Digestion
For the industrial process, see anaerobic digestion.
For the treatment of precipitates in analytical chemistry, see Precipitation
(chemistry)#Digestion.
"Gastrointestinal tract"
redirects here. For digestion in humans specifically, see Human
gastrointestinal tract.
Digestive system
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systema digestorium
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Digestion is the breakdown of food into
smaller components that can be more easily absorbed and assimilated by the
body. In certain organisms, these smaller substances are absorbed through the small intestine into the blood stream. Digestion is a form ofcatabolism that is often divided into two
processes based on how food is broken down: mechanical and chemical digestion.
The termmechanical digestion refers to the physical breakdown of
large pieces of food into smaller pieces which can subsequently be accessed by
digestive enzymes. In chemical digestion, enzymes break down food into the small molecules the
body can use.
In the human digestive
system, food enters the mouth and mechanical digestion of the food
starts by the action of mastication, a form of
mechanical digestion, and the wetting contact of saliva. Saliva, a liquid secreted by the salivary glands, contains salivary amylase, an enzyme which starts the
digestion of starch in the food. After undergoing mastication and starch
digestion, the food will be in the form of a small, round slurry mass called a bolus. It will then travel down the esophagus and into the stomach by the action ofperistalsis. Gastric juice in the stomach starts protein digestion. Gastric juice mainly
contains hydrochloric acid and pepsin. As these two chemicals may damage the stomach wall, mucus is
secreted by the stomach, providing a slimy layer that acts as a shield against
the damaging effects of the chemicals. At the same time protein digestion is
occurring, mechanical mixing occurs by peristalsis, which is waves of muscular
contractions that move along the stomach wall. This allows the mass of food to
further mix with the digestive enzymes.
After some time (typically 1–2 hours in
humans, 4–6 hours in dogs, 3–4 hours in house cats),[citation needed] the
resulting thick liquid is called chyme.
When the pyloric sphincter valve opens, chyme
enters the duodenum where it mixes with digestive
enzymes from the pancreas, and then passes
through the small intestine,
in which digestion continues. When the chyme is fully digested, it is absorbed
into the blood. 95% of absorption of nutrients occurs in the small intestine.
Water and minerals are reabsorbed back into the blood in the colon (large intestine) where the pH is
slightly acidic about 5.6 ~ 6.9. Some vitamins, such as biotin and vitamin K (K2MK7) produced by
bacteria in the colon are also absorbed into the blood in the colon. Waste
material is eliminated from the rectum during defecation
Digestive systems
Digestive systems take many forms. There is
a fundamental distinction between internal and external digestion. External
digestion is more primitive, and most fungi still
rely on it.In this process, enzymes are secreted into the environment surrounding
the organism, where they break down an organic material, and some of the
products diffuse back
to the organism. Animals have a tube (gastrointestinal
tract) in which internal digestion occurs, which is more efficient
because more of the broken down products can be captured, and the internal
chemical environment can be more efficiently controlled.
Some organisms, including nearly all spiders, simply secrete biotoxins and
digestive chemicals (e.g., enzymes) into the
extracellular environment prior to ingestion of the consequent
"soup". In others, once potential nutrients or food is inside the organism, digestion can be conducted to a vesicle or a sac-like structure, through
a tube, or through several specialized organs aimed at making the absorption of
nutrients more efficient.
Schematic
drawing of bacterial conjugation. 1- Donor cell produces pilus. 2-Pilus
attaches to recipient cell, bringing the two cells together. 3- The
mobile plasmid is nicked and a single strand of DNA is transferred to the
recipient cell. 4- Both cells recircularize their plasmids,
synthesize second strands, and reproduce pili; both cells are now viable
donors.
Secretion
systems
Bacteria use several systems to obtain
nutrients from other organisms in the environments.
Channel
transport system
In a channel transupport system, several
proteins form a contiguous channel traversing the inner and outer membranes of
the bacteria. It is a simple system, which consists of only three protein
subunits: the ABC protein, membrane fusion
protein (MFP), andouter membrane
protein (OMP)[specify].
