Download Session 19 Introduction to the Gastrointestinal System

Unit 6
Gastro intestinal System
Session 19
Introduction to the Gastrointestinal System
Session Outline
Introduction
19.1 Organs of the gastrointestinal system
19.2 Functions of the gastrointestinal system
19.3 Basic structure
19.4 Function of the smooth muscle of the alimentary canal
19.5 Blood and nerve supply
19.6 Regulation of the functions of the gastrointestinal system
Summary
Learning Outcomes
Review Questions
References
Introduction
.
Food is essential to maintain life. The necessity to produce or buy food is an important reason
why people work hard all over the world. The food we eat provide the energy to carry out daily
functions. Therefore the body needs food as a primary requirement to sustain life.
The alimentary canal, also known as the gastrointestinal (GI) tract or gut, is the continuous,
muscular digestive tube that winds through the body. It digests food—breaks it down into smaller
fragments (digest = dissolved)—and absorbs the digested fragments through its lining into the
blood.
The part of the digestive tract below the diaphragm, is called the gastrointestinal (GI) tract. The
alimentary canal consists of the mouth, pharynx, esophagus, stomach, small intestine, rectum,
anal canal, and anus. From mouth to anus this canal is about 9 m (30 ft) long. The associated
structures of the digestive system include the teeth, lips, tongue, cheeks, salivary glands,
pancreas, liver, gallbladder, and bile duct.
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Self Assessment Questions
Can you list the organs in the alimentary canal?
What are the associated structures of the digestive system and can you think out their
functions?
19.1 The organs of the gastro intestinal system
The main organs of the alimentary canal ( the passage through which the food passes) are the
mouth, pharynx, oesophagus, stomach, small intestine, and large intestine. The large intestine
ends at the bottom in a terminal opening, or anus. In a cadaver, the alimentary canal is
approximately 9 m (about 30 ft) long, but in a living person, it is considerably shorter because of
its muscle tone. Food material in this tube is technically outside the body because the canal is
open to the external environment at both ends ( ie. through the mouth and the anus).
The accessory digestive organs are the teeth, tongue, gallbladder, and a number of large
digestive glands—the salivary glands, liver, and pancreas. The teeth and tongue are in the mouth,
(oral cavity), while the digestive glands and gallbladder lie outside the GI tract and connect to it
by ducts. The accessory digestive glands produce many secretions that contribute to the
breakdown of food.
Self Assessment Questions
Can you list the organs of the alimentary system?
What are the accessory digestive organs?
19.2 The functions of the gastrointestinal system
The digestive tract is similar to a ―disassembly line‖ as the food that is eaten becomes less
complex at each step of digestion. The processing of food by the digestive system involves six
essential activities. They are ingestion, propulsion, mechanical digestion, chemical digestion,
absorption, and defecation (Figure19.2).
1. Ingestion- is taking food into the digestive tract usually through the mouth (eating).
2. Propulsion- is the movement of food through the alimentary canal. It includes
- Swallowing- commenced voluntarily and later becomes involuntary process along the
pharynx - oesophagus
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Peristalsis- ( peri – around , stalsis = contraction ) involves alternating waves of
contraction and relaxation of the smooth muscle in the walls of the tract. By this process
food travels along the entire tract from the stomach upto the rectum / anus.
3. Mechanical digestion- prepares food for chemical digestion by enzymes. the process
involves – chewing – breaking the food into small particles in the mouth and mixing with
saliva, churning- food in the stomach to break in to small particles, and segmentation- or
rhythmic local contractions of the intestine to mix the food better with digestive juices and
increase the efficiency of absorption.
4. Chemical digestion – During this process food molecules are broken down to their basic
molecules and then converted into small particles that can be absorbed. The enzymes
secreted are active in promoting the process of digestion.
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5. Absorption – It is the process through the digested food products are taken up into the blood.
This happens through the mucosa of the intestine into the blood stream by active and passive
transport processes. The absorbed water and nutrients enter into the blood stream (
portal circulation) and lymphatic system.
6. Defaecation- It is the process where undigested substances are removed from the body in the
form of faeces.
