Introduction to the Functions and Control of the Alimentary Tract

Question 1

What are the digestive functions of the stomach?

Answer 1

1. Accomodation and storage.
2. Mechnical and enzymatic breakdown.
3. Slow delivery of chyme to the duodenum.

Chyme meaning - It is the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food.

Question 2

Describe where the storage of digestive products takes place in the body.

Answer 2

1. STOMACH:
   
   • Food may be stored here during the first stage of digestion; it may remain there for around an hour, unmixed (where it acts as a reservoir).
   • The fundus and the body of the stomach (with thinner muscle tone) relax, allowing a large volume of food storage (≈ 1.5L but can be up to 4L in people who can binge eat).
   • The vagal reflex inhibits smooth muscle tone. The mechanoreceptors are very important in bringing about fundic relaxation [Vasoactive intestinal peptide (VIP) and nitric oxide (NO) are very important for this (accomodation)].
   • The antral region mixes and grinds food with gastric secretions (leading to digestion).
2. COLON / RECTUM:
   
   • Here, the storage of indigestive residues and faecal matter occur.

Question 3

Describe the storage of gastric secretions.

Answer 3

• The stomach stores 2-3 litres of gastric juice ever 24 hours (made up of mucous, pepsinogen, intrinsic factor, lipase, etc.) which helps in the digestion and absorption of food.
• Some examples of gastric secretions: -
  
  1. MUCOUS:
     
     • It is secreted by goblet cells and mucus neck cells.
     • It acts as a lubricant by acting as a barrier that protects the stomach and colon, especially from gastric acid (prevents trauma).
  2. LIPASE:
     
     • It converts triglycerides to fatty acids and glycerol.
  3. PEPSIN:
     
     • It is secreted by chief cells or peptic cells as pepsinogen.
     • It helps in protein digestion.
  4. HCL (hydrochloric acid):
     
     • It is secreted by parietal cells.
     • It is important for defence of the gut.
     • (Pepsinogen upon mixing with the HCL of the gastric juice is activated to pepsin).
  5. INTRINSIC FACTOR:
     
     • It is secreted by parietal cells.
     • It helps with Vitamin B12 absorption.

Question 4

Describe the pancreatic secretions into the gut.

Answer 4

• These are often called ‘local hormones’.
• They are secreted from cells in the mucosa, but, unlike hormones, the chemical acts locally on adjacent cells via the interstitial fluid.
• E.g. Somatostatin which is released by D cells inhibits the gastrin release by G cells in the stomach.

Interstitial fluid meaning - It is a thin layer of fluid which surrounds the body’s cells.

Gastrin meaning - It is a peptide hormone that stimulates of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility. It is released by G cells in the pyloric antrum of the stomach, duodenum, and the pancreas.

Question 5

List some exocrine secretions that aid in digestion.

Answer 5

1. SALIVARY GLANDS:
   
   • Mucous [lubrication for mastication (chewing) and speech].
   • Lingual lipase (Refer below).
2. GASTRIC GLANDS:
   
   • HCl.
   • Pepsin.
   • Mucous.
3. PANCREAS:
   
   • Bicarbonate ions (stimulated to be released by a peptide hormone called secretin to regulate the pH of the duodenum).
   • Enzymes (eg. amylase, lipase, carboxypeptidase).
4. LIVER:
   
   • Bile salts, bile acids which are important for the emulsification of fats (Refer below).
5. Secretions from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection.

Lingual lipase what is it? - It is an enzyme that breaks down fatty acids, specifically triacylglycerols. It is excreted as part of saliva and it doesn’t finish its job until it gets to the stomach. The amount of lingual lipase in your saliva decreases as you get older, and gastric and pancreatic lipase lower down in your digestive system take over the job of digesting fats. Lingual lipase is very important for infants because it helps them digest the fats in milk, making digestion much easier for their immature systems.

Emulsification of fats what is it? - It is the breakdown of fat globules in the duodenum into tiny droplets, which provides a larger surface area on which the enzyme pancreatic lipase can act to digest the fats into fatty acids and glycerol.

Question 6

List some endocrine secretions that aid in digestion.

Answer 6

• Secretions called ‘hormones’ are synthesised by ductless glands, and enter the blood stream, travel to their target tissue(s) where they bind to specific receptors to elicit their effects.
• Examples include: -
  
  1. GASTRIN:
     
     • Stomach (G-cells in antrum).
  2. SECRETIN:
     
     • Duodenal mucosa (when acidic chyme is secreted, we want to neutralise it so that the enzymes can work very well. This is when secretin causes the release of bicarbonate ions).
  3. CHOLECYSTOKININ / PANCREOZYMIN (CCK):
     
     • Duodenal mucosa (It is responsible for stimulating the digestion of fat and proteins. It also acts as a hunger suppressant).
  4. INSULIN:
     
     • Pancreas (β-cells).
• Exocrine, endocrine and paracrine secretions allow for active digestion and the control of digestion and gastric motility (and energy homeostasis).

