The GI System

Question 1

The GI System

Answer 1

digests/biochemically breaks down ingested food into small nutrients.

absorbs and transfers nutrients + electrolytes and water from ingested food into the systemic circulation so that it is ready to be distributed to the body cells as building blocks or sources of fuel/energy.

Question 2

what are the 5 basic digestive processes?

Answer 2

motility
secretion
digestion
absorption
storage & elimination of ingestible food.

Question 3

Motility

Answer 3

muscular contractions
mix and propel the contents of the digestive tract forward.

two types:
propulsive movements (propel)
mixing movements (mixed contents with digestive contents to promote digestion)

Question 4

Secretion

Answer 4

juices secreted in the tract in response to neural/hormonal stimulation.

digestive juices consists of water, electrolytes, and specific organic constituents (enzymes, mucus etc)

include endocrine (e.g. gastrin) and exocrine (e.g. H2O) secretions.

Question 5

Digestion

Answer 5

biochemical breakdown of complex food into smaller, absorbable units.

accomplished by enzyme-mediated hydrolysis.

carbohydrates - monosaccharides
proteins - amino acids
fats - glycerol and fatty acids.

Question 6

Absorption

Answer 6

Transfer of small digested units along with water, vitamins, and electrolytes from the GI tract into the blood/lymph.

occurs largely or most completely in the small intestine.

Question 7

what are the parts to the alimentary canal?

Answer 7

upper
middle
lower

Question 8

what does the upper part of the alimentary canal consist of and what is its role?

Answer 8

mouth, esophagus, stomach.

• serves as food intake source + holder

Question 9

what does the middle part of the alimentary canal consist of and what is its role?

Answer 9

small intestine, duodenum, jejunum and ileum.

• site of main digestive and absorptive processes.

Question 10

what does the lower part of the alimentary canal consist of and what is its role?

Answer 10

large intestine - caecum, colon, rectum.

Question 11

how many layers does the wall structure of the alimentary canal consist of?

Answer 11

4 layers
- mucosa
-submucosa
-muscularis externa
-serosa

Question 12

mucosa

Answer 12

innermost, moist, folded lining of epithelial cells, connect tissue and smooth muscle cells.

functions:
-protective barrier against pathogens
-secretes mucus, digestive enzymes and hormones.
-absorbs digestive nutrients into the blood therefore undergoes rapid cell diviision and regeneration.

Question 13

submucosa

Answer 13

dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles and nerve fibres.

contains submucosal nerve plexus of Meissner.

main functions:
- provides digestive tract with distensibility and elasticity.
-glands secrete digestive enzymes
-nerve plexus helps regulate gut motility.

Question 14

muscularis externa

Answer 14

• consists of inner layer of circular smooth muscle and outer layer of longitudinal smooth muscle.
  -myenteric plexus of Auerbach lies in between.

main functions:
- contractions facilitate mixing & movement of food contents (peristalsis and segmentation)

• intrinsic nerve plexus helps regulate gut motility.

Question 15

mastication (chewing)

Answer 15

• aids the mechanical breakdown of large foods.
• mixes food with saliva that contains salivary amylase. (partial digestion of starch).
• saliva moistens ingested food, lubricates and exerts antibacterial effects.

Question 16

mastification

Answer 16

chewing

helps with the mechanical breakdown of larger food.

mixes food with saliva that contains salivary amylase - partial digestion of starch.

saliva moistens ingested food, lubricates and exerts antibacterial effects.

Question 16

serosa

Answer 16

outer connective tissue and epithelial lining covering.
continuous with the mesentery.

main functions:
- secretes a watery, slippery serious fluid for lubrication and reduces friction between digestive organs and surrounding viscera.

Question 17

deglutition

Answer 17

swallowing - two stages

deglutition stage - the movement of the bolus through the pharynx into the esophagus.

oesophagus stage:
- as it goes through the oesophagus:
circular muscles contract which constrict the passageway, pushing the bolus down.
- longitudianl muscles contract, shortening passageway.
-gastroesophageal sphincter opens allowing bolus to go through.

