Lecture 5 - The Digestive System Flashcards
What are the functions of the digestive system?
Ingestion
Mechanical processing - big lumps to smaller lumps
Digestion - breakdown by chemical or biological as large molecules cant be absorbed
Secretion
Absorption - Nutrients in food taken up
Excretion - defecation - removing waste and faeces
What are the major subdivisions of the Digestive Tract?
Oral cavity, teeth and tongue - mechanical processing and salivary secretion
Pharynx - propulsion
Oesophagus - transport of materials into the stomach
Stomach - chemical breakdown and mechanical processing
Small intestine - Enzymatic digestion and absorption
Large intestine - enzymatic digestion and absorption
What are the accessory organs of the digestive system?
Salivary glands - Secretion of saliva
Liver - Secretion of bile and many other functions
Gall bladder - storage of bile
Pancreas - Secretion of buffers and enzymes
What is the peritoneum?
Serous membrane - visceral layer which covers organs and a parietal layer which lines cavities
Peritoneal fluid - 7L / day produced which provides lubrication to allow sliding
What are the mesenteries?
They suspend portions of digestive tract
Allow passage of blood vessels, nerves and lymphatic vessels
What is the digestive tract?
Protects against : Digestive acids and enzymes, Mechanical stress and bacteria
Histological organisation
1. Mucosa - have folds called Plica circulares to slow down the passage of food so it can be absorbed
2. Submucosa
3. Muscularis externa - Slow contraction and relaxation
4. Serosa - Visceral peritoneum - smooth surface
Outline the digestive tract mucosa
Mucosal Epithelium - Stratified squamous epithelium in oral cavity, pharynx and 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 - this contracts to dislodge food
Outline the digestive tract Submucosa
Layer of dense irregular connective tissue
Has large blood vessels and lymphatic vessels
May contain exocrine glands - secrete buffers and enzymes
Submucosal Plexus - neural network - innervates the mucosa and submucosa
Outline the digestive tract Muscularis Externa
Smooth muscle cells
Movement co-ordinated by enteric nervous system- sensory ,inter and motor neurons - parasympathetic n.s - via the vagus nerve
Inner is circular - Moves food down the tract
Outline the digestive tract serosa
Serous membrane covering muscularis externa in most of the digestive tract
Replaced by the adventitia (dense sheet of collagen fibres to attach adjacent structures) in oral cavity, pharynx, oesophagus and rectum
How do digestive materials move down the digestive tract?
Pacesetter cells (in muscularis mucosae and muscularis externa ) activate smooth muscle in rhythmic cycles
Longitudinal muscle first to move the tract to the food then circular to push the food from behind
The cells undergo spontaneous depolarisation and contraction passes slowly along the muscular wall
Peristalsis - waves of muscular contractions
Segmentation - Mix and churn food no pattern
Outline the functions of the oral cavity
Sensory analysis
Mechanical processing
Lubrication
Limited digestion - carbs - salivary amylase - Lipids - lingual lipase form tongue
Opens to the oropharynx (continuous with the nasopharynx)
Outline the functions of saliva and the salivary glands
Saliva - 1-1.5L per day - 99% water, electrolytes, buffers, mucins, antibodies
Functions - Lubes the mouth and contents, dissolving chemicals, initiating digestion of complex carbs
Salivary glands - Parotid, sublingual and submandibular
Production of saliva is controlled by ps and s stimulation
How do we swallow?
- Buccal phase - tongue lifts up and moves the bolus to the back
- Pharyngeal phase - Passes the epiglottis so the food doesn’t go into the air pathway
- Oesophageal phase - through the oesophagus
- Bolus enters stomach
Outline the oesophagus
Conveys solid food and liquids to the stomach
Resting muscle tone in the superior 3cm to prevent air from entering
Adventitia anchors to surrounding structures
No serous membrane - really thick layer of mucosa to provide more protection
Outline the function of the stomach
Storage of ingested food
Mechanical breakdown
Chemical breakdown by enzymes - pepsin - starts protein continued activity of salivary amylase and lingual lipase By acid - HCl pH of 2
Production of intrinsic factor - needed for the absorption of B12 (lack of B12 = anaemia)
Outline the structure of the stomach
Fundus
Cardia
Body
Pylorus
Pyloric sphincter - stays contracted to keep food from moving on
3 layers of muscle for churning food
Outline the stomach lining
Simple columnar epithelium lines the stomach- produces mucus
Gastric pits - connect gastric glands in mucosa to the surface, cells at the base divide to to replace superficial cells from damage
Smooth muscle - Oblique layer in addition to circular and longitudinal
Outline the stomach glands
Gastric glands - in fundus and body
Parietal cells - secrete intrinsic factor and HCl
Chief cells - Secrete pepsinogen (inactive) converted to pepsin by HCl
G cells - Secretes gastrin - increases activity of stomach
Pyloric glands - produce mucous - lube
G cells secrete gastrin
D cells release somatostatin - inhibits gastrin
How is HCl secreted by the parietal cells?
- Carbonic anhydrase converts CO2 and H20 to carbonic acid which dissociate to form H+ ions
- Counter transport mechanism ejects bicarbonate ions and imports chloride ions
- Chloride ions diffuse across the cell and exit through open chloride channels into the lumen of the gastric gland
- H+ ions are actively transported into the lumen of the gastric gland
What mechanisms help protect the stomach?
