THE DIGESTIVE SYSTEM

• Food is carried from the mouth to the stomach by the oesophagus (also esophagus). This tube descends from the mouth, and through an opening in the diaphragm (a dome shaped muscle that separates the lungs and heart from the abdomen and assists in breathing), connecting with the stomach.

• Immediately after passing through the diaphragm's opening, the esophagus empties into the stomach where the acids that break down the food are produced. From the stomach, the food flows directly into the first part of the small intestine, called the duodenum. It is here in the duodenum that bile and pancreatic fluids enter the digestive system.

Structure: organs

Mastication

• Mastication or chewing is the process by which food is crushed and ground by teeth. It is the first step of digestion and it increases the surface area of foods to allow more efficient break down by enzymes. During the mastication process, the food is positioned between the teeth for grinding by the cheek and tongue. As chewing continues, the food is made softer and warmer, and the enzymes in saliva begin to break down carbohydrates in the food. After chewing, the food (now called a bolus) is swallowed. It enters the esophagus and via peristalsis continues on to the stomach, where the next step of digestion takes place.

Insalivation• Insalivation is the mixture of the nourishments (aliments)

crushed by the teeth with the saliva that moistens, lubricates, and begins their chemical treatment.

• After mastication and insalivation, aliments crushed by the teeth form a mass called food bolus.

• The saliva also has functions: -It eases the passage of the food bolus to the pharynx and

the esophagus.

-It begins the digestion of the starch molecules in many aliments (bread, potatoes, rice…),transforming them into simpler sugars. This is due to amylase (a digestive enzyme present in saliva).

-It Destroys some of the bacteria existing in the food.

Swallowing

• Is the process that makes something (e.g. the food bolus) pass from the mouth, to the pharynx, and into the esophagus, while shutting the epiglottis. If the latter fails, and the object goes through the trachea, then choking or pulmonary aspiration can occur.

In the stomach • Food enters the stomach through the cardiac sphincter. In

the stomach, the food is further broken down and mixed thoroughly with gastric juice (gastric acid, pepsin and other digestive enzymes to break down proteins).

• The enzymes in the stomach have an acid optimum pH. This means that they work better at a specific acid pH.

• The gastric acid itself does not break down food molecules. It rather provides an optimum pH for the reaction of the pepsin enzyme, and kills many microorganisms that are ingested with food.

• The gastric acid can also denature proteins.

• Other small molecules such as alcohol are absorbed in the stomach, passing through the membrane of the stomach directly entering the circulatory system.

• Food in the stomach is in semi-liquid form, which upon completion is known as chyme. After food consumption, the digestive "tonic" and peristaltic contractions begin. This helps to break down the food and move it through.

• When the chyme reaches pylorus (the sphincter connecting the stomach to the duodenum) contractions "squirt" the food back into the stomach through a process called retropulsion, which exerts additional force and further grinds down food into smaller particles.

• Gastric emptying is the release of food from the stomach into the duodenum. This process is tightly controlled where liquids are emptied much quicker than solids.

• Gastric emptying has attracted medical interest as rapid gastric emptying is related to obesity, and delayed gastric emptying syndrome is associated with diabetes mellitus, aging, and gastroesophageal reflux.

The Liver and the Pancreas • The liver (located under the ribcage in the right upper part of the abdomen),

the gallbladder (hidden just below the liver), and the pancreas (located beneath the stomach) are gland organs. These organs are not part of the alimentary canal, but they are essential to digestion.

• The liver also plays a major role in the handling and processing of nutrients, which are carried to the liver in the blood from the small intestine.

• The liver produces bile, which helps the body absorb fats. Bile is stored in the gallbladder until it is needed.

• The pancreas produces enzymes that help digest proteins, fats, and carbohydrates. It also synthesises sodium bicarbonate that neutralizes stomach acid. These enzymes, along with the bile, travel through channels (called ducts) directly into the small intestine, where they help break down food. It is in the duodenum that the bile and the pancreatic fluids enter the digestive system.

• The pancreas is an integral part of the digestive system. The flow of the digestive system is often altered during the surgical treatment of pancreatic cancer. Therefore, it is helpful to review the normal flow of food before reading about surgical treatment.

The liver

The pancreas

Nutrient absorption

The nutrients in digestion must come to the cells, thus passing through the intestine into the blood. This process is called absorption.

Microvilli are a large number of folds that line the inner surface of the small intestine. This increases the surface and efficiency of absorption.

The products of fats digestion are collected by lymphatic capillaries.

Small intestine

• In vertebrates, the small intestine is the part of the gastrointestinal tract (gut) following the stomach and followed by the large intestine, and is where the vast majority of digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to describe the entire intestine.

• The primary function of the small intestine is the absorption of nutrients found in food.

Small intestine

The microvilli

Large Intestine

• -Caecum (also cecum): It contains the appendix.-Colon: It has 3 segments (ascending, transverse and descending) in which symbiotic bacteria grow and generate vitamins.-Rectum: Flows into the anus.

• When the absorption of nutrients is finished, the remains go to the large intestine that doesn’t have villi.In the large intestine water is absorbed and faeces are constructed.The food waste is expelled through the anus, where in addition to food waste, it contains cells of the intestinal mucosa and bacteria.

The large intestine

Digestive System: Diseases

• Some diseases can be of infectious or noninfectious origin:

• -Oral cavity: • Cavities: caused by bacteria that live in the mouth and break down food scraps that fall between the teeth (mainly sugars).

Cavities

• Gingivitis: painful inflammation of the gums that are red and bleed.

• -Stomach: • Gastritis: due to irritation of the gastric mucosa which often causes inflammation.

• Ulcer: injury caused by the destruction of the gastric mucose. Caused by the bacteria Helicobacter pilori whose optimum pH is quite acid.

• -Liver: •Hepatitis: inflammation of the liver caused by a virus, abuse of alcohol or medications.

• Cirrhosis: A chronic disease of the liver characterized by the replacement of normal tissue with fibrous tissue and the loss of functional liver cells.

• Gallstones: solid particles formed in the gallbladder by cholesterol and mineral salts found in bile.

• - Intestine: • Intestinal infections: infections caused by bacteria from water or food.

• Salmonellosis: infectious disease caused by the bacteria Salmonella tiphy present in spoiled food.

• Appendicitis: inflammation of the appendix which causes intense pain, caused by the retention of waste in it’s interior. This enhances the action of bacteria.

• Constipation: difficulty passing stools due to low mobility of the large intestine. The remains of digestion stay in the colon for a long time where a high absorption of water hardens the stool.• Diarrhoea (also diarrhea): an intestinal disorder characterized by abnormal frequency and fluidity of fecal evacuations.

Digestive System: Healthy Habits

• These are some healthy diet and hygiene habits :

Chew food slowly and grind it up. This will facilitate the digestive process and stimulate salivation.

Brush your teeth after every meal and floss thoroughly to clean the space between the teeth.

Respect meal times and try not to eat out which can overwork your digestive system.

Do not consume food or hot drinks that may cause irritation of the mucous membranes of the mouth, pharynx and even the esophagus, or eat spicy foods, as they can cause irritation of stomach lining.

Wash your hands before eating and preparing meals.

Ensure that food and drinks are in good condition to prevent infections and food poisoning.

Eat foods with fiber which help prevent constipation.

Do regular physical activity as it helps prevent the onset of intestinal gas and constipation.

Avoid alcoholic beverages because they cause irreversible damage to the liver and the pancreas, and also faciliate stomach and esophageal cancer.

Do not smoke because it causes cancer.

CREDITS

• Alberto Gullón• José Carlos González• Rosario Guerrero• Adela Granero• Marta García