GASTROINTESTINAL AND HEPATIC SYSTEM Flashcards
WHAT IS THE GASTROINTESTINAL SYSTEM
- digestive system (mouth to anus)
activities of the digestive system:
> ingestion
> propulsion
> mechanical & chemical digestion
> absorption
> defaecation
Mechanical processing:
- Crushing and shearing of food make it easier to move along the digestive tract
- increasing surface area = easier for enzymes to break it down
teeth tear and shear food, tongue squashes food into small fragments = easier to swallow, continuing in the stomach after ingestion = mixing and churning the food
INGESTION: enters the digestive tract via the mouth.
- An active process needs conscious choice
DIGESTION: chemical breakdown of food into small fragments = absorption by digestive epithelium.
- Protein molecules need to be broken down to be absorbed
SECRETION: release of water, acids, enzymes & salts by the epithelium of the digestive tracts
ABSORPTION: movement of electrolytes, vits & water across digestive epithelium into interstitial fluids of digestive tracts
EXCREATION: removal of waste from body fluids
COMPONENTS OF GASTROINTESTINAL SYSTEM
- mouth
- pharynx
- Oesophagus
- stomach
- small intestine
- large intestine
- rectum
- anal canal
3 pairs of salivar glands: liver & billary duct
EMBRYOLOGY OF GI TRACT
- foregut: mouth, oesophagus, stomach, duodenum
- Midgut: duodnum, jejunum, ileum 2/3 colon
- Hindgut: transverse, desending, sigmoid and anal canal
FOETAL DEVELOPMENT OF THE GI TRACT
6-8 weeks: rapid growth, gut = extrusion from the abdominal cavity
10 weeks: withdraw back into the abdomen
14 weeks: peristalsis evident
16 weeks: swallow amniotic fluid
24 weeks: anatomically complete
24-28 weeks: Digestive enzymes start to function
34 weeks: co-ordination of sucking, swalling and peristalsis
Birth: reduce stomach acid, meconium passed
HSITOLOGY OF GI TRACT
osephugus - anus, walls of alimentary canal
Mucosa: secreate hormone & mucous, absorbs & production of digestion & protects against infection
- inner lining of the digestive tract
- Mucous membrane: functions to absorb nutrients from the end products of the digestive system to prevent infection
- epithelium: simple or stratified
Submcosa: contains lymphoid follicles and elastic tissue
- a layer of dense irregular connective tissue supporting the mucosa + joins overlying muscles that cover it
- consists of blood vessels and exocrine glands that secrete enzymes into the lumen of the digestive tract.
- serous membrane = visceral peritoneum
Muscularis externa: segmentation & peristalsis, contains inner circular layer & outer longitudinal layer
Serosa: visceral peritoneum, made of areolar connective tissue
- lining of digestive tract = longitutidnal folds & transverse fold (PLICAE) = expands & disappear as tract fills = expanding surface area avaliable for absorption of liquid and nutrients
- external part of the layer consisting of blood vessels, nerve endings, smooth muscle cells called LAMINA PROPRIA (LP)
- The upper GI tract contains secretory cells of mucous glands
- Smooth muscle cells in the area contract to change the shape of the lumen and move the epithelial pleats and folds
Muscularis Externa: dominated by the smooth muscle & surrounds the submucosal plexus
- 2 layers: an inner circular layer and an outer longitudinal layer.
