Topic 3- Digestive system Flashcards
functions of the digestive system
Takes in food, breaks it down, absorbs the molecules and excretes the waste
Difference between alimentary canal and accessory organs
GI: Continual digestive muscular tubes, alimentary canal
Accessory: Aid in digestive process
Ingestion
Taking in food via mouth
Propulsion
Aids in moving food along gastrointestinal tract
Involves the voluntary process of swallowing
Automatic process of perastalsis:
Involuntary
Alternating waves of contraction and relaxation
Each waves starts distal to previous
Where small remnants, bacteria and debris is moved from the small to the large intestine.
Travel time from duodenum to ileum is 2 hrs.
Mechanical Digestion
Increase surface area of ingested food to physically prepare it for digestion by enzymes
Eg. Chewing in mouth, churning of food in stomach, segmentation along small intestine
Segmentation:
Mixes food with digestive juices
Enhances absorption
Moves contents toward ileocecal valve via alternating contracting smooth muscles at a rate slow enough for absorption and digestion to occur.
Chemical digestion
Complex food molecules are broken down to their chemical building blocks by enzymes
Begins in mouth, and is completed in the small intestine
Absorption
Passage of the digest end products along with vitamins, minerals and water from the lumen of the gastrointestinal tract to the blood or lymph
Major site of absorption is small intestine
Defecation
Indigestible substances are eliminated from the body
Mouth
composition, function and digestive process
Composition
Oral cavity:
-Bounded by lips, cheeks palate and tongue
Lips and cheeks:
-Core of skeletal muscle covered by skin
-Palate:
Hard palate
Soft palate
Function
Mechanical digestion
Digestive Processes
Ingestion
Mechanical digestion
Chewing (Partly voluntary and partly reflexive)
Chemical digestion
Propulsion
Tongue
composition and function
composition=Skeletal Muscles
Function= Repositioning and mixing food during chewing
Formation of the bolus (compact mass of food)
Initiation of swallowing (pushing bolus into pharynx), speech and taste
Salivary Glands function
Secretion:
Cleanses the mouth
Moistens and dissolves food chemicals
Aids in bolus formation
Contains enzymes that begin the breakdown of starch
Antimicrobials that protect against microbial invasion.
Pharynx
function= (No digestive role)
Oropharynx and laryngopharynx
Allow passage of food, fluids and air
Stratified squamous epithelial lining
Skeletal muscles layers contract to propel food into the oesophagus
Digestive processes=Deglutition (swallowing)
-Buccal phase
——Voluntary
——-Forces food into oropharynx
-Pharyngeal-oesophageal phase
——–Food enters the oesophagus
——–Momentarily, respiration is inhibited
———8 seconds for solid food to reach stomach, 1-2 seconds for liquids.
Oesophagus composition, function and digestive processes
composition=Flat muscle tubes from laryngopharynx to stomach. Esophageal mucosa contains stratified sqamous epithelium
Function= Esophageal glands in submucosa secrete mucus to aid in bolus movement
Stomach composition and function
composition=Mucosal lining composted of:
Simple columnar epithelial cells
——-Secrete mucous
Gastric glands composed of
-Mucous neck cells
-Parietal cells (secrete HCl which makes stomach content acidic.
-Chief cells (secrete lipases which are fat digestive enzymes)
-Enteroendocrine cells (release chemical messengers that aid in digestion)
Function=Temporary storage tank (4L)
Physical digestion
Denaturation of proteins
Delivers chyme to the small intestine
Regulation of gastric secretion
Stimuli acting 3 distinct sites being the head, stomach and small intestine provoke or inhibit gastric secretions.
there are 3 phases
Phase 1 of the regulation of gastric secretion
Cephalic(reflex) phase
-triggered by the taste, sight, smell or thought of food
- prepares stomach for it’s digestive roles
- Hypothalamus receives signals from tastebuds, olfactory receptors and then stimulates the medulla, which transmits impulses by vagus nerves, which affects secretory and contractile activity of the stomach
Phase 2 of the regulation of gastric secretion
Gastric phase
-Occurs once food enters the stomach
-Hormonal and neural stimuli
Phase 3 of the regulation of gastric secretion
Intestinal phase
-Stimulatory: as food fills the duodenum (mucosal cells release gastrin)
-Inhibitory: enterogastric reflex prevent further food entry into small intestine which causes a decline in gastric secretory activity.
