Digestive System

Question 1

Digestive System Functions

Answer 1

• Ingestion—taking in food
• Digestion—breaking food into nutrient molecules
• Absorption—movement of nutrients into the bloodstream
• Defecation—excretes to rid the body of indigestible waste

Question 2

Two Main Groups of Organs

Answer 2

Digestive tract (gastrointestinal, or GI, tract)
 Continuous, coiled, hollow tube
 These organs ingest, digest, absorb, defecate
 Begins with the mouth –> ends with the anus

Accessory digestive organs
 Include teeth, tongue, and several large digestive organs
 Assist digestion in various ways
 Connected to the GI tract by ducts
• Secrete chemicals which aid in chemical breakdown and absorption of food

Question 3

Organs of the Digestive Tract

Answer 3

The digestive tract is a continuous, coiled, hollow tube that runs through the ventral cavity from stomach to anus 
o	Mouth 
o	Pharynx 
o	Oesophagus 
o	Stomach 
o	Small / Large intestine 
o	Anus

Question 4

Anatomy of the Mouth

Answer 4

Mouth (oral cavity)
o Mucous membrane–lined cavity

Lips (labia)
o Protect the anterior opening
o Assist with injection by grabbing food and pulling into mouth
o Under voluntary control

Cheeks
o Form the lateral walls of oral cavity

Hard palate
o Forms the anterior roof of oral cavity

Soft palate
o Forms the posterior roof oral cavity
o Formed from a fold of mucous membrane

Uvula
o Fleshy projection of the soft palate
o Helps prevent food from entering nasal cavity when swallowing

Vestibule
o Space between lips externally and teeth and gums internally

Oral cavity proper
o Area contained by the teeth

Tongue
o Attached at hyoid bone and styloid processes of the skull, and by the lingual frenulum to the floor of the mouth
o Lingual Frenulum –> anchors tongue to floor of mouth and limits posterior movements

Tonsils
o	Part of the bodies defence system 
o	Palatine
	Located at posterior end of oral cavity 
o	Lingual
	Located at the base of the tongue

Question 5

Functions of the Mouth

Answer 5

• Mastication (chewing) of food
• Tongue mixes masticated food with saliva
  o Chemical breakdown of food
• Tongue initiates swallowing
  o Propulsion by pushing food
• Taste buds on the tongue allow for taste

Question 6

Pharynx

Answer 6

Serves as a passageway for foods, fluids, and air

Food passes from the mouth posteriorly into the:
o Nasopharynx
 Connects nasal cavity to the oropharynx
o Oropharynx
 Posterior to oral cavity
 Foods, liquids and air passes through
o Laryngopharynx
 Below the oropharynx and continuous with the oesophagus

Food is propelled to the oesophagus by two skeletal muscle layers in the pharynx
o Longitudinal outer layer
o Circular inner layer

Alternating contractions of the muscle layers (peristalsis) propel the food through pharynx inferiorly

Question 7

Oesophagus

Answer 7

Anatomy
o About 10 inches long
o Runs from pharynx to stomach through the diaphragm
o Passes through opening in diaphragm

Physiology
o Conducts food by peristalsis (slow rhythmic squeezing) to the stomach
 When food reaches the top of the oesophagus following swallowing –> a wave of peristalsis beings –> pushes food into the stomach
o Passageway for food only (respiratory system branches off after the pharynx)

Question 8

Layers of Tissue in the Digestive Tract Organs

Answer 8

Summary of the four layers from innermost to outermost, from oesophagus to the large intestine (detailed next) 
o	Mucosa
o	Submucosa
o	Muscularis externa 
o	Serosa

Question 9

Mucosa

Answer 9

Innermost, moist membrane consisting of:
o Surface epithelium that is mostly simple columnar epithelium
 Except for oesophagus — stratified squamous epithelium
 Covered in mucous secreted by cells or glands

o Small amount of connective tissue (lamina propria)
 Contains blood vessels, lymphatic vessels and mucous secreting vessels (some parts of the body)

o Scanty smooth muscle layer (thin)
- Lines the cavity (known as the lumen)

