Digestive System

Diagram showing the body, mainly showing the digestive system. Click for enlarged version.

Digestion is the breaking down of chemicals in the body to a form that can be absorbed and used. Digestion beings, in mammals, with the saliva in the mouth.

The Mouth

This is the buccal cavity in a human and is where mechanical digestion and mastication (chewing) takes place.
Saliva in the mouth consists of mucin (binds food together), amylase (digests starch to maltose, 2x glucose) and mineral salts (regulates pH around 7, neutral).

Oesophagus

Peristalsis of the circular muscle contracts and relaxes to push food down.
The upper part of the oesophagus is under conscious control until a point when it becomes involuntary.
It usually takes between 4 and 8 seconds for food to travel from mouth to stomach.

Stomach

Acidic conditions (pH1/2, enzyme optimum pH).
Mechanical digestion takes place here (churning of the stomach makes a larger surface area).
While food is in the stomach it mixes with gastric juice by churning.
Food in the stomach stimulates stomach wall to produce gastrin (transported in the blood).
With a fatty meal enterogasterone is produced to slow churning, it also weakens the acidic pH of the stomach.
The gastric juices produced consist of, pepsinogen, made inactive to prevent autolysis and when added to HCl become active and is called pepsin used to convert proteins. Pro-rennin, a part of the gastric juices and when added to HCl forms rennin used to coagulate caseinogen, reducing surface area allowing a chance to digest. Hydrochloric acid (HCl) and mucus are also part of these gastric juices.
Pro-rennin is only found in babies though, used for the digestion of the mothers milk.
Mucus is used for the protection from excess stomach acid and most of all autolysis. It also acts as a lubricant to reduce friction.

Note: [ Chyme - semi-liquid mixed with food ]

Duodenum (early small intestine)

Further digestion takes place.
Input of pancreatic juices.
Lipids broken down into fatty acids.
Protein broken down into amino acids.
Large surface area due to a large number of villi (villus).
The digestion of carbohydrates takes place inside cells.

Liver

Produces bile (stored in gall bladder, transported by the bile duct).
Bile made from bile salts and mineral salts.
Bile salts help with the digestion of fats by breaking them down from large globules to smaller globules, lowering surface tension and making a larger surface area.
Mineral salts neutralise the stomach acid (HCl) to around 7/8 pH in the small intestine.

Pancreas

The production of pancreatic juices takes place in the pancreas, containing amylase, lipase and trypsinogen (actively known as trypsin).
Amylase converts the remaining amylose (starch) into maltose.
Lipase converts lipids into fatty acids and glycerol
Trypsin converts proteins into peptides and amino acids.

Brunner's gland

Located in the wall of the duodenum.
Secretes alkaline juices and mucus.

Ileum

This is the longest part of the small intestine.
Secretes endopeptidases and exopeptidases.
Endopeptidases breaks proteins into small polypeptides by breaking the bonds in the middle.
Exopeptidases also breaks down protein but by taking single amino acids of the end. Both work together to break down the proteins into small amino acids.
Sucrose is converted into fructose and glucose by sucrase.
Maltose is also converted into two glucose by maltase.

Small Intestine in General

There are three types of movement across small intestine, diffusion, facilitated diffusion and active transport.
- Diffusion (or osmosis) is the movement of water.
- Facilitated diffusion moves some glucose and amino acids.
- Active transport moves the rest of the glucose and amino acids but also mineral ions.
Once absorbed from the small intestine and into the capillaries, the products travel through the venules and the hepatic portal vein into the liver. Here in the liver toxins like alcohol, drugs, spent hormones and other toxins get converted into a non-toxic form. When they're safe and non-toxic they can be transported to the kidney in the form of urea.
The absorption of lipids don't enter the capillaries instead entering the lymphatic system and then via the thoracic duct into the blood system.
Products from digestion do one of three things, they either get stored (assimilation), used or converted (deamination). An example of assimilation is excess glucose being stored in the liver as glycogen. In the case of deamination an example could be an excess of amino acids, they then get converted into urea by the removal of the amino group (NH₂) which combines with CO₂ to form CONH₂ (urea). The remaining organic molecule can be broken down and respired.

Large Intestine

The large intestine absorbs water (by osmosis), minerals and vitamins.
Chyme in the large intestine consists of water, bile, mucus, dead cells, bacteria and undigested food.
The large intestine has a larger lumen, is smaller in length and has a thinner wall than the small intestine.
Faeces is stored in the rectum and then egested by the anus (sphincter muscle).
Diarrhea causes dehydration because of the lack of absorption. Pernicious Anemia is another disease and is caused by not absorbing enough vitamin B and can be a side effect of diarrhea.

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