Brain | Mouth to Stomach | Stomach | Small Intestine | Large Intestine | Auxiliary Digestive Organs | Hormones Involved

So here you are... Sitting at lunch table, enjoying some grilled-chicken pizza and coke. When you're finished, you take a last drink of coke, wipe your mouth, and head straight to your desk. You start working and get busy with whatever you are doing. After few hours you start feeling hungry again. So what happened to the grilled chicken pizza you had for lunch?!!? It got absorbed into the blood stream after mechanical (chewing process) and chemical breaking down of food into smaller components that can be easily absorbed and gave you energy to do your job efficiently. Yes that's the whole digestion process. It is just like a scientific experiment that happens all the time!!

So, where do we digest our food?

The digestion of food in humans takes place in the gastrointestinal tract (also referred to as the GI Tract, the alimentary canal, the digestive tract, or the gut). GI tract is a series of hollow organs (mouth, esophagus, and stomach, large and small intestines) connected to form a long tube of about 24 feet in length which extends from the mouth to the anus.

Above the large intestine, the digestive system is sometimes called the upper gastrointestinal tract, while everything below is the lower gastrointestinal tract. The tract has muscular walls that propel food along the tube (a process called peristalsis) breaking it down and mixing it with digestive juices for optimum absorption.

Back to Top

Digestion begins in the brain!

Your digestive system starts working even before you took the first bite of your pizza. Even before you eat, when you smell a tasty food, see it, or think about it, digestion begins. Saliva or spit begins to form in your mouth. This saliva or spit whatever you want to call it consists of digestive enzymes. So as soon as you take your first bite more digestive enzymes are released and the process begins.

When you do eat, the saliva breaks down the chemicals in the food a bit, which helps make the food mushy and easy to swallow.

This is what actually happens -

The GI tract (where digestion process begins) starts in the oral cavity (mouth) where your teeth grind and chew food, breaking it into small manageable pieces. This chewing process, known as mastication, is dependent upon powerful muscles (masseter and temporalis), as well as smaller muscles that permit fine control; they move the mandible (lower jawbone) against the upper jaw and enable crushing of relatively hard food.

Mastication causes exocrine glands under the tongue and in the back of the mouth to secrete watery liquid called saliva which performs two essential functions. It moistens and compacts the chewed food so your tongue can roll it into a ball (bolus) and push it to the back of your mouth for swallowing and easy passage through the pharynx and esophagus.

In addition, saliva contains digestive enzymes (e.g. salivary amylase) which begin the breakdown of carbohydrates. Mastication and saliva secretion work in harmony: chewing increases the surface area of foods which helps to accelerate the breakdown of complex molecules in the food into simple ones by the digestive enzymes.

Only complex carbohydrates and starches are broken down into simpler molecules in the mouth. Almost no protein or fat digestion occurs in the mouth.

Your tongue helps out, pushing the food around while you chew with your teeth. When you're ready to swallow, the tongue pushes a tiny bit of mushed-up food (called bolus) towards the back of your throat and into the opening of your esophagus, the second part of the digestive tract.

Back to Top

On the way down...

After the food is passed by the tongue, it is then swallowed and passes into the pharynx, or throat. It is transported to the stomach via a long muscular tube called as esophagus. The food reaches the stomach when it passes through esophagus. It takes about 2-3 seconds for food to pass through the esophagus and enter the stomach.

Esophagus lies behind the trachea (throat) and heart, and in front of the spinal column. It passes through the muscular chest diaphragm before reaching the stomach.

When we swallow food (bolus), passages to the lungs (windpipe) and the nasal cavity are automatically closed, and the food goes into the esophagus (a muscular tube extending from the pharynx to the stomach). The function of the esophagus is to push food from the back of your throat to your stomach. It can contract or expand to allow for the passage of food.

The esophagus is about 10 inches in length. Each end of it is closed off by a ring of muscle (sphincter). At the top end, is the upper esophageal sphincter (UES), while at the bottom is the lower esophageal sphincter (LES), also known as the gastro esophageal junction, cardiac valve, or cardia. The upper esophageal sphincter or UES is usually closed but when the food enters the back of the throat the sphincter muscle relaxes and opens, allowing the food to enter the esophagus. The UES then closes to prevent regurgitation into the throat.