Question 10

Submucosa

Answer 10

• Just beneath the mucosa
• Soft, irregular dense connective tissue with blood vessels, nerve endings, mucosa-associated lymphoid tissue, and lymphatic vessels
  o Glands and tissues secrete substances that aid in secretion and absorption

Question 11

Muscularis Externa - Smooth Muscle

Answer 11

• Surrounds submucosal layer
• Propels food through Digestive Tract by peristalsis
• Performs mechanical digestion occurs
  o Churns food
• Has two layers:
  o Inner circular layer
  o Outer longitudinal layer

o Layer of nerve fibers between the layers regulate the activity of each layer

Question 12

Serosa

Answer 12

• Thin, serous membrane which helps to reduce friction
• Outermost layer of the wall
• Contains fluid-producing cells

Divided into:
o Visceral peritoneum
 Innermost layer that is continuous with the outermost layer
o Parietal peritoneum
 Outermost layer that lines the abdominopelvic cavity by way of the mesentery

Question 13

Digestive Tract Nerve Plexuses

Answer 13

Digestive tract wall contains two intrinsic nerve plexuses that are part of the autonomic nervous system
o Submucosal nerve plexus
o Myenteric nerve plexus

• Regulate mobility and secretory activity of the GI tract organs

Question 14

Stomach

Answer 14

• C-shaped organ located on the left side of the abdominal cavity
• Stomach can stretch and hold 4 L (1 gallon) of food when full
• Varies from 15 – 25 cm in length

Diameter and volume depend on how much food it contains
o Rugae
 Internal folds of the mucosa present when the stomach is empty

Question 15

Stomach Regions

Answer 15

Cardial (cardia)
 Near the heart and surrounds the cardio oesophageal sphincter (allows food to enter from oesophagus)

Fundus
 Expanded portion lateral to the cardiac region

Body
 Midportion
 Greater curvature is the convex lateral surface
 Lesser curvature is the concave medial surface
 As it shrinks anteriorly –> body becomes pyloric antrum –> funnel shaped pylorus

Pylorus
 Funnel-shaped terminal end
 Continuous with small intestine through the pyloric sphincter or pyloric valve

Question 16

Stomach Omentum

Answer 16

Lesser Omentum
o Double layer of the peritoneum
o Extends from liver to the lesser curvature of stomach

Greater Omentum
o Another extension of the peritoneum
o Covers the abdominal organs and attaches to posterior boy wall
o Fat insulates, cushions, and protects abdominal organs
o Contains collections of lymphoid follicles
 Contains macrophages and defensive cells of the immune system

Question 17

Structure of the Stomach Mucosa

Answer 17

• Simple columnar epithelium composed almost entirely of mucous cells
• Mucous cells produce bicarbonate-rich alkaline mucus
  o Clings to the stomach mucosa and protections from damage against acid
• Dotted by gastric pits leading to gastric glands that secrete gastric juice, including:
  o Intrinsic factor, which is needed for vitamin B12 absorption in the small intestine

Question 18

Stomach Mucosa Cells

Answer 18

Chief cells
o Produce inactive protein-digesting enzymes (pepsinogens)

Parietal cells
o Produce hydrochloric acid that activates enzymes
 Conversion of pepsinogen and pepsin
o Makes stomach contents acidic

Mucous neck cells
o Produce thin acidic mucus (different from the mucus produced by mucous cells of the mucosa) with an unknown function

Enteroendocrine cells
o Produce local hormones such as gastrin

Question 19

Stomach Functions

Answer 19

• Temporary storage tank for food
• Site of food breakdown
  o Three layers of muscle allow to move food along tract and breakdown
• Chemical breakdown of protein begins
  o Most occurs within the pyloric region
• Delivers chyme (thick) (processed food) to the small intestine via the pyloric sphincter

Question 20

Small Intestine

Answer 20

• The body’s major digestive organ
• Longest portion of the digestive tract (2–4 m, or 7–13 feet, in a living person)
• Site of nutrient absorption into the blood and all water absorption
• Muscular tube extending from the pyloric sphincter to the ileocecal valve
• Suspended from the posterior abdominal wall by the mesentery

Question 21

Small Intestine Subdivisions

Answer 21

• Duodenum (5%)
• Jejunum (40%)
• Ileum (60%)

