The digestive system (from the mouth to the anus) functions as follows:
- receive food
- breaking down food into nutrients (a process called digestion)
- Absorb nutrients into the bloodstream
- Removes undigested food from the body.
The digestive tract consists of the mouth, throat, esophagus, stomach, small intestine, large intestine, rectum and anus. The digestive system also includes organs located outside the digestive tract, namely the pancreas, liver and gallbladder.
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Mouth, Throat & Esophagus
The mouth is the entrance for the digestive system and the respiratory system.
The inside of the mouth is lined by a mucous membrane. Ducts from the salivary glands on the cheeks, under the tongue and under the jaw drain their contents into the mouth.
At the floor of the mouth is the tongue, which functions to taste and mix food. Behind and below the mouth is the throat (pharynx).
Taste is felt by the taste buds on the surface of the tongue.
Smell is sensed by the olfactory nerve in the nose. Taste is relatively simple, consisting of sweet, sour, salty and bitter. Olfaction is more complicated, consisting of a variety of odors. Food is cut up by the front teeth (incisors) and chewed by the back teeth (molars, molars), into smaller pieces that are easier to digest.
The saliva from the salivary glands will wrap the parts of the food with digestive enzymes and start digesting it. At the time of eating, the flow of saliva cleans the bacteria that can cause tooth decay and other disorders. Saliva also contains antibodies and enzymes (eg lysozyme), which break down proteins and attack bacteria directly.
Swallowing process starts consciously and continues automatically.
The epiglottis will be closed so that food does not enter the air tube (trachea) and into the lungs, while the roof of the back of the mouth (palatum mole, soft palate) is lifted so that food does not enter the nose.
The esophagus (esophagus) is a thin-walled muscular tube lined by a mucous membrane. The esophagus connects the throat to the stomach. Food is pushed through the esophagus not by gravitational forces, but by rhythmic waves of muscle contraction and relaxation called peristalsis.
Stomach
The stomach is a large, hollow muscular organ shaped like a donkey's cage, consisting of 3 parts, namely the cardia, fundus and antrum. Food enters the stomach from the esophagus through a ring-shaped muscle (sphincter), which can open and close. Under normal circumstances, the sphincter blocks the re-entry of gastric contents into the esophagus. The stomach functions as a food storehouse, contracting rhythmically to mix food with enzymes. The cells lining the stomach produce 3 important substances:
- lender
- hydrochloric acid
- a precursor to pepsin (an enzyme that breaks down proteins).
The mucus protects the gastric cells from being damaged by stomach acids and enzymes.
Any abnormality in this mucus lining (whether due to infection with the bacterium Helicobacter pylori or due to aspirin), can cause damage that leads to the formation of gastric ulcers.
Hydrochloric acid creates a very acidic environment, which is needed by pepsin to break down protein. The high acidity of the stomach also acts as a barrier against infection by killing various bacteria.
Acid release is stimulated by:
- nerves leading to stomach
gastrin (a hormone released by the stomach)
histamine (a substance released by the stomach).
Pepsin is responsible for the breakdown of about 10% of protein. Pepsin is the only enzyme that digests collagen, which is a protein and the main ingredient of meat. Only some substances can be absorbed directly from the stomach (eg alcohol and aspirin) and even then only in very small amounts.
Small intestine
The stomach releases food into the duodenum, which is the first part of the small intestine. Food enters the duodenum through the pyloric sphincter in amounts that can be digested by the small intestine.
When full, the duodenum will send a signal to the stomach to stop passing food. Duodenum receives pancreatic enzymes from the pancreas and bile from the liver. This fluid (which enters the duodenum through an opening called the sphincter of Oddi) is an important part of digestion and absorption.
Peristalsis also aids digestion and absorption by stirring and mixing it with substances produced by the intestines.
The first few inches of the lining of the duodenum are smooth, but the rest have folds, tiny bumps (villi) and smaller bumps (microvilli). The villi and microvilli increase the surface area of the duodenal lining, thereby increasing the amount of nutrients absorbed. The remainder of the small intestine, which lies below the duodenum, consists of the jejunum and ileum.
This section is mainly responsible for the absorption of fat and other nutrients. This absorption is enhanced by its large surface because it consists of folds, villi and microvilli. The intestinal wall is rich in blood vessels that transport absorbed substances to the liver via the portal vein.
The intestinal wall releases mucus (which lubricates the intestinal contents) and water (which helps dissolve digested food fragments). The intestinal wall also releases small amounts of enzymes that digest protein, sugar and fat. The density of the intestinal contents changes gradually, as it travels through the small intestine.
In the duodenum, water is rapidly pumped into the intestinal contents to dissolve the acidity of the stomach. When passing through the lower small intestine, the intestinal contents become more fluid because it contains water, mucus and pancreatic enzymes.