This secretion system transports various molecules, from ions, drugs, to
proteins of various sizes (20 - 900 kDa). The molecules secreted vary in size
from the small Escherichia coli peptide colicin V, (10 kDa) to
thePseudomonas fluorescens cell adhesion protein LapA of 900 kDa.
Molecular
syringe
One molecular syringe is used through which
a bacterium (e.g. certain types of Salmonella, Shigella, Yersinia)
can inject nutrients into protist cells. One such mechanism was first
discovered in Y. pestis and showed that toxins could be
injected directly from the bacterial cytoplasm into the cytoplasm of its host's
cells rather than simply be secreted into the extracellular medium
Conjugation
machinery
The conjugation machinery
of some bacteria (and archaeal flagella) is capable of transporting both DNA
and proteins. It was discovered in Agrobacterium tumefaciens, which
uses this system to introduce the Ti plasmid and proteins into the host, which
develops the crown gall (tumor). The VirB complex of Agrobacterium
tumefaciens is the prototypic system.
The nitrogen fixing Rhizobia are an interesting case,
wherein conjugative elements naturally engage in inter-kingdom conjugation. Such elements as the Agrobacterium Ti or Ri plasmids
contain elements that can transfer to plant cells. Transferred genes enter the
plant cell nucleus and effectively transform the plant cells into factories for
the production ofopines, which the bacteria use as carbon and
energy sources. Infected plant cells form crown gall or root tumors.
The Ti and Ri plasmids are thus endosymbionts of the bacteria, which are
in turn endosymbionts (or parasites) of the infected plant.
The Ti and Ri plasmids are themselves
conjugative. Ti and Ri transfer between bacteria uses an independent system
(the tra, or transfer, operon) from that for inter-kingdom transfer
(the vir, or virulence, operon). Such
transfer creates virulent strains from previously avirulent Agrobacteria.
Release
of outer membrane vesicles
In addition to the use of the multiprotein
complexes listed above, Gram-negative bacteria possess another method for
release of material: the formation of outer membrane vesicles. Portions of the outer membrane
pinch off, forming spherical structures made of a lipid bilayer enclosing
periplasmic materials. Vesicles from a number of bacterial species have been
found to contain virulence factors, some have immunomodulatory effects, and
some can directly adhere to and intoxicate host cells. While release of
vesicles has been demonstrated as a general response to stress conditions, the
process of loading cargo proteins seems to be selective.
Venus
Flytrap (Dionaea muscipula) leaf
Gastrovascular
cavity
The gastrovascular cavity functions
as a stomach in both digestion and the distribution of nutrients to all parts
of the body. Extracellular digestion takes place within this central cavity,
which is lined with the gastrodermis, the internal layer of epithelium. This cavity has only one opening
to the outside that functions as both a mouth and an anus: waste and undigested
matter is excreted through the mouth/anus, which can be described as an
incomplete gut.
In a plant such as the Venus Flytrap that can make its own food through
photosynthesis, it does not eat and digest its prey for the traditional
objectives of harvesting energy and carbon, but mines prey primarily for
essential nutrients (nitrogen and phosphorus in particular) that are in short
supply in its boggy, acidic habitat.
Trophozoites ofEntamoeba histolytica with
ingested erythrocytes
Phagosome
A phagosome is a vacuole formed around a particle absorbed
by phagocytosis. The vacuole is formed by the
fusion of the cell membrane around
the particle. A phagosome is a cellular compartment in
which pathogenic microorganisms can be killed
and digested. Phagosomes fuse with lysosomes in their maturation process,
forming phagolysosomes. In
humans, Entamoeba histolytica can
phagocytose red blood cells
Specialised
organs and behaviours
To aid in the digestion of their food
animals evolved organs such as beaks, tongues, teeth, a crop, gizzard, and others.
A Catalina Macaw's seed-shearing beak
Squid
beak with ruler for size comparison
Beaks
Birds have bony beaks that
are specialised according to the bird's ecological niche. For example, macaws primarily
eat seeds, nuts, and fruit, using their impressive beaks to open even the
toughest seed. First they scratch a thin line with the sharp point of the beak,
then they shear the seed open with the sides of the beak.
