The Digestive System

Chapter 22

The Digestive System The digestive system

– Takes in food– Breaks it down into nutrient molecules– Absorbs the nutrient molecules into the

bloodstream– Rids the body of indigestible remains

The Digestive System

The organs of the digestive system can be separated into two main groups; those of the alimentary canal and the accessory organs

The Digestive System

The alimentary canal or gastrointestinal (GI) tract is the continuous muscular digestive tube that winds through the body

The Digestive System The organs of the alimentary canal are

– Mouth, pharynx, esophagus, stomach, small intestine and large intestine

– Food in this canal is technically out of the body

The accessory digestive organs are– Teeth, tongue, gallbladder, salivary glands,

liver and pancreas– The accessory organs produce saliva, bile and

digestive enzymes that contribute to the breakdown of foodstuffs

Digestive Processes The digestive tract

can be viewed as a process by which food becomes less complex at each step of processing and nutrients become available to the body

Ingestion Ingestion is simply

the process of taking food into the digestive tract via the mouth

Propulsion Propulsion is the

process that moves food through the alimentary canal

It includes swallowing (voluntary process) and peristalsis (involuntary process)

Propulsion

Peristalsis involves alternate waves of contraction and relaxation of muscles in the organ walls

Its main effect is to squeeze food from one organ to the next

Some mixing occurs as well

Mechanical Digestion Mechanical

digestion physically prepares food for chemical digestion by enzymes

Mechanical Digestion Mechanical processes

include chewing, mixing of food with saliva by the tongue, churning of food by the stomach, and segmentation

Segmentation mixes food with digestive juices and increases the rate of absorption by moving food over the intestinal wall

Chemical Digestion Chemical digestion

is a series of catabolic steps in which complex food molecules are broken down to their chemical building blocks

Chemical Digestion Chemical digestion is accomplished by

enzymes secreted by various glands into the lumen of the alimentary canal

The enzymatic breakdown of foodstuffs begins in the mouth and is essentially complete in the small intestine

Absorption Absorption is the

passage of digested end products (plus vitamins, mineral and water) from the lumen of the GI tract into the blood or lymph capillaries located in the wall of the canal

Absorption For absorption to occur these substances

must first enter the mucosal cells by active or passive transport processes

The small intestine is the main absorption site

Defecation Defecation is the

elimination of indigestible substances from the body as feces

Basic Functional Concepts Most organ systems respond to changes in

the internal environment either by attempting to restore some plasma variable or by changing their own function

The digestive system creates an optimal environment for its functioning in the lumen of the GI tract

Essentially all digestive tract regulatory mechanisms act to control luminal conditions so that digestion and absorption can occur there as effectively as possible

Basic Functional Concepts Digestive activity is provoked by a range

of mechanical and chemical stimuli– Receptors are located in the walls of the tract

organs– These receptors respond to several stimuli– The most important being the stretching of

the organ by food in the lumen, osmolarity (solute concentration) and pH of the contents and the presence of substrates and end products of digestion

Basic Functional Concepts When appropriately

stimulated, these receptors initiate reflexes that– Activate or inhibit

glands that secrete digestive juices into the lumen or hormones into the blood

– Mix lumen contents along the length of the tract by stimulating the smooth muscle of the GI tract walls

Basic Functional Concepts Controls of digestive activity are both

extrinsic and intrinsic– Another novel trait of the digestive tract is

that many of its controlling systems are intrinsic - a product of in-house nerve plexuses or local hormone-producing cells

– The walls of the alimentary canal contain nerve plexuses

– These plexuses extend essentially the entire length of the GI tract and influence each other both in the same and in different organs

Digestive Processes Two kinds of reflex

activity occur Short reflexes are

mediated entirely by the local enteric plexuses in response to GI tract stimuli

Long reflexes are initiated by stimuli arising from within or outside of the GI tract and involve CNS centers and ANS

Digestive Processes The stomach and small intestine also

contain hormone-producing cells that, when stimulated by chemicals, nerve fibers, or local stretch, release their products to the extracellular space

These hormones circulate in the blood and are distributed to their target cells within the same or different tract organs, which they prod into secretory or contractile activity

Digestive System Organs Most of the digestive

organs reside in the abdominal-pelvic cavity

All ventral body cavities contain serous membranes

The peritoneum of the abdominal cavity is the most extensive serous membrane of the body

Digestive System Organs The visceral peritoneum

covers the external surface of most digestive organs and is continuous with the parietal peritoneum that lines the walls of the abdomino-pelvic cavity

Between the two layers is the peritoneal cavity, a slitlike potential space containing fluid secreted by the serous membranes

Digestive System Organs The serous fluid

lubricates the mobile digestive organs, allowing them to glide easily across one another as they carry out their digestive activities

Digestive System Organs

A mesentery is a double layer of peritoneum - a sheet of two serous membranes fused back to back - that extends to the digestive organ from the body wall

Digestive System Organs

Mesenteries provide routes for blood vessels, lymphatics and nerves to reach the digestive viscera

Digestive System Organs

Mesenteries also suspend the visceral organs in place as well as serving as a site for fat storage

Digestive Processes

Not all alimentary canal organs are suspended with the peritoneal cavity by a mesentery

Some parts of the small intestine originate the cavity but then adhere to the dorsal abdominal wall (Figure 22.5) above

Digestive Processes

Organs that adhere to the dorsal abdominal wall lose their mesentery and lie posterior to the peritoneum

These organs, which also include most of the pancreas and parts of the large intestine are called retro-peritoneal organs

Digestive Processes

Digestive organs like the stomach that keep their mesentery and remain in the peritoneal cavity are called interperitoneal or peritoneal organs

It is not known why some digestive organs end up in the retroperitoneal position

Blood Supply The splanchnic circulation includes those

arteries that branch off the abdominal aorta to serve the digestive organs and the hepatic portal circulation

The hepatic, splenic and left gastric branches of the celiac trunk serve the spleen, liver, and stomach

The mesenteric arteries (superior and inferior) serve the small and large intestine

Blood Supply The arterial supply to the abdominal organs

is approximately one quarter of the cardiac output

The hepatic portal circulation collects nutrient-rich venous blood draining from the digestive viscera and delivers it to the liver

The liver collects the absorbed nutrients for metabolic processing or for storage before releasing them back to the bloodstream for general cellular use

Histology of the Alimentary Canal From the esophagus to the anal canal, the

walls of every organ of the alimentary canal are made up of the same four basic layers or tunics– Mucosa– Submucosa– Muscularis externa– Serosa

Each tunic contains a predominant tissue type that plays a specific role in food breakdown

Histology of the Alimentary Canal From internal to

external the four layers of the alimentary canal are– Mucosa

– Submucosa

– Muscularis Externa

– Serosa

Histology: Mucosa The mucosa is the

moist epithelial membrane that lines the length of the lumen of the alimentary canal

Major functions are– Secretion of mucus,

digestive enzymes and hormones

– Absorption

– Protection

Histology: Mucosa The mucosa is the

moist epithelial membrane that lines the length of the lumen of the alimentary canal

Major functions are– Secretion of mucus,

digestive enzymes and hormones

– Absorption

– Protection

Histology: Mucosa More complex than most other mucosae

the typical digestive mucosa consists of three sublayers– A surface epithelium– A lamina propria– A deep muscularis mucosae

Histology: Mucosa The epithelium of

the mucosa is a simple columnar epithelium that is rich in mucus secreting goblet cells

Histology: Mucosa The slippery mucus it produces protects

certain digestive organs from digesting themselves by enzymes working within their cavities and eases food passage

In the stomach and small intestine the mucosa contain both enzyme-secreting and hormone-secreting cells

Thus, in such sites, the mucosa is a diffuse kind of endocrine organ as well as part of the digestive organ

Histology: Mucosa The lamina propria

which underlies the epithelium is loose areolar connective

Note lymph nodule

Histology: Mucosa Its capillaries nourish the epithelium and

absorb digested nutrients Its isolated lymph nodules are part of the

mucosa associated lymphatic tissue (MALT) which collectively act as a defense against bacteria and other pathogens

Large collections of lymph nodules occur at strategic locations such as within the pharynx (tonsils) and appendix

