What structures make up the digestive system and what are their functions?
- oral cavity, teeth, tongue
Mechanical processing, moistening, mixing with salivary secretions - pharynx
Muscular propulsion of materials into the esophagus - esophagus
Transport of materials to the stomach - stomach
Chemical breakdown of materials by acid and enzymes; mechanical processing through muscular contractions - small intestine and large intestine
Enzymatic digestion and absorption of water, organic substrates, vitamins and ions
What are the accessory organs of the digestive system?
- salivary glands
Secretion of lubricating fluid containing enzymes that breakdown carbohydrates - liver
Secretion of bile (important for lipid digestion), storage of nutrients, and many other vital functions - gallbladder
Storage and concentration of bile - pancreas
Exocrine cells secrete buffers and digestive enzymes; endocrine cell secrete hormones
What are the 4 layers of the digestive system tissue?
- mucosa
- sub mucosa
- muscularis externa
- serosa
What is the peritoneum?
- blood vessels and nerves reach the small and large bowel through the mesenteries
- enables folding of the lengths of bowel into the abdominal space whilst keeping these vessels and nerves untangled
- provides serous fluid, minimising friction due to movement within the lumen
What is mechanical and chemical digestion?
MECHANICAL
- through chewing and muscular action, the food is broken down in to smaller pieces until it has a fluid like consistency
- as it leaves the stomach, what was food is now called chyme
CHEMICAL
The addition of enzymes to the food causes chemical bonds to be broken.
- proteins become their component amino acids
- starches and larger polysaccharide sugars become monosaccharides
- larger lipid molecules become smaller component molecules
How does food move through the GI tract?
- swallowing into the esophagus - peristalsis
- churning in the stomach
- expulsion in to the small intestine ( small bowel) by peristalsis
- mass movement in the large colon ( large bowel)
- defecation from the anus
How is the function of and movement within the GI tract controlled?
- Local factors
- primary stimulus for digestive activities
Eg. PH of contents in the lumen, physical distortion of digestive tract wall, presence of chemicals - Neural control mechanism
SHORT REFLEXES
- myenteric plexus neurone to local changes
LONG REFLEXES
- interneurones and motor neurones of CNS generally controlling movement from one digestive tract region to another - Hormonal control mechanism
- peptides produced by enteroendocrine cells within epithelium of digestive tract
- at least 18 hormones that effect digestive and other systems
What role does the mouth and it’s structures play in digestion?
HARD PALATE
underlain with bone, hard surface to aid with mechanical digestion
SOFT PALATE
Skeletal muscle, rise to close off nasopharynx when we swallow
TONGUE
skeletal muscle and mucous membrane
Papillae- taste buds grip and repositions food, mixes food with saliva and form bolus.
Forms words
What are the main functions of saliva?
- Salivary glands add enzymes to the food and moisten it
- salivary amylase begins starch digestion
- mucous moistens food
- antibodies and proteins released for immune protection
How is food swallowed - BUCCAL PHASE
- Compression of the bolus against the hard palate.
- tongue retracts, forces bolus into oropharynx and assists in elevation of soft palate, sealing of nasopharynx
- buccal phase is voluntary
- once bolus enters oropharynx, reflex responses are initiated and bolus is moved toward the stomach
What is the overarching role of the digestive system?
BREAKS DOWN INGESTED FOOD FOR USE BY THE BODY
- chemical breaking of bonds releases energy and helps to maintain body temperature
- rebuilding the food molecules into those needed by the body requires energy.
EXCRETES WASTE PRODUCTS AND UNDIGESTIBLE FIBRE AS FAECES THROUGH THE PROCESS OF DEFAECATION
How is food swallowed - PHARYNGEAL PHASE
Begins with receptors on palatial arches and uvula are stimulated. Motor commands from swallowing centre in the medulla oblongata then direct a coordinated pattern of muscle contraction in pharyngeal muscles. Elevation of the larynx and folding of epiglottis results from contraction of the pharyngeal muscles, while palatial muscles elevate uvula and soft palate to block the entrance of the nasopharynx. Pharyngeal constrictions force bolus through pharynx, past the close epiglottis and in to the esophagus.
