Our digestive system is a combination of

mechanical and chemical actions.

Imagine, putting your food in a petri dish,

chopping it up, and exposing it

to a bunch of chemicals and microbes.

Can you imagine what it would look like in the end?

This is what the digestive system does.

MOUTH

The journey down the alimentary canal

begins in the mouth.

Here, the food is broken down

into smaller chewable pieces.

Chewing, breaks the food into pieces,

while the saliva mixes with food

to begin the process of breaking it down

into a form your body can absorb and use.

Rolling action of the tongue

and secretion of saliva

rolls food into a bolus.

The saliva contains water,

electrolytes,

antibacterial components,

and enzymes such as amylase.

Amylase converts carbohydrates into sugars.

THROAT

The throat is the region where the mouth cavity

and the nasal passages join.

Swallowing pushes the food

through the throat or pharynx

and into the esophagus.

An important function of the throat

is that it prevents the food

from entering into the trachea,

more commonly known as the windpipe.

When the food enters our throat,

the larynx or our voice box closes.

This results in epiglottis covering

the entrance of the trachea or windpipe.

The epiglottis is a flap of tissue.

ESOPHAGUS

Now that the food has reached the esophagus,

a wave of smooth muscle contractions occurs,

pushing the food into the stomach.

These smooth muscle movements

are called peristalsis.

The importance of sphincter muscles.

Three types of sphincter muscles

help in the digestive system.

Here, at the junction of the esophagus

and the stomach

is a thick ring of circular-smooth-muscle,

that prevents the movement of food

pass from esophagus into the stomach.

It is called the esophageal sphincter.

Another sphincter muscle, the pyloric sphincter,

directs the passage of food

from the stomach into the intestine.

The third sphincter muscle surrounds the anus.

STOMACH

Most of us eat our food in a matter of minutes,

but digesting it can take hours.

One of the important functions of our stomach

is to store food until it is digested.

Food can be stored here for 2-6 hours.

It also kills the microorganisms

we consume unconsciously along with our food,

and begins the digestion of the proteins

we took in our diet.

The stomach secretes gastric juice,

hydrochloric acid, water, mucus, pepsin, and renin,

that continue the process of breaking down the food.

Pepsin is secreted as pepsinogen

by cells in the gastric glands

that are present in the deep folds

of the stomach lining.

Other cells in the gastric glands

produce hydrochloric acid,

which has a pH balance between 1 and 3.

The low pH helps convert pepsinogen to pepsin

and is also the right pH

for pepsin enzymatic action.

Hydrochloric acid or HCl

also helps break the bonds

holding the ingested contents together.

The breakdown of these food contents

exposes more surface area

to the action of pepsin,

and later to the other digestive enzymes

in the small intestine.

Mucus secreted by the stomach

lines the walls of the stomach and protects them

from being digested by HCl and pepsin.

If this coating is eroded

at some place of the stomach,

for instance by the attack of

bacteria Helicobacter pylori,

it can cause an ulcer.

Contractions of the smooth muscles

in the walls of the stomach, roll around its contents,

mixing partly digested food with enzymes and acids.

This acidic fluid mixture of gastric juice

is called chyme.

Peristaltic movements of the stomach walls

push chyme towards the end of the stomach.

These waves of peristalsis

cause the pyloric sphincter

to relax briefly,

so that very little amount of chyme

can enter the small intestine.

In this way, our stomach empties itself gradually

over a period of almost four hours.

The small intestine works

on a small amount of food at a time.

We'll continue to explain the small intestine,

pancreas, liver, gall bladder,

down to colon and rectum in the next video.

Actually, we were told to make

this title into two parts,

so we can publish more.

It's a cheat, really.

But a new project is a kind of refreshing

to our animators and designers,

giving them some false sense of

novelty and freedom.

So, at least we've got that going for us,

which is nice.