Question 22

Small Intestine - Chemical Digestion

Answer 22

• Begins in the small intestine –> can only process a small amount of food at a time –> pyloric sphincter controls movement of chyme into small intestine –> prevents small intestine from being overwhelmed
• Enzymes produced by intestinal cells and pancreas are carried to the duodenum by pancreatic ducts
  o Chemical breakdown of food ends in the pancreatic ducts
• Bile, formed by the liver, enters the duodenum via the bile duct
• Hepatopancreatic ampulla is the location where the main pancreatic duct and bile ducts join

Question 23

Small Intestine Structural Modifications

Answer 23

Increase surface area for food absorption

Decrease in number toward the end of the small intestine
o Villi
 Finger like projections formed by the mucosa
• House a capillary bed and lacteal
 Contains tubular indentations (intestinal crypts)
 Each contain a lymphatic capillary (lacteal) as well as blood capillaries

o Microvilli
 Tiny projections of the plasma membrane (brush border enzymes)

o Circular folds (plicae circulares)
 Deep folds of mucosa and submucosa

Question 24

Small Intestine - Peyer’s Patches

Answer 24

• Collections of lymphatic tissue
• Located in submucosa
• Increase in number toward the end of the small intestine
• More are needed there because remaining food residue contains much bacteria
  o Must be prevented from entering the blood stream

Question 25

Large Intestine

Answer 25

- Larger in diameter, but shorter in length at 1.5 m, than the small intestine - Extends from the ileocecal valve to the anus - Main functions --> propulsion and elimination of waste Absorption of water, electrolytes and some vitamins are additional but limited functions Distinctive Feature  Presence of large colon ease bacteria • Responsible for B vitamin synthesis and Vitamin K ``` Subdivisions o Cecum o Appendix o Colon o Rectum o Anal canal ```

Question 26

Cecum

Answer 26

- Saclike first part of the large intestine (first to receive food) Connects to large intestine by the ileocecal valve o Usually, closed --> opens in response to gastrin (released by the stomach) o Partially controlled by the nervous system o When open --> digestive remnants of food travel from the ileum to the cecum

Question 27

Appendix

Answer 27

o Hangs from the cecum Accumulation of lymphoid tissue that sometimes becomes inflamed (appendicitis)  Contains lymphocytes that help protect the body from infection organisms • Does not protect against appendicitis

Question 28

Colon

Answer 28

Ascending o Travels up right side of abdomen and makes a turn at the right colic (hepatic) flexure Transverse o Travels across the abdominal cavity and turns at the left colic (splenic) flexure Descending o Travels down the left side Sigmoid o S-shaped region; enters the pelvis --> exits at the rectum Sigmoid colon, rectum, and anal canal are located in the pelvis

Question 29

Anal Canal

Answer 29

- Anal canal ends at the anus Anus o Opening of the large intestine o External anal sphincter  Formed by skeletal muscle and is voluntary o Internal anal sphincter  Formed by smooth muscle and is involuntary o These sphincters are normally closed due to constriction except during defecation The large intestine delivers indigestible food residues to the body’s exterior

Question 30

Large Intestine

Answer 30

- No villi are present - Goblet cells produce alkaline mucus to lubricate the passage of feces to the end of the digestive tract - Longitudinal layer of the muscularis externa layer is reduced to three bands of muscle, called teniae coli - These bands of muscle cause the wall to pucker into haustra (pocketlike sacs)

Question 31

Accessory Digestive Organs

Answer 31

- Teeth - Salivary glands - Pancreas - Liver - Gallbladder

Question 32

Salivary Glands

Answer 32

- Three pairs of salivary glands empty secretions into the mouth # Located within tissue surrounding the oral cavity o Parotid glands (largest)  Found anterior to the ears  Mumps affect these salivary glands o Submandibular glands  Lie on the medial side of the mandible o Sublingual glands  Both submandibular and sublingual glands empty saliva into the floor of the mouth through small ducts

