Function of the digestive system and the two basic processes
Breaks down food into forms that can be absorbed and used by body cells. •Absorbs water, vitamins, and minerals, and it eliminates wastes from the body. oTwo Basic Process: •Mechanical digestion •Chemical digestion
Explain mechanical digestion and chemical digestion
Mechanical - chewing, swallowing, mixing Chemical - chemicals secreted into the digestive system like acids, enzymes secreted in saliva, enzymes secreted in the stomach, enzymes in small intestine
Organs of the digestive system
Two groups of organs:
o The gastrointestinal (GI) tract:
•a continuous tube that extends from the mouth to the anus through the thoracic and abdominopelvic cavities
•Mouth, most of the pharynx, oesophagus, stomach, small intestine, and large intestine
oThe accessory digestive organs: •Teeth, tongue, salivary glands, liver, gallbladder, and pancreas - These are the organs that secrete some really important liquids into GI tract to help GI tract do its thing
List in order the functions of the disgestive system
•Ingestion: taking food into mouth.
•Secretion: release of water, acid, buffers, and enzymes into lumen of GI tract.
•Mixing and propulsion: churning and propulsion of food through GI tract.
•Digestion: mechanical and chemical breakdown of food.
•Absorption: passage of digested products from GI tract into blood and lymph.
•Defecation: elimination of faeces from GI tract.
Layers of the GI tract
o Four-layered arrangement of tissues: in the wall of the GI tract from the lower oesophagus to the anal canal
oFrom deep to superficial:
•Serosa/Adventitia (visceral peritoneum)
Mucousa ( layer of GI tract)
•A mucous membrane
•Inner lining of the GI tract
•A layer of epithelium: in direct contact with the contents of the GI tract
•Lamina propria: a layer of connective tissue
•Muscularis mucosa: a thin layer of smooth muscle ( Important for allowing the epithelial layer to move around and come in contact with more food )
Mucosa epithelium consists of?
Nonkeratinized stratified squamous:
‒in mouth, oesophagus and anus
‒serves as a protective layer
–in the stomach and intestines
–functions in secretion and absorption
•Exocrine cells: secrete mucus and fluid into the lumen of the tract,
•Enteroendocrine cells: secrete hormones
Mucosa (Layer of GI tract )
The lamina propria consists of ?
Loose areolar connective tissue: adheres the epithelium to the lower layers,
• Blood and lymph vessels: transport absorbed food to other tissues,
• Mucosa associated lymphatic tissue (MALT):
‒Prominent lymphatic nodules containing immune system cells.
‒Present all along the GI tract, especially in the tonsils, small intestine, appendix, and large intestine.
Function of the muscularis mucosa:
•Throws the mucous membrane of the stomach and small intestine into many small folds which creates an Increase in surface area for digestion and absorption. Movement of the epithelial layer allowing it to come in contact with food
The submucosa consists of?
oBlood and lymphatic vessels:receive absorbed food molecules.
o Submucous Plexus:
•An extensive network of neurons from the autonomic nervous system.
•Innervates mucosa, blood vessels and secretory cells of mucosal glands.
o Glands and lymphatic tissue
The Muscularis consists of:
Skeletal muscle: in mouth, pharynx , upper oesophagus and anus
•control over swallowing and defaecation
oSmooth muscle: rest of the tract (about 90%)
•mixes, crushes and propels food along by peristalsis
oMyenteric plexus (Motor, command centre of the gut)
•Innervate circular (below) and longitudinal (above)smooth muscle layers
The Serosa consists of
•a serous membrane
•Superficial layer to the GI tract organs that are suspended in the abdominopelvic cavity
• part of the visceral peritoneum
o Secretes slippery fluid = Important as it allows the GI tract to sit in the right orientation
What is the GI tract regulated by?
•Enteric Nervous System (ENS): an intrinsic set of nerves
Is able to perfrom all the functions of GI tract independent of any other neural input. regulates the amount of absorption to occur, how much and what type of motility (motion), amount of secretions
The activity of the enteric nervous system can be influenced by other nervous systems so our ANS can Influence the activity of our gut
•Autonomic Nervous System (ANS):an extrinsic set of nerves
•Gastrointestinal Reflex Pathways
What are the two plesuses of the Enteric nervous system
•Myenteric plexus:control gastric motility
•Submucosal plexus:control the secretory cells
Enteric Nervous System (ENS): the “brain of the gut,” can function...
oCan function independently of the CNS and ANS
Enteric Nervous System (ENS)
consists of 100 million neurons that extend from the oesophagus to the gut
• motor neurons, interneurons, and sensory neurons
- Contains more nerves than your spinal cord!