Question 7

What is the point of absorption?

Answer 7

• For food to be of use to the body, the nutrients resulting from digestion must be transported across the intestinal epithelium into the blood (eg. glucose, amino acids, etc.) or lymph via lacteals (fats/lipids).
• Absorption occurs mainly in the small intestine.
• The absorption of fluid occurs in the small intestine and colon.
• The colon absorbes 90% of water, reducing the volume to 200 ml of semi-solid faecal matter.
• Disorders of fluid secretion and absorption are important (together with motility) in the pathogenesis of diarrhoea.

Motility meaning - It is the ability of an organism to move independently, using metabolic energy.

Question 8

What is motility?

Answer 8

• It is the movements of the muscular wall (which is mostly smooth muscle except extreme ends of the upper oesophagus/rectum) which allow: -
  
  1. Movement from one region to another (law of gut); mass evacuation (excretion).
  2. Mechanical degradation (eg. gastric antrum).
  3. Mixing lumen contents (eg. small intestines).
  4. Transport of nutrients, water and of urea and electrolytes.
  5. Digestion and absorption.

Question 9

What are the different methods of excretion?

Answer 9

• Drugs and some products of normal metabolism may leave the body in: -
  
  • Saliva.
  • Bile.
  • Faeces.
  • (Vomit).
• Indigestible food residues (eg. tomato skin) leave the body in the faeces.

Question 10

How does the gut contribute to defence in the body?

Answer 10

• Like the skin and airways, the gut epithelium is an interface with the ‘contaminated’ outisde world (it is exposed to the external environment - the mouth to the anus is one big hole).
• The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens.
• Like the skin, if there is a breach in the barrier, ‘toxins’ will enter the blood.
• It is also the largest lymphoepithelial organ.

Lymphoepithelial organ meaning - It is an organ containing or composed of epithelial and lymphocytic tissue.

Question 11

What different defence mechanisms does the gut have?

Answer 11

• The gut is unsterile as it is open to the external environment, and the following help to protect the gut:
  
  1. Sight, smell and taste alerts us to harmful food substances (so we get rid of the food before swallowing or even putting into our mouth).
  2. Vomit reflex.
  3. Acid in the stomach (HCl) kills most harmful bacteria.
  4. Mucous secretions (allow the epithelial surface of the gut to be protected).
  5. Natural bacterial flora prevents the colonisation of harmful bacteria.
  6. Aggregation of lymphois tissue (eg. Peyer’s patches) are able to mount a response to food-borne antigens - analyse and response to pathogenic microbes.
  7. Peyer’s patches are located in the lamina propria layer of the mucosa and extend into the submucosa of the ileum. They are involved in immune surveillance and facilitate the generation of an immune response.

Lamina propria layer meaning - It is a thin layer of loose (areolar) connective tissue, which lies beneath the epithelium, and together with the epithelium and basement membrane constitutes the mucosa (mucus membrane).

Question 12

Describe the metabolic function of the gut.

Answer 12

• The liver is a major metabolic organ in the abdominal cabity and weighs about 1.3 kg in an adult.
• It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as the production of bile and excretion of bilirubin (It is a yellow compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body’s clearance of waste products that arise from the destruction of aged red blood cells).

Question 13

The gut is autonomously innervated. List what the parasympathetic and sympathetic systems stimulate and inhibit.

Answer 13

1. PARASYMPATHETIC: - (Rest and digest)
   
   • Stimulates digestive behaviour.
   • Stimulates gall bladder.
   • Relaxes the rectum.
2. SYMPATHETIC: - (Fight of flight)
   
   • Inhibits digestive activity.
   • Stimulates glucose release by liver.

• The gut is encircled by nerves (from the oesophagus to the stomach) some of which go into the brain and the spinal chord.
• The enteric nervous system allows the gut initiate its own responses to gut stimuli without requiring the higher function of the brain. [It is called the second brain]
• If the gut needs to move materials along there are contractions that are initiated within the gut that are dependent on the distension or the material within the bolus of food.
• They are initiated by exitatory motor reflexs or mediated by Ach or SubP (Substance P).
• The relaxation effects that are inhibitory are mediated by VIP and NO.

Question 14

Describe the steps of food movement through the stomach and into the duodenum.

Answer 14

1. Ripples of contraction move the food towards the antrum (thicker muscle layer). It also allows grinding to occur.
2. The pyloric sphincter is often relaxed, but closes upon the arrival of the peristaltic wave.
3. The repulsion of chyme causes the opening of the pyloric sphincter.
4. Small, partially digested material (1-2mm) is squirted through the pyloric sphincter into the duodenum.
5. Repulsion of antral contents backwards towards the body allow for mixing/grinding by the antrum.

• The sieving effect means that viscous and solid matter are retained in the stomach.
• Gastrin promotes contraction of the antrum.

Question 15

What are the different types of control of the alimentary tract?

