Digestive System Anatomy, Physiology, & Disease

Chapter 13: The Digestive System

Learning Outcomes

• 13.1 Use medical terminology related to the digestive system.
• 13.2 Differentiate between mechanical digestion and chemical digestion.
• 13.3 Describe the digestive anatomy of the oral cavity.
• 13.4 Explain the physiology of mechanical and chemical digestion in the mouth.
• 13.5 Describe the digestive anatomy from the mouth to the stomach.
• 13.6 Explain how materials move from the mouth to the stomach.
• 13.7 Describe the digestive anatomy of the stomach.
• 13.8 Explain the physiology of mechanical and chemical digestion in the stomach.
• 13.9 Explain the feedback mechanism of how food moves from the stomach to the small intestine.
• 13.10 Describe the anatomy of the digestive accessory organs connected to the duodenum by ducts.
• 13.11 Describe the digestive anatomy of the small intestine.
• 13.12 Explain the physiology of chemical digestion in the duodenum, including the hormones and digestive secretions involved.
• 13.13 Explain how nutrients are absorbed in the small intestine.
• 13.14 Describe the anatomy of the large intestine.
• 13.15 Explain the physiology of the large intestine in terms of absorption, preparation of feces, and defecation.
• 13.16 Summarize the types of nutrients absorbed by the digestive system from the diet.
• 13.17 Trace the circulation of the nutrients once they have been absorbed.
• 13.18 Explain the control of digestion.
• 13.19 Summarize the functions of the digestive system.
• 13.20 Summarize the effects of aging on the digestive system.
• 13.21 Describe common diagnostic tests used for digestive system disorders.
• 13.22 Describe digestive system disorders and relate abnormal function to pathology.

Word Roots and Combining Forms

• chol/e: gall, bile
• col/o: colon
• cyst/o: bladder, sac
• duoden/o: duodenum
• emet/o: vomit
• enter/o: intestine
• esophag/o: esophagus
• gastr/o: stomach
• gingiv/o: gums
• gloss/o: tongue
• hepat/o: liver
• peps/o: digestion
• rect/o: rectum
• sigmoid/o: sigmoid colon

Overview of the Digestive System

• Hollow tube structures form the alimentary canal.
• Accessory structures include:
  • Teeth
  • Tongue
  • Salivary glands
  • Liver
  • Gallbladder
  • Bile ducts
  • Pancreas
• The digestive system and accessory structures include:
  1. The mouth
  2. Oral cavity
  3. Tongue
  4. Pharynx
  5. Esophagus
  6. Stomach
  7. Gallbladder
  8. Liver
  9. Pancreas
  10. Small intestine
  11. Large intestine
  12. Rectum
  13. Anus
• Measurements:
  • Mouth to duodenum: 0.5 meters
  • Small intestine: 5.5 to 6 meters
  • Large intestine to anus: 1.5 meters
• Nutrients are required for normal body function:
  • Carbohydrates, proteins, lipids, vitamins, minerals
  • Derived from food
  • Food is too big to be directly passed into blood
• Goals of the digestive system:
  • Digest food into smaller molecules
  • Absorb smaller molecules into the blood

Mechanical Digestion

• Physical breakdown of food into smaller pieces.
• Forms a greater surface area for contact with digestive enzymes.

Chemical Digestion

• Splitting of complex, nonabsorbable food molecules into small, absorbable nutrient molecules by hydrolysis.
• Enzymes speed up the reaction and enable digestion to occur.

Anatomy and Physiology of the Digestive System

Anatomy in the Mouth

• Oral Cavity:
  • Hard palate – bony roof
  • Soft palate - soft tissue roof
  • Uvula
  • Lined by stratified squamous epithelial tissue
• Deciduous teeth are replaced by permanent teeth.
  • Alveolus
  • Periodontal ligaments
  • Tooth structure:
    • Crown
    • Enamel
    • Dentin
    • Root
    • Pulp cavity
• The purpose of the tongue is to manipulate what is ingested and to provide the sense of taste.
• Salivary glands produce saliva:
  • Parotid glands
  • Submandibular glands
  • Sublingual glands
  • Enzymes amylase and lingual lipase, along with mucus, lysozymes, and antibodies

Physiology of Digestion in the Mouth

• Mechanical and chemical digestion takes place in the mouth.
• The masseter and temporalis muscles move the jaw for mastication.
• Amylase in saliva partially digests carbohydrates.
• Mucus mixes with the food to make it easier to swallow.
• The bite of food is called a bolus before it is swallowed.