The mouth of the squid is
equipped with a sharp horny beak mainly made of cross-linked proteins. It is used to kill and tear prey
into manageable pieces. The beak is very robust, but does not contain any
minerals, unlike the teeth and jaws of many other organisms, including marine
species. The beak is the only
indigestible part of the squid.
Tongue
The tongue is skeletal
muscle on the floor of the mouth that manipulates food for
chewing (mastication) and swallowing (deglutition). It is sensitive
and kept moist by saliva. The underside of
the tongue is covered with a smooth mucous membrane. The tongue also has a touch
sense for locating and positioning food particles that require further chewing.
The tongue is utilized to roll food particles into a bolus before being transported down the esophagus through peristalsis.
The sublingual region underneath the front of
the tongue is a location where the oral mucosa is very thin, and underlain
by a plexus of veins. This is an ideal location for introducing certain
medications to the body. The sublingual route takes advantage of the highly vascular quality of the oral cavity, and
allows for the speedy application of medication into the cardiovascular system,
bypassing the gastrointestinal tract.
Teeth
Teeth (singular tooth) are small whitish
structures found in the jaws (or mouths) of many vertebrates that are used to
tear, scrape, milk and chew food. Teeth are not made of bone, but rather of
tissues of varying density and hardness, such as enamel, dentine and cementum.
Human teeth have a blood and nerve supply which enables proprioception. This is
the ability of sensation when chewing, for example if we were to bite into
something too hard for our teeth, such as a nectarine pip, our teeth send a
message to our brain and we realise that it cannot be chewed, so we stop
trying.
The shapes, sizes and numbers of types of
animals' teeth are related to their diets. For example, herbivores have a
number of molars which are used to grind plant matter, which is difficult to
digest. Carnivores have canine teeth which are used to kill and
tear meat.
Crop
A crop, or croup, is a thin-walled expanded
portion of the alimentary tract used
for the storage of food prior to digestion. In some birds it is an expanded,
muscular pouch near thegullet or throat. In
adult doves and pigeons, the crop can produce crop milk to feed newly hatched birds.
Rough
illustration of a ruminant digestive system
Abomasum
Herbivores have evolved cecums (or
an abomasum in the case of ruminants). Ruminants
have a fore-stomach with four chambers. These are therumen, reticulum, omasum, and abomasum. In the first two chambers, the rumen
and the reticulum, the food is mixed with saliva and separates into layers of
solid and liquid material. Solids clump together to form the cud (or bolus). The cud is then regurgitated, chewed
slowly to completely mix it with saliva and to break down the particle size.
Fibre, especially cellulose and hemi-cellulose, is primarily broken down into
the volatile fatty acids, acetic acid, propionic acid and butyric acid in these chambers (the
reticulo-rumen) by microbes: (bacteria, protozoa, and fungi).
In the omasum, water and many of the inorganic mineral elements are absorbed
into the blood stream.
The abomasum is the fourth and final
stomach compartment in ruminants. It is a close equivalent of a monogastric
stomach (e.g., those in humans or pigs), and digesta is processed here in much
the same way. It serves primarily as a site for acid hydrolysis of microbial
and dietary protein, preparing these protein sources for further digestion and
absorption in the small intestine. Digesta is finally moved into the small
intestine, where the digestion and absorption of nutrients occurs. Microbes
produced in the reticulo-rumen are also digested in the small intestine.
A
flesh fly "blowing a bubble", possibly to concentrate its food by
evaporating water
Specialised
behaviours
Regurgitation has
been mentioned above under abomasum and crop, referring to crop milk, a
secretion from the lining of the crop ofpigeons and doves with which the parents
feed their young by regurgitation.
Many sharks have
the ability to turn their stomachs inside out and evert it out of their mouths
in order to get rid of unwanted contents (perhaps developed as a way to reduce
exposure to toxins).
Other animals, such as rabbits and rodents, practise coprophagia behaviours - eating
specialised faeces in order to re-digest food, especially in the case of
roughage. Capybara, rabbits, hamsters and other related species do not have a
complex digestive system as do, for example, ruminants. Instead they extract
more nutrition from grass by giving their food a second pass through the gut.
Soft faecal pellets of partially digested food are excreted and generally
consumed immediately. They also produce normal droppings, which are not eaten.
