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Human Anatomy, Second EditionMcKinley & O'Loughlin

Chapter 26 Lecture Outline: Digestive System

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Overall Function: Disassembly Line hydrolysisPolymers---------------->monomers < --------------- dehydration synthesis

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General Structure and Functions of the Digestive System The GI tract organs:

oral cavity pharynx esophagus stomach small intestine large intestine

A continuous tube about 30 feet (9–10 meters) from the mouth to the anus (longer in dead than living).

Smooth muscle in most of the GI tract wall pushes materials from one end to the other.

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General Structure and Functions of the Digestive System Accessory digestive organs:

often develop as outgrowths from the tube and are connected to the GI tract

assist the GI tract in the digestion of food.

teeth, tongue, salivary glands, liver, gallbladder, and pancreas

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Digestive System Functions Ingestion Digestion

mechanical digestion chemical digestion

Propulsion peristalsis segmentation (mainly churns and mixes)

Secretion Absorption Elimination of wastes (defecation)

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Oral Cavity (mouth) Entrance to the GI tract. Initial site of mechanical digestion (via

mastication) and chemical digestion (via enzymes in saliva: salivary amylase and lipase).

Bounded by the teeth and lips, the oropharynx, the hard and soft palates, and the tongue

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Palate Anterior two-thirds of the palate is the

bony hard palate), while the posterior one-third is the soft palate, primarily skeletal muscle).

Extending inferiorly from the posterior part of the soft palate is the uvula.

When swallowing, the soft palate and the uvula elevate to close off the opening of the nasopharynx.

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Tongue An accessory digestive organ formed

from skeletal muscle and covered with lightly keratinized stratified squamous epithelium.

Manipulates and mixes ingested materials during chewing and

Helps compress these materials into a bolus.

Performs important functions in swallowing.

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Salivary Glands Collectively produce and secrete saliva.

Most is produced during mealtime, but smaller amounts are produced continuously.

Water makes up 99% of the volume of saliva.

Also contains a mixture of other components.

Three pairs of large, multicellular salivary glands

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The Parotid Glands Largest salivary glands. Near the ear, partially overlying the

masseter muscle. Produce about 25–30% of the saliva. Site of mumps

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The Submandibular Glands Inferior to the body of the mandible. Produce most of the saliva (about

60–70%).

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The Sublingual Glands Inferior to the tongue and internal to the

oral cavity mucosa. Contribute only about 3–5% of the total

saliva.

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Functions of Saliva Moistens food and helps turn it into a semisolid

bolus that is more easily swallowed. Moistens and cleanses the oral cavity

structures. First step in chemical digestion occurs

when amylase in saliva begins to break down carbohydrates. Also a lingual lipase.

Contains antibodies and antibacterial lysozyme that help inhibit bacterial growth.

Watery medium into which food molecules are dissolved so taste receptors can be stimulated.

Produce about 1-1 1/2 liter/day.

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Teeth Responsible for mastication (chewing), the

first part of the mechanical digestion process. Increases the surface area and, therefore, increases the efficiency of digestive enzymes. Incisors and canines cut and tear Premolars and molars crush and grind.

A tooth has an exposed crown, a constricted neck, and one or more roots that anchor it the jaw.

Roots of the teeth fit tightly into dental alveoli.

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Teeth Two sets of teeth develop and erupt

during a normal lifetime. 20 deciduous teeth, “milk teeth,” erupt

between 6 months and 30 months after birth. Replaced by 32 permanent teeth. The last teeth to erupt are the third molars,

“wisdom teeth,” in the late teens or early 20’s. May be absent.

Often the jaw lacks space and they may either emerge only partially or grow at an angle and become impacted.

