Digestive System - Human Anatomy

myenteric plexus:

(plexus of Auerbach) major nerve supply to alimentary canal wall; controls motility

submucosal plexus

(plexus of Meissner) nerve supply that regulates activity of glands and smooth muscle

Mouth - Major Function

-Ingests food -Chews and mixes food -Begins chemical breakdown of carbohydrates -Moves food into the pharynx -Begins breakdown of lipids via lingual lipase

Mouth - other functions

-Moistens and dissolves food, allowing you to taste it -Cleans and lubricates the teeth and oral cavity -Has some antimicrobial activity

Esophagus - Major function

-Propels food to the stomach -Mixes and churns food with gastric juices to form chyme -Begins chemical breakdown of proteins -Releases food into the duodenum as chyme

Small intestine

Absorbs breakdown products of carbohydrates, proteins, lipids, and nucleic acids, along with vitamins, minerals, and water Performs physical digestion via segmentation

Cardiovascular

Blood supplies digestive organs with oxygen and processed nutrients

Large intestine

Further breaks down food residues Absorbs most residual water, electrolytes, and vitamins produced by enteric bacteria Propels feces toward rectum Eliminates feces

Assist the GI tract in the digestion of food.

Include: Teeth Tongue Salivary glands Liver Gallbladder Pancreas

Lymphatic

Mucosa-associated lymphoid tissue and other lymphatic tissue defend against entry of pathogens; lacteals absorb lipids; and lymphatic vessels transport lipids to bloodstream

Pharynx - Major function

Propels food from the oral cavity to the esophagus

Respiratory

Respiratory organs provide oxygen and remove carbon dioxide

explain why segmentation occurs and peristalsis slows in the small intestine.

The majority of digestion and absorption occurs in the small intestine. By slowing the transit of chyme, segmentation and a reduced rate of peristalsis allow time for these processes to occur.

continuous tube

about 30 feet (9-10 meters) from mouth to anus.

mastication:

chewing

chemical digestion:

enzymatic breakdown of food

Digestive organs collectively make up the:

gastrointestinal (GI) tract. Also called: the digestive tract alimentary canal.

accessory digestive organ

includes teeth, tongue, salivary glands, gallbladder, liver, and pancreas

mucosa:

innermost lining of the alimentary canal

submucosa:

layer of dense connective tissue in the alimentary canal wall that binds the overlying mucosa to the underlying muscularis

retroperitoneal

located posterior to the peritoneum

Pancreas - Digestive Organ

mixed gland because it exhibits both endocrine and exocrine functions Endocrine functions are performed by the pancreatic islets. Exocrine activity results in the secretion of digestive enzymes, collectively called pancreatic juice, into the duodenum.

motility

movement of food through the GI tract

muscularis:

muscle (skeletal or smooth) layer of the alimentary canal wall

peristalsis:

muscular contractions and relaxations that propel food through the GI tract

The GI tract organs:

oral cavity pharynx esophagus stomach small intestine large intestine

serosa:

outermost layer of the alimentary canal wall present in regions within the abdominal cavity

absorption:

passage of digested products from the intestinal lumen through mucosal cells and into the bloodstream or lacteals

Smooth muscle in the wall

responsible for motility pushes materials from one end to the other.

chyme:

soupy liquid created when food is mixed with digestive juices

ingestion:

taking food into the GI tract through the mouth

propulsion

voluntary process of swallowing and the involuntary process of peristalsis that moves food through the digestive tract

mechanical digestion:

chewing, mixing, and segmentation that prepares food for chemical digestion

Gallbladder-Accessory Digest Organ

concentrates bile produced by the liver and stores this concentrate until it is needed for digestion cystic duct connects the gallbladder to the common bile duct can hold approximately 40 to 60 milliliters of concentrated bile

alimentary canal:

continuous muscular digestive tube that extends from the mouth to the anus

Composed of two separate categories of organs:

digestive organs accessory digestive organs.

