Download The Digestive System and Homeostasis

The Digestive System and Homeostasis
The digestive system contributes to homeostasis by breaking down food into forms that
can be absorbed and used by the body cells. It also absorbs water, minerals and
eliminates wastes from the body.
The food we eat contains a variety of nutrients which are used for building new body
tissue and repairing damaged tissues. Food is also vital for life because it is our only
source of chemical energy. The food we eat consists of molecules that are too large to be
used by body cells. Therefore food must the broken down into molecules that are small
enough to enter the body by process known as digestion.
Overview of the digestive system
Two groups of organs compose the digestive system, organs of the gastrointestinal tract
and the accessory digestive organs. The gastrointestinal (GI) tract or alimentary canal is a
continuous tube that extends from the mouth to the anus. Organs of the gastrointestinal
tract include the mouth, most of the pharynx, esophagus, stomach, small intestine and
large intestine. The accessory digestive organs are the teeth, tongue, salivary glands liver,
gall bladder and pancreas. The accessory digestive organs produce and/or store secretions
that flow into the GI tract through ducts. Their secretions aid in a chemical breakdown of
food.
The functions of the digestive system:
1. Ingestion: taking food into the mouth.
2. Secretion: release of water, acids, buffers and enzymes into the digestive tract.
3. Mixing and propulsion: churning and propulsion of food through the digestive tract.
4. Digestion: mechanical and chemical breakdown of food.
5. Absorption: passage of digestive products from the digestive tract into the blood and
lymph.
6. Defecation: elimination of feces from the digestive tract.
Layers of the GI tract
Mucosa
The mucosa or an inner lining of the GI tract is mucous membrane. It is composed of a
layer of epithelium that is in direct contact with the contents of the GI tract, a layer of
connective tissue, the lamina propria and a thin layer of smooth muscle, muscularis
mucosae.
1. Epithelium - simple columnar epithelium which functions in secretion and in
absorption. Located among the epithelial cells are exocrine cells that secrete mucus
and fluid into the lumen of the tract and enteroendocrine cells that secrete hormones
into the blood.
2. Lamina propria – is composed of areolar connective tissue. This layer supports the
epithelium and binds the epithelium to the muscularis mucosae. The lamina propria
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contain lymphatic nodules (mucosa associated lymphatic tissue, MALT) that have
immune system cells that protect against disease.
3. Muscularis mucosae – a thin layer of smooth muscle that throws the mucous
membrane into many small folds, which increases the surface area for digestion and
absorption. Movements of the muscularis mucosae allow all the absorptive cells to be
fully exposed to the contents of the GI tract.
Submucosa
The submucosa consists of connective tissue that binds the mucosa to the muscularis
externa. Located in the submucosa is a network of neurons known as the submucosal
plexus.
Muscularis Externa
The muscularis externa consist of two layers, an inner layer of circular smooth muscle
and an outer layer of longitudinal smooth muscle. The contractions of the smooth muscle
breakdown food, mixes it was digestive secretions and propels it along the tract. Between
the layers of the muscularis is plexus of neurons, the myenteric plexus.
Serosa
The serosa is a serous membrane composed of areolar connective tissue and simple
squamous epithelium.
Neural Innervation of the Digestive Tract
Enteric Nervous System (ENS)
The enteric nervous system extends from the esophagus to the anus. The neurons of the
ENS are arranged into two plexuses.
1. The myenteric plexus is located between the longitudinal and circular smooth muscle
layers of the muscularis.
2. The submucosal plexus is located within the submucosa.
The motor neurons of the myenteric plexus supply the smooth muscle layers of the
muscularis to control GI tract motility in regard to frequency and strength of contraction.
The motor neurons of the submucosa plexus supply the secretory cells of the mucosal
epithelium controlling secretions of the organs of the GI tract. The interneurons of the
ENS interconnect the neurons of the myenteric and submucosal plexuses. The sensory
neurons of the ENS in the mucosal epithelium function as chemoreceptors and stretch
receptors.
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Autonomic Nervous System (ANS)
The neurons of the ENS can function independently but subject to regulation by the
neurons of the ANS. The vagus (X) nerves supply parasympathetic neurons to most parts
of the GI tract. The last half of the large intestine is supplied by parasympathetic neurons
from the sacral spinal cord. The parasympathetic nerves that supply the GI tract form
neural connections with the ENS. Stimulation of the parasympathetic nerves causes
increased GI tract secretion and motility by increasing the activity of ENS neurons.
Sympathetic nerves to the GI tract arise from thoracic and upper lumbar regions of the
spinal cord. These nerves also form connections with the ENS. Stimulation of the
sympathetic nerves causes decreased GI tract secretion and motility by inhibiting the
activity of ENS neurons.
Mouth
The mouth is also referred to as the oral or buccal cavity. The oral mucosa lines the
mouth is a mucous membrane. It consists of nonkeratinized stratified squamous
epithelium. The oral mucosa functions to protect the oral cavity against abrasion during
eating. The oral mucosa contains small salivary glands to keep the oral cavity moist.
Salivary Glands
The salivary glands release saliva into the mouth. Just enough saliva is secreted to keep
the mouth and pharynx moist and to cleanse the mouth and teeth. When food enters the
mouth secretion of saliva increases and it lubricates, dissolves and begins the chemical
digestion of food.
The 3 pairs of major salivary glands lie outside the mouth and ducts conduct the saliva
into the mouth.
Parotid glands – The parotid glands are located inferior and anterior to the ears. The
parotid glands secrete saliva into the mouth via the parotid ducts to open into the
vestibule opposite the 2nd maxillary (upper) molar tooth.
Submandibular glands – The submandibular glands lies in the floor of the mouth.
Sublingual glands- The sublingual glands lie beneath the tongue.
Composition and Functions of Saliva
Chemically saliva is 99.5% water and 0.5% solute. The solute ions are Na+, K+, Cl-,
HCO3- and PO4=. Organic substances are urea, uric acid, mucus, immunoglobulin A,
lysozyme and salivary amylase. The salivary amylase is the digestive enzyme that acts on
the starch.
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The parotid glands secrete a watery (serous) liquid containing salivary amylase. The
submandibular glands secrete a thicker fluid contain salivary amylase and mucus. The
sublingual glands secrete a much thicker fluid mainly mucus and only a small amount of
salivary amylase.
