digestive system Flashcards
The GI tract has
4 layers
Mucosa
- Protective
- Epithelium
- Muscularis mucosae - thin layer of muscles, helps make folds in mucosa, more SA
Submucosa
houses blood vessels and nerves
Muscularis
- Very muscular
- Longitudinal and circular muscle
Serosa
Outside
- connective tissues, nerves, fat cells, blood vessels.
- Slippery layer so GI tract can move freely
- If it touches another organ - adventitia
Oral (buccal) cavity
accessory organs:
* Teeth
* Tongue salivary glands
Tongue salivary glands
-
Parotid
- Largest
- Sits in front of ear -
Submandibular
- Lower jaw -
Sublingual
- Under tongue
Pharynx
- Moves bolus to oesophagus
- Made up of skeletal muscles
Bolus
chewed up food, in a ball
Oesophagus
Propel bolus to stomach
Anatomy
* Muscular tube - (~25cm long)
* Upper - skeletal muscles
* Middle - mixed
* Lower - smooth muscle
Anatomy of the stomach
j -shaped chamber
Very muscular
Rugae - ridges
4 main areas:
* Cardia, fundus, body, pylorus
Muscular valves (sphincter) of stomach
ensure movement is only one way
Top: gastro-oesophageal sphincter
Bottom: pyloric sphincter
Muscle layers of stomach
help with churning motion
1.Longitudinal layer
2.Circular layer
3.Oblique layer
Oriented diagonally
Chyme
what food is called after digestion in the stomach
function of the stomach
- Holding reservoir
- Mixing chamber (chyme)
- Secretion of gastric juices
- Secretion of gastrin
Anatomy of Small intestine
3 regions
1. Duodenum (shortests)
2. Jejunum (2-2.5m) - bulk of where chemical digestion and absorption occurs
3. Ileum (3m) - at the end ileocecal sphincter (ensures chyme travels one way)
Joins large intestine at ileocecal sphincter
At mucosa of small intestine
(at mucosa) Villi: finger-like projections of mucosa
Microvilli: hair-like projections of absorptive cells
function of small intestine
- Receives pancreatic juice and bile
- Secretes intestinal juice
- Major site of digestion and absorption
Pancreas
Endocrine cells releases insulin and glucagon
Exocrine cells release pancreatic juice
Liver
Largest gland
Secretes bile
Gallbladder
Store and concentrate bile
Anatomy of Large intestine
4 major regions:
* caecum, colon, rectum, anal canal
Sphincters:
* Internal anal sphincter (involuntary)
* External anal sphincter (voluntary)
Functon of the large intestine
- Absorption
- Production of certain vitamins
- Formation and elimination of faeces
- Housing microbes
Mechanical digestion
involves the physical breakdown of food into smaller particles
Chemical digestion
assisted via enzymatic activity to form absorbable compounds
If we consume carbohydrates (mostly starches)
we can absorb Monosaccharides (mostly glucose)
If we consume proteins
we can absorb amino acids
If we consume lipids (mostly triglycerides)
we can absorb fatty acids
Oral cavity
- Saliva (99.5% water, 0.5% solutes)
- Mucus lubricates food
- Immunoglobulin A (IgA) prevents microbes from attaching to epithelium
- Mechanical digestion via mastication
- Chemical digestion via action of two enzymes in saliva
1. Salivary amylase digests starch to maltose
2. Lingual lipase digests triglycerides
Oesophagus: motility
Propulsion of bolus via peristalsis
Peristalsis
continuous waves of contraction and relaxation of muscles down the tract
Stomach: Mechanical digestion, motility
Peristaltic contractions ~3/min push bolus towards pylorus (propulsion)
1. Contractions strengthen towards the pylorus grinding contents
2. Pylorus allows 1/10 of chyme it holds to **pass through pyloric valve per contraction
3. Contractions also close pyloric value and remaining contents are propelled back (retropulsion**)
Gastric Pits in the Stomach consist of:
- Pareital cell
- Chief cell
- Mucous cell
- G cell
Parietal cell
- secretes HCl - kills microbes, denatures proteins, converts pepsinogen to pepsin
- Secretes intrinsic factor - needed for transportation & absorption of vitamin B12 in ileum
Chief cell
- release pepsinogen (inactive enzyme) and active form (pepsin) and breaks proteins into peptides
- Secretes gastric lipase - breaks triglycerides into fatty acids and monoglycerides
Mucous cell
release bicarbonate-rich mucus
* lubrication, protects stomach wall from protein digestion, corrosion
G cell
releases gastrin (hormone) into bloodstream, stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen
Bile secreted by liver
- Bile salts: emulsifies fats, facilitate absorption of fat and cholesterol
- Bilirubin: pigment that is a waste product from the breakdown of red blood cells.
- Gallbladder stores and concentrates bile by absorbing water and ions
intestinal juice
- secretion stimulated by acidic chyme
- 1-2 L/day
- pH 7.4-7.8
- Water, mucus, enzyme-poor
Pancreatic juice
- 1.2-1.5 L/day
- Water
- Bicarbonate ions: neutralizes HCl from stomach
- Pancreatic proteases released in active form e.g. trypsinogen activated to trypsin
- Pancreatic amylase, lipases, nucleases
Small intestine: chemical digestion
- Largely occurs in the duodenum
- All forms of organic compounds are digested in the small intestine
- Most enzymes are transported from pancreas
- Brush border enzymes: on plasma membrane of microvilli
Optimal digestive activity relies on…
slow delivery of chyme from stomach
* Regulate against hypertonicity of entering chyme
* Allows low pH of chyme to be neutralised
Small intestine: motility & absorption
Peristalsis moves chyme through intestinal tract, ~16 contractions/min
Segmentation the most common motion in the jejunum
* Localised contractions and relaxations move chyme back and forth a few centimetres
* Promotes mixing, absorption of nutrients and water
Small intestine: absorption
Water-insoluble products of lipid digestion, fat-soluble vitamins, cholesterol form micelles with bile salts
* Components transferred to absorptive cells
* Lipids would ‘float’ in chyme without micelles
* Normally occurs in ileum
large intestine: motility
Haustral contractions in ascending and transverse colon
* Slow segmenting movements lasting ~1 min aids with mixing and water absorption
* Occurs every 30min
Peristalsis occurs 3-4 times/day
* Allows for mass movement of content towards rectum
Large intestine: digestion, absorption
Some digestion of resident bacteria
- Insoluble carbohydrates broken down via fermentation -> short-chain fatty acids
Absorbs remaining water and electrolytes (Na+, Cl -)
Absorbs vitamins synthesised by resident bacteria (vitamin B complexes, vitamin K)