Anatomy from the Mouth to the Stomach

• Pharynx is composed of:
  1. Nasopharynx
  2. Oropharynx
  3. Laryngopharynx
• Epiglottis: elastic cartilage that guards the opening of the larynx
• The esophagus is a tube that goes through the diaphragm to connect with the stomach.

Physiology of Digestion from the Mouth to the Stomach

• Deglutition (swallowing) involves four cranial nerves.
• The epiglottis closes off the glottis so that the bolus moves to the esophagus.
• Peristalsis moves the bolus through the esophagus.

Anatomy of the Stomach

• The stomach has three layers of smooth muscle in its walls, each oriented in a different direction.
• The lining of the stomach has rugae for more surface area to accommodate gastric pits that lead to gastric glands.
  • Esophagus
  • Cardia
  • Fundus
  • Body
  • Pylorus
  • Duodenum
• Gastric pits and gastric glands are composed of five types of cells:
  • Mucous cells produce alkaline mucus.
  • Endocrine cells produce gastrin.
  • Parietal cells produce hydrochloric acid and intrinsic factor.
  • Chief cells produce pepsinogen and gastric lipase.
  • Regenerative cells are stem cells to replace all other cells.

Physiology of Digestion in the Stomach

• During swallowing, the medulla oblongata sends signals to the stomach, telling it to relax.
• The cardiac sphincter opens to allow the bolus to enter.
• Stretching of the stomach walls starts peristaltic contractions.
• The pyloric sphincter remains closed until the pH of the stomach contents reaches 2.
• Hydrochloric acid changes pepsinogen to pepsin so that proteins are partially digested.
• Hydrochloric acid activates lingual lipase, which partially digests lipids along with gastric lipase.
• Intrinsic factor binds to vitamin B12 so that it can be absorbed later.
• Once gastric secretions are mixed with the bolus, it is called chyme.

Negative Feedback: Moving Food to Small Intestine

• pH of chyme falls, stomach pH approaches 2
• Endocrine cells stop producing acids
• Causes pyloric sphincter to open and chyme to leave the stomach into the duodenum

Peritoneum

• Membrane that lines the abdominopelvic cavity
  • Parietal peritoneum
  • Visceral peritoneum
  • Mesentery
  • Mesocolon
  • Greater omentum
  • Lesser omentum
  • Greater sac
  • Lesser sac
  • Cul De Sac

Anatomy of Digestive Accessory Structures

Liver

• The liver’s four lobes are arranged in hepatic lobules.
• Hepatocytes produce bile that contains bile acids and lecithin, both of which aid in the chemical digestion by emulsifying lipids.
• Bile is released into hepatic ductules leading to the hepatic duct.
• Liver lobes are divided into hepatic lobules (the functional units of the liver).
• Lobules consist of plates of hepatic cells radiating out from a central vein.
• Hepatic sinusoids run between plates of cells.
• Oxygen-poor blood from the digestive tract is transported to the liver through the hepatic portal vein.
• It mixes with oxygen-rich blood from the hepatic artery.
• This blood filters through the hepatic sinusoids.
• Hepatocytes adjust nutrient concentrations, as the nutrients from the digestive tract flow by.
• Bile produced by hepatocytes leaves the liver through bile ducts.

Liver Functions

1. Produces glycogen from glucose
2. Breaks down glycogen into glucose
3. Converts non-carbohydrates to glucose
4. Oxidizes fatty acids
5. Synthesizes lipoproteins, phospholipids, and cholesterol
6. Converts excess carbohydrates and proteins into fats
7. Deaminates amino acids
8. Forms urea
9. Synthesizes plasma proteins
10. Converts some amino acids to other amino acids
11. Stores glycogen, iron, and vitamins A, D, and B12
12. Phagocytizes damaged RBCs and foreign substances/antigens
13. Acts as blood reservoir
14. Detoxification of the blood, removing alcohol and certain drugs
15. Role in digestion is to secrete bile

Gallbladder

• The common bile duct is a tube common to the hepatic duct, the cystic duct, and the pancreatic duct.
• The hepatopancreatic sphincter controls the opening of the common bile duct to the duodenum.
• The gallbladder collects the overflow of bile from the common bile duct and concentrates it.

