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Functional Human Physiologyfor the Exercise and Sport Sciences
The Gastrointestinal System
Jennifer L. Doherty, MS, ATC
Department of Health, Physical Education, and Recreation
Florida International University
Overview of Gastrointestinal System Function
Digestion Absorption Secretion Motility
Organs of the Gastrointestinal System
The alimentary canal Continuous hollow tube extending from the
mouth to the anus Called the gastrointestinal (GI) tract as is passes
inferior to diaphragm Functions:
Digestion Absorption of digested fragments into blood
Organs of the Gastrointestinal SystemAccessory organs Assist in the
chemical process of digestion by secreting saliva, enzymes, and bile Salivary glands Pancreas Liver Gallbladder
Assist in the mechanical process of digestion Teeth Tongue
Digestion
The process mechanically breaking down food into particles small enough to be absorbed through cell membranes
Two methods of food breakdown Chemical Mechanical
Digestion: 5 Integrated Steps
(1) Ingestion Foodstuff enters the GI tract via the mouth
(2) Propulsion The process that moves foodstuff through the GI
tract via coordinated reflexive contraction activity (3) Digestion
The process of breaking down large food particles into smaller particles via chemical and mechanical action
Digestion: 5 Integrated Steps (4) Absorption
The movement of digested end products through the intestinal wall and into the blood or lymph
End products include small organic molecules, electrolytes, & H2O
(5) Defecation The process of discharging undigested and
unabsorbed foodstuff
The GI Tract
The Gastrointestinal Wall 4 layers (1) Muscosa
Lamina propria Muscularis mucosa
(2) Submucosa (3) Muscularis Externa (4) Serosa
Mucosa
Inner most mucous membrane Lines lumen Composed of simple columnar
epithelium Contains many mucus secreting goblet
cells
Mucosa Lamina propria
Underlying loose CT Contains blood vessels, sensory nerve
endings, lymph vessels, and scattered lymph tissue
Muscularis mucosa Layer of smooth muscle Produces local movements that change
the shape of the lumen
Mucosa
Functions: Protect underlying tissue Absorb digested material Secrete mucous or digestive juices Increase surface area
Folds in the mucosa Villi present in the small intestine
Submucosa
Composed of loose CT Contains blood vessels and lymphatics
Function to circulate absorbed nutrients Contains nerves from the ANS
Form the submucosal plexus A component of the intrinsic nervous
system of GI tract
Muscularis Externa
Composed of two layers of smooth muscle Inner layer - circular fiber arrangement Outer layer - longitudinal fiber arrangement
Muscularis Externa
Inner layer - circular fiber arrangement When fibers contract = ↓ lumen size Forms sphincter muscles
Prevent backflow of materials
Outer layer - longitudinal fiber arrangement When fibers contract = mix and propel food
along the alimentary canal
Muscularis Externa
Myenteric plexus Extensive nerve network between the
smooth muscle layers Regulate motility
Movement/contraction of the GI tract walls Regulate glandular secretions
Secretions into the lumen of the GI tract
Serosa
Outermost layer of the GI wall Inner layer
Consists of fibrous CT Provides structural support
Outer layer - Mesothelium Consists of epitelium
Secretes a water lubricating fluid allowing organs to slide past one another
Peritoneal Cavity
The space between the visceral peritoneum and parietal peritoneum
Both visceral and parietal peritoneum secrete serous fluid into the peritoneal cavity Lubricates and protects abdominal tissues
as they slide past one another
GI Motility Patterns
Contractions of the muscularis externa Two methods
Peristalsis Segmentation
GI Motility Patterns
Peristalsis Propelling motion produced by alternate
waves of contraction and relaxation of muscularis externa layer
Occurs due to contraction of one part of wall with simultaneous relaxation of the wall ahead
Propels food along tube
GI Motility Patterns
Segmentation Occurs due to rhythmic, local
contractions of the smooth muscle in the muscularis externa layer
Mechanically grinds foodstuff in the stomach and intestines mixing it with digestive juices