Young elephants, pandas, koalas, and hippos
eat the faeces of their mother, probably to obtain the bacteria required to
properly digest vegetation. When they are born, their intestines do not contain
these bacteria (they are completely sterile). Without them, they would be
unable to get any nutritional value from many plant components.
In
earthworms
An earthworm's digestive system consists of a mouth, pharynx, esophagus, crop, gizzard, and intestine. The mouth is surrounded by strong
lips, which act like a hand to grab pieces of dead grass, leaves, and weeds,
with bits of soil to help chew. The lips break the food down into smaller
pieces. In the pharynx, the food is lubricated by mucus secretions for easier
passage. The esophagus adds calcium carbonate to neutralize the acids formed by
food matter decay. Temporary storage occurs in the crop where food and calcium
carbonate are mixed. The powerful muscles of the gizzard churn and mix the mass
of food and dirt. When the churning is complete, the glands in the walls of the
gizzard add enzymes to the thick paste, which helps chemically breakdown the
organic matter. By peristalsis, the
mixture is sent to the intestine where friendly bacteria continue chemical
breakdown. This releases carbohydrates, protein, fat, and various vitamins and
minerals for absorption into the body.
Overview of vertebrate digestion
In most vertebrates, digestion is a multistage process
in the digestive system, starting from ingestion of raw materials, most often
other organisms. Ingestion usually involves some type of mechanical and chemical
processing. Digestion is separated into four steps:
1.
Ingestion: placing food into the mouth (entry
of food in the digestive system),
2.
Mechanical and chemical breakdown:
mastication and the mixing of the resulting bolus with water, acids, bile and enzymes in the stomach and intestine to break down
complex molecules into simple structures,
3.
Absorption: of nutrients from the digestive
system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and
4.
Egestion (Excretion): Removal of undigested
materials from the digestive tract through defecation.
Underlying the process is muscle movement
throughout the system through swallowing and peristalsis. Each step in digestion requires
energy, and thus imposes an "overhead charge" on the energy made
available from absorbed substances. Differences in that overhead cost are
important influences on lifestyle, behavior, and even physical structures.
Examples may be seen in humans, who differ considerably from other hominids
(lack of hair, smaller jaws and musculature, different dentition, length of
intestines, cooking, etc.).
The major part of digestion takes place in
the small intestine. The large intestine primarily serves as a site for
fermentation of indigestible matter by gut bacteria and for resorption of water
from digests before excretion.
In mammals, preparation for digestion begins with the cephalic phase in which saliva is produced in the mouth and digestive enzymes are produced in the stomach. Mechanical and chemical digestion
begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down
mechanically and chemically through churning and mixing with both acids and
enzymes. Absorption occurs
in the stomach and gastrointestinal
tract, and the process finishes withdefecation.
Human digestion process
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It has been suggested that this section be split into
a new article titled Human digestive
system. (Discuss) Proposed since April 2014.
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Upper
and Lower human gastrointestinal tract
The human
gastrointestinal tract is around 9 meters long. In a healthy
human adult this process can take between 24 and 72 hours. Food digestion
physiology varies between individuals and upon other factors such as the
characteristics of the food and size of the meal
Phases
of gastric secretion
·
Cephalic phase - This phase occurs before
food enters the stomach and involves preparation of the body for eating and
digestion. Sight and thought stimulate the cerebral cortex. Taste and smell stimulus is
sent to the hypothalamusand medulla oblongata. After this it is routed
through the vagus nerve and
release of acetylcholine. Gastric secretion at this phase rises to 40% of
maximum rate. Acidity in the stomach is not buffered by food at this point and
thus acts to inhibit parietal (secretes acid) and G cell (secretes gastrin) activity via D cell secretion of somatostatin.
·
Gastric phase - This phase takes 3 to 4
hours. It is stimulated by distension of
the stomach, presence of food in stomach and decrease in pH.
Distention activates long and myenteric reflexes. This activates the release ofacetylcholine, which stimulates the release of
more gastric juices. As
protein enters the stomach, it binds tohydrogen ions, which raises the pH of
the stomach. Inhibition of gastrin and gastric acid secretion is lifted. This
triggers G cells to release gastrin, which in turn stimulates parietal cells to secrete gastric acid.
