the digestive system. 9.1 the digestive tract the digestive system is involved in the ingestion and...

THE DIGESTIVE SYSTEM

9.1 The Digestive Tract

The digestive system is involved in the ingestion and digestion of food and elimination of indigestible material.

• Digestion takes place within the digestive tract, which begins with the mouth and ends with the anus.

• Digestion involves mechanical and chemical digestion.o Mechanical digestion: chewing of food, and churning

and mixing of food in the stomacho Chemical digestion: enzymes break macromolecules

down into small organic molecules

UNIT B Chapter 9: Digestive System

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Section 9.1

ABSORPTION: the passage of digested nutrients from the gut lumen into the blood or lymph, which distributes them through the body.

ELIMINATION: the expulsion of indigestible residues from the body.


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Section 9.1

Figure 9.1 The human digestive tract. The upper part of the tract includes the mouth, pharynx, esophagus, stomach, and small intestine. The large intestine consists of the cecum, the colon (ascending, transverse, descending, and sigmoid colons), the rectum, and the anus. Note also the location of the accessory organs of digestion: the pancreas, the liver, and the gall bladder.

The Mouth

The mouth takes food into the body.

• Teeth: involved in chewing food• Tongue: composed of skeletal muscle and involved in forming

the bolus (a mass of food that is ready for swallowing)• Roof of the mouth: composed of a hard palate and a soft palate;

prevents ingested food from entering the nasal cavity• Tonsils: contain lymphoid tissue that protect against infections• Salivary glands: produce saliva to keep the mouth moist; saliva

contains an enzyme that digests starch


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Section 9.1

The Pharynx

The pharynx is a passageway that

receives air from the nasal cavities

and food from the mouth.

Swallowing (a reflex action) occurs in the pharynx.• The soft palate moves back to close off the nasopharynx• The trachea moves up under the epiglottis to cover the glottis

(the opening to the larynx (voice box))• During swallowing, food enters the epiglottis because the air

passages are blocked


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Section 9.1


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Section 9.1

Figure 9.2 Swallowing. When food is swallowed, the soft palate closes off the nasopharynx and the epiglottis covers the glottis, forcing the bolus to pass down the esophagus. Therefore, a person does not breathe while swallowing.


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Section 9.1

The Esophagus

The esophagus is a long muscular tube that moves food from the mouth to the stomach by peristalsis.

• Peristalsis (rhythmic muscular contractions) pushes food along the esophagus and through the digestive tract to the stomach.


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Section 9.1

The Stomach

The stomach is an organ that receives food from the esophagus, mechanically and chemically digests food, and moves food into the small intestine.


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Section 9.1

Structure and Function of the Stomach

The human stomach has thick walls with folds (called rugae) that allow it to expand and fill with food.

• Lining of the stomach: has gastric glands that secrete gastric juice containing mucus and digestive enzymes

• Wall of the stomach:o Three muscle layers (longitudinal, circular, oblique)o Responsible for moving and churning food (mechanical

digestion), and mixing food with gastric juice to break it down (chemical digestion)


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Section 9.1

• Alcohol and other liquids are absorbed in the stomach, but most solid food is not.

• When food leaves the stomach, it is a thick, soupy liquid called chyme.o Chyme enters the small intestine by way of the

pyloric sphincter.


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Section 9.1

Figure 9.3 Anatomy of the stomach. a. The stomach has a thick wall with folds that allow it to expand and fill with food. b. The mucosa contains gastric glands, which secrete gastric juice containing mucus and digestive enzymes.

The Small Intestine

The small intestine receives chyme from the stomach and completes the digestion of food. Macromolecules are broken down into nutrients, which are absorbed in the small intestine and pass into the blood.


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Section 9.1

Structure and Function of the Small Intestine

The small intestine is composed of three parts.

• Duodenum: upper part of the small intestineo Contains a bile duct that delivers bile from the liver and

pancreatic juice from the pancreas− Enzymes in pancreatic juice complete food digestion

• Jejunum: middle part of small intestine• Ileum: lower part of small intestine

o Contains lymphoid tissues involved in immune response to intestinal pathogens


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Section 9.1

Structure and Function of the Small Intestine

The wall of the small intestine contains villi, which increase the surface area to improve the absorption of nutrients.