Pancreas
The pancreas is an organ that consists of 2 basic tissues:
- Asini, produces digestive enzymes
- Pancreatic islets, produce hormones.
The pancreas releases digestive enzymes into the duodenum and releases hormones into the blood. Digestive enzymes are produced by the acini cells and flow through various ducts into the pancreatic duct.
The pancreatic duct joins the bile duct at the sphincter of Oddi, where it enters the duodenum. Enzymes released by the pancreas will digest proteins, carbohydrates and fats. Proteolytic enzymes break down proteins into a form that can be used by the body and released in an inactive form. This enzyme will only be active if it has reached the digestive tract.
The pancreas also releases large amounts of sodium bicarbonate, which protects the duodenum by neutralizing stomach acid.
The 3 hormones produced by the pancreas are:
- Insulin, which lowers blood sugar levels
Glucagon, which functions to increase blood sugar levels
- Somatostatin, which functions to block the release of the other two hormones (insulin and glucagon).
Heart
The liver is a large organ and has many functions, some of which are related to digestion. Nutrients from food are absorbed into the intestinal wall which is rich in small blood vessels (capillaries).
These capillaries drain into veins which join larger veins and eventually enter the liver as portal veins.
The portal vein is divided into small vessels in the liver, where the incoming blood is processed.
Blood is processed in 2 ways:
- Bacteria and other foreign particles that are absorbed from the intestine are removed
- Various nutrients that are absorbed from the intestine are then broken down so that they can be used by the body.
The liver performs the process at a high speed, after the blood is enriched with nutrients, the blood is circulated into the general circulation. The liver produces about half of all cholesterol in the body, the rest comes from food.
About 80% of the cholesterol produced in the liver is used to make bile.
The liver also produces bile, which is stored in the gallbladder. Gallbladder & Bile Duct. Bile drains from the liver through the left and right hepatic ducts, which then join to form the common hepatic duct.
This duct then joins with a duct from the gallbladder (cystic duct) to form the common bile duct.
The pancreatic duct joins the common bile duct and enters the duodenum.
Before eating, bile salts accumulate in the gallbladder and only a small amount of bile flows from the liver. Food in the duodenum triggers a series of hormonal signals and nerve signals so that the gallbladder contracts. As a result, bile flows into the duodenum and mixes with the food.
Bile has 2 important functions:
- Helps digestion and absorption of fat
- Plays a role in the removal of certain wastes from the body, especially hemoglobin that comes from the destruction of red blood cells and excess cholesterol.
Specifically, bile plays a role in the following processes:
- Bile salts increase the solubility of cholesterol, fat and fat-soluble vitamins to help the absorption process
- Bile salts stimulate the release of water by the large intestine to help move its contents
- Bilirubin (the main pigment of bile) is excreted in the bile as waste from red blood cells that are destroyed
- Drugs and other wastes are excreted in the bile and then excreted from the body
- Various proteins that play a role in bile function are excreted in the bile.
- Bile salts are reabsorbed into the small intestine, distilled by the liver and flowed back into the bile.
This circulation is known as the enterohepatic circulation.
All bile salts in the body circulate as much as 10-12 times / day. In each circulation, small amounts of bile salts pass into the large intestine (colon). In the colon, bacteria break down bile salts into various constituents. Some of these constituents are reabsorbed and the rest is excreted in the feces.
Colon
The large intestine consists of:
- ascending colon (right)
- Transverse colon
- Descending colon (left)
- Sigmoid colon (connected to the rectum).
The appendix (appendix) is a small protrusion shaped like a tube, which is located in the ascending colon, at the junction of the ascending colon with the small intestine. The large intestine produces mucus and functions to absorb water and electrolytes from the stool. When it reaches the large intestine, the contents of the intestine are liquid, but when it reaches the rectum it becomes solid. The large number of bacteria in the large intestine functions to digest some materials and help absorb nutrients.
Bacteria in the large intestine also function to make important substances, such as vitamin K. These bacteria are important for the normal function of the intestines. Some diseases and antibiotics can cause interference with the bacteria in the large intestine. The result is irritation that can cause mucus and water to be released, and diarrhea occurs. The rectum is a space that begins at the end of the large intestine (after the sigmoid colon) and ends at the anus.
Usually the rectum is empty because the stool is stored in a higher place, namely in the descending colon. When the descending colon is full and stool passes into the rectum, there is an urge to defecate. Adults and older children can resist this urge, but infants and younger children lack the muscle control necessary to delay bowel movements. .
The anus is the opening at the end of the digestive tract, through which waste materials exit the body. Part of the anus is formed from the surface of the body (skin) and part of the intestine. A muscular ring (anal sphincter) keeps the anus closed.
Sumber: disini