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About 5 inches long Nasopharynx, oropharynx, and

laryngopharynx

Pharynx (Throat)

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Peritoneum and its Extensions Greater omentum (do not cut away!) Lesser omentum Falciform ligament Round ligament Mesentery Mesocolon Parietal peritoneum Visceral peritoneum

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General Histology of GI Organs From the esophagus through the large intestine is a tube of four

concentric layers called tunics: the mucosa

superficial epithelium lamina propria: areolar CT muscularis mucosa

the submucosa dense irregular CT lymphatic tissue in some areas mucin-secreting glands many large blood vessels and lymphatic vessels submucosal nerve plexus (Meissner plexus)

the muscularis two smooth muscle layers typically except in the esophagus and stomach myenteric nerve plexus (Auerbach plexus)

the adventitia or serosa (covered by visceral peritoneum)

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Tunics of the Abdominal GI Tract See figure and model.

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See figure.

Phases of Swallowing

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Esophagus (Gullet) About 10 inches long Muscular tube: 1/3 skeletal, 1/3

mixed, and 1/3 smooth 2 sphincters regulate food in and out Mucus lubricates No digestion or absorption! Esophageal hiatus through the diaphragm Hiatal hernia

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Reflux esophagitis is when acidic chyme refluxes into the esophagus, “heartburn”

Poorly protected so it becomes inflamed and irritated Overweight, large meals, smoking, lying down, and hiatal

hernias contribute GERD

Chronic reflux esophagitis can lead to scar tissue and narrowing of lumen

Advanced cases may get Barrett esophagus where stratified squamous becomes columnar secretory epithelium and increases the risk of developing cancer

May also lead to ulcers or cancerous growths Treat with medications to limit or neutralize stomach acid

Reflux Esophagitis and Gastroesophageal Reflux Disease (GERD)

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Stratified squamous in esophagus changes to simple columnar in stomach and stays simple columnar until the end of the GI tract where it becomes stratified squamous again.

Three muscular layers produce mixing waves Rugae: deep folds in mucosa and submucosa Openings in mucosal folds are gastric pits

Stomach

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Storage: 2-6 hours Mixing and propulsion: break down bolus into

chyme Protein digestion begins with pepsin HCl kills microbes, pH <2, denatures proteins,

and important in conversion of inactive pepsinogen into active pepsin

Little absorption: alcohol and aspirin (they damage stomach)

Stomach Functions

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Lack of vitamin B12 leads to pernicious anemia.

Gastrectomy

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Mucus (bile can produce gastritis)

Tight junctions Rapid turnover (every three

days) Pepsinogen is inactive until

activated by HCl

Stomach Protected by

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Peptic Ulcers Chronic, solitary erosion of the lining of the stomach

(gastric ulcer) or small intestine (duodenal ulcer) Symptoms: gnawing, burning pain that may be worse

after a meal; nausea; vomiting; and extreme belching Irritation of the gastric mucosa (gastritis) in many

cases (NSAIDS), but most cases related to Helicobacter pylori (present in over 70% of gastric ulcers and over 90% of duodenal ulcers) that break down the mucus and WBCs that destroy mucous neck cells

Treatment: antibiotics and reducing and/or neutralizing the stomach acid

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Pyloric Sphincter Acts as a “gatekeeper” allowing

only small amounts of chyme to enter duodenum at a time

The chyme contains partially digested carbohydrates, proteins, and lipids.

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Small Intestine Finishes the chemical digestion process and is

responsible for absorbing most of the nutrients and most of the water.

Chemical and mechanical digestion depend also on accessory structures: pancreas, liver, and gall bladder.

Ingested nutrients spend at least 12 hours in the small intestine as chemical digestion and absorption are completed.

Coiled, thin-walled tube about 6 meters (20 feet in dead and 10 feet in living) in length.

Extends from the pylorus of the stomach to the ileocecal valve between the ileum and the cecum of the large intestine, and thus occupies a significant portion of the abdominal cavity.

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Small Intestine The duodenum is retroperitoneal, about 25

centimeters (10 inches) long and originates at the pyloric sphincter.

The jejunum is about 2.5 meters (7.5 feet) primary region for chemical digestion and nutrient

absorption The ileum is about 3.6 meters (10.8 feet)

terminates at the ileocecal valve, a sphincter that controls the entry of materials into the large intestine.

Two layers to the muscularis Serosa surrounds except for major part of the

duodenum.

