functional human physiology for the exercise and sport sciences the gastrointestinal system jennifer...

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

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

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

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

(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

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

Teeth Tear and grind food

Mastication Mechanical breakdown of foodstuff into

smaller fragments

Lips and Cheeks Keep food in mouth Involved in speech

Palate Forms roof of mouth 2 distinct parts

Hard Palate Soft Palate

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

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

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

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

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

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

Both the pharynx and the esophagus are only passageways for foodstuff Peristalsis

The pharynx and the esophagus are not directly involved in digestive activities

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

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

Stomach Stores foodstuff

Gradual delivery to the small intestine Mixes foodstuff with gastric juices Initiated the digestion of proteins

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

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

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

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

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

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

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

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

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

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

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

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

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

Gastric emptying of nutrients Carbohydrates are emptied first Followed by proteins Fats take the longest to leave the stomach

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

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***

The SI extends from the pyloric sphincter of the stomach to the iliocecal valve in the large intestine

Subdivisions Duodenum Jejunum Ileum

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

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

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

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

Lipases Fat digestion: breakdown fats into glycerol and fatty acids

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

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

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

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

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)

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

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

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

Electrolytes Na+ and Cl-

Absorbed via active transport K+

Absorbed via passive diffusion H2O

Absorbed via osmosis Moves freely across intestinal mucosa

Digestive Function: Defecation

The Large Intestine 2 major regions

Colon Rectum

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

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

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

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 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

Feces are forced into the rectum via mass movements This causes distension of the LI wall

Triggers the defecation reflex

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

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