Gastrointestinal Regulation
Intrinsic Control Submucosal Plexus Myenteric Plexus
Extrinsic Control Parasympathetic nerve fibers Sympathetic nerve fibers
Gastrointestinal Regulation: Intrinsic Control Provided through the
Submucosal Plexus Myenteric Plexus
Local stimulus = distension of submucosa or muscularis externa walls
Local response = activation of stretch receptors ↑ glandular secretions ↑ smooth muscle contractions in the immediate area
Gastrointestinal Regulation: Extrinsic Control
Includes Parasympathetic and Sympathetic input from the ANS
Fibers within the muscularis externa layer specifically assist in controlling the rate and strength of contractions
Gastrointestinal Regulation: Extrinsic ControlParasympathetic
activity Impulses carried by
the vagus nerve ↑ motility ↑ glandular
secretions
Gastrointestinal Regulation: Extrinsic Control
Sympathetic activity Opposes parasympathetic activity
↓ motility ↓ glandular secretions
Causes sphincters to contract thus slowing the movement of foodstuff through the GI tract
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation
Digestive Function: Ingestion
The digestive process begins in the mouth
Includes… Mechanical fragmentation of foodstuff Foodstuff mixes with saliva
Digestive Function: Ingestion
Organs and associated structures
Tongue Mixes foodstuff with saliva during chewing Initiates swallowing Contains taste buds
Sensitive to chemical differences among food molecules
Differentiate sweet, sour, salty, or bitter tastes
Digestive Function: Ingestion
Organs and associated structures
Teeth Tear and grind food
Mastication Mechanical breakdown of foodstuff into
smaller fragments
Digestive Function: Ingestion
Organs and associated structures
Lips and Cheeks Keep food in mouth Involved in speech
Digestive Function: Ingestion
Organs and associated structures
Palate Forms roof of mouth 2 distinct parts
Hard Palate Soft Palate
Digestive Function: Ingestion
Organs and associated structures Hard palate
Anterior region composed of bone Forms a hard surface against which foodstuff is pushed
during chewing
Soft palate Posterior region composed of skeletal muscle Rises reflexively to close off nasopharynx during
swallowing
Digestive Function: Ingestion
Organs and associated structuresSalivary glands - 3 pairs 1). Parotid glands
Largest salivary glands Anterior to the ears
2). Submandibular glands Inferior to the jaw
3). Sublingual glands Inferior to the tongue
Digestive Function: Ingestion
Organs and associated structuresSaliva Produce 1 - 1.5 L/day Basic composition
Water (98 - 99%) Salivary Amylase: a digestive enzyme Mucins: mucous that lubricates the mouth and food Ions, buffers, metabolites, antibodies, etc
Dissolves foodstuff
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation
Digestive Function: Propulsion
Swallowing Reflex response which moves foodstuff
through the pharynx and down the esophagus
The swallowing reflex is triggered when material moves into the pharynx
Digestive Function: Propulsion
Components of the swallowing reflex: The soft palate rises The epiglottis covers the opening of the larynx
Prevents foodstuff from entering the air passageways Peristaltic contractions along the pharyngeal and
esophageal walls propel foodstuff through the GI tract
Relaxation of lower esophageal sphincters allows foodstuff to enter the stomach
Digestive Function: Propulsion
Esophagus A muscular tube which collapses when it is
not in use Contains 2 sphincters
Upper esophageal sphincter Lower esophageal sphincter
The sphincters contract to prevent the backflow of stomach acids into the esophagus
Digestive Function: Propulsion
Both the pharynx and the esophagus are only passageways for foodstuff Peristalsis
The pharynx and the esophagus are not directly involved in digestive activities
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation
Digestive Function: Digestion
Saliva Dissolves foodstuff via chemical processes
Contains salivary amylase Chemical breakdown of starches
Allows food to be tasted Moistens foodstuff and converts it into bolus
Mass of moistened food that can be easily swallowed
