Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.

Chapter 15

The Digestive System

Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.


© 2013 by John Wiley & Sons, Inc. All rights reserved.

• The digestive system has two major parts– The gastrointestinal system or GI tract

• The GI tract begins at the oral cavity• Winds through the abdominal cavity• And ends at the anus

– A set of accessory organs • The major accessory organs include the

– Pancreas– Liver– Gallbladder

GI Tract


• The GI tract and accessory organs carry out five basic processes– Ingestion

• Foods and liquids are taken into the mouth and eaten– Mixing

• The smooth muscle in the walls of the GI tract mixes the food and sends it on its way through the tract

– Digestion• Food is broken down by both mechanical and chemical processes

– Absorption • The epithelial cells lining the GI tract absorb the digested food molecules

and pass them to the blood or lymph– Defecation

• Any substance not digested or absorbed, for whatever reason, is passed along to the end of the GI tract, the anus, and leaves the body

The GI Tract – “Structure”


• The structure of the GI tract is essentially the same along its entire length and is composed of four layers– Mucosa - innermost layer is composed of a mucous membrane

• Absorption, secretion, etc.– Submucosa - under the mucosa

• includes glands, nerves, and blood supply for the tract itself– Muscularis - enables the movement of substances lengthwise

• Mostly composed of one layer of longitudinal muscle above another layer of circular muscle

– Serosa - the outermost layer of the GI tract• These layers work in unison to create the peristaltic wave that propels

food through the tube

The GI Tract


The GI Tract – “Peristalsis”


Mechanical Digestion

• Mechanical digestion – Increases the efficiency of enzymes

in the stomach and small intestine – By creating small bits of food with a

great deal of surface area– Enzymes can carry out the process

of chemical digestion• Teeth function as

– Cutting tools (incisors)– Piercing and ripping utensils

(canines)– Grinding instruments (molars and

premolars)– Small bits of food are macerated

with saliva


The Tongue

• The tongue manipulates food into a bolus– Positions bolus at the back of the oral cavity so it can be swallowed– This muscle can move in almost any direction in the oral cavity

• On its surface, keratinized epithelium covers each papilla– Creating a rough texture to help move the slippery food into position where

the teeth can masticate it – Taste buds reside along the sides of these papillae

• The tongue also secretes watery mucus and a lipase – Lingual lipase

• A digestive enzyme, from sublingual salivary glands on its undersurface• Begins chemical digestion of lipids by breaking down triglycerides


The Tonsils

• At the back of the oral cavity– First line of defense against any microbes that may enter your mouth along

with food • When bacteria invade the oral cavity

– The tonsils swell as they attempt to destroy the pathogen through the action of specific immune tissues


MALT - Mucosa-Associated Lymphatic Tissue


• Starting with the tonsils, the mucosa of the GI tract contains a disease-prevention tissue called MALT – Mucosa-associated lymphatic tissue– MALT is also prevalent in the small intestine, large intestine, and appendix

• MALT consists of nodules of lymphatic tissue– They prevent pathogens from taking over the lumen of the digestive tract– Also are important for preserving homeostasis

• MALT tissues represent a large percentage of the entire immune system– Including about half of the body's total lymphocytes and macrophages– Without MALT, pathogens could grow within the digestive tract, penetrate the

epithelial lining, and cause serious internal infections

The Salivary Glands

• Located within the oral cavity – Secrete small amounts of watery saliva to moisten the oral mucosa – The thought and smell of food can increase saliva production– When food is in the mouth

• Excess saliva is needed to mix with the food and form the slippery bolus required for swallowing

• The major salivary glands are the – Parotid glands - located below and in front of the ears

• Produce watery saliva that includes some ions (sodium, potassium, chloride, bicarbonate, and phosphate) and organic substances

– Submandibular glands - under the tongue• The submandibular glands produce thicker, ropey saliva with similar ion

content but a larger concentration of mucus• Saliva contains lysozyme

– An enzyme complex that helps destroy bacteria in the oral cavity – Also contains salivary amylase

• A digestive enzyme that breaks carbohydrate polysaccharides into monosaccharides


Swallowing and Deglutition


• Swallowing or deglutition– Occurs as the bolus of macerated food is moved to the back of the throat– The tongue positions the bolus at the opening to the esophagus

• This is the last muscular movement you control until the food has worked its way to the other end of your GI tract– The tongue is composed of voluntary, consciously controlled skeletal muscle– The muscularis of the GI tract is smooth muscle, controlled by the autonomic

nervous system– The anal sphincter is again skeletal muscle

Swallowing Involves Three Stages


• Voluntary stage - food is consciously swallowed • Pharyngeal stage - the bolus involuntarily passes through the pharynx

– The epiglottis is closed against the larynx to allow the bolus to bypass the respiratory system and enter the esophagus

– The uvula covers the nasal opening and the larynx moves upward against the epiglottis

– The epiglottis covers the opening to the respiratory system, and the bolus slides back toward the esophagus instead of dropping into the respiratory system

• Esophageal stage - food moves through the esophagus into the stomach via peristalsis– The bolus is pushed along the esophagus in a controlled manner – The esophagus terminates at its lower end with a sphincter muscle

• The lower esophageal sphincter (LES) – At the base of the esophagus – Opens as the bolus touches it, dropping the bolus into the upper portion of

the stomach

Swallowing


The Stomach

• The stomach is a J-shaped organ that lies beneath the esophagus– The stomach has three layers of muscles

• circular, longitudinal, and oblique– The muscularis in the rest of the GI tract has only two sets of muscles

• the circular and longitudinal• The stomach is separated from the esophagus and the small intestine by

two sphincter muscles– The lower esophageal sphincter is the upper boundary of the stomach– The pyloric sphincter marks the lower end of the stomach

• The pyloric sphincter– The strongest sphincter muscle of the digestive tract– Opens to allow chyme to enter the small intestine only when chemically ready– This sphincter is so powerful that it can cause projectile vomiting in infants


The Stomach

• The walls of the stomach contain folds, or rugae– Permit expansion

somewhat like a deflated punching ball

• The function of the stomach is to – Churn and mix the bolus

with the acid environment of the stomach

– To begin protein digestion


The Stomach

• The pH of the stomach is only 2 – The high acidity breaks down large macromolecules– And destroys many microbes– May also harm the stomach lining

• The stomach also secretes enzymes that digest protein– The stomach produces a protective layer of thick, viscous, alkaline mucus– Protection against the acid and protein-digesting enzymes it produces


Digestion


• The walls of the stomach contain gastric pits which secret 2-3 quarts of gastric juice daily

• Composed of two cell types– Chief cells

• Secrete pepsinogen and gastric lipase• Pepsinogen is an inactive precursor of pepsin, the

enzyme that digests proteins– Parietal cells

• Produce hydrochloric acid and intrinsic factor• The hydrochloric acid causes the low pH of the

stomach– Also activates the pepsin and kills microbes

• The intrinsic factor is needed for the absorption of Vitamin B12– A micronutrient that helps produce red

blood cells

Digestion


• As the bolus of food reaches the stomach, small mixing waves are initiated– Waves occur every 15 seconds or so and help to break up the food

• The food may stay in the fundus of the stomach for as long as an hour before being moved into the body of the stomach

• With each wave– A small portion of the chyme is forced through the pyloric sphincter and into

the small intestine

Digestion Occurs in Three Phases in the Stomach

1.Cephalic phase In the first phase, thoughts of food and the feel of food in the oral cavity stimulate increased secretion from the gastric pits. The stomach begins to churn more actively in preparation for the incoming food.

2.Gastric phase When the bolus reaches the stomach, the second phase of gastric digestion begins. Here the stomach produces gastrin as well as continuing the production of pepsin and HCl. Gastrin aids in stimulation of the gastric pits, providing a feedback system that speeds digestion. Impulses from the stomach also go back to the brain, maintaining contact with the nervous system.

3.Intestinal phase In the final phase of gastric digestion, the chyme begins to leave through the pyloric sphincter. As the chyme leaves the stomach, gastrin production decreases, the impulses to the brain indicate a lessening of chyme, and the brain begins to slow the stimulation of the gastric pits. At the same time, hormones from the beginning portion of the small intestine initiate activation of the small intestine.


The Small Intestine

• Once in the small intestine– The chyme itself stimulates the release of hormones

• Chyme containing glucose and fatty acids causes the release of the hormones cholecystokinin (CCK) and secretin – CCK inhibits stomach emptying– Secretin decreases gastric secretions

• Both also affect the liver, pancreas, and gallbladder, the accessory organs of the gastrointestinal tract

• Their combined action holds the food in the stomach for a prolonged period, ensuring the food is sufficiently broken down

• Sometimes food in the stomach does not “agree” with the stomach because it contains bacteria or toxins that irritate the stomach lining– May cause vomiting– A mechanism to quickly rid the pathogens from the body


The Small Intestine


The Small Intestine

• This organ is the only portion of the GI tube where nutrients are taken into the cells– Nutrients are absorbed

• The small intestine has three regions– The duodenum – next to stomach– The jejunum– The ileum – next to large intestine

• The total surface area of the small intestine is about 260 square meters, or 2800 square feet


The Small Intestine

• The small intestine also has an abundance of MALT, in the form of Peyer’s patches– These nodules of lymphatic tissue are similar to the tonsils– Peyer’s patches are an important part of the immune system

• Protect the lumen of the digestive tract from bacterial invasion• Both mechanical and chemical digestion occur in the small intestine

