UNIT 6 - DIGESTIVE SYSTEM Flashcards
1Digestive system
Consists of the gastrointestinal tract and accessory organs or digestion and involves breakdown of food
Functions of digestive system (4)
- Takes in food
- Breaks food down into nutrient molecules
- Absorbs molecules into bloodstream
- Get rid of any indigestible remains
Ingestion
Eating, or taking in food into the digestive tract
Digestion
Process by which insoluble food consisting of large molecules is broken down into soluble compounds
Mechanical digestion
Chewing of food into smaller pieces involving smooth muscle layers, teeth, tongue
Chemical digestion
Enzymes and acids breaking down the food involving pepsinogen, pepsin, HCl, gastric lipase
Absorption
Passage of food molecules from the digestive tract into circulatory/lymphatic system for transport to body cells
Defecation
Elimination or passing of feces from rectus out the body
Extracellular digestion
Food is broken down OUTSIDE the cell by enzymes produced by digestive tract cells
Intracellular digestion
Food is broken down INSIDE a cell (eg. Bacteria phagocytosed by WBC and digested by enzymes with lysosomes)
2 compartments of organs of digestive system:
- Alimentary canal (gastrointestinal)
- Accessory organs
Alimentary canal
Continuous muscular rube that runs from the mouth to the anus, consisting of mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus. Functions in digesting food and absorbing fragments through lining into blood
Accessory organs
Organs that aid in process of digestion consisting of teeth, tongue, gallbladder and digestive glands such as salivary gland, liver, pancreas
Digestive glands
Produce secretions that break down food
Mouth
Where food is chewed and mixed with enzyme containing saliva that begins process od digestion and where swallowing is initiated (eg. Mouth, tongue, salivary gland, teeth)
Tongue
Mixes food with saliva for easy swallowing
Buccal cavity
AKA oral cavity consisting of lips, cheeks, palate, tongue lined with stratified squamous epithelium
Uvula
Hanging from middle of posterior edge of soft palate
Soft palate
Behind hard palate separating nasal passage from food passage
Hard palate
Tissue covering part of maxilla in the anterior roof of mouth
Mastication
Chewing of food to make smaller food particles
Bolus
Small rounded mass of chewed food at moment of swallowing
Saliva
Clear liquid made by several glands in mouth and made up of 99.5% water and 0.5% solutes including salt, gases, bacteriostatic lysozyme and salivary amylase
Major salivary glands (3):
- Parotid
- Submandibular
- Sublingual
Saliva functions (4)
- Cleanse mouth
- Dissolve food chemicals to be tasted
- Moisten food and make bolus
- Contain enzyme amylase that begins digestion with starch
Pharynx and esophagus
Conduits to pass food from mouth to stomach and have a major function in propulsion that starts with deglutition
Deglutition
Swallowing and involves coordination of 22 muscle groups and 3 phases
3 phases of deglutition (swallowing) (3)
- Buccal phase (voluntary)
- 2 pharyngeal esophageal phases (involuntary)
- Controlled by swallowing center in medulla and lower pons
Buccal phase (voluntary)
Upper esophageal sphincter is closed, tongue moves upward and backwards against palate and pushes bolus to back of oral cavity into oropharynx
Pharyngeal esophageal phase (involuntary) 1
Uvula and soft palate move upward and close off nasopharynx, and larynx rises to epiglottis and blocks trachea. Upper esophageal sphincter relaxes and food enters esophagus
Pharyngeal esophageal phase (involuntary) 2:
Constrictor muscles of esophagus contract and force food inferiorly, upper esophageal sphincter closes
Esophagus
Muscular collapsible tube extending from pharynx to stomach located behind trachea and in front of vertebral column. It runs through neck, diaphragm and connects to stomach
Esophageal sphincter
Point where the esophagus connects to stomach
Peristalsis
Rhythmic muscular movements of alternating contractions of circular and longitudinal muscles that pushes food down esophagus towards stomach. Food is moved distally along tract
Circular muscle contractions
Lumen (internal diameter of digestive tract) contracts
Longitudinal muscle contraction
Length of digestive tract decreases and lumen widens
Stomach
Muscular J shaped organ in upper part of abdomen below diaphragm consisting of 4 major parts. Functions in digesting proteins and lipids (very small amounts of digestion)
4 major parts of stomach:
- Cardia
- Fundus (left)
- Body
- Pylorus (right)
Cardia
Lower portion where the esophagus is joined
Fundus
Rounded part of stomach above and left to cardia
Body
Largest central portion of stomach
Pylorus
Lower part of stomach attached to first part of small intestine (duodenum)
