Gastrointestinal physiology
Encyclopedia
Gastrointestinal physiology is a branch of human physiology
addressing the physical function of the gastrointestinal (GI) system. The major processes occurring in the GI system are that of motility, secretion, regulation, digestion and circulation. The function and coordination of each of these actions is vital in maintaining GI health, and thus the digestion of nutrients for the entire body.
subunits linked by gap junctions. These subunits fire spontaneously in either a tonic or a phasic fashion. Tonic contractions are those contractions that are maintained from several minutes up to hours at a time. These occur in the sphincters of the tract, as well as in the anterior stomach. The other type of contractions, called phasic contractions, consist of brief periods of both relaxation and contraction, occurring in the posterior stomach and the small intestine, and are carried out by the muscularis externa
.
s that can cause action potentials in smooth muscle cells. They are associated with the contractile smooth muscle via gap junctions. These slow wave potentials must reach a threshold level for the action potential to occur, whereupon Ca2+ channels on the smooth muscle open and an action potential occurs. As the contraction is graded based upon how much Ca2+ enters the cell, the longer the duration of slow wave, the more action potentials occur. This in turn results in greater contraction force from the smooth muscle. Both amplitude and duration of the slow waves can be modified based upon the presence of neurotransmitters, hormones or other paracrine signaling. The number of slow wave potentials per minute varies based upon the location in the digestive tract. This number ranges from 3 waves/min in the stomach to 12 waves/min in the intestines.
and segmentation
. Occurring between meals, the migrating motor complex
is a series of peristaltic wave’s cycles in distinct phases starting with relaxation followed by an increasing level of activity to a peak level of peristaltic activity lasting for 5–15 minutes.
This cycle repeats every 1.5–2 hours but is interrupted by food ingestion. The role of this process is likely to clean excess bacteria and food from the digestive system.
Peristalsis is the second of the three patterns and is one of the patterns that occur during and shortly after a meal. The contractions occur in wave patterns traveling down short lengths of the GI tract from one section to the next. The contractions occur directly behind the bolus of food that is in the system, forcing it toward the anus into the next relaxed section of smooth muscle. This relaxed section then contracts, generating smooth forward movement of the bolus at between 2–25 cm per second. This contraction pattern depends upon hormones, paracrine signals, and the autonomic nervous system
for proper regulation.
for proper absorption.
, facilitated diffusion
and open channel ion movement. The arrangement of these proteins on the apical
and basolateral sides of the epithelium determines the net movement of ions and water in the tract.
H+ and Cl- are secreted by the parietal cells into the lumen
of the stomach creating acidic conditions with a low pH of 1. H+ is pumped into the stomach by exchanging it with K+. This process also requires ATP as a source of energy; however, Cl- then follows the positive charge in the H+ through an open apical channel protein.
HCO3- secretion occurs to neutralize the acid secretions that make their way into the duodenum
of the small intestine. Most of the HCO3- comes from pancreatic acinar cells in the form of NaHCO3 in a watery solution. This is the result of the high concentration of both HCO3- and Na+ present in the duct creating an osmotic gradient to which the water follows.
, which is secreted in the stomach by chief cells. Pepsin in its secreted form is inactive (pepsinogen). However, once it reaches the gastic lumen it becomes activated into pepsin
by the high H+ concentration, becoming an enzyme vital to digestion. The release of the enzymes is regulated by neural, hormonal, or paracrine signals. However, in general, parasympathetic stimulation increases secretion of all digestive enzymes.
is released in the stomach and intestine, and serves to lubricate and protect the inner mucosa of the tract. It is composed of a specific family of glycoproteins termed mucins and is generally very viscous. Mucus is made by two types of specialized cells termed mucus cells in the stomach and goblet cells in the intestines. Signals for increased mucus release include parasympathetic innervations, immune system response and enteric nervous system messengers.
. It is produced in liver cells and stored in the gall bladder until release during a meal. Bile is formed of three elements: bile salts, bilirubin
and cholesterol. Bilirubin is a waste product of the breakdown of hemoglobin. The cholesterol present is secreted with the feces. The bile salt component is an active non-enzymatic substance that facilitates fat absorption by helping it to form an emulsion with water due to its amphoteric nature. These salts are formed in the hepatocytes from bile acids combined with an amino acid
. Other compounds such as the waste products of drug degradation are also present in the bile.
