What happens to the fat we take in meals? When we eat fat, it is digested and absorbed in our gastrointestinal tract. Some fat is not absorbed and excreted. Fat is one of the three macronutrients of human food, along with carbohydrates and proteins. Fat is a very good source of energy and essential for cell formation and vitamin absorption. It is vital for good health. However, an excess of fat is hazardous for our health. Let us delve into the fate of fat in our body. Digestion of fat: Lipids are large molecules, and unlike carbohydrates and proteins, they are not soluble in water and do not mix with the watery blood. So, they like to cluster together in large droplets in a liquid surrounding the gastrointestinal tract. The digestive process breaks those large droplets of fat into smaller droplets, and then enzymes digest lipid molecules using enzymes called lipase. In the mouth: Chewing breaks food into smaller particles and mixes them with saliva. Lingual lipase be
Introduction of Gastrointestinal Tract
This article discusses different parts of the gastrointestinal tract. Learn about the structure, blood supply, and nervous supply here.
Keywords: Meissner's plexus, Gastrointestinal bleeding, Gastrointestinal agents, Gastrointestinal symptoms, Myenteric plexus, Muscularis mucosa.
Table of
contents
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1. |
Parts |
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2. |
Functions |
|
3. |
Structure |
|
4. |
Nervous control |
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5. |
Parasympathetic & sympathetic system |
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6. |
Blood supply |
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Different parts of the gastrointestinal tract from above downwards are:
1. Mouth
2. Tongue
3. Pharynx
4. Oesophagus
5. Stomach
6. Small intestine
7. Large intestine
8. Rectum
9. Anus
The
total length is 30 ft /9 meters at autopsy but shorter in the living due to
contraction.
Accessory organs of the gastrointestinal tract are
1. Tongue
2. Salivary Glands
3. Liver
4. Pancreas
5. Gall Bladder
A. Provides the body with a continual supply of:
1. Nutrients
2. Water
3. Electrolytes
4. Vitamins
B. And finally, expel the unabsorbable excreta outside the body.
To perform its functions, the gastrointestinal tract causes :
1. Mastication
of food
2. Movement of food through the alimentary tract.
3. Secretion of digestive juices from the
gastrointestinal tract and accessory organs of the gastrointestinal tract.
4.
Digestion of the food and.
5. Absorption of the product of digestion,
water, electrolytes, and vitamins.
6. Blood circulation through the alimentary
tract to carry away the absorbed substances to the liver and
The gastrointestinal tract functions are
controlled by the local nervous system, central nervous system, and hormones secreted
in the body.
The general structure of the gastrointestinal tract :
The
alimentary tract has a more or less uniform structure in a cross-section with
some regional variations.
The
following layers are present in a typical cross-section of the alimentary
tract from the outer surface inwards:
The
serosal layer.
A smooth
muscle layer of longitudinal and circular muscles
The submucosa and The mucous membrane
The
mucous membrane has 3 layers from inside to outside:
The
epithelial layer: a single layer of columnar cells arising from the basement
membrane. It lines the lumen of the alimentary tract.
Lamina
propria, or basement membrane, is a layer
of reticular tissue and is next to the epithelial layer and
Muscularis
mucosa : a thin layer of muscules that
separates lamina propria from the
submucosa. The muscularis mucosa is outside the lamina propria and separates it from the submucosa. It is present
from the esophagus to the upper rectum. The muscularis mucosa is also known as
lamina muscularis mucosa. Lamina stands for a thin layer. Muscularis mucosa comprises several thin layers of smooth muscle fibers arranged in different
ways, keeping the mucosal surface and underlying glands in a constant state of
gentle movements to expel glandular contents and enhance contact between
epithelium and the contents of the lumen. It folds in the lumen, so the surface
area increases for digestion and absorption. It goes into the villi, which
protrudes into the gastric lumen. When it contracts, it pushes the absorbed fat
into the lacteals and, when it contracts alternately, causes side-to-side movements
to absorb the nutrients. It has no role in major movements of the small
intestine.
The
submucosa is present between the mucous membrane and the muscular layer. It
contains areolar tissue with blood vessels, nerves, and lymphatics.
The
muscular layer has two parts: the outer longitudinal and the inner circular.
The
longitudinal layer of smooth muscle is present throughout the alimentary tract.
In the large intestine, this layer exhibits bands -thickening at fixed positions of the large
intestine. These bands are known as Taenia coli.
The
inner circular layer of smooth muscle is regularly present
around the alimentary tract with some regional variations:
The smooth muscles of the circular layer become thick around orifices to form
sphincters.
An oblique layer of smooth muscles in the stomach is present deep in the circular
layer.
The
serosal layer is the outermost layer of the alimentary tract. The
visceral peritoneum forms it. It is worth mentioning that the visceral
peritoneum forms the outer layer of the whole gastrointestinal except the esophagus and the distal rectum.
Neural
control of the alimentary tract :
The
alimentary tract has a nervous system called the enteric nervous system. It
lies entirely in the gut wall from the esophagus to the anus. The
number of neurons in this enteric nervous system is about 100 million. The
Enteric nervous system is essential in controlling alimentary tract
movements and secretion.
Types
of Enteric Nervous System This is composed mainly of two interconnected
plexuses:
1. The
myenteric plexus is the outer plexus between the longitudinal and circular
muscle layers and mainly controls gastrointestinal movements.
2.
The Meissner's or submucosal plexus is the inner plexus present in the
submucosal layer. It is mainly concerned with secretion and local blood flow.
The
plexuses are connected. They are also associated with the extrinsic
sympathetic and parasympathetic fibers.
The
Enteric nervous system can function independently, but extrinsic sympathetic
and parasympathetic nervous systems may influence it.
Functions
of sympathetic and parasympathetic nervous systems :
Sympathetic
nervous system: its stimulation decreases motility by causing relaxation of
intestinal smooth muscles and contracting the sphincters.
Parasympathetic
nervous system: Its stimulation increases motility by contracting intestinal smooth muscles and relaxing the sphincters. It also increases gland secretion.
Reflexes
in the alimentary tract:
Two
types of reflexes are present in the gastrointestinal tract
Long
reflexes: efferents from GIT enter the spinal cord or brain –the center of
reflex- afferents emerge from the central nervous system and finally reach the
GIT effector organ.
In short, the reflexes afferent nerve, the center,
and the efferent nerve area in the wall of the GIT.CNS is not involved.
GIT blood flow is from the splanchnic circulation
–blood flows through the gut, liver, spleen, and pancreas to the liver.
In the portal circulation, two capillary beds are in series, so blood collected from one capillary bed drains into another.
The
first capillary bed is in the gut.
The
second capillary bed is in the liver.
In
splanchnic circulation, blood flows from the spleen, pancreas, and liver to the
gut. From the gut, the blood flows into the liver by the hepatic portal
vein. Finally, the hepatic portal vein drains into the vena cava.
Hashtags: Meissner's plexus#Gastrointestinal bleeding# Gastrointestinal agents# Gastrointestinal symptoms# Myenteric plexus# Muscularis mucosa.
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Internal links:
https://blog.totalphysiology.com/2021/06/movements-of-stomach-we-must-know.html
https://blog.totalphysiology.com/2021/05/deglutition-edited-we-must-know.html
https://blog.totalphysiology.com/2021/05/gut-brain-axis-you-must-know.html
https://blog.totalphysiology.com/2021/03/small-intestine-we-must-know.html
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