Pyloric sphincter
Band of circular muscle that can open or close and control movement of food from stomach into duodenum
Gastrin
Hormone produced by stomach that stimulates release of gastric juice (hydrochloric acid)
Gastric juice
Converts bolus to acid chyme with a pH of 2
Cells of gastric gland (5)
- Surface mucous cell
- Mucous neck cell
- Parietal cell
- Chief cell
- G cell
Surface mucous cell
Secretes mucus
Mucous neck cell
Secretes mucus
Parietal cell
Secretes hydrochloric acid and intrinsic factor
Chief cell
Secretes pepsinogen and gastric lipase
G cell
Secretes hormone gastrin
Liver
Largest gland in body weighing 3 pounds and consists of 4 primary lobes
4 lobes of liver:
- Right
- Left
- Caudate
- Quadrate
Falciform ligament
Separates larger right and smaller left lobe of liver
Round ligament (ligamentum teres)
Rope like band of connective tissue of fetal umbillical vein along falciform ligament
Liver functions (9):
- Produce bile salts from cholesterol for lipid emulsification
- Fat/cholesterol absorption
- Regulate blood glucose by producing and storing glycogen
- Detoxifies poisonous substances
- Produce plasma protein
- Stores fat soluble vitamins ADKE
- Storage for elements (Cu, Fe)
- Fatty acids converted to storage forms
- Degradation of hormones
Bile
Consists of bile salts, phospholipids, cholesterol, electrolytes and water used to break down fats during digestion and is produced by liver and is stored/concentrated in gallbladder
Left and right hepatic duct
Made up of several small bile ducts in the right lobe of liver that join
Common hepatic duct
Made up of the left hepatic duct joining with the right hepatic duct
Common bile duct
Made up of the common hepatic duct joining with the cystic duct (duct from gallbladder)
Ampulla of Vater
Made up of the common bile duct joining with the pancreatic duct that projects slightly into the duodenum
Hepatic portal vein
Unions of veins of stomach, small intestine and large intestine that take blood to the liver. The liver does receive some arterial blood through hepatic artery but majority is from hepatic portal vein
What happens to hepatic portal vein and artery in the liver
Both hepatic artery and hepatic portal vein break up into blood capillaries and release blood that mix together. The blood gets drained later on into two hepatic veins and enters inferior vena cava
Hepatic portal system significance
Allows modification of nutrients absorbed from the digestive tract by the liver cells (eg. Level of blood glucose). The liver removes toxins from the blood and adds plasma proteins and therefore the liver modifies, stores, and controls various nutrients in the blood and detoxifies blood before entering circulatory system
Pancreas
Soft gland about 12-15cm long and 2.5cm wide located below the curvature of stomach divided into three parts
3 parts of pancreas:
- Head
- Body
- Tail
Exocrine functions of pancreas
Production of pancreatic juice (enzyme for digestion of fats, proteins, carbohydrates, nucleic acids)
Bicarbonate ion:
Neutralizes acid chyme coming from the stomach and provides a basic pH for the enzyme to function
Endocrine functions of pancreas
Produces insulin to lower blood sugar and glucagon to raise blood sugar via islets of langerhans
Small intestine
Longest section of the alimental canal (3m), forms the pyloric sphincter to caecum and is made up of 3 parts. Functions in mechanical (segmentation) and chemical digestion of food, and absorption of water, digestion, vitamins, salts, and monosaccharides
3 parts of small intestine:
- Duodenum (25cm)
- Jejunum (1m empty)
- Ileum (2m twisted)
Circular folds
Permanent folds on the lining of the small intestine (1cm deep) that force chyme to slowly spiral through lumen and allow more time for nutrient absorption
Villi
Fingerlike projections of mucosa (1mm high) in small intestine with a core that contains dense capillary bed and lymphatic capillary called a lacteal for absorption
Microvilli
Cytoplasmic extensions of mucosal cell that gives fuzzy appearance called brush border that contains membrane bound enzymes (brush border enzymes) used for final carbohydrate and protein digestion
Segmentation
Most common motion of small intestine (mix/moves contents toward ileocecal valve) after a meal
Peristalsis vs segmentation:
- Peristalsis: Adjacent segment of alimentary canal organs alternately contract and relax. Food is moved distally along tract and some mixing may occur but primarily propulsive
- Segmentation: Nonadjacent segment of alimentary canal organs contract and relax. Food is moved forward, then backward. Mixing of food occurs and breaks it down mechanically with some propulsion
Large intestine