How is food swallowed? - OESOPHAGEAL PHASE
Begins as contraction of pharyngeal muscles forces the bolus though the entrance to oesophagus. Bolus is then pushed towards stomach by peristalsis. Approach of bolus triggers the opening on the lower oesophageal sphincter, and the bolus then continues to the stomach
What are the main functions of the oesophagus?
- passes through the diaphragm and terminates at oesophageal sphincter
Digestive functions of the oesophagus
- muscles secreted and muscular contractions propel food into the stomach
What are the functions of the stomach?
- mixing chamber and storage area for ingested food
- rugby allow for increased volumes
- pyloric sphincter regulates movement of chyme into the small intestine
- site of mechanical and chemical digestion
MECHANICAL
mixing waves causes peristalsis
CHEMICAL
formation of hydrochloric acid
What are the main stomach secretions and role in digestion?
MUCOUS EPITHELIAL CELLS
- simple squamous epithelium that produces alkaline mucous to protect epithelial cells from acids and enzymes
PARIETAL CELLS
- secrete intrinsic factor and HCI
- intrinsic factor aids B12 absorption
- HCI activates pepsinogen (keeps stomach at pH 2)
CHIEF CELLS
- secrete pepsinogen
- activates in acid to become pepsin
- activates proteolytic (protein binding) enzyme
G CELLS
- release gastric hormones in to blood in response to muscle stretch receptors and chemical receptors
- produce a variety of hormones
How is digestion controlled in the stomach?
CEPHALIC PHASE - you see, smell, taste - signal from CNS to Autonomic NS - vagus nerve stimulates mucous cells, chief cells, parietal cells and G cells - results in mucous cells releasing mucous ~ chief cells = pepsinogen ~ parietal cells = HCI ~ G cells = gastrin
GASTRIC PHASE
- food arrives at stomach
- causes distension, pH increases
- distension and increased pH stimulates receptors that cause increased cell secretions and mixing wave
- peptides and amino acids in chyme stimulates secretion of hormone gastrin.
INTESTINAL PHASE
- chyme enters small intestine
- in the stomach, contents are moving toward the small intestine, so there is no more distension and no more stimulation of stretch receptors
- stretch receptors and chemoreceptors of duodenum stimulated. This triggers enterogastric reflexes
- enterogastric reflexes inhibits gastrin production and gastric contractions and stimulates contractions of pyloric sphincter
What is the main role of the liver?
Metabolism of:
~carbohydrates
~lipids
~proteins
- processes drugs and hormones
- excretes bilirubin
- synthesis of bike salts and bile
- storage of glycogen, vitamins and minerals
- phagocytosis
- activates vitamin D
What is the main role of the gall bladder?
Secretes bile to emulsify lipid droplets
What is the main role of the pancreas?
- pancreatic islets (of langerhans)
- glucagon: increases rate of glycogen to glucose conversion
- insulin: fir glucose storage
- somatostatin: slows down digestion by inhibiting digestion and absorption
What is the structure of the small intestine?
- main role is digestion and absorption
- 6m in length
- divided into sections
~ duodenum
~ jejunum
~ ileum - connected to large intestine by ileocaecal sphincter
How is the small intestine adapted for digestion and absorption?
- brush border enzymes
- accepts bike and pancreatic secretions
- large surface area
- good blood supply and lymphatic capillaries
What is the structure of the large intestine?
Colon
- ascending
- transverse
- descending
- sigmoid
CAECUM
- pouch
- attached appendix
RECTUM
ANAL CANAL
- anal columns
- anus
- internal and external sphincter
What are the major roles of the large intestine?
- production and storage of faeces
- water reabsorption to produce semi-dried chyme
- absorption of bacterial products and vitamins
