2. Digestive System
The gastrointestinal tract (digestive tract, digestional
tract, GI tract, GIT, gut, or alimentary canal) is an
organ system which takes in food, digests it to extract
and absorb energy and nutrients, and expels the
remaining waste as feces. E. g. mouth, esophagus,
stomach, and intestines
Gastrointestinal means pertaining to the stomach and
intestines and a tract is a collection of related anatomical
structures or a series of connected body organs.
3. Digestive System Facts
Digestion is the process by which food is broken down into
smaller pieces so the body can use them to build and nourish
cells and to provide energy.
Digestion involves the mixing of food, its movement through
the digestive tract (also known as the alimentary canal), and
the chemical breakdown of larger molecules into smaller
molecules.
Every piece of food eaten has to be broken down into smaller
nutrients that the body can absorb, which is why it takes
hours to fully digest food.
The digestive system is made up of the digestive tract. This
consists of a long tube of organs that runs from the mouth to
the anus and includes the esophagus, stomach, small
intestine, and large intestine, together with the liver,
gallbladder, and pancreas, which produce important
secretions for digestion that drain into the small intestine.
The digestive tract in an adult is about 30 feet long.
4. Functions/ Steps/ Process of
Digestive System
To achieve the goal of providing energy and
nutrients to the body, six major functions take
place in the digestive system:
Ingestion
Secretion
Mixing and movement
Digestion
Absorption
Excretion
5. Accessory organs
GI tract Upper GI tract
Lower GI tract
Teeth,
Tongue,
Accessory organs Salivary glands,
Liver,
Gallbladder, and pancreas.
6.
7. Accessory organs
Accessory organs of the digestive system include the
teeth, tongue, salivary glands, liver, gallbladder, and
pancreas.
These are the organs which works as accessory organ to
perform the functions of digestion but is not directly
involved or participates in digestion.
8. Mouth
Food begins its journey through the digestive system in
the mouth, it is also known as oral cavity.
Inside the mouth are many accessory organs that aid in
the digestion of food—the tongue, teeth, and salivary
glands.
Teeth chop food into small pieces, which are moistened
by saliva before the tongue and other muscles push the
food into the pharynx.
9. Teeth
The teeth are 32 small hard organs found along the
anterior and lateral edges of the mouth.
The teeth are designed for cutting and grinding food
into smaller pieces.
10. Tongue
The tongue is located on the inferior portion of the
mouth just posterior and medial to the teeth.
It is a small organ made up of several pairs of muscles
covered in a thin, bumpy, skin-like layer.
The taste buds on the surface of the tongue detect taste
molecules in food and connect to nerves in the tongue
to send taste information to the brain.
The tongue also helps to push food toward the posterior
part of the mouth for swallowing.
11. Salivary Glands
The salivary glands are accessory organs that produce a
watery secretion known as saliva.
Saliva helps to moisten food and begins the digestion
of carbohydrates.
The body also uses saliva to lubricate food as it passes
through the mouth, pharynx, and esophagus.
Surrounding the mouth are 3 sets of salivary glands.
12. In healthy individual, between 0.5 and 1.5 liters of saliva
are produced every day.
The secretion of saliva (salivation) is mediated by
parasympathetic stimulation
13. Composition of Saliva
Saliva is mainly water. In fact, it’s 97-99.5% water which makes
it hypo osmotic.
Generally, saliva is a bit acidic (6.75-7.00), but the pH can vary.
Its solutes include electrolytes (mainly sodium, potassium,
chloride, and bicarbonate); the digestive enzymes like: salivary
amylase and lingual lipase; the proteins mucin, IgA, and
lysozyme; metabolic wastes (uric acid, urea). When dissolved
in water, the glycoprotein mucin forms thick mucus that
lubricates the oral cavity and hydrates foodstuffs.
Saliva protects against microorganisms because it has:
IgA antibodies
Lysozyme- a bactericidal enzyme (it inhibits bacterial growth in
the mouth)
Defensins- defensins function as cytokines and call defensive cells
(lymphocytes) into the mouth.
14. Parotid glands
The two parotid glands are major salivary glands.
It has taken from two words: Par=near, otid=ear
It is the largest salivary gland, which secrete saliva to
facilitate mastication and swallowing, and amylase to
begin the digestion of starches.
