Digestive System - Biology

The digestive system consists of the organs that food passes through, from the mouth to the anus, as well as the accessory organs that influence digestion. This includes organs that produce materials aiding in digestion, for example the liver, which produces bile the digest fats. The mouth is the first step in the digestive tract, the duodenum is a region of the small intestine, and the colon is a region of the large intestine.

The lymph nodes are part of the lymphatic system and do not play a role in the digestive system.

Initial protein digestion takes place in the stomach. Chief cells are responsible for releasing pepsinogen into the stomach, an inactive enzyme (zymogen) that must be cleaved in order to form pepsin. The activation of this enzyme is accomplished by the release of hydrochloric acid into the stomach by parietal cells.

Goblet cells release mucus into the stomach to protect the epithelium from the acidic environment. G cells release gastrin, which stimulates parietal cells to release acid.

All pancreatic enzymes are released as inactive enzymes, or proenzymes, into the small intestine. Enterokinase is an enzyme that activates trypsin in the small intestine. Once activated, trypsin is able to activate the rest of the pancreatic enzymes. Trypsin activates chymotrypsin to digest proteins and pancreatic amylase to digest carbohydrates. Ribonuclease is used to break down old RNA in the cytoplasm of cells, and is not a pancreatic digestive enzyme.

The large intestine, or colon, has the major function of water and electrolyte reabsorption. If this function is inhibited, by a pathogen for example, the body cannot retain water and becomes dehydrated.

Enzyme secretion occurs in the duodenum of the small intestine, where digestive enzymes from the pancreas are released. Protein digestion primarily occurs in the small intestine, but begins in the stomach. The stomach and mouth are primarily responsible for mechanical digestion.

Critical to proper digestion, bile and pancreatic juices must enter the intestine rather early so that they have ample time to digest foods passing by. After preliminary protein digestion in the stomach, food passes through the pyloric sphincter into the duodenum. The pyloric sphincter simply separates the stomach from the small intestine. The duodenum serves as the main region of chemical digestion in the body. It is in this region that bile and pancreatic enzymes are introduced. Food undergoing digestion subsequently passes through the jejunum, the ileum and the colon, or large intestine. These regions are mostly used for absorption of nutrients and water, rather than chemical digestion.

Blood that is rich in nutrients from digestion is collected in various mesenteric veins and then passes through the portal vein directly into the liver, where the nutrients are metabolized. The portal vein supplies about two thirds of the blood to the liver. The hepatic artery supplies the other third. The cisterna chyli collects lymph, not blood, and the femoral vein is associated with the leg.

In the digestive system, food is broken apart mechanically (by the teeth), then chemically (by the stomach). Food then travels to the intestines, where nutrients are absorbed and water is removed from the gut tube, leaving only the waste. The waste is then eliminated from the body. Production of gametes is the function of the reproductive system, and does not take place in the digestive system.

The small intestine has three parts: the duodenum, jejunum and ileum. Note that the ilium is one of the three bones that makes up the pelvis. The small intestine is the primary site of nutrient absorption. Whatever is not absorbed by the small intestine will then be absorbed my the large intestine (mainly water and vitamins). The stomach produces hydrochloric acid, which, like all acids, decrease the pH of their environment below 7.

No chemical digestion occurs in the esophagus. The esophagus is used in order to move food into the stomach using peristaltic motion.

Amylase is introduced in the mouth and begins breaking down carbohydrates. Pepsin in the stomach helps digest proteins. Lipase, trypsin, and other digestive enzymes are introduced in the small intestine, where the majority of chemical digestion takes place.

The duodenum is the first section of the small intestine. It is the site of the most digestion in the small intestine (and the digestive tract in general) because pancreatic enzymes and bile are released into its lumen. These enzymes break down proteins and carbohydrates, while the bile helps to digest lipids and fats. The jejunum and ileum are primarily involved in absorbing nutrients. The cecum is part of the large intestine.

While the pancreas is responsible for the secretion of the majority of digestive enzymes, the liver is responsible for the creation of bile. The gall bladder is responsible for the storage of bile. When the gall bladder is stimulated, bile is released into the small intestine, where it helps with the emulsification and digestion of fats.