Gastric contractile activity
After a meal, peristalsis begins near the gastropharyngeal sphincter.
As contractions approach pyloric sphincter, contractions become stronger retropulsion occurs at pyloric valve where food is broken up due to the backwards and forwards motion of the chyme between the duodenum and the stomach
The stomach usually empties completely within 4 hours after a meal
Liver digestive function, liquid produced each day
Produce bile for export to the duodenum (first part of small intestine
Bile:
Alkaline solution containing:
Bile salts:
Emulsifies fats
Yellow green solution
Approx 900ml
Pancreases digestive function and liquid produced each day
Enzyme production that breaks down all food stuffs
Pancreatic juice:
-Drains from the pancreas to duodenum
-pH8 and neutralises chyme
Enzymes:
-Amylase, lipases and nucleases
——–Secreted in active but require ions or bile for optimal activity
-Proteases (protein digesting enzymes) are secreted in inactive form
———–They are activated in the duodenum
1.5L daily
water, electrolytes, enzymes
Gall bladder digestive function
Stores bile
Concentrates it by absorbing water and ion
Small intestines info?
Major organ of digestion and absorption
All of the foodstuffs
805 of the electrolytes
Most of the water
Up to 4 metres in length
Layers of small intestines
1.Mucosa
2.Innermost layer
3.Three parts:
-Epithelium: secretes mucous, digestive enzymes and hormones
-Lamina propria: contains capillaries for nourishment and absorption
-Muscularis mucosa: smooth muscle that produces local movements of the mucosa
4.Submucosa
-External to mucosa
-Rich supply of blood
-Lymphatic vessels and follicles
-Nerve fibres
-Muscularis externa
-Responsible for segmentation and peristalsis of the small intestine
-Serosa
-Protective outermost layer
Divisions of small intestines
Duodenum
-Receives bile from the liver
-Receive pancreatic juice from the pancreas
Jejunum
Ileum
Motility
Motility of the small intestine
There are two motility patterns in the small intestine
After a meal - segmentation
——–Ensures that the chyme is thoroughly mixed with bile, pancreatic and intestinal juices
———Ensures that the absorbable products of digestion comes into contact with the mucosa of the small intestine for absorption
Between meals - peristalsis
——Migrating motor complex (form of peristalsis)
——-Sweeps contents down to large intestine
In order for contents to pass into the large intestine:
-The ileocecal sphincter (b/w small and large intestine) relaxes by:
———Gastrolieal reflex enhances force of segmentation in ileum
————————-Triggerd by stomach activity
———-V Gastrin increases motility of ileum
Modifications of the small intestines
increased surface area for nutrient absorption
-Circular folds: deep permanent folds of the mucosa and submucosa
It forces the chyme to slowly spiral through the lumen
Villi: finger-like extensions of the mucosa
-Largest in the duodenum (where absorption)
-Narrow and shorten along the length of the intestine
Microvilli- brush border
-Found on enterocytes
-Contain enzymes that complete digestion of carbs and proteins
Small intestines digestion and absorption
Digestion and absorption in the small intestine requires:
-Slow delivery (3-6 hours) of chyme which contains:
————-Partially digested carbohydrates and proteins
————–Undigested fats
-Delivery of bile, enzymes and bicarbonate
————–From the liver and pancreas
-Mixing
Absorption of:
-All of the foodstuffs
-80% electrolytes
-Most of water (95%)
Function of Large intestines
Absorb remaining water, vitamins and electrolytes from indigestible food residues
-Temporarily store the residues
-Eliminate residues from the body (propulsion of faeces towards the anus)
-*also absorbs metabolites produced by resident bacteria as they ferment carbs not absorbed in the small intestine
Motility of Large intestines
Haustral contractions
-Slow segmenting movements
Gastrocolic reflex
-Initiated by presence of food in stomach
-Activates three to four slow powerful peristaltic waves per day in colon (mass movements)
Defecation Large Intestines
Mass movements force faeces toward rectum
Muscles of rectum contract to expel faeces
Distension initiates spinal defecation reflex
Assisted by Valsalva’s maneuver
-Closing of glottis, contraction of diaphragm and abdominal wall muscles leads to increased intra-abdominal pressure