Mucous lining
Rapid cell division
Separate secretion of H+ and Cl - in parietal cells
Secretion of inactive precursors e.g. pepsinogen
Stimulation of secretion only need
What are the regions of the small intestine?
Duodenum - 25 cm long - neutralise chyme, receives secretions from pancreas and liver, few plica circulares, small villi
Jejunum - 2.5m long - location of most chemical digestion and nutrient absorption, prominent plica circulares and villi
Ileum - 3.5 m long - lymphoid nodules - Peyer’s patches
Outline the organisation of the intestinal wall
Villi to increase s.a
Lacteals - lymphatic vessels that absorb fat
Lymphoid nodules
Mucosa
Muscularis mucosae
Sub mucosa
Muscularis externa
Serosa
Outline the function of the small intestine
Intestinal glands - epithelial cells divide at the base are displaced towards tips of villi then disintegrated adding enzymes to lumen
Brush border enzymes - Integral membrane proteins on intestinal microvilli, breakdown materials in contact with brush border , enteropeptidase - activates pancreatic proenzyme trypsinogen
Enteroendocrine cells - Produce hormones in response to pH changes - CCK and secretin- increase bile Gastric inhibitory peptide - inhibits gastrin
Duodenal glands - produce mucus - raises pH
Intestinal juice - Moistens chyme and keeps intestinal contents in solution, buffers acids
Outline the pancreas
Secretes pancreatic juice - 1/L per day - contains pancreatic enzymes including pancreatic alpha-amylase, pancreatic lipase, nucleases, proteases and peptidases - secreted as inactive proenzymes, activated after reaching the s.i
Outline the structure of the pancreas
Pancreatic duct
Connective tissue septum
Exocrine cells in pancreatic acini - secretes pancreatic juice
Endocrine cells in pancreatic islet - secrete insulin and glucagon into blood
Outline the hepatic blood supply
2/3 of blood from hepatic portal vein originating from oesophagus, stomach, small intestine and large intestine - all the nutrients absorbed into the capillaries from the intestine will be filtered and processed in the liver - protects and removes toxic substances
1/3 blood from hepatic artery proper
Outline the histology of the liver
100,000 hexagonal lobules 1mm diameter
Portal areas contain - branch of hepatic portal vein, branch of hepatic artery proper and a branch of bile duct
Blood flows past hepatocytes - absorb solutes, secrete proteins
Drains into - sinusoid - central vein
Bile ductiles - bile ducts
What are the functions of the liver?
Metabolic regulation - metabolism of CHO, lipids, and aa, Waste product removal, Vitamin and mineral storage, drug inactivation
Haematological regulation - Phagocytosis and antigen presentation, synthesis of plasma proteins, removal of circulating hormones, antibodies and toxins
Bile production - Bile salts break insoluble lipid droplets apart (emulsification), increases s.a exposed to enzymatic attack
Outline how bile is transported and stored
- The liver secretes 1L bile per day
- Bile is stored and concentrated in the gall bladder
- 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
Outline the structure of the large intestine
Ascending colon
Transverse colon
Descending colon
Sigmoid colon
Rectum
Outline the function of the large intestine
Absorption - reabsorption of water and bile salts, absorption of vitamins and organic wastes
Site of bacterial vitamin production - Vit K, biotin and Vit B5
Compaction of intestinal contents into faeces
Storage of faeces until defaecation
What is the histology of large intestine?
Lacks villi
Abundant mucous cells for lubrication
The longitudinal layer of the muscularis externa - taeniae coli - thin strips
Haustrum
How is intestinal movement controlled neurally?
Local reflexes - weak myenteric reflexes - local responses to stretch/pH change - gastrin secretion
Central control - parasympathetic stimulation - motility and secretion increases sympathetic stimulation - inhibits
The gastroenteric reflex - stimulates motility and secretion along the entire small intestine
The gastroileal reflex - Triggers the opening of the ileocaecal valve allowing materials to pass from small to large intestine
Outline the hormonal control of the intestines
Gastrin - Acid production by parietal cells and stimulation of gastric motility
GIP - Release of insulin from pancreas which facilitates the nutrient utilisation
Secretin and CCK - Release of pancreatic enzymes and buffers and bile from gall bladder
VIP - Dilation of intestinal capillaries facilitates nutrient absorption
Outline digestion
The processing and absorption of nutrients
Macronutrients
Complex carbs - polysaccharides- monosaccharides
Lipids - fatty acids
Proteins - polypeptides - amino acids
Micronutrients - vits and minerals
Water
Molecules released into bloodstream are - absorbed by cells, broken down to provide energy for ATP synthesis or used to synthesise carbs, proteins and lipids
Outline the pathway of carbs
Oral cavity - Salivary amylase
Stomach - disaccharides and trisaccharides
Small intestine - Pancreatic alpha amylase
Intestinal muscosa - lactase, maltase, sucrase - monosaccharides
Route to bloodstream - capillary
Outline the pathway of lipids
Oral cavity - lingual lipase
Small intestine - Bile and pancreatic lipase - fatty acids
Intestinal mucosa - monoglycerides, triglycerides chylomicrons exocytosis to lacteal
Outline the pathway of proteins
Stomach - pepsin - polypeptides
Small intestine - trypsin, chymotrypsin, elastase, carboxypeptidase - short peptides and a/a
Intestinal mucosa - facilitated diffusion and co-transported in and out to the capillary