Gastric acid is about 0.5% hydrochloric acid (HCl), which lowers the
pH to the desired pH of 1-3. Acid release is also triggered byacetylcholine and histamine.
·
Intestinal phase - This phase has 2 parts,
the excitatory and the inhibitory. Partially digested food fills theduodenum. This triggers intestinal gastrin to
be released. Enterogastric reflex inhibits vagal nuclei, activatingsympathetic
fibers causing the pyloric sphincter to tighten to prevent
more food from entering, and inhibits local reflexes.
Oral
cavity
In humans, digestion begins in
the Mouth, otherwise known as the "Buccal Cavity", where food is chewed. Saliva is secreted in large amounts (1-1.5 litres/day)
by three pairs of exocrine salivary glands (parotid, submandibular, and
sublingual) in the oral cavity, and is mixed with the chewed food by the
tongue. Saliva cleans the oral cavity, moistens the food, and contains
digestive enzymes such as salivary amylase, which aids in the chemical breakdown ofpolysaccharides such as starch into disaccharides such as maltose. It also contains mucus, a glycoprotein that helps soften the food
and form it into a bolus. An
additional enzyme, lingual lipase,
hydrolyzes long-chain triglycerides into partial glycerides and free fatty
acids.
Swallowing transports the chewed food
into the esophagus, passing through the oropharynx and hypopharynx. The mechanism for swallowing is
coordinated by the swallowing center in the medulla oblongata and pons.
The reflex is initiated by touch receptors in the pharynx as the bolus of food
is pushed to the back of the mouth.
Pharynx
The pharynx is the part of the neck and
throat situated behind the mouth and nasal cavity, and cranial, or superior, to
the esophagus. It is part of the digestive system andrespiratory system.
Because both food and air pass through the pharynx, a flap of connective
tissue, the epiglottis closes
over the trachea when food is swallowed to prevent choking or asphyxiation.
The oropharynx is that part of the pharynx
behind the oral cavity. It is lined with stratified
squamous epithelium. The nasopharynx lies behind the nasal cavity
and like the nasal passages is lined with ciliated columnar pseudostratified
epithelium.
Like the oropharynx above it the
hypopharynx (laryngopharynx)
serves as a passageway for food and air and is lined with a stratified squamous
epithelium. It lies inferior to the upright epiglottis and extends to the
larynx, where the respiratory and digestive pathways diverge. At that point,
the laryngopharynx is continuous with the esophagus. During swallowing, food
has the "right of way", and air passage temporarily stops.
Esophagus
The esophagus is a narrow muscular tube
about 20-30 centimeters long, which starts at the pharynx at the back of the mouth, passes
through the thoracic diaphragm,
and ends at the cardiac orifice of
the stomach. The wall of the esophagus is made up
of two layers of smooth muscles,
which form a continuous layer from the esophagus to the colon and contract
slowly, over long periods of time. The inner layer of muscles is arranged
circularly in a series of descending rings, while the outer layer is arranged
longitudinally. At the top of the esophagus, is a flap of tissue called the epiglottis that closes during swallowing
to prevent food from entering the trachea (windpipe).
The chewed food is pushed down the esophagus to the stomach through peristaltic contraction of these muscles.
It takes only about seven seconds for food to pass through the esophagus and
now digestion takes place.
Stomach
The stomach is a small, 'J'-shaped pouch
with walls made of thick, distensible muscles, which stores and helps break down food. Food
reduced to very small particles is more likely to be fully digested in the
small intestine, and stomach churning has the effect of assisting the physical
disassembly begun in the mouth. Ruminants, who are able to digest fibrous
material (primarily cellulose), use
fore-stomachs and repeated chewing to further the disassembly. Rabbits and some
other animals pass some material through
their entire digestive systems twice. Most birds ingest small stones to assist
in mechanical processing in gizzards.
Food enters the stomach through the cardiac
orifice where it is further broken apart and thoroughly mixed with gastric acid, pepsin and other digestive enzymes to break down proteins.