The water in saliva provides a medium for dissolving foods so that the food can be tasted
by the taste receptors. The water also allows the digestive reactions to occur. Cl- ions
activate the salivary amylase to start the digestion of the starch. HCO3-1 and PO4-2 ions
buffer acidic foods in the mouth, so the saliva is only slightly acidic (pH 6.35-6.85).
Mucus lubricates the food so it can be moved easily in the mouth and formed into a ball
to be swallowed. Immunoglobulin A and lysozyme have antimicrobial activity.
Salivation
Salivation (secretion of saliva) is controlled by the ANS. 1.0-1.5 liters of saliva is
secreted daily. The PNS stimulation promotes secretion of saliva. SNS stimulation
inhibits secretion of saliva. The feel and taste of food are stimulators of salivation.
Chemicals in the food stimulate taste receptors in the taste buds and the impulses are
conveyed from the taste buds to the salivary nuclei in the brain stem. Then from the
salivary nuclei, PNS neurons in the facial (VII) and glossopharyngeal (IX) nerves
stimulate the salivary glands to secrete saliva. The smell, sight, sound or thought of food
also stimulate secretion of saliva.
Tongue
The tongue assists in chewing and swallowing.
Teeth
The teeth aid in the physical breakdown of the food.
Mechanical and Chemical Digestion in the Mouth
Mastication is the mechanical digestion of food in the mouth due to chewing. During
mastication the food is manipulated by the tongue, ground by the teeth and mixed with
the saliva. The food is reduced to a soft, flexible easily swallowed mass called the bolus.
The food molecules begin to dissolve in the water of the saliva because enzymes can only
react with the food molecules in a liquid medium.
Salivary amylase initiates the breakdown of starch into the disaccharide maltose, the
trisaccharide maltotriose and short-chain polymers of glucose, α-dextrins. Food is
swallowed too quickly for all the starches to be completed digested in the mouth. The
salivary amylase continues to act on the starches in the stomach until the stomach acids
inactivates the amylase. Lingual lipase secreted by the lingual glands of the tongue
activated by the acidic environment of the stomach digests dietary triglycerides into fatty
acids and diglycerides.
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Pharynx
When food is swallowed it passes from the mouth into the pharynx. The pharynx is a tube
that extends posterior to the esophagus and anterior to the larynx. The pharynx is
composed of skeletal muscle and lined by a mucous membrane is divided into three parts.
The nasopharynx functions only in respiration. The oropharynx and laryngopharynx
function in respiratory and digestive functions. Swallowed food passes from the mouth
into the oropharynx and laryngopharynx; the muscular contractions of these areas propel
food into the esophagus and then into the stomach.
Esophagus
The esophagus is a collapsible muscular tube posterior to the trachea. The esophagus
begins at the inferior end of the laryngopharynx passes through the mediastinum. It
pierces the diaphragm through the esophageal hiatus and ends in the superior portion of
the stomach.
Histology of the Esophagus
Mucosa - The mucosa of the esophagus consists of nonkeratinized stratified squamous
epithelium, lamina propria (areolar connective tissue) and muscularis muscosae (smooth
muscle). The stratified squamous epithelium gives protection against abrasion and wearand tear from the swallowed bolus.
Submucosa – The submucosa contains areolar connective tissue and submucosal mucous
glands.
Muscularis Externa – The superior 1/3 is skeletal muscle, intermediate 1/3 is skeletal and
smooth muscle and the inferior 1/3 is smooth muscle. At each end of the esophagus the
muscularis forms two sphincters – the upper esophageal sphincter (UES) consists of
skeletal muscle and the lower esophageal sphincter (LES) consists of smooth muscle. The
upper sphincter regulates movement of food from the pharynx into the esophagus; the
lower sphincter regulates movement of food from the esophagus into the stomach.
Adventitia – The outer layer of the esophagus is a layer of areolar connective tissue not
covered by mesothelium (simple squamous epithelium). The adventitia attaches the
esophagus to the surrounding structures.
Physiology of the Esophagus
The esophagus secretes mucus and transports food into the stomach. It does not produce
digestive enzymes and it does not carry on absorption.
Deglutition
The movement of food from the mouth into the stomach is achieved by deglutition (the
act of swallowing). Deglutition involved the mouth, pharynx and esophagus and is aided
by the secretion of saliva and mucus.
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Deglutition occurs in three stages.
1. Voluntary Oral Stage – Swallowing starts when the bolus is forced to the back of the
oral cavity and into the oropharynx by the tongue moving upward and backward against
the palate.
2. Involuntary Stage – a. Pharyngeal stage – The passage of the bolus into the oropharynx
begins the pharyngeal stage. The bolus stimulates receptors in the oropharynx, which
send impulses to the deglutition center in the medulla oblongata and pons. The impulses
from the deglutition center cause the soft palate and uvula to move upward to close the
nasopharynx, which prevents swallowed foods and liquids from entering the nasal cavity.
The epiglottis closes off the opening of the larynx, which prevents the bolus from
entering the trachea. The bolus then moves through the oropharynx and the
laryngopharynx. When the upper esophageal sphincter relaxes, the bolus moves into the
esophagus.
b. Esophageal Stage – The passage of the bolus into the esophagus begins the esophageal
stage. During this stage peristalsis pushes the bolus onward. Peristalsis is a progression of
coordinated contractions and relaxations of the circular and longitudinal layers of the
muscularis. In the section of the esophagus superior to the bolus the circular smooth
muscle contracts constricting the esophageal wall and squeezes the bolus toward the
stomach. The longitudinal smooth muscle inferior to the bolus also contracts shortening
this inferior section and pushes its wall outward so it can receive the bolus. The repeated
contractions in waves push the bolus toward the stomach. As the bolus approaches the
end of the esophagus, the lower esophageal sphincter relaxes and the bolus moves into
the stomach. Mucus secreted by the glands of the esophagus lubricates the bolus to
reduce friction.
Stomach
Anatomy – Refer to the laboratory manual.
Histology of the Stomach
The stomach wall has the same four basic layers found in the GI tract.