The esophagus is lined with a protective mucous membrane and its walls are lined with muscles that slowly squeeze the food through the esophageal passage - a process known as peristalsis.

When food reaches the end of the esophagus, the LES opens to allow food to pass through into the stomach, and then closes to stop food or acidic gastric juice from the stomach from backing up.

Malfunctions of these sphincters can lead to a variety of digestive or gastric conditions, including achalasia, heartburn or acid reflux and gastro esophageal reflux disease (GERD).

Know what foods should be eaten to prevent acid reflux/heartburn right here!

Back to Top

In the stomach...

The first stop after the esophagus is the stomach. Shaped like the letter 'J', the stomach is a large stretchy bag situated in the middle of the chest behind the liver, between the esophagus and the first section of the small intestine (the duodenum).

It is subdivided into 4 regions each with different cells and gastric functions:

1.The Cardiac region, where the contents of the esophagus empty into the stomach through the lower esophageal or cardiac sphincter
2.The Fundus, an expanded area curving up above the esophageal opening
3.The Body, the central and largest region
4.The Pylorus, the narrow end of the stomach that joins the small intestine at the pyloric sphincter. Like the cardiac sphincter, the pyloric sphincter is a ring of muscle that regulates the movement of food into and out of the stomach.

It has three important jobs:

• to store the food you've eaten
• to break down the food into a liquidy mixture
• to slowly empty that liquidy mixture into the small intestine

Stomach is the major organ acting in the digestion process after which the food then enters the small intestine. To break down the food, the stomach releases a number of powerful gastric juices containing hydrochloric acid and other digestive enzymes. In addition to breaking down food, these acidic juices (PH 1-3) also kill bacteria in the food.

Stomach operates like a food mixer, churning the food bolus to a pulp called chyme, and releasing numerous chemicals such as digestive hormones, enzymes and gastric juices which help to break down food molecules in the chyme into small particles for absorption into the bloodstream.

Absorption of food and water in the stomach is negligible. But iron and highly fat-soluble substances like alcohol are absorbed directly from the stomach. Stomach is made up of finger-like folds called villi which give the stomach a huge surface both to absorb food and secrete digestive juices. Special cells secrete a protective coating called mucus, on the stomach walls to prevent its damage from the gastric acids.

Most of the nutrients from the foods are absorbed in the small intestine. The digestion process happening in the small intestine is extremely slow. It typically takes about 4-5 hours to pass the food from the stomach into the duodenum, the first part of the small intestine.

While the intestine is full and still digesting food, the stomach acts as storage area for food. Another sphincter valve, the pyloric sphincter, keeps the food in the stomach until it has been properly churned and mixed. This allows us to eat a large number of food calories in a relatively short time and then digest it over a longer period.

Several factors affect emptying of the stomach, including the kind of food and the degree of muscle action of the emptying stomach and the small intestine. Carbohydrates, for example, spend the least amount of time in the stomach, while protein stays in the stomach longer, and fats the longest. As the food dissolves into the juices from the pancreas, liver, and intestine, the contents of the intestine are mixed and pushed forward to allow further digestion.

Once the enzymes that require an acid environment have had time to do their job, the pyloric sphincter opens and the food passes into the small intestines where most of the absorption of nutrients takes place.

Back to Top

Next Stop: Small Intestine

After being churned and mixed with digestive juices in the stomach, food chyme moves slowly into the folds of the small intestine through the pyloric sphincter or valve.

The small intestine (don't go by the name small!) is the longest section of the digestive tract (approx 17 feet) and is divided into three segments: the duodenum, jejunum and ileum, each of which performs different digestive functions.

It is inside the small intestine that we absorb most of the nutrients in our food. Although the small intestine has a relatively small diameter, the intestinal walls are covered in millions of finger-like projections called villi, which are themselves studded with millions of smaller projections called microvilli. Inside each villus is a series of lymph vessels (lacteals) and blood vessels (capillaries). This provides a surface-area of about the size of a tennis court for nutrient absorption!

Inside the duodenal tube (The first segment of the small intestine), chyme is mixed with fluids from the gallbladder (bile) and pancreas (pancreatic juice, amylase, maltase, lactase, sucrase etc.).

As a result of this, peptides (complex chains of protein molecules) are broken down into amino acids; lipids (fats) are broken down into fatty acids and glycerol; and carbohydrates are broken down into simple sugars like glucose which help in nutrient absorption. Bile also contains sodium bicarbonate to neutralize stomach acid.

Absorbed nutrients flow in the bloodstream to the liver where they are further metabolized and then either stored or sent to cells in other parts of the body. In total, food typically takes 4-5 hours to transit all three sections of the small intestine. Along the way its consistency changes from chyme to a thin watery mixture.

Roughly 4-7 feet in length, the jejunum (middle part of the small intestine) is where chemical breakdown of the food chyme is completed. Pancreatic enzymes, along with enzymes produced by the jejunum wall, finalize the food digestion process.

Roughly 7-5 feet in length, the ileum is the final section of the small intestine, linked to the large intestine by the ileocecal valve. The main function of the ileum is to absorb nutrients. Bile is also absorbed here and returns to the liver through blood vessels in the intestinal walls.

From the ileum that is the last part of the small intestine, after all the nutrients have been absorbed from ingested food the undigested or the unabsorbed part and watery waste remains are passed into the large intestine.

Back to Top

In the Large Intestine...

The large intestine (also referred to as the colon) is a thick tube of about 5 feet in length which gets progressively narrower in diameter. It consists of 4 regions: the cecum, colon, rectum, and anal canal. It is the final section of the GI tract and its main function is to remove water (plus any remaining minerals) from the food waste and compress it into a form for easy expulsion from the body.

The undigested remains of the food enter the cecum (or caecum) which is the first part of the large intestine from the ileum (via the ileocecal valve) and exits into the ascending colon (colon is the longest segment of the large intestine, subdivided into 4 sections i.e. the ascending colon, transverse colon, descending colon and sigmoid colon) of the large intestine. As the chyme passes through the large intestine, the water is removed and the chyme is combined with mucus and bacteria (gut flora), and is converted into feces.

As mentioned above (in the esophagus and small intestine), undigested food is propelled through the large intestine by waves of muscular contraction and expansion, called peristalsis. However, unlike in the small intestine where these waves occur at irregular intervals, peristalsis in the large intestine is continuous.

In addition to this, about 2-3 times a day, a more vigorous type of movement (gastrocolic reflex) occurs which propels material towards the rectum and anus. As waste matter is pushed into the rectum, it triggers a desire to defecate.

Feces formed in the colon gets collected in the rectum (3rd part of the large intestine) before being excreted via the anus.

After the rectum, the last part of the large intestine is the anal canal. It is a short passage about 1.5 inches long, terminating in two muscular rings: the internal and external sphincters.

As waste products from the rectum pass into the anal canal, nerves in the rectum cause the internal sphincter to relax and open. Then the external sphincter also relaxes, permitting fecal discharge. And the waste remains of the food are finally eliminated or excreted form the body!!

Apart from the GI tract organs there are few other organs which help in the whole process of digestion. Let us have a look at them and their specific functions in related to the digestive process.

Back to Top

Auxiliary digestive organs

Liver: Liver is probably the most important organ in the digestive process, without which the process of digestion would become almost impossible. The nutrient-rich blood comes directly to the liver for processing.

It acts as a blood reservoir and a storage organ for vitamins such as A and D and for digested carbohydrate (glycogen), which is released to sustain blood sugar levels. It manufactures enzymes, cholesterol, proteins, vitamin A (from carotene), and blood coagulation factors.

One of the prime functions of the liver is to produce bile. Bile contains salts that promote efficient digestion of fats. The liver even filters out harmful substances or wastes, turning some of the waste into more bile. The liver even helps figure out how many nutrients will go to the rest of the body, and how many will stay behind in storage.