The liver has a variety of functions, including blood detoxification, glycogen formation, and blood filtration. The liver is not responsible for chemical digestion, but is needed to synthesize and store key nutrients and macromolecules. Protein digestion primarily takes place in the small intestine, though the process begins in the stomach.

The large intestine is located near the end of the gastrointestinal tract. Its primary function is to reabsorb water and nutrients prior to elimination of waste. Mechanical digestion of food occurs in the mouth and stomach. Chemical digestion of food occurs in the mouth (saliva has amylase), stomach, and early in the small intestine. Blood glucose levels are maintained by the pancreas.

Bile is a dark greenish brown substance continuously produced by the liver. It is stored and concentrated in the gallbladder, and is released into the small intestine after eating. In the small intestine, bile aids in the digestion of lipids (this is a very important part of the absorption of fat soluble substances such as certain vitamins).

The digestive system begins at the mouth where food is chewed into pieces small enough to be digested. In the mouth, food is already starting to be digested by enzymes, and is lubricated by saliva (saliva starts to be secreted even before you take your first bite). Next the pharynx, also called the throat, receives the food from the mouth. The food is then moved to the esophagus, which carries food to the stomach. From the stomach food is moved to the small intestine, then to the colon, rectum, and anus. During this process there are a number of motility types, hormones, secretions, and accessory digestive organs involved in the digestive process.

The trachea, also known as the windpipe, moves air from the pharynx (shared with the digestive system) to the bronchi and then into the lungs. It plays no role in digestion.

The pancreas is an important organ in digesting fats. The fat soluble vitamins (A, D, E, K) rely on the pancreatic function for their absorption into the body. Without the pancreas, an individual with cystic fibrosis will have fat soluble vitamin deficiencies and steatorrhea (fatty stools).

The stomach contains the enzyme pepsin, which helps sever the peptide bonds between amino acids and breaks protein molecules into smaller fragments. As the contents of the stomach enters the small intestine, more digestive enzymes are added to further digest the proteins; however, the first step occurs in the stomach.

It is important to note that the mouth is the first location at which carbohydrates are digested, due to the protein amylase found in saliva.

This is a fundamental question that checks for clear understanding of the purpose of the digestive system. The digestive system includes the mouth, esophagus, stomach, small intestine, and large intestine. The primary functions of these structures are to break down and absorb nutrients from ingested food.

The respiratory system is responsible for providing a site for gas exchange (namely the alveoli). The circulatory system delivers oxygen to tissues, while the excretory system removes nitrogenous wastes and maintains water balance via kidney function.

It is important to know sites of production and action of the digestive enzymes. In the stomach, the very low pH environment facilitates gastric enzyme function, so we can already assume that an acidic compound is involved in the stomach.

HCl, or hydrochloric acid, is the acidic compound released by parietal cells and functions to kill microorganisms, digest acid labile substances, and activate pepsinogen to create pepsin.

The second main digestive compound of the stomach is pepsin, a protease, which digests amino acids into smaller peptides. It is released as a zymogen, or inactive form, by chief cells before it is activated to pepsin. The inactive form of pepsin is called pepsinogen.

Bile is a substance produced by the liver, stored by the gall bladder, and secreted into the duodenum of the small intestine for emulsification of fat globules. HCO3-, or bicarbonate ion, is a basic substance released by the pancreas into the duodenum to neutralize the entering chyme, reducing acidity to allow for optimal enzyme activity in the duodenum. Neither bile, nor bicarbonate are active in the stomach.

The chemical digestion of food is facilitated by the secretion of the hormone gastrin, which stimulates the secretion of hydrochloric acid. Insulin is secreted by the pancreas in response to high levels of blood glucose. Glucagon is secreted in response to low levels of blood glucose. Aldosterone is secreted by the adrenal cortex and increases reabsorption of salt in the kidney, subsequently increasing water retention and blood volume. Thyroxine is secreted by the thyroid and increases the metabolic rate of most body cells.