Transcript Chapter_24x

Anatomy
and
Physiology
Biology 2402
Chapter-24
The Digestive System
OVERVIEW
Organization
• The Digestive system is organized into two groups of
organs:
– The Gastrointestinal Tract or alimentary canal: a long tube
extending from the mouth to the anus.
• Includes mouth, pharynx, esophagus, stomach, small intestine, large
intestine, and anus
– The accessory structures: glands, organs, and other structures
that produce and release secretions into the gastrointestinal
tract, and contribute to the mechanical and chemical digestion of
food.
• Includes teeth, tongue, salivary glands, liver, gall bladder, and
pancreas.
An Introduction to the Digestive System
• The Digestive System
– Acquires nutrients from environment
– Anabolism
• Uses raw materials to synthesize essential compounds
– Catabolism
• Decomposes substances to provide energy cells need to function
• Breakdown of carbohydrates, proteins, lipids and nucleic acids.
The Digestive Processes
Digestion includes six basic processes.
• Ingestion is taking food into the mouth (eating).
• Secretion is the release, by cells within the walls of the
GI tract and accessory organs, of water, acid, buffers,
and enzymes into the lumen of the tract.
• Mixing and propulsion result from the alternating
contraction and relaxation of the smooth muscles within
the walls of the GI tract.
• Digestion proper is the breakdown of large food
molecules into simple molecules. It includes:
– Mechanical digestion: consists of movements of the GI
tract that mix, soften, and liquefy the food.
– Chemical digestion: a series of enzymatic reactions that
break down polysaccharides, lipids, proteins, and nucleic
acids into smaller molecules.
• Absorption is the passage of nutrients generated by
digestion from the lumen of the GI tract into blood or
lymph.
• Defecation is the release of solid waste (generated
by digestion) from the rectum.
LAYERS OF THE GI TRACT
The basic layers of the GI tract from the inside to the
outside are :
1. Mucosa
2. Submucosa
3. Muscularis
4. Serosa
LAYERS OF THE GI TRACT
Mucosa
• The mucosa lines the lumen of the GI tract; it consists of an
epithelium, lamina propria, and muscularis mucosa.
–The epithelium consists of various cells: stratified cells for
protection, simple columnar cells for secretion and absorption,
mucus secreting cells for protection and lubrication, and
enteroendocrine cells that release hormones involved in the
regulation of digestion.
–The lamina propria anchors the mucosa and provides a
framework for blood vessels, lymph vessels, and nerves. Many
mucosa-associated lymph tissues (MALT) provide on-site
defense against potential pathogens present in food.
– The muscularis mucosa provides elasticity to the mucosa to
enhance digestion and absorption.
Submucosa
• The submucosa is made of areolar connective tissue
housing a large number of glands, nerves, blood
vessels, and lymphatic tissue.
Muscularis
 The muscularis is made of at least two layers of
smooth muscle fibers (involuntary):
– the innermost layer is the circular layer
– the outermost layer is the longitudinal layer
 Together the two layers contract rhythmically during
a process called peristalsis to mix, crush, and propel
the food through the GI tract.
 The muscularis is highly vascular and houses the
myenteric plexus or plexus of Auerbach. This vast
network of nerves, neurons, and ganglia controls
both parasympathetic & sympathetic innervation of
circular and longitudinal muscle layers
Movement
Serosa
• The serosa is made of two layers:
– An inner connective tissue layer which is made of areolar
connective tissue: it covers the other layers, and lead blood
vessels and nerves into them.
– An outer epithelial layer which attaches the GI tract to the
peritoneum folds.
Structure of the Peritoneum
• The peritoneum is the largest serous membrane of the body.
• The parietal peritoneum lines the wall of the abdominal
cavity.
• The visceral peritoneum covers some of the organs and
constitutes their serosa.
• The space between the parietal and visceral layers of the
peritoneum is called the peritoneal cavity and it contains
serous fluid.
