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
