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The Digestive system

Digestive system

The digestive system is a group of organs working together to convert food into energy and basic nutrients to feed the entire body. 

Food passes through a long tube inside the body known as the alimentary canal or the gastrointestinal tract (GI tract).

 The digestive system is made up of the oral cavity, pharynx, esophagus, stomach, small intestines, and large intestines. In addition to the digestive system, there are several important digestive organs that help your body to digest food but do not have food pass through them.

 Accessory organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Digestion is the process of turning large pieces of food into its component chemicals. Mechanical digestion is the physical breakdown of large pieces of food into smaller pieces. 

This mode of digestion begins with the chewing of food by the teeth and is continued through the muscular mixing of food by the stomach and intestines.

 Bile produced by the liver is also used to mechanically break fats into smaller globules. While food is being mechanically digested it is also being chemically digested as larger and more complex molecules are being broken down into smaller molecules that are easier to absorb.

 Chemical digestion begins in the mouth with salivary amylase in saliva splitting complex carbohydrates into simple carbohydrates. 

The enzymes and acid in the stomach continue chemical digestion, but the bulk of chemical digestion takes place in the small intestine thanks to the action of the pancreas.

 The pancreas secretes an incredibly strong digestive cocktail known as pancreatic juice, which is capable of digesting lipids, carbohydrates, proteins and nucleic acids. By the time food has left the duodenum, it has been reduced to its chemical building blocks—fatty acids, amino acids, monosaccharides, and nucleotides.

Why is digestion important?

Digestion is important for breaking down food into nutrients, which the body uses for energy, growth, and cell repair. 

Food and drink must be changed into smaller molecules of nutrients before the blood absorbs them and carries them to cells throughout the body. 

The human body breaks down nutrients from food and drink into carbohydrates, protein, fats, and vitamins.

To achieve the goal of providing energy and nutrients to the body, six major functions take place in the digestive system:

Mixing and movement
The human digestive system
The digestive system 



The first function of the digestive system is ingestion, or the intake of food.\

 The mouth is responsible for this function, as it is the orifice through which all food enters the body. The mouth and stomach are also responsible for the storage of food as it is waiting to be digested.

 This storage capacity allows the body to eat only a few times each day and to ingest more food than it can process at one time.In the course of a day, the digestive system secretes around 7 liters of fluids. 

These fluids include saliva, mucus, hydrochloric acid, enzymes, and bile. Saliva moistens dry food and contains salivary amylase, a digestive enzyme that begins the digestion of carbohydrates. Mucus serves as a protective barrier and lubricant inside of the GI tract. Hydrochloric acid helps to digest food chemically and protects the body by killing bacteria present in our food. 

Enzymes are like tiny biochemical machines that disassemble large macromolecules like proteins, carbohydrates, and lipids into their smaller components. Finally, bile is used to emulsify large masses of lipids into tiny globules for easy digestion.

Food begins its journey through the digestive system in the mouth, also known as the oral cavity.

The mouth is the first part of the gastrointestinal tract and is equipped with several structures that begin the first processes of digestion.

These include salivary glands, teeth and the tongue. The mouth consists of two regions, the vestibule and the oral cavity proper. The vestibule is the area between the teeth, lips and cheeks, and the rest is the oral cavity proper.

 Most of the oral cavity is lined with oral mucosa, a mucous membrane that produces a lubricating mucus, of which only a small amount is needed.

 Mucous membranes vary in structure in the different regions of the body but they all produce a lubricating mucus, which is either secreted by surface cells or more usually by underlying glands. The mucous membrane in the mouth continues as the thin mucosa which lines the bases of the teeth.

 The main component of mucus is a glycoprotein called mucin and the type secreted varies according to the region involved. Mucin is viscous, clear, and clinging. Underlying the mucous membrane in the mouth is a thin layer of smooth muscle tissue and the loose connection to the membrane gives it its great elasticity. It covers the cheeks, inner surfaces of the lips, and floor of the mouth.
The oral cavity
The oral cavity

Inside the mouth are many access The organs that aid in the digestion of food—the tongue, teeth, and salivary glands. Teeth chop food into small pieces, which are moistened by saliva before the tongue and other muscles push the food into the pharynx.Oral cavity, cross-section

Saliva produced by the salivary glands moistens food so it moves more easily through the esophagus into the stomach. Saliva also contains an enzyme that begins to break down the starches from food.

Saliva functions initially in the digestive system to moisten and soften food into the formation of a bolus. The bolus is further helped by the lubrication provided by the saliva in its passage from the mouth into the esophagus.

 Also of importance is the presence in saliva of the digestive enzymes amylase and lipase.