It is the serous type of gland which secretes ptyalin.
It enters the oral cavity via the parotid duct (Stensen
duct).
They produce 20% of the total salivary content in the
oral cavity.
15. Submandibular glands
The submandibular glands (previously known as
submaxillary glands) are a pair of major salivary glands
located beneath the lower jaws, superior to the digastric
muscles.
The secretion produced is a mixture of both serous fluid
and mucus, and enters the oral cavity via the
submandibular duct or Wharton duct.
Approximately 65-70% of saliva in the oral cavity is
produced by the submandibular glands, even though
they are much smaller than the parotid glands.
This gland can usually be felt as it is in the superficial
cervical region and feels like a rounded ball. It is located
about two fingers above the Adam's apple and about
two inches apart under the chin.
16. Sublingual glands
The sublingual glands are a pair of major salivary glands
located inferior to the tongue, anterior to the
submandibular glands.
The secretion produced is mainly mucous in nature;
however, it is categorized as a mixed gland.
Unlike the other two major glands, the ductal system of
the sublingual glands does not have intercalated ducts
and usually does not have striated ducts either, so saliva
exits directly from Rivinus ducts.
Approximately 5% of saliva entering the oral cavity
comes from these glands.
17. Minor salivary glands
There are 800 to 1,000 minor salivary glands located
throughout the oral cavity within the submucosa & oral
mucosa, the lateral parts of the hard palate, and the floor
of the mouth or between tongue.
They are 1 to 2 mm in diameter and unlike the major
glands, they are not encapsulated by connective tissue.
A minor salivary gland may have a common excretory
duct with another gland, or may have its own excretory
duct.
Their secretion is mainly mucous in nature.
18. Von Ebner's glands
Von Ebner's glands are glands found in circumvallate
papillae on the dorsal surface of the tongue.
They secrete a purely serous fluid that begins lipid
hydrolysis.
They also facilitate the perception of taste through
secretion of digestive enzymes and proteins.
The arrangement of these glands around the
circumvallate papillae provides a continuous flow of
fluid over the great number of taste buds lining the
sides of the papillae, and is important for dissolving
the food particles to be tasted.
19. Nerve supply in salivery gland
Salivary glands are innervated, either directly or
indirectly, by the parasympathetic and sympathetic arms
of the autonomic nervous system. Parasympathetic
stimulation evokes a copious flow of saliva. In contrast,
sympathetic stimulation produces either a small flow,
which is rich in protein, or no flow at all. Parasympathetic
innervation to the salivary glands is carried via cranial
nerves. The parotid gland receives its parasympathetic
input from the glossopharyngeal nerve (CN IX) via the
otic ganglion,[while the submandibular and sublingual
glands receive their parasympathetic input from the
facial nerve (CN VII) via the submandibular
ganglion.These nerves release acetylcholine and
substance P, which activate the IP3 and DAG pathways
respectively.
20. Control of Salivation
The minor salivary glands secrete saliva continuously, keeping the
mouth optimally moist.
When food enters, the major glands activate and large amounts of
saliva pour out.
The average human being produces around 1500ml of saliva per
day.
Salivation is controlled by the parasympathetic division of the
autonomic nervous system. When food is ingested, chemoreceptors
and mechanoreceptors in the mouth send signals to the salivatory
nuclei in the brain stem to the pons and medulla. As a result,
parasympathetic nervous system activity increases.
Impulses sent by motor fibers in the facial (VII) and
glossopharyngeal (IX) nerves dramatically increase the output of
watery saliva.
The chemoreceptors are activated the most by acidic foods and
liquids (vinegar, pickles, ect). The mechanoreceptors are activated by
almost any type of mechanical stimulus in the mouth (chewing).
21. Pharynx
The pharynx, or throat, is a funnel-shaped tube
connected to the posterior end of the mouth.
The pharynx is responsible for the passing of masses of
chewed food from the mouth to the esophagus.
The pharynx also plays an important role in the
respiratory system, as air from the nasal cavity passes
through the pharynx on its way to the larynx and
eventually the lungs.
Because the pharynx serves two different functions, it
contains a flap of tissue known as the epiglottis that
acts as a switch to route food to the esophagus and air
to the larynx.
22. Liver and Gallbladder
The liver is a roughly triangular accessory organ of the
digestive system located to the right of the stomach, just
inferior to the diaphragm and superior to the small
intestine.