(CNS), short reflexes from the enteric nervous system
(ENS) and reflexes from GI peptides working in harmony with each other.
and secretin
families, with the third composed of all the other hormones unlike those in the other two families. Further information on the GI peptides is summarized in the table below.
Human physiology
Human physiology is the science of the mechanical, physical, bioelectrical, and biochemical functions of humans in good health, their organs, and the cells of which they are composed. Physiology focuses principally at the level of organs and systems...
addressing the physical function of the gastrointestinal (GI) system. The major processes occurring in the GI system are that of motility, secretion, regulation, digestion and circulation. The function and coordination of each of these actions is vital in maintaining GI health, and thus the digestion of nutrients for the entire body.
Motility
The GI tract generates motility using smooth muscleSmooth muscle
Smooth muscle is an involuntary non-striated muscle. It is divided into two sub-groups; the single-unit and multiunit smooth muscle. Within single-unit smooth muscle tissues, the autonomic nervous system innervates a single cell within a sheet or bundle and the action potential is propagated by...
subunits linked by gap junctions. These subunits fire spontaneously in either a tonic or a phasic fashion. Tonic contractions are those contractions that are maintained from several minutes up to hours at a time. These occur in the sphincters of the tract, as well as in the anterior stomach. The other type of contractions, called phasic contractions, consist of brief periods of both relaxation and contraction, occurring in the posterior stomach and the small intestine, and are carried out by the muscularis externa
Muscularis externa
The muscular coat is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa membrane...
.
Stimulation
The stimulation for these contractions likely originates in modified smooth muscle cells called interstitial cells of Cajal. These cells cause spontaneous cycles of slow wave potentialSlow wave potential
In physiology, a slow-wave potential is a membrane potential that cycles between depolarizations and repolarizations. Slow wave potentials are generated by myocytes. Due to temporal summation, a slow-wave potential will periodically reach threshold and generate an action potential. This in turn...
s that can cause action potentials in smooth muscle cells. They are associated with the contractile smooth muscle via gap junctions. These slow wave potentials must reach a threshold level for the action potential to occur, whereupon Ca2+ channels on the smooth muscle open and an action potential occurs. As the contraction is graded based upon how much Ca2+ enters the cell, the longer the duration of slow wave, the more action potentials occur. This in turn results in greater contraction force from the smooth muscle. Both amplitude and duration of the slow waves can be modified based upon the presence of neurotransmitters, hormones or other paracrine signaling. The number of slow wave potentials per minute varies based upon the location in the digestive tract. This number ranges from 3 waves/min in the stomach to 12 waves/min in the intestines.
Contraction Patterns
The patterns of GI contraction as a whole can be divided into two distinct patterns, peristalsisPeristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...
and segmentation
Segmentation
Segmentation may mean:*Market segmentation, in economics and marketingBiology*A process of morphogenesis that divides a metazoan body into a series of semi-repetitive segments*Segmentation , a series of semi-repetitive segments...
. Occurring between meals, the migrating motor complex
Migrating Motor Complex
Migrating motor complexes are waves of activity that sweep through the intestines in a regular cycle during fasting state....
is a series of peristaltic wave’s cycles in distinct phases starting with relaxation followed by an increasing level of activity to a peak level of peristaltic activity lasting for 5–15 minutes.
This cycle repeats every 1.5–2 hours but is interrupted by food ingestion. The role of this process is likely to clean excess bacteria and food from the digestive system.
Peristalsis
Autonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
for proper regulation.
Segmentation
The third contraction pattern is segmentation, which also occurs during and shortly after a meal within short lengths in segmented or random patterns along the intestine. This process is carried out by longitudinal muscles relaxing while circular muscles contract at alternating sections thereby mixing the food. This mixing allows food and digestive enzymes to maintain a uniform composition, as well as to ensure contact with the epitheliumEpithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
for proper absorption.
Secretion
Every day, seven liters of fluid are secreted by the digestive system. This fluid is composed of four primary components: ions, digestive enzymes, mucus, and bile. About half of these fluids are secreted by the salivary glands, pancreas, and liver, which compose the accessory organs and glands of the digestive system. The rest of the fluid is secreted by the GI epithelial cells.Ions
The largest component of secreted fluids is ions and water, which are first secreted and then reabsorbed along the tract. The ions secreted primarily consist of H+, K+, Cl-, HCO3- and Na+. Water follows the movement of these ions. The GI tract accomplishes this ion pumping using a system of proteins that are capable of active transportActive transport
Active transport is the movement of a substance against its concentration gradient . In all cells, this is usually concerned with accumulating high concentrations of molecules that the cell needs, such as ions, glucose, and amino acids. If the process uses chemical energy, such as from adenosine...
