Digestive system

The digestive system is responsible for breaking down food into smaller, absorbable components that can be used by the body for energy, growth, and repair. It consists of both the alimentary canal (digestive tract) and associated organs that aid in the process of digestion and nutrient absorption.

Ingestion

Ingestion is the process of taking substances, such as food and drink, into the body through the mouth. The mouth and esophagus make up the alimentary canal.

Think of your mouth as a door to your digestive system. When you eat a piece of pizza or drink a glass of water, you are opening the door and allowing these substances to enter your body through the mouth.

Mouth 

The mouth is the starting point of the digestive system. In the mouth, there are two types of digestion. This includes physical digestion by teeths (chewing and mastication) and chemical digestion (which occurs via enzymes released by salivary glands). Salivary gland in the mouth releases:

(a) Salivary Amylase (Digest / Breakdown carbohydrate Starch to Maltose)
(b) Enzyme Lysozyme: Kills bacteria in the food.
(c) Water
(d) Mucus

In the mouth, food is broken down mechanically through chewing and mixing it with saliva. The food is mixed with saliva by the tongue, which performs an important role. It contains an enzyme, Amylase, that initiates the chemical breakdown of carbohydrates. Amylase breaks down Starch (Polysaccharide) to Maltose (Disaccharide).

Analogy: Imagine you are eating a piece of bread. When you take a bite, your teeth start breaking it down into smaller pieces, and as you chew, the food mixes with saliva, making it easier to swallow.

As the digestion is completed, the food is brought in the form of a ball-like appearance, called bolus, which is swallowed. 

Oesophagus 

The Oesophagus or the Food pipe is a muscular tube that transports chewed food from the mouth to the stomach through a process called peristalsis. 

Think of the esophagus as a water slide in a theme park. Just as the water slide carries you smoothly from the top to the bottom, the esophagus carries the food from the mouth to the stomach in a smooth, controlled manner.

Peristalsis

Peristalsis is defined as the wave-like coordinated, rhythmic muscular contraction and relaxation of the muscles primarily the esophagus, stomach, and intestines, that propels the contents through the digestive system. 

The process of peristalsis is involuntary (that means it’s not in conscious control). The muscle contractions involved in peristalsis are usually smooth muscle contractions, circular muscles and longitudinal muscles. These muscles are antagonistic muscles, which means if circular muscles contract, longitudinal muscles will relax and if longitudinal muscles contract, circular muscles will contract.

As shown in the diagram, the food is moving in the forward direction. This involves the circular muscles at point A to contract and longitudinal muscles to relax. At point B, longitudinal muscles contract while circular muscles are relaxed. 

Peristalsis is a series of coordinated muscular contractions that move ingested food and waste materials through the digestive system, allowing for digestion, nutrient absorption, and elimination of waste. It ensures that food and other materials move in one direction through the digestive tract, helping to facilitate the various processes of digestion and absorption.

Digestion

Digestion is the breakdown of food into smaller, absorbable molecules that can be used by the body.

Imagine your digestive system as a recycling plant for food. Just as a recycling plant breaks down large objects like cardboard boxes into smaller recyclable materials, your digestive system breaks down large food molecules into smaller nutrients like glucose, amino acids, and fatty acids.

Stomach

As the food arrives at the stomach, it enters via a muscle called the sphincter muscle.  The stomach stores the food and continues the mechanical and chemical breakdown of food using stomach acids and enzymes. It churns the food to form a semi-liquid mixture called chyme.

Picture the stomach as a mixing bowl in a kitchen. Just as you mix different ingredients in a bowl to make a batter, the stomach mixes the chewed food with digestive juices to create chyme.

Absorption

• Absorption is the movement of nutrients from the intestines into the blood, so they can be transported to different parts of the body.

Imagine the small intestine as a gateway to the bloodstream. After the nutrients are broken down during digestion, they pass through the walls of the small intestine like travelers passing through customs at an airport. Once cleared, these nutrients enter the bloodstream and travel to different organs and tissues in the body.

Assimilation

• Assimilation is the uptake and use of nutrients by cells for energy, growth, and repair.

Think of assimilation as the delivery of packages to different parts of your body. Just as a delivery person brings packages to your home, the bloodstream delivers nutrients to cells, providing them with energy and the building blocks they need for growth and repair.

Small Intestine – Duodenum and Ileum

The small intestine is the primary site of nutrient absorption. It receives chyme from the stomach and further breaks down food using enzymes produced by the pancreas and intestinal wall. Nutrients are then absorbed into the bloodstream.

Imagine the small intestine as a conveyor belt in a factory. As the chyme moves through the small intestine, nutrients are absorbed from it and delivered into the bloodstream, just like products being sorted and transported along a conveyor belt in a factory.

Egestion

• Egestion is the removal of undigested food from the body as feces.

Imagine the large intestine as a waste disposal system. After the nutrients have been absorbed, the indigestible and unused parts of the food, along with some water, form feces. The large intestine acts like a garbage collector, gathering the waste and storing it until it’s ready to be eliminated from the body through the rectum and anus.

Large Intestine – Colon, Rectum and Anus

The large intestine absorbs water and electrolytes from the remaining undigested food, forming feces. It stores feces until elimination through the rectum and anus.

Think of the large intestine as a sponge that absorbs excess water from the undigested food, leaving behind a more solid mass, similar to how a sponge soaks up water from a surface.

Associated Organs

Salivary Glands

Salivary glands secrete saliva into the mouth, containing enzymes like amylase that start breaking down carbohydrates.

Imagine the salivary glands as factories that produce a special liquid (saliva) containing enzymes like amylase. When you start eating food, these factories become active and release the liquid to help break down the carbohydrates in the food.

Pancreas

The pancreas is a gland that produces digestive enzymes and releases them into the small intestine to further break down proteins, fats, and carbohydrates.

Think of the pancreas as a chef in the kitchen. Just like a skilled chef adds different spices and flavors to a dish to enhance its taste, the pancreas releases various digestive enzymes into the small intestine to further break down the food and make it easier to absorb nutrients.

Liver and Gallbladder

The liver produces bile, a substance that helps digest fats, and stores it in the gallbladder. When needed, the gallbladder releases bile into the small intestine to emulsify fats, making them easier to digest.

Imagine the liver as a factory that produces a special detergent (bile) to break down fats, and the gall bladder as a storage tank that holds this detergent until it’s needed. When you eat a fatty meal, the gall bladder releases the detergent (bile) to help digest the fats, just like detergent helps to remove grease from dirty dishes.

In summary, the organs of the digestive system work together to ensure the proper processing of food and drinks. 

• The mouth allows us to take substances into the body – Ingestion
• Then the process of digestion breaks down the food into smaller nutrients. These nutrients are absorbed into the bloodstream – Absorption
• Delivered to cells for energy, growth, and repair – Assimilation
• Any remaining undigested food is removed from the body as feces through – Egestion. 

The digestive system is like a well-coordinated team, making sure that the body gets the essential nutrients it needs to function properly.