Great big tube that extends from your mouth all the way to your anus that’s packed full of nervous.
Fully independent, fully functioning nervous system
The enteric nervous system lets the CNS and ANS know what's going on but can function on its own
What is the connection between the ANS and ENS ?
oAutonomic Nervous System (ANS):
Forms neural connections with ENS and regulates functions of ENS - The ANS talks to the ENS but the ENS can function on its own
- Parasympathetic nerves: (rest and digest nerves) increase GI secretion and motility by increasing the activity of ENS neurons.
- Sympathetic nerves:decrease GI secretion and motility by inhibiting the neurons of the ENS.
What are the Gastrointestinal Reflex Pathways?
and their function?
chemoreceptors and stretch receptors are associated with the sensory neurons of the ENS synapse with other neurons located in the ENS, CNS, or ANS.
o Regulate secretions and motility in response to stimuli present in the lumen.
What is the peritoneum ?
name and define the two divisions and cavity
•The largest serous membrane of the body. (serous membrane = lines outside of organs)
•The parietal peritoneum: lines wall of abdominal cavity, surrounding entire abdominal cavity
•The visceral peritoneum: covers some of the organs of the GI tract and constitutes their serosa, surrounds the gut (GI tract)
oPeritoneal cavity: The potential space between the parietal and visceral portions of the peritoneum
•contains serous fluid.
What are the peritoneal folds?
They are a part of the peritoneal layers that are folds
•Weave between the viscera.
•Bind the organs to one another and to the walls of the abdominal cavity.
•Contain blood vessels, lymphatic vessels, and nerves of the abdominal organs.
What are the five major peritoneal folds ?
What are the small and major salivary glands of the mouth?
Salivary Glands: secrete saliva into the oral cavity
oSmall salivary glands: In mucous membrane of the mouth and tongue
oMajor salivary glands: lie outside the oral mucosa (exocrine glands)
Where do you find the major salivary glands?
oParotid:inferior and anterior to the ears, between the skin and the masseter muscle
oSubmandibular: medial and partly inferior to the body of the mandible
oSublingual glands: beneath the tongue and superior to the submandibular glands.
Name some functions of saliva
o Water:dissolve foods to be tasted by gustatory receptors and begin digestive reactions.
o Mucus:lubricates food to facilitate bolus formation and swallowing
o Bicarbonate and phosphate ions: buffer acidic foods that enter the mouth
o Chloride ions: activate salivary amylase
o Salivary amylase: starts the breakdown of starch in the mouth
o Lysozyme:kills bacteria
o Immunoglobulin A (IgA):prevents attachment of microbes to the epithelium,
o Help remove waste molecules from the body, which accounts for the presence of urea and uric acid in saliva
o Protects mouthfrom infection with its rinsing action -1 to 1.5 liters/day
o Keep the mucous membranes of the mouth and pharynx moist
What controls salivation?
Salivation is entirely under autonomic nervous control
What increases salivation and what stops salivation ?
•sight, smell, sounds, memory of food, tongue stimulation -rock in mouth
•parasympathetic nerves (CN 7 & 9) (digestive nervous system, saliva compliments disgestion)
• dry mouth when you are afraid
• sympathetic nerves
What part of the Pharynx has digestive functions ?
What is the function?
oropharynx and laryngopharynx have digestive as well as respiratory functions.
oFunction: Propel food into the oesophagus and then into the stomach.
Location & Function of the oesophagus?
a collapsible, muscular tube that lies behind the trachea and connects the pharynx to the stomach.
o Function: Secrete mucus and transport food to the stomach.
Moves a bolus from the mouth to the stomach.
oFacilitated by saliva and mucus
oOrgans involved: The mouth, pharynx, and tongue
What are the 3 stages of swallowing/deglutition ?
