Digestive System Flashcards
Digestive System Organs
Mouth Oropharynx Oesophagus Stomach Small Intestine Large Intestine
Accessory organs:
Salivary Glands
Pancreas
Liver
Gall bladder and biliary tract
Oral Cavity
- Inner surfaces are lined with mucous membranes.
- Hard palate -bony partition between oral & nasal cavities. Allows simultaneous chewing and breathing.
- Soft palate –an arch of muscle.
- Uvula–swings up and blocks the nasopharynx preventing food from entering the nasal cavity.
- Tongue–voluntary skeletal muscle structure attached to the hyoid bone and mandible. The superior surface covered with ‘papillae’, which contain taste buds.
Oral Cavity: Functions
- Mastication
- Speech
- Taste
- Swallowing–bolus (mixed digested food & digestive juices) is pushed by the tongue into the pharynx closing the nasopharynx –‘pharynx reflex action’.
Teeth
- Adults have 32 teeth (20 temporary).
- Teeth have a crown, neck & root.
- ‘Dentin’ makes up the bulk of a tooth internally. Teeth are covered externally by enamel.
- Sits within the gum (gingiva) and periodontal membrane (a ligament that fixes to bone/connects teeth).
- Functions in Mastication (what chewing does not accomplish mechanically must be completed by the digestive tract chemically).
Saliva Composition
- Water(99.5%).
- Mineral Salts (Na, K, Ca, Cl, Bicarbonate, P).
- Enzymes:salivary amylase (parotid), lysozymes (found in many body secretions, breaks down bacterial cell walls).
- Mucous.
- Immunoglobulins(IgA).
- Blood clotting factors.
- pH 6.35-6.85 (mildly acidic).
Saliva Functions
- Digestion-chemical breakdown of polysaccharides.
- Lubricating & dissolving food.
- Cleansing of oral cavity and teeth.
- Defence-non-specific (IgA & lysozymes).
- Taste.
- Buffer–for acidic foods.
- Waste removal–urea / uric acid from the body.
Oesophagus
- A 25cm long muscular tube, attached to the larynx. Posterior to trachea, passes through the diaphragm (T10).
- Lined with squamous epithelium for protection, lubricated with mucous.
- The superior/middle oesophagus contains skeletal muscle & the lower contains smooth.
- Food travels to the stomach via muscular contractions (‘peristalsis’).
- Epiglottis–a flap of elastic cartilage which prevents food entering the trachea.
- The lower oesophageal sphincter acts as a seal on the stomach to prevent reflux of stomach contents into the oesophagus.
Stomach
- The stomach is a J-shaped organ with 4 main regions: cardia, fundus, body, pyloric with lesser and greater curvatures.
- Same layers as the rest of the GIT, but with 3 layers of muscle (rather than 2) churning & mixing food with gastric juice.
- 2 sphincters: cardiac and pyloric.
Stomach: Neural feedback
- Food distends the stomach stimulating stretch receptors in its walls.
- Chemoreceptors monitor pH changes.
- Activates submucosal plexus causing waves of peristalsis and gastric juice flow.
Stomach: Composition
- The stomach secretes 2-3L of highly acidic (pH 2-3) gastric juice and mucous a day.
- The mucous produced by goblet cells in the stomach provide a ‘barrier’ against the acidity present within the stomach.
- The stomach contains simple columnar epithelial cells. These have a fast turnover (replacing the lining every 3 days)
Stomach: Exocrine Cells
Parietal cells
Chief cells
Goblet cells
Parietal cells
Intrinsic Factor: necessary for vitamin B12 absorption
HCl:
• Secretes H+and Cl-separately which combine in the stomach.
• HCl activates pepsin, acts as an anti-microbial agent and assists in activating bile and pancreas flow.
Chief cells
Pepsinogen: Protein digestionHCl converts pepsinogen to the active enzyme pepsin (digests protein).
Gastric Lipase:lipid digestion
Goblet cells
Mucous: protects against acid
Stomach: Functions
- Mixing chamber –churns up food.
- Holding reservoir –storage.