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Small Intestine Almost all the digestion and absorption

occur in the small intestine! 90% of absorption in SI 10% of absorption in stomach and LI

Large surface area (600x more than if just a tube) 10 feet long Plicae circulares (circular folds about 10

mm high) spiral through Villi (0.5 - 1 mm high) Microvilli (brush border)

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Small Intestine Mucosal Cavities lined with glandular epithelium

Intestinal glands (crypts of Lieberkuhn) produce intestinal juice (1-2 liters/day)

Paneth cells produce lysozyme and also do phagocytosis Goblet cells produce mucus

Most of the digestion by the SI itself is by brush border enzymes on the surface or in the lumen. Help digest proteins, carbohydrates, and nucleotides.

Submucosa of duodenum has duodenal glands (Brunner’s glands) that produce an alkaline mucus

A lot of MALT in the lamina propria: many solitary lymphatic nodules and Peyer’s patches in ileum

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Absorption in SI Active transport mainly into villi and then

diffusion into capillaries then nutrients mainly go to the liver

Lipids diffuse into capillaries and lacteals which lead to cisterna chyli, etc. Fat soluble vitamins, ADEK, enter through lipids

Undigestibles, some water and a lot of bacteria go through the ileocecal valve to the cecum of the large intestine.

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Large Intestine About 1.5 meters (5 feet) long and 6.5 centimeters (2.5

inches) wide. Absorbs most of the water and electrolytes from the

remaining digested material. Absorbs a very small percentage of nutrients still remaining

in the digested material. Vitamin manufacture here by bacteria Watery material that first enters the large intestine soon

solidifies and becomes feces. Stores this fecal material 3-10 hours until the body is ready

to defecate. Composed of four segments:

the cecum, colon, rectum, anal canal Attached to the posterior abdominal wall by the mesocolon.

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Movements in the Large Intestine Peristalsis - slower than before Haustral churning: haustra fill

and then contract Mass peristalsis: strong

movements 3-4x/day during or after meal

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No circular folds and no villi Intestinal glands

Microvilli present in absorptive cells

Goblet cells secrete mucus

Histology of LI

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Digestion in LI Chemical digestion is bacterial by E. coli not

enzymatic Typically, feces has about 25% bacteria. Bacteria in

the SI and LI are in the lumen! Bacteria ferment carbohydrates and produce gases

(flatus). Bacteria synthesize B vitamins and vitamin K. Vitamins are absorbed here.

Amino acids are broken down to small substances that have an odor. Some of these go to the liver for detoxification and leave through the urine

Bilirubin may be converted to urobilinogen and leave through the urine or to stercobilin and produce the brown color of feces

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Defecation Mass peristalsis from the sigmoid

colon initiates defecation reflex (parasympathetic)

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Appendicitis Perforation allows bacteria to enter

the body and cause peritonitis, massive infection Pain first over umbilicus, then in right lower

quadrant Shock Renal failure Respiratory insufficiency Liver failure

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Second most common type of cancer in US especially found in sigmoid colon, and distal descending colon

Most arise from polyps (most polyps benign) Risk factors: low fiber diet, family history, ulcerative colitis,

and increased age. Symptoms: often none at first, bleeding, change in bowel

habits. Later, pain, fatigue, weight loss, and anemia. Treatment: surgery, chemo, radiation. Screening at 50 or earlier if at risk:

Fecal occult blood test yearly Sigmoidoscopy every 5 years Colonoscopy every 10 years

Colorectal Cancer

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Accessory Digestive Organs The liver, about 3# (heaviest gland)

composed of four incompletely separated lobes

Right lobe Left lobe Caudate lobe Quadrate lobe

supported by two ligaments Falciform ligament (attaches to peritoneum) Round ligament (ligamentum teres) from umb. vein

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Functions of The Liver Produces bile, an emulsifier, not an enzyme.

a greenish fluid that breaks down fats into small droplets to assist in their chemical digestion

Detoxifies drugs, metabolites, and poisons. Stores excess nutrients and vitamins and release

them when they are needed (a key metabolic organ). Synthesizes blood plasma proteins such as albumins,

globulins, and proteins required for blood clotting. Phagocytizes debris in the blood. Helps break down and recycle components of aged

erythrocytes and damaged or worn-out formed elements.

