The esophagus is the closed pipe at the back of your throat.

It contains two layers of muscles.

• circular - these are like your fingers around a nail or chisel

• Longitudinal - these run down the length of the esophagus from top to bottom

Together they push the bolus down to the stomach

As the bolus enters, it relaxes, then as the bolus passes the circular muscles contract above it squeezing the food downwards. This reminds me of a piping bag, putting the decorations on a cake.

This pushing of food is called peristalsis

Because of the closed pipe, the muscles of the esophagus doing peristalsis and the closed esophageal sphincter - you can eat in space.

Furthermore, you can do a handstand without the contents of your stomach spilling onto your face

In fact, you can eat while doing a handstand

Sometimes the Esophageal sphincter does not close properly

This can lead to heartburn

You can also get heartburn if you go onto the trampoline straight after having all you can eat for dinner. Or do a handstand after eating a KFC Family pack all by yourself. In these cases its not a problem

If a person gets heartburn regularly then it is a problem

In this case it is called Gastroesophageal Reflux Disease - GERD

GERD is a problem because the stomach acid burns the esophagus, this causes the cells to change. This change is not for the better. There is an increased risk of cancer

It is important that someone with GERD manages it with anti-acids and has a consult with their doctor

Anti-acids work in two ways

1. They slightly neutralize the stomach acid

2. They form a layer on the top of the stomach acid, like the oil layer in your salad dressing, this makes it harder for the acid to re-enter the inferior esophageal sphincter

The stomach is a muscular bag in which chemical digestion occurs with some mechanical assistance

When you eat the stomach expands to hold the food. It can hold as much food as a soft-drink bottle (2 Liters)

The stomach has 3 layers of muscle that contract in 3 different directions, thus allowing the churning and mixing of the chyme

This mixing action of the muscles is important to churn the bolus, allowing acid and pepsin to mix with it thoroughly, creating chyme.

In other words, the food was bitten then chewed, saliva was mixed in and it became a bolus. In the stomach the muscles churn it and mix in acid and pepsin and it becomes chyme

The pyloric sphincter is the gate-way between the stomach and the small intestine. It only allows small amounts of chyme into the small intestine at a time.

The stomach has a layer of mucus, this mucus stops the acid and the enzymes from dissolving the walls of the stomach

Almost all of the bacteria that you eat will die in the stomach

The acid and the digestive enzymes rip them apart, and they become part of your food

Some bacteria survive, these ones are the ones that will make you sick they are the pathogenic food poisoning bacteria

They include: Salmonella, E.coli, Listeria, Campylobacter and Colera

This happens due to Evolution - they have evolved to briefly tolerate the low pH of the stomach

Salmonella is a good example. If Salmonella is left in the stomach for long enough and it will die. Salmonella is only briefly resistant to low pH.

So Salmonella doesn't stay in the stomach. It doesn't reside here.

If you eat some raw eggs (cause your bulking up) and they contain salmonella, it won't be a few bacterium, it'll be in the thousands if not millions. Most of these will be killed as they sit in the stomach acid. However a few will be hidden deep enough in the egg, and will get close enough to the pyloric sphincter quickly enough that they can get through. Once they are in the small intestine they start feeding and multiplying and you'll get sick

There is a pathogenic bacteria that has evolved to live in teh stomach. This is Helicobacter pylori buries itself into the mucus and consumes the mucus, as it does this it exposes the stomach to the acid. The acid can eventually burn through the wall, causing a Stomach Ulcer. This is painful. It bleeds and it requires medical intervention.

Helicobacter plyori can be caught by sharing cups and utensils as well as people preparing food not washing their hands thoroughly

Usually Pepsin doesn't have time to cut it all the way down to the thousands of amino acids that make up the protein. So, it will just have time to cut it into smaller strings of amino acids

These smaller strings have tens of amino acids on them and are called polypeptide (many peptides)

Some of the strings will be cut even smaller and will just be peptides (two or more amino acids stuck together)

Some of the stings will bet a chance to be cut into amino acids before finally the chyme is passed through the pyloric sphincter.

Pepsin will only works in the Low pH of the stomach.

Pepsin has an optimal pH of around 2.

As pepsin moves with the chyme into the small intestine it is exposed to a higher pH and it denatures and stops working.

This keeps the intestines safe from pepsin.

Pepsin - made by Gastric Chief Cells in the wall of the stomach. It is made in an inactive form called pepsinogen that is only activated once it is in the acid of the stomach - this keeps the cells safe from pepsin