• Villi: fingerlike projections that contain microvillio Microvilli increase the surface area of the villus for the

absorption of nutrientso Each villus contains blood capillaries and a small

lymphatic capillary called a lactealo Nutrients are absorbed into the blood capillaries and the

lacteals, which carry them to body cells


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Section 9.1


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Section 9.1

Figure 9.4 Anatomy of the small intestine. The wall of the small intestine has folds that bear fingerlike projections called villi. Microvilli, which project from the villi, absorb the products of digestion into the blood capillaries and the lacteals of the villi.

Regulation of Digestive Secretions

Digestive secretions are controlled by the nervous system and hormones.

After eating a meal:• The stomach produces the hormone gastrin

o Gastrin: stimulates the gastric glands to secrete more gastric juice

• The duodenal wall produces the hormones secretin and CCKo Secretin and CCK: stimulate the pancreas to secrete

pancreatic juice and the gall bladder to secrete bile


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Section 9.1

Hormone Released by What Part/ in response to what?

Acts on What Part?

What does it do?

GASTRIN upper part of stomach/in response to protein in the stomach

Gastric juice secreting cells at top of stomach

Causes secretion of gastric juicesAlso increases the churning action of the stomachIs inhibited by the presence of HCl, Secretin, and GIP – negative feedback loop

SECRETIN Small intestine/Acid chyme from stomach

Pancreas and parietal cells of stomach

Causes pancreas to release NaHCO3 and pancreatic enzymesAlso inhibits gastric acid secretion by the stomach

CCK (Cholecystokinin)

Small intestine/Acid chyme in stomach

Pancreas and Liver (gall bladder)

Causes liver to secrete bile and pancreas to secrete pancreatic juice.Also acts as a hunger supressant

Hormones that Control Digestion:


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Section 9.1

Figure 9.5 Hormonal control of digestive gland secretions. Gastrin (blue), produced by the lower part of the stomach, enters the bloodstream and thereafter stimulates the stomach to produce more digestive juices. Secretin (green) and CCK (purple), produced by the duodenal wall, stimulate the pancreas to secrete its digestive juices and the gall bladder to release bile.

The Large Intestine

The large intestine absorbs water, salts, and some vitamins. It also stores indigestible material until it is eliminated as feces.


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Section 9.1

Structure and Function of the Large Intestine


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Section 9.1

The large intestine includes the cecum, colon, rectum, and anal canal.

• Cecum: a small pouch that forms the first part of the large intestineo Contains the appendix or

vermiform appendix, which may play a role in fighting infection

Figure 9.6 Junction of the small intestine and the large intestine. The cecum is the blind end of the large intestine. The appendix is attached to the cecum.

Structure and Function of the Large Intestine

• Colon: includes the ascending, transverse, descending, and sigmoid colon

• Rectum: the last part of the large intestine; opens at the anus• Anus: rectum opening; site of defecation (expulsion of feces)


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Section 9.1

Defecation Reflex

• When feces are forced into the rectum by peristalsis, a defecation reflex occurs


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Figure 9.7 Defecation reflex. The accumulation of feces in the rectum causes it to stretch, which initiates a reflex action resulting in rectal contraction and expulsion of the fecal material.

Defecation Reflex

• Eliminating feces from the body is a way the digestive system maintains homeostasis

• Feces are about three-quarters water and one-quarter solidso Solids: bacteria, fibre, and other indigestible materialso Bacteria break down some indigestible material, and

produce some vitamins that our bodies can absorbo Water that is unsafe for drinking has a high number of

coliform (intestinal) bacteria, indicating that a significant amount of feces has entered the water


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Section 9.1

9.2 Accessory Organs of Digestion

The accessory digestive organs are the pancreas, liver, and gall bladder.


