Digestive System Flashcards
Digestive System Functions
- 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
Two Main Groups of Organs
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
Organs of the Digestive Tract
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
Anatomy of the Mouth
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
Functions of the Mouth
- 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
Pharynx
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
Oesophagus
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)
Layers of Tissue in the Digestive Tract Organs
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
Mucosa
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)
Submucosa
- 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
Muscularis Externa - Smooth Muscle
- 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
Serosa
- 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
Digestive Tract Nerve Plexuses
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
Stomach
- 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
Stomach Regions
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
Stomach Omentum
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
Structure of the Stomach Mucosa
- 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
Stomach Mucosa Cells
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
Stomach Functions
- 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
Small Intestine
- 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
Small Intestine Subdivisions
- Duodenum (5%)
- Jejunum (40%)
- Ileum (60%)
Small Intestine - Chemical Digestion
- 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
Small Intestine Structural Modifications
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
Small Intestine - Peyer’s Patches
- 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
Large Intestine
- 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
Cecum
- 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
Appendix
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
Colon
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
Anal Canal
- 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
Large Intestine
- 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)
Accessory Digestive Organs
- Teeth
- Salivary glands
- Pancreas
- Liver
- Gallbladder
Salivary Glands
- 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
Saliva
- 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
Pancreas
- 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
Liver
- 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
Liver - Digestive Role to Produce Bile
- 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
Gall Bladder
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
Overview of Gastrointestinal Processes and Controls
Essential processes of the GI tract
- Ingestion
- Propulsion
- Segmentation
- Absorption
- Defecation
Ingestion
Placing of food into the mouth
Include teeth, lips and tongue
Propulsion
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
Segmentation
Movement of materials back and forth to foster mixing in the small intestine
Mixes the materials with the digestive juices
Mechanical digestion
Mechanical Breakdown
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
Chemical Breakdown
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
Absoprtion
End products of digestion are absorbed in the blood or lymph
Food must enter mucosal cells and then move into blood or lymph capillaries
Defectation
Elimination of indigestible substances from the GI tract in the form of feces
• Exits the body through the anus
Activities occuring in mouth, pharynx and oesophagus - Ingestion and Breakdown
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
Activities occuring in mouth, pharynx and oesophagus - Propulsion, Swallowing and Peristalsis
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
Activities of the Stomach - Food Break Down
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
Food Propulsoin
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
Activities of the Small Intestine - Chyme Breakdown and Absorption
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
Activities of the Small Intestine: Pancreatic Enzymes
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
Activities of the Small Intestine: Alkaline Content
- 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
Activities of the Small Intestine: Pancreatic Juice Release
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
Activities of the Small Intestine: Hormones
Secretin and CCK also target the liver and gallbladder to release bile
Bile
• Acts as a fat emulsifier or separator
• Needed for fat
Activities of the Small Intestine: Water
o Water is absorbed along the length of the small intestine
Activities of the Small Intestine: End Products of Digestion
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
Activities of the Large Intestine: Nutrient Breakdown and Absorption
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
Activities of the Large Intestine: Propulsion of Food Residue and Defecation
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
Gastroesophageal reflux
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)
Ulcers (Peptic Ulcers)
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
Gastroenteritis
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
Inflammatory Bowel Disease
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
Diarrhoea
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
Constipation
Could be caused by weakness of the bowel muscle, lack of fibre, lack of exercise and some prescription drugs
Hepatitis
Characterised by inflammation and damage to the liver
Most common feature among hepatitis
Can cause a variety of symptoms
Pancreatitis
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
Gallstones
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