About 1.3m x 6.5cm and consists of 4 main parts. Although most of water absorption occurs in small intestine, it functions in absorption of water, minerals, vitamins. Synthesis of vitamins K and B5 by resident bacteria, production of feces/defecation and chemical digestion of food
4 parts of large intestine:
- Caecum (appendix)
- Colon (main part, 4 divisions)
- Rectum
- Anal canal
Feces
Undigested material composed of bacteria, water, hemoglobin breakdown products
Anal canal
Last segment of large intestine that opens to body exterior at anus consisting of two sphincters
2 sphincters of anus:
- Internal anal sphincter: smooth muscle
- External anal sphincter: skeletal muscle (voluntary control)
Regulation of defecation (5)
- Feces moving to rectum cause distension of rectal walls and stimulate pressure receptors on wall
- Signal sent to bottom portion of spinal cord (sacral)
- Impulse sent back to rectum and stimulate contraction of longitudinal muscles on rectal walls
- Pressure on internal anal sphincter relaxes and opens
- If external sphincter is relaxes, defecation occurs. If constricted, defecation is delayed
Abdominal muscle function in defecation
Voluntary contractions of abdominal muscles help defecation by putting pressure on rectum
6 essential nutritional factors:
- Carbohydrates
- Fats
- Proteins
- Water
- Vitamins
- Minerals
Digestion of carbohydrates
Begins in the mouth
What is starch broken down into
Glucose
Salivary amylase
Initiates hydrolysis of starches (carbohydrate) in the mouth and breaks them into maltose (disaccharide of two glucose molecules). Sometimes there is not enough time so it only breaks them into shorter chains of starch
Pancreatic amylase
Further digestion of carbohydrates breaking down starch into maltose in lumen of small intestine
Maltase
Breaks down maltose to glucose
Sucrase
Digests sucrose
Lactase
Digests lactose
Brush border
Cells lining small intestine
Where does the absorption of monosaccharides occur
Small intestine
Process of monosaccharide absorption (4)
- Pancreatic amylase breaks down starch and glycogen into oligosaccharides/disaccharide
- Brush border enzymes break oligosaccharides/dissacharides into monossachride- Monosaccharides transported across apical membrane of absorptive epithelial cell via active transport and Na+ K+ pump in basolateral membrane
- Monosaccharides exit across basolateral membrane by facilitated diffusion and enter capillary via intracellular cleft
How do monosaccharides get absorbed in epithelial cells
Glucose and galactose get transported by active cotransport with sodium ions, and fructose by facilitated diffusion
How do monosaccharides get absorbed in blood
Facilitated diffusion
What are proteins broken down into
Amino acids
Digestion of proteins
Begins in the stomach
Gastric juice
Composed of mucous, HCl, pepsinogen (inactive form of enzyme) formed within the stomach lining and secreted
Pepsin
Stomach enzyme originating from pepsinogen, activated by HCl and secreted by the stomach. Functions in breaking down proteins into polypeptides and works well in very acidic pH
Trypsin
Secreted in inactive form by the pancreas into duodenum and gets activated by enzyme in small intestinal wall and then activates others. Functions in digesting polypeptides into smaller peptide units
Chymotrypsin
Secreted by the pancreas into duodenum and digests peptides into amino acids
What happens to pepsin after digestion
It becomes inactive when the acidic environment of the stomach is neutralized in the small intestine and other enzymes like trypsin and chymotrypsin help with protein digestion
Carboxypeptidase
Secreted by pancreas into duodenum and cuts off one amino acid from the COOH end
Aminopeptidase
Secreted by brush border and cuts off one amino acid at amino end
Dipeptidase
Splits dipeptides
Endopeptidases
Cut peptide bonds within the protein molecule other than the ones at each end of protein molecule (eg. pepsin, trypsin, chymotrypsin)
Exopeptidases
Cut peptide bonds at the ends of the protein molecule (eg. Carboxypeptidase, aminopeptidase)
Process of protein absorption (4):
- Pancreas proteases break down proteins into smaller pieces and some amino acids
- Brush border enzymes break protein fragments into amino acids
- Amino acids cotransported across apical membrane of absorptive epithelial cells via Na+ K+ pump in basolateral membrane
- Amino acid exit across basolateral membrane via facilitated diffusion and enter capillary via intracellular cleft
How do proteins get absorbed into epithelial cells
By active cotransport of amino acids with sodium
How do proteins get absorbed into blood
By facilitated diffusion
What are fats broken down into
Fatty acids and glycerol
Digestion of lipids