- control the peristalsis (waves of contractions) controlled by the parasympathetic division of the autonomic nervous system
- Parasympathetic stimulation increases muscular relaxation
- Sensory neurons of the enteric nervous system between two muscles layers
LAYERS OF THE GI TRACT
Circular
Inner
Longitudinal
Outer
LAYERS OF THE WALL OF THE ALIMENTARY CANAL
Outer layer: adventitia or serosa - loose fibrous tissue in the thorax
- The abdomen is a serous membrane - the peritoneum
Muscle layer: 2-layer smooth involuntary muscles (lymph & blood vessels
- Peristalsis & mixing food with digestive juices
Submucosa: loose areolar connective tissue = collagen & elastic fibres = binds muscles to mucosa, contain:
> blood vessels
> nerve
> lymph vessels
> lymphoid tissue
Mucosa: urocal lining: outer layer of mucous membrane - columnar epithelium = protection, secretion & absorption
- connective tissue supports blood vessels and lymphatic tissue
- deepest layer = thin layer of smooth muscle
PERIONEUM AND MESENTERIRES
NERVE SUPPLY
- alimentary canal + accessory organs supplied with parasympathetic and sympathetic divide of the autonomic nervous system
- parasympathetic nervous system
parasympathetic on digestive system =
> increased muscular activity
> increased glandular secreation
ORAL CAVITY
- formed by the hard palate, formed by the palatine process of maxillary bones, covered with a hard ridge (raphe) (tongue compressed with food against the ridge provides traction with moving the food around the mouth
- soft palate covered by thinner mucosa
- tongue dominates the space on the floor of the buccal cavity and joins the uvula = helping to prevent food from entering the pharynx first
DIGESTION IN ORAL CAVITY:
- mouth responsible for mechanical breakdown of food from bolus before swallowing
CHEMICAL DIGESTION:
> lingual lipase (fats)
> Salivary amylase (carbs)
SALIVARY GLANDS
3 PAIRS OF SALIVARY GLANDS:
> PAROTID: salivary amylase (duct called Stensen’s duct draining saliva from the parotid gland into the mouth (upper cheek) Salivary amylase is an enzyme that breaks down carbs or starch
> SUBLINGUAL: lubricant (buffer) the floor of the mouth & keep the oral cavity moist and lubricated through the ducts
> SUBMANDIBULAR: glycoproteins & analyses - The submandibular glands are found on both sides, just under and deep to the jaw, towards the back of the mouth.
- The submandibular duct, called Whartin’s duct, enters the floor of the mouth under the front of the tongue.
- glands have fibrous capsuals = lobules small acini lined with secretory cells
- Saliva is released into ductules, which become larger ducts.
- Saliva is from the salivary glands and mucus-secreting glands in the oral mucosa.
- Secretion of saliva is controlled by the autonomic nervous system
Parasympathetic stimulation results in profuse secretion of watery saliva
Sympathetic stimulation results in secretion of small amounts of saliva rich in organic material especially from the submandibular glands
Saliva production is also a reflex action when food is in the mouth
FUNCTIONS OF SALIVA
- Chemical digestion of polysaccharides by amylase
- lubricates food
- cleaning and lubricating the mouth
- non-specific defence
- taste
THE PHARYNX
PHASES OF SWALLOWING (DEGUSTATION)
- Deglutition or swallowing occurs when the bolus has been prepared in the oral cavity.
- The bolus is swallowed down the common passageway for food, liquid and air known as the pharynx.
- The nasopharynx, laryngopharynx and the oropharynx are lined with protective stratified squamous epithelium, which protects against abrasion.
- Food is propelled towards the naso and oropharynx by the tongue and muscles of the mouth and towards the oesophagus.
- The pharyngeal constrictor muscle pushes the food towards the oesophagus, the palatopharyngeus and stylopharyngeus muscles elevate the larynx, and the palatal muscles elevate the soft palate.
- The bolus passes the epiglottis and makes its way down the oesophagus and into the stomach.
PERISTALSIS AND SEGMENTATION
During the oesophageal phase of swallowing, the food bolus is pushed towards the stomach by a peristaltic wave.
The approach of bolus triggers the opening of the lower oesophageal or cardiac sphincter.
Peristaltic waves are controlled by the glossopharyngeal and vagus nerves.
Liquids may make the journey in a few seconds.
Secondary peristaltic waves are triggered by the stimulation of the sensory receptors in the walls of the esophagus.
THE OESPHAGUS
- A muscular tube connects the pharynx to the stomach
- Pierce’s diagram at the oesophageal hiatus
- lined with squamous epithelium changes to columnar epithelium when it meets the stomach
- this is how reflux happens
OESOPHAGUS AND ASSOCIATED STRUCTURES
- Oesophageal glands – produce mucus to lubricate the bolus.
- They are located near the cardiac orifice in the lamina propria
- Esophageal sphincter – prevents backflow into oral cavity
Cardiac sphincter- prevents backflow into esophagus - Pyloric sphincter - allows food to pass slowly into the duodenum.
THE STOMACH
- Temp food storage
- chemical processing of food (acid and enzyme)
- mechanical processing
- release of intrinsic factors for the absorption of Vitamin B12
- Anatomically, the stomach is divided into sections:
MAIN FUNCTION OF THE STOMACH:
> Storage of ingested food
> mechanical breakdown
> acids and enzymes break food down
> production of intrinsic factors
Intrinsic factors, the glycoprotein of vitamin B12, in the small intestine
- ingested materials combine with acids and enzymes = soup-type mixture, digested food called chyme
The stomach is divided into 4 regions:
> cardia: the smallest region & which contains mucous glands, helps to protect the connection with the oesophagus from harmful stomach acids
> cundus: superior portion of stomach & posterior surface of the diaphragm
> Body: largest section, acts as a mixing tank
> Gastric glands secrete the acids and enzymes involved in gastric digestion, located in the mucosa
> pylorus dividing into two sections
> pyloric antrum connects to the body, and the pyloric canal empties into the duodenum.