Histology: Mucosa The muscularis

mucosae is a scant layer of smooth muscle cells that produces local movements of the mucosa

Histology: Mucosa The twitching of this muscle layer dislodges

food particles that have adhered to the mucosa

In the small intestine, it throws the mucosa into a series of small folds that immensely increase its surface area

Histology: Submucosa The submocosa is a

moderately dense connective tissue containing blood and lymphatic vessels, lymph nodules, and nerve fibers

Its rich supply of elastic fibers enables the stomach to regain its normal shape after storing a large meal

Histology: Submucosa The submocosa is a

moderately dense connective tissue containing blood and lymphatic vessels, lymph nodules, and nerve fibers

Its rich supply of elastic fibers enables the stomach to regain its normal shape after storing a large meal

Histology: Muscularis Externa The muscularis

externa is responsible for segmentation and peristalsis

It mixes and propels foodstuffs along the digestive tract

This thick muscular layer has an inner circular and an outer longitudinal layer

Histology: Muscularis Externa In several places along the GI tract, the

circular layer thickens to form sphincters Sphincters act as valves to prevent backflow

and control food passage from one organ to the next

Histology: Serosa The serosa is the

protective outermost layer of inter- peritoneal organ

This visceral peritoneum is formed of areolar connective tissue covered with meso- thelium, a single layer of squamous epithelial cells

Histology: Serosa In the esophagus, which is located in the

thoracic instead of the abdominopelvic cavity, the serosa is replaced by an adventitia

The adventitia is an ordinary fibrous connective tissue that binds the esophagus to surrounding structures

Retroperitoneal organs have both a serosa (on the side facing the peritoneal cavity) and an adventitia (on the side abutting the dorsal body wall)

Enteric Nervous System The alimentary

canal has its own in-house nerve supply

Enteric neurons communicate widely with each other to regulate digestive system activity

IntrinsicNervePlexes

Enteric Nervous System These enteric

neurons constitute the bulk of the two major intrinsic nerve plexuses found within the walls of the alimentary canal– Submucosal nerve

plexus

– Myenteric nerve plexus

Myentericplexus

Submucosalplexus

Enteric Nervous System A smaller third

plexus is found within the serosa layer– Subsersora nerve

plexus

Subserosa nerveplexus

Enteric Nervous System The submucosal

nerve plexus chiefly regulates the activity of glands and smooth muscle in the mucosa tunic

The myenteric nerve plexus lies between the circular and longitudinal layers of smooth muscle of the muscularis externa

Myentericplexus

Submucosalplexus

Enteric Nervous System Via their communication with each other,

with smooth muscle layers, and with submucosal plexus, the enteric neurons of the myenteric plexus provide the major nerve supply to the GI tract

This plexus controls GI tract mobility by controlling the patterns of segmentation and peristalsis

Control comes from local reflex arcs between enteric neurons in the same or different plexus or organs

Enteric Nervous System The enteric nervous system is also linked

to the CNS by afferent visceral fibers and sympathetic and parasympathetic branches of the ANS

Digestive activity is subject to extrinsic control exerted by ANS which can speed up or slow secretory activity and mobility

Digestive System

Mouth, Pharynx, and Esophagus The mouth is the only part of the

digestive system that is involved in the ingestion of food

Most digestive function of the mouth reflect the activity of accessory organs chewing the food and mixing it with salvia to begin the process of chemical digestion

The mouth also begin the propulsive process by which food is carried through the pharynx and esophagus to the stomach

The Mouth The oral cavity is a

lined with mucosa It bounded by the

lips anteriorly, and the tongue inferiorly and the cheeks laterally

Its anterior opening is the oral orifice

Posteriorly the oral cavity is continuous with the oropharynx

The Mouth The walls of the

mouth are lined with stratified squamous epithelium

The epithelium is highly ketatinized for extra protection against abrasion during eating

The mucosa also produces defensins to fight microbes in the mouth

The Lips and Cheeks The labia and the

cheeks have a core of skeletal muscle covered by skin

The orbicularis oris muscle forms the bulk of the lips

The cheeks are formed largely by the buccinators

The area between the teeth and gums is the vestibule

The Lips and Cheeks The lips extend

from the inferior margin of the nose to the superior boundary of the chin

The reddened area is called red margin

The labial frenulum is a median fold that joins the internal aspect of each lip to the gum

The Palate The palate which

forms the roof of the mouth has two distinct parts– Hard palate

– Soft palate

The Palate The hard palate is

underlain by bone and is a rigid surface against which the tongue forces food during chewing

There exists a centerline ridge called a raphe

The mucosa is corrugated for friction

The Palate The soft palate is a

mobile fold formed by skeletal muscle

Projecting down from its free edge is the uvula

The soft palate rises reflexively to close off the nasopharynx when swallowing

The Palate The soft palate is

anchored to the tongue by the palantoglossal arches and to the wall of oropharynx by the palantopharyngeal arches

These arches form the boundary of the facuces

The Tongue The tongue occupies the floor of the

mouth and fills most of the oral cavity when closed

The tongue is composed of interlacing masses of skeletal muscle fibers

The tongue grips the food and constantly repositions it between the teeth

The tongue also mixes the food with salvia and form it into a mass called a bolus and then initiates swallowing by moving the mass into the pharynx

The Tongue The tongue has both

intrinsic and extrinsic skeletal muscles

The intrinsic muscles are confined within the tongue and are not attached bone

The fibers allow the tongue to change its shape for speech and swallowing but not its position

The Tongue The extrinsic muscles

extend the tongue from their points of origin

The extrinsic muscles allow the tongue to be protruded, retracted and moved side to side

The tongue is divided by a median septum of connective tissue

The Tongue A fold of mucosa

called the lingual frenulum secures the tongue to the floor of the mouth

This frenulum limits the posterior move- ment of the tongue

You cannot swallow your tongue

The Tongue The conical filaform

papillae give the tongue surface a roughness that aids in manipulating foods in the mouth

They align in parallel rows on the dorsum

They contain keratin which stiffens them

House taste buds

The Tongue The mushroom

shaped fungiform palillae are scattered over the surface

Each has a vascular core that gives it a reddish hue

Houses taste buds

The Tongue The circumvallate

are located in a V-shaped row at the back of the tongue

Appear similar to the fungiform papillae but with an additional surrounding furrow

The Salivary Glands A number of glands both inside and

outside the oral cavity produce and secrete saliva

Saliva functions to – Cleanses the mouth– Dissolves food chemical so that they can be

tasted– Moistens food and aids in compacting it into

a bolus– Contains enzymes that begin the chemical

breakdown of starches

The Salivary Glands Most saliva is

produced by three pairs of extrinsic salivary glands– Parotid

– Submandibular

– Sublingual These glands lie

outside the oral cavity and empty their secretions into it

The Salivary Glands The intrinsic

salivary glands are small and are scattered throughout the oral cavity

The Salivary Glands The salivary glands are composed of two

types of secretory cells; mucus and serous The serous cells produce a watery

secretion containing enzymes and the ions of saliva

The mucus cells produce mucus a stringy viscous solution

The Teeth The teeth lie in sockets in the gum

covered margins of the mandible and maxilla

Teeth function to tear and grind food and begin the mechanical process of digestion

Dentition Ordinarily we have two sets of teeth the

primary and permanent dentitions The primary dentition consists of

deciduous teeth The first teeth appear at six months and

additional teeth continue to erupt until about 24 months when all 20 teeth have emerged

Dentition As the deeper permanent teeth enlarge

and develop, the root of the milk teeth are resorbed from below causing them to loosen and fall out between the ages of 6 and 12 years

Generally, all the teeth of the permanent dentition have erupted by adolescence

The Teeth Teeth are classified

according to their shape and function– Incisors / cutting

– Canines / tear

– Premolars / grind

– Molars / crush There are 20 milk

teeth and 32 permanent teeth

Tooth Structure Each tooth has two

major regions; the crown and the root

The crown represents the visible portion of the tooth exposed above the gum

The root is the portion of the tooth that is imbedded in the jawbone

The Pharynx From the mouth, the

food passes posteriorly into the oropharnyx

The mucosa consists of stratified squamous epithelium

The epithelium is supplied with mucus producing glands for lubrication

The Pharynx The external muscle

layer consists of two skeletal muscle layers

The cells of the inner layer run longitudinally

The outer layer of muscles pharyngeal constrictor muscles, encircle the wall

Sequential contractions propel food into esophagus

The Esophagus

The esophagus takes a fairly straight course through the mediastinum of the thorax, pierces the diaphragm at the esophageal hiatus to enter the abdomen

