..................... THE HUMAN DIGESTIVE SYSTEM AND THE DIGESTION

Activities to do. Read the explanations on the digestive system and perform the first Multiple Choice Test and the Mute Picture. Then read the explanations on the process of digestion and perform the second Multiple Choice Test and the Crossword.

1. The human digestive system. It is the device responsible for ingesting food by degrading it to small molecules capable of enter to the cells, called the nutrients and expelling nondigestible rests (feces).

2 . Parts of the digestive system. The human digestive system is a tube with an inlet opening (mouth) and output (anus), in which different regions can be distinguished (oral cavity, pharynx, esophagus, stomach, small intestine and large intestine) and various annexed glands (salivary glands, liver and pancreas).

1 . The oral cavity. It is the cavity where the food is ingested. It is delimited by the lips, cheeks, hard palate, soft palate (called "velum") and the base of the mouth. Internally it is covered with a moist epithelium called the oral mucosa. Inside of it there are the tongue and the teeth, and the salivary glands terminate within it. The adults have 32 teeth. On the each jaw there are 4 incisors, 2 canines, 4 premolars and 6 molars (for chewing). Between the oral cavity and the pharynx there are tonsils which perform the function of immune defensive barrier. At the end of this section there is a description of the internal structure of the teeth.

2. The salivary glands. There are three pairs of glands that secrete saliva. It is constituted of water, digestive enzymes (ptyalin and amylase) and mucin (a mucosa substance). Thanks to saliva moistening the food, it is easier to swallow, some of the accompanying bacterias are being removed and the digestion of carbohydrates is being initiated.

3. The phaynx. It is a very short conduit (12cm) that runs from the end of the oral cavity to the top of the esophagus. It also connects with the larynx through the glottis, with the nostrils through the choanae (see picture) and the middle ear through the eustachian tubes.

4. The estophagus. It is the conduit between the pharynx and stomach. It has a length of about 25cm. When the food is introduced it originate the annular muscle contraction and relaxation (peristaltic waves) causing the advancement of the bolus.

5. The stomach. It is a sack-shaped organ with the capacity of 2.5 liter and very thick walls due to having three layers of muscle cells. Inside of it, three regions can be distinguished:

• 5.1 Cardia section. Is the one that connects with the esophagus through the sphincter "cardia".
• 5.2 Fundus section. Is the largest one that corresponds to the big curvature.
• 5.3 Pylorus section. Is the one that connects with the duodenum through the sphinctes "pylorus".

6. The liver. It is a large organ located under the right lung which performs several functions. One of them is to secrete biles which is stored in the gallbladder. The presence of food in the duodenum stimulates the secretion of bile by the cystic duct and then by the common bile duct, which empties into the ampulla of Vater, where it goes into the duodenum. The biles are responsible for the emulsion of fats.

7. The pancreas. It is a double gland that has a function of exocrining (secretion to the outside, specifically secretion of the pancreatic digestive juice into the duodenum) and an endocrine function (secretion within the body, ie the blood, particularly the secretion of insulin and glucagon hormones). The pancreatic juice passes through the secretory canals to the central duct, the Wirsung canal, which flows into the ampulla of Vater and thence passes into the duodenum. There also may be another duct which empties into the duodenum that is called the duct of Santorini.

8. The small intestine. It is a tube about 7 meters long and about 2.5 inches in diameter. It can differentiate in three areas called:

• 8.1 Duodenum. Is the first part of the small intestine. It connects with the stomach by a valve called pylorus. It has a length of about 30cm. Within it meet the biles, pancreatic juice and intestinal juice originated from glands that are present within its walls.
• 8.2 Jejunum. Is the middle part of the small intestine and also the largest one. It has many curvatures on itself, called intestinal loops.
• 8.3. Ileum. Is the last part of the small intestine. Connects with the intestine through the ileocecal valve.
  

9. The large intestine. It is the final part of the digestive tube. It is a conduit about 1.7 meters long and about 8 inches in diameter. Inside of it the bacterias abound and are called bacterial flora. In the large intestine three sections can be differentiated, which are:

• 9.1 Cecum. Is the first part of the large intestine. It is a conduit without an end (blind). At the end it has an vermiform appendix (a form of a worm), which if not emptied continuously can become infected and inflamed (appendicitis) and if it is drilled a generalized infection occurs (septicemia) that can cause death.
• 9.2 Colon. Is the second part of the large intestine. It runs from the end of the small intestine, the ileum, with which it connects through the ileocecal valve, down to the rectum. In the large intestine three sectors can be distinguished, called: ascending colon, treansverse colon and descending colon.
• 9.3 Rectum. Is the last part of the large intestine. It ends at the anus.

The teeth have an outer part (crown), an internal part (root) and an intermediate part (neck). The teeth are made of a substance called dentine or ivory (mainly calcium phosphate). The outside part also has a cover of a very hard material called enamel. The root is attached to the jawbone by a substance called cementum.

In adults 32 teeth are distinguished. In each jaw are:

• 4 incisors (for cutting),
• 2 canines (for ripping or tearing),
• 4 premolars (for crushing) and
• 6 molars (for chewing).

The last molars are called the wisdom teeth and appear between 18 and 20 years. The first dentition, the so-called milk teeth, only has 20 teeth and begins to fall from 5 or 6 years.

