The digestive tube is like a long road that begins in the mouth and ends in the anus. It transforms the food we eat in:
A. Energy for the cells;
B. Raw material to produce elements that compose the body.

Food is divided in three major groups: carbohydrates, proteins, and fats.

Carbohydrates are constituted of basic units called monosaccharides, which combine themselves to form different carbohydrates.

Proteins are the combination of amino acids (basic units of protein). The variations of amino acid groups produce different proteins.

Fats are constituted of fatty acids and glycerol (basic units of fat). Different combinations of these elements produce different fat.

When we eat steak, for instance, we ingest protein from bovine meat, which is different from human protein. So the digestive tube transforms the bovine protein into amino acids and also promotes its absorption into the blood. Amino acids are stored in the liver where they are recombined to produce proper protein for the human muscle. The same happens to the carbohydrates and fats. The process of deconstructing, absorbing and reconstructing is called digestion.

So the food runs through a "road" called Digestive Tube and it is pushed down through wavelike muscular contractions called peristalsis. The road has many "stops", and each has peculiar functions.

Mouth
Solid food particles are broken up by the teeth and then mixed with saliva, which contains enzymes (substances capable of provoking chemical reactions that transform other substances). One of these salivary enzymes is called ptyalin that initiates the digestion of carbohydrates. The salivary production moisturizes the food and makes the swallowing process easier.

Esophagus
It is a tube that produces lubricant mucous and leads the food from mouth to stomach. It does not have a digestive function.

Stomach
It is a sack-like organ where food is stored for approximately 2 hours. The stomach produces hydrochloric acid and pepsin (this mixture is called gastric juice). The acid activates the pepsin and also sterilizes the ingested food. The enzyme pepsin initiates protein digestion. The organ produces 2 liters of gastric acid per day.

Stomach has two flap valves: esophageal sphincter and pylorus. The esophageal sphincter allows the food to go down into the stomach and also keeps the food from washing back into the esophagus. The pylorus separates stomach from intestines (it only opens two hours after a meal). Thus, the food stays between the two valves for 2 hours after each meal.

The stomach hydrochloric acid is very strong, pH around 3, and it would probably dissolve the stomach walls, if were not for some internal "carpeting" that protects it.

During those two hours, the stomach squeezes itself to mix food, saliva, hydrochloric acid and pepsin, like a washing machine. Proteins and carbohydrates are partially digested in the stomach, however fats are still intact at this point. After this period, pylorus opens and all that mixture is released to the duodenum.

Food fibers stretch the stomach walls and stimulate the volume receptors of stomach, that in turn, through hypothalamic stimuli, produce a sensation of fullness and make the person stops eating. It is the first warning that digestive tube sends to the brain to stop food ingestion. That's why it is so important to ingest fibers. People who only eat carbohydrates do not have the fullness sensation and therefore do not stop eating.

The gastric antrum (final part of stomach next to pylorus) produces the intrinsic factor, a protein that binds with vitamin B12, protecting it from enzymatic action of the digestive tube, keeping the vitamin intact to be absorbed by small intestine (terminal ileum). Vitamin B12 (like folic acid) is very important for the cell development. Low levels of vitamin B12 cause anemia.

The major part of the iron we ingest is in the ferrous form (positive bivalence), which is not absorbed by the digestive tube. The stomach hydrochloric acid transforms it into the ferric form (positive trivalence), that is well absorbed. Drugs used to inhibit the acid secretion ends up inhibiting iron absorption. The lack of iron absorption causes severe anemia (reduction of red blood cells).

The stomach also produces ghrelin hormone. Nowadays, it is the only known hormone that is classified as an orexigenic, i.e., increases or stimulates the appetite. It is like if the stomach asked for the brain to be hungry. Another function of this hormone is the diminishing of basal metabolism, which causes laziness and sleepiness. Thus, the hormone generates energetic gain; therefore the individual eats more and burns less calories.

Small Intestine
It is a long tunnel measuring from 3 to 10 meters and produces 2 liters of secretion (water and enzymes). The small intestine initial half is called jejunum and its distal half is called ileum. The ingested compounds get in touch with the inner surface walls (like carpeting) of the small intestine called mucosa or mucous membrane.

That carpeting produces 3 kinds of enzymes, which digest the principal types of carbohydrates such as sucrase, maltase and lactase. The enzymes transform the sugars in monosaccharides (basic units of carbohydrates) that are absorbed by active transport (with expenditure of metabolic energy) into the blood.

The majority of carbohydrates we eat are constituted of a monosaccharide called glucose (80%). The other monosaccharides are fructose and galactose that are quickly converted in glucose. Thus, 100% of the monosaccharides transported in the blood stream are glucose. When the glucose passes through the liver, it is stored for future use in the form of glycogen. Glycogen is released into the blood as needed by the body to maintain the glucose level (between 70 and 100mg%). In turn, glycogen is transformed in glucose by the hormone glucagon, produced by the pancreas. This way, glucose is released into the blood stream. Such molecule is the main energetic source for the cell. The glucose must go from the blood to inside the cells.

