Digestive System

∞ generated and posted on 2016.02.27 ∞

The alimentary canal from the stomach to just prior to the large intestine, i.e., digestion and most absorption.

The job of the stomach through to the start of the large intestine, at the ileocecal valve, is to chemically breakdown food into absorbable subunits, such as monosaccharides, amino acids, and fatty acids, and then to absorb those subunits either into the blood or into lymph vessels.

This page contains the following terms: Stomach, Pepsin, Pyloric sphincter, Chyme, Liver, Gallbladder, Bile, Bile salts, Pancreas, Small intestine, Absorption, Duodenum, Intestinal villi, Microvilli, Jejunum, Ileum



Stomach

Compartment of alimentary canal possessing low pH along with protein-digesting enzymes.
In us, the stomach represents the next compartment within the alimentary canal following the mouth. Its purpose is to break down or otherwise disrupt the structure of food beyond that achieved by mastication (chewing) in the mouth. This is accomplished in part via high acidity, that is, low pH, which can have the effect of breaking the peptide bonds that make up proteins. Enzymes are also present that break apart proteins, and an ongoing churning action disrupts food further and otherwise brings acid and enzymes (gastric proteases) into contact with the digesting food. Thus, acid, enzymes, and churning are the means by which the stomach effects food digestion.

The stomach acid and to some degree the gastric proteases also serve to limit the ability of microorganisms to pass from the mouth through the stomach into the rest of the alimentary canal. The product of digestion of food by the stomach is called chyme.


Links to terms of possible interest: Cardioesophageal sphincter, Duodenum, Esophagus, Fundus, Greater curvature, Lesser curvature, Muscularis externa, Pyloric antrum, Pyloric sphincter, Pylorus, Rugae of mucosa, Serosa, Stomach

The above, short video, mostly considers the function of the stomach, though ultimately the function of the stomach is to convert a bolus of food that has passed down the esophagus and into the stomach instead into what is known as chyme, which then enters the stomach into the duodenum.

The above videos takes a look at a cheeseburger dissolving in hydrochloric acid (HCl). While it's true that the pure HCl solution does not have all of the digestive components that are found in gastric juices, it's not true, as the suggested in the video, that bile salts are found in gastric juice. They are not, but instead are mixed with chyme after the stomach.



Pepsin

Stomach enzyme that digests proteins and which is initially secreted in an inactive form.
That inactive form of pepsin is called pepsinogen and both stomach acid and pepsin itself convert pepsinogen to pepsin. Pepsin is relatively unusual among digestive enzymes secreted by our bodies in that it is active especially at low pH, that is, within the high acidity of the stomach. This low pH is neutralized upon movement (of chyme) out of the stomach.

In this way pepsin is present and active only in the course of protein digestion within the stomach. The original pepsinogen is secreted from the mucosa of the stomach (the gastric mucosa), which is protected from pepsin, as well as gastric juices generally, due to the presence of mucus found on the mucosal surface.

The above video does a good job of walking through the physiology of gastric secretion, including prominently that of pepsin, in the stomach.



Pyloric sphincter

Valve found at the exit of the stomach that controls especially movement forwards in the alimentary canal, out of the stomach.
The pyloric sphincter prevents the movement of food out of the stomach until sufficient digestion there has taken place, with the esophageal sphincter and pyloric sphincter otherwise sealing the stomach at both "ends" (the in-coming "end" and the out-going "end"). In this way the stomach, despite being a segment of a much longer tube, is able to be temporarily sealed closed such that digestion in a separate chamber from the rest of the alimentary canal can occur.


Links to terms of possible interest: Duodenum, Lesser curvature, Pyloric antrum, Pyloric sphincter, Pylorus, Stomach



Chyme

Fluid product of food digestion by the stomach.
The stomach, that is, digests what mostly is solid food, breaking it down both chemically and mechanically (in addition to the action of mastication in the mouth), converting that solid food into a liquid form called chyme. The chyme leaves the stomach into the duodenum at a pH that is highly influenced by gastric juices but is then neutralized via sodium bicarbonate (essentially a solution of baking soda) that is secreted by the pancreas as well as by Brunner's glands which are found closer to the pyloric sphincter.