The Esophagus The esophagus joins

the stomach at the cardiac orifice

The cardica orifice is surrounded by the cardiac esophogeal sphincter

The Pharynx The esophageal mucosa contains a non-

ketatinized stratified squamous epithelium which changes abruptly simple columnar epithelium upon reaching the stomach

When empty the esophagus is empty with its mucosa drawn into folds which flatten out when food is in passage

The mucosa contains mucus secreting esophageal glands which are compressed by a passing bolus of food resulting in the glands secreting a lubricant

The Pharynx The muscularis externa changes from

skeletal muscle to a mix of skeletal and smooth to finally all smooth as it approaches the stomach

Instead of a serosa, the esophagus has a fibrous adventitia composed entirely of connective tissue, which blends with surrounding structures along its route

Digestive Processes The mouth and its accessory digestive

organs are involved in most digestive processes– The mouth ingests food– Begins mechanical digestion by chewing– Initiates propulsion by swallowing– Starts the process of chemical digestion

– The pharynx and the esophagus serve as conduits to pass food from the mouth to the stomach

Digestive Processes: Mastication Mastication is the mechanical process of

breaking down food The cheeks and closed lips hold the food

between the teeth The tongue mixes the food with saliva to

soften it The teeth cut and grind food into smaller

pieces

Digestive Processes: Deglutition In deglutition, food is

first compacted by the tongue into a bolus and swallowed

Swallowing is a process that requires the coordination of tongue soft palate, pharynx, esophagus and 22 separate muscles

Digestive Processes: Deglutition In deglutition, food is

first compacted by the tongue into a bolus and swallowed

Swallowing is a process that requires the coordination of tongue soft palate, pharynx, esophagus and 22 separate muscles

Digestive Processes: Deglutition Food passage into

respiratory passageways by rising of the uvula and larynx

Relaxation of the upper esophageal sphincter allows food entry into the esophagus

Digestive Processes: Deglutition The constrictor muscles

of the pharynx contract, forcing food into the esophagus inferiorly

The upper esophageal sphincter contracts after entry

Digestive Processes: Deglutition Food is conducted along

the length of the esophagus to the stomach by peristaltic waves

Digestive Processes The gastroesophageal

sphincter enters opens and food enters the stomach

The Stomach The stomach functions as a temporary

storage tank where the chemical breakdown of protein begins and food is converted to a creamy paste called chyme

The stomach lies in the upper left quadrant of the abdominal cavity

Though relatively fixed at both ends, it is free to move in between

The Stomach: Gross Anatomy The stomach varies

from 6 to 10 inches in length, but its diameter and volume depend on how much food it contains

Empty it may contain on 50 ml but can expand to hold about 4 liters of food

The Stomach: Gross Anatomy

When empty, the stomach collapses inward, throwing its mucosa into large, longitudinal folds called rugae

The Stomach: Gross Anatomy

The major region of the stomach are the cardia region, the fundus, body, pyloric region, and the greater and lesser curvatures

The Stomach: Gross Anatomy

The lesser omentum runs from the liver to the lesser curvature where it becomes continuous with the visceral peritoneum of the stomach

The Stomach: Gross Anatomy

The greater omentum drapes inferior from the greater curvature of the stomach to cover the coils of the small intestine

Stomach: Microscopic Anatomy The stomach wall exhibits the four tunics

of most of the alimentary canal but its muscularis and mucosa are modified for the special roles of stomach

The muscularis externa has an extra oblique layer of muscle that enables it to mix, churn and pummel food

The epithelium lining the stomach mucosa is simple columnar epithelium composed entirely of goblet cells, which produce a protective coating of mucus

Microscopic Anatomy The four tunics

typical of the alimentary canal– Mucosa

– Submucosa

– Muscularis Externia

– Serosa

Microscopic Anatomy The otherwise smooth

lining is dotted with millions of gastric pits which lead to gastric glands that produce gastric juice

The glands of the stomach body are substantially larger and produce the majority of the stomach secretions

Microscopic Anatomy Mucus neck cells

produce a different type of mucus from that secreted by the mucus secreting cells of the surface epithelium

The special function of this unique mucus is not yet understood

Microscopic Anatomy Parietal cells scattered

among the chief cells secrete hydrochloric acid (HCl) and intrinsic factor

The parietal cells have a large surface area adapted for secreting HCl in the stomach

Intrinsic factor is required for absorption of B12 in the small intestine

Microscopic Anatomy Chief cells produce

pepsinogen, the inactive form of the protein- digesting enzyme pepsin

The cells occur mainly in the basal regions of the gastric glands

Pepsinogen is activated by HCl

Microscopic Anatomy Parietal cells scattered

among the chief cells secrete hydrochloric acid (HCL) and intrinsic factor

The parietal cells have a large surface area adapted for secreting HCL in the stomach

Intrinsic factor is required for absorption of B12 in the small intestine

Microscopic Anatomy Enteroendocrine

release a variety of hormones directly into the lamina propria

These products diffuse into capillaries and ultimately influence several digestive system target organs which regulate stomach secretion and mobility

Mucosal Barrier Gastric juice is 100,000 more concentrated

than that found in the blood Under such harsh conditions the stomach

must protect itself from self digestion with a mucosal barrier– Bicarbonate rich mucus is on the stomach wall– Epithelial cells are joined by tight junctions– Glandular cells are impermeable to HCl– Surface epithelium is replace every 3 to 6 days

Digestive Processes: Stomach The stomach is involved in the whole

range of digestive activities– It serves as a holding area for ingested food– Breaks down food further chemically and

mechanically– It delivers chyme to the small intestine at a

controlled rate

Digestive Processes: Stomach Protein digestion is initiated in the

stomach and is essentially the only type of enyzmatic digestion that occurs there

The most important protein digesting enzyme produced by the gastric mucosa is pepsin

In children, the stomach glands also secrete rennin, an enzyme that acts on milk protein converting it to a curdy substance appearing like sour milk

Digestive Processes: Stomach Despite its many functions in the

digestive system the only one that is essential for life is secretion of intrinsic factor

Intrinsic factor is required for intestinal absorption of vitamin B12, needed to produce mature erythrocytes

Without B12 the individual will develop prenicious anemia unless administered by injection

Regulation of Gastric Secretion Gastric secretion is controlled by both

neural and hormonal mechanisms Under normal conditions the gastric

mucosa creates as much as 3 liters of gastric juice every day

Gastric juice is an acid solution that has the potential to dissolve nails

Regulation of Gastric Secretion Nervous control is regulated by long

(vagus nerve mediated) and short (local enteric) nerve reflexes

When the vagus nerves actively stimulate the stomach, secretory activity of virtually all of its glands increase

The sympathetic nerves depress secretory activity

Regulation of Gastric Secretion Hormonal control of gastric secretion is

largely from the presence of gastrin Gastrin stimulates the secretion of both

enzymes and HCL in the stomach Hormones produced by the small

intestine are largely gastrin antagonists

Regulation of Gastric Secretion Stimuli acting at three distinct sites, the

head, stomach, and small intestine, provoke or inhibit gastric secretory activity

Accordingly the three phases are called cephalic, gastric, and intestinal phases

However, the effector site is the stomach in all cases and once initiated, one or all threephases may be occurring at the same time

Phase 1: Cephalic reflex The cephalic reflex phase of gastric

secretion occurs before food enters the stomach

It is triggered by the aroma, taste, sight, or though of food

During this phase the brain gets the stomach ready for food

Phase 1: Cephalic reflex Inputs from activated olfactory receptors

and taste buds are relayed to the hypothalamus which in turn stimulates the vagal nuclei of the medulla oblongata, causing motor impulses to be transmitted via the vagus nerves to the parasympathetic nerve ganglia

Eneteric ganglionic neurons in turn stimulate the stomach glands

Phase 1: Cephalic reflex The enhanced secretory activity that

results when we see or think of food is a conditioned reflex and occurs only when we like or want the food