The enzymes in the stomach also have an optimum conditions, meaning that they
work at a specific pH and temperature better than any others. The acid itself
does not break down food molecules, rather it provides an optimum pH for the
reaction of the enzyme pepsin and kills many
microorganisms that are ingested with the food. It can also denature proteins.
This is the process of reducing polypeptide bonds and disrupting salt bridges,
which in turn causes a loss of secondary, tertiary, or quaternary protein
structure. The parietal cells of
the stomach also secrete a glycoprotein called intrinsic factor, which enables the absorption
of vitamin B-12. Mucus
neck cells are present in the gastric glands of the stomach. They secrete mucus,
which along with gastric juice plays
an important role in lubrication and protection of the mucosal epithelium from
excoriation by the highly concentrated hydrochloric acid. Other small molecules such
as alcohol are absorbed in
the stomach, passing through the membrane of the stomach and entering thecirculatory system directly.
Food in the stomach is in semi-liquid form, which upon completion is known as chyme.
After consumption of food, digestive
"tonic" and peristaltic contractions
begin, which helps break down the food and move it onward. When the chyme reaches the
opening to the duodenum known as
the pylorus, contractions "squirt" the
food back into the stomach through a process called retropulsion, which exerts
additional force and further grinds down food into smaller particles.[15] Gastric emptying is the
release of food from the stomach into the duodenum; the process is tightly
controlled with liquids being emptied much more quickly than solids.[15] Gastric emptying has attracted
medical interest as rapid gastric
emptying is related to obesity and delayed gastric emptying syndrome is
associated with diabetes mellitus, aging, and gastroesophageal reflux.
The transverse section of the alimentary
canal reveals four (or five, see description under mucosa) distinct and well
developed layers within the stomach:
·
Serous membrane, a thin layer of mesothelial cells that is the outermost
wall of the stomach.
·
Muscular coat, a well-developed layer of
muscles used to mix ingested food, composed of three sets running in three
different alignments. The outermost layer runs parallel to the vertical axis of
the stomach (from top to bottom), the middle is concentric to the axis
(horizontally circling the stomach cavity) and the innermost oblique layer,
which is responsible for mixing and breaking down ingested food, runs diagonal
to the longitudinal axis. The inner layer is unique to the stomach, all other
parts of the digestive tract have only the first two layers.
·
Submucosa, composed of connective tissue that links the inner
muscular layer to the mucosa and contains the nerves,
blood and lymph vessels.
·
Mucosa is the extensively folded innermost layer. It
can be divided into the epithelium, lamina propria, and the muscularis mucosae,
though some consider the outermostmuscularis mucosae to be a
distinct layer, as it develops from the mesoderm rather than the endoderm (thus
making a total of five layers). The epithelium and lamina are filled with connective tissue and covered in gastric glands that may be simple or
branched tubular, and secrete mucus, hydrochloric acid, pepsinogen and rennin. The mucus lubricates the food and also prevents
hydrochloric acid from acting on the walls of the stomach.
Small
intestine
After being processed in the stomach, food
is passed to the small intestine via
the pyloric sphincter. The majority of digestion
and absorption occurs here after the milky chymeenters
the duodenum. Here it is further mixed with three different
liquids:
·
Bile, which emulsifies fats to
allow absorption, neutralizes the
chyme and is used to excrete waste products such as bilin and bile acids. Bile is produced by the liver and
then stored in the gallbladder where
it will be released to the small intestine via the bile duct. The bile in the gallbladder is much
more concentrated.[clarification
needed]
·
Pancreatic juice made by the pancreas, which secretes enzymes such as pancreatic amylase, pancreatic lipase, and trypsinogen (inactive form of protease).
·
Intestinal juice secreted by the intestinal glands in the small intestine. It contains enzymes such as enteropeptidase, erepsin, trypsin, chymotrypsin, maltase, lactase andsucrase (all three of which process only sugars).
The pH level increases in the small
intestine as all three fluids are alkaline. A more basic environment causes more
helpful enzymes to activate and begin to help in the breakdown of molecules
such as fat globules.