PERITONEUM
Regions of the Peritoneum
•
Parietal peritoneum
• Visceral peritoneum
• Mesentery
• Mesocolon
• Falciform ligament
• Lesser omentum
•
Greater omentum
Functions of the Peritoneum
•
•
Organs located on the posterior abdominal wall
behind the peritoneum are said to be
retroperitoneal. In the adult, the aorta, inferior
vena cava, kidneys, adrenal glands, the pancreas,
duodenum, ascending and descending colons are
retroperitoneal.
Organs covered by the folds of the peritoneum are
said to be intraperitoneal: they include stomach,
small intestine (jejunum and ileum), transverse
colon, liver and gallbladder.
• Mesenteries
– Are double sheets of peritoneal membrane
– Suspend portions of digestive tract within peritoneal cavity by sheets
of serous membrane that connect parietal peritoneum with visceral
peritoneum.
• The Lesser Omentum
– Stabilizes position of stomach
• The Greater omentum
• Extends inferiorly between the body wall and the anterior surface
of small intestine
• Hangs like an apron From lateral and inferior borders of stomach
Figure 24-2b Mesenteries
Lesser
omentum
Transverse
colon
Ascending
colon
Greater
omentum (cut)
Transverse
mesocolon
Mesocolon of ascending
and descending colons
fused to posterior portion
of the parietal peritoneum
Mesentery
proper
(mesenterial
sheet)
Descending
colon
Small
intestine
Sigmoid
colon
A diagrammatic view of the organization of
mesenteries in an adult.
MOUTH (Oral Cavity)
Parts of the Mouth Lips and Cheeks
• Vestibule
• Teeth
• Hard & soft palate
• Uvula
• Tongue
• Pharyngeal Arches
• Fauces
Structure of the Oral Cavity
• The mouth or oral cavity is formed by the
cheeks, hard and soft palate, lips , and tongue.
• The vestibule of the mouth is the space between
the cheeks and lips (external boundary), and the
gums and teeth (internal boundary).
• The oral cavity proper is the space that extends
from the gums and teeth (outermost boundary)
to the fauces (innermost boundary).
• The fauces are the opening that leads to the
pharynx and larynx.
Oral Cavity (continued)
• Lips and cheeks keep the food within the mouth.
• The tongue is a muscular organ that occupies most of the floor
of the mouth. It moves food around the mouth during
mastication, mixes it with saliva, and shapes it into the bolus
which is swallowed later. Lingual papillae (taste buds) detect
food flavor.
• The roof of the mouth is called the palate, which is subdivided
into hard and soft palates.
• The uvula is an extension of the soft palate: it contributes to
sound modulation, and prevents the entry of food in the nasal
cavity during swallowing.
Salivary Glands
The Three Major Pairs of
Salivary Glands:
• parotid
• Submandibular (submaxillary)
• sublingual
Salivary Glands (continued)
• The salivary glands are located around the oral cavity:
- The parotid glands are located below the ears and over the masseters.
- The submandibular glands are located under the lower edge of
mandible.
- The sublingual glands are located underneath the tongue in the floor of
the mouth
• These glands are exocrine glands: they produce saliva which is
delivered via ducts that empty into the oral cavity.
Saliva
• Saliva is a watery secretion from the salivary glands.
• Water makes up 99.5% of saliva, and the remaining
0.5% is a mixture of mucus, solutes such as salts,
various organic substances, and enzymes.
• Functions of Saliva
– Lubricating the mouth
– Moistening and lubricating materials in the mouth
– Dissolving chemicals that stimulate taste buds and provide sensory
information
– Initiating digestion of complex carbohydrates by the enzyme salivary
amylase.
Salivation
• Salivation is the secretion of saliva.
• Salivation is a nearly constant phenomenon that helps
cleanse the mouth and keep it moist.
• Salivation can be triggered by a number of stimuli
including the smell, taste, sight, or even the thought of
food.
• It can be interrupted by emotions such as fear (dry
mouth).
• Salivation is under sympathetic and parasympathetic
control.