Mucosa
Epithelium – The surface epithelium is a layer of simple columnar epithelial cells mainly
of goblet cells. These cells continue down to line narrow channels, the gastric pits. At the
base of the gastric pits the epithelial cells extend down into the lamina propria where they
form the gastric glands. Several gastric glands open into a gastric pit.
The gastric glands contain three types of exocrine gland cells that secrete their products
into the gastric pits and then into the stomach’s lumen.
Surface mucous cells and mucous neck cells secrete mucus. The mucous neck cells are in
that part of the gastric gland called the neck of the gland.
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Parietal cells produce HCl and intrinsic factor (needed for vitamin B12 absorption) are
scattered among the chief cells.
Chief cells are mainly towards the base of the gastric gland secrete pepsinogen and
gastric lipase.
The secretions from all of these gastric gland cells form the gastric juice.
The gastric glands contain an enteroendocrine cell, the G cells. The G cells secrete the
hormone gastrin. The G cells are mainly located in the gastric glands of the pyloric
antrum. The gastrin stimulates several gastric activities. The G cells are in the base of the
gastric glands.
Lamina propria – Areolar connective tissue.
Muscularis mucosae – A thin layer of inner circular and outer longitudinal smooth
muscle.
Submucosa – Composed of areolar connective tissue.
Muscularis Externa
In the body of the stomach the muscularis has three layers of smooth muscle, an inner
oblique, middle circular and an outer longitudinal. The rest of the muscularis externa of
the stomach is inner circular and outer longitudinal smooth muscle.
Serosa
The serosa is composed of areolar connective tissue and covered by simple squamous
epithelium (mesothelium).
The serosa covering the stomach is part of the visceral peritoneum. The visceral
peritoneum at the lesser curvature of the stomach that extends superiorly to the liver is
the lesser omentum. The visceral peritoneum at the greater curvature of the stomach
which extends inferiorly and drapes over the intestine is the greater omentum.
Functions of the Stomach
1. Mixes saliva, food and gastric juice to form chyme.
2. Serves as a reservoir for food before release into the small intestine.
3. Secretes gastric juice, which contains HCl (kills bacteria and denatures protein), pepsin
(begins digestion of proteins), intrinsic factor (aids absorption of vit B12) and
gastric lipase (aids digestion of triglycerides).
4. Secretes gastrin into the blood.
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Mechanical and Chemical Digestion in the Stomach
Mechanical Digestion
After the bolus enters the stomach gentle peristaltic mixing waves macerate the bolus,
mix it with gastric juice and reduce it to a soupy liquid called chyme. More vigorous
mixing waves begin at the body of the stomach and intensify as they reach the pylorus.
As the chyme reaches the pylorus, the mixing waves forces the chyme through the
pyloric sphincter into the duodenum, this is called gastric emptying. The mixing waves of
the stomach are responsible for most of the mixing in the stomach.
Chemical Digestion
The chyme remains in the fundus for about an hour without becoming mixed with the
gastric juice. During this time, digestion by salivary amylase continues. Soon the
churning action mixes the chyme with the acidic gastric juice, inactivating salivary
amylase and activating lingual lipase which starts the digestion of triglycerides into
diglycerides and fatty acids.
Parietal Cells and HCl Secretion
The parietal cells secrete HCl. Proton pumps in the apical membrane of the parietal cells
powered by H+/K+ ATPase actively transport H+ ions into the lumen of the stomach and
K+ ions into the parietal cells. At the same time, Cl- and K+ ions diffuse out into the
lumen through channels in the apical membrane. The enzyme carbonic anhydrase in the
parietal cells catalyzes the formation of H2CO3 from H2O and CO2. The H2CO3
dissociates into H+ and HCO3- ions. The H+ ions are transported out of the parietal cells
into the lumen of the stomach by the proton pumps. The HCO3- ions as they build up in
the parietal cells are exchanged for Cl- ions by the Cl-/HCO3- antiporters in the basolateral
membrane of the parietal cells. The HCO3- ions diffuse into nearby capillaries. As the
HCO3- ions enter the blood after a meal they elevate the blood and urine pH, this is
known as the “alkaline tide”.
HCl secretion by the parietal cells is stimulated by acetylcholine, gastrin and histamine.
Histamine enhances the effects of acetylcholine and gastrin.
Due to HCl the highly acidic fluid of the stomach kills microbes in the food. HCl
partially denatures proteins in the food and stimulates secretion of hormones from the
stomach that promote the flow of bile and pancreatic juice.
Chief Cells and Its Enzymes
The enzyme pepsin secreted by the chief cells of the stomach breaks the protein chain
into smaller peptide fragments. Pepsin acts in the acidic environment (pH 2) of the
stomach. The pepsin is secreted in the inactive form, pepsinogen. The pepsinogen is
converted into the active form of the enzyme, pepsin, on coming into contact with HCl or
with active pepsin.
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The pepsinogen and the alkaline mucus, secreted by the mucous cells of the stomach
(surface cells, cells of the gastric pits, and mucous neck cells of the gastric glands)
protect the stomach epithelial cells from being digested.
Gastric lipase secreted by the chief cells digests short-chain triglycerides in fat molecules
into fatty acids and monoglycerides.
Gastrin a hormone secreted by the enteroendocrine cells of the stomach stimulates gastric
activity. The gastrin stimulates secretion of HCl by the parietal cells, secretion of
pepsinogen by the chief cells, increases contraction of the LES, increases stomach
motility and relaxes the pyloric sphincter.
Absorption in the Stomach
A small amount of nutrients are absorbed in the stomach, because the epithelial cells are
impermeable to most substances. The mucous cells of the stomach absorb some water,
ions, short-chain fatty acids, drugs and alcohol.
Within 2-4 hours of eating, the stomach empties its contents into the duodenum. Foods
high in carbohydrates spend the least time in the stomach; protein foods remain
somewhat longer and fats containing large amounts of triglycerides empty the slowest.
Accessory Digestive Organs
The pancreas, liver and gall bladder are accessory digestive organs that are found within
the abdominal cavity and are connected to the GI tract by ducts. Chemical digestion in
the small intestine depends on the activity of these organs.
Pancreas
Anatomy of the Pancreas
The pancreas is a retroperitoneal gland, which lies posterior to the greater curvature of
the stomach. Retroperitoneal means that the organ lies on the posterior abdominal wall
and covered only by peritoneum on its anterior surface.
