D2. Digestion

image credit: Quora

image credit: Quora

Lesson One: Objectives How are the secretions of digestive juices eg. gastrin regulated? Recognise exocrine gland cells, know that fibre content dictates speed of movement through the gut and be familiar with two gut maladies, cholera and the stomach ulcer – and know the case of William Beaumont and Alexis St. Martin.


  • Exocrine gland: A gland which has a duct or pipe, and secretes to the surface of the body (eg. sweat gland), or lumen of the gut (eg. gastric gland)
  • Cholera: a disease caused by Cholera vibrio bacteria, affecting the colon, and characterised by extreme diarrhoea and vomiting and associated dehydration
  • Ulcer: a sore in the stomach lining, linked to stress, chewing gum (controversially), and heliobacter pylori
  • Gastric juices: the secretions of the lining of the stomach, containing hydrochloric acid, salts, and digestive enzymes (e.g.. pepsin). 
  • Gastrin: a hormone that stimulates gastric secretion
  • Secretin, somatostatin: two hormones that inhibit gastric secretion
  • William Beaumont, Alexis St. Martin: a doctor and his patient
  • Acinus: a group of secretory cells in a globular arrangement.

The most bizarre experiment in history: William Beaufort (warning graphic images)


  •  Dr William Beaumont experimented on an open gunshot wound to one Alexis St Martin. 
  • He first observes the process of digestion
  • He dangles pieces of food into the gunshot wound hole and then withdraws then to observe the effects at different times. 
  • He drinks the digestive juice, and discovers the acidity of gastrin.
  • Dr William Beaumont becomes the father of modern gastrology

Q) What ethical questions might be raised by the case of Dr William Beaumont and Alexis St Martin?

Regulation of digestive secretions:

Regulation is achieved by altering bloodflow, and using nerves and hormones. The vagus nerve is the nerve by which the brain communicates with the gut lining.


Sometimes digestion is not required, because either food is not present, or a human body is active – or even running for it’s life. In these times the secretions of digestive juices is halted. Blood is diverted away from the gut and towards the muscles.


Sometimes digestion is a priority activity, such as when resting after a large meal. This leads to a diversion of blood flow to the gut, and away from the brain causing the classic early afternoon sleepiness.

How are these secretions regulated?

  1. The sight of food (or even the thought in cases), causes the brain to send signals to the stomach lining via the vagus nerve, stimulating gastric secretion.
  2. Chemoreceptors in the lining of the stomach detect the presence of peptides, signals travel to the brain and then back to the stomach via the vagus nerve, stimulating gastric secretion
  3. Stretch receptors in the stomach as the stomach fills send impulses to the brain, which responds with signals via the vagus nerve to endocrine cells in the lining of the stomach, causing them to secrete gastrin. Gastrin is a hormone that stimulates gastric secretion
  4. If the pH drops too low (i.e. too much gastrin), two other hormones are released to inhibit gastric secretion (secretin and somatostatin).

Q) do these things stimulate or inhibit gastric secretion?

full stomach, proteins in stomach, low pH in stomach, looking at tasty food

The gut contains exocrine and endocrine glands


image credit: Quora

image credit: Quora


The pancreas contains both exocrine and endocrine glands

Encocrine glands:

  • ductless, secrete directly into the blood

Exocrine glands:

  • ducted, secrete via a duct into the lumen of the gut or surface of the skin

Q) What do the endocrine and exocrine glands of the pancreas secrete?

Recognise exocrine glands and villi


Examples of exocrine glands include:

  • Salival glands (secrete water, mucus, salivary amylase, lysozyme)
  • Gastric glands (gastrin which is water, mucus, enzymes (eg. pepsin), hydrochloric acid
  • Pancreatic glands (pancreatic juice, which is water, bicarbonate (to neutralise acid), pancreatic amylase, lipase, trypsin)


How do you recognise them?

1) Structure

image credit: bioninja

image credit: bioninja


The structure includes a duct lined by duct cells, a basement membrane and secretory cells ( in globular arrangements called acini pl. or acinus sing.), which are filled with mitochondria, vesicles, and rough endoplasmic reticulum.


2) That they are filled with mitochondria, vesicles, and rough endoplasmic reticulum

Q) Explain why the secretory cells are filled with these mitochondria, vesicles, and rough endoplasmic reticulum?

3) Histology:

one view

image credit: pathguy

image credit: pathguy


another view (intestines). Glands are located in the submucosa layer (stomach and intestine). Mucus is produced by a ‘cup-shaped cells’ called goblet cells, in the mucosa (layer closest to the food).

image credit:human biology lab wiki

image credit:human biology lab wiki


The Villus adaptations part of the mucosa layer – already covered in 6.2 Absorption. A reminder:

  • High surface area due to finger like shape, and presence of micro-villi
  • Good blood supply to maintain concentration gradients
  • Single cell thick epithelial layer to speed up diffusion
  • Lacteal in centre to absorb fatty acids and glycerol


  • Each epithelial cell adheres tightly to its neighbour (tight junctions), this ensures most material goes into the blood vessels
  • Large amounts of mitochondria are present to drive active transport
  • The apical surface (next to the food) contains sodium-potassium protein pumps for active transport e.g. in absorption of glucose

Ouch – two digestion maladies!


  1. Stomach ulcers. These are open sores in the stomach lining. These are caused by partial digestion of the stomach lining itself, by pepsin and hydrochloric acid (self-digestion). The acid is important to kill bacteria (barrier system), but it does pose a risk to the stomach lining. So does the pepsin, which is actually secreted in an inactive form -pespsinogen. Even so, sometimes the stomach starts digesting itself…why?

What causes stomach ulcers? Chewing gum, coffee, stress? Never proven. Actually scientists have recently identified a bacterium Helicobacter pylori has been significantly associated with stomach ulcers. 

Stomach acid which splashes out into the oesophagus causes heart burn or classic indigestion.

Q) Doctors may prescribe protein pump inhibitors (which slow down the production of acid in the stomach as it pumps acid ions into the ducts), for stomach ulcers or indigestion. Why? 

2. Cholera. When a bacterium called Vibrio Cholera infects the colon, it releases a toxin that reverses the absorption of water and salts through the lining of the large intestine. This causes dramatic dehyrdration from watery diarrhoea. Cholera is a killer, and the cause of death is usually dehydration.

 Fiber speeds it along

Some food passes through your digestive system in a matter of 24 hours. Other foods take up to 48 hours or even more. What makes the difference? Apparently the major factor is fibre content. fibre is indigestible food. It allows the food to be moved by peristalsis, by providing bulk to the material.

Higher fibre – more rapid transmission through gut

Lower fibre – slower transmission though gut.


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