The liver weighs about 3 pounds and is the second
largest organ in the body.
The liver has main function to produce bile and its
secretion into the small intestine.
Bile is a dark-green-to-yellowish-brown fluid produced
by the liver that aids the digestion of lipids in the small
intestine. Bile is produced continuously by the liver
(liver bile) and stored and concentrated in the
gallbladder.
23. Liver is the largest gland (weighing an average of 1500 g).
It lies under the diaphragm in the right upper abdomen
and mid-abdomen and extends to the left upper
abdomen.
The liver has the general shape of a prism or wedge, with
its base to the right and its apex to the left (see the image
below).
It is pinkish brown in color.
The gallbladder is a small, pear-shaped organ located
just posterior to the liver. The gallbladder is used to store
and recycle excess bile from the small intestine so that it
can be reused for the digestion of meals
24. Location
This is useful for remembering the anatomical relations
of the liver:
Anterior to the liver is the anterior abdominal wall and
ribcage.
Superior to the liver is the diaphragm (separating the
abdominal cavity from the thoracic cavity)
Posterior to the liver are the oesophagus, stomach,
gallbladder, first part of the duodenum (the foregut-
derived organs).
25. Structure
The entire liver is covered by a fibrous layer, known
as Glisson’s capsule. The ligaments and surface depressions
of the liver divide it into four lobes.
It is divided into a right lobe and left lobe by the attachment
of the falciform ligament.
There are two further ‘accessory’ lobes that arise from the
right lobe, and are located on the visceral surface of liver:
The caudate lobe is located on the upper aspect of the visceral
surface.
The quadrate lobe is located on the lower aspect of the visceral
surface.
Between the caudate and quadrate lobes is a deep fissure,
known as the porta hepatis. It transmits all the vessels,
nerves and ducts entering or leaving the liver.
26. The cells of the liver (known as hepatocytes) are
arranged into lobules. These are the structural units of
the liver.
Each lobule is hexagonal-shaped, and is drained by a
venule in its centre, called a central vein. At the
periphery of the hexagon are three structures collectively
known as the portal triad:
Arteriole – a branch of the hepatic artery entering the liver.
Venule – a branch of the hepatic portal vein entering the
liver.
Duct – branch of the bile duct leaving the liver.
The portal triad also contains lymphatic vessels and vagus
nerve (parasympathetic) fibres.
27.
28. Pancreas
The pancreas is a large gland located just inferior and
posterior to the stomach.
It is about 6 inches long and shaped like short, lumpy
snake with its “head” connected to the duodenum and
its “tail” pointing to the left wall of the abdominal cavity.
The pancreas secretes digestive enzymes into the small
intestine to complete the chemical digestion of foods.
29. Process
Mouth and Salivary Glands
Digestion begins in the mouth, where chemical and mechanical
digestion occurs. Saliva or spit, produced by the salivary glands
(located under the tongue and near the lower jaw), is released into
the mouth. Saliva begins to break down the food, moistening it and
making it easier to swallow. A digestive enzyme i. e. amylase in the
saliva begins to break down the carbohydrates (starches and
sugars). One of the most important functions of the mouth is
chewing. Chewing allows food to be mashed into a soft mass that is
easier to swallow and digest later.
Movements by the tongue and the mouth push the food to the back
of the throat for it to be swallowed. A flexible flap called the
epiglottis closes over the trachea (windpipe) to ensure that food
enters the esophagus and not the windpipe to prevent choking.
30. Upper gastrointestinal tract
The upper gastrointestinal tract consists of the buccal cavity,
pharynx, esophagus , stomach, and duodenum. The exact
demarcation between the upper and lower tracts is the suspensory
muscle of the duodenum. This delineates the embryonic borders
between the foregut and midgut, and is also the division commonly
used by clinicians to describe gastrointestinal bleeding as being of
either "upper" or "lower" origin. Upon dissection, the duodenum
may appear to be a unified organ, but it is divided into four
segments based upon function, location, and internal anatomy. The
four segments of the duodenum are as follows (starting at the
stomach, and moving toward the jejunum): bulb, descending,
horizontal, and ascending. The suspensory muscle attaches the
superior border of the ascending duodenum to the diaphragm.