Self Assessment Questions
Can you list the functions of the Gastro intestinal system
Explain the process of peristalsis
19.3 The basic structure of the alimentary canal
Commencing from the esophagus to the anal canal, the walls of the alimentary canal
have the same four basic layers, or tunics (Figure 19.4)—mucosa, submucosa, muscularis
externa, and serosa These layers each contain a predominant tissue type and is important
as it plays a specific role in food breakdown.
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The Mucosa
The mucosa, or mucous membrane — It is the innermost layer and is a moist epithelial
membrane that lines the alimentary canal lumen from mouth to anus.
Its major functions are listed below
(1) secretion of mucus, digestive enzymes, and hormones
(2) absorption of the end products of digestion into the blood,
(3) protection against infectious disease.
The mucosa in a particular area of the GI tract may have one or all three of these functions
depending on the function of that area ( eg the stomach will perform all three above) .
The typical digestive mucosa consists of three sub layers:
(1) lining epithelium, - It is a simple columnar epithelium with a large number of mucus
secreting goblet cells (protect the intestinal mucosa). In the stomach and small intestine, the
mucosa also contains both enzyme-synthesizing and hormone-secreting cells.
(2) lamina propria,- is loose areolar connective tissue. It has capillaries to nourish the
epithelium and absorb digested nutrients. A few lymphoid follicles, which help defend us
against bacteria and other pathogens. Large collections of lymphoid follicles are seen in the
pharynx (as the tonsils) and in the appendix.
(3) muscularis mucosae. It is external to the lamina propria. It is a thin layer of smooth muscle
cells that produces local movements of the mucosa. For example, twitching of this muscle layer
dislodges food particles that have got stuck to the mucosa. In the small intestine, it throws the
mucosa into a series of small folds that immensely increase its surface area and these are known
as villi.
The Submucosa
The submucosa, is just external to the mucosa, is connective tissue layer . It contains blood and
lymphatic vessels, lymph follicles, and nerve fibers. In the stomach it has a rich supply of elastic
fibers to enable the stomach to return to its normal shape after temporarily storing a large meal. It
has a network of blood vessels that supplies the surrounding tissues of the GI tract wall.
The Muscularis Externa
Just deep to the submucosa is the muscularis externa ( muscularis propria). This layer is
responsible for segmentation and peristalsis. It typically has an inner circular layer and an outer
longitudinal layer of smooth muscle cells (Figure 19.1). In several places along the tract, the
circular layer thickens, forming sphincters that act as valves to prevent backflow and control the
passage of food from one organ to the next.
The Serosa
The serosa is the protective outermost layer of the intraperitoneal organs. It is formed of
connective tissue covered with mesothelium, a single layer of squamous epithelial cells.
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Figure 19. 1 Organisation of the intestinal wall into functional layers
basic structure of the alimentary canal
Self Assessment Questions
Explain the basic layers of a typical digestive mucosa
Can you list the functions of each layer?
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19.4 The functions of the smooth muscle in the alimentary canal
Figure 19.2 Movements along the gastrointestinal tract
The smooth muscle of the alimentary canal is involved in three kinds of movement.
1. Isolated contraction- This happens in the mouth when food particles are moved
about in the mouth during chewing
2. Segmentation- This slows the movement of the intestinal contents along the
intestinal tract. It provides time for digestion and absorption. It helps to mix the
intestinal contents (known as chyme) with the digestive juices. A segment of
bowel contracts at both ends, and then a second contraction occurs in the center of
the segment to force the chyme both backward and forward. ( fig 19.2 (b))
3. Peristalsis – It is a reflex response that is initiated when the gut wall is stretched
by the contents inside the lumen. It is seen in all parts of the gastrointestinal tract
from the oesophagus to the rectum. The stretch initiates a circular contraction
behind the stimulus and an area of relaxation in front of it as shown in the figure.
(Figure 19.2(a)). The wave of contraction then moves in an oral-to-caudal ( ie.
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from the mouth to the rectum) direction, pushing the contents of the lumen
forwards.
Peristaltic activity can be increased or decreased by the autonomic nerve
stimulation. Cholinergic neuron activation is responsible for causing smooth
muscle contraction behind the food bolus and the relaxation of the smooth
muscles in front of the food bolus.
Self Assessment Questions
Can you describe the process of segmentation and peristalsis?
What is the importance of each of these in the function of the GIT?
