The Digestive System

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Many times as children, we got ourselves involved in a lot of things and ate stuffs like sand, nails, pins, stones and even intentionally swallowed insects. We didn't really care what we put in our mouth but we were able to discern if they were sweet or bitter. And of course, we thought our abdomen was just a big drum that food just drop into, and we never knew why we ate.

Nutrition is a very important element of a healthy life. We are taught about what to and not to eat by doctors and food scientist so as to boost our immune system, watch our weight, elevate our mood and prevent diseases. It's so amazing how what we eat can have so much effect on our productivity. These food can not just be used up by the body or cause an effect in the form at which we ingest them; they need to go through some chemical reactions to break them down. This is a set of metabolic reactions called Digestion, and the organs involved in digestion are the digestive organs.

Digestion is the process by which large food substances are breakdown into simple chemical substances that can be utilized by the body. This process is made possible by a group of specialized organs known as the digestive organs
These organs work in solidarity to make sure that food taken by an individual is transformed to a size and form that they can easily diffuse into the blood. The organs of digestion are collectively referred to as the Digestive System or Alimentary System.

The Alimentary Canal extends from the mouth by which food enters into the body to the anus by which undigested constituent of the food is excretedThe digestive organs are series of hollow organs attached in a long, bending tube from Mouth to the Anus. These organs are actually continuous from the mouth to the anus with exception of accessory organs that also take part in food breakdown. The organs of digestion hence can be classified into two type:

The Primary Digestive Organs
i. Mouth
ii. Pharynx
iii. Esophagus
iv. Stomach
v. Small intestine
vi. Large intestine
The Accessory Digestive Organs
i. Teeth
ii. Tongue
iii. Salivary gland
iv. Liver
v. Gall bladder
vi. Pancreas

Primary Digestive Organs

These include the mouth, pharynx, oesophagus, stomach, intestines(small and large) and Anus. These organs are continuous and are the essential organs of digestion.

Mouth (Buccal Cavity): The mouth is also known as the buccal or the oral cavity. It encloses the teeth, tongue and the salivary glands; It is formed by the cheeks, the lips and palate. The function of the mouth include speech, mixing of food with saliva to initiate digestion, appreciation of taste etc.
Pharynx: It is the portion of the alimentary canal between the mouth and the Esophagus. It comprises of the nasopharynx which is the nasal portion of the pharynx and the oropharynx, the oral or hind portion of the pharnyx.
Esophagus: This is a muscular and tubular structure through which the food passes from the mouth to the stomach. The esophagus is approximately 25cm in length. The walls of the esophagus is consituted by muscles that generate peristaltic movement that aids a gentle transport of the swallowed food into the stomach.
Stomach: This is a J-shaped organ that is located in the upper abdominal region of the body. It is the most stretchable part of the gastro-intestinal tract. It continues with esophagus in the upper view and the duodenum of the small intestine at the back view. The major functions of the stomach are to store food temporarily; and to digest the digestables substance of the food. Not all constituents of the food is digested in the stomach, the reminants are futher conveyed to the intestines for futher digestion.
Small Intestine: The small intestine is the longest part of the gastro-intestinal tract. It is the segment between the pyloric sphincter of the stomach and the ileocecal valve opening of the large intestine. The function of of the small intestine is to further digest food substances and then absorb the digested molecules.
Large Intestine: The large intestine is the part of the gastro intestinal tract that passes waste product out of the body through the Rectum and anal canal. It is the continuation of the small intestine. It is segmented into sections that aids different stages of absorption and excretion.

But what is actually responsible for digestion of food substances in these organ?

Enzymes.

Enzymes are organic catalysts that accelerates the chemical reactions. The enzymes involved in breaking down of food substances are called Digestive Enzymes. Digestive enzymes are found in different sections of the alimentary canal catalyzing reactions by action on the different molecular constituents of the food. Food constituents are majorly classified into Carbohydrates, Proteins and the Lipids. Enzymes are specific in their actions; for example, an enzyme involved in the breakdown of proteins is specific to proteins and won't take part in the digestion of carbohydrates or lipids.

Action of an enzyme: In this case, an amylolytic enzyme called sucrase acting on sucrose (table sugar)

Carbohydrate digestions are catalyzed by amylolitic enzymes. These enzymes are found in the mouth, stomach and intestines. They catalyze large carbohydrate molecules such as starch, maltose and sucrose into smaller soluble and absorb-able molecules such as glucose, fructose and galactose. These absorbed molecules serve as fuel for the body to carry out daily activities. Examples of common carbohydrate food includes rice, bread, grains, malt, pasta etc. Examples of amylolytic enzymes are amylase, sucrase, maltase, galactase etc.

Proteins are digested majorly in the stomach by proteolytic enzymes. They breakdown large protein molecules into absorb-able small chained peptide and finally amino acids. Amino acids are important substances needed in building up and repairing of body tissues. They are essential in growth and synthesis of substances such as hormones, enzymes and receptors. Examples of common proteins in our meal are casein in milk, actin and myosin in meat etc. Examples of proteolytic enzymes are pepsin, elastase, chymotrypsin etc

Lipids are organic molecules that are insoluble in water or polar solvents. They form an essential constituent of meals. Their major function is to provide amount of energy and as well store energy in the body. They are stored in the form of triacylglycerides in adipose tissues of the body. These triacylglycerides are converted to energy in fasting state or when there is no carbohydrate fuel. Lipids in meals are digested by lipolytic enzymes. Examples of these enzymes are lipase and phospholipase.

Water doesn't need to be digested since it is in a simple form that can easily be absorbed into the body.

Processes occurring during digestion.

Ingestion: Taking food into the digestive tract through the mouth.
Propulsion: Propulsion means moving or pushing an object forward. The following processes makes up Propulsion;
- Deglutition (Swallowing of food)
- Peristalsis (Contraction and relaxation of the digestive organ wall).
Mechanical Breakdown: This includes;
- Chewing of food
- Mixing food with saliva by the tongue
- Fermentation of food in the stomach
- Sectioning of food to the appropriate site of action
Digestion: This involve a series of catabolic steps in which enzymes is secreted into the lumen of GIT break down complex food molecules to their chemical building blocks.
Absorption: The process by which the digested food end product moves from the lumen of the GIT to the mucosal cell.
Defecation:This means eliminating of indigestible substances from the body through the anus.

All said and done, digestion is essential to life, and this is made possible by some organs in our body which have dedicated their existence to make it possible.

The Digestive System

Primary Digestive Organs

But what is actually responsible for digestion of food substances in these organ?

Processes occurring during digestion.

Further Reading: