AAP Digestive System

digestive system functions

• ingestion: taking in food

• digestion: breaking food down both physically & chemically

• absorption: movement of nutrients into bloodstream

• defecation: rids body of indigestible waste

alimentary canal

AKA gastrointestinal (GI) tract – 9m long

• pathway of digestion:

1. Mouth (oral cavity)

2. Pharynx

3. Oesophagus

4. Stomach

5. Small intestine

6. Large intestine

mouth (oral cavity)

mastication (chewing) of food

• mixing masticated food w saliva

• initiation of swallowing by tongue

• allows for sense of taste via taste buds (taste receptors)

pharynx (throat)

serves as passageway for foods, fluids & air

• food is propelled to oesophagus by 2 skeletal muscle layers in pharynx

oesophagus

• ~25cm long

• runs from pharynx to stomach thru diaphragm

• conducts food by peristalsis (slow rhythmic squeezing) to stomach

• passageway for food only (respiratory branches off after pharynx)

stomach

• food enters at cardiac sphincter

• food empties into small intestine at pyloric sphincter (valve)

• rugae: folds that appear in mucosa when stomach is empty – stomach can stretch & hold 4L of food when full

functions:

• site of mechanical food breakdown

• chemical breakdown of proteins begins (breaks down into peptides & amino acids)

• delivers chyme (processed food) to small intestine

structure of stomach mucosa

simple columnar epithelium composed almost entirely of cells that produce alkaline mucus

• dotted by gastric pits leading to gastric glands that secrete gastric juice, including:

1. chief cells – produce protein-digesting enzymes (pepsinogens)

2. parietal cells – produce hydrochloric acid that activates enzymes

3. mucous neck cells – produce thin acidic mucus (diff from mucus produced by mucous cells of mucosa)

4. enteroendocrine cells – produce local hormones (e.g. gastrin)

small intestine – major regions

main site of chemical digestion

• longest portion of alimentary canal (2-4m)

• site of nutrient absorption into blood

• muscular tube extending from pyloric sphincter to ileocecal valve

3 major regions:

1. duodenum

2. jejunum

3. ileum

small intestine – function

chemical digestion of proteins, fats & carbs

• enzymes produced by intestinal cells & pancreas (carried to duodenum by pancreatic ducts)

• bile, formed by liver, enters duodenum via bile duct

small intestine – projections in small intestine

nearly all nutrient absorption in bloodstream occurs in small intestine

• lined w projections that increase SA:

1. villi: finger-like structures formed by mucosa

2. microvilli: tiny projections of plasma membrane

3. lacteal: extension of lymphatic system that absorbs fats

large intestine

• larger in diameter but shorter in length (1.5m) than small intestine

• extends from ileocecal valve to anus

subdivisions:

• caecum

• appendix: hangs from caecum (lymphoid tissue)

• colon: 4 parts – ascending, transverse, descending, sigmoid

• rectum

• anus

— external anal sphincter: voluntary control

— internal involuntary sphincter: formed by smooth muscle

accessory digestive organs – salivary glands

• 3 pairs

saliva:

• mixture of mucus & serous fluids

• helps moisten & bind food tgt into mass called bolus

• contains salivary amylase (enzyme that breaks down starch (complex carb) into smaller molecules, i.e., glucose, sucrose)

• dissolve chemicals so they can be tasted

• contains lysozomes & antibodies – inhibits bacteria

accessory digestive organs – pancreas

• pancreatic juice: secreted into duodenum from pancreas

• contains digestive enzymes that break down proteins, fats & carbs

• alkaline, so acidic chyme coming from stomach

• has separate cells that produce insulin + glucagon, which enter bloodstream & control BGLs

accessory digestive organs – liver

digestive role:

• produces bile

• bile emulsifies fats (physically breaks down large fat globules into smaller ones)

• leaves liver thru common hepatic ducts & enters duodenum thru bile duct

other roles of liver:

• makes vital proteins & hormones

• stores glucose

• stores Fe

• breaks down harmful substances

• clearance of bilirubin – byproduct of RBC breakdown (if there’s a buildup of bilirubin, skin & eyes turn yellow)

accessory digestive organs – gall bladder

• when no digestion occurs, bile is stored in gallbladder

• when digestion of fatty food occurs, bile is introduced into duodenum from gallbladder

digestive activities of the mouth, pharynx & oesophagus

1. Mechanical breakdown

• food physically breaks down by chewing

2. Chemical digestion

• food is mixed w saliva

• carb digestion begins w salivary amylase (enzyme)