, facilitated diffusion
Facilitated diffusion
..Facilitated diffusion is a process of passive transport, facilitated by integral proteins. Facilitated diffusion is the spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins...
and open channel ion movement. The arrangement of these proteins on the apical
Apical
Apical, from the Latin apex meaning to be at the apex or tip, may refer to:*Apical , an anatomical term of location for features associated with the base of an organism or structure...
and basolateral sides of the epithelium determines the net movement of ions and water in the tract.
H+ and Cl- are secreted by the parietal cells into the lumen
Lumen (anatomy)
A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...
of the stomach creating acidic conditions with a low pH of 1. H+ is pumped into the stomach by exchanging it with K+. This process also requires ATP as a source of energy; however, Cl- then follows the positive charge in the H+ through an open apical channel protein.
HCO3- secretion occurs to neutralize the acid secretions that make their way into the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
of the small intestine. Most of the HCO3- comes from pancreatic acinar cells in the form of NaHCO3 in a watery solution. This is the result of the high concentration of both HCO3- and Na+ present in the duct creating an osmotic gradient to which the water follows.
Digestive Enzymes
The second vital secretion of the GI tract is that of digestive enzymes that are secreted in the mouth, stomach and intestines. Some of these enzymes are secreted by accessory digestive organs, while others are secreted by the epithelial cells of the stomach and intestine. While some of these enzymes remain embedded in the wall of the GI tract, others are secreted in an inactive proenzyme form. When these proenzymes reach the lumen of the tract, a factor specific to a particular proenzyme will activate it. A prime example of this is pepsinPepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...
, which is secreted in the stomach by chief cells. Pepsin in its secreted form is inactive (pepsinogen). However, once it reaches the gastic lumen it becomes activated into pepsin
Pepsin
Pepsin is an enzyme whose precursor form is released by the chief cells in the stomach and that degrades food proteins into peptides. It was discovered in 1836 by Theodor Schwann who also coined its name from the Greek word pepsis, meaning digestion...
by the high H+ concentration, becoming an enzyme vital to digestion. The release of the enzymes is regulated by neural, hormonal, or paracrine signals. However, in general, parasympathetic stimulation increases secretion of all digestive enzymes.
Mucus
MucusMucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
is released in the stomach and intestine, and serves to lubricate and protect the inner mucosa of the tract. It is composed of a specific family of glycoproteins termed mucins and is generally very viscous. Mucus is made by two types of specialized cells termed mucus cells in the stomach and goblet cells in the intestines. Signals for increased mucus release include parasympathetic innervations, immune system response and enteric nervous system messengers.
Bile
Bile is secreted into the duodenum of the small intestine via the common bile ductCommon bile duct
The common bile duct is a tube-like anatomic structure in the human gastrointestinal tract. It is formed by the union of the common hepatic duct and the cystic duct . It is later joined by the pancreatic duct to form the ampulla of Vater...
. It is produced in liver cells and stored in the gall bladder until release during a meal. Bile is formed of three elements: bile salts, bilirubin
Bilirubin
Bilirubin is the yellow breakdown product of normal heme catabolism. Heme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases...
and cholesterol. Bilirubin is a waste product of the breakdown of hemoglobin. The cholesterol present is secreted with the feces. The bile salt component is an active non-enzymatic substance that facilitates fat absorption by helping it to form an emulsion with water due to its amphoteric nature. These salts are formed in the hepatocytes from bile acids combined with an amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
. Other compounds such as the waste products of drug degradation are also present in the bile.
Regulation
The digestive system has a complex system of motility and secretion regulation which is vital for proper function. This task is accomplished via a system of long reflexes from the central nervous systemCentral nervous system
The central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
(CNS), short reflexes from the enteric nervous system
Enteric nervous system
The enteric nervous system is a subdivision of the autonomic nervous system that directly controls the gastrointestinal system in vertebrates.It is derived from neural crest.-Function:...
(ENS) and reflexes from GI peptides working in harmony with each other.
