Story of the Human Digestive System


Hello, guys let me tell you a story about
a friend of mine, a human’s digestive system. Let’s now imagine going to McDonald’s and
getting a cheeseburger. I know you are so excited to eat that cheeseburger,
but do not worry you will be eating the burger very soon. However, before you do eat, do
not forget to wash your hands! To prevent yourself from diarrhea, please
wash your hands!!! By the first bite of that cheeseburger, the mission of my friend will
be started!!! The mouth is the first station; our teeth
and tongue are the soldiers that move food around and break it apart by chewing. From this station, we enter this delicious
cheeseburger into our valuable friend. Here, our friend has three types of glands
that help the system to digest it. Their names are salivary glands, these glands
make saliva. The saliva moistens our cheeseburger and begins
its chemical breakdown. The final bite which is named “Bolus”.
According to the content of the cheeseburger, our PH of the mouth will be changed from neutral
7.0 to the more acidic or basic environment. Both of them can demineralize our teeth, which
is resulted in tooth decay. The safe limit for this PH of the mouth is
6.2-7.6. So the mouth’s PH level can never drop below 6.3. The cheeseburger breaks down to small bits
as well as undergoes some chemical changes “hydrolysis” via salivary amylase and due to mucinous secretions of salivary
glands, the bolus can be swallowed very smoothly through the pharynx or the throat. Our oral cavity has three main pairs of salivary
glands: “the parotid glands, submandibular glands, and the sublingual glands”. The mouth amylase helps us to detect the taste
of sweet foods. In the throat, there is a cartilage flap that
is named epiglottis. Epiglottis blocks the entrance of the trachea
and prevents the entrance of food particles to the lung. Bolus smoothly slides over the epiglottis
and enters the esophagus. Now the particles of the cheeseburger are
on the way to the stomach. When the bolus is swallowed, it enters down
in the esophagus and reaches to the next destination: the stomach. In the esophagus, bolus propelled down by
a specific type of transport named peristalsis which is a unique type of sequential contraction
and relaxation of a tube-shaped muscle. At the end of the esophagus, our cheeseburger
particles reach the stomach. The volume of the stomach is so huge, as it
can contain about four liters of water. The stomach is responsible for three functions,
Accepting the bolus, Churning the bolus, and Hydrolysis or chemical digestion which is
enzyme assisted degradation of the bolus. The final product of the stomach’s action
is a fluid-like material named chyme. The chyme includes food particles, gastric
juice, and gastric enzymes and its PH ranging from 1.5 to 3.5. Why so acidic!!! The acidity of the chyme is due to HCL that
is secreted by parietal cells of the stomach. However, the stomach has other cells name
chief cells that secrete pepsinogen. Pepsinogen, by the act of HCL, is converted
to the pepsin and pepsin hydrolyzes the proteins and makes some dipeptides and tripeptides
in the stomach. The third type of stomach cells is mucus cells
that produce mucin. Mucin makes a thick protective layer over
the gastric lining and protects the stomach itself from the acidic corrosive effect of
the HCL. If the mucinous cells of the stomach do not
produce enough mucus to protect the stomach lining, the acidic content of the stomach may damage
the stomach and leads to peptic ulcers. The other function of the stomach is storing
of the chyme and squeezing it to the duodenum at the appropriate time. Normally it takes around 2 hours to digest
chemically and physically, the chyme in the stomach. At the end of the stomach, there is a sphincter
called pylorus that prevents the backward return of intestinal contents to the stomach. This sphincter also protects duodenum from
entering the acidic contents of the stomach to the intestine in an inappropriate time. After enough proteolysis of proteins in the
stomach, the chyme is allowed to enter the duodenum which is the first part of the small
intestine. Small Intestine has three parts, duodenum
is the most important for digestion, jejunum the most important part for absorption
of nutrients, and the ileum the main site for absorption of vitamins such as Vitamin
B12, A, D, C, and vitamin K. Let’s talk about some accessory but very
important organs of the GI system, Live, Panaceas, and Bile system. The liver is a bulky organ that sits on the
right side of the stomach. The liver is responsible for the main metabolic
actions of the body; it stores fats and carbohydrates in the form
of triglycerides lipoproteins and glycogen. Proteins are not stored in the liver but they
are taken from the blood in the liver catabolized to amino acids and use these amino acids to make new proteins
and release new proteins into the bloodstream. While its most commonly known function is
to filter toxins out of the body, it has 7 major functions. The other function of the liver is detoxification
of the chemicals such as drugs using a specific type of enzymes named Cytochrome P450 enzymes. The last function of the liver cell is bile
production. The produced bile transfers through the common
hepatic duct to the gall bladder, store there and used for the breakdown of the fat content
of the food. Some bacteria and also some foreign materials
are also removing from the bloodstream by the specialized hepatic cells. Bile consists of some pigments and some salts. Bile pigments have no crucial effect but bile
salts have a crucial effect in fat metabolism through the emulsification of the large fat globules
and making small droplets of fat, easier-to-digest molecules. The liver is vulnerable to some types of disease
problems. Toxins, alcohol, viruses are the most important
factors that can damage hepatic cells and may lead to liver dysfunction. Some liver disease may be inherited genetically
and makes life troublesome or impossible. Obesity is one of the conflicts of people
that can damage the liver and make fatty liver disease. Cirrhosis, Hepatitis A, B, and C, are caused
by damaging the hepatic cells and should be considered important health problems. The liver makes multiple proteins and hormones
for our body function and failure of the liver could be lethal. The liver has a unique capacity to regenerate
small pieces of the liver after implantation which can work as a mature liver. Sometimes, surgeons remove more than two-thirds
of the liver and still patients will survive without major problems. The liver has three different types of blood
circulation. Portal Vein circulation that gathers the blood
from the gastrointestinal tract and directs them to the liver, this blood so full of nutrients. Hepatic Artery circulation that brings oxygenated
blood from the lung to the hepatic cells for their function. Hepatic Vein circulation that transfers deoxygenated
blood from the liver to the lung. The gallbladder is a small organ beneath the
liver that stores the produced bile by the liver and squeezed and evacuated bile into the duodenum
in response to entering fat droplets into the duodenum. This process is mediated by the secretion
of the cole systo kinin cholecystokinin (CCK) that squeezes the gall bladder. Bile in the duodenum reacts with fat droplets
and makes them easier to metabolize through the fat emulsification. Of course, without the gall bladder, we can
survive because bile directly enters the duodenum freshly. The pancreas is the most important and dangerous
part of the digestive system and sometimes surgeons called the pancreas the belly lion
because of pancreas secrets very powerful enzymes that can be very dangerous to the
human body if they splash in the body cavity. The pancreas has two different parts exocrine
and endocrine. The exocrine part produces some enzymes and
salt including. Bicarbonate that used to neutralize stomach
juice, Amylase for the breakdown of carbohydrates and starch, Lipase for the breakdown of lipids
and triglycerides, Proteolytic enzymes for the breakdown of some proteins. The endocrine part of the pancreas is responsible
for producing some of the most important hormones of the body through their eyelet islet cells. The pancreas has three types of islet cells.
Alpha cells produce glucagon. This hormone converts glycogen to glucose molecules. Beta cells produce insulin. This hormone produces glycogen from glucose
molecules and stores glucose molecules inside the cells and prevents glucose accumulation
in the blood (Diabetes). Delta cells produce somatostatin. This hormone is a unique hormone that stops
the action of other active hormones such as insulin, glucagon, and CCK. Exocrine part of the pancreas also produces
trypsinogen and chymotrypsinogen that are inactive forms of proteolytic enzymes and converted to the trypsin and chymotrypsin
in the intestine through the endopeptidase enzyme. The trypsin and chymotrypsin are the active
enzymes that degrade the proteins. The small intestine is the longest part of
the GI tract with about 20 feet in length. In the first part, the duodenum is the busiest
section. Chyme and stomach juice enter here, liver
secretions and gall bladder bile and pancreatic enzymes all release in this area, and most important brush border enzymes that
are very important in chemical digestion and nutrients are produced in the vili of this
part of the small intestine. The microvilli, are tiny little hair-like
projections covering the lining of the small intestine and have crucial effects in the digestion of the
chyme and absorption of the nutrients and minerals. The second part of the small intestine is
jejunum. About 90% of all nutrients and minerals are absorbed here. The last part of the small intestine is ileum.
Most vitamins absorbed from this part of the intestine. The PH of the intestine contents is approximately
6 in the duodenum but reaches to 7.4 in the terminal ileum. As previously mentioned, most of the digestive
enzymes in the small intestine are produced by the pancreas and secreted to the small
intestine via the pancreatic duct. Bile enters the small intestine in response
to the hormone cholecystokinin, which is secreted in the presence of fatty nutrients in the
duodenum. Among the different nutrients in our food,
there are four main macromolecules. Proteins, Carbohydrates, Nucleotides, and Fats. Proteins converted to amino acids by brush
border enzymes mediated action of trypsin and chymotrypsin. Carbohydrates converted to monosaccharides
and glucose through the action of pancreatic amylase and brush border lactase. Glucose and other monosaccharides are absorbed
into the bloodstream and delivered to the liver via the portal vein. Nucleic acids such as DNA and RNA molecules
are digested here to their elements such as phosphate group, ribose or deoxyribose,
and some bases (adenine, guanine…) through the action of deoxyribonuclease, and ribonuclease
secreted by the pancreas. Fat contents of the food affected by the bile
and pancreatic lipase and breakdown to the free fatty acids, monoglycerides, diglycerides,
and glycerol. The route of absorption of fat droplets is
through the lymphatic vesicles. After the ileum, the intestinal contents pour
into the large intestine. The length of the large intestine is about
5 feet and its diameter is 3 inches. In the large bowl majority of water content
and the remaining nutrients are absorbed and the remaining material is stored for removing
from the body by defecation. So this waste material is commonly named as
“feces”, essentially consists of undigested and non-absorbable materials. There are some bacteria in the large intestine
that act in the body to produce some important substances, such as vitamin K. These bacteria are a part of the normal coexistence
of our body system. Without them, we will develop some debilitating
disease. In the large intestine, the net movement of
water across cell membranes always occurs through osmosis. At the end of the large intestine, there is
anus that controls voluntarily the time of passage of the stool.

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