Lecture 4- Digestive tract and the small intestine Flashcards
What is the primary role of the digestive system?
• Primary role: transfer of nutrients, water and electrolytes from the food into the body • The act of eating does not make nutrients available to the body, they need to be digested or biochemically broken down • ~95% of ingested food is made available to the body
What does the digestive tract look like?
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What are the 4 basic digestive processes?
- Motility • Muscular contractions that mix and move forward the contents of the digestive tract 2. Secretion • A number of secretions are produced by the lumen of exocrine glands • Water, electrolytes, enzymes, bile salts, mucus etc. 3. Digestion • Biochemical breakdown into smaller absorbable units • Carbohydrates, proteins or fats 4. Absorption • Completed in the small intestine • Transfer of units into the blood or lymph
What happens in the mouth?
•Entry to digestive system is via the mouth •Palate •Tongue & taste buds •Teeth •Secretion of saliva •Amylase •Mucus •Lysozyme •Carbohydrate digestion begins •No absorption of foodstuffs •Some medications can be absorbed
What is chewing/mastication?
• Slicing/tearing/grinding/mixing of food • Tooth enamel cannot be regenerated (cavities!) • Malocclusion • Grind and break food to facilitate swallowing, mix with saliva and stimulate the taste buds • Molars can exert 200 pounds of force • More than necessary • Mix food with saliva and aid in swallowing • Taste – important! • Pleasure • Stimulates secretions of saliva, gastric and pancreatic secretions
What does saliva do?
• Produced by salivary glands in the mouth • 99.5% water & 0.5% electrolytes and proteins • Low salt concentration (important for taste of salt) • No glucose present (important for sweet tastes) • Begins digestion of carbohydrates • Amylase • Polysaccharides (carbs) → maltose • Provides moisture and some antibacterial properties (lysosome) • Aids speech • Cleaning of mouth and teeth • Bicarbonate buffers • NOT ESSENTIAL FOR DIGESTION
What is the salivary secretion like in humans?
• Continuous • 1-2 L / day • Can be increased by reflexes • Unconditioned • Chemoreceptors / pressure receptors • Conditioned / acquired • No oral stimulation • Smell, sight, memory – anticipation.
What happens in the esophagus?
• Swallowing • Pressure receptors in pharynx stimulate the brain to begin swallowing • Swallowing begins voluntarily but cannot be stopped once initiated • Upper (pharyngoesophageal) sphincter maintains pressure gradients • Prevent excess air → stomach • Peristaltic waves • Ringlike contractions forcing food to stomach • Lower (gastroesophageal) sphincter • Remains closed except during swallowing • Prevents reflux of acidic gastric contents
What happens in the stomach?
• J shaped • Stores food prior to digestion/absorption in SI • The entire digestive process takes hours • Secretes HCl & enzymes • Protein digestion begins • Mixes & pulverises food into chyme • Divided into 3 sections • Fundus–above esophagea lopening • Body–middle/main section • Antrum – heavy musculature, peristaltic contractions • Absorbs alcohol and aspirin, but not food or water
How does gastric emptying work?
• Gastric emptying controlled by factors in stomach & duodenum • Volume of chime • Empty stomach has a volume of ~50mL, can increase to 1000 mL during a meal • Distension of stomach triggers gastric motility • Gastrin • Duodenum must be ready – neural and hormonal responses • Influencedbyfats,acids,hypertonicityanddistention • Hormone secretions • Cholecystokinin (CCK) – also causes contraction of the gallbladder to secrete bile • Secretin – main target is the pancreas to secrete bicarbonate • Emotions & stress
What are the gastric secretions?
1.• Hydrochloricacid • conversion of pepsinogen to pepsin by HCl • bacteriostatic effect 2.• Pepsin • protein digestion • replaceable by pancreatic enzymes 3.• Mucus • protective coating (inhibits pepsin, neutralizes HCl), lubricant • part of gastric mucosal barrier 4.• Intrinsic factor • binds B12 vitamin, absorption in the ileum • the only indispensable substance in gastric juice • Deficiency leads to pernicious anemia
What is the importance of the pancreas?
-The pancreas secretes enzymes responsible for, approximately, 50% of carbohydrate digestion 50% of protein digestion 90% of lipid digestion The pancreas also secretes bicarbonate to assist in the neutralisation of chyme in duodenum (change from gastric outflow (pH 2.2) to mild alkaline conditions (pH 7.5)
What is the function of the pancreas?
• Pancreatic duct penetrates duodenal wall • Endocrine functions • Insulin and glucagons • Exocrine functions • Majority of pancreatic secretions • Pancreatic juice secreted into small intestine • Carbohydrases • Lipases • Nucleases • Proteolytic enzymes
What are the pancreatic and bile secretions?
• Exocrine & endocrine pancreas • Exocrine secretion regulated by CCK • Secretin regulates pancreatic duct cells • Liver produces bile • Biliary system = liver, gallbladder & ducts • Pancreas secretes enzymes, amylase and lipase • 1-2 L per day
What is the gall bladder and what is it for?
• Bile is continuously secreted by the liver and stored in the gall bladder between meals • Diverted UP from the doudenum when the sphincter of Oddi is closed • 250 – 1000 mL bile secreted per day • Bile = aqueous alkaline fluid • + bile salts, cholesterol, lecithin & biliruben • Gallstones • Cholesterol, bile salt & lecithin secretion out of proportion
What are gallstones?
• Excess cholesterol precipitates into microcrystals • Aggregate into gallstones • Gall bladder removal (cholecysterectomy) • No site of bile storage • Changes to digestion • Fat digestion issues • Bile salts have detergent actions • Lipid emulsion – lipase needs to contact the triglycerides but they are not water soluble & form large droplets • Fat digestion is prolonged when bile is not present • Micellar formation • Dissolve water insoluble (lipid soluble) substances
What is the liver and what is it for?
• Largest metabolic organ • ‘biochemical factory’ • Metabolic processing of nutrients post-absorption • Detoxification • Synthesis of plasma proteins • Storage of glycogen, fats, iron, copper and vitamins • Activation of Vitamin D • in conjunction with kidneys • Removal of bacteria and RBC • Excretion of cholesterol and bilirubin
What is the hepatic portal system in the liver like?
• Hepatic portal systems • Connects digestive system and liver • Veins from stomach and intestines enter the hepatic portal vein to allow processing, detoxification and storage • Organised into lobules • Each with 3 vessels: hepatic artery, portal vein and bile duct • Kupffer cells destroy old RBC and bacteria from blood • Hepatocytes secrete bile continuously into channels (carried to bile ducts then stored in gallbladder between meals)
What are the enzymes in the digestive tract suited for?
-The array of enzymes in the digestive tract are suited for nutrient processing in different sections of the gut.
What are the enzymes used to utilise lipids?
- lipase and colipase
- origin: pancreas
- product: monoglycerides and free fatty acids
How does nutrient absorption occur in the small intestine?
- Variety of mechanisms
- Diffusion
- Facilitated diffusion
- Active transport
- Pinocytosis or endocytosis
Dependent upon
- Solubility of the nutrient (not necessarily related to water solubility)
- Concentration or electrical gradient
- Size of the molecule to be absorbed
How is digestion controlled in the small intestine?
-Digestion is regulated by hormones and the autonomic nervous system
Duodenal hormones:
1) secretin – stimulates pancreas & liver to secrete alkaline fluids
2) cholecystokinin – triggers release of enzymes from pancreas and gall bladder (amylase, lipase, deoxyribonuclease, proteases)
