Digestive+Anatomy+and+Physiology

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Absorbtion:
Water and Mineral Salts: absorb 8/9 of intake, a fraction is absorbed in small intestine, the rest is reabsorbed by the colon (it's main function) Vitamins: -water soluble: absorbed into the blood stream -fat soluble: absorbed into lymphatic system Carbohydrates: takes place mostly in duodenum and jejunum. Proteins: mostly in the first third of the small intestine. Lipids (fats): broken down into fatty acids and glycerol, absorbed throughout the small intestine

Digestive Enzymes
Hydrolysis: breaking down of large molecoles into smaller molecules with the use of water

Saccharidases: Break Down Saccharides (sugars) Peptidases: Break down proteins Lipases: Break down lipids and fats

Movements within the digestive system
Peristaltic: adjacent segments of the intestine contract and relax, moving the food

Segmentation: Single segments of the intestine alternately contract and relax. This moves the food forward AND BACKWARD, mixing the food rather than simply propelled.

Extrinsic Motor Innervation
Sympathetic and Parasympathetic systems. Parasympathetic system is connected through the pneumogastric and pelvic nerves. Stimulation is both excitatory and inhibitory. Sympathetic is made up of splanchnic nerves w/ inhibiting effect on motricity.

Intrinsic Innervation
Occurs through a mass of neurons called intramural ganglions or plexuses (auerbach's and Meissner's plexuses)

Sensory Innervation
Send messages to the brain about intra-digestive events. Send sensory messages collected from the mucous membrane through receptors spread out along the digestive tract.

Surface continuity
0.5mm. thick viscous layer that acts as an impassable barrier to large molecules

Cell Renewal
50-100 billion cells are destroyed and replaced daily. atrophy, hyperplasia or tumors can result from a disruption. Increase renewal: gastrin, hydrocortisone, enteroglucagon decreases renewal:prostaglandins (PGE2)

Cytoaggression and cytoprotection
Digestive cells are subject to constant aggression. Alchohol/aspirin destroys the mucous layer of the stomach causing small hemorrhages and ulcers 3 cytoprotection systems: mucus, cappillary circulation and prostaglandins

Enzymes
Biological compounds, protein in nature, that are water-soluble and catalyze biochemical reactions. Also called bio-catalysts.

Synthesized by living organisms differently by species but are passed on hereditarily.

Most are a result of a binding of 2 molecules: apoenzyme + coenzyme = enzyme

__"apoenzyme"__: protein and is destroyed by heat. responsible for substrate specificity.

on active site there are generally 2 sites: biding site: where substrate and coenzyme become attached catalytic site: where enzymatic action occurs

"coenzyme": not a protein, not destroyed by heat. most often result of a nucleotide and and a B-group vitamin participates directly in the reaction.

=Mouth=

Saliva
97-99.5% water, rest is potassium salts, bicarbonate salts, sodium salts and chloride salts AWA proteins and enzymes.

Slightly acidic (6.8) until contacts CO2 and become alkaline (7.2)

Glands for saliva secretion
Parotid Glands: largest, located below the earlobes, 25% of saliva Submandibular: middle of lower jaw, 70% of saliva Sublingual: under the tongue, 5% of saliva

Paste created by mixture of food and saliva = alimentary bolus

=Stomach=

Gastric Juices
2-3 litres produced daily (~700 Ml/meal)

6ppm of "mineral material" (chlorides, Ca phosphates, and Hydrochloric acid) 4ppm of "organic material" or mucin that protects the gastric mucous membrane

Gastrin stimulates stomach cells to release HCI and pepsin once food has entered stomach

Hydrochloric Acid: Ph= 1-1.8, produced by parietal cells, gives juice it's acidity. Causes sweling and gelation of collagen in conjuctive tissue of muscles. Acts on Carbohydrates by initiating hydrolysis of saccharose, which is broken down into glucose and fructose. Antiseptic role by killing bacteria through lysozymes

__2 enzymes:__ Pepsin: secreted by gastric gland which contain inactive substance pepsinogen (propepsin). Converts into pepsin when reacts w/ HCI. Dissociates protein molecule by hydrolyzing them into peptones and polypeptides or aggregates of many amino acids

Rennet: coagulant diastase of milk. secreted abundantly in infants. presence of calcium salts promotes coagulation while potassium oxalate stops the coagulation. Women's breast milk is more digestible than cow's milk.

Lipase (In infants): acts on fats, turning them into fatty acids and glycerin following pyloric-duodenal regurgitation

Roles of Gastric Juices
Break down proteins. keratin, mucin, elastin resist gastric juices

Gastric Secretions
__3 types:__ Mucous cells: produce alkaline mucous to neutralize acidity

Parietal Cells: produce hydrochloric acid to facilitate digestion and destroy bacteria

Main Cells: produce pepsin to partially digest proteins

__3 Phases:__ Cephalic: comes from brain following visual, taste or olfactive stimulation. (Pavlov's reflex)

Gastric: comes from stomach after distension of stomach walls, hormonal secretions, excess alchohol, coffee, histamine, ect.

