GIT Flashcards
6 major functions of GIT
- ingestion
- digestion
- motility
- secretion
- absorption
- elimination
Also… - immune defence
- nutrition & metabolism
Layers of the GIT
1) Mucosa
- epithelium
- lamina propria (loose CT, rich in blood)
2) Submucosa
- dense irregular CT, large vessels
- submucosal glands, secrete buffers and enzymes
- submucosal plexus
3) Muscularis externa
- smooth muscle
- myenteric plexus
4) Serosa
- serous membrane
lining of the small intestine is made up of…
Plicae circularis - folds supported by submucosa Villi - finger like projections of mucosa Microvilli - tiny structures on enterocytes (absorptive cells) - increase surface area 600-fold
what is each villus lined by?
simple columnar epithelium
- enterocytes
- goblet cells
- enteroendocrine cells
- paneth cells
- stem cells
regulation of motility and secretion of GIT through neural mechanisms
Extrinsic (autonomic) - parasympathetic (stimulate) - sympathetic (inhibit) Intrinsic (enteric nervous system) - submucosal plexus (local effects) - myenteric plexus (whole tract motor control)
regulation of motility and secretion of GIT through hormonal mechanisms
enteroendocrine cells (EEC) are present (peptides)
1) gastrin - from stomach
2) cholecystonkinin (CKK) - from SI
3) secretin - from duodenum
4) GIP - from SI
5) motilin - throughout gut
6) somatostatin - stomach and duodenum
GIT smooth muscles
- small spindle shaped cells form tube
- in muscularis mucosa and externa
- are syncytium-electrically coupled
- joined by gap junctions
- contractions are slow, waved
types of intestinal motility
Peristalsis - move intestinal content distally - more longitudinal muscle cells Segmentation - mixing of chyme - more circular muscle cells
3 paired salivary glands in oral cavity
Parotid - water and enzymes
Submandibular - mucous, enzymes and water
Sublingual - mucous
content and function of saliva
- lubrication, coating and protection
- 99% water
- high K+ and HCO3-
- IgA (immune defence)
3 stages of swallowing
1) oral stage
- voluntary
- upper oesophageal sphincter (UES) closed
2) pharyngeal stage
- involuntary
- UES open
- epiglottus blocks larynx
2) oesophageal stage
- involuntary
- USE re-closed
dysphagia
difficulty in passage from mouth to stomach
layers of the stomach wall
Mucosa
- simple columnar epithelium and lamina propria
Gastric pits
- depression in mucosa
Gastric glands
- secrete mucous, gastric juice and gastrin
HCL secretion in the stomach
1) formation of carbonic acid (CO2 + H2O -> H2CO3)
2) dissociation of carbonic acid (H2CO3 -> H(+) + HCO3-)
3) H+ pumped from cell to lumen by proton pump (H-K-ATPase)
4) Cl- and HCO3- antiporter transports HCO3- into plasma and Cl- from plasma into cell
5) Cl- pumped into lumen through facilitated diffusion
6) Cl- and H+ join in lumen to form HCl
roles of HCL
- chief cells secrete pepsinogen
- HCL activates pepsinogen to pepsin
- HCL converts ingested Fe3+ to Fe2+
stomach protective mechanisms
1) Mucosa coat
- highly alkaline mucous produced by neck cells
- resists action of acids and enzymes
- protects from auto-digestion by pepsin and HCl
2) Tight gap junctions between epithelial cells
3) Cell replacement
Phases of digestion
1) cephalic phase
- prepare stomach for food
2) gastric phase
- stomach acts on food
3) intestinal stage
- maximise absorption
segments of the SI
duodenum - like a mixing bowl - mixing chyme with secretions for pancreas and liver jujunum - most chemical digestion and absorption - increase in SA ileum - B12
exocrine cells of the pancreas
Acinar cells
- contain zymogen granules with digestive enzymes
Ductal cells
- produce alkaline solution to help move enzymes
- neutralise acidic chyme
endocrine cells of the pancreas
- Insulin
- Glucagon
- released from beta-cells
regulation of pancreatic exocrine secretion
- primarily controlled by hormones from duodenum
- also by the digestion and absorption of nutrients in the small intestine
- sectretin - works on ductal cells (release buffers HCO3-)
- CCK - works on acinar cells (releases digestive enzymes)
functions of the liver
1) metabolic functions
- carb metabolism (maintain blood glucose)
- lipid metabolism
- synthesis proteins and hormones
- drug metabolism
- RBC recycling
- storage
2) Bile production
- bile salts break down fat particles into tiny particles so they can be attacked by lipase
- bile salts are reabsorbed in the SI
3) Blood circulation and filtration
regulation of bile secretion
trigger for bile production is fatty acids and amino acids entering the duodenum and is stimulate by
- PSNS
- sectretin - works on ductal cells
- CCK - works on acinar cells
nutrient absorption
carbs - monosaccharides - blood
proteins - amino acids - blood
fats - glycerol, fatty acids - lymph/blood
lipid digestion
1) fatty acids enter epithelial cell
2) bind together to form triglyceride
3) combine with protein to for chylomicron
4) exit cell into lymph
5) lymph takes it away
carbohydrate digestion
carbohydrate - salivary a-amylase polysaccharide - pancreatic a-amylase monosaccharide - active transport monosaccharide in blood
protein digestion
- pepsin breaks down amino acids in the stomach
- enzymes begin acting in small intestine
- absorption - Na co-transporter
elimination of waste
Defecation reflex - started by distention (stretch receptors in sigmoid colon and rectum) Internal sphincter - smooth muscle - PSNS - relax - SNS - constrict External sphincter - skeletal muscle - somatic spinal efferent - voluntary control
Vitamin A
Biochemistry - retinol - transport and storage of Vit A - retinal - vision - retinoic acid - cell differentiation and growth Functions - maintains epithelial cells - night and peripheral vision - bone growth Sources - liver - yellow/orange veges - fats of milk and eggs
Vitamin D
Occurs in 2 forms - Vit D3 or cholecalciferol - from sunlight - Vit D2 or ergocalciferol - limited range of foods Functions - enhance dietary Ca absorption - enhance dietary P absorption - bone mineralisation Calcitriol is active form
Vitamin E
- prevents oxidisation of polyunsaturated fats in the body (phospholipid cell membranes)
- reduces risk of CVD
Vitamin K
- synthesised by bacteria in the gut
- assists in blood clotting
- found in green leafy veges, wheat bran and plant oils