The pancreatic juice is secreted by exocrine cells into small ducts that unite to form the
pancreatic duct and the accessory duct. These ducts convey the secretions into the small
intestine. The pancreatic duct joins the common bile duct and enters the duodenum as a
common duct, the hepatopancreatic ampulla (ampulla of Vater). The ampulla opens on an
elevation of the duodenal mucosa, the duodenal papilla. The duodenal papilla lies inferior
to the pyloric sphincter. Control of the secretions is regulated by smooth muscle, the
sphincter of the hepatopancreatic ampulla (sphincter of Oddi). The accessory duct
empties into the duodenum superior to the hepatopancreatic ampulla.
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Histology of the Pancreas
The exocrine portion of the pancreas is mainly made up of small groups of glandular
epithelial cells, called acini. The acinar cells secrete the pancreatic juice, which contains
digestive enzymes. The endocrine portion of the pancreas consists of clusters of cells, the
pancreatic islets of Langerhans. These cells secrete the hormones insulin and glucagon.
Composition and Functions of Pancreatic Juice
The pancreatic juice is a clear, colorless liquid consisting of water, some salts, H2CO3
and several enzymes. The H2CO3 gives the pancreatic juice a slightly alkaline pH (7.18.2) that buffers the acidic gastric juice in the chyme, stops the action of pepsin and
creates the proper pH for the activity of the digestive enzyme in the small intestine. The
enzymes in pancreatic juice are: starch-digesting pancreatic amylase; protein-digesting
trypsin, chymotrypsin, carboxypeptidase, and elastase; triglyceride-digesting pancreatic
lipase and nucleic acid-digesting ribonuclease and deoxyribonuclease.
The protein digesting enzyme of the pancreas are produced and secreted in an inactive
form so they do not digest the pancreatic cells. Trypsin is secreted in an inactive form,
trypsinogen. Pancreatic acinar cells also secrete a trypsin inhibitor that combines with
any trypsin and blocks its activity. Upon reaching the lumen of the small intestine the
trypsinogen is activated by a brush-border enzyme in the cells of the small intestine,
enterokinase. The entrokinase splits off part of the trypsinogen molecule to form the
active trypsin. The trypsin then acts on the inactive precursors (chymotrypsinogen,
procarboxypeptidase and proelastase) to produce the active forms of the enzymes
chymotrypsin, carboxypeptidase and elastase.
Control of secretion of pancreatic juice is through the nervous system via the ANS division and
by hormones secreted by the enteroendocrine cells in the intestinal glands of the small intestine.
These hormones are secreted simultaneously by specific enteroendocrine cells.
a. Nervous System Control
The parasympathetic nervous system division (PNS) neurons in the Vagus nerve (X) stimulate
directly secretion of pancreatic digestive enzymes and the hormone insulin. The sympathetic
nervous system division (SNS) neurons in the greater splanchic nerve inhibit directly
secretion of pancreatic digestive enzymes and insulin, but promotes secretion of the hormone
glucagon.
b. Hormonal Control by the Enteroendocrine Cells
1. Cholecystokinin (CCK) is secreted by CCK cells due to partially digested proteins and
triglycerides in the chyme in the lumen of the small intestine. The CCK then circulate in the
blood to the pancreas. At the pancreas the CCK stimulates the pancreatic exocrine acinar
cells to secrete pancreatic juice rich in digestive enzymes to continue digestion of the
partially digested proteins and triglycerides of the chyme in the small intestine.
2. Secretin (S) is secreted by the S cells due to acidic chyme in the lumen of the small
intestine. The S then circulates in the blood to the pancreas. At the pancreas the S stimulates
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the pancreatic exocrine acinar cells to secrete pancreatic juice rich in HCO3- ions.
The HCO3- ions help buffer the acidic chyme that enters the duodenum from the stomach.
Liver and Gallbladder
Anatomy of the Liver and Gallbladder
The liver has two main lobes, a large right lobe and a smaller left lobe. Between the two
main liver lobes is the falciform ligament which helps suspend the liver from the anterior
abdominal cavity wall. The liver is the only digestive system organ suspended from the
anterior abdominal cavity wall. All other digestive system organs are suspended from the
posterior abdominal cavity wall. The right lobe has an inferior quadrate lobe (square or
rectangle shaped) and a posterior caudate lobe (tail shaped). The gallbladder projects
beyond the inferior border of the right lobe of the liver. The gallbladder consists of a
fundus, body and neck.
Histology of the Liver and Gallbladder
The lobes of the liver are made up of the functional units, the liver lobules. A lobule is a
six-sided structure that consists of epithelial cells, the hepatocytes. The hepatocytes are
arranged in interconnected plates around a central vein. Between the plates of
hepatocytes are highly-permeable capillaries, the sinusoids. Blood passes through the
sinusoids into the central vein. Present in the walls of the sinusoids are fixed phagocytes,
the stellate reticuloendothelial (Kupffer) cells. As the blood flow through the sinusoids
these cells phagocytose and destroy old white and red blood cells, bacteria and other
foreign matter in the venous blood draining from the GI tract.
Bile secreted by the hepatocytes enters the bile canaliculi, narrow intercellular canals that
empty into small bile ductules. The bile ductules pass the bile into bile ducts at the
periphery of the lobules. The bile ducts merge to form the larger right and left hepatic
ducts, which unite and exit the liver as the common hepatic duct. The common hepatic
duct joins the cystic duct from the gallbladder to form the common bile duct.
Contraction of the smooth muscle of the muscularis of the gallbladder ejects the stored
bile from the gallbladder into the cystic duct. The functions of the gallbladder are to store
and concentrate bile produced by the liver until needed in the small intestine.
Role and Composition of Bile
The hepatocytes secrete about a liter of bile /day. The bile an olive-green liquid has a pH
of 7.6-8.6 and consist water, bile salts, cholesterol, the phospholipid lecithin, bile
pigments especially bilirubin and ions.
The bilirubin, derived hemoglobin of old red blood cells, is secreted into the bile and is
further broken down in the intestine into stercobilin. The sterocobilin gives the feces their
normal brown color.
