for pepsin to do its work, the enzyme that
digests proteins. The stomach needs a mucosal barrier to withstand
these acidic conditions, so that it is not broken down along with the food, and this
is achieved by a lot of mucus sitting on top of tightly joined epithelial cells that are
constantly regenerated by stem cells beneath. The stomach narrows to form the pyloric part,
made of the pyloric antrum and the pyloric canal, which ends at the pyloric sphincter,
and then leads to the duodenum. Chyme travels through here into the small intestine. This is where digestion is completed and almost
all of the absorption occurs. There are three sections, the first of which
is the duodenum, followed by the jejunum and the ileum. The duodenum contains the hepatopancreatic
ampulla, which is where the bile duct, delivering bile from the liver, and the main pancreatic
duct, delivering pancreatic juice from the pancreas, join and merge with the small intestine. The other two sections are longer and hang
in coils, joining the large intestine at the ileocecal valve. The small intestine is perfect for absorption
of nutrients. It is very long to begin with, and the circular
folds, fingerlike villi, and much tinier microvilli amplify the absorptive surface area even more so. Crypt epithelial cells secrete intestinal
juice, which contains mucus and helps with the absorption. The mucosa of the small intestine is also
where we will find Peyer’s patches, the lymphoid nodules we learned about when we
looked at the lymphatic system. At this point we should mention three more
accessory organs which are associated with the small intestine. These are the liver, gallbladder, and pancreas. The liver performs many metabolic and regulatory
tasks, but in a digestive context, its purpose is to produce bile, which enters at the duodenum. This yellow-green substance breaks down fats
for digestion. And the gallbladder is a tiny muscular sac
adjoining the liver that serves mainly to store bile. Looking more closely at the liver, we see
four primary lobes separated by ligaments. Zooming in more closely still, we see smaller
units called liver lobules. These are hexagonal structures made of liver
cells called hepatocytes radiating outward from a central vein, and with a portal triad
at each corner. This name refers to the fact that each triad
consists of a bile duct, as well as an arteriole and a venule, carrying blood to and from the liver. Bile is secreted and flows through bile canaliculi,
towards the bile ducts in the portal triads, eventually leaving the liver through the common
hepatic duct towards the duodenum. Moving on to the pancreas, this is a large
gland attached to the duodenum via the main pancreatic duct, and it produces pancreatic
juice, which contains a wide variety of enzymes that help break down food. We can see clusters of acinar cells surrounding
ducts, this is where enzyme production takes place. And lastly, once food has made it all the
way through the small intestine, almost all of the water and nutrients have been absorbed,
and what remains is more or less indigestible once it enters the large intestine, which
frames the small intestine on three sides. This serves to continue absorbing more water
from these food residues, and also compact them into fecal matter, which is eliminated
from the body through the anus. The large intestine is comprised of three
bands of smooth muscle called the tenia coli, sacs called haustra, and tiny fat-filled pouches
called epiploic appendages. It is also split into regional subdivisions,
these being the cecum, appendix, colon, rectum, and anal canal. The cecum is the first section when entering
from the small intestine. Attached to this is the appendix, which contains
lymphoid tissue. The colon is split up into the ascending colon,
transverse colon, and descending colon, for obvious reasons relating to the direction
of travel during those segments, as well as the sigmoid colon. This then feeds into the rectum, and finally
the anal canal. So that covers the basics regarding the digestive
system, and provides a rough picture of what happens to food from the moment it goes in
your mouth, all the way through to the other side. There is plenty more to be said regarding
the mechanism of digestion. Each type of biomolecule, whether protein,
carbohydrate, fat, or otherwise, will require specific enzymes to be broken down, and these
pathways are worth examining in detail. But that will have to wait for a nutrition
series in the near future. For now, let’s keep going and wrap things
up with a few more systems.