Small, finger-like structures called villi, and their epithelial cells is covered with numerous microvilli to improve the absorption of
nutrients by increasing the surface area of the intestine and
enhancing speed at which nutrients are absorbed. Blood containing
the absorbed nutrients is carried away from the small intestine via the hepatic portal vein and
goes to the liver for filtering, removal of toxins,
and nutrient processing.
The small intestine and remainder of the
digestive tract undergoes peristalsis to transport food from the
stomach to the rectum and allow food to be mixed with
the digestive juices and absorbed. The circular muscles and longitudinal
muscles are antagonistic muscles,
with one contracting as the other relaxes. When the circular muscles contract,
the lumenbecomes
narrower and longer and the food is squeezed and pushed forward. When the
longitudinal muscles contract, the circular muscles relax and the gut dilates
to become wider and shorter to allow food to enter.
Large
intestine
After the food has been passed through the
small intestine, the food enters the large intestine. Within it, digestion is
retained long enough to allow fermentation due to the action of gut bacteria,
which breaks down some of the substances that remain after processing in the
small intestine; some of the breakdown products are absorbed. In humans, these
include most complex saccharides (at most three disaccharides are digestible in
humans). In addition, in many vertebrates, the large intestine reabsorbs fluid;
in a few, with desert lifestyles, this reabsorbtion makes continued existence
possible.
In general, the large intestine is less
vigorous in absorptive activity. It produces sacculation, renews epithelial
cells, and provides protective mucus and mucosal immunity. In humans, the large
intestine is roughly 1.5 meters long, with three parts: the cecum at
the junction with the small intestine, the colon, and the rectum. The colon itself has four parts: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The large intestine absorbs
water from the chyme and stores feces until
it can beegested. Food products that cannot go through
the villi, such as cellulose (dietary fiber), are mixed with other waste
products from the body and become hard and concentratedfeces.
The feces are stored in the rectum for a certain
period and then the stored feces are eliminated from the body, through the anus,
by relaxation of the anal sphincterassisted
by contraction of the abdominal muscles and diaphragm.
Breakdown into nutrients
Protein
digestion
Protein digestion occurs in the stomach and duodenum in which 3 main enzymes, pepsin secreted by the stomach and trypsin and chymotrypsin secreted by the pancreas,
break down food proteins into polypeptides that are then broken down by
various exopeptidases and dipeptidases into amino acids. The digestive enzymes however are
mostly secreted as their inactive precursors, the zymogens. For example, trypsin is secreted by
pancreas in the form of trypsinogen, which is
activated in the duodenum by enterokinase to form trypsin. Trypsin
then cleaves proteins to smaller polypeptides.
Fat
digestion
Digestion of some fats can begin in the
mouth where lingual lipase breaks
down some short chain lipids into diglycerides. However fats are mainly digested
in the small intestine. The presence of fat in the
small intestine produces hormones that stimulate the release of pancreatic lipase from the pancreas and bile from the liver which helps in the
emulsification of fats for absorption fatty acids Complete digestion of one
molecule of fat (a triglyceride) results
in 3 fatty acid molecules and one glycerol molecule
Carbohydrate
digestion
In humans, dietary starches are composed of glucose units arranged in long chains
called amylose, a polysaccharide.
During digestion, bonds between glucose molecules are broken by salivary and
pancreatic amylase, resulting in progressively smaller
chains of glucose. This results in simple sugars glucose and maltose (2 glucose molecules) that can be
absorbed by the small intestine.
Lactase is an enzyme that breaks down the disaccharide lactose to its component parts, glucose
and galactose. Glucose and galactose can be
absorbed by the small intestine. Approximately half of the adult population
produce only small amounts of lactase and are unable to eat milk-based foods.
This is commonly known as lactose intolerance.
Sucrase is an enzyme that breaks down the
disaccharide sucrose, commonly known as table sugar, cane
sugar, or beet sugar. Sucrose digestion yields the sugars fructose and glucose which are readily
absorbed by the small intestine.
DNA
and RNA digestion
DNA and RNA are broken down into mononucleotides by the nucleases deoxyribonuclease and ribonuclease (DNase and RNase) from the
pancreas.
Non-destructive digestion
Some nutrients are complex molecules (for
example vitamin B12)
which would be destroyed if they were broken down into their functional groups. To digest vitamin B12 non-destructively, haptocorrin in saliva strongly binds and protects the B12 molecules
from stomach acid as they enter the stomach and are cleaved from their protein
complexes.
