9.1 The Digestive System

Question 1

Four layers of the gut wall

Answer 1

Mucosa, submucosa, muscularis externae, serosa

Question 2

Passage from mouth to anus?

Answer 2

Mouth, oesophagus, stomach, small intestine(duodenum, jejunem, ileum), large intestine (caecum, ascending, transverse, descending, sigmoid), rectum, anus

Question 3

Function of GI tract?

Answer 3

Port of entry for food
Disrupt, store, digest food 
Kill pathogens
Move food along tract
Absorb nutrients
Remove waste

Question 4

Function of the mouth?

Answer 4

1) physical - teeth, tongue and muscle of mastication chew food.
2) chemical - salivary amylase and lipase break down food
3) forms bolus assisted by saliva which enters oesophagus

Question 5

What is the function of the saliva? (4)

Answer 5

1) digestion starts by amylase and lipase
2) it is bacteriostatic containing immunoglobulin A antibody (IgA)
3) protects teeth and gum decay and mouth using high calcium
4) assists swallowing

Question 6

What can be said about the upper and lower ends of the oesophagus?

Answer 6

Upper end - voluntary control, some striated muscle

Lower end - involuntary control, solely smooth muscle

Question 7

What is peristalsis?

Answer 7

Contraction and relaxation of muscles creating wave-like contractions transporting bolts to stomach from oesophagus in 8 seconds

Question 8

Function of the stomach?

Answer 8

Physical - churning
Chemical - forms hypertonic chyme from food, HCl acid and proteolytic enzyme
Storage of food as we eat faster than we digest

Question 9

Why doesn’t the stomach digest itself?

Answer 9

It produces mucous which contains HCO3- ions that neutralise the acid. It is resistant to the pepsin too.

Question 10

What causes mucus release?

Answer 10

Distension, food contents, gastric gland secretions

Question 11

What structure are the following cells a part of? Isthmus, parietal, chief and enteroendocrine cells?
What are their functions?

Answer 11

Gastric Gland

Isthmus - stem cells divide and populate gland
Parietal - secrete H+ ions into lumen, HCO3- ions into blood which then move into surface mucous cells
Chief cells - secrete Pepsinogens which convert into pepsin
Enteroendocrine - secrete gastrin

Question 12

Function of the duodenum? (2) What two organs are associated with it and its function?

Answer 12

1) Dilution and neutralisation of hypertonic chyme making it isotonic. It brings in water from ECF.
2) absorption of iron

Pancreas and liver

Question 13

Function of liver in association to duodenum?

Answer 13

Releases bile to emulsify fat

Assists duodenum to neutralise hypertonic chyme as bile is made of water, alkali and bile salts

Question 14

Function of pancreas in association to duodenum?

Answer 14

Secretes digestive enzymes

• cleave peptides to AA
• cleave polysaccharides to monosaccharides
• break down and reform lipids
• break down nucleic acids

Helps with neutralisation

Question 15

The small intestine is in charge of absorption of different things. What is the duodenum, jejunum and ileum responsible for absorbing?

Answer 15

Duodenum - absorbs iron
Jejunum - sugars, AA, fatty acids
Ileum - Vit B12, bile acids, remaining nutrients

Question 16

How does the small intestine increase its surface area and why would it need to?

Answer 16

Villi and microvilli maximise absorption

Question 17

What is the function of the large intestine?

Answer 17

Water recovery from the isotonic chyme so its semi solid near end
It stores faeces
Contains most of GI tracts bacteria which are involved in
-the synthesis of vitamins K, B12, thiamine and riboflavine.
-breakdown of primary bile to secondary bile
-conversion of bilirubin to non-pigmented metabolites (all readily absorbed)

Question 18

Where are most of the GI tracts bacteria found and what are their functions?

Answer 18

The large intestine

Synthesis of Vit K, B12, thiamine, riboflavine
Breakdown of primary bile to secondary
Breakdown of bilirubin (yellow) to non-pigmented molecules which are all readily absorbed

Question 19

Why is fluid balance so important in the body?

Answer 19

Gut is dealing with large quantities of water (14L without drinking) and so if this is messed with, life threatening dehydration and electrolyte imbalance.

Its also why diarrhoea and constipation are so common

Question 20

Precise control in the digestive system are achieved by which 3 systems?

Answer 20

Neural, paracrine, endocrine

Question 21

Explain neural control of the digestive system.

Answer 21

1) somatic Nervous system controls ingestion - the mouth and top 1/3 of oesophagus

2) autonomic controls the rest by postganglionic plexuses in two areas
- between muscle layers and Gut wall
- between submucosa and muscularis externae

Question 22

Explain paracrine control of the digestive system.

Answer 22

Substances act locally

Histamine - controls production of acid in stomach
Vasoactive substances - affect blood flow in gut

Question 23

Explain endocrine control of the digestive system

Answer 23

1) secretin
- HCO3- secretion from duct cells of pancreas
- bile production by the liver
- inhibition of acid by parietal cells in stomach

2) Cholecystokinin (CCK)
- enteroendocrine cells of duodenum (gastric glands)
- promotes release of digestive enzymes by pancreas
- promotes release of bile from gall bladder
- Hungersuppressant

3) Gastrin
- released by pancreas, duodenum
- promotes production of HCl by parietal cells of stomach

Question 24

If stools are pale and greasy, what could account for this?

Answer 24

No bile production

Question 25

Why dont we inhale what we swallow?

Answer 25

The Epiglottis and Upper Esophageal Sphincter (UES) open and close as required.

When we swallow a bonus, epiglottis closes airway (larynx) and soft palate blocks the nasal cavity
When we breath, UES closes and blocks esophagus