Long Reflexes
Long reflexes to the digestive system involve a sensory neuron sending information to the brain, which integrates the signal and then sends messages to the digestive system. While in some situations, the sensory information comes from the GI tract itself; in others, information is received from sources other than the GI tract. When the latter situation occurs, these reflexes are called feedforward reflexes. This type of reflex includes reactions to food or danger triggering effects in the GI tract. Emotional responses can also trigger GI response such as the butterflies in the stomach feeling when nervous. The feedforward and emotional reflexes of the GI tract are considered cephalic reflexes.Short Reflexes
Control of the digestive system is also maintained by ENS, which can be thought of as a digestive brain that can help to regulate motility, secretion and growth. Sensory information from the digestive system can be received, integrated and acted upon by the enteric system alone. When this occurs, the reflex is called a short reflex. Although this may be the case in several situations, the ENS can also work in conjunction with the CNS; vagal afferents from the viscera are received by the medulla, efferents are effected by the vagus nerve. When this occurs, the reflex is called vagovagal reflex. The Myenteric plexus and Submucosal plexus are both located in the gut wall and receive sensory signals from the lumen of the gut or the CNS.GI Peptides
GI peptides are signal molecules that are released into the blood by the GI cells themselves. They act on a variety of tissues including the brain, digestive accessory organs, and the GI tract. The effects range from excitatory or inhibitory effects on motility and secretion to feelings of satiety or hunger when acting on the brain. These hormones fall into three major categories, the gastrinGastrin
In humans, gastrin is a peptide hormone that stimulates secretion of gastric acid by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the antrum of the stomach, duodenum, and the pancreas...
and secretin
Secretin
Secretin is a hormone that controls the secretions into the duodenum, and also separately, water homeostasis throughout the body. It is produced in the S cells of the duodenum in the crypts of Lieberkühn...
families, with the third composed of all the other hormones unlike those in the other two families. Further information on the GI peptides is summarized in the table below.
| Secreted By | Target | Effects on Endocrine Secretion | Effects on Exocrine Secretion | Effects on Motility | Other Effects | Stimulus for Release | |
|---|---|---|---|---|---|---|---|
| Gastrin | G Cells in stomach | ECL cells; parietal cells | None | Increases acid secretion, increases mucus growth | None | None | Peptides and amino acids in lumen; gastrin releasing peptide and Ach in nervous reflexes |
| Cholecystokinin (CCK) | Endocrine cells of the small intestine; neurons of the brain and gut | Gallbladder, pancreas, gastric smooth muscle | None | Stimulates pancreatic enzyme and HCO3- secretion | Stimulates gallbladder contraction; Inhibits stomach emptying | Satiety | Fatty Acids and some Amino acids |
| Secretin | Endocrine Cells of the Small Intestine | Pancreas, stomach | None | Stimulates pancreatic and hepatic HCO3- secretion; Inhibits acid secretion; Pancreatic growth | Stimulates gallbladder contraction; Inhibits stomach emptying | None | Acid in small intestine |
| Gastric inhibitory Peptide | Endocrine K Cells of the small intestine | Beta Cells of the pancreas | Stimulates pancreatic insulin release | Inhibits Acid Secretion | None | Satiety and lipid metabolism | Glucose, Fatty Acid, and amino acids in small intestine |
| Motilin | Endocrine Cells in Small intestine | Smooth muscle of antrum and duodenum | None | None | Stimulates Migrating motor complex | Action in Brain?, Stimulates Migratory Motor Complex | Fasting: Cyclic release every 1.5–2 hours by neural stimulus. |
| Glucagon Like Peptide 1 | Endocrine Cells in Small Intestine | Endocrine Pancreas | Stimulates Insulin release; inhibits glucagon release | Possibly Inhibits Acid Secretion | Slows gastric Emptying | Satiety | Mixed meals of Fats and Carbohydrates. |
Digestion
- carbohydrates (monosaccharideMonosaccharideMonosaccharides are the most basic units of biologically important carbohydrates. They are the simplest form of sugar and are usually colorless, water-soluble, crystalline solids. Some monosaccharides have a sweet taste. Examples of monosaccharides include glucose , fructose , galactose, xylose...
, disaccharideDisaccharideA disaccharide or biose is the carbohydrate formed when two monosaccharides undergo a condensation reaction which involves the elimination of a small molecule, such as water, from the functional groups only. Like monosaccharides, disaccharides form an aqueous solution when dissolved in water...
) - proteins
- lipids
External links
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