5: The Digestive System Flashcards
what’s included in gastrointestinal tract?
oral cavity
pharynx
oesophagus
stomach
small intestine
large intestine
functions of digestive system
ingestion
mechanical processing (chewing)
digestion
secretion (enzymes, acids)
absorption
excretion
peritoneum
sheet of smooth tissue that lines the abdominopelvic cavity and surrounds abdominal organs
serous membrane: visceral layer covers the organs, parietal layer lines cavity
peritoneal fluid: 7 L/day produced, provides lubrication to allow sliding of two membranes
mesenteries
ligaments that suspend portions of the digestive tract, keeping them in right position but still allowing them to expand contract
allow passage of blood vessels, nerves and lymphatic vessels
digestive tract
protects against digestive acids and enzymes, mechanical stresses and bacteria
4 major layers: mucosa, submucosa, muscularis externa, serosa
mucosa
inner layer is mucosal epithelium: stratified squamous epithelium in oral cavity, pharynx and oesophagus for protection, simple columnar with mucous cells elsewhere for absorption
lamina propria: middle layer made of areolar tissue with blood and lymphatic vessels and nerve endings
muscularis mucosae: outer layer of mucosa, thin layer of smooth muscle, inner circular layer, outer longitudinal layer
surface covered in villi to increase surface area
plica circulares are folds to increase surface area
submucosa
layer of dense, irregular connective tissue
large blood vessels and lymphatic vessels
may contain exocrine glands to secrete buffers and enzymes into digestive tract
submucosal plexus is a neural network to innervate the mucosa and submucosa
muscularis externa
smooth muscle cells, inner circular layers and outer longitudinal layers
movements coordinated by enteric nervous system - sensory neurons, inter neurons and motor neurons innervated primarily by parasympathetic division of ANS
serosa
serous membrane covering muscularis externa in most of digestive tract
replaced by adventitia in oral cavity, pharynx, oesophagus, and rectum
adventitia is dense sheath of collagen fibres to attach adjacent structures
firm attachment so thinks aren’t moving round in these areas
movement of digestive materials
rhythmic cycles of smooth muscle activity is controlled by pacesetter cells (located in muscularis mucosae and muscularis externa)
cells undergo spontaneous depolarisation causing waves of contraction through entire muscular sheet
peristalsis = waves of muscular contraction that move food through digestive system
segmentation = cycles of contraction, no set pattern
1. contraction of circular muscle behind bolus
2. contraction of longitudinal muscles ahead of bolus
3. contraction in circular muscle layer forces bolus forward
oral cavity
functions:
- sensory analysis (smell/taste)
- mechanical processing
- lubrication
- limited digestion (carbohydrates start breaking down by salivary amylase, some lipids start break down by lingual lipase)
passageway for food, liquids and air
salivary glans and saliva
saliva: 1-1.5 L/day produced, contains water (>99%), electrolytes, buffers, mucins and antibodies
functions of saliva: lubricate, dissolve chemicals, initiating digestion of complex carbs
3 salivary glands: parotid in front of ear, sublingual under tongue, submandibular under mandible
production of saliva controlled by parasympathetic and sympathetic stimulation
swallowing
buccal phase (back of mouth)
pharyngeal phase
oesophageal phase
bolus enters stomach
oesophagus
conveys solid food and liquids to stomach
resting muscle tone in the superior 3cm prevents air from entering, closed and contracted when not swallowing
adventitia anchors to surrounding structures
thick stratified squamous epithelium
major functions of stomach
storage of ingested food
mechanical breakdown
chemical breakdown by enzymes and acid
production of intrinsic factor (needed for vitamin B12 absorption)
pylorus is the exit of stomach to duodenum, pyloric sphincter is ring of muscle at end to keep food in until ready to move to intestines
stomach lining
simple columnar epithelium lines stomach - produces mucus to protect lining of stomach from acid
gastric pits connect gastric glands in mucosa onto the gastric surface - cells at base divide and replace superficial cells
smooth muscle - oblique layer in addition to circular and longitudinal layers
folds on lining of stomach called rugae
stomach glands
gastric glands: in fundus (top of stomach) and body, parietal cells secrete intrinsic factor and HCl, chief cells secrete pepsinogen which only become active when converted to pepsin in presence of HCl, G cells secrete gastrin which helps to increase activity of stomach when eating
pyloric glands: in pylorus, produce mucous, G cells secrete gastrin, D cells release somatostatin (inhibits gastrin)
hydrochloric acid secretion
enzyme in parietal cells called carbonic anhydrase converts carbon dioxide and water to carbonic acid
acid will disassociate to form bicarbonate ions and protons
hydrogen ions (protons) actively transport into the gastric tract
bicarbonate ions are exchanged for chloride ions, bicarbonate ions travel into bloodstream
chloride ions diffuse through cells and exit into lumen
how the stomach is protected from acid
mucous lining
rapid cell division
separate secretion of H+ and Cl- in parietal cells
secretion of inactive precursor pepsinogen
stimulation of secretion only when needed
inhibition of secretion: CCK, GIP, secretin
regions of small intestine
