Digestion can be defined as the mechanical and chemical breakdown of food from large, insoluble molecules into smaller, soluble molecules which can be used by the body.

To facilitate digestion, the body has a digestive system, which like any organ system, is made up of several organs, working together for the body to be able to obtain useful nutrients from food.

https://i.postimg.cc/0jBbvD67/Digestive-System-Drawing.png

Overview of digestion:

1.      As food enters the mouth, it meets saliva, which aids in softening food, as it is physically broken down through the process of chewing/ mastication.

2.      The chewed and softened food forms a bolus (a ball of softened food), which enters the oesophagus.

3.      The bolus moves down the oesophagus through peristalsis- rhythmic muscular contractions of the oesophagus wall.

4.      The bolus then enters the stomach, where it is broken down by stomach acid (hydrochloric acid), and many enzymes(mainly pepsin) to break down the different components of food into smaller components. These enzymes will be investigated in a subsequent note.

5.      The food in the stomach is now known as chyme.

6.      From the stomach, the chyme enters the first part of the small intestine, the duodenum, where it is mixed with bile (from the liver), and pancreatic juice (from the pancreas). Bile emulsifies fats, turning them into small droplets (more exposed surface area for enzymes to act), while pancreatic juice contains enzymes to further break down certain food components.

7.      The final part of the small intestine, the ileum, is responsible for the absorption of several nutrients, such as amino acids, glucose and fatty acids. The ileum contains many microvilli, which increase the surface area for absorption of these nutrients back into the bloodstream.

8.      The large intestine/ colon is mainly involved in the reabsorption of water and some minerals. After the water is absorbed, what remains becomes solid waste, and is now known as faeces.

9.      After travelling through the large intestine, whatever remains is no longer of any use to the body; all the usable nutrients have been removed and must be egested from the body.

10. This solid waste is stored in the rectum, until defecation happens, where faeces pass through the anus, and are removed from the body.

Many questions may ask for the specific functions of parts of the digestive system, or require annotations for diagrams. Candidates may thus find the following list of structures and their functions useful:

Mouth- contains saliva and salivary amylase to soften food simultaneously while mastication takes place, forming a bolus.

Oesophagus- muscular tube through which food moves down via peristalsis.

Stomach- contains hydrochloric acid and enzymes to break down food into smaller constituent molecules which can be used by the body.

Liver- produces bile, which emulsifies fats, making them smaller and increases the surface area for enzymes to act on.

Pancreas- produces pancreatic juice, which contains several enzymes to break down food further.

Duodenum- Where chyme is mixed with bile and pancreatic juice.

Ileum- involved in the absorption of nutrients such as amino acids, glucose and fatty acids, and contains microvilli to increase the surface area for this absorption.

Large intestine/ colon- mainly involved in the absorption of water.

Rectum- stores faeces until defecation

Anus- contains sphincter muscles, which dilate during defecation, for removal of faeces from the body.