If we are depressed or have no appetite, this part of the cephalic reflex is suppressed

Phase 2: Gastric reflex Once food reaches the stomach, local

neural and hormonal mechanisms initiate the gastric phase

This phase provides about two-thirds of the gastric juice released

The most important stimuli are distension, peptids, and low acidity

Phase 2: Gastric reflex Stomach distension activates stretch

receptors and initiates both local (myentertic) reflexes and the long vagovagal reflexes

In vagovagal reflex, impulses travel to the medulla and then back to the stomach via vagal fibers

Both types of reflexes lead to acetylcholine (ACH) release, which in turn stimulates the output of more gastric juice by cells

Phase 2: Gastric reflex Though neural influences initiated by

stomach distension are important, the hormone gastrin probably plays a greater role in stimulating stomach gland secretion during the gastric phase

Chemical stimuli provided by partially digested proteins (peptids)caffine (colas, coffee) and rising pH directly active gastrin secreting entoendocrine cells called G cells

Phase 2: Gastric reflex Although gastrin also stimulates the

release of enzymes, its main target is the HCL secreting parietal cells, which it prods to spew out even more HCL

Highly acidic (pH below 2) gastric contents inhibit gastrin secretion

Phase 2: Gastric reflex When protein foods are in the stomach,

the pH of the gastric contents generally rises because proteins act as buffers to tie up H+

The rise in pH stimulates gastrin and subsequently HCL release, which in turn provides the acidic conditions needed for protein digestion

Phase 2: Gastric reflex The more protein in the meal, the greater

the amount of gastrin and HCL released As proteins are digested, the gastric

contents gradually become more acidic, which again inhibits the gastrin secreting cells

This negative feedback mechanism helps maintain optimal pH and working conditions for the gastric enzymes

Phase 2: Gastric reflex G cells are also activated by the neural

reflexes already described Emotional upsets, fear, anxiety, or

anything that triggers the fight-or-flight response inhibits gastric secretion because (during such times) the sympathetic division overrides parasympathetic controls of digestion

Phase 2: Gastric reflex The control of the HCL secreting parietal

cells is unique and multifaceted Basically, HCL secretion is stimulated by

three chemicals, all of which work through second-messenger systems Ach released by parasympathetic nerve fibers and gastrin secreted by G cells

Phase 2: Gastric reflex Ach released by

para-sympathetic nerve fibers and gastrin secreted by G cells bring about their effects by increasing intercellular Ca+

+ levels

Phase 2: Gastric reflex Histamine

released by mucosal cells called histaminocytes acts through cyclic AMP (cAMP)

Phase 2: Gastric reflex When only one of the three chemicals

binds to the parietal cells, HCL secretions are minimul

When all three of the chemicals bind to the parietal cells volumes of HCL pour forth as if pushed out under pressure

Phase 2: Gastric reflex The process of HCL formation within the

parietal cells is complicated and poorly understood

The consensus is that H+ is actively pumped into the stomach lumen against a tremendous concentration gradient

Phase 2: Gastric reflex As hydrogen ions are secreted, chloride

ions (Cl-) are also pumped into the lumen to maintain an electrical balance in the stomach

The Cl- is obtained from blood plasma, while the H+ appears to come from a breakdown of carbonic acid formed by the combination of carbon dioxide and water and within the parietal cells

Phase 2: Gastric reflex CO2 + H2O

H2CO3 H+ + HCO3

-

As H+ is pumped from the cell and HCO3

- is ejected through the basal cell membrane into the capillary blood

Phase 2: Gastric reflex The result of ejection of the bicarbonate

ion into the capillary blood is that blood draining from the stomach is more alkaline than the blood serving it

The phenomenon is called the alkaline tide

Phase 3: Intestinal The intestinal phase of gastric secretion

has two components– One excitatory– One inhibitory

Phase 3: Intestinal The excitatory aspect is set into motion as

partially digested food begins to fill the initial part (duodenum) of the small intestine

This stimulates intestinal mucosal cells to release a hormone that encourages the gastric glands to continue their secretory activity

Phase 3: Intestinal The effects of this hormone imitate those

of gastrin, so it has been named intestinal (enteric) gastrin

However, intestinal mechanisms stimulate gastrin secretion only briefly

As the intestine distends with chyme containing large amounts of H+, fats, partially digested proteins, and irritating substances, the inhibitatory component is triggered in the form of the enterogastric reflex

Phase 3: Intestinal The enterogastric reflex is actually a trio

of reflexes that– Inhibit the vagal nuclei in the medulla– Inhibit local reflexes– Activate sympathetic fibers that cause the

pyloric sphincter to tighten and prevent further food entry into the small intestine

As a result, gastric secretory activity declines

Phase 3: Intestinal These inhibitions on gastric activity

product the small intestine to harm due to excessive acidity and match the small intestine’s processing abilities to the amount of chyme entering it at a given time

Phase 3: Intestinal In addition, the factors just named trigger

the release of several intestinal hormones collectively called enterogastrones which include– Secretin– Cholecystokinin (CCK)– Vasoactive intestinal peptide (VIP)– Gastric inhibitory peptide (GIP)

All of these hormones inhibit gastric secretion when the stomach is very active

Gastric Motility and Emptying Stomach contractions, accomplished by the

tri-layered muscularis, not only cause its emptying but also compress, knead, twist, and continually mix the food with gastric juice to produce chyme

Because the mixing movements are accomplished by a unique type of peristalis (bidirectional) the process of mechanical digestion and propulsion are inseparable in the stomach

Gastric Motility: Stomach Filling Although the stomach stretches to

accommodate incoming food, internal stomach pressure remains constant until about 1 liter of food has been ingested

The relatively unchanging pressure in the filling stomach is due to 1) reflex mediated relaxation of the stomach muscle and 2) plasticity of visceral smooth muscle

Gastric Motility: Stomach Filling Reflexive relaxation of stomach muscle in

the fundus and body occurs both in anticipation of and in response to food entry into the stomach

As food travels through the esophagus, the stomach muscles relax

This receptive relaxation is coordinated by the swallowing center in the brain stem and mediated by the vagus nerves

Gastric Motility:Stomach Filling The stomach also actively dilates in

response to gastric filling, which activates stretch receptors in the wall

The phenomenon called adaptive relaxation appears to depend on local reflexes involving nitric oxide (NO) releasing hormones

Gastric Motility: Stomach Filling Plasticity is the intrinsic ability of

visceral smooth muscle to exhibit the stress- relaxation response, that is, to be stretched without greatly increasing its tension and contractile strength

Gastric Motility and Emptying

After a meal peristalsis begins near the cardiac sphincter, where it produces only gentle rippling movements of the stomach wall

Gastric Motility and Emptying

As contractions approach the pylorus, where the stomach musculature is thicker, the contractions become more powerful

Gastric Motility and Emptying

Consequently, the contents of the fundus remain relatively undisturbed, while the foodstuffs close to the pylorus receive a very active mixing

Gastric Motility and Emptying

The pyloric region of the stomach, which holds about 30 ml of chyme, acts as a “dynamic filter” that allows only liquids and small particles of food to pass

Gastric Motility and Emptying

Normally, each peristaltic wave reaching the pyloric muscle squirts 3 ml or less of chyme into the small intestine

Gastric Motility and Emptying

While the stomach delivers small amounts of chyme into the doudenum it also simultaneously forces most of the contained material backward into the stomach for further mixing

Gastric Motility and Emptying Although the intensity of the stomach’s

peristaltic waves can be modified, the rate is always constant at around 3 per minute

The contractile rhythm is set by the spontaneous activity of pacemaker cells located at the margins of the longitudinal smooth muscle layer

Gastric Motility and Emptying The pacemaker cells, are believed to be

muscle-like noncontractile cells called interstitial cells of Cajal which depolarize the repolarize spontaneously three times each minute

This depolarization and repolarization establish the so-called cyclic slow waves of the stomach or its basic electrical rhythm (BER)

Gastric Motility and Emptying Since the pacemakers are electrically

coupled to the rest of the smooth muscle sheet by gap junctions, their “beat” is transmitted efficiently and quickly to the entire muscularis