duodenum = 25cm, neutralises chyme, receives secretions from pancreas and liver, few plica circulares, small villi
jejunum = 2.5m, location of most chemical digestion and nutrient absorption, prominent plica circulares and villi
ileum = 3.5m, lymphoid nodules which have immune function
intestinal wall structure
villi - extensive network of capillaries
intestinal crypts (openings) at base of villi
lacteals up the centre of villi - branch off lymphatic vessel
brush border - enzymes located there, important for digestion
the small intestine
intestinal glands
- epithelial cells divide at base are displaced towards tip of villi then disintegrate adding enzymes to lumen
- brush border enzymes = integral membrane proteins on intestinal microvilli, breakdown materials in contact, enteropeptidase activates pancreatic proenzyme trypsinogen to trypsin which breaks down proteins
- enteroendocrine cells = produce hormones in response to pH change/nutrients when chyme enters small intestine (cholecystokinin CCK and secretin increases enzyme/bile secretion, gastric inhibitory peptide inhibits gastrin activity)
duodenal glands: produce mucus to raise pH as contents are highly acidic
intestinal juice - moistens chyme and keeps intestinal contents in solutions, contains buffers to buffer acidic contents
the pancreas
ducts that lead to small intestine
secretes pancreatic juice (1 L/day)
contains pancreatic enzymes including pancreatic alpha amylase (carbs), pancreatic lipase (fats), nucleases (nucleic acids), proteases and peptides (70% of all pancreatic enzyme production, secreted as inactivated proenzymes, activated at small intestine)
functions
- endocrine cells (1%) secrete insulin and glucagon into bloodstream, found in pancreatic islets
- exocrine cells secrete pancreatic juice, secrete to outside surface pancreatic ducts, found in pancreatic acini
hepatic blood supply (blood for liver)
2/3 blood from hepatic portal vein, originating from: oesophagus, stomach, small intestine, much of large intestine (deoxygenated)
all of blood from digestive organs effectively filtered through liver
1/3 blood from hepatic artery proper - brings oxygenated blood
liver histology
100,000 hexagonal lobules (functional unit of liver) - 1mm diameter
portal area at lobules corner contain: branch of hepatic portal vein, branch of hepatic artery proper and branch of bile duct
blood flows past hepatocytes to absorb solutes and secrete proteins
sinusoids drain to central vein
bile ductules drain to bile ducts
liver function: metabolic regulation
metabolism of carbohydrate, lipids and amino acids
waste product removal (ammonia, toxins)
vitamin storage (fat soluble vitamins, B12)
mineral storage (ferritin)
drug inactivation
liver function: haematological regulation
phagocytosis and antigen presentation
synthesis of plasma proteins
removal of circulating hormones, antibodies, toxins
liver function: bile production
bile salts break insoluble lipid droplets apart (emulsification)
increase surface area of fats exposed to enzymatic attack
gallbladder and bile transport/storage
liver produces 1L bile a day
bile is stored and concentrated in gallbladder until small intestines need it
duodenal CCK release triggers dilation of the hepatopancreatic sphincter and contraction of the gallbladder
bile then ejected into duodenum
bile salts break apart lipid droplets by emulsification to give them a greater surface area for enzymes
the large intestine
ileum has a valve that controls movement of material into large intestine
ascending colon travels up, transverse colon across, descending colon down and sigmoid colon curves into rectum
large intestine functions
absorption (<10% as most happened in small intestine) - reabsorption of water and bile salts, absorption of vitamins and organic wastes
site of bacterial vitamin production: vitamin K, biotin, vitamin B5
compaction of intestinal contents into faces
histology of large intestine
lacks villi - smooth surface
abundant mucous cells - lubrication
the longitudinal layer of the muscularis externa is incomplete - thin strips called taeniae coli
neural control of intestinal movement
central control = parasympathetic stimulation increases motility and secretion, sympathetic inhibits
local reflexes = weak myenteric reflexes, local responses to stretch/pH change
central gastric reflexes = the gastroenteric reflex stimulates motility and secretion along entire small intestine, gastroileal reflex triggers the opening of the ileocecal valve allowing materials to pass from small to large intestine
hormonal control
food in stomach increases gastrin production which increases acid production by parietal cells and increases mixing in stomach as muscular activity increases
when food enters duodenum, stimulate gastrin inhibitory peptide GIP which slows down activity of stomach and stimulates release of insulin from pancreas, lowering glucose levels
food in jejunum stimulates secretin and CCK which releases pancreatic enzymes and buffers and bile is secreted
food in jejunum also stimulates vasoactive intestine intestine peptide VIP which dilates the intestinal capillaries
digestion
defined as the processing and absorption of nutrients
macronutrients: complex carbohydrates > polysaccharides > monosaccharides, lipids > fatty acids, proteins > polypeptides > amino acids
water
molecules released into bloodstream are: absorbed by cells, broken down to provide energy for ATP synthesis, or used to synthesise carbohydrates, proteins, lipids