Digestive Function: Digestion
Control of Salivation Regulated by both the PNS and SNS
Receptors in mouth send signals to brain Controlled primarily by the PNS
Causes ↑ salivation
Rate of secretion and composition of saliva changes in response to… sight, sounds, smells, pressure of food in mouth
Digestive Function: Digestion
Stomach Stores foodstuff
Gradual delivery to the small intestine Mixes foodstuff with gastric juices Initiated the digestion of proteins
Digestive Function: Digestion
Stomach Protein digestion occurs via pepsin
Enzyme that breaks down proteins into smaller polypeptide and amino acid fragments
Protein digestion is the only type of chemical digestion occurring in stomach
Digestive Function: Digestion
Stomach Foodstuff mixed with gastric juices
forming a creamy paste called Chyme Chyme is passed out of the stomach into the
small intestine Limited absorption occurs in the stomach
Absorb small amounts of H2O, glucose, salts, alcohol, and lipid-soluble drugs
The Stomach
Distends to accommodate foodstuff Empty stomach
Walls collapse forming folds (rugae) in the inner lining
Full stomach Rugae smooth out
Able to hold 1 - 1.5 L of foodstuff
The Stomach
Regions of the stomach Body - large, main portion of stomach Cardiac region - surrounds opening
where esophagus enters the stomach Fundus - domed-shaped region that
projects above the cardiac region Serves as a storage area
The Stomach
Regions of the stomach Pylorus - exit Antrum (pyloric antrum) - funnel-shaped
region near the pylorus Pyloric canal - narrow region
terminating at the pylorus Pyloric sphincter - controls stomach
emptying
The Stomach
Modifications of the Stomach Wall The muscularis layer is modified according to the
functions of the stomach Greater churning/mixing ability Mechanical breakdown of foodstuff into smaller
pieces Composed of 3 layers
Circular Longitudinal Oblique layer - additional innermost layer
The Stomach
Modifications of the Stomach Wall The mucosa layer contains gastric pits
Millions of pockets in the epithelium Gastric pits lead into gastric glands
Gastric glands are located deep in the lamina propria
Gastric glands produce gastric juices Stomach secretions
Gastric Glands
4 types of gland cells: 1) Chief cells
Zymogenic cells 2) Parietal cells
Oxyntic cells 3) Mucous neck cells 4) Enteroendocrine cell
Gland Cells: Chief cells (Zymogenic cells)
Secrete pepsinogen Inactive form of pepsin
Protein-digesting enzyme Activated by stomach acids
Also secrete gastric lipase Fat-digesting enzyme Not very active in stomach due to low pH Main action is on butter fat
Gland Cells: Parietal cells (Oxyntic cells)
Secretes HCl HCl is a strong acid that ↑ the acidity of the
stomach (pH = 1.5 - 3.5) Activates pepsin
Also secretes Intrinsic factor Required for vitamin B12 absorption in the
small intestine Only stomach function essential for life
Gland Cells: Mucous Neck Cells
Secretes alkaline mucous Protects stomach lining from…
Damaging acidity in the stomach Damaging action of protein-digesting
enzymes
Gland Cells: Enteroendocrine Cells
Secrete a variety of compounds directly into the lamina propria
These compounds then diffuse into blood capillaries
Function as local hormones to regulate the functions of various digestive organs i.e.) Gastrin – regulates the stomach
Gland Cells
Gastric juices do not digest the walls of the stomach due to… Mucous barrier Epithelial cells
The mucous barrier is alkaline Neutralizes acid on the stomach lining
Tight junctions between epithelial cells prevent acid from leaking to underlying tissue Rapid turnover of epithelial cells Replaced every 3 days
Regulation of Gastric Secretion
Neural control Parasympathetic input
Conducted via the vagus nerve
Increases secretion from all gastric gland cells ↑ Pepsin concentration in stomach
↑ HCl concentration in stomach ↑ Gastrin concentration in stomach
Regulation of Gastric Secretion
Neural control Sympathetic input
Overrides Parasympathetic input Inhibits gastric gland secretion
↓ Pepsin concentration in stomach
↓ HCl concentration in stomach ↓ Gastrin concentration in stomach
Occurs during times of… Physical activity Emotional stress