Structure of a Tooth
• Crown
• Neck
• Roots
• Pulp cavity
Composition of Teeth
• The crown is the visible portion of the enamel above the
gums (gingivae).
• The enamel is the hardest part of the tooth. It is made of
calcium phosphate like bone, but unlike bone it does not
contain collagen.
• The crown of a tooth is made of a thick enamel surface
overlaying dentin, the main tissue of the tooth.
• Dentin is a calcified connective tissue. It is protected by
cementum in the root of the tooth.
• Cementum is a thin bone like tissue linked to the alveoli of the
maxilla and the mandible by periodontal ligaments. (This
immovable joint is called a gomphosis).
Dental Pulp
• Dental Pulp is located at the center of a tooth within
the pulp cavity and the root canal(s).
• Pulp is a soft tissue made of various connective tissue
cells such as odontoblasts, and fibroblasts. It produces
dentin and other tissues of the tooth.
• Blood vessels and nerves enter and exit a tooth via the
root canals, supplying blood and innervation.
• Gingivae (gums) cover the maxilla and mandible.
Dentition
• Dentition is the kind, the number, and the
arrangement of teeth in an individual.
• Deciduous or primary dentition (baby teeth) is
present in infants and children between 6 months,
when it starts appearing, to 6 years of age, when it
starts being replaced.
• Permanent or secondary dentition starts to appear in
6 years old children and is completed between age
17 and 21.
• In the adult there are 4 types of teeth: incisors,
canines, premolars and molars.
Deciduous and Permanent Dentition
Mastication
• Mastication or chewing is the mechanical breakdown
of food in the mouth. Teeth tear, crush, and grind the
food into a soft paste called bolus.
• The tongue moves food around the oral cavity, mixing
crushed food with saliva: this allows chemical
digestion to begin, softening of the food, and
neutralization of acids in food.
• At the end of mastication, the bolus is swallowed.
Pharynx (The Throat)
• The pharynx is a funnel shaped tube that links the nasal
cavity, the oral cavity, and the larynx.
• It is subdivided into the nasopharynx, the oropharynx,
and the laryngopharynx (hypopharynx).
• The nasopharynx is not involved in the digestive system,
and the uvula is designed to prevent food from entering
the nasopharynx during swallowing.
• The oropharynx and laryngopharynx are involved in
swallowing.
Esophagus
• The esophagus is a long tube that connects the pharynx to the
stomach.
• Layers lining the esophagus
– Mucosa: made of Stratified Squamous Non-keratinized
epithelium.
– Submucosa: contains mucus secreting glands that lubricate
the lining of the esophagus.
– Muscularis: made of longitudinal and circular smooth
muscle layers that allow propulsion of food.
– Adventitia: Outermost connective tissue layer.
• The pharyngeal (upper) opening of the esophagus is guarded
by the upper esophageal sphincter, while the gastric (lower)
opening is guarded by the lower esophageal sphincter.
Esophagus
Deglutition
• Deglutition or swallowing is the passage of food from the mouth to
the esophagus through the pharynx.
• Is divided into three phases
• Buccal phase
• Pharyngeal phase
1.
2.
• Esophageal phase
During the buccal phase of deglutition the tongue pushes the soft
and humid bolus towards the back of the oral cavity, into the
oropharynx.
The pharyngeal stage starts when the presence of food in the
oropharynx triggers the deglutition reflex: the larynx is lifted, the
epiglottis bends and covers the air passages; the uvula blocks access
to the nasopharynx, and pharyngeal muscles push the bolus
towards the esophagus.
Deglutition (continued)
3. The esophageal stage starts when the lifted pharynx causes the
upper esophageal sphincter to relax.
– The bolus enters the esophagus causing the esophageal muscle layers to
contract rhythmically: this is peristalsis.
– Deglutition ends when the upper esophageal sphincter contracts, closing
the esophagus, and the larynx and epiglottis move back to their initial
positions.
– One can breath again while food moves towards the stomach.
– As the peristaltic wave reaches the lower (cardiac) esophageal sphincter, it
relaxes and opens the stomach.