The material then passes through the small intestine and ultimately is converted to fecal material in the large intestine, though prior to this conversion to feces the material similarly is described as chyme (and indeed is the same but more fully digested as well as nutrient-depleted chyme as that which is released from the stomach). Protein-digesting enzymes provided by the pancreas are released into the chyme after it leaves the stomach and these replace the high acidity-functional protease of the stomach, that is, protein-digesting enzyme called pepsin. These neutral pH pancreatic proteases are called trypsin and chymotrypsin, and these too are initially produced in inactive, precursor forms known respectively as trypsinogen and chymotrypsinogen.


Links to terms of possible interest: Chyme, Duodenum, Lower esophageal sphincter, Pyloric sphincter

The above video is a nice overview of stomach-mediated digestion and therefore, prominently, the formation of chyme.



Liver

Large organ found adjacent to the stomach and diaphragm that possesses secretory as well as substantial synthetic and degradative functions.
The liver is a large, somewhat easily regenerating – from existing liver tissue – and multi-lobed organ. It plays important roles in digestion along with post-absorption modification of nutrients as well as drugs. It is a storehouse of carbohydrate, in the form of the starch known as glycogen, and as part of that storage plays important roles in the regulation of glucose levels in the blood (that is, by taking up excess glucose in response to higher blood levels or instead releasing glucose in response to reduced blood levels). In addition to glucose storage, the liver is involved in the storage of various vitamins and minerals.

In addition to the liver's contribution to the maintenance of glucose levels in the blood, it contributes as well to the maintenance of levels of amino acids as well as fats. The liver also produces a number of blood proteins, as well as cholesterol, and is responsible for the modification of numerous substances including the interconversion of otherwise useful materials, such as the conversion of lactic acid to glucose or various energy-containing substances to fat, and the conversion of potentially toxic substances to less-toxic ones (detoxification).


Links to terms of possible interest: Bile ducts, Common bile duct, Common hepatic duct, Cystic duct, Gallbladder, Liver, Pancreas, Small intestine



Links to terms of possible interest: Heart, Hepatic portal vein, Intestines, Kidneys, Liver



Links to terms of possible interest: Duodenum, Ileum, Jejunum, Pancreas, Spleen, Stomach

Quick, really nice introduction to the liver and functions.

The above video discusses the liver as viewed on more microscopic as well as macroscopic scales. The video becomes less introductory at 2:17.



Gallbladder

Bile-storing reservoir that is connected to the liver via a duct and connects to the duodenum via the common bile duct.
As bile is involved in fat digestion, the gallbladder is involved in the storage of this material until fat is found in the duodenum. It is because of this function that removal of the gallbladder can impair fat digestion, particularly given consumption of large, fatty meals at a single sitting. As gallstones can develop within the gallbladder, gallbladder treatment can involve removal of the gallbladder.


Links to terms of possible interest: Common bile duct, Cystic duct, Duodenum, Gallbladder, Pancreas, Pancreatic duct, Pancreatic juices, Pyloric valve, Stomach

The above video provides an overview of gallbladder functioning particularly through 0:38.



Bile

Liver-produced fluid that is stored in the gallbladder and involved in fat digestion.
The predominant constituents of bile are water and bile salts, and the latter can viewed as the primary "active ingredient". In addition, the excretory role of the liver, that is, in removing unwanted materials from blood, is mediated via excretion of these various substances into bile.


Links to terms of possible interest: Ampulla of Vater, Bile, Canaliculi, Common bile duct, Common hepatic duct, Cystic duct, Duodenum, Gallbladder, Hepatic ducts, Hepatocyte, Interlobular bile duct, Liver lobule, Pancreatic duct, Perilobular bile duct, Small intestine, Sphincter of Oddi, Terminal bile ductules,

The above video provides a quick as well as rather well done overview of bile and its movement to the duodenum as well as the gallbladder.



Bile salts

Emulsifying agent produced by the liver and then released into the duodenum towards fat digestion.
Bile salts are detergents – which basically are surfactants/emulsifying agents – and the action of bile salts is the same as the action of laundry detergent, which is to convert lipid-soluble materials into smaller droplets of lipid-soluble material. This works because detergents possess two chemically distinct ends, one water soluble (hydrophilic) and the other water insoluble (hydrophobic). The hydrophobic end inserts into the fat droplet and the hydrophilic end remains on the outside, interacting with water. The result is a disruption of the "enmity" between water and fat, which allows the fat droplet to increase its surface area by breaking up into smaller droplets, called micelles, which in turn increases the surface area upon which lipases, which are fat-digesting enzymes, can interact with the fats that they are digesting.