19.5 The blood and nerve supply of the alimentary canal
The splanchnic circulation is the main blood supply of the GIT. It originates from the arteries
that branch off the abdominal aorta. It supplies the digestive organs and the hepatic portal
circulation.
The arterial supply is via the
-main celiac trunk – it give off branches that supply the spleen, liver, and stomach,
- mesenteric arteries - supply the small and large intestines It normally receives 25%
of the cardiac output.
After a meal the blood volume to the GIT increases..
The hepatic portal circulation collects venous blood draining from the digestive organs and
delivers it to the liver. In the liver this venous blood is filtered. The liver collects the absorbed
nutrients for metabolic processes or for storage. Some of the nutrients are then released back to
the bloodstream for the use of the other cells of the body .
The alimentary canal has its own nerve supply, by the enteric neurons (enter = gut). These nerves
communicate with one another to regulate the activity of the digestive system. These nerves are in
two major intrinsic nerve plexuses in the walls of the alimentary canal. They are the submucosal (
Meissners) and myenteric nerve (Auerbach’s) plexuses. The submucosal nerve plexus is in the
submucosa. It has both sensory and motor neurons. This plexus regulates the activity of glands
and smooth muscle in the mucosa through parasympathetic and sympathetic nerves.
The large myenteric nerve plexus is located between the circular and longitudinal muscle layers
of the muscularis externa. This plexus provide the major nerve supply to the GI tract wall and
controls the motility of the GI tract. They control the segmentation and peristalsis in the intestine.
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The digestive activity is controlled by the autonomic nerves. Parasympathetic nerve impulses
enhance secretory activity and motility. The parasympathetic function is brought about by the
Vagus Nerves on both sides of the body.
The sympathetic function is brought about by the thoraco lumbar nerves of the sympathetic tract.
The sympathetic impulses inhibit secretions and inhibit motility.
Self Assessment Questions
Can you list the organs that are supplied by the splanchnic circulation?
Explain the variation in the circulation after a meal
Can you explain the nerve supply to the digestive tract?
Can you list the nerves regulating the function of the gastrointestinal tract?
List the actions of myenteric and submucosal nerve plexuses?
19.6 Regulation of the function of the Gastrointestinal system
The functions of the gastrointestinal tract, such as secretion, digestion, absorption and
motility must be regulated in an integrated way to ensure absorption of nutrients after a
meal. Endocrine regulation is brought about by hormones triggered in association with
the meal. Some of these hormones and their actions are briefly explained below.
Gastrin stimulates the secretion of gastric juice in the stomach and also increases gastric
motility. Passage of food and acid into the duodenum stimulates the secretion of
cholecystokinin- pancreozymin (CCK-PZ). CCK-PZ stimulates the release of pancreatic
juice and bile into the duodenum.
Self Assessment Questions
Can you list the hormones regulating gastrointestinal function and their main actions?
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Summary
In this session you learnt about the main organs of the digestive system and the basic anatomical
structure of the gastrointestinal system. You learnt about the function of main organs. The
gastrointestinal movements of isolated contraction, segmentation and peristalsis to promote the
digestion and absorption of food are briefly described. You learnt about the importance of the
splanchnic circulation and the importance of the blood passing through the liver before it returns
to the inferior vena cava and back to the right atrium. The importance of the autonomic nerve
supply regulating gastrointestinal function describes the increased motility and increased
secretary activity of the glands brought about by the parasympathetice nerves and the decreased
motility and decreased secretary activity of the glands brought about by the sympathetice nerves
The functions of the hormones in gastrointestinal secretion digestion and absorption was further
explained in this main introductory session.
Learning Outcomes
i.
List the organs of the digestive system
ii.
iii.
What are the main functions of the digestive system?
List the functions of the digestive system
iv.
v.
vi.
Can you list the tissue layers of the gastrointestinal tract and their functions.
Explain the importance of the smooth muscle layers lining the gastrointestinal tract
Describe the basic structure of the digestive system
vii.
List the functions of the smooth muscle in the alimentary canal
viii.
outline the process of peristalsis and segmentation
ix.
Outline the nerve and blood supply of the alimentary canal
x.
Briefly describe the role of the following in regulating secretions of the GIT and
motility

enteric nervous system

autonomic nervous system

hormones
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