Question 33

Saliva

Answer 33

- Mixture of mucus and serous fluids - Composed mostly of water - Helps to moisten and bind food together into a mass called a bolus ``` Contains: o Salivary amylase  Begins starch digestion  Breaks down complex carbohydrates into shorter chains of sugar chains o Lysozymes and antibodies  Inhibit bacteria o Lingual lipase  Initiates the breakdown of fats ``` Dissolves chemicals so they can be tasted

Question 34

Pancreas

Answer 34

- Functions as an exocrine and endocrine gland - Soft, pink triangular gland - Found posterior to the parietal peritoneum o Mostly retroperitoneal - Secretes products into the blood and lumen - Extends across the abdomen from spleen to duodenum - Produces a wide spectrum of digestive enzymes that break down all categories of food - Secretes enzymes and pancreatic juice into the duodenum o Pancreatic juices contain digestive enzymes (proteases --> break down proteins / amylases --> breaks down starches / lipases --> break down lipids) - Alkaline fluid introduced with enzymes neutralizes acidic chyme coming from stomach - Hormones produced by the pancreas o Insulin o Glucagon o Involved in regulating blood glucose levels

Question 35

Liver

Answer 35

- Largest gland in the body - Located on the right side of the body under the diaphragm - Consists of four lobes suspended from the diaphragm and abdominal wall by the falciform ligament

Question 36

Liver - Digestive Role to Produce Bile

Answer 36

- Bile leaves the liver through the common hepatic duct and enters duodenum through the bile duct o Duodenum uses bile to breakdown lipids Bile is yellow-green, watery solution containing: o Bile salts and bile pigments (mostly bilirubin from the breakdown of hemoglobin)  Are emulsifying agents --> greatly increases surface area of fats and assisting breakdown by lipase enzymes o Cholesterol, phospholipids, and electrolytes o Makes about 1L/day at a constant rate --> only needed following food intake Bile emulsifies (breaks down) fats

Question 37

Gall Bladder

Answer 37

Green sac found in a shallow fossa in the inferior surface of the liver o Releases bile after a meal that contains fat  Controlled by several bile sphincters (bile sphincters, pancreatic sphincter and hepatopancreatic sphincter) o When no digestion is occurring, bile backs up the cystic duct for storage in the gallbladder o While in the gallbladder, bile is concentrated by the removal of water o When chyme containing fat enters the duodenum --> gallbladder contracts --> the gallbladder spurts out stored bile  Common bile duct and hepatopancreatic duct sphincter relax to allow bile to flow to duodenum and emulsify the fat

Question 38

Overview of Gastrointestinal Processes and Controls

Answer 38

Essential processes of the GI tract - Ingestion - Propulsion - Segmentation - Absorption - Defecation

Question 39

Ingestion

Answer 39

 Placing of food into the mouth |  Include teeth, lips and tongue

Question 40

Propulsion

Answer 40

Movement of foods from one region of the digestive system to another Initiated by swallowing at the pharynx Peristalsis o Symmetrical, alternating waves of contraction and relaxation that squeeze food along the GI tract

Question 41

Segmentation

Answer 41

Movement of materials back and forth to foster mixing in the small intestine  Mixes the materials with the digestive juices  Mechanical digestion

Question 42

Mechanical Breakdown

Answer 42

Examples • Mixing of food in the mouth by the tongue • Churning of food in the stomach • Segmentation in the small intestine Mechanical digestion prepares food for further degradation by enzymes

Question 43

Chemical Breakdown

Answer 43

Digestion occurs when enzymes chemically break down large molecules into their building blocks Each major food group uses different enzymes • Carbohydrates are broken down to monosaccharides (simple sugars) o Three monosaccharides are common in diet --> glucose, fructose and galactose o Blood Sugar Level --> extremely dependent on glucose o Fructose --> found in fruits o Galactose --> found in milk o Only carbs GI tract breaks down  Sucrose, Maltose, Lactose and Starch  Starch is a polysaccharide • Proteins are broken down to amino acids o Intermediate product is poly peptides • Fats are broken down to fatty acids and glycerol

Question 44

Absoprtion

Answer 44

 End products of digestion are absorbed in the blood or lymph  Food must enter mucosal cells and then move into blood or lymph capillaries

Question 45

Defectation

Answer 45

 Elimination of indigestible substances from the GI tract in the form of feces • Exits the body through the anus