Regulation of Gastric Secretion
Hormonal control Provided primarily by Gastrin Increases secretion from all gastric gland cells
↑ Pepsin concentration in stomach
↑ HCl concentration in stomach ↑ Gastrin concentration in stomach
Three phases of gastric secretion Occur almost simultaneously
Phases of Gastric Secretion
1). Cephalic phase (Reflex phase) Triggered by sight, smell, thought, or taste of
food Sensory input is relayed to hypothalamus
Sensory input is integrated in hypothalamus Vagus nerve is stimulated
Vagus nerve sends impulses to gastric glands Increase secretions from all gastric gland cells
Prepares the stomach in advance for the arrival of foodstuff
Phases of Gastric Secretion
2). Gastric phase Begins when foodstuff reaches the stomach Stimulation of the vagus nerve occurs due to 2
stimuli a). Distension of stomach walls
Stimulation of stretch receptors b). Chemical stimuli
Provided by partially ingested foodstuff Increase secretions from all gastric gland cells
Phases of Gastric Secretion
3). Intestinal Phase Purpose = control the rate of gastric emptying
Provides the small intestine with enough time for digestion and absorption
Phase begins when chyme enters the duodenum First portion of the small intestine
Phases of Gastric Secretion
3). Intestinal Phase cont. Presence of foodstuff in the stomach stimulates
the release of intestinal gastrin Hormone similar to gastrin
Increase secretions from all gastric gland cells Helps speed digestion in the stomach before
stomach-emptying is complete Released only when stomach begins to empty
Inhibition of Gastric Secretions
Can be accomplished via… Emotional upset Sympathetic innervation
The rate of gastric secretion may also be inhibited by the… Stomach Small intestine
Inhibition of Gastric Secretions
Gastric secretion may be inhibited during the gastric phase
Gastric secretion decreases when the mucosa becomes too acidic (pH < 2) Slows the rate of further gastric secretion
Occurs as the stomach empties and buffering capacity of food material decreases
Inhibition of Gastric Secretions
Gastric secretion may be inhibited during the intestinal phase
Most regulatory controls during the intestinal phase are inhibitory
Duodenum responds to various stimuli through the enterogastric reflex
Inhibition of Gastric Secretions
Enterogastric reflex This is an inhibitory reflex in the small intestine
that slows gastric secretions Stimuli
Distension of the duodenum Presence of acidic (H+) and/or hypertonic chyme Presence of fats, partially digested proteins,
and/or irritants
Inhibition of Gastric Secretions
Inhibitory effects: enterogastric reflex PNS input to the stomach mucosa SNS input to the pyloric sphincter
Prevents additional foodstuff from entering the small intestine
gastric motility hormone release from the small intestine,
which inhibits gastric secretions
Digestive Function: Digestion
Mechanical Digestion and Propulsion Mechanical digestion is achieved through
contractile activity of the stomach Mechanically mix and breakdown foodstuff
Propulsion is achieved through peristalsis and segmentation Propel chyme from the stomach to small intestine
These mechanisms are triggered via distension of the stomach ~ 1 L of food enters
Digestive Function: Digestion
Mechanical Digestion and Propulsion cont. Peristaltic waves sweep across the
stomach toward the pylorus Foodstuff is pushed against the pyloric
sphincter, which is normally closed Large food materials are churned and
mixed in the stomach until the food particles are small enough to pass through the pyloric sphincter
Regulation of Gastric Emptying
The rate of gastric emptying depends on the type of food ingested
Fluids Pass through quickly 90 minutes
Solids Remain in the stomach until reduced to very small
particles Remain in the stomach until dissolved in gastric juices 3 - 4 hours
Regulation of Gastric Emptying
Gastric emptying of nutrients Carbohydrates are emptied first Followed by proteins Fats take the longest to leave the stomach
Regulation of Gastric Emptying
Dependent upon the duodenum Gastric emptying is regulated along with gastric
secretions Stimuli
