My chosen structure is the Digestive System in mammals. It is
arguably the most important system that humans and other mammals have. The
function of the Digestive System is to absorb the nutrients and energy that are
released from digested food so that the organism can function. The system
consists of many organs and involves many enzymes, that undergo many processes,
to successfully break down the food into small enough molecules for
reabsorption. Some organs such as the; pancreas, gall bladder, liver and
salivary glands do not have direct contact with the food but are essential for
the process of digestion. Most of the hollow digestive organs have four layers
to their cell wall; the muscous layer which contains mucus producing goblet
cells, the submucosa layer that has the blood vessels and the nerve fibres, the
Muscularis which is made up of bands of longitudinal and circular smooth muscle
and the serosa which acts as a skin around the tract and secretes fluid that
reduces friction with other organ parts. Mucus provides protection and
lubrication throughout the digestive tract. Food is moved around the hollow
organs by Peristalsis; waves of contraction from the longitudinal and circular
smooth muscle in the wall.

 

The first part of the digestive system is the mouth. Here
food enters the system and is mechanically broken food by the teeth into small
enough pieces to pass through the oesophagus. The salivary glands are exocrine
glands that secrete saliva into the mouth via multiple ducts. Saliva’s high-water
content softens the food and additionally the teeth grind the food down into small
enough sizes that enzyme activity can occur. This process of breaking the food
down by teeth is called mastication. The saliva also contains mucus and amylase
that breaks down starch into smaller sugar molecules such as maltose. The surface
of the tongue contains hundreds of protrusions called papillae which are
responsible for the taste sense and for friction to grip the food to aid its
movement around the mouth. The tongue moves the food from one side of the mouth
to the other and moves it to the back of the mouth where it enters the pharynx
which joins the mouth to the oesophagus. The high-water content of the food
helps it to move down the oesophagus. The upper oesophageal sphincter opens
when food is being swallowed and closes after to stop air entering the
oesophagus.  The wall of the oesophagus
contains the submucosal glands that contain myoepithelial cells which secrete
mucus onto the epithelium to protect it from getting damaged by stomach acid
and food. At the base of the oesophagus is the cardiac sphincter which prevents
stomach acid entering the tract.

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At the end of the oesophagus is the stomach and they are
joined at the gastroesophageal junction. The food is already crushed down into
a pulp but for efficient nutrient transfer into the body by the intestines, it
needs to be broken down further. The main role of the stomach is for partial
chemical digestion and it contains Hydrochloric Acid and many enzymes. There
are 3 glands in the stomach; the Cardiac gland which contains mucous cells that
secrete mucous to protect the stomach epithelium from getting damaged by the
gastric acid, the fundic glands that contain parietal cells which make
hydrochloric acid and chief cells that release pepsinogen which activate into
pepsin when in contact with the acid and finally the Pyloric gland that has G
cells which secrete the hormone gastrin which stimulates the parietal cells to
make gastric acid. Hydrochloric acid has a few roles such as activation of
proteins, denaturing of food proteins and breakdown of food substances. the
stomach has folds called Rugae, using these, the stomach can increase its
volume to store and digest food. Muscles in the stomach wall contract to move
around and help the digestion of the food. A band of smooth muscle called the
Pyloric sphincter, which lies in-between the stomach and small intestine,
remains shut whilst food is being broken down in the stomach.

 

The solid organs of the digestive system are important as
they provide the digestive enzymes. The hepatocytes in the liver produce bile
and transfer it to the Gall bladder, through the hepatic bile duct, where it is
stored. The Gall bladder then secretes the bile into the Duodenum via the
common bile duct. Bile emulsifies fats making it easier for lipase enzymes to
break down the molecules for energy. The Pancreas secretes many important
enzymes such as lipase, trypsin, amylase and nuclease enzymes into the main pancreatic
duct that leads into the duodenum.

 

 

The small intestine has three main parts; the Duodenum, Jejunum
and the Ileum. Which all contain the hollow organ wall structure describes in
the introduction. When the food is ready to be passed on to the next stage of
digestion, the Pyloric sphincter opens, and Peristalsis push the Chyme, the
word used to describe the partially digested food from the stomach, into the
Duodenum where digestive enzymes from the exocrine glands are added. The
stretch of the Duodenum stimulates G cells in the wall to produce more gastrin,
which stimulates the stomach Parietal cells to make more Hydrochloric acid thus
aiding digestion. The Duodenum has a thin mucus layer and when the acidity of
the chyme is too much for the wall of the Duodenum, it secretes secretin which
stimulates the pancreas to release Sodium Bicarbonate into the duodenum, to neutralise
the pH. From the Duodenum the Chyme passes through the Duodenojejunal flexure
into the second part of the small intestine, the Jejunum. The wall of the
Jejunum is highly folded consisting of millions of long microvilli which are
the site of nutrient absorption and are responsible for absorbing ~90% of
nutrients from the Chyme. These long microvilli have a make up a massive
surface area for fast nutrient diffusion to occur, with just one cell thickness
and a blood supply, it is excellent for transporting nutrients away from the
intestine in the blood stream. Goblet cells in the wall of the Jejunum produce
mucus which helps the food move through the intestine. The circular smooth
muscle present in the wall contracts and moves around the food in the Jejunum
to ensure most nutrients are absorbed.  The
small intestine contains many healthy bacteria which help to fend off pathogens
by adjusting the levels of pH, toxins and oxygen levels. Some bacteria however
provide useful enzymes that break down indigestible molecules. The chyme now
passes into the Ileum by Peristalsis. The Ileum also has microvilli, which are
smaller than those in the Jejunum, however they are still responsible for
absorbing remaining food molecules such as glycerol and fatty acids into the
blood stream. From here the mostly digested food passes into the Cecum of the
large intestine, but before this is the terminal ileum which contains the
ileocecal sphincter. This muscle sphincter controls the amount of chyme
entering the Cecum.

 

 

The Cecum is the first part of the large intestine and it is
here where the Chyme is combined with more bacteria to turn it into faeces. The
epithelial cell wall absorbs water and more food molecules, whilst goblet cells
produce mucus to protect the epithelium from getting damaged by Gastric acid.
Whilst the faecal matter is being moved up the ascending colon, the bacteria
that were added in the Cecum digests the food that is indigestible and releases
vital vitamins such as B12, K and B2. Like most of the digestive system, below
the mucous layer of the tract wall, is the submucosa layer which harbours the
blood vessels, these run close to the epithelial cells for efficient diffusion
of released vitamins and ions. The ascending, transverse and descending colon
all have ‘Haustra’, which are sacks in the colon wall that increase the surface
area ensuring maximum uptake of water, salt, minerals, vitamins and ions
released by digestion and fermentation of the Chyme. The faeces is moved into
the transverse colon. The transverse colon continues to absorb water, salt and
leftover nutrients and begins to shape the faeces. It then passed into the
descending colon where further absorption takes place, but it is mainly it is
where faeces is left to accumulate before passing into the Sigmoid colon. Here
the faeces are stored slightly until the body is ready to release it, however
it still absorbs water and food molecules. Longitudinal smooth muscles propel it
into the rectum where it is stored until defecation occurs. Stretch receptors
in the rectum send signals to the brain when enough faeces has accumulated and
the inner anal sphincter is relaxed to allow expulsion of the matter through
the anus.

 To conclude, many
different structures are involved in digestion. The liver draining bile from
the blood for fat ingestion, the Muscularis layer of the intestine and colon wall
which drives the food down the digestive tract via Peristalsis but also the importance
of mucus for lubrication and protection of the Oesophagus, stomach and
intestines. 

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