Accessory digestive organs

do not form the GI tube can develop as outgrowths are connected to the GI tract (some by ducts)

defecation:

elimination of undigested substances from the body in the form of feces

segmentation

alternating contractions and relaxations of non-adjacent segments of the intestine that move food forward and backward, breaking it apart and mixing it with digestive juices

Stomach

-Releases food into the duodenum as chyme -Absorbs some fat-soluble substances (for example, alcohol, aspirin) -Possesses antimicrobial functionsMixes chyme with digestive juices -Propels food at a rate slow enough for digestion and absorption

Alimentary Canal Organs

Also called the gastrointestinal (GI) tract or gut, the alimentary canal (aliment- = "to nourish") is a one-way tube about 7.62 meters (25 feet) in length during life and closer to 10.67 meters (35 feet) in length when measured after death, once smooth muscle tone is lost. The main function of the organs of the alimentary canal is to nourish the body. This tube begins at the mouth and terminates at the anus. Between those two points, the canal is modified as the pharynx, esophagus, stomach, and small and large intestines to fit the functional needs of the body. Both the mouth and anus are open to the external environment; thus, food and wastes within the alimentary canal are technically considered to be outside the body. Only through the process of absorption do the nutrients in food enter into and nourish the body's "inner space."

Submucosa

As its name implies, the submucosa lies immediately beneath the mucosa. A broad layer of dense connective tissue, it connects the overlying mucosa to the underlying muscularis. It includes blood and lymphatic vessels (which transport absorbed nutrients), and a scattering of submucosal glands that release digestive secretions. Additionally, it serves as a conduit for a dense branching network of nerves, the submucosal plexus, which functions as described below.

Skeletal

Bones help protect and support digestive organs

#4 Digestion

Digestion is the process of turning large pieces of food into its component chemicals. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines. Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb. Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas. The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum, it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.

Accessory Structures

Each accessory digestive organ aids in the breakdown of food. Within the mouth, the teeth and tongue begin mechanical digestion, whereas the salivary glands begin chemical digestion. Once food products enter the small intestine, the gallbladder, liver, and pancreas release secretions—such as bile and enzymes—essential for digestion to continue. Together, these are called accessory organs because they sprout from the lining cells of the developing gut (mucosa) and augment its function; indeed, you could not live without their vital contributions, and many significant diseases result from their malfunction. Even after development is complete, they maintain a connection to the gut by way of ducts.

Endocrine

Endocrine hormones help regulate secretion in digestive glands and accessory organs

Mucosa

Epithelium—In the mouth, pharynx, esophagus, and anal canal, the epithelium is primarily a non-keratinized, stratified squamous epithelium. In the stomach and intestines, it is a simple columnar epithelium. Notice that the epithelium is in direct contact with the lumen, the space inside the alimentary canal. Interspersed among its epithelial cells are goblet cells, which secrete mucus and fluid into the lumen, and enteroendocrine cells, which secrete hormones into the interstitial spaces between cells. Epithelial cells have a very brief lifespan, averaging from only a couple of days (in the mouth) to about a week (in the gut). This process of rapid renewal helps preserve the health of the alimentary canal, despite the wear and tear resulting from continued contact with foodstuffs. Lamina propria—In addition to loose connective tissue, the lamina propria contains numerous blood and lymphatic vessels that transport nutrients absorbed through the alimentary canal to other parts of the body. The lamina propria also serves an immune function by housing clusters of lymphocytes, making up the mucosa-associated lymphoid tissue (MALT). These lymphocyte clusters are particularly substantial in the distal ileum where they are known as Peyer's patches. When you consider that the alimentary canal is exposed to foodborne bacteria and other foreign matter, it is not hard to appreciate why the immune system has evolved a means of defending against the pathogens encountered within it. Muscularis mucosa—This thin layer of smooth muscle is in a constant state of tension, pulling the mucosa of the stomach and small intestine into undulating folds. These folds dramatically increase the surface area available for digestion and absorption.

Functions of digestive system

Ingestion Mechanical processing Digestion Secretion Absorption Excretion

Urinary

Kidneys convert vitamin D into its active form, allowing calcium absorption in the small intestine

Accessory organs

Liver: produces bile salts, which emulsify lipids, aiding their digestion and absorption Gallbladder: stores, concentrates, and releases bile Pancreas: produces digestive enzymes and bicarbonate

Pharynx _ other functions

Lubricates food and passageways

#5 Absorption

Once food has been reduced to its building blocks, it is ready for the body to absorb. Absorption begins in the stomach with simple molecules like water and alcohol being absorbed directly into the bloodstream. Most absorption takes place in the walls of the small intestine, which are densely folded to maximize the surface area in contact with digested food. Small blood and lymphatic vessels in the intestinal wall pick up the molecules and carry them to the rest of the body. The large intestine is also involved in the absorption of water and vitamins B and K before feces leave the body.