Distention of the wall in the small intestine Presence of acid (H+), fat, or hypertonic solution in the small
intestine Responses
Trigger the enterogastric reflex ↓ gastric motility Slows rate of gastric emptying
Purpose Provides time for the small intestine to digest and absorb foodstuff
Small Intestine (SI)
Chyme leaves the stomach ~3-4 hours after ingestion
Carbohydrates and proteins are partially digested upon entering the SI Too large to be absorbed through SI wall
Fats are undigested upon entering the SI
Small Intestine (SI)
In the duodenum, bile from liver is added to chyme
Also in the duodenum, enzymes from pancreas are added to chyme
Enteroendocrine Cells in the duodenum also secrete enzymes
***All nutrient absorption occurs in small intestine***
Small Intestine (SI)
The SI extends from the pyloric sphincter of the stomach to the iliocecal valve in the large intestine
Subdivisions Duodenum Jejunum Ileum
Small Intestine (SI)
The SI contains ducts that carry bile and pancreatic juices to the duodenum
Hepatopancreatic Ampulla A single duct that empties into duodenum
Sphincter of Oddi Also called hepatopancreatic sphincter Duct that controls entry of bile and
pancreatic juices into duodenum
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation
Digestive Function: Absorption Anatomy of the SI
Function to increase the surface area Increased surface area = Increased absorption
Plicae Permanent transverse folds in mucosa & submucosa layers
Villi Finger-like projections in mucosa layer
Microvilli Finger-like projections on the lumenal surfaces of mucosa
cells
Digestive Function: Absorption
SI Secretions: Intestinal juice A watery secretion with neutral pH (7.0) Serves as a medium for digestion and
absorption of nutrients Glands normally secrete ~1-2 L/day Stimuli for secretion
Distension of SI walls due to the presence of chyme
Irritation of SI mucosa caused by hypertonic or acidic chyme
Digestive Function: Absorption
SI Secretions: Digestive enzymes Secreted by epithelial cells in the SI
Disaccharidases Carbohydrate digestion: breakdown disaccharides into
monosaccharides
Peptidases Protein digestion: breakdown polypeptides into amino
acids
Lipases Fat digestion: breakdown fats into glycerol and fatty acids
Digestive Function: Absorption
Gastric Secreting Gland Cells Located in the mucosal epithelium of the SI
Goblet cells Secrete alkaline mucous
Enteroendocrine cells Secrete local hormones to regulate activity
of the SI
Intestinal Glands Location
At base of villi in intestinal crypts Secrete intestinal juices
A watery mixture with neutral pH (7.0) Glands normally secrete 1 - 2 L/day
Function Serves as medium for digestion and absorption of
nutrients Stimuli
Distension of SI walls due to the presence of chyme Irritation of SI mucosa caused by hypertonic or acidic
chyme
Intestinal Glands Specialized structures located in submucosa Peyer's patches
Collections of lymph tissue Preventing bacteria in undigested food from
entering the systemic circulation Brunner's glands
Only located in the initial part of duodenum Secrete alkaline mucous Protects duodenal walls by neutralizing the acidic
chyme entering the SI from the stomach
Regulation of Intestinal Secretions
Stimuli to ↑ intestinal secretions Presence of chyme in duodenum Distension of SI walls
Stimuli elicits a response from the Parasympathetic nervous system Impulses carried via vagus nerve Results in the secretion of…
Alkaline mucous Digestive enzymes
The Pancreas
Both endocrine and exocrine function Endocrine Function
Alpha cells release glucagon Beta cells release insulin
Exocrine Function Acinar cells
Secrete pancreatic juice into the pancreatic duct Watery alkaline fluid (pH = 8) Contains HCO3- and various digestive enzymes
The Pancreas
Pancreatic Enzymes Trypsin, Chymotrypsin, and Carboxypeptidase
All are proteolytic or protease enzymes Protein-digesting
Require alkaline environment Provided by HCO3-
Pancreatic Enzymes
Secreted in inactive form Activated in the duodenum
Prevents self-digestion of pancreas Pancreatic amylase
Breaks down almost all carbohydrates Pancreatic lipase
Breaks down fats Pancreatic nuclease
Breaks down nucleic acids