Bile is partially an excretory product and partially a digestive secretion. The bile salts
play a role in emulsification of fats which is the breakdown large lipid globules into a
suspension of small lipid droplets. The small lipids droplets present a large surface area
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for the pancreatic lipase to digest the triglycerides. Bile salts also aid in the absorption of
lipids after their digestion.
Between meals bile flows into the gallbladder for storage because the sphincter of the
hepatopancreatic ampulla closes off the entrance to the duodenum.
Functions of the Liver
In addition to secreting bile for absorption of dietary lipids, the liver performs other vital
functions some of which are:
Carbohydrate metabolism – The liver maintains a normal blood glucose level. When blood glucose
is low, the liver converts glycogen to glucose and releases it into the blood (glycogenolysis). When
blood glucose is high, the liver converts glucose to glycogen (glycogenesis) and triglycerides for
storage. By the process of gluconeogenesis, the liver can convert certain amino acids, lactic acid,
fructose and galactose into glucose.
Lipid metabolism – The hepatocytes store some triglycerides; breakdown fatty acids to
make ATP; synthesize cholesterol; use cholesterol to make bile salts and synthesize
lipoproteins, which transport fatty acids, triglycerides and cholesterol to and from body
cells.
Protein metabolism – Hepatocytes remove amino (NH2) groups (deaminate) from amino acids.
The NH2 groups are converted to toxic ammonia (NH3); however the liver cells convert the NH3
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into less toxic urea (NH2-C-NH2) which is excreted in the urine. Hepatocytes also synthesize
most of the plasma proteins, such as the α- and ß-globulins, albumin, prothrombin and
fibrinogen.
Functions of bile salts – The bile salts are used in the small intestine for the
emulsification and absorption of lipids.
Control of Production and Delivery (Secretion) of Bile
Control of secretion of bile is through the nervous system via the ANS division and by a
hormone secreted by the enteroendocrine cells in the intestinal glands of the small intestine.
a. Nervous System Control
The parasympathetic nervous system division (PNS) neurons in the Vagus nerve (X) stimulate
directly secretion of bile and glycogenesis. The sympathetic nervous system division (SNS)
neurons in the greater splanchic nerve inhibit directly secretion of bile but stimulate glycogenesis
and gluconeogensis.
b. Hormonal Control by the Enteroendocrine Cells
Cholecystokinin (CCK) is secreted by CCK cells due to partially digested proteins and
triglycerides in the chyme in the lumen of the small intestine. The CCK then circulate in the
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blood to the liver. The CCK causes contraction of the walls of the gall bladder, which
squeezes the stored bile from the gall bladder into the cystic duct and common bile duct.
The CCK also causes relaxation of the sphincter of the hepatopancreatic duct ampulla to
allow pancreatic juice and bile to flow into the duodenum.
Small Intestine
Most of the digestion and absorption occur in the small intestine. The small intestine
length provides a large surface area for these functions. The surface area is further
increased by circular folds (plicae circulares), villi and microvilli.
Anatomy of the Small Intestine
The small intestine is divided in to regions – duodenum is retroperitoneal, jejunum and
ileum. The small intestine starts at the pyloric sphincter and ends at the ileocecal
sphincter.
Histology of the Small Intestine
The wall of the small intestine is made up of the same four layers of the GI tract.
Mucosa – The epithelial layer consists of simple columnar epithelium that contains various cell
types. Absorptive cells digest and absorb nutrients in the chyme. Goblet cells secrete mucus. The
mucosa contains intestinal glands (crypts of Lieberkühn) which secrete intestinal juice. The
intestinal glands reside in the lamina propria. The intestinal glands contain absorptive cells, goblet
cells, paneth cells and enteroendocrine cells. Paneth cells secrete lysozyme, a bactericidal enzyme
and are capable of phagocytosis. Paneth cells regulate the microbial population in the small intestine.
There are three types of enteroendocrine cells in the small intestine. They are S cells secrete secretin,
CCK cells secrete cholecystokinin, and K cells secrete gastric inhibitory peptide or glucosedependent insulinotropic peptide (GIP).
Lamina Propria – Consist of areolar connective tissue and has an abundance of lymphoid
tissue as solitary lymphatic nodules (MALT) and the intestinal glands. Groups of
lymphatic nodules in the ileum aggregate as lymphatic follicles, the Peyer’s patches.
Submucosa –Consist of areolar connective tissue. The submucosa of the duodenum
contains duodenal (Brunner’s) glands, which secrete alkaline mucus that neutralizes
gastric acid in the chyme.
Muscularis Externa – Consists of an inner thick layer of circular smooth muscle and an
outer thinner layer of longitudinal smooth muscle.
Serosa or Visceral Peritoneum – Surrounds the small intestine except for the duodenum.
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Special structures of the small intestine to increase surface area which facilitate digestion
and absorption.
Circular folds or plicae circulares are folds of the mucosa and submucosa. The circular
folds enhance absorption by increasing surface area. The circular folds extend either all
the way or partly around the circumference of the small intestine. The circular folds cause
the chyme to spiral as it passes through the small intestine.
Villi – Villi are fingerlike projections of the mucosa to increase the surface area of the
small intestine for digestion and absorption. The villi give the intestinal mucosa a velvety
appearance. Each villus is covered by columnar epithelium and has a core of lamina
propria; in the areolar connective tissue of the lamina propria are an arteriole, a venule, a
blood capillary network and a lacteal. A lacteal is a lymphatic capillary. Nutrients
absorbed by the epithelial cells pass through the wall of the capillaries or lacteals to enter
the blood or lymph.
Microvilli – Microvilli are projection of the apical (free) cell membrane of the absorptive
cells. Viewed in the E/M, each microvillus is a core of cytoplasm containing a bundle of
actin filaments and covered by the cell membrane. Viewed in the L/M, the microvilli
form a brush border. The microvilli greatly increase the surface area of the absorptive
cells so large amounts of digested nutrients can quickly diffuse into the absorptive cells.
Intestinal Juice and Brush-Border Enzymes
About 1-2 liters of intestinal juice, a clear, yellow fluid are secreted each day. Intestinal
juice has a pH of 7.6 contains water, intestinal enzymes and mucus. The pancreatic and
intestinal juices and bile provide the medium for digestion and absorption of nutrients
from the chyme in the small intestine.