Answer 15

• Endocrine.
• Paracrine.
• Neural - Includes the vago-vagal reflex and the enteric or local reflex.
• Metabolic (type of food eaten)

Question 16

Describe the endocrine control of the alimentary tract.

Answer 16

• Hormones are carried in the blood from their site of production to their target site.
• All hormones produced by the gut are peptides.

Question 17

How does gastrin mediate its effects?

Answer 17

1. When the stomach is distended, neurons are stimulated, releasing GRP (Gastrin-releasing peptide), which acts on G cells to release gastrin.
2. The gastrin travels through the blood and affects two cell types: enterochromaffin-like cells (EMF cells) and parietal cells.
3. ECL cells release histamine, which promotes acid secretion from parietal cells.
4. Acetylcholine also affects ECLs and parietal cells in a similar manner.
5. The gut can also be influenced by hormones produced elsewhere in the body (eg. oestradiol, progesterone).

Question 18

Describe the paracrine control of the alimentary tract.

Answer 18

• Note that hormones (endocrine) go to the target tissue via the blood, but paracrine agents go via the interstitial fluids.
• An example of paracrine control would somatostatin. It is only released when there is a hyper-secretion of acid.
• Increased H+ stimulates D cells, which release somatostatin.
• This then acts on G cells to inhibit the release of gastrin.

Question 19

Describe the neural control of the alimentary tract.

Answer 19

1. VAGO-VAGAL REFLEX:
   
   • It is described as a type of reflex in which both the afferent (‘sensory’) and efferent (‘motor’) axons are in the vagus nerve trunk.
   • It is a reflex circuit within the GIT (gastrointestinal tract).
   • The pathway is via the brain stem (medulla).
   • It describes the reflex control of responses to gut stimuli via the NTS (Nucleus tractus solitarius) and DMVN (dorsal vagal complex in the brain).
   • The NTS is the main site of termination of vagal afferents. The DMVN is the main site of origin of vagal efferents (motor) supplying the gut.
   • Thus the vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing – reflex goes from stomach to brain and then back to stomach → active relaxation of smooth muscles of the stomach
2. ENTERIC NERVOUS SYSTEM/ LOCAL REFLEX:
   
   • Two nerve fibres are intrinsic to the gut:
     
     1. Myenteric Plexus (Auerbach’s plexus): motor function (P + S).
     2. Submucosal Plexus (Meissner’s plexus): intestinal secretions (only P).
        
        • Auerbach’s plexus has both parasympathetic and sympathetic input, whereas the Meissner’s plexus has only parasympathetic fibres and provides secretomotor innervation to the mucosa nearest the lumen of the gut.
   • They reflexly regulate GI functions entirely within the wall of the gut.
   • It is connected to the CNS via the parasympathetic and the sympathetic fibres but can function autonomously without these connections.
   • These effects are mediated entirely by the enteric nervous system (the third component of the ANS).
   • Its nerotransmitters are Ach, NO (nitric oxide), noradrenaline, 5-HT, GABA, ATP.

Question 20

Give an example of how the innervation of the gut recognises food in the system.

Answer 20

When there is food in the oesophagus, it will stimulate mechanoreceptors, which will send impulses to the brain and back via the vagal afferent (and efferent) fibres.

Question 21

What is the colo-colonic reflex?

Answer 21

• It is when the presence of food or food products/distension of the stomach increases the motility of the colon in response - again, it is due to the stretch in the stomach and the by-products of digestion in the small intestine.
• This reflex is responsible for the urge to defecate following a meal.

Question 22

How does stimulation of the autonomic nervous system affect colon motility?

Answer 22

• An increase in parasympathetic activity will increase contraction in the proximal colon.
• An increase in sympathetic activity will decrease colon movement.

Question 23

How does stimulation of the autonomic nervous system affect Auerbach’s plexus (myenteric plexus)?

Answer 23

• An increase in parasympathetic (cholinergic) activity increases gastric motility and secretion, while an increase in sympathetic (adrenergic) activity will result in decreased gastric motility and secretion.

Question 24

Describe the metabolic control of the alimentary tract.

Answer 24

• The rate of emptying is dependent upon the material’s ability to be absorbed.
  
  • Carbohydrates are emptied quickly into the duodenum.
  • Proteins are slow-emptying.
  • Fatty foods are even slower.
• Fatty acids in the duodenum lead to a decrease in gastric emptying by increasing the contractility of the pyloric sphincter. This is because the lipid content in the food needs to be emmulsified which requires bile, and bile is secreted when CCK acts on the gall bladder.

Question 25

Describe the peristalsis of the alimentary tract.

Answer 25

• It is a wave of propulsive contractions that moves contents of the gut towards the anus.
• It is slower in the large intestine when compared to the small intestine.
• Distention initiates the contraction, while vagal inhibitory and excitatory fibres control movement. (With the peristaltic waves passing over the stomach, some luminal material is forced into the duodenum.)