5: The Digestive System

Question 1

what’s included in gastrointestinal tract?

Answer 1

oral cavity
pharynx
oesophagus
stomach
small intestine
large intestine

Question 2

functions of digestive system

Answer 2

ingestion
mechanical processing (chewing)
digestion
secretion (enzymes, acids)
absorption
excretion

Question 3

peritoneum

Answer 3

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

Question 4

mesenteries

Answer 4

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

Question 5

digestive tract

Answer 5

protects against digestive acids and enzymes, mechanical stresses and bacteria
4 major layers: mucosa, submucosa, muscularis externa, serosa

Question 6

mucosa

Answer 6

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

Question 7

submucosa

Answer 7

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

Question 8

muscularis externa

Answer 8

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

Question 9

serosa

Answer 9

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

Question 10

movement of digestive materials

Answer 10

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

Question 11

oral cavity

Answer 11

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

Question 12

salivary glans and saliva

Answer 12

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

Question 13

swallowing

Answer 13

buccal phase (back of mouth)
pharyngeal phase
oesophageal phase
bolus enters stomach

Question 14

oesophagus

Answer 14

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

Question 15

major functions of stomach

Answer 15

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

Question 16

stomach lining

Answer 16

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

Question 17

stomach glands

Answer 17

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)

Question 18

hydrochloric acid secretion

Answer 18

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

Question 19

how the stomach is protected from acid

Answer 19

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

Question 20

regions of small intestine

Answer 20

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

Question 21

intestinal wall structure

Answer 21

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

Question 22

the small intestine

Answer 22

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

Question 23

the pancreas

Answer 23

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

Question 24

hepatic blood supply (blood for liver)

Answer 24

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

Question 25

liver histology

Answer 25

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

Question 26

liver function: metabolic regulation

Answer 26

metabolism of carbohydrate, lipids and amino acids
waste product removal (ammonia, toxins)
vitamin storage (fat soluble vitamins, B12)
mineral storage (ferritin)
drug inactivation

Question 27

liver function: haematological regulation

Answer 27

phagocytosis and antigen presentation
synthesis of plasma proteins
removal of circulating hormones, antibodies, toxins

Question 28

liver function: bile production

Answer 28

bile salts break insoluble lipid droplets apart (emulsification)
increase surface area of fats exposed to enzymatic attack

Question 29

gallbladder and bile transport/storage

Answer 29

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

Question 30

the large intestine

Answer 30

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

Question 31

large intestine functions

Answer 31

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

Question 32

histology of large intestine

Answer 32

lacks villi - smooth surface
abundant mucous cells - lubrication
the longitudinal layer of the muscularis externa is incomplete - thin strips called taeniae coli

Question 33

neural control of intestinal movement

Answer 33

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

Question 34

hormonal control

Answer 34

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

Question 35

digestion

Answer 35

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