The suspensory muscle is an important anatomical landmark which
shows the formal division between the duodenum and the jejunum,
the first and second parts of the small intestine, respectively.This is a
thin muscle which is derived from the embryonic mesoderm.
31. Esophagus
The esophagus is a muscular tube connecting the
pharynx to the stomach that is part of the upper
gastrointestinal tract.
It carries swallowed masses of chewed food along its
length.
At the inferior end of the esophagus is a muscular ring
called the lower esophageal sphincter or cardiac
sphincter.
The function of this sphincter is to close of the end of
the esophagus and trap food in the stomach.
32. Once food is swallowed, it enters the esophagus, a
muscular tube that is about 10 inches long. The
esophagus is located between the throat and the
stomach.
Muscular wavelike contractions known as peristalsis
push the food down through the esophagus to the
stomach.
A muscular ring (cardiac sphincter) at the end of the
esophagus allows food to enter the stomach, and, then,
it squeezes shut to prevent food and fluid from going
back up the esophagus.
33. Stomach
The stomach is a muscular sac that is located on the left
side of the abdominal cavity, just inferior to the
diaphragm.
In an average person, the stomach is about the size of
their two fists placed next to each other.
This major organ acts as a storage tank for food so that
the body has time to digest large meals properly.
The stomach also contains hydrochloric acid and
digestive enzymes that continue the digestion of food
that began in the mouth.
34. Position
The stomach is located in the superior aspect of the
abdomen. It lies in the epigastric, protected by the
lower portion of the rib cage.
The stomach, part of the gastrointestinal tract, is a
digestive organ located between the esophagus and the
duodenum.
It has a ‘J’ shape, and features a lesser and greater
curvature. The anterior and posterior surfaces are
smoothly rounded with a peritoneal covering.
The exact size, shape and position of the stomach can
vary from person to person.
36. Greater and Lesser Curvatures
The structure of the stomach are curved, forming the
lesser and greater curvatures:
Greater curvature – It forms the long, convex,
lateral border of the stomach. Arising at the cardiac orifice,
it arches backwards and passes inferiorly to the left. It
curves to the right. The short gastric arteries and the right
and left gastro-arteries have supply to the greater
curvature.
Lesser curvature – It forms the shorter, concave,
medial surface of the stomach. The most inferior part of
the lesser curvature, indicates the junction of the body and
pyloric region. The lesser curvature gives attachment to
the hepatogastric ligament and is supplied by the left
gastric artery and right gastric branch of the hepatic
artery.
37. Structure
The stomach has four main regions; the cardia, fundus,
body and pylorus:
Cardia
Fundus
Stomach Body
Pylorus
38. Cardia – It surrounds the superior opening of the
stomach. The cardia is where the contents of the
oesophagus empty into the stomach. The cardia is
defined as the region following the "z-line" of the
gastroesophageal junction. Near the cardia is the lower
oesophageal sphincter.
Fundus – It is the rounded portion superior to and left
of the cardia. It means "bottom" is formed by the upper
curvature of the organ.
Body – It is the large central portion inferior to the
fundus.
Pylorus – It connects the stomach to the duodenum.
"gatekeeper") is the lower section of the organ that
facilitates emptying the contents into the small intestine.
39. Blood supply
Schematic image of the blood supply to the human
stomach: left and right gastric artery, left and right
gastroepiploic artery and short gastric artery.
The lesser curvature of the human stomach is supplied
by the right gastric artery inferiorly, and the left gastric
artery superiorly, which also supplies the cardiac region.
The greater curvature is supplied by the right
gastroepiploic artery inferiorly and the left gastroepiploic
artery superiorly.
The fundus of the stomach, and also the upper portion
of the greater curvature, is supplied by the short gastric
artery which arises from the splenic artery.
40. Lower gastrointestinal tract
The lower gastrointestinal tract includes small intestine
and the large intestine.
In human anatomy, the intestine (bowel, or gut) is the
segment of the gastrointestinal tract extending from the
pyloric sphincter of the stomach to the anus.
The intestines are a long, continuous tube running from
the stomach to the anus. Most absorption of nutrients
and water happen in the intestines. The intestines
include the small intestine, large intestine, and rectum.
41. In humans, the small intestine is further subdivided into
the duodenum, jejunum and ileum while the large
intestine is subdivided into the cecum, colon, rectum,
and anal canal.