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Section 9.2

Figure 9.8 Liver, gall bladder, and pancreas. a. The liver makes bile, which is stored in the gall bladder and sent (black arrow) to the small intestine by way of the common bile duct. The pancreas produces digestive enzymes that are sent (black arrows) to the small intestine by way of the pancreatic duct.

The Pancreas

The pancreas is an organ that has both endocrine and exocrine functions.

• Exocrine function of the pancreaso Pancreatic cells produce pancreatic

juice (to neutralize stomach acid) and digestive enzymes

• Endocrine function of the pancreaso Pancreas secretes insulin and

glucagon, hormones that regulate blood glucose (sugar) levels


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Section 9.2

Structure and Function of the Pancreas

The pancreas contains pancreatic islets (islets of Langerhans), which are clusters of at least three types of endocrine cells:

• Alpha cells: produce glucagon• Beta cells: produce insulin• Delta cells: produce somatostatin


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Section 9.2


Insulin• Hormone secreted when blood glucose level is high• Stimulates the uptake of glucose by cells (liver, muscle,

adipose tissue) to lower blood glucose

Glucagon• Hormone secreted when blood glucose level is low • Stimulates the liver to break glycogen down into glucose to

increase blood glucose• Stimulates adipose tissue to break fat down to glycerol and

fatty acids (to make glucose)


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Section 9.2


Somatostatin• A growth-hormone-inhibiting hormone• Also produced by cells in the stomach and small intestine• Inhibits the release of growth hormone• Suppresses the release of insulin and glucagon• Decreases the absorption of nutrients


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Section 9.2


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Figure 9.9 Regulation of blood glucose level. Top: When the blood glucose level is high, the pancreas secretes insulin. Insulin promotes the storage of glucose as glycogen and the synthesis of proteins and fats (as opposed to their use as energy sources). Therefore, insulin lowers the blood glucose level to normal.


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Section 9.2

Bottom: When the blood glucose level is low, the pancreas secretes glucagon. Glucagon acts opposite to insulin. Therefore, glucagon raises the blood glucose level to normal.

The Liver

The liver is the largest gland in the body.

The liver has many functions, including:

• Detoxifying blood • Making plasma proteins• Maintaining blood glucose levels• Producing bile, which contains bile salts

that emulsify fat in the small intestine• Producing urea, a nitrogenous waste

product from the breakdown of amino acids


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Section 9.2

Structure and Function of the Liver

The liver contains about 100 000 lobules that serve as its structural and functional units.

Three structures are located between the lobules:• Bile duct: takes bile away from the liver• Hepatic artery branch: brings oxygen-rich blood to the liver• Hepatic portal vein: transports nutrients from the intestines


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Section 9.2


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Figure 9.8 b. A hepatic lobule. The liver contains over 100 000 lobules, each lobule composed of many cells that perform the various functions of the liver. They remove and add materials to the blood and deposit bile in a duct.


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Section 9.2

Figure 9.10 Hepatic portal system. The hepatic portal vein takes the products of digestion from the digestive system to the liver, where they are processed before entering a hepatic vein.

The Gall Bladder

The gall bladder is a muscular sac attached to the surface of the liver.

• Excess bile from the liver is stored in the gall bladder

• Bile leaves the gall bladder and proceeds to the duodenum via the common bile ducto Bile emulsifies fat to prepare it

for further breakdown by digestive enzymes


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Section 9.2

9.3 Digestive Enzymes

Digestive enzymes help break down the major components of food: carbohydrates, proteins, nucleic acids, and fats.


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Section 9.3

Carbohydrate Digestion by Enzymes

The digestion of starch (a carbohydrate) begins in the mouth.• Salivary amylase (produced by the salivary glands) digests

starch into maltose (a disaccharide)

• Pancreatic amylase (produced by the pancreas) and maltase (produced by the small intestine) then convert maltose in the small intestine to glucose (a monosaccharide). Glucose can be absorbed by the small intestine.


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Section 9.3


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Section 9.3

Figure 9.11 Digestion and absorption of nutrients.

a. The breakdown of carbohydrates, such as starch, involves amylase enzymes.

