Introduction

Question 1

What is the main function of the GI System?

Answer 1

To convert food, which consists of a variety of large molecules, into small, soluble molecules which can be used in metabolic processes.

Question 2

What does digestion achieve?

Answer 2

Converts food to a sterile, neutral, isotonic solution predominantly consisting of:
Small sugars (carbohydrates)
Amino acids / small peptides (proteins)
Lipids in very small particles (fats)

Question 3

Define absorption.

Answer 3

Active or passive uptake of nutrients, water and electrolytes.

Question 4

What materials does the body excrete?

Answer 4

Waste material from food, gut debris. Materials excreted via liver to gut must be processed, stored and released.

Question 5

Outline digestion in brief.

Answer 5

Physical disruption
Ingestion and transport to storage
Chemical disruption - chyme
Disinfection
Controlled release of chyme
Dilution and neutralisation of chyme
Absorption of nutrients and electrolytes, water further through the GI Tract (in the colon)
Producing faeces for controlled excretion

Question 6

What processes assist physical disruption of food?

Answer 6

Mastication and salivating.

Question 7

How does saliva assist in digestion?

Answer 7

Protects the mouth - wet, bacteriostatic, alkaline, containing high [Ca2+]
Lubricates food for mastication and swallowing - it is wet and contains mucus.
Contains amylase - starts digestion of carbohydrates.

Question 8

Swallowing is required for ingestion to take place. Describe how food is transported for storage.

Answer 8

Formation of bolus (due to mouth, tongue as well as mucin) which can be transported rapidly down the oesophagus to the stomach (<10s).

Question 9

Why does the stomach relax and contract?

Answer 9

It will relax to accommodate food. Once the food is stored in the stomach it will contract rhythmically to mix and disrupt it.

Question 10

How does the stomach assist in the break down of the bolus of food?

Answer 10

Secretion of HCl and proteolytic enzymes. HCl will dissolve all biological material, disinfecting the food as well.

Question 11

What is chyme?

Answer 11

A combination of the food, acid and enzymes having been agitated.

Question 12

Where does the chyme go now?

Answer 12

It slowly enters the duodenum where dilution and neutralisation will take place (it will continue into the jejunum).

Question 13

How does dilution and neutralisation of chyme take place?

Answer 13

Water is drawn in from the ECF, diluting it. The liver secretes bile. Pancreatic secretions are added. Both organs release HCO3-. They also (in addition to the intestines) secrete enzymes and bile acids.

Question 14

How do enzymes released from the pancreas and the intestines aid digestion?

Answer 14

Cleave peptides into amino acids
Cleave polysaccharides into monosaccharides
Breakdown and re-form liids
Breakdown nucleic acids

Question 15

Where does the bulk of absorption take place?

Answer 15

Small intestine. The process is very energy-demanding.

Question 16

How is the small intestine adapted for absorption?

Answer 16

It has a very large surface area and a brush border, covered in micro-villi, which are absorptive cells with a high turnover.
Molecules are absorbed actively and passively - with Na+ absorption often being coupled with it. These molecules pass into the hepatic portal circulation.
Fluid passes very slowly through the small intestine (~7m of it)

Question 17

What is absorbed in the large intestine?

Answer 17

Lots of water and some electrolytes. Slow process (from in to out is ~10 hours). Faeces will accumulate in the descending and sigmoid colon.

Question 18

How is excretion controlled?

Answer 18

Faeces periodically pushed into the rectum. Leads to an urge to defaecate. There is controlled relaxation of sphincters and expulsion of faeces.

Question 19

Outline these processes: Secretion, Motility, Absorption.

Answer 19

Secretion: adding to contents
Motility: controlled movement of contents
Absorption: removal from contents

Question 20

What do the contents in the gut consist of? Roughly how much contents travels through the gut on an average day?

Answer 20

At most 14L
Food (1kg)
Saliva (1.5L)
Gastric secretions (2.5L)
Water & Alkali (up to 9L)

Question 21

Where does the 14L of contents that goes through the gut end up?

Answer 21

Absorption in Small Intestine (12.5L)
Absorption in Large Intestine (1.35L)
Leaving 0.15kg which is excreted as faeces.

Question 22

What will happen if the balance between secretion and absorption is disturbed?

Answer 22

There will be considerable loss of water and electrolytes, mainly from bodily fluids. Therefore motility and secretion need precise control.

Question 23

What are the three mechanisms of control for motility and secretion?

Answer 23

Neural
Paracrine
Endocrine

Question 24

How is neural control involved in digestion?

Answer 24

Somatic motor system active for ingestion and excretion.
Autonomic nervous system otherwise (mostly PNS):
Post ganglionic neurones form plexuses (~a nervous system for the gut) which coordinate both secretion and motility. Neurotransmitters are used.

Question 25

How is paracrine control involved in digestion?

Answer 25

There are chemical messengers that diffuse locally. An example is histamine (a vasodilator) in the stomach. The key is these are vasomodulating chemicals.

Question 26

How is endocrine control involved in digestion?

Answer 26

Hormones have an important role. They control secretion of HCL and HCO3- as well as enzyme secretion. The gut hormones are all peptide hormones with similar structures.