Week 5 - Digestive System Flashcards
List some functions of the digestive system
- Ingestion
- Mechanical processing
- Digestion
- Secretion
- Absorption
- Excretion
Name the parts of the gastrointestinal tract
Oral Cavity, teeth tongue - Mechanical processing, moistening,
salivary secretions
Pharynx - Propulsion
Oesophagus - Transport of materials to stomach
Stomach - Chemical breakdown; mechanical processing
Small Intestine - Enzymatic digestion and absorption
Large intestine - Enzymatic digestion and absorption.
Accessory organs of digestive system:
Salivary glands - Secretion of saliva
Liver - Secretion of bile, many other functions
Gallbladder - Storage of bile
Pancreas - Secretion of buffers and enzymes
What is the peritoneum and mesentries?
Peritoneum:
Serous membrane - visceral layer covers organs, parietal layer lines cavities.
Peritoneal fluid - 7l/day produced; provides lubrication to allow sliding.
Mesenteries:
Suspend portions of digestion tract
Allow passage of blood vessels, nerves and lymphatic vessels.
What does the digestive tract protect against and what is its histological organisation?
Protects against:
Digestive acids and enzymes
Mechanical stresses
Bacteria
4 major layers:
1.Mucosa
2.Submucosa
3.Muscularis externa
4.Serosa
Describe the mucosa
Mucosal epithelium
• Stratified squamous
epithelium in oral cavity,
pharynx & oesophagus
• Simple columnar with
mucous cells elsewhere
Lamina Propria
• Areolar tissue with blood
and lymphatic vessels
and nerve endings
Muscularis Mucosae
• Inner circular layer
• Outer longitudinal layer
Describe the submucosa
• Layer of dense, irregular connective tissue
• Has large blood vessels and lymphatic vessels
• May contain exocrine glands
• Secrete buffers and enzymes into digestive tract
• Submucosal Plexus- neural network
• Innervates the mucosa and submucosa
Describe the muscularis externa
• Smooth muscle cells:
• Inner circular layer
• Outer longitudinal layer
• Movements coordinated by enteric
nervous system (ENS)
• Sensory neurons, Interneurons and
Motor neurons
• Innervated primarily by parasympathetic
division of ANS
Describe the serosa
• Serous membrane covering muscularis externa in most of digestive tract
• Replaced by adventitia (dense sheath of collagen fibers to attach adjacent structures) in oral cavity, pharynx, oesophagus, and rectum
Describe how movement of Digestive materials occurs
Rhythmic cycles of smooth muscle activity controlled by pacesetter cells
• Located in muscularis mucosae and muscularis externa
• Cells undergo spontaneous depolarization
• Waves of contraction through entire muscular sheet
Peristalsis
• Waves of muscular contractions
Segmentation
• Cycles of contraction, does not follow a set pattern
Give functions of the oral cavity
• Sensory analysis
• Mechanical processing
• Lubrication
• Limited digestion
-carbohydrates (salivary
amylase from salivary
glands)
-Lipids (lingual lipase
produced by tongue)
Passageway for food, liquids and
air
• Opens to oropharynx
(continuous with
nasopharynx)
Describe salivary glands and saliva
Saliva
• 1.0-1.5 L/day produced
• Contains water (>99%),
electrolytes, buffers, mucins,
antibodies
Functions of Saliva:
• Lubricating the mouth and contents
• Dissolving chemicals
• Initiating digestion of complex
carbohydrates by salivary amylase
Salivary Glands
• Parotid, Sublingual and
Submandibular
• Produce saliva controlled by
parasympathetic and sympathetic
stimulation
What’s the role of the oesophagus
• Conveys solid food and liquids to the stomach
• Resting muscle tone in the superior 3 cm of prevents air from entering
• Adventitia anchors to surrounding structures
Give the major functions of the stomach
• Storage of ingested food
• Mechanical breakdown
• Chemical breakdown
- By enzymes
-pepsin starts protein breakdown
-Continued activity of salivary
amylase and lingual lipase
• By acid (HCl)
-pH reduces to 2
• Production of intrinsic factor
- Needed for vitamin B12
absorption
Describe the stomach lining
Simple columnar
epithelium lines stomach
- Produces mucus
Gastric pits connect
gastric glands in mucosa
onto the gastric surface
- Cells at the base
divide, replacing
superficial cells
Smooth Muscle
(muscularis mucosae and
muscularis externa)
- Oblique layer in
addition to circular and
longitudinal layers
Describe the stomach glands
Gastric Glands
- In fundus and body of stomach
- Parietal cells
-Secrete intrinsic factor and HCl
-Chief cells
-Secrete pepsinogen
- Converted to pepsin by HCl
- G cells
- Secrete gastrin
Pyloric Glands
-In pylorus!