Carbohydrate Digestion by Enzymes

Other disaccharides, such as lactose, have their own enzyme that digests them in the small intestine.

• Lactase is an enzyme that digests lactose, a sugar found in milk.o Individuals who have a lactase deficiency often have

symptoms of lactose intolerance (diarrhea, gas, cramps) caused by the fermentation of non-digested lactose by intestinal bacteria


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Section 9.3

Protein Digestion by Enzymes

The digestion of proteins begins in the stomach.• Pepsin is an enzyme produced by gastric glands that acts on

proteins to produce peptides.

• Trypsin (produced by the pancreas) and peptidases (produced in the small intestine) break down peptides into amino acids


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Section 9.3


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b. Protein digestion involves the action of protease enzymes.

Fat Digestion by Enzymes

• Lipase (produced by the pancreas) acts in the small intestine and digests fat molecules in the fat droplets after they have been emulsified by bile salts

• Glycerol and fatty acids enter the cells of the villi, where they are rejoined and repackaged as lipoprotein droplets (chylomicrons) before entering the lacteals


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Section 9.3


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c. For fat digestion, bile salts emulsify the fats so that lipase enzymes can digest the particles.

Regulation of Digestive Enzymes

Enzymes function best at an optimum temperature and pH that helps maintain the proper shape to fit their substrate.

• Since the digestive system is maintained at a constant 37ºC, enzymatic activity is largely controlled by pHo The pH of the stomach is between 1 and 2 but can

increase to around 7.4 to 7.8 when sodium bicarbonate in pancreatic juice is released from the pancreas

o This increase in pH occurs after chyme enters the duodenum, and allows different digestive enzymes to be active depending on the pH


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Section 9.3

9.4 Disorders of the Digestive System

Disorders of the digestive system can be grouped into two categories:

Disorders of the digestive tract itself• Stomach ulcers, intestinal disorders (diarrhea, Crohn’s disease,

constipation), polyps, and colon cancer

Disorders of the accessory organs• Pancreatic disorders (pancreatitis, pancreatic cancer), diabetes

mellitus, hepatitis, cirrhosis, gallstones


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Section 9.4

Disorders of the Digestive Tract

Stomach Ulcers

• A stomach ulcer is an open sore in the stomach wall caused by a gradual disintegration of the tissue.o Mainly caused by infection by Helicobacter pylori, a

bacterium that impairs the ability of mucous cells to produce protective mucus for the stomach wall

o Also caused by viral infections, or overuse of anti-inflammatory medications that damage stomach lining

o Can be treated with antibiotics that kill H. pylori bacterium, medications that reduce stomach acid


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Section 9.4


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Section 9.4

Figure 9B Stomach ulcer. Drs. Barry Marshall and Robin Warren won a Nobel Prize in 2005 for discovering that most stomach ulcers are caused by a bacterial infection.

Intestinal Disorders

• Diarrhea is loose, watery feces.o Acute: can be caused by infections of the small or large

intestine with bacteria, viruses, or protozoas that cause the intestinal wall to be irritated.

− Peristalsis increases, and less water is absorbed, resulting in loose, watery feces

o Chronic: Persistent inflammation of the intestine due to a misdirected immune response against one’s own intestinal tissues and bacteria (Crohn’s disease)

− Genetic predisposition and environmental triggers are factors.


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Section 9.4

• Constipation results in dry and hard feces.o Caused by ignoring the urge to defecate or by

inadequate fluid or fibre in the dieto Chronic constipation can lead to hemorrhoids, which are

inflamed blood vessels in the anuso Can be prevented by increasing the amount of water and

fibre in diet− Laxatives and enemas can be used but can irritate the

colon


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Section 9.4

Polyps and Colon Cancer

• Polyps are small growths arising from the epithelial lining in the colono Can be benign or cancerouso Usually detected by a colonoscopy, where an endoscope is

inserted in the colon to enable a viewing of the wall of the large intestine

o Colon cancer can be cured if detected early and surgically removed while it is still confined to a polyp