The absorptive epithelial cells of the small intestine synthesize several digestive enzymes,
the brush-border enzymes. These enzymes are inserted into the cell membrane of the
microvilli. Some enzymatic digestion occurs at the surface of the absorptive cells. As
these cells slough off into the lumen of the small intestine, they break apart and release
their enzymes to help in digestion here.
The brush-border enzymes are: carbohydrate-digesting enzymes – α-dextrinase, maltase,
sucrase, and lactase; protein-digesting enzymes – aminopeptidase and dipeptidase; and
nucleotide-digesting enzymes – nucleosidases and phosphatases.
Mechanical Digestion in the Small Intestine
There are two types of movements of the small intestine that cause mechanical digestion.
Both movements are controlled by the myenteric plexus through the PNS.
Segmentations
Segmentations are localized, mixing contractions that occur in parts of the intestine
distended by the chyme. Segmentations are the most common movement of the small
intestine. Segmentations mix the chyme with the digestive juices to enhance digestion
and bring the digested chyme into contact with the absorptive epithelial cells of the
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mucosa. Segmentation starts with contractions of circular smooth muscle in a portion of
the small intestine, an action that constricts the intestine into segments. Then smooth
muscle in the middle of each segment contract, dividing each segment. Finally the
smooth muscle that first contracted relaxes and each small segment unites so that large
segments are formed again. As the sequences of segmentations repeats, the chyme is
churned and moves back and forth. Segmentations are more rapid in the duodenum than
in the ileum which causes a slow progression of the chyme toward the colon.
When most of the nutrients have been absorbed, segmentations stop and peristalsis
begins.
Migrating Motility Complex
This type of peristalsis in the small intestine is termed migrating motility complex
(MMC). A peristaltic wave begins in the pyloric region of the stomach. The peristaltic
movement pushes the chyme forward slowly along a short length of the small intestine
and then dies out. This wave is followed by another wave a little further along the tract
from the first wave. The successive peristalsis or MMC slowly migrates down the small
intestine moving the chyme with remains of undigested residue toward the colon. When
the MMC reaches the end of the ileum, then another MMC begins. Intake of the next
meal into the stomach suppresses the MMC and reactivates the segmentations.
Chemical Digestion in the Small Intestine
Chyme entering the small intestine contains partially digested foods. The completion of
the digestion of the ingested foods is due to the activity of the pancreatic juice enzymes,
intestinal juice enzymes and bile in the small intestine.
Digestion of Carbohydrates
The polysaccharides starch and glycogen are acted on by the pancreatic amylase by hydrolysis to
form maltose, maltotriose and α-dextrin. The pancreatic amylase does not act on cellulose. After
amylase has split the polysaccharides into smaller units the α-dextrinase acts on the α-dextrins
clipping off one glucose unit at a time.
Ingested disaccharides sucrose, lactose and maltose are not digested until they reach the small
intestine. Brush-border enzymes by hydrolysis digest these disaccharides into monosaccharides.
Sucrase breaks sucrose into glucose and fructose. Lactase digests lactose into glucose and
galactose. Maltase splits maltose and maltotriose into glucose molecules. The digestion of
carbohydrates ends with the formation of monosaccharides, which can then be absorbed by the
absorptive epithelial cells of the small intestine.
H H
H
H
|
|
|
|
-C-O-C- + HOH → -C-OH + HO-CGlycoside linkage
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Digestion of Proteins
The pancreatic enzymes, trypsin, chymotrypsin, carboxypeptidase and elastase by hydrolysis
digest proteins into peptides. The actions of these enzymes differ because each breaks peptides
between different amino acids. Trypsin, chymotrypsin and elastase cleave the peptide bond
between specific amino acids. Carboxypeptidase splits off the amino acid at the carboxyl end of
the peptide. Two peptidases in the brush-border complete the protein digestion. Aminopeptidase
cleaves off an amino acid at the amino end of a peptide. Dipeptidase splits dipeptides into amino
acids.
OH
O
H
|| |
||
|
-C-N- + HOH → -C-OH + H-NPeptide bond
Digestion of Lipids
Most of the digestion of the triglycerides in the ingested lipids occurs in the small intestine. The
triglycerides by hydrolysis are broken down into long and/ or short chain fatty acids and
monoglycerides.
H
O
H
O
|
||
|
||
H-C-O-C-C- + HOH → H-C-OH + HO-C-C| Ester linkage
H
Before a large fat globule can be digested in the small intestine it must undergo emulsification.
Emulsification is a process in which large lipid globules are broken down into small lipid
globules. The bile salts in the bile emulsify large lipid globules into small lipid globules. The
small lipid globules provide a large surface area for the pancreatic lipase to act.
Digestion of Nucleic Acids
Pancreatic ribonulease digests RNA, deoxyribonuclease by hydrolysis digests DNA to
nucleotides. The nucleotides are further by hydrolysis digested by brush-border enzymes
nucleosidases and phosphatases into pentoses, phosphates and nitrogen bases.
Absorption in the Small Intestine
Movement of digested nutrients from the GI tract into the blood or lymph is absorption. The
purpose of mechanical and chemical digestion from the mouth through the small intestine is to
change complex food molecules into simpler forms that can pass through the absorptive
epithelial cells of the mucosa into the blood and lymphatic vessels.
Absorption occurs by diffusion, facilitated diffusion, osmosis and active transport. 90% of the
absorption occurs in the small intestine, 10% occurs in the stomach and large intestine. Any
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undigested and unabsorbed material in the small intestine passes on to the large intestine for
elimination.
Absorption of Monosaccharides
All dietary carbohydrates are digested to monosaccharides are absorbed. Monosaccharides pass
from the lumen through the apical cell membrane of the absorptive cells by facilitated diffusion
or active transport. Fructose is transported by facilitated diffusion. Glucose and galactose are
transported by secondary active transport (symport) coupled to the active transport of Na+ ions.
The transporter has three binding sites, one for the glucose molecule and two for Na+ ions; all
three sites must be filled if glucose is to be transported. Galactose competes with glucose for the
sites on the transporter. Indigestible cellulose and other polysaccharides pass into the large
intestine. Monosacchaides move out of the absorptive cells through the basolateral cell
membrane by facilitated diffusion and enter the capillaries of the villi.