Question 18

the stomach

Answer 18

• J shaped sac like chamber
• most distensible part of the GI tract.

main functions:
-stores ingested food.
-secretes gastric juices (acid, mucus, intrinsic factors, digestive enzymes- pepsin, lipase).

mixes food with gastric juices which initiates the digestion of proteins and fats + kills bacteria.

chymes (moves) food at a controlled and steady rate into the duodenum.

Question 19

gastric secretion

Answer 19

3 phases of gastric secretion

cephalic: ‘head’ phase - triggered by factors arising before food enters the stomach, e.g. smell. sight.

gastric phase: triggered by factors resulting in the presence of food in the stomach.

intestinal phase: triggered by factors resulting form the movement of food from the stomach to the duodenum.

Question 20

digestion and absorption in the stomach

Answer 20

• carbohydrate digestion by salivary amylase, partial inhibition of amylase by acidity.
  -proteins partially digested by pepsin (main digestive enzyme) in the antrum.
  -only alcohol and aspirin are absorbed.

Question 21

gastric mucosal barrier

Answer 21

protects stomach from damage

-cell membrane of gastric mucosal cells are impermeable to HCL.
-tight junctions between adjacent gastric mucosal epithelial cells are impermeable to HCL.
-HC03- rich mucus serves as a protective physical barrier and as a chemical barrier that neutralizes acid close to the gastric mucosa.
-rapid rate of cell division, entire lining replaced in 3 days.
-prostaglandins inhibit acid secretion + promote alkaline mucus secretion.

Question 22

the small intestine

Answer 22

main site for digestion and absorption of ingested food.

secretes aqueous salt and mucus solution for lubrication and protection

uses segmentation and peristaltic contractions to mix + propel + chyme and facilitate absorption.

digestion occurs via the action of bile, pancreatic enzymes + intestinal (brush border) enzymes.

absorption occurs along the entire length, but mostly in the duodenum + jejunum.

Question 23

three main subdivisions of the small intestine

Answer 23

duodenum + hepatopancreatic organs

jejunum

ileum

Question 24

duodenum

Answer 24

-first ten inches of the small intestine
-receives bile + pancreatic juice via common bile and pancreatic duct.
-the arrival of chyme stimulates the release of hepatic and pancreatic secretion

-fatty chyme in the duodenum → release of CCK → release of bile and pancreatic enzymes

• acid in chyme → release of secretin → release of pancreatic bicarbonate → neutralizes the acidity.

Question 25

functional anatomy of the pancreas

Answer 25

duct cells - secretes aqueous bicarbonate solution

acinar cells - secretes pancreatic juice: protease, amylase, lipase.

endocrine - islets of Langerhans
β-cells secrete insulin
alpha cells secrete glucagon

Question 26

the liver and hepatic portal circulation

Answer 26

liver receives blood from two sources

arterial blood delivered by the hepatic artery. provides the liver with O2 supple and carries blood-borne metabolites for hepatic processing.

venous blood - drains the digestive tract. carried by the hepatic portal vein to the liver for processing and storage of newly absorbed nutrients.

blood leaves the liver via the hepatic veins.

Question 27

the liver

Answer 27

the largest gland in the body

composed of ‘plates’ of liver cells (hepatocytes)

main digestive function is the production of bile.

Question 28

what is bile?

Answer 28

-fat emulsifier
-yellow/green alkaline solution
-contains bile salts, bile pigment, cholesterol, neutral fats, phospholipids, and electrolytes.
-stored and released from the gallbladder.

Question 29

duodenum + jejunum (small intestine)

Answer 29

-the main site of absorption
-freely absorb all products of carbohydrates, proteins, and fat digestion
-freely absorb most ingested electrolytes, vitamins + water.
-absorption of calcium + iron is adjusted to the body’s needs.

Question 30

ileum (small intestine)

Answer 30

absorbs bile salts, vitamin B12, electrolytes and water.