The pacemakers set the maximum rate of contraction, but they do not initiate the contractions or regulate their force

They generate subthreshold depolarization waves, which are then enhance by neural and hormonal factors

Gastric Motility and Emptying Factors that increase the strength of

stomach contractions are the same factors that enhance gastric secetory activity

Distension of the stomach wall by food activates stretch receptors and gastric secreting cells, which both ultimately gastric smooth muscle and so increase gastric motility

Gastric Motility and Emptying Thus, the more food there is in the

stomach, the more vigorous the stomach mixing and emptying movements will be evident

The stomach usually empties completely within four hours after a meal

However, the larger the meal (greater distension) and the more liquid the meal, the faster the stomach empties

Gastric Motility and Emptying Fluids pass quickly through the stomach Solids linger, remaining until they are

well mixed with gastric juice and converted to a liquid state

Gastric Motility and Emptying The rate of emptying depends as much on the

contents of the duodenum as on whats happening in the stomach

The stomach and duodenum act in tandem As chyme enters the duodenum, receptors in

its wall respond to chemical signals and to stretch, initiating the enterogastric reflex and hormonal mechanisms described earlier

These factors inhibit gastric secretory activity and prevent further duodenal filling by reducing the force of pyloric contractions

Gastric Motility and Emptying A carbohydrate-rich meal moves through

the duodenum rapidly, but fats form an oily layer at the top of the chyme and are digested more slowly by enzymes acting in the intestines

Thus, when chyme entering the duodenum is fatty, food may remain in the stomach six hours or more

The Small Intestine and Associated Structures

In the small intestine, usable food is finally prepared for its journey into the cells of the body

However, this vital function cannot be accomplished without the aid of secretions from the liver (bile) and pancreas (digestive enzymes)

Thus the accessory organ are discussed in this section

Small Intestine The small

intestine is a convoluted tube extending from the pyloric sphincter in the epigastric region to the iliocecal valve where it joins the large intestine

Small Intestine It is the longest part of the alimentary

tube, but its diameter is only about 2.5 cm In the cadaver, the small intestine is 6 - 7

meters long because of loss of muscle tone, while it is only 2 - 4 meters long in the living individual

The small intestine has three subdivisions– Duodenum– Jejunum– Ileum

Gross Anatomy

The relatively immovable duodenum which curves about the head of the pancreas

Small Intestine The duodenum is about 10 inches long Although it is the shortest subdivision,

the duodenum has the most features of interest– The bile duct– Main pancreatic duct– Hepatopancreatic ampulla– Major duodenal papilla

Gross Anatomy

The bile duct, delivering bile from the liver The main pancreatic duct, carries pancreatic juice from

the pancreas

Gross Anatomy

The hepatopancreatic ampulla is where these two ducts unite in the wall of the duodenum

The papilla is where this sphincter enters the duodenum

Small Intestine The jejunum is

about 8 ft long and extends from the duodenum to the ileum

This central section twists back and forth within the abdominal cavity

Small Intestine The ileum is

approximately 12 ft. in length

It joins the large intestine at the ileocecal valve

Small Intestine The jejunum

and ileum hang in coils in the central and lower part of the abdominal cavity

Small Intestine The jejunum

and ileum are suspended from the posterior abdominal wall by the fan shaped mesentery

Small Intestine Nerve fibers serving the small intestine

include the parasympathetics from the vagus nerves and sympathetics from the long splanchic nerves

These are relayed through the superior mesenteric and celiac plexus

Small Intestine The arterial supply is primarily from the

superior and mesenteric artery The veins run parallel to the arteries and

typically drain into the superior mesenteric vein

From the mesenteric vein, the nutrient rich venous blood from the small intestine drains into the hepatic portal vein which carries it to the liver

Microscopic Anatomy The small intestine is highly adapted for

nutrient absorption Its length provides a huge surface area

for absorption There are three structural modifications

which increase the surface area for absorption– Plicae circulares– Villi– Microvilli

Microscopic Anatomy

Digestive System Organs

In this view you can see the plicae circulares and the villi of the small intestine

Microscopic Anatomy Structural modifications increase the

intestinal surface area tremendously It is estimated that the surface area of the

small intestine is equal to 200 square meters or roughly equivalent to the floor space of a two story house

Most absorption occurs in the proximal part of the small intestine, with these structural modifications decreasing toward the distal end

Microscopic Anatomy The circular

folds or plicae circularis are deep permanent folds of the mucosa and submucosa

These folds are nearly 1 cm tall

Microscopic Anatomy The folds force

chyme to spiral through the lumen, slowing its movement and allowing time for full nutrient absorption

Microscopic Anatomy Villi are finger

like projections of the mucosa

Over 1 mm tall they give a velvety texture to the mucosa

Microscopic Anatomy The epithelial

cells of the villi are chiefly absorptive columnar cells called enterocytes

Microscopic Anatomy In each villus is a capillary

bed and a wide lymphatic capillary called a lacteal

Digested food is absorbed through the epithelial cells into both the capillary blood and the lacteal

Villi become gradually narrower and shorter along the length of the sm. intestine

Enterocyte

Microscopic Anatomy Microvilli are tiny

projections of the plasma membrane of the absorptive cells of the mucosa

It gives the mucosal surface a fuzzy appearance sometimes called a brush border

Microscopic Anatomy Beside increasing the absorptive surface,

the plasma membrane of the microvilli bear enzymes referred to as the brush border enzymes

These enzymes complete the final stages of digestion of carbohydrates and proteins in the small intestine

Histology of the Wall The four tunics of

the digestive tract are modified in the small intestine by variations in mucosa and sub- mucosa

Histology of the Wall The epithelium of the mucosa is largely

simple columnar epithelium serving as absorptive cells

The cells are bound by tight junctions and richly endowed with microvilli

Also present are many mucus-secreting goblet cells

Histology of the Wall Scattered among the epithelial cells of the

wall are T cells called intraepithelial lymphocytes

These T cells provide an immunological component

Finally, there scattered enteroendocrine cells which are the source of secretin and cholecystokinin

Histology of the Wall

Between villi the mucosa is studded with pits that lead into tubular intestinal glands called intestinal crypts or crypts of Lieberkuhn

Histology of the Wall The epithelial cells that line these crypts

secrete intestinal juice Intestinal juice is a watery mixture

containing mucus that serves as a carrier fluid for absorption of nutrients from chyme

Histology of the Wall Located deep on the crypts are

specialized secretory cells called Paneth cells

Paneth cells fortify the small intestine by releasing lysozyme an antibacterial enzyme

The number of crypts decreases along the length of the wall of the small intestine, but the number of goblet cells becomes more abundant

Histology of the Wall The various epithelial cells arise from

rapidly dividing stem cells at the base of the crypts

The daughter cells gradually migrate up the villi where they are shed from the villis tips

In this way the villus of the epithelium is renewed every three to six days

Histology of the Wall The rapid replacement of the intestinal

(and gastric) epithelial cells has clinical as well as physiological implications

Treatments for cancer, such as radiation therapy and chemotherapy preferentially target the cells in the body that divide most quickly

This kills cancer cells, but it also nearly obliterates the GI epithelium causing nausea, vomiting, and diarrhea after each treatment

Histology of the Wall The submucosa is typical areolar

connective tissue, and it contains both individual and aggregated lymphoid follicles (Peyer’s patches)

Peyer’s patches increase in abundance toward the end of the small intestine, reflecting the fact that the large intestine contains huge numbers of bacteria that must be prevented from entering the bloodstream

Histology of the Wall

A set of elaborated mucus-secreting duodenal glands (Brunner’s) is found in the submucosa of the duodenum only

Histology of the Wall These glands produce an alkaline

(bicarbonate-rich) mucus that helps neutralize the acidic chyme moving in from the stomach

When this protective mucus barrier is inadequate, the intestinal wall is eroded and duodenal ulcers results

Histology of the Wall The muscularis is typical and bilayered Except for the bulk of the duodenum,

which is retroperitoneal and has an adventitia, the external intestinal surface is covered by visceral peritoneum (serosa)

Intestinal Juice The intestinal glands normally secrete

between 1 and 2 liters of intestional juice daily

The major stimulus for its production is distension or irritation of the intestinal mucosa by hypertonic or acidic chyme