Absorption of Amino Acids, Dipeptides and Tripeptides
Amino acids are absorbed by active transport. Different transporters carry different types of
amino acids. Some amino acids are transported along with Na+ ions by a symporter; other amino
acids are actively transported by themselves. One symporter transports dipeptides and tripeptides
along with H+ ions. The peptides are then broken into amino acids inside the absorptive cells by
the brush-border enzymes. Amino acids diffuse out of the absorptive cells by diffusion and enter
the capillaries of the villi. The monosaccharides and the amino acids are transported in the blood
to the liver. If not absorbed by the liver cells, they enter the general circulation.
Absorption of Lipids
All dietary lipids are absorbed by simple diffusion. The triglycerides are emulsified and digested
to monoglycerides and fatty acids. Short-chain fatty acids dissolve in the watery chyme pass
through the absorptive cells by simple diffusion into the blood capillaries of the villi. Long-chain
fatty acids and monoglycerides are hydrophobic have difficulty of being dissolved in the watery
chyme. The bile salts make the long-chain fatty acids and monoglycerides more soluble by
surrounding these molecules to form tiny spheres called micelles. The micelles move from the
lumen of the small intestine to the brush-border of the absorptive cells. At the brush-border of
the absorptive cells, the long-chain fatty acids and monoglycerides leave the micelles and enter
the absorptive cells.
Once inside the absorptive cells, the long-chain fatty acids and the monoglycerides recombine to
form triglycerides. The triglycerides along with phospholipids and cholesterol become coated
with proteins aggregate into large spherical masses called chylomicrons. The chylomicrons by
excytosis leave the absorptive cells and enter the lacteals of the villi. The lacteals have larger
pores in their walls than the blood capillaries. From the lacteals, the chylomicrons are transported
by lymphatic veins to the thoracic duct and enter the blood at the left subclavian vein. The
protein coat that surrounds each chylomicron keeps the chylomicrons suspended in the blood and
prevents them from sticking to each other.
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The chylomicrons are removed from the blood as the blood passes through the capillaries in the
liver and adipose tissue. An enzyme, lipoprotein lipase, on the apical surface of the capillary
endothelial cells breaks down the triglycerides and other lipoproteins in the chylomicrons into
fatty acids and glycerol. The fatty acids and glycerol diffuse into the liver cells and adipose cells
and recombine into triglycerides.
The bile salts are reabsorbed by active transport in the ileum, returned to the liver by the hepatic
portal system for recycling. The recycling bile salts between the liver and the ileum is called
enterohepatic circulation.
Study the below topic on your own.
Absorption of Electrolytes
Absorption of Vitamins
Absorption of Water
Absorption of Alcohol
Large Intestine
Functions of the large intestine
1. Movements of the large intestine by haustral churning, peristalsis and mass peristalsis drive
the contents of the large intestine into the rectum.
2. Bacteria in the large intestine converts proteins to amino acids, breakdown amino acids and
produce some B vitamins and vitamin K.
3. Absorb some water, vitamins and ions.
4. Form feces and eliminate the feces (defecation).
Anatomy of the Large Intestine
The large intestine is the terminal part of the GI tract. It extends from the ileum to the anus.
The ileocecal sphincter allows chyme to pass from the small intestine into the large intestine.
Inferior to the ileocecal sphincter is the cecum. Attached to the cecum is the vermiform
appendix.
Superior to the cecum is the colon which is divided into ascending, transverse, descending and
sigmoid parts. The rectum is the last part of the GI tract. The terminal end of the rectum is the
anal canal. The opening of the anal canal to the exterior is the anus. The anus is guarded by
involuntary internal anal sphincter of smooth muscle and voluntary external anal sphincter of
skeletal muscle.
Histology of the Large Intestine
The large intestine contains the four layers found in the rest of the GI tract.
Mucosa – The mucosa consists of simple columnar epithelium, lamina propria and muscularis
mucosa. The epithelium is mostly absorptive cells which function to absorb water and goblet
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cells which secrete mucus. The mucus lubricates the passage of the contents along the large
intestine. The absorptive and goblet cells are located in long, straight, tubular intestinal glands
that resides in the lamina propria.
In the lamina propria solitary lymphatic nodules are present. The mucosa has no circular folds or
villi. Microvilli on the surface of absorptive cells are present.
Submucosa – The submucosa consists of areolar connective tissue.
Muscularis Externa – The muscularis externa consists of an internal layer of circular smooth
muscle. The external layer consists of three longitudinal bands of smooth muscle that run the
length of the large intestine, the teniae (taeniae) coli. Tonic contractions of the teniae coli bands
gather the colon into pouches, the haustra. The haustra give the colon a puckered appearance.
Serosa – The serosa is part of the visceral peritoneum. Small pouches of visceral peritoneum
filled with fat are attached to the teniae coli are the epiploic appendages.
Mechanical Digestion in the Large Intestine
The passage of chyme into the cecum is regulated by the action of the ileocecal sphincter. The
sphincter remains partially closed so passage of chyme into the cecum occurs slowly. A
gastroileal reflex increases the peristalsis in the ileum and relaxes the ileocecal valve this forces
the chyme into the cecum. Gastrin also relaxes the sphincter. When the cecum is distended, the
contraction of the ileocecal sphincter increases.
Movements of the large intestine begin when chyme passes through the ileocecal sphincter, fills
the cecum and ascending colon.
1. Haustral churning – In this process, the haustra remain relaxed and become distended while
they fill up. When distended to a point, the haustral walls contract and push the contents into the
next haustrum.
2. Peristalsis – Peristalsis also pushes the contents onwards.
3. Mass Peristalsis – Mass peristalsis, a strong peristaltic wave that begins in the middle of the
transverse colon and quickly drives the contents of the colon into the rectum. Food in the
stomach initiates this gastrocolic reflex in the colon causing the mass peristalsis. Mass peristalsis
takes place during or immediately after a meal.
Chemical Digestion in the Large Intestine
The digestion in the large intestine occurs through the activity of the intestinal bacteria within the
lumen. Mucus is secreted by the large intestinal glands, but no enzymes are secreted. Chyme is
prepared for elimination by the action of the bacteria. The bacteria ferment any remaining
carbohydrates release gases which contribute flatus (gas) in the colon. Bacteria convert any
remaining proteins to amino acids and breakdown the amino acids to simpler substances.