Functions of The Liver

Produce bile. a greenish fluid that breaks down fats into small droplets to assist in their chemical digestion Detoxify drugs, metabolites, and poisons. Store excess nutrients and vitamins and release them when they are needed. Synthesize blood plasma proteins such as albumins, globulins, and proteins required for blood clotting. Phagocytize debris in the blood. Help break down and recycle components of aged erythrocytes and damaged or worn-out formed elements

Nervous

Sensory and motor neurons help regulate secretions and muscle contractions in the digestive tract

Muscular

Skeletal muscles support and protect abdominal organs

Integumentary

Skin helps protect digestive organs and synthesizes vitamin D for calcium absorption

#3 Mixing/Moving

Swallowing. Swallowing is the process of using smooth and skeletal muscles in the mouth, tongue, and pharynx to push food out of the mouth, through the pharynx, and into the esophagus. Peristalsis. Peristalsis is a muscular wave that travels the length of the GI tract, moving partially digested food a short distance down the tract. It takes many waves of peristalsis for food to travel from the esophagus, through the stomach and intestines, and reach the end of the GI tract. Segmentation. Segmentation occurs only in the small intestine as short segments of intestine contract like hands squeezing a toothpaste tube. Segmentation helps to increase the absorption of nutrients by mixing food and increasing its contact with the walls of the intestine. 4

Biliary apparatus-Accessory Digest. Organ

The biliary apparatus. network of thin ducts that carry bile from the liver and gallbladder to the duodenum the left and right lobes of the liver drain bile into the left and right hepatic ducts, respectively the left and right hepatic ducts merge to form a single common hepatic duct the cystic duct attaches to the common hepatic duct and carries bile to and from the gallbladder

Digestive System Organs

The easiest way to understand the digestive system is to divide its organs into two main categories. The first group is the organs that make up the alimentary canal. Accessory digestive organs comprise the second group and are critical for orchestrating the breakdown of food and the assimilation of its nutrients into the body. Accessory digestive organs, despite their name, are critical to the function of the digestive system.

#6 Excretion

The final function of the digestive system is the excretion of waste in a process known as defecation. Defecation removes indigestible substances from the body so that they do not accumulate inside the gut. The timing of defecation is controlled voluntarily by the conscious part of the brain, but must be accomplished on a regular basis to prevent a backup of indigestible materials.

#1 Ingestion

The first function of the digestive system is ingestion, or the intake of food. The mouth is responsible for this function, as it is the orifice through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested. This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.

Mucosa

The mucosa is referred to as a mucous membrane, because mucus production is a characteristic feature of gut epithelium. The membrane consists of epithelium, which is in direct contact with ingested food, and the lamina propria, a layer of connective tissue analogous to the dermis. In addition, the mucosa has a thin, smooth muscle layer, called the muscularis mucosa (not to be confused with the muscularis layer, described below).

Serosa

The serosa is the portion of the alimentary canal superficial to the muscularis. Present only in the region of the alimentary canal within the abdominal cavity, it consists of a layer of visceral peritoneum overlying a layer of loose connective tissue. Instead of serosa, the mouth, pharynx, and esophagus have a dense sheath of collagen fibers called the adventitia. These tissues serve to hold the alimentary canal in place near the ventral surface of the vertebral column.

It has been several hours since you last ate. Walking past a bakery, you catch a whiff of freshly baked bread. What type of reflex is triggered, and what is the result?

The smell of food initiates long reflexes, which result in the secretion of digestive juices.

Muscalaris

The third layer of the alimentary canal is the muscalaris (also called the muscularis externa). The muscularis in the small intestine is made up of a double layer of smooth muscle: an inner circular layer and an outer longitudinal layer. The contractions of these layers promote mechanical digestion, expose more of the food to digestive chemicals, and move the food along the canal. In the most proximal and distal regions of the alimentary canal, including the mouth, pharynx, anterior part of the esophagus, and external anal sphincter, the muscularis is made up of skeletal muscle, which gives you voluntary control over swallowing and defecation. The basic two-layer structure found in the small intestine is modified in the organs proximal and distal to it. The stomach is equipped for its churning function by the addition of a third layer, the oblique muscle. While the colon has two layers like the small intestine, its longitudinal layer is segregated into three narrow parallel bands, the tenia coli, which make it look like a series of pouches rather than a simple tube.

Histology of the Alimentary Canal

Throughout its length, the alimentary tract is composed of the same four tissue layers; the details of their structural arrangements vary to fit their specific functions. Starting from the lumen and moving outwards, these layers are the mucosa, submucosa, muscularis, and serosa, which is continuous with the mesentery.

#2 Secretion

n the course of a day, the digestive system secretes around 7 liters of fluids. These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. Enzymes are like tiny biochemical machines that disassemble large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally, bile is used to emulsify large masses of lipids into tiny globules for easy digestion.