Intestinal Juice Normally, the pH range of intestinal juice

is slightly alkaline (7.4-7.8), and it is isotonic with blood plasma

Intestinal juice is largely water but it also contains some mucus, which is secreted both by the duodenal glands and by goblet cells of the mucosa

Intestinal juice is relatively enzyme poor because intestinal enzymes are largely limited to the bound enzymes of the brush border

The Liver and Gallbladder The liver and gallbladder are accessory

organs associated with the small intestine The liver has many metabolic and

regulatory roles Its digestive function is to produce bile

for export to the duodenum Bile is a fat emulsifier which breaks up

fat into tiny particles so that they are more accessible to digestive enzymes

The gallbladder is a storage site for bile

The Liver

The ruddy, blood rich liver is the largest gland in the body weighing about 1.4 kg in the average adult

The Liver Shaped like a wedge, it

occupies most of the right hypochondriac and epigastric regions extending farther to the right of the body midline than the left

The Liver Located under the diaphragm, the liver

lies almost entirely within the rib cage The location of the liver within the rib

cage offers this organ some degree of protection

The Liver

The liver has four primary lobes; right, left, caudate and quadrate

The Liver

A mesentery, the falciform ligament, separates the right and left lobes anteriorly and suspends the liver from the diaphragm

The Liver

Running along the free inferior edge of the falciform ligament is the ligamentum teres a remnant of the fetal umbilical vein

The Liver

Except for the superiormost liver area, which is fused to the diaphragm, the entire liver is enclosed by a serosa (visceral peritoneum)

The Liver A dorsal

mesentery, the lesser omentum, anchors the liver to the lesser curvature of the stomach

The Liver

The hepatic artery and hepatic portal vein, enter the liver at the porta hepatis

The Liver

The common bile duct, which runs inferiorly from the liver, travels through the lesser omentum

The Liver

The gallbladder rests in a recess of the inferior surface of the right lobe of the liver

The Liver

Bile leaves the liver through several bile ducts that ultimately fuse to form the large common hepatic duct which travels to the duodenum

The Liver

The common hepatic duct and the cystic duct fuse to form the bile duct

Microscopic Anatomy of Liver The liver is

composed of seed sized structural & functional units called liver lobules

Each lobule is roughly hexagonal

Microscopic Anatomy of Liver Hepatocytes

or live cells are organized to radiate out from a central vein running the length of the longitudinal axis of the lobule

Microscopic Anatomy of Liver To make a rough “model” of a liver

lobule, open a paperback book until the two covers meet

The pages represent the plates of hepatocytes and the hollow cylinder formed by the rolled spine represents the central vein

Microscopic Anatomy of Liver The liver’s main function is to filter and

process the nutrient rich blood delivered to it

At each of the six corners of a lobule is a portal triad so named because three basic structures are always present there:– A branch of the hepatic artery– A branch of the hepatic portal vein– A bile duct

Microscopic Anatomy of Liver

The hepatic artery supplies oxygen rich arterial blood to the liver

Microscopic Anatomy of Liver

The hepatic vein carries blood laden with nutrients from the digestive viscera

Microscopic Anatomy of Liver

A bile duct to carry secreted bile toward the common bile duct and ultimately to the duodenum

Microscopic Anatomy of Liver Between the

hepatocyte plates are enlarged, very leaky capillaries, the liver sinusoids

Microscopic Anatomy of Liver Blood from both

the hepatic portal vein and the hepatic artery percolates from the triad regions through these sinusoids and empties into the central vein

Microscopic Anatomy of Liver From the central

vein blood eventually enters the hepatic veins, which drain the liver, and empty into the inferior vena cava

Microscopic Anatomy of Liver Inside the

sinusoids are star shaped hepatic macrophages, also called Kupffer cells, which remove debris such as bacteria and worn-out blood cells

Microscopic Anatomy of Liver The hepatocytes (liver cells) are virtual

organelle storehouses with large amounts of both rough and smooth endoplasmic reticulum, Golgi apparatuses, peroxisomes, and mitochondria

Thus equipped, the hepatocytes produce not only bile but also– Process blood borne nutrients– Store Fat-soluble vitamins– Detoxify the blood

Microscopic Anatomy of Liver In processing nutrients the hepatocytes

store glycogen and make plasma proteins Fat soluble vitamins are stored until such

time as they are needed for metabolism Detoxification occurs are the hepatocytes

rid the blood of ammonia by converting it to urea

The net result is that the blood leaving the liver contains fewer nutrients and waste materials than the blood that entered

Microscopic Anatomy of Liver Secreted bile

flows through tiny canals, called bile canaliculi that run between adjacent hepato cytes toward the bile branch ducts in the portal triad

Microscopic Anatomy of Liver Note that the bile

and the blood flow in opposite directions in the liver lobule

Bile entering the bile ducts eventually leaves the liver via the common hepatic duct

Microscopic Anatomy of Liver Bile is a yellow-green, alkaline solution

containing– Bile salts– Bile pigments– Cholesterol – Neutral fats– Phospholipids (lecithin and others)– Electrolytes

Only bile salts and phospholipids aid the digestive process

Microscopic Anatomy of Liver Bile salts, primarily cholic acid and

chenodeoxycholic acids are cholesterol derivatives

Their role is to emulsify fats which distributes them throughout the watery intestinal contents

As a result, large fat globules entering the small intestine are physically separated into millions of small fatty droplets

Microscopic Anatomy of Liver Millions of tiny fat droplets vastly

increase the surface area for the fat digesting enzymes to work on

Bile salts also facilitate fat and cholesterol absorption and help solubilize cholesterol, both that contained in bile and that entering the small intestine for food

Microscopic Anatomy of Liver Although many substances secreted in bile

leave the body in feces, bile salts are not among them

Bile salts are conserved by a means of a recycling mechanism called enterohepatic circulation

In this process bile salts are– Reabsorbed into the small intestine– Returned to the liver via the hepatic portal vein– Resecreted in newly formed bile

Microscopic Anatomy of Liver The chief bile pigment is bilirubin, a

waste product of hemoglobin (heme) during the breakdown of worn-out erythrocytes

The globin and iron parts of hemoglobin are saved and recycled, but bilirubin is absorbed from the blood by the liver cells and actively excreted into the bile

Microscopic Anatomy of Liver Most of the bilirubin in bile is metabolized

in the small intestine by resident bacteria A breakdown by-product is urobilirubin

which give feces its brown color In the absence of bile, feces are grey-white

in color and have fatty streaks because essentially no fats are digested or absorbed

Microscopic Anatomy of Liver The liver produces 500 to 1000 ml of bile

daily, and bile production is stepped up when the GI tract contains fatty chyme

Bile salts themselves are a major stimulus for enhance bile secretion

Microscopic Anatomy of Liver The single most

important stimulus of bile secretion is an increased level of bile salts in the enterohepatic circulation

The Gallbladder The gallbladder is

a thin-walled, green muscular sac, rouhgly the size of a kiwi fruit

It snuggles in a shallow fossa on the ventral surface of the liver

The Gallbladder

The gallbladder stores bile that is not immediately needed for digestions

The Gallbladder Bile that is not needed is concentrated by

absorbing some of its water and ions When empty, its mucosa adopts the ridge

like folds or rugae of the stomach Its muscular walls can contract to expell

its contents into the cystic duct which then flows into the bile duct

Like most of the liver it is covered by visceral peritoneum

The Gallbladder

When digestion is not occurring, the hepatopancreatic sphincter is tightly closed

The Gallbladder

Bile then backs up the cystic duct into the gallbladder where it is stored until needed

The Gallbladder Although the liver makes bile continuously

bile does not usually enter the small intestine until the gallbladder contract

The major stimulus for gallbladder contraction is the intestinal hormone cholecystokinin (CCK)

CCK is released to the blood when acidic, fatty chyme enters the duodenum

The Gallbladder Besides causing the gallbladder to

contract, CCk has two other important effects– It stimulates secreation of pancreatic juice– It relaxes the hepatppancreatic sphincter so

that bile and pancreatic juice can enter the duodenum

Parasympathetic impulses delivered by the vagus nerves have a minor impact on stimulating gallbladder contraction