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Section 9.4

Disorders of the Accessory Organs

Disorders of the Pancreas

• Pancreatitis is an inflammation of the pancreas.o Caused by excessive alcohol consumption, gallstones that

block the pancreatic duct, or other unknown factorso Chronic pancreatitis: digestive enzymes secreted by the

pancreas damage the pancreas and decrease insulin secretion

o Pancreatic cancer is a cancer that is almost always fatal.o 20% of patients are alive one year after diagnosiso Resistant to treatment and spreads to other organs before

symptoms appear


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Section 9.4

• Diabetes mellitus is a condition that affects the regulation of glucose metabolism.o Type I diabetes: individuals do not produce enough insulino Type II diabetes: individuals cannot properly use the insulin

they produceo In either case, blood glucose levels rise, but the use of

glucose by the cells is impaired− Excess glucose in the blood is secreted into the urine− Because blood glucose cannot be used, the body

metabolizes fat, which leads to the buildup of ketones− Ketones metabolize into acids, which can build up in the

blood and lead to coma and death


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Section 9.4

• The glucose tolerance test is used to test for diabetes.o Person ingests a known

amount of glucose, and blood glucose concentration is measured at intervals

o In a person with diabetes, blood glucose rises greatly and remains elevated for hours


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Section 9.4

Figure 9.12 Glucose tolerance test. Following the administration of 100 g of glucose, the blood glucose level rises dramatically in the person with diabetes and glucose appears in the urine. Also, the blood glucose level at 2 hours is equal to or more than 200 mg/100 dL.

• Type 1 Diabetes (individuals do not produce enough insulin)o 10% of individuals with diabetes in Canada have type 1o Usually begins in childhood (also known as juvenile-onset

diabetes)o Can also occur due to a viral infection, autoimmune reaction,

or environmental agent that destroys the pancreatic islets that produce insulin

o Treatment is through daily insulin injections, which can be administered through an insulin pump


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Section 9.4


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Section 9.4

Figure 9.13 An insulin pump. Insulin pumps administer preprogrammed small doses of insulin throughout the day via an implanted catheter. Most insulin pumps can be worn under clothing.

• Type 2 Diabetes (individuals cannot use insulin properly)o 90% of individuals with diabetes in Canada have type 2.o Can occur in adults or childreno Individuals with type 2 diabetes are often overweight or

obese, and fat tissue may produce a substance that impairs insulin receptor functiono Normally, insulin binds to its receptor on cell surfaces to

cause the number of glucose transporters to increase in the plasma membrane. This does not occur in individuals with type 2 diabetes.

o Treatment involves weight loss, insulin injections, and medications to increase the effectiveness of the insulin produced

o ** see Insulin Sheet


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Section 9.4

Disorders of the Liver and Gall Bladder

o Hepatitis is inflammation of the liver.o Commonly caused by one of several viruseso Hepatitis A: acquired by consuming food or water that is

contaminated with sewage (vaccine is available)o Hepatitis B: spread by sexual contact, blood transfusions, or

contaminated needles (vaccine is available)o Hepatitis C: spread by sexual contact, blood transfusions, or

contaminated needles (no vaccine, but antiviral drugs are available)


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Section 9.4

• Cirrhosis is a chronic disease of the liver.o Often seen in alcoholics due to malnutrition and toxic

effects of excess amounts of alcoholo Liver becomes infiltrated with fat, and the fatty liver

tissue is replaced with non-functioning fibrous scar tissue

o Damage to the liver exceeds the rate of liver regeneration

o Treatment involves a liver transplant


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Section 9.4

• Gallstones are small, hard masses that form in the gall bladder.o Cholesterol can precipitate out of the bile and form

crystals that can grow into gallstones.o Passage of gallstones from the gall bladder may block

the common bile duct and cause jaundice, a yellowing of the skin and eyes due to buildup of bilirubin.

o If the gallstones cannot be removed, the gall bladder must be removed.


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Section 9.4

the digestive system. 9.1 the digestive tract the digestive system is involved in the ingestion and...

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