Helps activate vitamin D along with the skin and kidneys to its active form.

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Yellowish color to skin and mucous membranes

Three major types Increase production of bilirubin Liver disease Blockage from stones or cancer in the

area Also neonatal (physiological) jaundice

that disappears as the liver matures

Jaundice

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Cirrhosis of the Liver Destruction of hepatocytes and their replacement

with fibrous scar tissue which compresses the blood vessels and bile ducts leading to hepatic portal hypertension and obstruction to the flow of bile

Caused by chronic injury to the hepatocytes from chronic alcoholism, liver disease, or certain drugs or toxins.

Hepatitis A: virus enters through fecal/oral route Hepatitis B: serum hepatitis (can also cause cancer) Hepatitis C: blood transfusions, IV drug use, sex

Early stages may be asymptomatic. Later, fatigue, weight loss, and nausea, and maybe pain. Liver may be small and hard. Biopsy to confirm diagnosis.

Irreversible but may halt progression,.

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Cirrhosis of the Liver Complications if advanced:

Jaundice (yellowing of skin and sclerae from bilirubin)

Edema and ascites Intense itching Accumulation of toxins in blood and brain Portal hypertension can lead to dilated veins

in the inferior esophagus (esophageal varices) End-stage liver cirrhosis requires a liver

transplant

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Accessory Digestive Organs Gallbladder (about 3-4 inches long)

concentrates bile produced by the liver and stores this concentrate until it is needed for digestion

cystic duct connects the gallbladder to the common bile duct

can hold approximately 50 milliliters of concentrated bile

simple columnar with no submucosa has rugae hormones cause its smooth muscle to contract and

release bile into the cystic duct.

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Gallstones Concentration of materials: cholesterol or

calcium and bile salts Risk factors: obesity, age, female sex

hormones, Caucasian, lack of physical activity Usually asymptomatic until they become

lodged in the cystic duct, then severe pain in the r. hypochondriac region or r. shoulder, nausea and vomiting, indigestion and bloating may occur.

Treatment: removal of gall bladder (cholecystectomy)

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Accessory Digestive Organs The biliary apparatus.

network of thin ducts that carry bile from the liver and gallbladder to the duodenum

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Accessory Digestive Organs Pancreas

mixed gland: both endocrine and exocrine functions

Secretes digestive enzymes, collectively called pancreatic juice (1 1/2 liters/day), into the duodenum. 99% of the pancreas are are exocrine acinar

cells Digests carbohydrates, fats, proteins, and

nucleic acids Bicarbonate ions buffer HCl from stomach Under hormonal and neural control

Retroperitoneal

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Intestinal Disorders

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Celiac Disease Autoimmune disease of SI mucosa

where villi are attacked. Gluten sensitive

Gluten found in wheat, rye, and barley Symptoms like many other GI problems Must watch diet and avoid gluten

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Inflammatory Bowel Disease IBD: selective regions of the bowel become

inflamed. Two autoimmune disorders. Crohn disease

Young adults Periodic intense abdominal cramping and

diarrhea Usually in the distal ileum. Entire thickness of

the wall is involved. Linear ulcerations of the mucosa are frequent

and in some scarring may lead to bowel obstruction. Some may develop selective malabsorption of some vitamins.

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Inflammatory Bowel Disease Ulcerative colitis

Similar age distribution and symptoms, but only colon involved

Rectum and descending colon show first signs of inflammation and most severe effects usually

Inflammation confined to mucosa If patient has UC for more than 10 years, then chances

for colon cancer are increased 20-30x. Need careful monitoring.

Treatment for IBD Complex and multidisciplinary approach usually. Anti-inflammatory drugs, stress reduction, and maybe

nutritional supplementation. Surgery may be necessary.

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More common but not IBD, one in five have it. Cause and cure not known. Control by decreasing stress, changing one’s diet, and using certain medications.

Irritable Bowel Syndrome

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Other GI Problems Dental caries and periodontal

disease Eating disorders Diverticulitis Obstruction Diarrhea Constipation