The Pancreas

The pancreas is a soft, tadpole-shaped gland that extends across the abdomen

The Pancreas

Most the pancreas is retroperitoneal and lies deep to the greater curvature of stomach

The Pancreas An accessory organ, the pancreas is

important to the digestive process because it produces a broad spectrum of enzymes

These enzymes break down all categories of foodstuffs, which the pancreas then delivers to the duodenum

This exocrine product is called pancreatic juice

The Pancreas

Pancreatic juice drains from the pancreas via the centrally located main pancreatic duct

The Pancreas

The pancreatic duct generally fuses with the bile duct just as it enters the duodenum

The Pancreas

A smaller accessory pancreatic duct empties directly into the main duct

The Pancreas Within the

pancreas are the acini, clusters of secretory cells surrounding ducts

The Pancreas The acini cells

are full of rough endoplasmic reticulum and exhibit deeply staining zymogen granules containing digestive enzymes

The Pancreas The pancreas

also has an endocrine function

Scattered amidst the acini are the more lightly staining pancreatic islets

The Pancreas These Islets of

Langerhans release insulin and glucagon, hormones that regulate carbohydrate metabolism

Pancreatic Juice Approximately 1200 to 1500 ml of clear

pancreatic juice is produced daily It consists mainly of water and contains

enzymes and electrolytes The acinar cells produce the enzyme rich

pancreatic juice The epithelial cells lining the smallest

pancreatic ducts release the bicarbonate ions that make it alkaline (pH 8)

The Pancreas The high pH enables pancreatic fluid to

neutralize the acid chyme entering the duodenum

It also provides the optimal environment for activity of intestinal and pancreatic enzymes

Like pepsin of the stomach, pancreatic protein digesting enzymes are produced and released in active forms, which are then activated in the duodenum

The Pancreas Within the duodenum trypsinogen is

activated to trypsin by enterokinase an intestinal brush border enzyme

Trypsin in turn activates two other pancreatic enzymes– Procarboxypeptidase > carboxypeptidase– Chymotrypsinogen > chymotrypsin

Other pancreatic enzymes (amylase, lipase, and nucleases) are secreted in active form but require ions in the bile for activity

Regulation of Pancreatic Secretion Secretion of pancreatic juice is regulated

both by local hormones and by the parasympathetic nervous system

Regulation of Pancreatic Secretion Secretin is

released in response to the presence of HCL in the intestine

Cholecystokinin is released in response to the entry of proteins and fats

Regulation of Pancreas Secretion Both hormones act on the pancreas, but

secretin targets the duct cells, prompting their release of watery bicarbonate-rich pancreatic juice, Whereas CCK stimulates the acini to release enzyme-rich pancreatic juice

Vagal stimulation causes release of pancreatic juice primarily during the cephalic and gastric phases of gastric secretion

Regulation of Pancreatic Secretion Normally, the amount of HCL produced

in the stomach is exactly balanced by the amount of bicarbonate (HCO3) actively secreted by the pancreas

HCO3 is secreted into the pancreatic juice, and H+ enters the blood

Regulation of Pancreatic Secretion

Consequently, the pH of venous blood returning to the heart remains relatively unchanged because alkaline blood draining from the stomach is neutralized by the acidic blood draining the pancreas

Digestion: Small Intestine Although food reaching the small intestine

is unrecognizable, it is far from being digested chemically

Carbohydrates and proteins are partially degraded, but virtually no fat digestion has occurred to this point

The process of food digestion is accelerated during the chyme’s journey of 3 to 6 hours through the small intestine, it is here that virtually all nutrient absorption occurs

Optimal Intestinal Activity Although the primary functions of the small

intestine are digestion and absorption, intestinal juice provides little of what is needed to perform these functions

Most substances required for chemical digestion - bile, digestive enzymes (except for brush border enzymes) and bicarbonate ions (to provide the proper pH for enzymatic catalysis) are imported from the liver and pancreas

Optimal Intestinal Activity Anything that impairs liver or pancreatic

function or delivery of their juices to the small intestine severely hinders the individual’s ability to digest food and absorb nutrients

Optimal Intestinal Activity Optimal digestive activity in the small

intestine also depends on a slow, measured delivery of chyme from the stomach

The small intestine can process only small amounts of chyme at one time

Chyme enter the small intestine is usually hypertonic

Optimal Intestinal Activity If large amounts of chyme were rushed

into the small intestine, the osmotic water loss from the blood into the intestinal lumen would result in dangerously low blood volume

Additionally, the low pH of entering chyme must be adjusted upward and the chyme must be well mixed with bile and pancreatic juice for digestion to continue

These adjustments take time

Optimal Intestinal Activity

Food movement into the small intestine is carefully controlled by the pumping action of the stomach pylorus which prevents the duodenum from being overwhelmed

Motility of the Small Intestine Intestinal smooth muscle mixes chyme

thoroughly with bile and pancreatic and intestinal juices and moves food residues through the ileocecal valve and into the large intestine

In contrast to the peristaltic waves of the stomach, which both mix and propel food, segmentation is the most common motion of the small intestine

Motility of the Small Intestine In segmentation,

chyme is moved backward and forward a few centimeters at a time by alternating contraction and relaxation of rings of smooth muscles

Motility of the Small Intestine These segmenting

movements of the intestine are initiated by intrinsic pacemaker cells (interstitial cells of Cajal) in the longitudinal smooth muscle layer

Motility of the Small Intestine Unlike the somach pacemakers, which

have only one rhythm, the pacemakers in the duodenum depolarizes more frequently (12-14 contractions per minute) than those of the ileum (8-9 contractions per minute)

As a result, segmentation moves intestinal contents slowly and steadily toward the ileocecal valve at a rate that allows ample time to complete digestion and absorption

Motility of the Small Intestine The intensity of the segmentation is

altered by hormones and long and short reflexes– Parasympathetic enhances segmentation– Sympathetic decreases segmentation

The more intense the contractions, the greater the mixing effect, however the basic contractile rhythms of the various intestinal regions remain unchanged

Motility of the Small Intestine True peristalsis

occurs only after most nutrients have been absorbed

Segmentation movements wane, and peristaltic waves begin

Motility of the Small Intestine Peristaltic waves initiated in the duodenum

begin to sweep slowly along the intestine, moving 10 - 70 cm before dying out

Each successive wave is initiated a bit more distally, and this pattern of peristaltic activity is called the migrating mobility complex

A complete migration from the duodenum to the ileum takes about two hours and then repeats itself

Motility of the Small Intestine Peristalsis serves to sweep out the last

remnants of the meal plus bacteria, sloughed-off mucosal cells, and other debris into the large intestine

This “housekeeping” function is critical for preventing the overgrowth of bacteria that migrate from the large intestine to the small intestine

As food enters the stomach with the next meal segmentation replaces peristalsis

Motility of the Small Intestine The local enteric neurons of the GI tract

wall coordinate intestinal mobility patterns

The physiological diversity of the enteric neurons allows a variety of effects to occur depending on which neurons are activated or inhibited

Motility of the Small Intestine A given ACh-releasing (cholinergic)

sensory neuron in the small intestine, once activated, may simultaneously send messages to several different interneurons in the myenteric plexus that regulate peristalsis:– Impulses sent proximally by cholingeric

neurons cause contraction and shortening of the circular muscular layer

Motility of the Small Intestine …interneurons in the myenteric plexus

that regulate peristalsis:– Impulses sent distally to certain interneurons

cause shortening of the longitudinal muscle layers and distension of the intestine, in response to Ach-releasing neurons

– Other impulses sent distally by activated VIP or NO-releasing enteric neurons cause relaxation of the circular muscle

Motility of the Small Intestine As a result, as the proximal area constricts

and forces chyme along the tract, the lumen of the distal part of the intestine enlarges to receive it

Motility of the Small Intestine Most of the time, the ileocecal sphincter

is constricted and closed Two mechanisms, one neural and one

hormonal , cause it to relax when ileal mobility increases and allow food residues to entry the cecum

Enhance activity of the stomach initiates the gastroileal reflex, a long reflex than enhances the force of segmentation in the ileum