 Amylase starts to work on the starch in carbohydrates, breaking it down into the simple sugars of maltose and dextrose that can be further broken down in the small intestine. Saliva in the mouth can account for 30% of this initial starch digestion. Lipase starts to work on breaking down fats.

 Lipase is further produced in the pancreas where it is released to continue this digestion of fats. The presence of salivary lipase is of prime importance in young babies whose pancreatic lipase has yet to be developed.

As well as its role in supplying digestive enzymes, saliva has a cleansing action for the teeth and mouth. It also has an immunological role in supplying antibodies to the system, such as immunoglobulin This is seen to be key in preventing infections of the salivary glands, importantly that of parotitis.

Saliva also contains a glycoprotein called haptocorrin which is a binding protein to vitamin B12. It binds with the vitamin in order to carry it safely through the acidic content of the stomach. When it reaches the duodenum, pancreatic enzymes break down the glycoprotein and free the vitamin which then binds with intrinsic factor.

The large, hollow organs of the digestive system contain a layer of muscle that enables their walls to move. The movement of organ walls—called peristalsis—propels food and liquid through the GI tract and mixes the contents within each organ. Peristalsis looks like an ocean wave traveling through the muscle as it contracts and relaxes.


When a person swallows, food pushes into the esophagus, the muscular tube in the digestive system that carries food and liquids from the mouth to the stomach.

 Once swallowing begins, it becomes involuntary and proceeds under the control of the esophagus and brain. 

The lower esophageal sphincter, a ringlike muscle at the junction of the esophagus and stomach, controls the passage of food and liquid between the esophagus and stomach.

 As food approaches the closed sphincter, the muscle relaxes and lets food pass through to the stomach.

The act of swallowing takes place in the pharynx partly as a reflex and partly under voluntary control. The tongue and soft palate -- the soft part of the roof of the mouth push food into the pharynx, which closes off the trachea. The food then enters the esophagus.

The esophagus is a muscular tube extending from the pharynx and behind the trachea to the stomach. Food is pushed through the esophagus and into the stomach by means of a series of contractions called peristalsis.

Just before the opening to the stomach is an important ring-shaped muscle called the lower esophageal sphincter (LES). This sphincter opens to let food pass into the stomach and closes to keep it there. If your LES doesn't work properly, you may suffer from a condition called GERD, or reflux, which causes heartburn and regurgitation (the feeling of food coming back up).


The stomach is a sac-like organ with strong muscular walls. In addition to holding food, it serves as the mixer and grinder of food. The stomach is a muscular sac that is located on the left side of the abdominal cavity, just inferior to the diaphragm. In an average person, the stomach is about the size of their two fists placed next to each other. 

This major organ acts as a storage tank for food so that the body has time to digest large meals properly. The stomach also contains hydrochloric acid and digestive enzymes that continue the digestion of food that began in the mouth.

The stomach secretes acid and powerful enzymes that continue the process of breaking the food down and changing it to a consistency of liquid or paste. From there, food moves to the small intestine. Between meals, the non-liquefiable remnants are released from the stomach and ushered through the rest of the intestines to be eliminated.
The human stomach
The stomach 

The stomach stores swallowed food and liquid, mixes the food and liquid with digestive juice it produces, and slowly empties its contents, called chyme, into the small intestine.

 The muscle of the upper part of the stomach relaxes to accept large volumes of swallowed material from the esophagus. The muscle of the lower part of the stomach mixes the food and liquid with digestive juice.

Just before the opening to the stomach is an important ring-shaped muscle called the lower esophageal sphincter (LES). This sphincter opens to let food pass into the stomach and closes to keep it there. If your LES doesn't work properly, you may suffer from a condition called GERD, or reflux, which causes heartburn and regurgitation (the feeling of food coming back up).

Small intestine

The small intestine is a long, thin tube about 1 inch in diameter and about 10 feet long that is part of the lower digestive system. It is located just inferior to the stomach and takes up most of the space in the abdominal cavity. 

The entire small intestine is coiled like a hose and the inside surface is full of many ridges and folds. 

These folds are used to maximize the digestion of food and absorption of nutrients. By the time food leaves the small intestine, around 90% of all nutrients have been extracted from the food that entered it.

The muscles of the small intestine mix food with digestive juices from the pancreas, liver, and intestine and push the mixture forward to help with further digestion. 

The walls of the small intestine absorb the digested nutrients into the bloodstream. The blood delivers the nutrients to the rest of the body.

Made up of three segments -- the duodenum, jejunum, and ileum -- the small intestine also breaks down food using enzymes released by the pancreas and bile from the liver.

 The small intestine is the 'work horse' of digestion, as this is where most nutrients are absorbed.

 Peristalsis is also at work in this organ, moving food through and mixing it up with the digestive secretions from the pancreas and liver, including bile. 