The liver also functions in amino acid metabolism. In a process called deamination, it converts some amino acids to compounds that can be used in energy metabolism. In doing so, the liver removes the amino groups from amino acids and uses the amino groups to produce urea. Urea is removed from the body in the urine.

The liver's bile flow is also one of the main vehicles for the general detoxification of the organism. Bile eliminates heavier or fat soluble wastes from the body.

Unfortunately, modern dietary abuses, like excessive meat consumption and oily, greasy, fried foods, aggravate, obstruct or congest the bile, causing jaundice and fatty degenerative changes in the liver.

Hence it is very much essential to make changes in the diet to keep your liver in healthy condition. Know about foods which are healthy for liver, right here!!

Gall bladder: The gallbladder is a very important organ that is responsible for the storage of bile that has been produced by the liver. It is a sac like storage organ about three inches long. It holds bile, modifies it chemically, and concentrates it ten-fold.

Pancreas: The pancreas has both exocrine secretions that balance and regulate digestion, as well as endocrine secretions that balance and regulate metabolism.

This gland is about six inches long and provides the body's most important enzymes. It secretes:

• Insulin and Glucagon, which balance and regulate the metabolism of carbohydrates
• Pancreatic juice, which contains some of the body's most important digestive enzymes - lipases, which split fats; proteases, which split proteins; and amylases, which split starches.

Salivary Glands:  There are three pairs of salivary glands: the parotid, submandibular, and sublingual glands. Each gland releases salivary amylase that begins digestion by degrading starch in the mouth. The glands are composed of specialized cells that make enzymes. They are connected to the mouth by a duct. These ducts are lined with epithelium, which serves as a protective barrier from auto digestion. They also have some muscle in them that allows the ducts to contract. This enables the one-way flow of enzymes from the glands into the mouth.

Back to Top

The Hormones Involved...
Hormones control the regulation of the entire digestive process, some even regulate your appetite. The hormones produced in the mucosa cells of the stomach and small intestines work by stimulating these organs and their digestive juices.

The hormones responsible for the digestion of your pizza are -

Gastrin: It gives the stomach the signal to produce acid. It also plays an important role in the growth of the stomach, small intestine and colon lining -- which, as you now know, is needed for absorbing nutrients and excreting digestive juices. Secretion of gastrin is stimulated by food arriving in stomach. The secretion is inhibited by low pH.

Secretin: It communicates with all the major digestive accessory/auxiliary organs. In the pancreas, a call from secretin causes the excretion of those helpful digestive juices. Then secretin calls the stomach, causing it to produce pepsin, an enzyme used to digest protein. Secretin's final call is the liver, which then produces more of that much-needed bile.

It also stimulates pancreas to produce sodium bicarbonate which neutralizes the acidic chyme from the stomach.

CCK (Cholecystokinin): CCK production is stimulated by the presence of the food in the duodenum. With the help of this hormone the pancreas produces more enzymes. Once the gallbladder hears from CCK, it knows to release all the bile it has been storing for the liver. The production of this hormone is stimulated when we overeat!!

GIP (Gastric Inhibitory Peptide): Food in the duodenum stimulates certain endocrine glands to produce GIP. It has the opposite effects of gastrin. It inhibits gastric glands in the stomach and it inhibits the mixing and churning movements of the stomach muscles. This slows the rate of stomach emptying when the food is in duodenum.

As mentioned before, a few hormones also work to encourage you to start and stop eating that food. The first of these is ghrelin, which both the stomach and small intestine produce when there is no food in them. So, when ghrelin levels run high, your appetite is stimulated.

Peptide YY, a hormone made in the gastrointestinal tract, works in an opposite manner. Once you finish that pizza, it's excreted to quell your hunger.

So, Now that you know how digestive system works, it would be interesting to know more about the digestion of individual components of foods like carbohydrates, proteins, fats etc. Keep watching this space for more on digestion of carbs, proteins, fats and vitamins!!

Back to Top