The Gastrointestinal system

Question 1

What is the gastrointestinal system and its purpose

Answer 1

the gastrointestinal system is a tube that runs through the body

its role is the digestion and absorption of nutrients, aided by motility and secretion
- nutrients are needed for energy production and growth and repair of tissues

Question 2

What are the components of the gastrointestinal tract

Answer 2

oral cavity
pharynx and oesophagus
stomach
small intestine
large intestine

Question 3

What is the role of the oral cavity

Answer 3

the oral cavity is responsible for food intake, chewing and salivary secretion

the three salivary glands are the parotid, sublingual and submandibular glands

saliva lubricates food facilitating swallowing, contains alpha-amylase to breakdown carbohydrates and contains lipase for fat digestion

Question 4

What is the role of pharynx and oesophagus

Answer 4

• swallowing and the transport of food to the stomach

transport is aided by peristalsis - circular and longitudinal muscles relax and contract around bolus of food causing movement

Question 5

What is the role of the stomach in the GI tract

Answer 5

• mechanical breakdown of food
• digestion of proteins to polypeptides is initiated

Question 6

What is the structure of the stomach and stomach walls

Answer 6

• food passes the cardia sphincter which prevents food re-entering the oesphagus as food is covered in bile juice and wouold harm the cells
• rugae flatten out expanding the stomach volume to 4l
• vigorous movement from the pyloric antrum to mechanically breakdown food
• extra layer of muscle (oblique muscle layer) for grinding and churning of food

Question 7

What are the gastric glands in the stomach wall

Answer 7

gastric glands are composed of
- serosa
- muscularis externa
- sub-mucosa
- mucosa
- surface epithelium
- chief cells
- parietal cells
- endocrine cells
- gastric pits

surface epithelium cells secrete alkaline fluid containing mucus

Question 8

What do chief cells do

Answer 8

• secrete pepsinogen and gastric lipase

parietal cells secrete pepsinogen
chief cells secrete HCl

pepsinogen —>(HCl) pepsin

low pH causes activation of pepsinogen to pepsin, pepsin has positive feedback loop leading to quick activation of pepsin

entero-endocrine cells secrete gastrin, which activates motility of stomach which releases gastric acid

Question 9

What do parietal cells do

Answer 9

parietal cells secrete gastric acid
- gastric acid has a pH of 1-3 and has three main functions
- kill microorganisms that may cause infection, denature proteins by causing unfolding, activate pepsinogen

Question 10

How is the stomach not damaged by gastric acid

Answer 10

• secretion of alkaline mucus forming a layer to protect the stomach
• tight junctions between cells prevents gastric juice from entering
• quick replacement of surface cell so new undamaged cells take their place

Question 11

What is the role of the small intestine

Answer 11

• the small intestine is the primary site digestion and absorption
• duodenum, jejunum, ileum

segmentation = mixing of chyme, smooth muscles contract and relax

Question 12

What are the liquids in the small intestine

Answer 12

• bile and pancreatic juice secreted into the duodenum
• chyme mixes with bile, pancreatic juice and intestinal juice
• bile salts (emulsify fat), pancreatic juice (pH 8, has amylase and inactive lipase and peptidase), intestinal juice (Brunner’s glands secrete alkaline fluid and brush border enzymes)

Question 13

How are pancreatic enzymes activated

Answer 13

in the small intestine inactive enzymes enter
- brush border peptidase activates trypsinogen to trypsin

trypsin then activates
chymotrypsinogen —> chymotrypsin
procarboxypeptidase —> carboxypeptidase

lipase is also activated by trypsin

Question 14

What are carbohydrates, fats and proteins broken down into

Answer 14

carbohydrates
starch/glycogen —> (amylase) disaccharides - maltose, sucrose, lactose

maltose —> (maltase) two glucose
sucrose —> (sucrase) glucose and fructose
lactose —> (lactase) glucose and galactose

protein digestion
- ENDOpeptidase digests internal peptide bonds forming two smaller peptides (endopeptidase from gastric juice, pancreatic juice and intestinal brush border)
- EXOpeptidase digests terminal peptide bonds to release amino acids

fat digesiton
- bile salts emulsify fat globules into fat droplets increasing surface area and allowing more access for lipase
- fat is then broken down into monoglycerides and free fatty acids by lipase
- lipase is secreted from saliva, gastric juice and pancreatic juice

Question 15

How does the small intestine have such a large surface area

Answer 15

actual length of 4m
- circular folds, villi and brush borders of microvilli create a surface area of 200m squared

Question 16

How are carbohydrates, peptides and fatty acids and monoglycerides absorbed

Answer 16

monosaccharides
- driven by sodium potassium ATPase, pumps Na+ out for K+ which forms a sodium ion concentration gradient. when Na+ moves down the gradient it is coupled with glucose/galactose moving it into the cell
- fructose enters by facilitated diffusion

peptides/amino acids
- driven by sodium potassium ATPase, Na+ exchanged for K+ forming sodium ion concentration gradient, co-transport of Na+ and amino acids moves amino acids
- peptides are moved by transcytosis through the cell

fatty acids/monoglycerides
- diffuse across plasma membrane. mixed together forming microns and then transported by lymph vessels

Question 17

What is the role of the large intestine and the components

Answer 17

• caecum, colon, rectum and anal canal

storage of chyme
mucus secretion
absorption of remaining water and electrolytes

Question 18

What is the structure of the large intestine and other component

Answer 18

• the large intestine has a smooth inner surface as there are no villi
• sodium ions are actively absorbed and water is reabsorbed by osmosis
• longitudinal layers of muscle bend forming postural (pockets)
• when material reaches the rectum causes urge to defecate
• controlled by internal anal sphincter and external anal sphincter
• movement is under voluntary control