Pancreas

• The pancreas lies in the middle to the upper abdominal cavity, towards the back. It is about 12” long and tapers from right to left.
• The pancreas secretes bicarbonate ions and enzymes for the digestion of carbohydrates, lipids, and proteins.
• Anatomy of the Pancreas:
  • Relations:
    • Retroperitoneal
    • Stomach
    • Duodenum
    • Spleen
    • Kidney
    • Superior mesenteric vessels
    • Posterior abdominal wall
  • Rich blood supply:
    1. Hepatic A
    2. Gastroduodenal A
    3. Pancreaticoduodenal A
    4. Superior mesenteric A
    5. Splenic A

Anatomy of the Small Intestine

• The small intestine is composed of the duodenum, the jejunum, and the ileum.
• All parts of the small intestine have smooth muscle in their walls and are lined by villi.
• Endocrine cells of the duodenum secrete secretin and cholecystokinin.
• The ileocecal valve controls the movement of materials from the small intestine to the colon.
• Tiny projections of the mucosa, intestinal villi, greatly increase surface area for absorption of digestive end products
• Each villus contains blood vessels, a lacteal (lymphatic capillary), and nerve fibers
• Free surfaces of epithelial cells contain microvilli, which also increase surface area for absorption

Physiology of Digestion in the Small Intestine

• Secretin is released from endocrine cells of the duodenum in response to the acidic chyme.
• Secretin tells the pancreas to release bicarbonate ions to neutralize the chyme in the duodenum.
• Cholecystokinin is secreted by endocrine cells in the duodenum in response to the presence of lipids.
• Cholecystokinin targets the gallbladder (telling it to release bile) and the hepatopancreatic duct (telling it to relax).
• The release of bicarbonate ions from the pancreas carries the digestive enzymes through the pancreatic duct to the duodenum, where all further chemical digestion is completed.
• Segmentation ensures that all the contents of the small intestine come in contact with villi for absorption.
• Peristalsis further moves the contents through the jejunum and ileum to the ileocecal valve.

Absorption of Nutrients in the Small Intestine

• Monosaccharides and amino acids are absorbed through the epithelium of the villi into capillaries by facilitated diffusion.
• Fatty acids and glycerides are absorbed across the epithelial membranes of the villi by diffusion, coated with proteins, and exocytosed to lacteals.

Anatomy of the Large Intestine

• The colon is composed of the cecum, the ascending colon, the transverse colon, the descending colon, the sigmoid colon, and the rectum.
• The anus contains two sphincter muscles: the smooth muscle internal anal sphincter, controlled by the autonomic nervous system, and the skeletal muscle external anal sphincter, controlled by the somatic nervous system.

Physiology of Digestion in the Large Intestine

• The large intestine absorbs water, compacts materials to form feces, and then stores the feces until they are removed through defecation.
• Bacteria living in the large intestine produce vitamin K and flatus.
• Stretching of the stomach and duodenum causes a mass movement of fecal material from the transverse colon to the rectum.
• Stretch receptors in the rectal walls initiate the defecation reflex.
• Defecation happens voluntarily when the external anal sphincter is relaxed.

Types of Absorbed Nutrients

• Proteins
• Carbohydrates
• Lipids
• Vitamins
• Minerals
• Water

Circulation of Absorbed Nutrients

• The hepatic portal vein drains nutrient-rich blood from the capillaries in the villi and carries it to the capillary beds in the liver.
• The fatty acids and glycerides absorbed into lacteals in the villi join the bloodstream at the subclavian veins and eventually reach the liver through the hepatic artery.
• The splenic vein, inferior mesenteric vein, and superior mesenteric vein enters the hepatic portal vein.
• This vein enters the liver and passes through the liver and enters the IVC.