Question 46

Activities occuring in mouth, pharynx and oesophagus - Ingestion and Breakdown

Answer 46

Food is placed into the mouth  Physically broken down by chewing  Mixed with saliva, which is released in response to mechanical pressure and psychic stimuli  Salivary amylase begins starch digestion --> chemically breaks down into maltose Essentially, no food absorption occurs in the mouth

Question 47

Activities occuring in mouth, pharynx and oesophagus - Propulsion, Swallowing and Peristalsis

Answer 47

o Pharynx and oesophagus have no digestive function  Serve as passageways to the stomach Pharynx functions in swallowing (deglutition)  Two phases of swallowing • Buccal phase o Voluntary o Occurs in the mouth o Food is formed into a bolus o The bolus is forced into the pharynx by the tongue • Pharyngeal-oesophageal phase o Involuntary transport of the bolus by peristalsis  Controlled by the PNS promote the mobility of digestive organs from this point on o Nasal and respiratory passageways are blocked  Tongue blocks mouth  Soft palate blocks respiratory passageways o Peristalsis moves the bolus toward the stomach  Moved through pharynx and into oesophagus by peristaltic contractions of the muscle walls • Longitudinal --> Circular o The cardio oesophageal sphincter is opened when food presses against it  Allows food to enter when open

Question 48

Activities of the Stomach - Food Break Down

Answer 48

o Gastric juice is regulated by neural and hormonal factors o Presence of food or rising pH causes the release of the hormone gastrin Gastrin causes stomach glands to produce:  Protein-digesting enzymes (e.g., pepsinogen)  Mucus  Hydrochloric acid Hydrochloric acid makes the stomach contents very acidic  Can be dangerous as both hydrochloric acid and protein-digesting enzymes have the ability to eat the stomach themselves --> causing ulcers • As long as there is mucous --> stomach is protected Acidic pH  Activates pepsinogen to pepsin for protein digestion  Provides a hostile environment for microorganisms Protein-digestion enzymes  Pepsin • An active protein-digesting enzyme  Rennin • Works on digesting milk protein in infants; not produced in adults o Converts to substance looking like sour milk o Alcohol and aspirin are virtually the only items absorbed in the stomach

Question 49

Food Propulsoin

Answer 49

Peristalsis: o Waves of peristalsis occur from the fundus to the pylorus, forcing food past the pyloric sphincter  Force increases as food approaches the pyloric sphincter • Helps grind food into chyme Grinding: o The pylorus meters out chyme into the small intestine (3 ml at a time) Retropulsion: o Peristaltic waves close the pyloric sphincter, forcing contents back into the stomach; the stomach empties in 4–6 hours When duodenum is filled with chyme and its wall is stretched, a nervous reflex occurs o Puts the brakes on gastric activity --> slows the emptying of the stomach by inhibiting the vagus nerve --> tightens the pyloric sphincter --> allows time for intestinal processing to catch up

Question 50

Activities of the Small Intestine - Chyme Breakdown and Absorption

Answer 50

o Chyme reaching the small intestine is only partially digested o Process of chemical digestion is accelerated --> as food moves through the small intestine Intestinal enzymes from the brush border function to:  Break double sugars into simple sugars  Complete some protein digestion Intestinal enzymes and pancreatic enzymes help to complete digestion of all food groups  Intestinal juice has few enzymes and protected mucous is the most important intestinal gland secretion Foods entering the small intestine • Mixed with pancreatic juice delivered through a duct on the pancreas as well as bile from the liver

Question 51

Activities of the Small Intestine: Pancreatic Enzymes

Answer 51

Play the major role in the digestion of fats, proteins, and carbohydrates  Pancreatic juice contains enzymes that together with brush border enzymes • Complete the digestion of starch, carry out half of protein digestion, fat digestion and digest nucleic acids

Question 52

Activities of the Small Intestine: Alkaline Content

Answer 52

- Neutralizes acidic chyme and provides the proper environment for the pancreatic enzymes to operate  When pancreatic juice reaches the small intestine --> it neutralises acidic chyme from the stomach --> provides necessary environment for activation and activity of intestinal and pancreatic digestive enzymes