The small intestine (small bowel) is about 20 feet long
and about an inch in diameter. Its job is to absorb most
of the nutrients from what we eat and drink.
The large intestine (colon or large bowel) is about 5 feet
long and about 3 inches in diameter. The colon absorbs
water from wastes, creating stool. As stool enters the
rectum, nerves there create the urge to defecate.
42. Small Intestine
The small intestine is a long, thin tube about 1 inch in
diameter and about 10 feet long that is part of the lower
gastrointestinal tract.
It is located just inferior to the stomach and takes up
most of the space in the abdominal cavity.
The entire small intestine is coiled like a hose and the
inside surface is full of many ridges and folds.
These folds are used to maximize the digestion of food
and absorption of nutrients.
By the time food leaves the small intestine, around 90%
of all nutrients have been extracted from the food that
entered it.
43.
44. Small intestine
The small intestine begins at the duodenum and is a
tubular structure, usually between 6 and 7 m long. Its
mucosal area in an adult human is about 30 m2.
The small intestine is so called because its lumen
diameter is smaller than that of the large intestine,
although it is longer in length than the large intestine.
Its main function is to absorb the products of digestion
(including carbohydrates, proteins, lipids, and vitamins)
into the bloodstream. There are three major divisions:
Duodenum
Small intestine Jejunum
Ileum
45. Duodenum
A short structure (about 20–25 cm long) which receives
chyme from the stomach, together with pancreatic juice
containing digestive enzymes and bile from the gall
bladder.
The digestive enzymes break down proteins, and bile
emulsifies fats into micelles.
The duodenum contains Brunner's glands, which
produce a mucus-rich alkaline secretion containing
bicarbonate.
These secretions, in combination with bicarbonate from
the pancreas, neutralizes the stomach acids contained in
the chyme.
46. Jejunum
This is the midsection of the small intestine, connecting
the duodenum to the ileum.
The duodenum continues into the jejunum at the
duodenojejunal junction.
It is about 2.5 m long, and contains the circular folds,
and villi that increase its surface area.
Products of digestion (sugars, amino acids, and fatty
acids) are absorbed into the bloodstream here.
47. Ileum
The final section of the small intestine.
It is about 3 m long, and contains villi similar to the
jejunum.
It absorbs mainly vitamin B12 and bile acids, as well as
any other remaining nutrients.
48. Large intestine
The large intestine also called the colon, consists of the cecum, rectum, and
anal canal. It also includes the appendix, which is attached to the cecum. The
colon is further divided into:
Cecum (first portion of the colon) and appendix
Ascending colon (ascending in the back wall of the abdomen)
Right colic flexure (flexed portion of the ascending and transverse colon
apparent to the liver)
Transverse colon (passing below the diaphragm)
Left colic flexure (flexed portion of the transverse and descending colon
apparent to the spleen)
Descending colon (descending down the left side of the abdomen)
Sigmoid colon (a loop of the colon closest to the rectum)
Rectum
Anus
The main function of the large intestine is to absorb water. The area of the
large intestinal mucosa of an adult human is about 2 m2.
49.
50. Digestive System Physiology
The digestive system is responsible for taking whole foods
and turning them into energy and nutrients to allow the
body to function, grow, and repair itself. The six primary
processes of the digestive system include:
Ingestion of food
Secretion of fluids and digestive enzymes
Mixing and movement of food and wastes through the
body
Digestion of food into smaller pieces
Absorption of nutrients
Excretion of wastes
51. Ingestion
The first function of the digestive system is ingestion, or
the intake of food.
The mouth is responsible for this function, as it is the
orifice through which all food enters the body.
The mouth and stomach are also responsible for the
storage of food as it is waiting to be digested.
This storage capacity allows the body to eat only a few
times each day and to ingest more food than it can
process at one time.
52. Secretion
In the course of a day, the digestive system secretes around 7
liters of fluids.
These fluids include saliva, mucus, hydrochloric acid, enzymes,
and bile. Saliva moistens dry food and contains salivary
amylase, a digestive enzyme that begins the digestion of
carbohydrates.
Mucus serves as a protective barrier and lubricant inside of
the GI tract.
Hydrochloric acid helps to digest food chemically and protects
the body by killing bacteria present in our food.