Nutrient Absorption Mechanism

• Monosaccharide: Facilitated diffusion and active transport into blood capillaries
• Amino acids: Active transport into blood capillaries
• Fatty acids and glycerol:
  • Facilitated diffusion of glycerol; diffusion of fatty acids into cells or blood capillaries
  • Most fatty acids are resynthesized into fats and incorporated in chylomicrons into lymph in lacteals
  • Some fatty acids with relatively short carbon chains are absorbed without being changed back into fats into blood capillaries
• Electrolytes: Diffusion and active transport into blood capillaries
• Water: Osmosis into blood capillaries

Control of Digestion

• The autonomic nervous system controls digestion.
• Parasympathetic fibers of the vagus nerve stimulate digestion.
• Sympathetic neurons from the celiac ganglion suppress digestion in part by diverting blood to skeletal muscles and the heart.

Functions of the Digestive System

• Ingestion
• Digestion
• Absorption
• Defecation

Effects of Aging on the Digestive System

• Tooth enamel thins, and the gingiva recede.
• The lining of the stomach atrophies.
• The liver may metabolize drugs differently.
• Movement of material through the large intestine slows with age.

Diagnostic Tests

• A barium swallow/upper GI series is an X-ray test used to evaluate the upper GI tract, which includes the esophagus, stomach, and small intestine.
• A colonoscopy is a procedure in which a lighted colonoscope is used to visualize the colon.
• Computerized tomography (CT) is an imaging technique to visualize internal structures.
• A fecal occult blood test is a noninvasive procedure that detects blood in stool.
• Hepatic screening is a collection of several tests used to determine whether the liver is functioning properly.
• Laparoscopy is a procedure in which a lighted laparoscope is used to visualize, collect biopsies from or perform surgical procedures in the abdomen or pelvic region.
• Magnetic resonance imaging (MRI) is a imaging technique to visualize soft tissue internal structures.
• Proctoscopy is a procedure in which a lighted endoscope is used to visualize the rectum.
• Sigmoidoscopy is a procedure in which a lighted endoscope is used to visualize the lower colon and rectum.
• A stool culture is procedure that involves collecting a stool sample and performing various tests to detect the presence of disease-causing pathogens.
• An ultrasound is an imaging technique in which sound waves are used to visualize internal structures.

Digestive System Disorders

• Leukoplakia:
  • White patches that occur on the surface of the tongue, inside the mouth, or on the inside surfaces of the cheek
  • Usually from contact with rough surfaces like dentures, tobacco products, or teeth with rough surfaces
• Gastroenteritis: Inflammation of the gastrointestinal tract caused by a bacterial, viral, or parasitic infection.
• Diverticular Disease:
  • Small pouches (diverticula) in the lining of the large intestine as it ages
  • A person with multiple diverticula has a condition known as diverticulosis.
• Abdominal Hernias: Protrusions of the contents in the abdomen through a weak portion in the abdominal wall
  • Inguinal hernias—protrusions into the groin
  • Umbilical hernias—through the umbilicus where the umbilical cord was once attached
  • Incisional hernias—protrusions through an incision from past abdominal surgery
  • Hiatus Hernia —protrusions through an incision from past abdominal surgery
• Irritable Bowel Syndrome:
  • Abdominal pain and discomfort, a change in the frequency of bowel movements, and a change in the consistency of stool
  • Common, mainly in young adults or middle-aged women
  • Frustration, anxiety
• Crohn’s Disease: Autoimmune inflammatory bowel disease that causes chronic inflammation along the gastrointestinal tract
  • Usually associated with the intestines
• Ulcerative Colitis:
  • Starts in the sigmoid colon
  • Affects the Colon and rectum
• Peptic Ulcers: Erosions of the Lining of the Digestive Tract
  • Esophageal ulcers—may happen in the lower esophagus if there is reflux of gastric juices through the cardiac sphincter
  • Gastric ulcers—in the stomach, usually from bacteria
  • Duodenal ulcers—the most common; caused when the chyme entering the duodenum is not sufficiently neutralized
• Pyloric Stenosis: Stomach normal emptying time is 2-6 hours.
  • Congenital
  • Boys
  • Surgery
• Intussusception: When a portion of the intestines folds back into itself, similar to a telescope, resulting in obstruction of the intestines and possible ischemia
• Volvulus: twisting of sigmoid
  • Congenital
  • Foreign body
• Ileus: no peristalsis (paralytic ileus).
  • After surgery.
• Cirrhosis:
  • Formation of scar tissue in the liver
  • Scar tissue will eventually block blood flow to parts of the liver and interfere with the liver’s ability to function properly.
  • Two main causes of cirrhosis are excessive alcohol consumption and chronic hepatitis infection.
• Hepatitis—Inflammation of the Liver
  • Hepatitis A—virus causes acute liver disease
  • Hepatitis B—transmitted by contact with infected blood or other bodily fluids, sexual contact, mother to newborn, shared needles
  • Hepatitis C—more likely causes a chronic infection; spreads by contact with infected blood and by sharing contaminated needles
  • Hepatitis D—rare in U.S., very serious
  • Hepatitis E—Outbreaks are usually associated with contaminated water supplies.