Question 53

Activities of the Small Intestine: Pancreatic Juice Release

Answer 53

From the pancreas into the duodenum is stimulated by:  Vagus nerves  Local hormones that travel via the blood to influence the release of pancreatic juice (and bile) • Secretin • Cholecystokinin (CCK) o Also causes gallbladder to contract and release stored bile into bile duct

Question 54

Activities of the Small Intestine: Hormones

Answer 54

Secretin and CCK also target the liver and gallbladder to release bile Bile • Acts as a fat emulsifier or separator • Needed for fat

Question 55

Activities of the Small Intestine: Water

Answer 55

o Water is absorbed along the length of the small intestine

Question 56

Activities of the Small Intestine: End Products of Digestion

Answer 56

Most substances are absorbed by active transport through cell membranes • Through active transport • Enter capillary beds in the villi to be transport to liver by hepatic portal vein Lipids are absorbed by diffusion • Carried to the liver by both blood and lymphatic fluids Substances are transported to the liver by the hepatic portal vein or lymph

Question 57

Activities of the Large Intestine: Nutrient Breakdown and Absorption

Answer 57

o No digestive enzymes are produced Resident bacteria digest remaining nutrients  Produce some vitamin K and some B vitamins  Release gases (methane, hydrogen sulphide) • Contribute to flatulence and the odour of faeces • 500 mL are produced each day Water, vitamins, ions, and remaining water are absorbed Remaining materials are eliminated via feces

Question 58

Activities of the Large Intestine: Propulsion of Food Residue and Defecation

Answer 58

o Sluggish peristalsis begins when food residue arrives o Haustral contractions are the movements occurring most frequently in the large intestine  Occur approximately every 30 minutes o Mass movements are slow, powerful movements that occur three to four times per day  Typically occurs before or after eating when food begins to fill stomach or small intestine o Presence of feces in the rectum causes a defecation reflex  Internal anal sphincter is relaxed  Defecation occurs with relaxation of the voluntary (external) anal sphincter

Question 59

Gastroesophageal reflux

Answer 59

o Movement of chyme from stomach into lower oesophagus o Stomach wall is protected from harmful effects of acid by a thick layer of mucus  Oesophagus is not protected --> can cause painful burning sensation in the oesophagus near the heart (heartburn)

Question 60

Ulcers (Peptic Ulcers)

Answer 60

Break in the protective lining of the stomach, duodenum or lower oesophagus  Causes the deeper structures of the body to be exposed to acidic chyme and protein digesting enzymes

Question 61

Gastroenteritis

Answer 61

o Inflammation of the stomach or intestine o Produces a combination of nausea, vomiting, diarrhoea and abdominal pain o Often contagious and is sometimes called stomach flu

Question 62

Inflammatory Bowel Disease

Answer 62

Walls of either the small or large intestine become chronically inflamed  Often results in diarrhoea and pain  Cause is unclear o Ulcerative colitis  Typically, only affects colon and mucosal layer of the intestinal wall o Chrohn’s disease  Can affect all four layers of the digestive tract wall  Inflammation typically affects the small intestine or colon

Question 63

Diarrhoea

Answer 63

 Waste spends too much time in the colon  So much water is absorbed by the waste --> becomes nearly solid --> more difficult to pass  Characterised by abnormally frequent bowel movement  Occurs when water doesn’t spend enough time in the colon --> excess is absorbed  Occurs when excess water is in waste --> despite normal travel time through the intestine

Question 64

Constipation

Answer 64

 Could be caused by weakness of the bowel muscle, lack of fibre, lack of exercise and some prescription drugs

Question 65

Hepatitis

Answer 65

Characterised by inflammation and damage to the liver  Most common feature among hepatitis  Can cause a variety of symptoms

Question 66

Pancreatitis

Answer 66

o Inflammation of the pancreas o Occurs when pancreatic enzymes (proteases) become active while still in the pancreas --> pancreas begins to break down its own tissue --> leads to severe abdominal pain

Question 67

Gallstones

Answer 67

o Solid crystals that form from substances in bile Usually created in the gall bladder and then get stuck in the cystic duct or common bile duct  Blockage prevents further secretion of bile  Resulting in pain and inability to digest fat