Enzymes are like tiny biochemical machines that disassemble
large macromolecules like proteins, carbohydrates, and
lipids into their smaller components.
Finally, bile is used to emulsify large masses of lipids into tiny
globules for easy digestion.
53. Mixing and Movement
The digestive system uses 3 main processes to move and mix
food:
Swallowing: Swallowing is the process of using smooth and
skeletal muscles in the mouth, tongue, and pharynx to push
food out of the mouth, through the pharynx, and into the
esophagus.
Peristalsis: Peristalsis is a muscular wave that travels the
length of the GI tract, moving partially digested food a short
distance down the tract. It takes many waves of peristalsis for
food to travel from the esophagus, through the stomach and
intestines, and reach the end of the GI tract.
Segmentation: Segmentation occurs only in the small
intestine as short segments of intestine contract like hands
squeezing a toothpaste tube. Segmentation helps to increase
the absorption of nutrients by mixing food and increasing its
contact with the walls of the intestine.
54. Digestion
Digestion is the process of turning large pieces of food into its small
component.
Mechanical digestion is the physical breakdown of large pieces of
food into smaller pieces. This mode of digestion begins with the
chewing of food by the teeth and is continued through the muscular
mixing of food by the stomach and intestines.
Bile produced by the liver is also used to mechanically break fats into
smaller globules.
The food is also being chemically digested as larger and more
complex molecules are being broken down into smaller molecules
that are easier to absorb.
Chemical digestion begins in the mouth with salivary amylase in
saliva splitting complex carbohydrates into simple carbohydrates.
The enzymes and acid in the stomach continue chemical digestion,
but the bulk of chemical digestion takes place in the small intestine.
The pancreas secretes pancreatic juice, which is capable of digesting
lipids, carbohydrates, proteins and nucleic acids. By the time food has
left the duodenum, it has been reduced to its chemical building
blocks—fatty acids, amino acids, monosaccharides, and nucleotides.
55. Absorption
Once food has been reduced to its building blocks, it is
ready for the body to absorb.
Absorption begins in the stomach with simple molecules
like water and alcohol being absorbed directly into the
bloodstream.
Most absorption takes place in the walls of the small
intestine, which are densely folded to maximize the
surface area in contact with digested food.
Small blood and lymphatic vessels in the intestinal wall
pick up the molecules and carry them to the rest of the
body.
The large intestine is also involved in the absorption of
water and vitamins B and K before feces leave the body.
56. Excretion
The final function of the digestive system is the excretion
of waste in a process known as defecation.
Defecation removes indigestible substances from the
body so that they do not accumulate inside the gut.
57. Physiology of Saliva
It Cleanses the mouth.
It helps moisten and compact food into a round mass
called a bolus.
It contains enzymes that begin the chemical
breakdown of starch.
It dissolves food chemicals so they can be “tasted”.
58. Physiology of
Liver
The liver regulates most chemical levels in the blood.
It excretes a product called bile. This bile helps carry away waste products from the liver.
All the blood leaving the stomach and intestines passes through the liver. The liver
processes this blood and breaks down, balances, and creates the nutrients and also
metabolizes drugs into forms that are easier to use for the rest of the body or that are
nontoxic.
Production of bile, which breaks down fats in the small intestine during digestion
Production of certain proteins for blood plasma
Production of cholesterol and special proteins to help carry fats through the body
Conversion of excess glucose into glycogen for storage (glycogen can later be converted
back to glucose for energy) and to balance and make glucose as needed
Regulation of blood levels of amino acids, which form the building blocks of proteins
The liver stores iron.
Conversion of poisonous ammonia to urea.
Clearing the blood of drugs and other poisonous substances.
Regulating blood clotting.
Resisting infections by making immune factors (coagulation factors) and removing
bacteria from the bloodstream.
It helps in clearance of bilirubin, also from red blood cells. If there is an accumulation of
bilirubin, the skin and eyes turn yellow.
59. Stomach as nutrition sensor
The human stomach can "taste" sodium glutamate using
glutamate receptors and this information is passed to
the lateral hypothalamus and limbic system in the brain
through the vagus nerve.
The stomach can also sense, independently of tongue
and oral taste receptors, glucose, carbohydrates,
proteins, and fats.
This allows the brain to link nutritional value of foods to
their tastes.