Jaundice

• Types:
  1. Obstructive
  2. Hemolytic
  3. Infective

1) Obstructive Jaundice

• Causes:
  • Liver damage (Cirrhosis)
  • Cancer
  • Stone
  • Scar, or any blockage of any bile duct
• Escape of bile into blood:
  • Pale stools
  • yellowish skin and sclera.
• Also seen in newborns (immature liver)

2) Hemolytic Jaundice

• Causes: Excessive destruction of RBCs:
  • Sickle cell anemia
  • Overactive spleen
  • Causes of hemolytic anemia

3) Infective Jaundice

• Causes:
  • Hepatitis A, B, C, D, E
  • Malaria

Disorders of the Liver

• Liver Cancer: Metastasis (secondaries) through the portal system.
• Gallstones (Cholelithiasis)
  • Gallbladder normally concentrates bile salts, bile pigments, cholesterol
  • Sometimes cholesterol precipitates and forms solid crystals
  • Crystals can enlarge, forming gallstones
  • Gallstones can be found in the gallbladder, cystic duct, hepatic ducts, or the common bile duct
  • Associated with cholecystitis
  • Causes:
    • Excess bile concentration
    • Too much cholesterol secretion by the liver
    • Inflammation of the gallbladder

Gallstones Symptoms

1. Pain in right upper abdominal quadrant, back and right shoulder
2. Perhaps nausea and sweating

Gallstones Investigations

• Ultrasonography
• Cholecystography
• Cholangiography

Gallstones Treatments

• Cholecystectomy: surgical removal of the gallbladder
• Cholesystoscopy: to find stones or remove them from certain areas

Digestive System Disorders Cont.

• Vomiting: Vomiting can result from irritation anywhere along the digestive tract, and it is controlled in the medulla oblongata’s emetic center.
• Food Poisoning: Food poisoning is caused by bacteria or by toxins produced by bacteria.
  • Staphylococcus—usually contracted from a food handler
  • Salmonellosis—bacteria in contaminated food
  • Botulism—toxins made by a common bacterium found in the soil
• Parasites: Digestive parasites—such as pinworms, tapeworms, roundworms, and giardia—live off the food passing by or eat the host.
• Malabsorption: Inability to absorb the appropriate nutrients needed by the body

Disorders of the Esophagus

• Gastro-Esophageal Reflux Disease (GERD):
  • It feels like the pain is derived from the heart, heartburn.
  • Weakness of LES / Overfilling of stomach / Lack of HCl
  • ==> Esophagitis==> edema and scar tissue ==> or cancer.
  • Antacids
  • Avoid caffeine/ smoking/ alcohol
  • Sitting upright while eating/ no bending for a long time
  • No lying down after meals
  • Sleeping with head elevated
  • Weight loss
• Esophageal Varices

Disorders of the Intestine

• Hemorrhoids (Anal varices):
  • pain
  • bleeding
  • protrusion
  • infection
  • Surgery
• Colorectal Cancer:
  • The 4th most common cancer in USA
  • Males more
  • Direct examination
  • Screening tests include fecal occult blood test and colonoscopy
  • Early detection and treatment increase survival rates.

Disorders of the Pancreas

• Pancreatitis: If the bile ducts become blocked: Pancreatic enzymes back up into pancreas!
• Gastritis can spread and irritate the pancreas.
• Can be fatal (acute pancreatitis): nausea, vomiting, loss of appetite, diarrhea.

Word Anatomy