Intestinal: comes from duodenum and inhibits secretions to slow down the process if necessary (duodenum not ready for chyme)

=Small Intestine= Depend on 3 kinds of juices: Pancreatic, Biliary and Inestinal

Assimilates all food substances that can be. -Carbohydrates (starches) convert into hexoses, mainly glucose as well as levulose and galactose -Fats are split into fatty acids and glycerol. -Proteins are broken down into their component amino acids

__Pancreas__
__2 parts:__ Exocrine Pancreas: secretes digestive enzymes and bicarbonates into duodenum to neutralize stomach acid

Endocrine Pancreas: Secretes insulin and glucagon into the blood. Regulation of glycemia (blood glucose level). See Diabetes.

Pancreatic Secretions
2 litres of juice is emptied into duodenum via the panceatic duct.

viscous, clear, alkaline (Ph= 8 and 9) liquid 15ppt solid substances, ~1/2 being minerals (bicarbonates, chlorides, sodium chlorides, and calcium phosphate chlorides) and organic compounds (globulins) which make it coagulate when heated

__Controlled by:__ Humoral mechanism: chyme entering duodenum triggers secretin release which initiates the secretion of pancreatic juice.

Nervous Mechanism: pneumogastric nerve. Anticipatory secretion as food enters the mouth, esophagus and stomach cephalic stage = increase of digestive enzyme production gastric/intestinal phases = enzyme production maintained and bicarbonate production increases.

Role of Pancreatic Juice
__3 kinds of diastases:__ Amylase: same as salivary amylase but uses more energy as it works on stach, which is converted into dextrin, then maltose.

Lipase:emulsifies and saponifies fats in to fatty acids and glycerol. Increases considerably in presence of bile.

Proteolytic Diastases: main = trypsin. Inactive on proteins, however other pancreatic enzymes (trysinogen, chymotrysinogen) digest proteins. Acts on intact proteins which breaks down into polypeptides. Also attacks polypeptides by detaching their amino acid.

__Liver__
Largest gland in the body. Recieves blood from 2 sources, like all organs it has a feeding artery (hepatic artery, 400ml/minute) but also recieves blood directly from intestine through hepatic portal vein (1100 ml/minute)

"portal" allows the liver to filter everything that comes from the intestine (following digestion) to store or circulate different absorbed nutrients.

Also produces bile into duodenum via hepatic duct, which is connected to pancreatic duct.

Biliary Secretions
Bile is essential to digestion of fats.

500 ml/day, 97.5% water, rest is composed of sodium and bicarbonates, biliary pigments, biliary salts (10 ppt) and cholesterol (0.5-2 ppt)

Stored in the gallbladder and is also controlled by nervous factors (pneumogastric nerve) and hormonal factors (cholecystokinin secretion)

Role of Bile
digest fats through biliary salts.

Biliary salts allow bile to finely emulsify fat droplets, increasing surface action of pancreatic lipase.

Facilitates the intestinal absorption of calcium, reinforces peristaltic movement and in a way regulates intestinal flora.

__Intestinal Juice__
~ 1 litre/24 hours free flowing alkaline (Ph = 8.3) liquid that coagulates in heat

~20 ppt of solid mineral material: alkaline biocarbonates, chlorides, phosphates; and organic material: mucin (protects mucous membrane from the acidity of gastric chyme) and desquamated epithelial cells (make it look cloudy); and numerous enzymes

Product of Lieberkuhn's glands and Brunner's glands. Absent when stomach is empty. Found in the duodenum and first part of jejunum. Humoral mechanism as the cells of enteric mucous membrane release secretin or enterocrinin.

Role of intestinal juice
acts on all 3 chemical food groups by completing action of pancreatic juice.

__Diastases:__ maltase, invertase, and lactase that act on carbohydrates.

lipase that acts on fats

erepsin that acts on proteins.

=Colon=

Chemical Phenomena
Chyme from small intestine is subject to 2 chemical actions: dehydration and microbial action (through intestinal flora) Microbes aid in fermentation of proteins and carbohydrates.

Carb Fermentation
Cellulose is the main kind of fermentation in the colon. It is hydrolyzed by bacilus cellulosae and the sugars result in lactic acid and butyric acid. Also relases CO2 and H2. 50% of cellulose can be used by the body.

Protein Fermentation
Colon is the only place protein ferment. Caused by anaerobic bacteria (B. putrificus, B. perfringens, and B. sporogenes) Also produces basic compounds - phenol (from tyrosine), indole and skatole (metabolites of tryptophan)

Indole and skatole give feces its characteristic odour.

Finally, Cholesterol is converted in coprosterol and bilirubin is converted into stercolibin both giving feces it's soft texture.

Composition of Feces
75% water and undigested parts of food. mineral salts and toxic or unusable gasses.

~120g produced daily decreases w/ consumption of meat and increases w/ consumption of vegetables as they produce more waste