Bacteria of the large intestine produce some B vitamins and vitamin K that are absorbed.
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Absorption and Feces Formation in the Large Intestine
Absorption in the Large Intestine
The epithelial cells of the large intestine absorb enough water to make it an important organ in
maintaining the body’s water balance. Of the 1.0 liter of water that enters the large intestine all
but 100-200 mL is absorbed by osmosis. The large intestine also absorbs ions, such as Na+ and
Cl- ions and vitamins B and K.
Feces formation
As the chyme in the large intestine becomes solid or semisolid because of water absorption, it
becomes feces. The feces consists of water, mucus, inorganic salts, sloughed-off epithelial cells,
bacteria, products of bacterial decomposition, unabsorbed digested material and indigestible
parts of food.
The Defecation Reflex
Mass peristalsis pushes the feces from the sigmoid colon into the rectum. The distension of the
rectal wall stimulates stretch receptors, which initiates the defecation reflex.
In response to distension of the rectal wall, the stretch receptors send sensory nerve impulses to
the sacral spinal cord. Motor impulses from the spinal cord travel along parasympathetic neurons
in the pelvic splanchic nerves to the descending colon, sigmoid colon, rectum and anus. The
contraction of the teniae coli shortens the rectum increasing the pressure within it. Voluntary
contractions of the diaphragm and abdominal muscles pushes the walls of the sigmoid colon and
rectum inward and parasympathetic stimulation opens the internal anal sphincter.
The external anal sphincter is voluntarily controlled. If it is voluntarily relaxed, defecation occurs
and the feces are expelled through the anus. If the external anal sphincter is voluntarily
constricted defecation can be postponed and the feces back up into the sigmoid colon until the
next wave of mass peristalsis stimulates the stretch receptors again creating the urge to defecate.
The normal defecations are 2-3/day to 3-4 /week.
Phases of Digestion
Digestive activities occur in three phases.
Cephalic Phase
During the cephalic phase of digestion, the smell, sight, thought or taste of food activates neural
centers in the cerebral cortex, hypothalamus and brain stem. The brain stem then sends nerve
impulses along the facial (VII), glossopharyngeal (IX) and vagus (X) nerves. The facial and
glossopharyngeal nerves stimulate the salivary glands to secrete saliva. The vagus nerves
stimulate the gastric glands to secrete gastric juice. The purpose of the cephalic phase of
digestion is to prepare the mouth and stomach for food that is about to be eaten.
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Gastric Phase
When the food reaches the stomach, the gastric phase of digestion begins. Neural and hormonal
mechanisms regulate gastric secretion and motility.
Neural Regulation – Food in the stomach distends the stomach stimulates stretch receptors.
Chemoreceptors monitor the pH of the stomach’s chyme as it becomes alkaline due to incoming
food. The activation of these receptors set up a neural negative feedback condition. Nerve
impulses from the stretch receptors and chemoreceptors are sent to the submucosal plexus and
myenteric plexus, where they activate enteric neurons and to the medulla where they activate
parasympathetic neurons of the vagus nerve. The nerve impulses causes increased peristalsis of
the smooth muscle of the stomach and stimulate flow of gastric juice from the gastric glands.
The peristaltic waves mix the food with gastric juice; when the waves become strong enough
chyme undergoes gastric emptying into the duodenum. As the distension of the stomach walls
lessen and the pH of the stomach chyme decreases toward acid, the peristalsis and secretion of
gastric juice decreases.
Hormonal Regulation – Gastric secretion during the gastric phase of digestion is also regulated
by the hormone, gastrin. Gastrin is released from the G-cells of the gastric glands in response to
distension of the stomach by chyme, partially digested proteins in the chyme, the high (alkaline)
pH of chyme due to the presence of food in the stomach. The gastrin released into the blood
circulates to the gastric glands and stimulates these glands to secrete large amounts of gastric
juice. Gastrin also strengthens contraction of the LES to prevent reflux of the gastric contents
into the esophagus, increases gastric motility (peristalsis), and relaxes the pyloric sphincter to
promote gastric emptying. Gastrin secretion is inhibited when pH of the gastric juice drops
below pH 2.0 and is stimulated when the pH rises. This negative feedback provides an optimal
pH for the functioning of pepsin, killing of microbes and denaturing of proteins in the stomach.
Intestinal Phase
When food enters into the small intestine, the intestinal phase of digestion begins. Reflexes
occurring during the intestinal phase have inhibitory effects that slow the exit of chyme from the
stomach. This prevents the duodenum from being overload with more chyme than it can handle.
The responses during the intestinal phase promote the digestion of chyme that reached the small
intestine.
Neural Regulation – The presence of chyme in the duodenum causes the enterogastric reflex.
Stretch receptors in the duodenal wall send nerve impulses to the medulla oblongata; this inhibits
parasympathetic stimulation and stimulates sympathetic stimulation to the stomach. Gastric
motility is inhibited and pyloric sphincter contraction is increased, which decreases gastric
emptying. Stimulation of stretch receptors in the stomach as it fills causes the gastroenteric reflex
which stimulates motility and secretion along the entire small intestine. The gastroenteric and
gastroileal reflexes accelerate movement along the small intestine, while the enterogastric reflex
inhibits movement along the small intestine.
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Hormonal Regulation – Two hormones secreted by the cells of the small intestine glands regulate
the intestinal phase of digestion.
Cholecystokinin (CCK) is secreted in response to chyme containing amino acids from partially
digested proteins and fatty acids from partially digested triglycerides. The CCK stimulates the
secretion of pancreatic juice that contains enzymes that further digest proteins and triglycerides.
The CCK also causes contraction of the gallbladder to squeeze stored bile out of the gallbladder.
CCK causes relaxation of the hepatopancreatic ampulla sphincter, which allows pancreatic juice
and bile to flow into the duodenum. CCK slows gastric emptying by increasing pyloric sphincter
contraction.
Secretin is secreted as acidic chyme enters the duodenum. Secretin stimulates secretion of
pancreatic juice rich in HCO3- ions which buffer the acidic chyme from the stomach. Secretin
also inhibits the secretion of gastric juice.
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