> muscular pyloric sphincter regulates the chyme into the duodenum
> The muscular pyloric sphincter regulates the release of chyme into the duodeum (secreating hormones)
STOMACH CONT
- The stomach increases and decreases as chyme enters the small intestine
- When the stomach is relaxed, the mucosa folds = Rugae (temporary features allowing the stomach to expand and contract
- Chyme enters the small intestine
- From the lower oesophageal sphincter (not shown) to the pyloric sphincter, where food enters the duodenum. The pyloric sphincter is closed when food is in the stomach, but relaxes when the stomach is inactive.
3 regions fundus, body and pylorus
muscle layers:
3 layers of smooth fibrous
- The rest have 2 layers due to chewing and peristalsis
longitudinal folds
THE STOMACH
- Storage of ingested food
- degraded to chyme
- to the small intestine
gland cells: found in the fundus & body (digestion occurs)
secretory cells:
> mucous cells
> partial cells
> chief cells
> enteroendocrine cells
gland cells below the mucous membrane (simple columnar epithelium (contains gastric pits = secreate gastric juices)
Mucous neck cells produce a thin, soluble mucus, = functions an acid mucus
Parietal cells secrete intrinsic factors (glycoproteins) that absorb vitamin B12 in the small intestine.
This acidity enables the activation of pepsin, a protein-digesting enzyme. The acidity also helps digest food by denaturing (modifying the molecular structure by breaking the weak hydrogen bonds) proteins and breaking down the cell walls of plant foods. It is harsh enough to kill many of the bacteria ingested with foods.
- Chief cells: produce pepsinogen = inactive pepsin. When pepsin is present = it catalyses the conversion of pepsinogen to peptin
- Secretes lipase (digesting fat enzymes)
ENTEROENDOCRINE CELLS: release chemical messenger into the interstitial fluid and lamina propria
- histamine & serotonin locally in the pancreas
- Serotonin = contraction of stomach muscles
- Somatostatin acts as a paracrine and a hormone that diffuses into blood capillaries to influence several digestive system target organs. Somatostatin inhibits the gastric secretion of all products.
- gastrin = hormone & regulates stomach secretion and motility
LAMINA PROPRIA:
> A thin layer of connective tissue forms the moist lining = the mucous membrane or mucosa
>
GASTRIC JUICES
> water: liquifies food
Hydrochloric acid: acidfies food + provides acid to enviroemnt for pepsins
pepsinogen converted to pepsin: an enzyme digests proteins
intrinsic factors: absorb vit B12 in the small intestine
Mucus: protects stomachs from corrosve chemicals and enzymes
DIGESTIVE PROCESSES IN THE STOMACH
> PROPULSION: peristalsis
mechanical breakdown: churning action
digestion: protein. Digestion begins here
Absorption of lipid-soluble substances into the blood. Alcohol
digestion: HCI dietary protein for enzyme digestion by pepsin
INFANTS: stomach glands secrete renins, an enzyme acting on casein (Milk protein)
- converting milk to susbtance that looks sour milk
3 PHASES OF GASTRIC SECRETION
CEPHALIC:
> before the food enters the body
> sight, sound, and thought of food stimulate the nerve in the gastric gland, ready for digestion
>directed by the CNS and prepares the stomach for food
> parasympathetic nervous system innervates mucous cells
GASTRIC PHASE:
> arrival of food in the stomach
> stimulated by nerve reflexes and the vagus nerve
> gastrin stimulates parietal cells to release HCl
> Proteins are digested, and the gastric juice becomes more acidic, inhibiting gastrin-secreting cells = negative feedback mechanism
> Gastrin will stimulate contractions in the muscularis externa, which produce mixing waves that churn materials up.
INTESTINAL PHASE:
> Stimulated digested food fills the duodenum = stimulates the intestinal mucosal cells to release intestinal enteric gastrin
> inhibited by 4 factors:
- dimention of the duodenum
- presence of acid
- fatty or hypertonic chyme
> Signs stomach has stopped secreting because there’s enough HCI already.
NEURAL CONTROLS: vagus nerve-mediated and local enteric nerve reflexes
ACh (acetylcholine) released, stimulating the output of gastric juice
HORMONAL CONTROL: gastrin stimulates secretion of HCI
ACH gastrin and histamine mix to stimulate secreation of HCI