- Defence–non-specific defence from gastric acidity.
- Absorption (limited) –water, alcohol, drugs i.e. aspirin.
- Digestion–mechanical but also chemical digestion of proteins & lipids.
- Iron–made more soluble with stomach acid.
- Satiation–tells you to stop eating.
Stomach: Hormones
- Ghrelin–stimulates hunger.
* Gastrin(produced by G cells) –responds to stomach distension. Stimulates gastric juice secretion & gastric motility.
Pancreas
The pancreas is an accessory digestive organ of the digestive system that have both exocrine and endocrine functions.
Pancreas: Exocrine Function
Pancreatic juice is a clear liquid that is excreted into the small intestines (1.2-1.5L/day).
Sodium bicarbonate and water.
Protease enzymes: Trypsin, chymotrypsin, ribonuclease, deoxyribonuclease.
Lipase.
Pancreatic amylase.
Pancreas: Endocrine Function
Hormones secreted (into the blood).
- Insulin & Glucagon.
- Somatostatin (growth-hormone-inhibiting-hormone).
Pancreatic Enzymes
• Pancreatic enzymes are imperative for digestion.
• Pancreatic enzymes are secreted by the pancreas into the lumen of the duodenum.
Proteases are secreted in their inactive form.
Pancreatic amylase
Breakdown of starches into sugars
Pancreatic Lipase
Lipid/fat digestion
Typsin
Protein digestion
Chymotrypsin
Protein digestion
Ribonuclease
Digest RNA
Deoxyribonuclease
Digest DNA
Gallbladder
- The gallbladder is a pear-shaped sac in the liver, measuring 7-10 cm in length.
- Bile ducts project from the gallbladder and liver, meeting at the common bile duct.
- Bile ducts collect bile produced by hepatocytes which pools in the gallbladder.
- Bile enters the small intestine via the common bile duct.
- When the individual has ingested fat, the gall bladder contracts to eject bile down the common bile duct and into the duodenum.
Bile
- The gall bladder ejects ‘bile’, which is an agent that emulsifies fats.
- Emulsification breaks the lipid into smaller molecules. This increases the surface area for lipid enzymes (lipase) to work.
- Bile is composed of bile salts, cholesterol and bilirubin.
- pH 7.6-8.6 (mildly basic).
- 90-95% of bile are absorbed & transported back to the liver from the ileum: enterohepatic circulation
Small Intestine
- After food combines with stomach secretions, the resulting chyme is pushed through the pyloric sphincter into the small intestine.
- Most digestion & absorption occurs in the small intestine.
- A long structure (6.5 metres) with villi to maximise surface area.
- Contains a variety of cells e.g. goblet (mucous), endocrine, absorptive.
Duodenum
Emulsification & most digestion occurs here (30cm).
Jejunum
Most absorption occurs here (2.5m).
Ileum
Vitamin B12is absorbed (3.5m).
Cholecystokinin (CCK)
- Cholecystokinin (CCK) is a peptide hormone that is secreted by enteroendrocrine cells in the duodenum (small intestine).
- CCK is released in response to high fat or protein chyme entering the duodenum
CCK Functions
- Stimulates the pancreas to secrete pancreatic enzymes (& hence digest fat/protein/carbohydrates).
- Increases hepatic production of bile and stimulates contraction of the gall bladder (bile ->fat digestion).
- Mediates satiety (feeling of fullness).
Small Intestine: Functions
- Movement / peristalsis of food.
- Digestion.
- Absorption of nutrients & water.
- Hunger / satiety.
- Immunity-Peyer’s patches & bacterial microflora.
Small Intestine: Absorption
•90% of absorption occurs in small intestine.
• Carbohydrates and amino acids are transported into capillaries:
• Monosaccharides: active & passive transport
• Amino acids:active transport
• Fatty acids, glycerol, cholesterol and fat soluble vitamins (A,D,E,K) are:
1. Emulsified by bile.
2. Enterintestinal cells by simple diffusion.
3. Packaged into chylomicrons and absorbed into lacteals.
4. Travel through the lymphatic system and enter the blood in the subclavian vein.
• Vitamin B12is absorbed in the terminal ileum (needs intrinsic factor for absorption).