Bile salts are produced by the liver, stored in the gallbladder, and delivered to the duodenum via what ultimately is known as the common bile duct. The release of bile is stimulated by the presence of fat in the duodenum. Chemically, bile is similar to cholesterol and bile itself is mostly reabsorbed following fat digestion and subsequent fatty acid and glycerol absorption, where glycerol and fatty acids are the two products of fat digestion.


Links to terms of possible interest: Bile salt, Detergent, Emulsification, Fat digestion, Fat droplets, Fats, Hydrophilic, Hydrophobic, Nonpolar, Polar



Pancreas

Glandular organ responsible for releasing insulin, glucagon, and numerous digestive substances.
The release of hormones is described as an endocrine function while the release of digestive enzymes is described instead as an exocrine function. The location of the pancreas is within the vicinity of the stomach (below) and also the duodenum (left), with the duct associated with this exocrine function leading from the pancreas to the duodenum.

In addition to digestive enzymes, also released from the pancreas via the same duct to the duodenum is sodium bicarbonate – what we know of less formally as baking soda – which serves as an acid neutralizing agent. This acts on the chyme that is released from the stomach (via the pyloric sphincter) to increase its pH to approximately neutral pH.


Links to terms of possible interest: Acinar cells, Bile duct, Digestive enzyme, Duodenum, Endocrine gland, Exocrine gland, Islets of Langerhans, NaHCO3, Sodium bicarbonate, Stomach

The above video is more sober than that which follows, so is much less exciting but nonetheless is also more detailed and explicit in its presentation.

The first half or so of the above video – until about 1:59 – provides a nice introduction to the anatomy and functioning of the pancreas.

The above video is a bit superficial an overview of the pancreas but does consider both endocrine and exocrine functions.



Small intestine

Long, immediately post-stomach aspect of the alimentary canal in which the majority of nutrient absorption occurs.
The small intestine is described as small because its diameter is somewhat smaller than that of the large intestine. The small intestine takes the chyme released by the stomach and digests that material further, down to sugars, amino acids, nucleotides, fatty acids, etc. These macromolecule subunits, and other substances such as vitamins, are then absorbed across the mucosa of the small intestine and into body tissues, particularly blood and lymph.

The small intestine traditionally is differentiated into three parts, which I like to recall using the abbreviation, DUI (but replacing the U with a J). Thus, duodenum, jejunum, and ileum. It is into the duodenum that chyme flows through the pyloric sphincter from the stomach. Immediately following that entrance, the gastric acid is neutralized and new digestive enzymes introduced.

The above video provides a nice introduction to especially the gross anatomy of the intestines, with emphasis particularly on the small intestine, though the large intestines are discussed as well.

The above video is fairly heavy duty in its discussion but does allow for a superlative appreciation of what is going on in terms of the gross anatomy of the small intestine by supplying not one but two human dissections.



Absorption

Movement of substances from the lumen of the alimentary canal and into circulation within body tissues.
Nutrients are primarily absorbed within the small intestine, and particularly across the mucosa of the jejunum and ileum. Water is absorbed also from the large intestine, where its absorption results in a hardening of stools from the somewhat fluid chyme that enters and then flows through the small intestine. When this water absorption is insufficient then diarrhea can occur. Osmotic diarrhea, for example, occurs when undigested food materials, such as lactose in milk for the lactose intolerant, finds its way into the lumen of the large intestine where its presence impedes the absorption of water due to its countering attraction of that water. Excessive absorption of water from the large intestine, on the other hand, can result in excessive hardening of stools as well as constipation, and this can occur if the passage through the large intestine is extended such that water absorption is prolonged. Absorption is also used to describe the movement of drugs out of the lumen of the alimentary canal and into circulation within the body. The ingestion of drugs orally such that absorption can then occur is referred to as per os delivery.