- Produce mucous
- G cells secrete gastrin
- D cells release somatostatin (inhibits
gastrin release)
How is HCL secreted by parietal cells?
- Carbonic anhydride converts CO2 and H2O to carbonic acid which dissacociates to form h+ ions
- Countertransport mechanism ejects the bicarbonate ions and imports chloride ions
- chloride ions diffuse across the cell and exit through open chloride channels into lumen of the gastric gland
- Hydrogen ions are actively transported into the lumen of the gastric gland.
What are the three regions of the small intestine?
Duodenum
• 25cm long
• Neutralises chyme
• Receives secretions from pancreas & liver
• Few plica circulares, small villi
Jejunum
• 2.5 m long
• Location of most chemical digestion and nutrient absorption
• Prominent plica circulares and villi
Ileum
• 3.5 m long
• Lymphoid nodules
(Peyer’s patches)
Describe the small intestine
Intestinal glands
• Epithelial cells divide at base are displaced towards tips of villi then
disintegrate adding enzymes to lumen
• Brush border enzymes
• Integral membrane proteins on intestinal microvilli
• Break down materials in contact with brush border
• Enteropeptidase
• Activates pancreatic proenzyme trypsinogen
• Enteroendocrine cells
• Produce hormones in response to pH change/nutrients:
• cholecystokinin (CCK) and secretin – increase enzyme/bile secretion
• Gastric inhibitory peptide – inhibits gastrin activity
Duodenal glands
• Produce mucus- raises pH
Intestinal juice
• Moistens chyme and keeps intestinal contents in solution
• Buffers acids
Describe the pancreas
Secretes pancreatic juice
• ~ 1L/day
• Contains pancreatic enzymes,
including
• Pancreatic alpha-amylase
• Pancreatic lipase
• Nucleases
• Proteases & peptidases (70% of all pancreatic enzyme production)
• Secreted as inactive proenzymes, activated after reaching small intestine
Give the histology of the liver
~100,000 hexagonal lobules (functional unit of liver)
1mm diameter
Portal areas contain:
•Branch of hepatic portal vein
•Branch of hepatic artery proper
•Branch of bile duct
Blood flows past hepatocytes
•Absorb solutes
•Secrete proteins
Drains into
•Sinusoids>central vein
•Bile ductules>bile ducts
Give functions of the liver
• 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
• Haematological regulation
• Phagocytosis and antigen presentation
• Synthesis of plasma proteins
• Removal of circulating hormones, antibodies, toxins
• Bile production
• Bile salts break insoluble lipid droplets apart (emulsification)
• Increases surface area exposed to enzymatic attack
describe how the gallbladder transports/stores bile
- The liver secretes approx 1L bile per day
- Bile is stored and conc in the gallbladder
- Duodenal CCK release triggers dilation of the hepatopancreatic sphincter and contraction of the gallbladder. Bile is ejected into the duodenum.
- Bile salts break apart lipid droplets by emulsification.
Give the functions of the large intestine
• Absorption (<10% of all absorption)
• 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 faeces
• Storage of faeces until defaecation
Give the histology of the large intestine
-Lacks villi
- Abundant mucous cells
- The longitudinal layer of the muscularis externa =>taeniae coli
Describe neuronal control of intestinal movement
The gastroenteric Reflex stimulates motility and secretion along the entire small intestine.
The gastroileal (gas-tro-IL-e-al) reflex triggers the opening of the ileocaecal valve, allowing materials to pass from the small intestine into the large intestine
Central Control: Parasympathic stimulation -increases motility and secretion
Sympathetic stimulation - inhibits
Describe hormonal control
Slide 37
Describe Digestion
• The Processing and Absorption of Nutrients
• Macronutrients
• Complex carbohydrates=>polysaccharides=>monosaccharides
• Lipids=>fatty acids
• Proteins=>polypeptides=>amino acids
• Micronutrients
• Vitamins
• Minerals
• Water
• Molecules released into bloodstream are:
• Absorbed by cells
• Broken down to provide energy for ATP synthesis
• Or used to synthesize carbohydrates, proteins, and lipids