Phases of Deglutition
Deglutition Problems
• Problems may occur during deglutition
‒ When one swallows too fast, the epiglottis may fail to
cover the glottis in time to prevent food from entering the
air passages. In this case breathing is impaired. Coughing
may remove the obstruction, but more serious measures
may be attempted to unblock the air passages (backslaps,
abdominal thrusts, chest thrusts).
‒ When one swallows a large, moderately masticated
amount of food, propulsion is slow and painful.
‒ When the lower esophageal sphincter fails to close
properly, stomach acids may enter the lower esophagus
causing a burning sensation best known as a heartburn.
The Stomach
• Major Functions of the Stomach
1.
Storage of ingested food
2.
Mechanical breakdown of ingested food
3.
Disruption of chemical bonds in food material by acid and enzymes
4.
Production of intrinsic factor, a glycoprotein required for absorption
of vitamin B12 in small intestine
Stomach
• The stomach is shaped like an expanded J
• Short lesser curvature forms medial surface
• Long greater curvature forms lateral surface
• The entrance to the stomach is guarded by the cardiac
sphincter, and the exit is guarded by the pyloric sphincter.
Structure of the Stomach
Regions of the Stomach
•Cardia
•Fundus
•Body
•Pylorus
Anatomy of the Stomach
• The stomach is a very muscular organ covered by a tough
serosa (connective tissue layer) attached to the visceral
peritoneum.
• Unlike other organs of the GI tract the muscularis of the
stomach is made of three thick smooth muscle layers:
longitudinal layer (outermost), circular layer (middle), and
oblique layer (innermost).
– These 3 muscle layers allow the stomach to contract forcefully to mix
gastric juice with food, breakdown the bolus mechanically and liquefy
it. They also allow the stomach to eject this liquefied food (chyme)
into the duodenum of the small intestine.
• Submucosa
Gastric Mucosa
• The rough gastric mucosa (stomach lining) is made of
simple columnar epithelial cells.
• Ridges of the gastric mucosa called rugae give it a
rough appearance. They increase the mechanical
properties of the stomach and allow it to contract and
return to its original shape.
• Infoldings of the mucosa are called gastric pits shallow depressions that open onto the gastric surface.
Gastric Layers
Gastric Glands
• There are 4 main types of gastric gland cells in gastric
pits:
– Chief cells (exocrine) secrete the inactive enzymes pepsinogen
and gastric lipase (zymogens).
– Parietal cells (exocrine) secrete hydrochloric acid (HCl), and a
glycoprotein called intrinsic factor
– Mucous neck cells (exocrine) secrete mucus - protects the
mucosa from the very acidic gastric juice
– G cells (endocrine) secrete gastrin.
• Combined secretion of chief cells (Pepsin) + Parietal Cells
(HCl) + Mucous cells (mucous) is called gastric juice.
Gastric Gland Cells
Digestion in the Stomach
• In the stomach there are two types of digestion:
– Mechanical digestion occurs when peristaltic contractions
mix food and gastric juice: the muscularis contracts
forcefully to mix the contents of the stomach. This action
liquefies the food into acid chyme.
– Chemical digestion occurs when pepsin breaks down
proteins into peptides, and lipases (lingual and gastric)
break down certain fats.
– Meanwhile intrinsic factor binds vitamin B12 which will be
absorbed later in the ileum.
• The products of this digestion are not absorbed
through the gastric mucosa. However some drugs
such as aspirin and alcohol can pass through the
lining of the stomach.
Phases of Gastric Secretion
• Gastric secretion is divided into 3 phases: cephalic, gastric,
and intestinal.
– Cephalic phase:
taste, smell, thought, or presence of food in the mouth trigger gastric
juice secretion by the gastric glands
– Gastric phase:
Presence of food in the stomach stretches the gastric mucosa and
cause continued secretion of gastric juice in the stomach.