3. Swallowing

• food is formed into bolus, which is forced into pharynx by tongue

• all passageways except for stomach are blocked – epiglottis blocks larynx

4. Peristalsis moves bolus toward stomach

• cardiac sphincter opens when food presses against it

activities in the stomach

• presence of food & rising pH causes gastric glands to release gastrin (hormone)

• gastrin causes release of (components of gastric juice):

1. protein-digesting enzymes

2. mucus

3. HCl (makes stomach contents v acidic, which activates pepsinogen to pepsin (enzyme) for protein digestion & provides hostile environment for microorganisms)

• 2-3L of gastric juice produced every day

• alcohol & aspirin are the only items absorbed in stomach

mechanical digestion in the stomach

• food must be well mixed

• rippling peristalsis occurs in lower stomach

• pylorus chyme into small intestine (3mL at a time)

• stomach empties in 4-6 hrs

digestion in the small intestine

• Enzymes released by cells making up microvilli lining small intestine wall break down double sugars into simple sugars & complete protein digestion

• Pancreatic juice is released into duodenum, contains enzymes that:

— help complete digestion of CARBS (pancreatic amylase)

— carry out about half of all PROTEIN digestion

— digest FATS using lipases from pancreas

— digest nucleic acids using nucleases

• Pancreatic juice is alkaline & helps return pH of chyme to a lvl to activate intestinal + pancreatic enzymes & protects intestine

• Bile is released at the same time as pancreatic juice – acts like detergent to emulsify or mechanically break down large fat globules & necessary for fat-soluble vitamin absorption

absorption in the small intestine

• water is absorbed along length of small intestine

duodenum:

• site where most chemical digestion occurs & some absorption

• receives chyme from stomach, bile from liver + gallbladder, & pancreatic juice from pancreas

jejunum:

• bulk of nutrient absorption

• by the time chyme passes thru jejunum, ~90% of all available nutrients have been absorbed into body

ileum:

• any last nutrients not absorbed in jejunum are absorbed in ileum before food passes into large intestine

activities of the large intestine

• nutrient breakdown & absorption

• no digestive enzymes in large intestine

• resident bacteria digest remaining nutrients

— produce some vitamin K & B

— release gases

• water and vitamins K & B are absorbed

• faeces contains undigested food residues (mainly plant fibre), mucus, bacteria & water

• contractile waves over colon that occur just after eating force material to rectum which triggers urge to defecate

• defecation occurs w relaxation of voluntary (external) anal sphincter

nutrition and metabolism

• nutrient: substance used by body for growth, maintenance & repair

• major nutrients: carbs, lipids, proteins, water

• minor nutrients: vitamins, minerals

5 basic food groups & some of their major nutrients

carbohydrates

break down to glucose – body’s preferred source to produce cellular energy (ATP)

• body stores excess glucose in blood as glycogen (usually in liver)

• if BGLs remain high, then excess glucose is stored as fat

• when glucose lvls in blood drop, stored glycogen is converted to glucose & released into blood

lipids & lipid metabolism

lipids (fats):

• insulate body

• protect organs

• build some cell structures (membranes & myelin sheaths)

• provide reserve energy (excess dietary fat is stored in subcutaneous tissues & other fat depots)

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• when carbs are limited, more fats are oxidised to produce ATP

• excessive fat breakdown causes blood to become acidic (acidosis/ketoacidosis)

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breath has fruity odour – common with:

• no carb diet

• uncontrolled diabetes

• starvation

protein metabolism

• proteins form bulk of cell structure & most functional molecules

• amino acids are oxidised to form ATP mainly when other fuel sources are unavailable

• ammonia (released as amino acids) is detoxified by liver cells that combine w CO₂ to form urea

body energy balance – BMR

interference w body’s energy balance leads to:

• obesity

• malnutrition (leads to body wasting)

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Basic metabolism rate (BMR): amount of heat body produces/unit of time at rest

• avg BMR: 60-72 kcal/hr for an avg 70kg adult

body energy balance – body temp regulation

when foods are oxidised, more than 60% of energy escapes as heat, warming the body

• range of homeostatic temp (35.6-37.8°C)

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hypothalamus initiates mechanisms to maintain body temp:

• heat loss mechanisms: radiation of heat from skin & evaporation of sweat

• heat-promoting mechanisms: vasoconstriction of blood vessels & shivering

problems of digestive system

• gastroenteritis: inflammation of GI tract – can occur at any time

• appendicitis: inflammation of appendix – common in adolescents

• ulcers

• gallbladder problems

• obesity

• diabetes mellitus

• reflux (GORD)

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Activity of digestive tract in OLD AGE:

• fewer digestive juices

• peristalsis slows

• diverticulosis & GI cancers are more common