3. The digestion. It is the process that allows to take the advantage of the nutrients from food. Imcludes the following steps:

1. The ingestion. Is the entrance of the food.
2. The digestion of the food. Is the degradation of food in very small molecules that are able to enter the cells. Can be mechanical, such as crushing made by teeth, or chemical, as the action of the digestive enzymes.
3. The absorption. Is the passage of nutrients from the intestine into the blood and lymph.
4. The defecation. Is the expulsion to the outside of the substances that have not been able to be digested.

1 . THE DIGESTION IN THE MOUTH. In the mouth there are two types of digestion:

1. A mechanical digestion called "mastication" which is performed by the teeth, and
   
2. A chemical digestion performed by saliva through the contact with the food, a process called "salivation".

2. THE SALIVARY GLANDS. There are three pairs of glands called: parotid, submandibular and sublingual. Saliva contains:

1. Water (around 98%)
2. Mucin (mucous substance that facilitates the passage of the food).
3. Ptyalin, the digestive enzyme (carbohydrate enzyme that degrades starch until reaching the maltose molecules)
4. Maltase, the digestive enzyme (enzyme that breaks down maltose into two glucose molecules)

3. THE SWALLOWING. Swallowing is the passage of food from the mouth to the esophagus. It is performed in three stages:

1. Impulse of the bolus to the back of the mouth due to the tongue movement.
2. Entrance of the bolus into the pharynx.
3. Pass of the bolus from the pharynx to the esophagus.

4. THE PERISTALTIC WAVES IN THE ESOPHAGUS. Are the annular muscle contraction and relaxation that facilitate the advancement of the bolus.

5. THE CHEMICAL DIGESTION IN THE STOMACH. The stomach presents an inner layer called gastric mucosa that contains several types of glands specialized in secreting different substances in the gastric juice. These are:

1. Hydrochloric acid (HCl). Degrades the hard tissues of the food, kills many bacterias and transformes the pepsinogen into the pepsin
2. Pepsinogen. A substance that is converted into pepsin enzyme that degrades proteins into amino acids
3. Castle Factor. A substance that allows the vitamin B12 to be absorbed in the intestine.
4. Mucin. A substance that helps the passage of food
5. Sodium bicarbonate. A substance which neutralizes the hydrochloric acid before passing to the duodenum.
   

In the stomach peristaltic waves are being produced in order to move the food.

6. THE ACTION OF BILES IN THE DIGESTION. The biles are produced by the liver cells. If the person is fasting biles accumulate in the gallbladder, but if there is food in the duodenum, bile is released through it. Every day they secrete a 600ml. Apart from the water, the biles contain also the bile acids, cholesterol and lecithin, which are the substances emulsifying fats. Is to say that they perform the same function as detergents to disperse fat in water. So they facilitate chemical digestion and absorption. The biles also contains bilirubin (a yellow substance) and biliverdin (greenish substance) originating in the degradation of hemoglobin. They are responsible for the color of the stool.

7. THE DIGESTION DUE TO THE PANCREATIC JUICE

1. The pancreatic proteases (trypsin and chymotrypsin) degrades the proteins.
2. The pancreatic lipase degrades the lipids
3. The pancreatic amylase degrades carbohydrate starch.

8. THE FORMATION OF THE CHYLE. The doughy mass that leaves the stomach is called chyme. Subsequently, after undergoing through the intestinal digestion in the duodenum, it transforms into a thinner dough called chyle.

9. THE DIGESTION DUE TO THE INTESTINAL JUICE

1. The intestinal peptidases degrades proteins into amino acids.
2. The intestinal lipase degrades lipids.
3. The intestinal disaccharidases degrades disaccharides into glucose and other small carbohydrates.

10. THE INTESTINAL ABSORPTION. In the jejunum the obtained small molecules are absorbed through the intestinal villi.

1. Absorbed from the carbohydrate or protein nature, small molecules, such as glucose and amino acids respectively pass to the capillar venous.
2. Absorbed from the lipid nature, small molecules, such as fatty acids pass to the lymphatic venous

11. THE FORMATION OF THE STOOL. The chyle which passes into the large intestine contains 80% of water, the substances which were not able to digest and debris from the digestive juices. In the large intestine large part of this water is absorbed and, due to the bacterial flora, it is achieved to digest many of the resistant substances. The rest form the called fecal matter and goes out through the anus.

4 . The diseases of the human digestive system. The main ones are:

• Peptic ulcers not caused by bacteria. Erosions of the inner wall of the stomach or duodenum due to excessive acid secretion. Can lead to perforations.
• Hemorrhoids. Dilation of the veins surrounding the anus. They make it difficult to be able to sit down.
• Bladder stones. Deposits of the precipitated cholesterol in the gallbladder.
• Obesity. Excessive increase of the adipose tissue layer which is found in the certain areas of the skin. It may be due to an excess of food or a metabolism disorder.
• Anorexia. Mental impairment consistent in not wanting to eat in order to not be seen as obese, despite actually being very thin. May cause death from malnutrition.
• Bulimia. Psychic disturbance consisting of an intense hunger that involves high intakes of food followed by vomiting due to the sence of guilt.
• Infectious Diseases: Viral: mumps (salivary gland infection) and hepatitis (viral infection of the liver). Bacterial: dental caries, appendicitis (infection of the appendix), peptic ulcers caused by infection (stomach and intestine), salmonellosis (diarrhea), cholera (severe diarrhea and vomiting). Due to the protozoa: amoebic dysentery ( intermittent diarrhea)