– Intestinal phase:
Gastric emptying sends chyme into the duodenum. A very acidic
chyme (pH below 2), or presence of digested fats in chyme trigger a
series of events that result in decreased secretion of gastric juice.
• All these events involve the medulla oblongata, vagus nerves,
and hormones.
Phases of Gastric Secretion
Pancreas
• The pancreas is a leaf-shaped organ with three
main parts: the head, the body, and the tail.
• The head of the pancreas is attached to the
lesser curvature of the C-shaped duodenum via
the pancreatic duct (duct of Wirsung) and the
accessory pancreatic duct (duct of Santorini).
• It has both endocrine and exocrine functions: its
endocrine cells secrete hormones (insulin,
glucagon, somatostatin), and its exocrine cells
secrete pancreatic juice which contains digestive
enzymes and bicarbonate ions.
Pancreas (continued)
Pancreatic Islet (Islet of Langerhans)
• Pancreatic acini:
secrete pancreatic
juice
• Pancreatic islet
cells: secrete
hormones
Pancreatic Juice
• During digestion, the pancreas secrets pancreatic
juice which is a mixture of digestive enzymes and
bicarbonate ions.
• The digestive enzymes are secreted by basophilic
acinar cells.
– Pancreatic amylase for starch digestion.
– Trypsinogen, chymotrypsinogen, procarboxypeptidase, and
proelastase for protein digestion.
– Pancreatic lipase for lipid digestion.
– Deoxyribonuclease and ribonuclease for nucleic acid
digestion.
• Bicarbonate ions are produced by centroacinar cells.
Pancreatic Juice (continued)
• Pancreatic enzymes are secreted as zymogens
(inactive form of the enzyme).
• Most of these zymogen are activated by trypsin.
• Trypsin itself is activated by enterokinase which is
secreted by the absorptive cells of the small
intestine.
• Pancreatic juice is slightly alkaline, due to the
presence of bicarbonate ions. It neutralizes acid
chyme in the duodenum.
Secretion of Pancreatic Juice
• Pancreatic juice is collected by the pancreatic duct
through a network of tubes called the pancreatic
ductal system.
• The duct of Wirsung joins the common bile duct to
form the hepatopancreatic ampulla. There, bile and
pancreatic juice mix. They are released into the
duodenum through the major duodenal papilla.
• Pancreatic juice can be released alone into the
duodenum via the accessory pancreatic duct.
• Secretion and release of pancreatic juice are
regulated by 2 duodenal hormones: secretin and CCK
(cholecystokinin).
Hormonal
Regulation of the
Pancreas
Hormonal Regulation of the Pancreas
• Presence of fatty acids and amino acids in the
duodenum triggers the release of secretin and CCK by
the duodenal glands (enteroendocrine glands).
• Secretin and CCK have different effects on the
pancreas:
– Secretin stimulates secretion of bicarbonate ions.
– CCK stimulates secretion of digestive enzymes.
• Duodenal glands release also Gastric Inhibitory
Polypeptide (GIP). It inhibits stomach acid secretion,
and is now believed to also induce the secretion of
insulin in presence of glucose.
Liver and Gallbladder
Liver and Gallbladder
• The liver is the largest organ in the abdominopelvic
cavity.
• It is located in the upper abdominal cavity, right
underneath the diaphragm.
• The gallbladder is attached to the posterior surface of
the right hepatic lobe.
• Right and left lobes of the liver are unequally
separated by the falciform ligament: the right lobe is
significantly larger than the left lobe.
• The liver is an important, multitasking organ in the
body. However its main contribution to digestion is
the secretion of bile.
Liver Lobules
Liver Lobules (cont’d)
• Liver cells are called hepatocytes.
• They are arranged into regular structures called liver
lobules: the hepatocytes are lined up along sinusoids that
converge towards a central vein.
• Other cells called Kupffer cells are also associated with
liver lobules. They are macrophages specialized in red
blood cell recycling.
The Liver
• The Functions of the Liver
1.
