Digestive System Flashcards
What are the accessory organs of the digestive system?
Salivary glands, liver, gallbladder, pancreas
What is the alimentary canal composed of?
Mouth, pharynx, oesophagus, stomach, small intestine, large intestine, rectum, anus
What are the four layers of the gut?
Mucosa (innermost)
Submucosa
External muscle layers (muscularis externae)
Serosa
What does the mucosa consist of?
Epithelium, lamina propria and musculais mucosae
What is the submucosa of the gut wall?
A layer of connective tissue bearing glands, arteries, veins and nerves
What are the two layers of the external muscle of gut wall?
Outer longitudinal layer (cigar-shaped nuclei) and inner circular layer (central nuclei)
What does the muscularis externa create?
Peristaltic waves to move luminal contents along the gut
What does the serosa of the gut wall consist of?
Mesothelium and connective tissue
What are the major functions of the GI tract?
Mechanically disrupt food, temporarily store food chemically digest food, kill pathogens in food, move food along tract, absorb nutrients from resultant solution and eliminate residual waste material
What is digestion?
Conversion of what we eat by physical and chemical disruption into a solution that is relatively sterile, neutral in pH and isotonic from which we can absorb our nutrients.
What does saliva do?
It starts digestion as it contains amylase and lipase, is bacteriostatic as it contains immunoglobulin A antibody, protects teeth and assists swallowing.
How does physical digestion occur in the mouth?
By action of teeth, tongue and muscles of chewing
How does chemical digestion occur in the mouth?
By action of salivary amylase and lipase
What does the mouth form that enters the oesophagus?
A bolus
How does control differ between the upper and lower ends of the oesophagus?
Upper end - voluntary control (some striated skeletal muscle)
Lower end - involuntary control (solely smooth muscle)
How long does it take for the bolus to be transported to the stomach?
8-9 seconds
What are the layers of the oesophagus?
Mucosa (epithelium, lamina propria and muscularis mucosae) and submucosa
How is the oesophagus innervated?
Myentric plexus and submucosal plexus
How does physical digestion occur in the stomach?
Churning (3 muscle layers and mucosal rugae)
How does chemical digestion occur in the stomach?
By hydrochloric acid and enzymes eg pepsin
What is receptive relaxation of the stomach?
Wall relaxes so pressure doesn’t rise
How does the stomach protect its epithelium?
Secretes mucus
What is chyme?
Chewed up, partially digested stomach contents
The combined action of acid, enzymes and agitation of the stomach produces what?
Hypertonic chyme (hypertonic = draw in fluid)
What does gastric mucosa secrete?
Acid, digestive enzymes and gastrin
How many layers of smooth muscle in the mucularis externa of the stomach?
3 - oblique, circular and longitudinal
What are rugae?
Folds of gastric mucosa forming longitudinal ridges in empty stomach
Which type of epithelium in the stomach?
Simple columnar epithelia
What can damage mucous cells in the stomach?
Alcohol and aspirin
Which ions do the secreted mucous of the stomach contain?
Bicarbonate ions which neutralise the effect of H+ ions and thereby protect the stomach lining
Which cells do gastric glands consist of?
Mucus secreting cells, parietal cells, chief cells, enteroendocrine cells
What is the gastric pit of the gastric gland lined by?
Mucus secreting cells
What is the isthmus in a gastric gland?
Region in which stem cells divide to populate the gland by upward or downward migration
What do parietal cells in a gastric gland secrete?
H+ ions into the lumen and bicarbonate ions into nearby capillaries which move it to surface mucous cells
What do chief cells in a gastric gland secrete?
Pepsinogens which are converted to pepsins
What do enteroendocine cells in a gastric gland secrete?
Gastrin - the wider gastric muscosa responds to this hormone by secreting acid
What is the duodenum?
20-25cm long, C shaped, proximal portion of small intestine that curves around head of pancreas
What is contained in the walls of the duodenum?
Brunner’s glands which secrete bicabonate-rich mucus to neutralise acidic chyme
Which two organs connect with the duodenum?
Liver and pancreas
What does bile contain?
Water, alkali and bile salts to emulsify fat
What does the small intestine consist of?
Duodenum, Jejunum, Ileum
What does the duodenum absorb?
Iron
What does the jejunum absorb?
Most of sugars, amino acids and fatty acids
What does the ileum absorb?
Vitamin B12, bile acids and remaining nutrients
By the terminal ileum, all of the nutrients are absorbed but what is still to be absorbed?
Water
What does the large intestine consist of?
Caecum, ascending colon, transverse colon, descending colon, sigmoid colon
Approximately how long and wide is the large intestine?
1.2m long and 6-9cm wide
Which type of epithelia line the crypts of Lieberkuhn in the mucosa of the colon?
Simple columnar epithelia
What does the large intestine do?
Continues water recovery over a 20 hr transit
Where do the contents of the large intestine await expulsion?
In colon, not rectum
Where is most of the GI tract’s bacteria contained?
Colon
Are most bacteria in the colon aerobic or anaerobic?
Anaerobic
What are the bacteria of the colon involved in?
Synthesis of [vitamins K, B12, thiamine and riboflavine]
Breakdown of primary and [secondary bile acids]
Coversion of bilirubin to [non-pigmented metabolites]
*Things in square brackets are readily absorbed
What is the total volume of fluid produced by GI tract per day, without including fluid we drink?
14L
How much faeces produced in a day from 1kg of food?
0.15kg. So 1kg food to 0.15kg of faeces via 14L fluid
What does histamine do involving the stomach?
Controls production of acid in stomach
What do vasoactive substances do?
Affect blood flow in gut
What hormones is the gut controlled by?
Hormones it makes itself rather than hormones made elsewhere
What does secretin do?
Promotes bicarbonate secretion from duct cells of panceas, promotes bile production by the liver and inhibits secretion of acid by parietal cells of the stomach.
Where is cholecystokinin synthesised?
Enteroendocrine cells of the duodenum
What does cholecystokinin promote?
Release of digestive enzymes from the pancreas and release of bile from the gall bladder as it stimulates it to contract
Is cholecystokinin a hunger suppressant or stimulant?
Hunger suppressant
What is gastrin released by?
G cells of pyloric antrum of stomach, pancreas, duodenum
What does gastrin promote?
Production of gastric acid by parietal cells of the stomach
What are the overall roles of the gut?
Secretion, movement and absorption
What are the three salivary glands?
Sublingual gland, parotid gland and submandibular gland
What stops us from inhaling what we are trying to swallow?
The epiglottis, flap of elastic cartilage attached to the larynx entrance, is open during inhalation and closes while swallowing to force food down the oesophagus
Why is it more of a challenge to swallow fluids than solids?
More difficult to form a bolus as it needs more control and muscular contraction as well as more guidance into oesophagus
Why doesn’t the stomach digest itself?
Epithelial cells lining the stomach secrete mucus which provides a physical barrier as well as containing bicarbonate which neutralises the acid. Also, proton pumps are only active in the presence of food and there are inactive enzyme precursors.
What are the roles of bile?
Emulsify fats - increases surface area for enzymes to break them down
Facilitates fat absorption in the small intestine
Contains bile salts - deriveritive of cholesterol
Many waste products eg bilirubin are eliminated from the body by secretion into bile
Neutralises acid
