GI: Physiology Flashcards
What is secreted in the GI tract?
- Water
- Acid
- Alkali
- Enzymes
- Mucus
- Waste products
- Emulsifiers
- Intrinsic factor
Where do secretion come from in the GI tract?
- Saliva (acini of salivary glands)
- Gastric (gastric glands)
- Intestinal (Brunner’s glands, intestinal glands, Goblet cells)
- Liver (hepatocytes)
- Pancreas (exocrine pancreas)
What are the purposes of stomach acid?
- Innate barrier to infection
- Prepares proteins for digestion
- Activate enzymes
What are the emulsifiers in the GI tract and their purpose ?
- Bile salts
- Increase surface area of lipids
- Aids digestion by lipases
- Allows lipid breakdown products to be transported in the gut
What is the purpose of mucus in the GI tract?
- Protects against chemical damage due to acidic environment in stomach
- Protects against bacteria in small intestine
- Habours bacteria in large intestine
- Lubricates to reduce friction
- Forms physical barrier against bacterial inflammation
What are the principles of absorption in the GI tract?
- Movement across enterocyte
- Movement paracellularly
How is the large surface in the Gi tract created?
- Plica circulares (Permanent folds in small intestine)
- Villi
- Microvill
What are the effects of disrupting surface area in the gut?
- Diarrhoea
- Malnutrition
- Anaemia (Crohn’s, Coeliac)
What is absorbed in the GI tract?
- Nutrients (carbohydrates, proteins, lipids, fat soluble vitamins, Vitamin B12, Bile salts, Ca2+, Fe2+)
- Electrolytes
- Water
How is water reabsorbed in the GI tract?
Passive
-After a meal, water uptake is driven by nutrients coupled with Na+ (sodium co-transporters)
In between meals
-Na+ and Cl- absorbed (sodium/hydrogen and chloride/bicarbonate exchangers)
In colon
-Additional mechanism so that stool can be desiccated (ENaC)
What are the layers of the gut tube?
- Mucosa
- Submucosa
- External muscle layers
- Serosa
What are the regions of the mucosa?
- Epithelial layer
- Lamina propria
- Muscularis mucosa
What is the purpose of the epithelial layer?
- Selectively permeable barrier
- Facilitate transport and digestion of food
- Promote absorption
- Produce hormones
- Produce mucus
What is the purpose of the lamina propria?
- Lots of lymphoid nodules and macropahes
- Produces antibodies to protect against bacteria/viral invasion
What is the purpose of the Muscularis Mucosae?
-Layer of smooth muscle in different direction to help keep crypt contents dynamic and epithelium in contact with gut contents
What are the contents of the submucosa?
- Contains dense connective tissue, blood vessels, glands, lymphoid tissue
- Contain submucosal plexus (Meissner’s)
What are the contents inner circular muscle?
-Myenteric plexus
What are the contents of the serosa?
- Blood and lymph vessels and adipose tissue
- Continuous with mesenteries
What are the epithelial regions of the gut?
- Stratified squamous in oesaphagus and distal anus
- Everything in between is simple columnar
What is an enterocyte?
- Simple columnar epithelial cell that absorbs
- Predominant cell of small intestine and colon
- One cell thick
- Has microvilli
- Blood vessels/lymphatics lie immediately below the basolateral membrane of the enterocyte
What is the purpose of the enterocyte?
- Apical membrane (faces the lumen)
- Basolateral membrane (faces blood vessels)
What is the structure of the goblet cells?
- Has a wide top and pushed down base nucleus
- Has a terminal bar, mucus droplet and basal nucleus
- Produces mucus to protect the epithelia against pathogens and keeps some bacteria alive
- Scattered in between enterocytes in intestinal epithelia
What the defences of the stomach ageing acid?
- Surface mucus cells line gastric mucosa/gastric pits in stomach and secrete mucus/HCO3 that forms thick alkaline viscous layer that adheres to stomach epithelium acid to protect the stomach and keep the epithelial surface at a higher pH
- High turnover of epithelial cells to help keep epithelia intact
- Prostagladins to maintain mucosal blood flow to supply epithelium with nutrients
What are crypts of lieberkuhn (intestinal gland) and what do they contain?
Intestinal glands
- Stem cells
- Paneth cells
- Enteroendocrine cells
What is the purpose of the cells in the crypts of lieberkuhn?
Stem cells
- Constantly divide to replace epithelia (2-4 days)
- Mature as they migrate to surface
Paneth cells
- Located at base of crypts
- Secrete antibacterial proteins to protect stem cells
Enteroendocrine cells
- Secrete hormones to control functions of the gut
- Hormones such as gastrin, CCK and secretin
What do the acini glands tend to secrete?
- Serous (+enzymes) secretions
- Tubules tend to secrete mucous (Brunner’s glands)
*salivary glands can be mixed
What is ulceration in the GI tract?
- Erosion through muscularis mucosae
- Failure of protective merchiasnm such as mucus production
What is the oral preparatory phase?
- Voluntary
- Pushes the bolus towards the pharynx
- Once bolus touches the pharyngeal wall, pharyngeal phase begins
Describe the structure of the oesophagus and outline its functions
- Muscular layers
- Transport of bolus from oral cavity to the stomach by peristalsis
Outline the pharyngeal phase of swallowing?
- Involuntary
- Soft palate seals of the nasopharynx
- Pharyngeal constrictors push bolus downwards
- Larynx elevates closing epiglottis
- Vocal cords duct to protect the airway and breathing temporarily ceases
- The upper oesophageal sphincter opens
Describe the anatomical mechanisms that prevent gastro-oesophageal reflux.
- Lower oesophageal sphincter (diaphragm)
- Intra-abdominal oesophagus which gets compressed when intra-abdominal pressure rises
- Mucosal rosette at cardia to prevent back flow
- Acute angle of entry of oesophagus
Give an overview of the control of saliva production
-Autonomic control
-Mainly parasympathetically controlled to stimulate salivary secretion
-Sympathetic also causes small amounts of saliva
secretion and can also vasoconstrict blood vessels
What is the structure of a salivary cell?
- Acinus line with acinus cells
- Ductal portion with ductal cells
- Myoepithelial cells
Outline the production of saliva.
- Acinus produces initial saliva which is isotonic and releases it into the ductal portion
- Ductal cells modify the initial solution to produce hypotonic saliva.
- Myoepithelial cells which help move saliva from the structure into the mouth
What is the purpose of kallikrein released in the saliva?
-Helps to produces bradykinin to vasodilate in the mouth in periods of maximum activity to allow blood flow to get to salivary glands
How does saliva flow rate affect its modification by duct cells?
- Increased flow rate of saliva results in less modification
- Decreased flow rate of saliva results in more modification by duct cells
*Bicarbonate gets excreted more at higher flow rates as an exception
How do duct cells form hypotonic saliva?
- Exchanging increased amounts of sodium, chloride from the saliva in the lumen into the cell compared to excreting bicarbonate and potassium back into the lumen.
- This results in a hypotonic solution
Outline the oesohaphgeal phase of swallowing?
- Involuntary
- Closure of the upper oesophageal sphincter
- Peristaltic wave carries bolus downwards into oesophagus
Outline the neural control of swallowing and the gag reflex?
- Mechanoreceptors in wall of pharynx detect the bolus
- Glossopharyngeal nerve carries sensory impulses to medulla
- Vagus nerve carries impulses to the Pharyngeal constrictors which contract to cause an effect
- Pushes the bolus inferiorly
*Gag reflex works the same. Psychological possibly