Metabolic regulation: The liver regulates:
–
Composition of circulating blood
–
Nutrient metabolism and storage
–
Waste product removal
–
Drug inactivation
2. Hematological regulation: largest blood reservoir of the body.
–
Phagocytosis and antigen presentation
–
Synthesis of plasma proteins
–
Removal of circulating hormones
–
Removal of antibodies
–
Removal or storage of toxins
3. Bile production
Production of Bile
• Hepatocytes produce bile which is collected by bile
canaliculi. These small vessels converge to form bile
ducts.
• The main bile ducts of the liver are the right and the
left hepatic duct.
• These two ducts join to form the common hepatic
duct.
• The common hepatic duct joins the cystic duct from
the gallbladder to form the common bile duct.
• This duct delivers bile to the hepatopancreatic
ampulla where it mixes with pancreatic juice.
Bile
• Bile is a yellowish-green fluid produced by the liver
and stored in the gallbladder.
• It contains two main types of substances:
– Bile salts which are involved in digestion, Bile salts emulsify
fats, i.e. they break up fat globules into fat droplets that
lipases can break down.
– Bile pigments and cholesterol which are excretory products
of red blood cell recycling. Bile pigments contribute to the
color of feces.
• Gallstones (choleliths) are formed when cholesterol
concretizes. They can obstruct the cystic duct, causing
pain. They can be removed with drugs, sound waves,
and surgery.
Duodenal Hormones and
Bile Secretion
 Duodenal hormones regulate bile secretion.
 When chyme with fatty acids and amino acids enters the
duodenum, the duodenal glands secrete CCK and
secretin:
– secretin stimulates the production of bile by hepatocytes.
– CCK stimulates the gallbladder to contract and release bile.
• Gastric Inhibitory Peptide (GIP)
– Is secreted when fats and carbohydrates enter small
intestine
Gallbladder
• The gallbladder (or cholecyst) is a muscular, pear shaped
pouch that stores and releases bile.
• The gallbladder has three smooth muscle layers that
contract to eject bile into the cystic duct.
• The cystic duct leads to the common bile duct.
The Small Intestine
• The Small Intestine
– Plays key role in digestion and absorption of nutrients
– 90% of nutrient absorption occurs in the small intestine
– Divided into three sections: Duodenum, Jejunum, and Ileum
The Small Intestine
• The Duodenum
– The segment of small intestine closest to stomach
– 25 cm (10 in.) long
– “Mixing bowl” that receives chyme from stomach and digestive
secretions from pancreas and liver
– Functions of the duodenum:
• To receive chyme from stomach
• To neutralize acids before they can damage the absorptive
surfaces of the small intestine
The Small Intestine
• The Jejunum
– Is the middle segment of small intestine
– 2.5 meters (8.2 ft) long
– Is the location of most:
• Chemical digestion
• Nutrient absorption
– Has few plicae circulares
• The Ileum
– The final segment of small intestine
– 3.5 meters (11.48 ft) long
– Ends at the ileocecal valve
• A sphincter that controls flow of material from the ileum into the cecum
of the large intestine
Small Intestine
Layers of the Small Intestine
Intestinal Mucosa
– Circular folds or plicae circularis are folds of the mucosa and submucosa
that increase the surface of absorption
– Villi are finger-like projections of the mucosa that are lined with mucous
secreting goblet cells, and simple columnar absorptive cells called
enterocytes. They also increase the absorption surface area.
– The mucosal layer of each villus contains blood capillaries and a central
lymphatic capillary called a lacteal.
– Intestinal juice is secreted by epithelial cells of the crypts of Lieberkhun
(intestinal glands) located at the base of each villus. It contains
important enzymes called brush border enzymes that finish digestion.
– Microvilli: extensions of the plasma membrane of the luminal surface of
enterocytes. These bristle-like structures increase the surface of
absorption further, and release brush border enzymes for digestion.
Brush Border Enzymes
• Brush border enzymes include:
– Aminopeptidase and dipeptidase - split dipeptides into
amino acids.
– Sucrase, lactase, maltase - break disaccharides into
monosaccharides.
