MODULE 1 A JOURNEY THROUGH THE DIGESTIVE SYSTEM

M O D U L E 1A J O U R N E Y T H R O U G H

T H E D I G E S T I V E S Y S T E M

SALIVARY GLANDS

MOUTH

ESOPHAGUS

LIVER

APPENDIX

PANCREAS

STOMACH

GALL BLADDER

SMALL INTESTINE

LARGE INTESTINE

ANUS

RECTUM

The Amazing Digestive TractDigestion is a complex procedure of transforming the food we eat into nutrients, which the body utilises for cell

growth, repair, and the removal of waste.

The gastrointestinal tract starts at the mouth and ends at the anus where elimination happens. The tract is lined with muscle to help with movement of food, while digestive organs produce digestive enzymes to chemically break

food down.

Digestion really happens when we see or smell food. Smelling an appealing meal stimulates the olfactory nerve, which initiates the production of saliva and prepares our stomach in the anticipation of food arriving. This is known as the Cephalic phase. About 20% of total acid secretion occurs before food enters the stomach.

The process of chewing breaks food down mechanically and stimulates more saliva production, which contains enzymes to begin the chemical break down of food.

Food then travels down through the oesophagus via peristaltic movement and enters the stomach. In every section of the digestive tract there is a sphincter-like muscle to prevent the back flow of food upwards during

digestion.

In the StomachThe stomach is a muscular sack in which food is further broken down by acid and enzymes. The pH level of

the stomach ranges from 1.5 -3.5. At this stage, food is thoroughly broken down and is given the new name of chyme. From here, chyme enters the small intestine via the duodenum.

The Small IntestineThe small intestine is where most nutrients are absorbed in to the bloodstream. Digestive secretions from the pancreas and liver, including bile continue to break down the chyme in preparation for absorption through the

small finger-like protrusions called microvilli which cover the internal wall of the small intestine. Chyme is again pushed through this long tube-like structure via peristalsis and any remaining matter is moved through the

ileocecal valve and into the large intestine or ‘colon’.

The Colon and its MicrobiomeIn the colon, excess water is reabsorbed into the body, and the remaining waste debris is broken down by the

billions of bacteria present. These bacteria form our microbiome and are essential in synthesising various vitamins and maintaining the integrity of the colon wall by protecting it from harmful bacteria. Waste ultimately collects at

the rectum and is emptied via the anus.

Enzymes & SecretionsDigestive enzymes are found in saliva secreted by the salivary glands, in secretions from the pancreas, and in the secretions from cells lining the stomach, small intestine and large intestine. The liver produces bile which is stored

by the gallbladder until needed for digestion.

Digestive enzymes are classified based on their target substrates:• Lipases break fats and oils into fatty acids

• Proteases and peptidases split proteins into small peptides and amino acids.• Amylases split carbohydrates such as starch into simple sugars such as glucose.

• Nucleases split nucleic acids into nucleotides.

Digestive Transit Time – What is Normal?Transit time, or the time for food to travel through the entire digestive tract, varies from person to person.

Generally, it takes six to eight hours after eating for the food to pass in to the large colon. Once there it can take a further 36 hours for the food to pass through the colon itself. The whole digestive process can take anything

between 2-5 days. The length of transit also varies according to the hydration level of an individual and the type of food eaten.

The Role of the Nervous SystemThe Autonomic Nervous System (ANS) controls our involuntary physiologic processes such as heartbeat and

respiratory rate. It has three divisions; the Sympathetic Nervous System (fight or flight) Parasympathetic Nervous System (rest and digest) and Enteric System (known as the ‘second brain’). These systems all interact with one other

and also affect our digestion.

When we imagine, smell or see food the cerebral cortex is stimulated which then sends a message via the vagus nerve (the connection between the stomach and brain) to the Parasympathetic Nervous System (PNS) and a signal to the

stomach to activate the gastric glands in preparation for incoming food.

Our nervous system needs to be in its relaxed state (PNS) in order to optimise digestion and absorption. PNS stimulates salivation, facilitates peristaltic movement of the stomach and intestine, stimulates the gall bladder to

release bile for fat digestion and constricts the sphincter in the intestine to help with bowel movement and defecation. The importance of the PNS in digestion accounts for why stress is a contributing factor in so many digestive disorders.

Metabolism Metabolism is the set of chemical processes with which our body sustains life. Metabolism has two forms; the

constructive process of cell growth and energy storage known as anabolism and the breakdown of molecules to release energy, known as catabolism. In digestion, food molecules are broken down via catabolism in order to create

the energy to support anabolic processes of the growth and repair of the body’s cells.

The metabolism is influenced by the endocrine system, and particularly the hormone thyroxine which is produced by the thyroid gland, explaining why thyroid conditions have so much impact on our metabolism. Another important

hormone in the regulation of the metabolism is insulin which determines whether the body is in a catabolic or anabolic process, which is why balanced blood glucose levels are so important for stable energy levels.