- •Voluntary stage
- •Pharyngeal stage (involuntary)
- •Oesophageal stage (involuntary)
Explain the Voluntary stage of swallowing
Begins when the bolus is forced into the oropharynx by tongue movement upward and backward against the palate
Explain the pharyngeal stage of swallowing
The bolus in the oropharynx stimulates receptors, which send impulses to the deglutition centre in the medulla oblongata and lower pons of the brain stem.
•The soft palate and uvula move upward to close off the nasopharynx,
•The epiglottis closes off the opening to the larynx (so we don't breathe it in), The bolus moves through the oropharynx and the laryngopharynx.
•The upper oesophageal sphincter relaxes and the bolus moves into the oesophagus.
Explain the oesophageal stage of swallowing?
begins when the bolus enters the oesophagus.
o Peristalsis: A progression of coordinated contractions and relaxations of the circular and longitudinal layers of the muscularis,
•pushes the bolus further down the tract
o Lower oesophageal sphincter: Relaxes as the bolus approaches the end of the oesophagus, and the bolus moves into the stomach.
o Oesophageal glands: Secret mucus to lubricate the bolus and reduce friction.
Functions of the stomach
begins at the bottom of the oesophagus and ends at the pyloric sphincter.
•Mixes saliva, food, and gastric juice to form chyme. (digestive juices)
•Serves as reservoir for food before release into small intestine.
•Secretes gastric juice.
•Secretes gastrin into blood.
Layers of the stomach wall
Layers of the stomach wall:
What does the mucosa contain?
oSurface mucous cells/Epithelial layer: secrete mucus
oMuscularismucosae: smooth muscle
oGastric glands: Form gastric pits
What are the 3 types of exocrine gland cells?
and what is the enteroendocrine cell?
These cells promote chemical digestion
Three types of exocrine glands cells:
•Mucous neck cells
•Chief or zymogenic cells
•Parietal or oxyntic cells
Function of the G cells (enteroendocrine gastric cell)
G cell sits right at the bottom of gastric pit and secretes hormone into the bloodstream
And has a big effect on the motility and secretions of the gut itself
The gut can now sense what's inside it.
G cells can sense the chemical composition of what's coming into the stomach and it secretes varying amounts of gastrin in response to that composition
Name the 3 layers of the muscularis stomach layer
The serosa is part of the visceral peritoneum.
name the two curvatures
•At the lesser curvaturethe visceral peritoneum extends upward to the liver as the lesser omentum.
•At the greater curvature the visceral peritoneum continues downward as the greater omentum and drapes over the intestines.
Stomach: mechanical digestion
Explain the waves , Function and gastric empyting
oMixing waves: peristaltic movements pass over the stomach every 15 to 25 seconds.
Function: macerate food, mix it with secretions of the gastric glands, and reduce it to a soupy liquid called chyme.
o Gastric emptying: As food reaches the pylorus, each mixing wave periodically forces about 3 mL of chyme into the duodenum through the pyloric sphincter,
** Periodically = Allows duodenum to decide which parts need to be further digested in the stomach or to relax the pyloric sphincter and undergo gastric entering
Stomach: chemical digestion
Function of Hydrochloric acid?
•Kills microbes in food
•Denatures (unfolds) protein molecules - when unfolded it increases the surface area for chemical digestion
•Activates pepsinogen into pepsin
Stomach: Chemical Digestion
Function of intrinsic factor?
•Absorption of vitamin B12 for RBC production
Stomach: Chemical Digestion
Function of Pepsin?
Breaks down proteins into peptides
•Effective acidic environment.
activated by the acidic PH of the stomach
Stomach: Chemical Digestion
Function of Gastric lipase ?
Splits short chain triglycerides of milk into fatty acids and monoglycerides
•most effective at a pH of 5 to 6 (as in an infants stomach)
•has a limited role in the adult stomach, more imporant in infants
stomach: chemical digestion
Function of mucus ?
Forms protective barrier that prevents digestion of stomach wall.
Stomach: chemical digestion
Function of gastrin hormone ( G cell)
•Acts as a local endocrine hormone.
•Stimulates parietal cells to secrete HCland chief cells to secrete pepsinogen
•Contracts lower oesophageal sphincter,
•Increases motility of stomach,
•Relaxes pyloric sphincter