After the B12-haptocorrin
complexes pass from the stomach via the pylorus to the duodenum, pancreatic
proteases cleave haptocorrin from the B12 molecules which
rebind tointrinsic factor (IF).
These B12-IF complexes travel to the ileum portion of the small
intestine where cubilin receptors
enable assimilation and
circulation of B12-IF complexes in the blood
Digestive hormones
Action
of the major digestive hormones
There are at least five hormones that aid
and regulate the digestive system in mammals. There are variations across the
vertebrates, as for instance in birds. Arrangements are complex and additional
details are regularly discovered. For instance, more connections to metabolic
control (largely the glucose-insulin system) have been uncovered in recent
years.
·
Gastrin - is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the
enzymepepsin) and hydrochloric acid. Secretion of gastrin is
stimulated by food arriving in stomach. The secretion is inhibited by low pH.
·
Secretin - is in the duodenum and signals the secretion of
sodium bicarbonate in the pancreas and it
stimulates the bile secretion in the liver.
This hormone responds to the acidity of the chyme.
·
Cholecystokinin (CCK) - is in the
duodenum and stimulates the release of digestive enzymes in the pancreas and
stimulates the emptying of bile in the gall bladder. This hormone is secreted in
response to fat in chyme.
·
Gastric
inhibitory peptide (GIP) - is in the duodenum and decreases the
stomach churning in turn slowing the emptying in the stomach. Another function
is to induce insulin secretion.
·
Motilin - is in the duodenum and
increases the migrating
myoelectric complex component of gastrointestinal motility and
stimulates the production of pepsin.
Significance of pH in digestion
Digestion is a complex process controlled
by several factors. pH plays a crucial role in a normally
functioning digestive tract. In the mouth, pharynx, and esophagus, pH is
typically about 6.8, very weakly acidic. Saliva controls pH in this region of the digestive
tract. Salivary amylase is
contained in saliva and starts the breakdown of carbohydrates intomonosaccharides. Most digestive enzymes are
sensitive to pH and will denature in a high or low pH environment.
The stomach's high acidity inhibits the
breakdown of carbohydrates within
it. This acidity confers two benefits: it denatures proteins
for further digestion in the small intestines, and provides non-specific immunity,
damaging or eliminating various pathogens.[citation needed]
In the small intestines, the duodenum
provides critical pH balancing to activate digestive enzymes. The liver
secretes bile into the duodenum to neutralize the acidic conditions from the
stomach, and the pancreatic duct empties
into the duodenum, adding bicarbonate to neutralize the acidic chyme,
thus creating a neutral environment. The mucosal tissue of the small intestines
is alkaline with a pH of about 8.5.[citation needed]
Uses of animal's internal organs by humans
·
The use of animal gut strings by musicians can be traced back
to the third dynasty of
Egypt. In the recent past, strings were made out of lamb gut. With the advent of the modern
era, musicians have tended to use strings made of silk,
or synthetic materials such as nylon or steel.
Some instrumentalists, however, still use gut strings in order to evoke the
older tone quality. Although such strings were commonly referred to as "catgut" strings, cats were
never used as a source for gut strings.[citation needed]
·
Sheep gut was the original source for
natural gut string used in racquets, such as for tennis. Today, synthetic strings are much more common, but
the best gut strings are now made out of cow gut.
·
Gut cord has also been used to produce
strings for the snares that provide a snare drum's characteristic buzzing timbre.
While the modern snare drum almost always uses metal wire rather than gut cord,
the North African bendir frame drum still uses gut for this purpose.
·
"Natural" sausage hulls (or casings) are made of animal gut, especially
hog, beef, and lamb. Similarly, Haggis is traditionally boiled in, and served in, a
sheep stomach.
·
Chitterlings, a kind of food, consist of
thoroughly washed pig's gut.
·
Animal gut was used to make the cord lines
in longcase clocks and
for fusee movements
in bracket clocks,
but may be replaced by metal wire.
·
The oldest known condoms, from 1640 AD, were made from animal intestine.
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