Begins in the stomach
Lingual lipase
Enzyme for digesting triglycerides within stomach to 1 fa and diglyceride activated by HCl when bolus reaches stomach
Gastric lipase
Enzyme for digesting triglycerides within the stomach into 2 fa and 1 monoglyceride)
Pancreatic lipase
Enzyme for breaking down triglycerides to 2 fa and monoglycerides
Where do most lipid digestion occur
Small intestine
Is lipase soluble in fat
NO so it can only act on the surface of fat globule
Bile salts
Molecules polar at one end and nonpolar at other (similar to phospholipid) and functions to emulsify fats by breaking large globules of fat into smaller ones and preventing fat from forming a mass into larger droplets
Emulsification
Process in which fat globules must be broken into tiny droplets for lipase to work
Process of fat digestion in small intestine (6):
- Lipase is not soluble in fat so it can only act at surface of fat globule
- Bile emulsifies fat globule and break down into smaller droplets and increasing surface area and action of pancreatic lipase
- Pancreatic lipase hydrolyzes triglycerides into 2 fa and monoglycerides
- If the digested fa are short chains (less than 10-12 Carbon), they are soluble in water and can move easily to intestinal epithelial cells
- If fa is long chains, they are not soluble in water so bile salts must interact with them to produce small droplets called micelles
- Micelles diffuse over intestinal wall and the digested fat diffuses across intestinal wall leaving bile salts behind
Short fa chains (digestion of fat)
Soluble in water and can easily move to intestinal epithelial cells and diffuse into them
Long fa chains (digestion of fat)
Not soluble in water and must interact wih bile salts to produce small droplets
Micelles
Small droplets of digested fat
Absorption of lipids (3)
- Short fatty acids absorbed through the cell into blood capillaries in villus
- Monoglycerides and long fatty acids diffuse into villus epithelium with help of bile salts
- Inside epithelia, the fa and MG are reassembled into triglycerides and travel through blood via chylomicrons
Chylomicrons
Mixtures of triglycerides and cholesterol with a phospholipid and a protein coat to travel through blood for absorption
How do chylomicrons leave the epithelial cell
Exocytosis to lacteals, then lymph, then left subclavian vein, then the general systemic circulation
Nucleic acids
Found in cells of all plants and animals and are components of most vegetables and meats
Ribonuclease
Pancreatic enzyme that digests RNA into nucleotides
Deoxyribonuclease
Pancreatic enzyme that digests DNA into nucleotides
Intestinal enzymes of nucleotides
Digest nucleotides into pentose sugars, phosphate, nitrogenous base
Mechanism of control of digestion
Secretion of digestive juices in humans is under nervous and hormonal control
Nervous control secretions
Short lived
Hormonal regulation secretion
More sustained
3 phases that regulate secretion of digestive juices:
- Cephalic phase of digestion (neural)
- Gastric phase of digestion (neural & hormonal)
- Intestinal phase of digestion (neural & hormonal)
Cephalic phase of digestion
Smell, taste, sight or thought of food stimulates cerebral cortex, hypothalamus and brain stem. Nervous impulses travel via vagus nerve to stomach and stimulate secretion of gastric juice (only neural regulation, no hormonal regulation)
Gastric phase of digestion (neural)
Stretch and chemoreceptors in stomach activated as stomach distends and pH increases due to presence of proteins. Signals are sent by stretch and chemoreceptors to nerves within stomach wall that release HCl and greater stomach mobility
Gastric phase of digestion (hormonal)
Regulated by hormone gastrin produced by gastric glands and stimulated by stomach distension, partially digested protein, increased pH above 2, caffeine
Intestinal phase of digestion (neural)
Enterogastric reflex caused by distension of duodenum due to entry of stomach contents cause stretch receptors to send impulses to medulla to decrease gastric emptying which ensures acid chyme is not delivered to small intestine faster than it can be processed
Intestinal phase of digestion (hormonal)
Secretin and cholecystokinin hormones contribute to regulation of secretive digestive juices here
Secretin
Produced by endocrine glands in intestinal wall stimulate by acid chyme entering duodenum. Travels through the blood stream to pancreas and stimulates flow of pancreatic juice containing bicarbonate ion, decreasing acidity of chyme
Cholecystokinin
Stimulated by amino acids/fatty acids in duodenum and travels through bloodstream to pancreas, then gallbladder to stimulate the secretion of pancreatic juice containing enzymes and bile to emulsify fats