– Enterokinase - activates trypsinogen to produce
trypsin. Trypsin then activates the precursors of
chymotrypsin and carboxypeptidase.
– carbohydrases: dextrinase and glucoamylase.
– Nucleosidase and Phosphatase: remove phosphate
group and separate pentose sugar from nitogenous
base.
Absorption of Nutrients in the Small Intestine.
• Monosaccharides, amino acids, dipeptides and tripeptides, and
short chain fatty acids are transported through mucosa cells to
the blood
• Long fatty acids, cholesterol, and monoglycerides are
transported through the mucosa cells to the lymph.
• Vitamins and minerals are absorbed along with other nutrients:
lipid soluble vitamins are absorbed with lipids, while water
soluble vitamins are absorbed by diffusion.
• Absorption mechanisms vary depending on the nature of the
transported material.
– Transport mechanisms include: simple diffusion, facilitated diffusion,
primary and secondary active transport, exocytosis, and endocytosis.
Absorption Mechanisms
Facts About Absorption
• It lasts approximately 4 to 6 hours.
• The majority of nutrients are absorbed through the
intestinal mucosa.
• Most of the water that enters the GI tract, including
drinking water and water secreted by accessory
glands is absorbed in the small intestine.
• Peristalsis and segmentation propel food through the
small intestine.
• These movements are coordinated by the enteric
nervous system, and hormones that respond to the
presence of chyme in the small intestine.
Large Intestine
• The large intestine or colon is shorter in length than the small
intestine, but larger in diameter.
• The colon is made of haustrae, pouches which increase the
surface area of the colon for absorption of water and
electrolytes.
• The first part of the colon is a blind pouch called the cecum
which is connected to the ileum of the small intestine.
• The ileocecal sphincter (valve) separates the cecum from the
ileum.
• The vermiform (wormlike) appendix is attached to the cecum:
it may have a function in colic defense, but appendectomy
(removal of the appendix) does not appear to have serious
effect on the colon.
Regions of the Colon
•
•
•
•
•
•
•
•
cecum
ascending colon
transverse colon
descending colon
sigmoid colon
rectum.
anal canal
anus
Histology of the Colon
Functions of the Colon
•
•
•
Absorption of the remaining water in chyme
Production and storage of feces.
Long process (12 hours):
–
–
–
–
–
Peristaltic waves from the ileum relax the ileocecal valve.
Liquid chyme enters the colon.
The ileocecal valve prevents backflow of chyme
Colic muscle movements push chyme along:
3 types of colic movements:
•
•
•
minor peristaltic waves: slow and sluggish movement
haustral churning: segmentation contractions which serve to mix the
contents to enhance absorption.
mass peristalsis: large movements which occur at intervals, usually
associated with meals. Triggered by presence of food in the stomach.
Figure 24-28 Digestive Secretion and Absorption of Water
Digestive
Secretions
Dietary Input
Food and drink
2000 mL
Saliva
1500 mL
Gastric secretions
1500 mL
5000 mL
Water
Reabsorption
Liver (bile) 1000 mL
Pancreas (pancreatic
juice) 1000 mL
9000 mL
Small intestine
reabsorbs
7800 mL
Intestinal
secretions
2000 mL
1200 mL
Colon reabsorbs
1250 mL
150 mL lost
in feces
1400
mL
Colonic mucous
secretions
200 mL
Feces formation and Defecation Reflex
• Chyme turns from a liquid into a slush and then into firm feces.
• Vitamins such as vitamin K and certain B vitamins are produced
with the help of bacteria, and absorbed along with water and
electrolytes.
• Feces are stored in the rectum.
• Defecation occurs when:
– Pressure is exerted on the rectum, and on the internal anal
sphincter (made of smooth muscle = involuntary).
– The internal anal sphincter relaxes causing the need to
defecate.
– Feces are released when the external anal sphincter relaxes
(skeletal muscle, voluntary) , and the rectal muscles
contract.
Feces formation and
Defecation Reflex