Motility of the Small Intestine In addition, gastrin released by the

stomach increases the motility of the ileum and relaxes the ileocecal sphincter

Once the chyme has passes through, it exerts backward pressure that closes the valve’s flaps, preventing regurgitation into the ileum

Large Intestine

The large intestine frames the small intestine on three sides and extends from the ileocecal valve to the anus

Large Intestine Its diameter is greater than that of the

small intestine, but is less than half as long 1.5 meters

Its major function is to absorb water from indigestible food residues (delivered to it in fluid state) and eliminate them from the body as semisolid feces

Large Intestine

Over most of its length, the large intestine exhibits three features not seen elsewhere; teniae coli, haustra, epiploic appendages

Large Intestine

Teniae coli are three bands of smooth muscle which are the remnants of the smooth muscle layer

Large Intestine

The muscle tone of the teniae coli cause the wall of the large intestine to form pocketlike sacs called haustra

Large Intestine

Epiplocic appendages are small fat-filled pouches of visceral peritoneum that hang from its surface. Significance is not known

Large Intestine

The large intestine has the following subdivisions; cecum, appendix, colon, rectum, and anal canal

Large Intestine

The saclike cecum, or blind pouch, lies below the ileocecal valve is the first part of the large intestine

Large Intestine

Attached to the cecum is the blind, wormlike, vermiform appendix

Large Intestine The appendix contains masses of lymphoid

tissue, and as part of the MALT it plays an important role in body immunity

It has a significant structural problem in that its twisted tissue provides an ideal location for enteric bacteria to accumulate and multiply

Large Intestine

The colon has several distinct regions; ascending, transverse, and descending colon segments connected by flexures

Large Intestine

The ascending colon travels up the right side of the abdominal cavity to the level of the right kidney

Large Intestine

At the level of the kidney the colon makes a right-angle turn, the right colic, or hepatic flexure

Large Intestine

The transverse colon travels across the top of the abdominal cavity

Large Intestine

Directly anterior to the spleen, it bends downward to form the left colic or splenic flexure

Large Intestine

The descending colon descends down the left side of the abdominal cavity

Large Intestine

As the descending colon enters the pelvis it forms the S-shaped sigmoid colon

Large Intestine The transverse

and sigmoid portions of the colon are anchored to the posterior abdominal wall by mesentary sheets called mesocolons

Large Intestine

In the pelvis, at the level of the third sacral vertebra, the sigmoid colon joins the rectum, which is positioned anterior to the sacrum

Large Intestine The natural

orientation of the rectum allows for a number of pelvic organs to be examined digitally during a rectal exam

Large Intestine

The rectum has three lateral curves or bends represented internally are transverse folds called rectal valves

Large Intestine Rectal valves

separate feces from flatus, thus allowing gas to passed

Large Intestine The anal canal

lies entirely external to the abdominopelvic cavity

About 3 cm long the canal begins where the rectum penetrates the muscles of the pelvic floor

Large Intestine The anal canal

has two sphincters– External anal

sphincter– Internal anal

sphincter

Large Intestine The involuntary internal anal sphincter

is composed of smooth muscle The voluntary external anal sphincter is

composed of voluntary muscle These sphincters which act rather like

purse strings to open and close the anus, are ordinarily closed excepts during defecation

Large Intestine: Microscopic The wall of the large intestine differs in

several ways from that of the small intestine

The colon mucosa is simple columnar epithelium except in the anal canal

Because most food is absorbed before reaching the large intestine, there are no circular folds, no villi, and no cells that secrete digestive enzymes

Large Intestine: Microscopic Its mucosa is thicker, its abundant crypts

are deeper, and there are tremendous numbers of goblet cells in the crypts

Lubricating mucus produced by goblet cells eases the passage of feces and protects the intestinal wall from irritating acids and gases released by resident bacteria in the colon

Large Intestine: Microscopic The mucosa of

the anal canal is different from the rest of the colon, reflecting the greater abrasion that this region receives

Large Intestine: Microscopic The mucosa

hangs in long ridges or folds called anal columns and contains stratified squamous epithelium

Large Intestine The anal sinuses

are recesses between the anal columns which exude mucus when compressed by feces

This aids in the emptying of the canal

Large Intestine The horizontal

lines that parallels the inferior margin of the anal sinuses is called the pectinate line

The line separates areas of visceral and somatic sensory innervation

Pectinate line

Large Intestine: Microscopic The mucosa superior to the line is

innervated by visceral sensory fibers and so are relatively insensitive to pain

The are inferior to the pectinate line is innervated by somatic sensory fibers and is very sensitive to pain

Large Intestine: Microscopic Two superficial venous plexuses are

associated with the anal canal, one with the anal columns and the other with the anus itself

Where these veins (hemorhoidal) are inflamed, itchy varicosities called hemorrhoids result

Large Intestine: Microscopic In contrast to the more proximal regions

of the large intestine, teniae coli and haustra are absent in the rectum and anal canal

Consistent with its need to generate strong contractions to perform its expulsive role, the rectum’s muscularis muscle layers are complete and well developed

Bacterial Flora Although most bacteria entering the

cecum from the small intestine are dead having been killed by the action of lysozyme, defensins, HCL, and protein digesting enzymes

The bacteria that survive, together with the bacteria that enter the GI tract via the anus, constitute the bacterial flora of the large intestine

Bacterial Flora The bacterial flora colonize the colon and

ferment some of the indigestible carbo- hydrates (cellulose and others) releasing irritating acids and a mixture of gases– Dimethyl sulfide, H2, N2, CH4, and CO2

About 500 ml of gas is produced each day with much more when certain carbohydrate rich foods are eaten

The bacterial flora also synthesize B complex vitamins and most of vitamin K

Processes: Large Intestine What is finally delivered to the large

intestine contains few nutrients, but still has 12 to 24 hours more digestive system

Except for the small amount of digestion of residue by the enteric bacteria, no further food breakdown takes place in the large intestine

Processes: Large Intestine Although the large intestine harvests

vitamins made by the bacterial flora and reclaims most of the remaining water and some of the electrolytes (particularly sodium and chloride) absorption is not a major function of this organ

The primary concern of the large intestine are propulsive activities that force the fecal material toward the anus and then eliminate it from the body

Processes: Large Intestine While the large intestine is undeniably

essential for our comfort, it is not essential for life

Several different surgical procedures remove a part or all of the large intestine in order to save life

Motility: Large Intestine The large intestine musculature is

inactive much of the time, and when it is mobile, its contractions are sluggish and of short duration

The most frequent movements seen in the colon are haustral contractions, which are slow segmenting movements that occurs every 30 minutes or so

Motility: Large Intestine Haustral contractions reflect local controls

of smooth muscle within the walls of individual haustra

As a haustrum fills with food residue, the distension stimulates its muscle to contract, which propels the luminal contents into the next haustrum

These movements also mix the residue which aids in water absorption

Motility: Large Intestine Mass movements (mass peristalsis) are long,

slow-moving, but powerful contractile waves that move over large areas of the colon three or four times daily and force the contents toward the rectum

Typically these movements occur during or just after eating when the presence of food in the stomach activates the gastroileal reflex in the small intestine and the propulsive gastrocolic reflex in the colon

Motility: Large Intestine Bulk, or fiber, in the diet increases the

strength of colon contractions and softens the stool, allowing the colon to act more efficiently

Defecation The rectum is

usually empty, but when feces are forced into it by mass movements, stretching of the rectal walls initiates the defecation reflex

Defecation This is a spinal

cord mediated reflex that causes the walls of the sigmoid colon and the rectum to contract and the anal sphincters to relax

Defecation Distension or

stretch of the rectal walls triggers a depolarization of sensory (afferent) fibers which synapse with the spinal cord

Defecation Parasympathetic

motor (efferent) fibers, in turn, stimulate contraction of the rectal walls and relaxation of the internal anal sphincter

Defecation If it is convenient

to defecate, voluntary signals stimulate the relaxation of the external anal sphincter

Defecation As feces are forced into the anal canal,

impulses reach the brain allowing us to decide whether the external(voluntary) anal sphincter should remain open or closed

If defection is delayed, the reflex contractions end within a few seconds and the walls relax

With the next mass movement, the reflex is initiated again and again until one chooses to defecate