– Protein digestion continues using trypsin, chymotrypsin, carboxypeptidase, and elastase• All secreted from the pancreas

– Protein digestion is completed on the exposed edges of the intestinal cells themselves, using aminopeptidase and dipeptidase


The Small Intestine – “Absorption”


Ulcers

• Ulcers are wounds in the mucosa of the GI tract

– Persistent and painful

– Resist healing

• Mostly caused by infection with Helicobacter pylori,

– A spiral bacterium that thrives in the highly acidic stomach

– Antibiotics now cure ulcers


Accessory Organs

• Along the length of the gastrointestinal tract, several accessory organs assist in digestion– The pancreas– The liver– The gallbladder


The Pancreas

• The pancreas functions as an exocrine gland in the digestive system– Producing and secreting enzymes that are released via the pancreatic duct– Almost all of the digestive enzymes that act in the small intestine are made in

the pancreas• Pancreatic juice also buffers the acidity of the chyme as it leaves the

stomach – Mixes with chyme immediately as it enters the duodenum– Neutralizing the acidic chyme and the duodenum

• The pancreas is also responsible for secreting hormones into the bloodstream– The pancreas makes insulin and glucagon– For regulating glucose metabolism and glucose homeostasis


The Liver

• The liver is the largest organ of the body– The lobules monitor blood collected from the small intestine, adding and

subtracting materials to maintain fluid homeostasis• The liver is served by a portal system

– This portal system gives the individual hepatocytes access to the blood coming from the small intestine

– Hepatocytes “cleanse the blood” before it reaches the heart• Removing toxins and storing excess nutrients, such as iron, and fat-

soluble vitamins such as A, D, and E• Cholesterol, plasma proteins, and blood lipids are manufactured in the

hepatocytes• The liver also monitors the glucose level in the blood

– When it exceeds 0.1%, hepatocytes remove and store the excess as glycogen


The Liver


The Gallbladder

• Bile is formed by the liver as a by-product of the breakdown of hemoglobin and cholesterol– It is stored in the gallbladder, under the right lobe of the liver

• The concentrated bile salts act as an emulsifier or biological detergent, breaking larger fat globules into smaller ones– Bile aids in fat digestion

• By increasing the surface area of the fats through emulsification• Bile disperses the fat into small globules and prevents them from flowing

back into a larger fat complex• The greater surface area of the dispersed lipid globules allows for more

efficient fat digestion by pancreatic lipase • Another form of mechanical digestion


The Large Intestine – “Colon”


• The overall function of the large intestine is to reabsorb the water that was added to the chyme to begin digestion– Along with the water, the large intestine absorbs many dissolved minerals and

some vitamins

The Large Intestine – “Colon”

• The four divisions of the colon describe the direction of flow within them

- The ascending colon • Runs up the right side of the abdominal cavity

- The transverse colon • Cuts across the top of the abdominal cavity, underneath the stomach

- The descending colon • On the left side of the abdominal cavity the colon turns back down

- The sigmoid colon • At the lower left of the abdominal cavity, the colon makes an S turn to

wind up in the center of the body• This portion of the colon is where feces may sit for long periods of time

before moving out the rectum• Polyps can develop in the colon as feces rest against the mucosa


The Colon• The last 20 centimeters of the colon are the rectum and anus

– Chyme remains in the colon for 3–10 hours, during which time it becomes progressively drier - compacted chyme is called feces

• When feces enter the upper portion of the rectum– Triggers the opening of the internal anal sphincter, a smooth muscle

• The feces move into the rectum and press against the external anal sphincter – This triggers defecation, a skeletal muscle action– Material moves through the large intestine in mass movements, created using

a peristaltic wave• Diarrhea results from an irritation of the colon

– Chyme moves through colon too quickly for water or minerals to be absorbed– Prolonged diarrhea can lead to dehydration


Mechanical Digestion Occurs Mainly in the Mouth


• Once the bolus of food is passed to the esophagus– A small amount of mechanical digestion occurs in the stomach, as it rolls and

churns the food into chyme• The chyme then moves through the pyloric sphincter into the duodenum

– Where large droplets of fat are emulsified via bile• The action of bile is a form of mechanical digestion

– By emulsifying, breaking larger fat droplets into smaller ones without altering the chemical structure of the fats

• At this point, the chyme is ready for enzymatic degradation, and mechanical digestion is finished

Chemical Digestion Uses Enzymes

• Enzymes are functional proteins that work best under a set of optimal conditions of pH, temperature, substrate, and product levels – All enzymes are specific for a particular substrate and catalyze only one

reaction– Enzyme names are usually built from the name of the substrate, followed by

the suffix “-ase”


Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.

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Transcript of Copyright © 2013 by John Wiley & Sons, Inc. All rights reserved.