Links to terms of possible interest: absorptionApical membrane, Basolateral membrane, Brush border, Lumen, Mucosa, Small intestine

The above video provides a quick though not terribly detailed discussion of digestion and absorption as it occurs in the small intestine.



Duodenum

Region of the small intestine that receives chyme from the stomach and into which digestive elements are introduced from the gallbladder and the pancreas.
Ducts lead from the gallbladder, which otherwise collects material secreted by the liver, and also from the pancreas, together into the duodenum. The substances carried by these ducts include pH buffers (to neutralize the still acidic chyme), bile salts (which contribute to the digestion of lipids), and various digestive enzymes. The duodenum is the shortest of the segments of the small intestine and serves to set up chyme for further digestion and then absorption within the jejunum and ileum, including in terms of the production of mucus which serves to lubricate the passage of chyme through the intestine.

The duodenum starts at the pyloric sphincter and ends with the transition into the jejunum. Befitting this location immediately post the stomach, the duodenum also is involved hormonally in the regulation of the emptying of the stomach. Note that some authors discuss the duodenum as (arguably) distinct from the small intestine while others consider the duodenum as representing a portion, the first portion, of the small intestine.


Links to terms of possible interest: Ampulla of Vater, Bile, Chyme, Duodenojejunal flexure, Duodenum, Jejunum, Pancreatic juices, Pyloric sphincter, Pylorus, Stomach

The above video provides a fairly detailed though also a bit redundant (to itself) discussion of the duodenum.



Intestinal villi

Multi-celled outfoldings of mucosa which serve to increase mucosal surface area in contact with the alimentary canal lumen.
Villi are "finger-like" in that they are columnar extensions of the intestinal mucosa. They are small but macroscopic, on the order of 1 mm long. Within them are numerous capillaries as lymph vessels known as a lacteals. It is into these capillaries and lacteals that absorbed nutrients pass in the course of absorption. Note that it is on the surface of villi that one finds microvilli. Note also that the singular form of villi is villus.


Links to terms of possible interest: Capillary network, Lacteal, Microvilli, Small intestine, Villi, Villus



Microvilli

Finger-like outfoldings of plasma membrane of cells making up the intestinal mucosa.
Microvilli serve to further increase the digestive or absorptive area of villi. The mucosa of the small intestine thus consists of finger-like outfoldings (villi) that themselves have finger-like outfoldings (microvilli). The microvilli are supported internally by cytoskeleton members called actin filaments. Microvilli are found only on the luminal side of mucosal cells, and substantially increase the surface area and therefore absorptive as well as digestive area of these cells.


Links to terms of possible interest: absorption, Brush border, Microvilli, Villi

The above video provides a very quick overview of microvilli, such as one sees associated with the cells lining the small intestine, that is, small intestinal mucosal cells.



Jejunum

Region of the small intestine specialized for nutrient absorption.
The jejunum, which follows the duodenum in the alimentary canal, possess longer villi than the duodenum, as well as than the ileum. The jejunum also secretes less mucus (than the duodenum, as mediated in the duodenum by what are known as Brunner's glands, a.k.a., duodenal glands) and is less involved in immune functions (as mediated by Peyer's patches, and found predominantly in the ileum). The jejunum thus is the part of the small intestine that is especially involved in absorption, versus particularly digestion and other manipulations of chyme (duodenum), though versus also immunological functions. Towards increasing surface area beyond that mediated by villi (and microvilli), it is also within the jejunum that circular folds are best developed.


Links to terms of possible interest: Appendix, Colon, Duodenum, Ileum, Jejunum, Rectum, Small intestine, Stomach

The above video provides a somewhat detailed discussion of the distinction between the jejunum and the other regions making up the small intestine; excellent video.



Ileum

Region of the small intestine specialized for immune functioning as well as nutrient absorption.
The ileum is the last as well as largest segment of the small intestine, following the jejunum. It possesses numerous regions known as Peyer's patches, which have immune functions, basically sampling molecules (called antigens) that are found within the small intestine. These antigens are associated with as-yet undigested food along with microorganisms. It is particularly the identification of pathogenic bacteria that is thought to represent a key target for the action of Peyer's patches.


Links to terms of possible interest: Appendix, Cecum, Colon, Large intestine, Ileocecal junction, Ileum, Mesenteric blood supply


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