The insulin hormone, also produced in the pancreas, is responsible for transporting glucose by facilitated diffusion of the blood to inside the cells. The poor production of insulin results in a disease called diabetes, so a diabetic person produces little insulin and much glucose in the blood, however little glucose inside the cells.

The small intestine mucosa also produces peptidase, an enzyme that digests proteins and also transforms them into basic units of proteins, the amino acids. The ingested proteins are first digested in the stomach by pepsin and then by the intestinal peptidases, being reduced into amino acids. Those are absorbed from the intestine into the blood and then getting into the liver. Liver utilizes these basic units to create the proteins needed by the body, as muscle fibers, hormones and so on.

The mucosa also produces small amount of lipases that helps digest fat already broken down by biliary salts in fatty acids and glycerol (basic units of fat). Actually, the ingested fat from food is broken down in smaller pieces by the biliary salts in the duodenum. The pancreatic lipase, by hydrolysis, breaks apart fat in basic units, i.e. fatty acid and cholesterol. Once again, the bile salts combine the two types of basic fat units to transport them from intestine into the blood and then to the liver. This is called enterohepatic cycle of the bile salts, that is, the biliary salts are produced in the liver, released to the intestine, where they help to break down the fats so that the lipases can digest them (transform them into basic units) and then transport those into the blood. After that, the bile salts return to the liver to be reused, and once again released into the duodenum.

Every ingested food is transformed in the duodenum and small intestine into basic units: monosaccharides, amino acids, fatty acids and glycerol. These are therefore absorbed from the intestine into blood that runs through microscopic vessels, which flows towards the superior mesenteric vessel, which, in turn, empties into the portal vein. The portal vein leads all basic units to the liver.

Liver is a great body laboratory that utilizes all the basic units to compound the complex substances needed to nourish the human body. Is like Lego Toy, considering that intestine disassembles the nutrients, like Lego simplest pieces, and the liver assembles those pieces together in other shapes (proteins, sugars, cholesterols and so on). So, whenever someone is said to have high bad cholesterol levels, it is wrong to blame the food, because it is the liver that misused the glycerol basic units to produce too much of the bad cholesterol (LDL).

The total amount of food is digested and absorbed in the duodenum and small intestine. Protein and carbohydrate can be digested and assimilated completely in the small intestine, autonomously from liver and pancreas. For instance, if a patient hypothetically had some sort of disease whose solution treatment were the complete removal of stomach, liver and pancreas, he/she would still eat and properly absorb proteins and carbohydrates. It is estimated that a small intestine with 3 meters long is capable of digesting and absorbing all the necessary protein and carbohydrate. However, fat is not well absorbed in the small intestine, because it depends on bile salts that come from the liver.

The end of small intestine (terminal ileum) absorbs many liposoluble vitamins, such as vitamins A, D, E, K, and also vitamin B12, which is pushed by biliary salts. When nutrients and fibers get into the terminal ileum, they stimulate chemical receptors that induces this part of intestine to produce hormones that have two functions:

• Warn that there is too much food in the digestive tube and for that reason the body should stop eating immediately. To do so, there are some hormones with different mechanisms:
o PYY hormone
The major human appetite suppressor, produced in the terminal ileum and colon. It stimulates the hypothalamus, causing the fullness sensation, which makes the body rejects more food. The hormone produces anti-peristaltic response to excite nausea. It also promotes the pyloric valve constriction, provoking nausea. These actions together are called ileal brake;

o Other hormones
GLP1 and Oxintomodulina.

• Warn pancreas that the body had a meal and the sugar level in the blood will increase soon, and insulin must be produced to move the glucose inside the cells. This insulin production stimulus is called incretin effect and the two hormones are called incretins, as follows:

o GLP1
Produced in the terminal ileum and colon. It stimulates pancreatic beta cells to produce insulin. This hormone is so powerful that it reactivates lazy cells. The hormone inhibits alpha cell, the producer of glucagon, which increases glycemia. It is an anorexigen and also ileal brake, which betters diabetes type 2 better, however it does not improve diabetes type 1, since beta cells
have already failed;

o GIP
Same effect as GLP1. It is mostly produced in the duodenum and jejunum.

Large Intestine
It is a 2 meter long tube that connects small intestine to anus. Similarly to the esophagus, it does not digest or absorb food. This body organ absorbs almost the whole amount of water that gets in the colon (around 10 liters per day). It produces mucous (jelly-like substance) in order to facilitate the anal evacuation of waste (undigested) food.