• Vitamins, mineral salts & water enter blood capillaries and are sent to the liver (hepatic first pass).
• Bile salts are reabsorbed & recycled from the ileum.
Large Intestine
- Terminal portion of the GIT: where the final stages of digestion, absorption, some vitamin synthesis (through bacterial activity) & stool formation occurs.
- The ileo-caecal valve allows one way flow.
- Mucous is produced but no enzymes.
- Absorption of water, minerals, vitamins & some drugs
Large Intestine Regions
Caecum
Colon
Rectum
Anal canal
Caecum
Appendixis attached -a twisted tube about 8cm in length. Part of our immune system: contains macrophages, lymphocytes, bacteria.
Colon
ascending
transverse
descending
sigmoid colon
Rectum
Anal Canal
mucous membranes folds. Contains an internal and external anal sphincter.
Liver
- The liver is the heaviest gland of the body & second largest organ (skin being the largest).
- Uses 27% of our resting metabolism.
- It filters 1.4 litres of blood every minute.
- The hepatic artery brings oxygenated blood from the heart.
- Kuppfer cells are phagocytic cells which remove foreign bodies from the blood (they are a type of white blood cell that specialise in engulfing microbes).
Portal Vein
- The portal vein brings in nutrient (& toxin) rich blood from the GIT.
- “Hepatic first pass”: all blood from the GIT is transported to the liver (via the portal vein) to be filtered / metabolised before entering systemic circulation.
- Blood from the portal vein & hepatic artery mix in the ‘sinusoids’ (columns between hepatocytes).
- The filtrate enters the central vein which enters the hepatic vein & in to the systemic circulation.
Liver Functions
The liver is estimated to have over 500 functions.
- Cleansing blood of microbes.
- Detoxification-metabolising drugs, toxins & alcohol.
- Bile production & secretion.
- Haemolysis (kuppfercells)
- Synthesis of plasma proteins -blood clotting & coagulation factors.
- Hormone homeostasis –deactivating all hormones.
- Metabolism of glucose (glycogen), fats (hepatocytes store triglycerides) & amino acids.
- Heat production –thermogenesis.
- Synthesis -vitamin A (from beta carotene), Co-Q10 & activation of vitamin D.
- Storage-vitamins (A,D,E,K, B12), iron, copper, glycogen.
Liver Metabolism
Carbohydrate
• Excess glucose is converted to glycogen for storage.
• Glycogen to glucose as required.
Fat
• Metabolises fat from storage as required.
• Synthesises cholesterol & triglycerides.
Protein
•Converts essential amino acids into non-essential amino acids .
• Removes nitrogen groups from amino acids to form urea to be excreted.
• Breaks down nucleotides to form uric acid to be excreted.
Liver Detoxification
• Hepatocytes convert toxins into nontoxic metabolites which can then be excreted from the body.
• A healthy liver deals with thousands of toxins each day. These include airborne pollutants, those in food, drugs etc.
• Highly energy dependent (ATP) and nutrient dependent.
• Induced by toxicants, drugs, phytonutrients: enzymes made on the spot.
1. Hydrophilic: Excreted in urine or bile.
2. Lipophilic: Must be chemically altered into hydrophilic compounds to facilitate elimination
Liver Detox: Phase I
Bio-activation
- Involves CYP450 family of enzymes (a class of more than 50 enzymes). These enzymes are particularly important in metabolising toxins and medications.
- The enzymes are mostly found in liver cells but are also found in the small intestine, lungs, placenta and kidney.
- Converts water insoluble toxins into water soluble substances to be excreted by the kidneys
- Converts toxins to more reactive substances which can be metabolisedin Phase II.
Liver Detox: Phase II
- Conjugation reactions –molecules are attached to the toxins to neutralise them making them stable (non-reactive) & water-soluble to be excreted.
- Various enzymes are involved to induce many chemical reactions
‘Glutathione’ is a key anti-oxidant in phase II liver detoxifcation.