Regulation of Pancreatic Secretion
Begins during the cephalic and gastric phases of gastric secretion via PNS activation
Major stimulus for secretion is… Presence of chyme in the duodenum Occurs during the intestinal phase
Regulation of Pancreatic Secretion
Chyme in duodenum stimulates the secretion of… Secretin
Targets duct cells to secrete watery alkaline fluid Cholecystokinin (CCK)
Targets acinar cells to secrete pancreatic juice containing digestive enzymes
Pancreatic secretions trigger the enterogastric reflex
The enterogastric reflex slows gastric emptying
The Liver
Liver is the largest visceral organ Blood circulation in the liver
O2 blood delivered via hepatic artery Nutrient-rich blood delivered via hepatic portal
vein from the intestines
Blood from these 2 sources mixes as it flows through the liver sinusoids Blood leaves the liver via central veins -->
hepatic veins --> heart
The LiverFunctions: Contributes to blood maintenance
Phagocytosis Pathogens and old RBCs
Synthesize plasma proteins Detoxify drugs and poisons
Metabolic regulation Aids in digestion
Synthesis and secretion of bile
Bile Yellow-green alkaline solution Composition
Water Bile salts
Synthesized in the liver from cholesterol Function: Emulsification of fat globules
Mechanical break down of fat globules into small droplets
Small droplets have larger surface area thus allowing lipases to act more effectively
Bile Pigments
Biliverdin Metabolized by bacteria in
SI Exit body in feces (gives
brown color)
Bilirubin Metabolized by bacteria in
SI Exit body in feces (gives
brown color)
Bile salts Aid in digestion
Phospholipids Aid in digestion
Lecithin Cholesterol Neutral fats Electrolytes
Bile Pathway
Bile exits the liver via canaliculi Tiny bile canals
Bile ducts from the liver join together to form the common hepatic duct Extends downward toward the duodenum
Bile Pathway
Cystic duct from the gall bladder joins common hepatic duct as well Forms the common bile duct
Common bile duct Joins with the pancreatic duct via the
hepatopancreatic ampulla Pancreatic Duct
Empties bile and pancreatic juices into the initial portion of the duodenum
Sphincter of Oddi guards this entry into duodenum
Gallbladder
Small green sac located on the inferior surface of the liver
Concentrates and stores bile Does not synthesize bile
Sphincter of Oddi When closed, bile cannot enter duodenum Bile is then stored in the gallbladder
Regulation of Bile Release
Bile release from the gall bladder into the duodenum…
The presence of fat and protein in the duodenum stimulates the secretion of Cholecystokinin (CCK) Targets acinar cells to secrete pancreatic juice
containing digestive enzymes CCK stimulates the gallbladder to contract and
sphincter of Oddi to relax Results in bile release into the duodenum
Digestive Function: Digestion and Absorption
The SI is the major site of digestion and absorption
Requires that chyme be mixed with… Bile, and Digestive enzymes
Necessary to expose foodstuff to the SI mucosa for absorption
Digestive Function: Digestion and Absorption
Segmentation Begins as soon as chyme enters the SI Do not propel chyme onward
Peristalsis Propels chyme onward Requires time to move chyme
Allows more time for digestion and absorption
Digestion and Absorption of Major Nutrients
Carbohydrates (CHO) Amylase
Salivary amylase Began CHO digestion in mouth
Pancreatic amylase Continues digestion of CHO in the SI
Digestion and Absorption of Major Nutrients
Carbohydrates (CHO) Glucose is the major end-product of CHO
digestion Cellulose and other indigestible CHO provides
dietary fiber Cannot be digested by humans Passed to the large intestine
Simple sugars are absorbed across intestinal mucosa of the SI
Digestion and Absorption of Major Nutrients
Proteins (PRO) In Stomach
Dietary PRO is initially broken into smaller polypeptide fragments by pepsin
In Small Intestine Trypsin and chymotrypsin are enzymes from
pancreas that further break down polypeptides into smaller fragments
Digestion and Absorption of Major Nutrients
Proteins (PRO) Complete PRO digestion occurs in the SI via
the following enzymes… Carboxypeptidase (from pancreas) Peptidases (from SI)
