Digestion is the process of breaking down large, insoluble food molecules into small and soluble molecules.
The human digestive system consists of the alimentary canal and accessory organs.
- Alimentary canal is the passage that runs from the mouth to the anus. Food goes through the alimentary canal once it has been ingested.
- Food travels through this path: mouth and pharynx → esophagus → stomach → small intestine → large intestine → anus
- Accessory organs are organs that help food to get digested but are not included in the alimentary canal.
After food is ingested through the mouth, it is chewed into a ball, called the bolus.
The food bolus moves down the digestive tract by peristalsis, which is a series of muscle contractions that moves the food bolus down the digestive tract.
👇 How food bolus moves down the digestive tract by peristalsis👇
Mechanical and Chemical Digestion
- Mechanical digestion is the physical breakdown of food molecules without any enzymes. Examples of mechanical digestion are:
- chewing in the mouth
- muscular churning in the stomach
- Chemical digestion is the breakdown of food molecules using digestive enzymes.
- Amylase breaks down starch into maltose, and maltase breaks down maltose into glucose.
- Protease breaks down protein into amino acid.
- Lipase breaks down lipid into glycerol and fatty acid.
👇 Summary of digestive enzymes 👇
Other substances that help digestion:
- Hydrochloric acid: The protease that breaks down protein at the stomach works best at an acidic environment. To provide an acidic environment, hydrochloric acid is released at the stomach. Hydrochloric acid also helps to kill any microorganisms that may have entered with food.
- Bile: Bile is a substance that is produced in the liver, stored in the gallbladder and released into the duodenum of the small intestine. Bile has two functions:
- It neutralizes the hydrochloric acid from the stomach: Digestive enzymes at the small intestine have a slightly alkaline optimal pH, so bile is released to neutralized the acid.
- It emulsifies lipids: Bile helps to break down large drops of lipid into small drops. The larger surface area helps to chemically digest lipids faster.
- After food has been digested, it needs to be absorbed into the blood vessels to be transported throughout the body.
- Most of the absorption happens at the ileum of the small intestine.
- The ileum is lined with tiny, finger-like structures called villi that increase the surface area for absoption..
- By the time contents in the gut reaches the large intestine, most of the contents have already been digested and absorbed. Only water and some indigestable substances, such as cellulose, are left.
- Most of the water is absorbed at the colon of the large intestine.
- The remaining substance is called feces and is stored at the rectum. Later, it is removed through the anus.
👇 Summary of all that we have learned 👇