-Levator ani muscles contracts -> anal canal lifted superiorly -> faeces leave body
Carbohydrates
Intestine can only absorb monosaccharides; therefore carbs must be broken down
-Only monosaccharides (glucose, galactose, fructose) can be absorbed as they are
-The more complex carbs (lactose, starch, glycogen) need to be broken down into monosaccharides before they can be absorbed
Digestion of starch
-In mouth with salivary amylase
-Continues via secretions from the pancreas
——–Pancreatic amylase
-Is completed by intestinal brush border enzymes Eg. Lactase, maltase, sucrase
Protein
Begins in stomach
-pepsin
-rennin(infants only)
Continues via secretions from the pancreases
-pancreatic protease(e.g Trypsin)
Is completed by the brush border enzymes
- Eg. Aminopeptidases, carboxypeptidases and dipeptidases
Lipids
Begins in the mouth
-Lingual lipase
Continues in the stomach
-Gastric lipase
Emulsification occurs in the small intestine
-Bile salts
Digestion into fatty acids and monoglycerides
-Via pancreatic lipases
Primary site of lipid digestion is the small intestine
Emulsification by bile salts
-Pre-treatment step to break large fat globules into small fat droplets
Digestion
-Lipases split triglycerides into fatty acids and monoglycerides
Micelle formation
-Fatty acids + monoglyceride + bile salts
Diffusion
-Lipids leave the micelles by simple diffusion
Chylomicron formation
-Smooth ER converts the lipids back into triglycerides, which combine with other lipids to form chylomicrons
Chylomicron transport
-Enter the lymph
Nucleic Acids
Occurs in the small intestines
- enzymes
——-Pancreatic ribonuclease and deoxyribonuclease -> nucleotide monomers
———Brush border enzyme nucleosidases and phosphatases -> free bases, pentose sugars, phosphate ions
Anabolic
Anabolic: synthesizing larger molecules
Catabolic
Catabolic: degrading larger molecules into smaller ones
What to metabolic controls act to equalize
Metabolic controls act to equalize the balance between the absorptive state (when you’ve eaten) and postabsorptive state (when you haven’t eaten in a long time)
Amino acids can only be used as energy when converted to a carbohydrate intermediate (ketoacid)
Carbohydrates can be directly oxidised to produce cellular energy
Absorption state
Anabolism exceeds catabolism
The absorptive state is the fed state
-Time during each meal
-And for four hours after that meal
Dietary amino acids and fats are used to remake degraded body protein or fat and small amounts of these are used to provide ATP
Excess metabolites are transformed to fat (if not used for anabolism)
Carbohydrates
Delivered to the liver
Fructose and galactose are converted to glucose
-Glucose is converted in liver to glycogen or fat
———Is the major energy fuel
Triglycerides
Most glycerol and fatty acids converted to triglycerides for storage
Triglycerides used in adipose tissue, liver, and skeletal and cardiac muscle as primary energy source
- Excess fats stored in adipose tissue
Amino Acids
Excess amino acids are used for ATP synthesis or stored as fat in liver
Most amino acids used in protein synthesis
Hormonal Control
Absorptive state primarily controlled by
Insulin secretion stimulated by increase in BGL
-Insulin binds to membrane receptors of target cells
-GLUT4 transporter moves to plasma membrane
—————-Enhances uptake of glucose into target cells (eg. Muscle and adipose cells
Postabsorptive state
GI tract is empty - occurs late arvo, all night, etc.
Catabolism of fat, glycogen and proteins exceeds anabolism
Goal is to maintain blood glucose between meals
-Makes glucose available to blood
-Promotes use of fats for energy
-Glucose sparing - save glucose for organs that need it most
———-Brain uses blood glucose
———–Other organs use fatty acids
Sources of blood glucose:
Glycogenolysis: stores in liver
-Stores in skeletal muscle must be converted to pyruvic acid/lactic acid because the enzyme required to break glycogen to glucose is not present
Lipolysis: adipose tissues and liver
-Glycerol used for gluconeogenesis in liver
———-Making glucose from non-carb sources
Catabolism of cellular protein
-Major source during prolonged fasting
Hormonal controls
Post-absorptive state triggered by decreased insulin release as blood glucose levels drop
Glucagon released:
-Declining blood glucose
-Rising amino acid levels
Glucagon promotes:
-Glycogenolysis and gluconeogenesis in the liver
-Lipolysis