The duodenum is largely responsible for the continuing breakdown process, with the jejunum and ileum being mainly responsible for absorption of nutrients into the bloodstream.

A more technical name for this part of the process is "motility," because it involves moving or emptying food particles from one part to the next.

 This process is highly dependent on the activity of a large network of nerves, hormones, and muscles. Problems with any of these components can cause a variety of conditions.

While food is in the small intestine, nutrients are absorbed through the walls and into the bloodstream. What's leftover (the waste) moves into the large intestine (large bowel or colon).
The human intestine
The intestines 

Large Intestine

The colon (large intestine) is a five- to seven -foot -long muscular tube that connects the small intestine to the rectum. It is made up of the cecum, the ascending (right) colon, the transverse (across) colon, the descending (left) colon and the sigmoid colon, which connects to the rectum. The appendix is a small tube in the digestive system attached to the ascending colon. The large intestine is a highly specialized organ that is responsible for processing waste so that defecation (excretion of waste) is easy and convenient.

Stool, or waste left over from the digestive process, passes through the colon by means of peristalsis, first in a liquid state and ultimately in solid form. As stool passes through the colon, any remaining water is absorbed. Stool is stored in the sigmoid (S-shaped) colon until a "mass movement" empties it into the rectum, usually once or twice a day.

The large intestine is a long, thick tube about 2 ½ inches in diameter and about 5 feet long. It is located just inferior to the stomach and wraps around the superior and lateral border of the small intestine. The large intestine absorbs water and contains many symbiotic bacteria that aid in the breaking down of wastes to extract some small amounts of nutrients. Feces in the large intestine exit the body through the anal canal.The waste products of the digestive process include undigested parts of food and older cells from the GI tract lining. Muscles push these waste products into the large intestine. The large intestine absorbs water and any remaining nutrients and changes the waste from liquid into stool. The rectum stores stool until it pushes stool out of the body during a bowel movement.

It normally takes about 36 hours for stool to get through the colon. The stool itself is mostly food debris and bacteria. These bacteria perform several useful functions, such as synthesizing various vitamins, processing waste products and food particles, and protecting against harmful bacteria.

 When the descending colon becomes full of stool, it empties its contents into the rectum to begin the process of elimination.

The rectum is an eight-inch chamber that connects the colon to the anus. The rectum:
Receives stool from the colon
Lets the person know there is stool to be evacuated
Holds the stool until evacuation happens
When anything (gas or stool) comes into the rectum, sensors send a message to the brain. The brain then decides if the rectal contents can be released or not. If they can, the sphincters relax and the rectum contracts, expelling its contents. If the contents cannot be expelled, the sphincters contract and the rectum accommodates so that the sensation temporarily goes away.

What happens to the digested food molecules?

The small intestine absorbs most digested food molecules, as well as water and minerals, and passes them on to other parts of the body for storage or further chemical change. Specialized cells help absorbed materials cross the intestinal lining into the bloodstream. The bloodstream carries simple sugars, amino acids, glycerol, and some vitamins and salts to the liver. The lymphatic system, a network of vessels that carry white blood cells and a fluid called lymph throughout the body, absorbs fatty acids and vitamins.

How is the digestive process controlled?

Hormone and nerve regulators control the digestive process.

Hormone Regulators

The cells in the lining of the stomach and small intestine produce and release hormones that control the functions of the digestive system. These hormones stimulate production of digestive juices and regulate appetite.

Nerve Regulators

Two types of nerves help control the action of the digestive system: extrinsic and intrinsic nerves.

Extrinsic, or outside, nerves connect the digestive organs to the brain and spinal cord. These nerves release chemicals that cause the muscle layer of the GI tract to either contract or relax, depending on whether food needs digesting. The intrinsic, or inside, nerves within the GI tract are triggered when food stretches the walls of the hollow organs. The nerves release many different substances that speed up or delay the movement of food and the production of digestive juices.

Points to Remember
The digestive system
Summary of the digestive system 

Digestion is important for breaking down food into nutrients, which the body uses for energy, growth, and cell repair.
Digestion works by moving food through the gastrointestinal (GI) tract.

Digestion begins in the mouth with chewing and ends in the small intestine.

As food passes through the digestive system it mixes with digestive juices, causing large molecules of food to break down into smaller molecules. 
The body then absorbs these smaller molecules through the walls of the small intestine into the bloodstream, which delivers them to the rest of the body.
Waste products of digestion pass through the large intestine and out of the body as a solid matter called stool.
Digestive juices contain enzymes that break food down into different nutrients.

The small intestine absorbs most digested food molecules, as well as water and minerals, and passes them on to other parts of the body for storage or further chemical change. Hormone and nerve regulators control the digestive process.

1 comment:

  1. Very informative and enlightening... Good job!


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