Human digestive system

                                                                                                                  The human digestive system is very complex

and has evolved over millions of years.

It basically consists of the rectum,

the large intestine, the small intestine, the pancreas,

the stomach, also called gaster and ventriculus, and the liver

with the gallbladder.

The esophagus is also part of this sy

stem,

as well as various salivary glands near the mouth.

First the food is broken up in the mouth by the teeth,

and then mixed with saliva with the help

of the salivary glands.

Saliva contains a digestive enzyme

called amylase that already begins to digest carbohydrates

in the mouth.

It splits carbohydrates into smaller units.

The ball like mixture of food with saliva,

also known as bolus, is pushed into the throat by the tongue

and finally into the esophagus which propels

the bolus to the stomach.

The esophageal lumen, that is the opening

inside the esophagus, is very flexible,

which allows boluses of different sizes

to be transported.

The esophagus consists of several layers.

These layers occur throughout the interdigestive tract.

The two outer muscle layers are responsible for peristalsis.

Through these two muscles the bolus

can be transported from the mouth to the stomach,

even if the person is standing on his head.

The stomach is often divided into six areas.

The stomach is composed of a similar structure

to the esophagus.

It has a longitudinal muscle layer on the outside.

Underneath we can find circular muscle fibers.

In addition to this, there is an oblique muscle layer

overlaying the mucosa.

On the inside there are rugae that

allow the stomach to enlarge when food is consumed.

The stomach wall contains gastric glands.

They produce mucus, which is able to protect the stomach

wall from the secreted gastric acid.

Gastric acid is produced by simply smelling or seeing food,

but also spices, and the stretching of the stomach

causes the secretion.

That is, the release of gastric acid.

About one to two liters of gastric juice

are produced per day.

Since the esophagus does not have a protective mucus layer,

like the stomach, stomach and esophagus

are separated by a sphincter.

It relaxes when a bolus is pushed from the esophagus

into the stomach, and then contracts

to prevent acid and food from going back up.

Gastric juice consists, among other things,

of hydrochloric acid, the enzyme pepsin, the intrinsic factor,

and lipase for the digestion of fats.

In addition to nutrients, food also

contains bacteria that can damage the body.

The components of hydrochloric acid

are able to destroy harmful bacteria.

In addition, hydrochloric acid converts

pepsinogen also released by the gastric glands into pepsin.

Pepsin is able to break down proteins in the stomach.

For a vitamin B12 absorption in the small intestine,

the intrinsic factor is needed, which is

produced by the gastric glands.

The vitamin must combine with intrinsic factor,

then it can be absorbed later by the small intestine.

Vitamin B12 helps keep the body's nerve and blood cells

healthy, and helps make DNA.

It also contains gastric lipase, an acid resistant

enzyme for fat digestion.

In the stomach, gastric lipase splits a triglyceride

into a free fatty acid, and a diglyceride

whereby only the free fatty acid can be absorbed by the body.

More effective fat digestion takes

place in the small intestine.

Through gastric juice and stomach movements,

which take place approximately every 20 seconds,

the individual boluses are mixed to a semi fluid

mass of partly digested food the so-called chyme.

The chyme cannot enter the duodenum at first because there

is a sphincter at the stomach exit.

The pyloric sphincter resembles the esophageal sphincter.

The pyloric sphincter opens only a few millimeters,

so that larger pieces remain inside the stomach.

In the first section of the small intestine,

the duodenum bile, and pancreatic secretions

are mixed with the chyme via the ampulla of vater.

Pancreatic juice contains numerous digestive proenzymes

and enzymes.

In order for these to do their job,

a higher pH value than that in the stomach is necessary.

For this reason pancreatic juice contains

sodium hydrogencarbonate.

Hydrogencarbonate is able to neutralize

the acid in the chyme and thus produce

the optimum pH value of 7 or 8.

Pancreatic juice also contains proenzymes.

It is only through enterokinase released by the duodenum wall

that the proenzyme trypsinogen and becomes

tripsin, which can split proteins

and activate other tripsinogens.

We also find alpha amalyse, which we had already

found in the mouth.

It now does the rest regarding the splitting

of carbohydrates which it converts

into maltose and isomaltose.

Furthermore pancreatic lipase is able to split triglycerides

into two free fatty acids.

The gastric lipase, as we have seen before,

can produce only one free fatty acid.

The pancreatic lipase can cleave triglycerides excellently

because the bile breaks the fats down into tiny droplets.

This is called emulsification.

Numerous other enzymes are part of pancreatic juice,

but these will not be explained in detail here.

Bile is produced by the liver cells

and transported to the gallbladder.

The bile is stored in the gallbladder,

and finally added to the food in the duodenum

via the ampulla of Vater.

Of the pancreas also releases juice via the ampulla of Vater.

The small intestine consists of three sections,

duodenum, jejunum, and ileum.

The ileum continues into the large intestine

in the right lower abdomen.

The duodenum and the jejunum have circular folds

to increase the contact surface with the food.

These circular folds extend about centimeter

into the lumen of the small intestine.

These folds are covered with small finger

like projections called villi.

Villi increase the surface considerably.

Villi I are about one millimeter long.

Each villus contains blood capillaries

and a lymphatic capillary called lacteal,

which we will see later.

The nutrients, marked green here,

are absorbed by the villus and transferred

to the blood capillaries.

Some nutrients, such as glucose, do not require a carrier.

They are transported freely in the bloodstream.

Other nutrients, such as iron, require transport proteins

transferrin.

Fats are transported by chylomicrons,

which are lipoproteins.

The triglycerides to be transported

are virtually enclosed in the lipoprotein.

Chylomicrons and triglycerides are then

transported through the lacteal of the villus.

Each villus is covered by even smaller microvilli.

They multiply the intestinal surface considerably.

The microvilli absorb nutrients and transport them

to the inside.

The last part of the small intestine is the ileum.

It does not have, unlike the duodenum and jejunum,

circular folds.

The ileum absorbs electrolytes, such as calcium for building

bones, hair, and teeth.

trace elements, such as zinc for sperm production,

and the immune system, vitamins such as B12 for the formation

and maturation of red blood cells, and remaining bile acid,

which is transported back to the liver via the bloodstream.

As with the esophagus the food is transported by peristalsis.

In contrast, segmentation contractions

served to mix the chyme, which is shown here

in yellow and red, to make it easier

to see the mixing process.

The large intestine is thicker than the small intestine.

It is about one meter long and surrounds the small intestine.

The small intestine is connected to the large intestine

via the Bauhin's valve.

It opens when chyme is to pass from the small intestine

to the large intestine.

The large intestine does not have

any villi like the small intestine,

because most digestible substances have already

been absorbed in the small intestine.

However the large intestine has an estimated 100 billion

bacteria inside.

They are important for many other tasks,

such as the production of vitamins,

and the decomposition of fiber for the body's energy

production.

Many of these bacteria are an important part

of the immune system by killing harmful germs.

Through peristalsis, the chyme is transported

from the ascending colon to the transverse colon

to the descending colon.

On its way through the large intestine water

is removed from the chyme.

Furthermore, mucus is added for proper excretion of waste.

Substances that cannot be absorbed through the small

intestine or the large intestine remain in the rectum and are

finally excreted through the anus.