Final breakdown products are amino acids Absorbed in the SI and travel to the liver via the
hepatic portal vein
Digestion and Absorption of Major Nutrients Fats (Lipids) The SI is basically the only site of fat digestion Triglycerides (TG) are the most abundant
dietary fat As TGs enter the SI they are emulsified by bile
salts Emulsification droplets
Pancreatic lipase Enzyme that breaks down TGs into…
2 free fatty acids (FFA) 1 monoglyceride (glycerol)
Digestion and Absorption of Major Nutrients
Fats (Lipids) FFAs and Glycerides interact with bile
salts to form micelles Fatty elements clustered around bile salts for
the purpose of increasing solubility Micelles enhance fat absorption in the SI Once absorbed,
2 FFAs and 1 Glycerol form a TG
Digestion and Absorption of Major Nutrients
Fats (Lipids) TGs then combine with other lipid material to
form chylomicrons Water-soluble lipoprotein droplets that are
processed in the cell Chylomicrons are secreted into interstitial fluid
Carried by lacteals through the lymphatic system Ultimately enter the systemic circulation
Digestion and Absorption of Major NutrientsFats (Lipids) Chylomicrons are present in the systemic
circulation Systemic capillaries contain lipoprotein lipase
Enzyme that breaks down TGs into… 2 FFA 1 Glycerol
FFAs and Glycerol may then diffuse out of the blood and into cells Used for energy, or Stored as fat
Digestion and Absorption of Major Nutrients
Vitamins Fat-soluble vitamins
(A, D, E, K) Bind to ingested lipids Absorbed with lipids via
lacteals
Water-soluble vitamins Mostly absorbed with
water Exception - vitamin B12
Must bind with intrinsic factor, which is produced by parietal cells located in the stomach
Absorbed via endocytosis
Digestion and Absorption of Major Nutrients
Electrolytes Na+ and Cl-
Absorbed via active transport K+
Absorbed via passive diffusion H2O
Absorbed via osmosis Moves freely across intestinal mucosa
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation
Digestive Function: Defecation
The Large Intestine 2 major regions
Colon Rectum
Digestive Function: Defecation
The Colon The largest portion of the large intestine Extends from the ileocecal valve to the
rectum Subdivisions:
Ascending colon, Transverse colon, Descending colon, and Sigmoid (S-shaped) colon
Digestive Function: Defecation
The Rectum Distal portion of GI tract Includes last 6" of large intestine Extends to anal canal
Opens to the exterior of body at the anus Sphincters
Internal sphincter External sphincter
The Large Intestine (LI)
Anatomical Modifications Mucosa contains many goblet cells
No villi or microvilli Alkaline mucosa
Holds feces together Protects the lining of the large intestine
Longitudinal muscle layer arranged in 3 bands Produces out-pocketings of the LI wall Called haustra
The Large Intestine (LI)
Motility Patterns (1) Haustral churning
Similar to segmentation in the SI Local contractile activity mixes chyme Places chyme close to the haustra Facilitates absorption of water
(2) Peristalsis Very sluggish Contributes little to movement of chyme
The Large Intestine (LI)
Motility Patterns cont. (3) Mass movements
Powerful peristaltic waves Pass over large areas (8" segments) of the
colon ~ 3 times/day Responsible for…
Moving foodstuff to the rectum Producing the urge to defecate
The Large Intestine (LI)
The LI has limited digestive and absorptive function…
Digestion: the lumen contains millions of bacteria Metabolize remaining nutrients Produce gases
Hydrogen, methane, CO2, hydrogen sulfide
Absorption: limited Some vitamins, electrolytes, & H2O
Digestive Function: Defecation
Feces are forced into the rectum via mass movements This causes distension of the LI wall
Triggers the defecation reflex
Digestive Function: Defecation
Regulation of the defecation reflex is provided by centers in the sacral cord
Stimulates… Contractions in rectum and distal LI Relaxation of internal anal sphincter
Input to the cerebral cortex allows decision regarding appropriate behavior Relaxation of both the external and internal
sphincter if appropriate
Digestion: 5 Integrated Steps
(1) Ingestion (2) Propulsion (3) Digestion (4) Absorption (5) Defecation