How Digestion Works: 5 Stages of Human Digestion

Updated on March 8, 2020
JR Cuevas profile image

Ray was a member of Science Olympiad, participates in science and health writing competitions, and studied at a sci-tech school.

The gastrointestinal tract, also called the digestive tract, alimentary canal, or gut, is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste.
The gastrointestinal tract, also called the digestive tract, alimentary canal, or gut, is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. | Source

Introduction to Digestive System

The human digestive system consists of the food tube, organs, and glands, which secrete juices into it to help in the digestion of food. They are listed in the table below. The process of digestion includes a mechanical and a chemical phase. Digested food is absorbed by the body with the help of the circulatory and lymphatic systems. Undigested materials are passed through the anus into the external environment.

Food Tube
Accessory Organs and Glands
Mouth Cavity
Salivary Glands
Pharynx
Liver
Esophagus
Gall Bladder
Stomach
Pancreas
Small Intestine
 
Large Intestine
 
Man's Digestive System

1. Ingestion

Ingestion is the first stage of digestion. The food tube in man is about nine meters long (9m), extending from the mouth down to the anus. Food travels through the entire length of the food tube in 24 hours. This is why defecation is usually done once a day. It is not advisable to keep the feces in the intestine longer than three days. The decomposition products can reach the bloodstream and poison the body. Listed below is the step-by-step procedure on how we excrete food in our digestive system.

• The food we swallow goes down the esophagus with the help of peristalsis. Peristalsis is the wave-like contraction of muscles that push food down the digestive tube.

• The food stays a while at the lower end of the esophagus, the cardiac sphincter, which is a circular muscular valve that relaxes to allow the food into the stomach.

• After two hours, the pyloric sphincter which guards the opening on the lower end of the stomach relaxes.

• Food enters the duodenum. This is the upper part of the small intestine.

• Final digestion occurs in the small intestine. Undigested food passes on to the large intestine, where it undergoes decomposition by the action of bacteria.

• The resulting feces are thrown out of the body through the anus by the process of defecation or bowel movement.

2. Mechanical Phase of Digestion

Mechanical digestion, the second stage, involves a change in the physical properties of food.

• Food is cut and chewed into small pieces with the use of our teeth.

• Saliva produced from three pairs of salivary glands moistens the food. The tongue mixes the food with saliva. The back of the tongue mixes the food with saliva. The back of the tongue secretes mucus, which makes the food easier to swallow.

• The food tube churns and mixes the food with digestive juices in the stomach and small intestine.

• When the body happens to take in harmful substances, peristalsis in reverse direction helps protect our body by causing us to vomit.

3. Chemical Phase of Digestion

The chemical phase of digestion involves the change in the chemical composition of food, converting the complex molecules of proteins, carbohydrates, and fats into the simpler molecules of amino acids, simple sugars, fatty acids, and glycerol. This takes place in the presence of special protein molecules called enzymes.

3a. Chemical Digestion of Carbohydrates

Enzymes that are involved in the digestion of proteins are known as proteinases. Those involved in the digestion of carbohydrates (such as starches and double sugars) are known as carbohydrates. The enzyme involved in the digestion of fats, which are also called lipids, is known as lipase. These names give you an idea of how digestive enzymes are named. The names have two parts:

a. The substance on which they act, or the substrates; and
b. The suffix -ase.

Chemical digestion in the presence of enzymes
Chemical digestion in the presence of enzymes

The figure above shows that the products of chemical digestion of food are amino acids, fatty acids, glycerol, and simple sugar. Chemical digestion of carbohydrates brought about? Chemical digestion of starch starts from the mouth. Man has three parts of salivary glands. They are the parotid glands, submaxillary glands, and sublingual glands.

Organs, Glands, and Enzymes
Carbohydrates
Digestion Products
Salivary Glands (Amylase or Ptyalin)
Starch
Maltose
Pancreas (Amylase or Ptyalin)
Starch
Maltose
Intestinal Glands (Maltase, Sucrase, Lactase)
Maltose, Sucrose, Lactose
Glucose, Fructose, Galactose
Digestion of Carbohydrates

Saliva contains a starch digesting enzyme called salivary amylase, or ptyalin. Amylase is an example of a carbohydrate. It changes starch, also called amylum, into a double sugar called maltose. Maltase in the small intestine completes the digestion of starch by changing maltose to simple sugar.

When we eat and swallow starchy food without chewing it well, there is hardly any digestion of starch in the mouth. Fortunately, the pancreas produces a digestive juice which contains another starch-digesting enzyme called pancreatic amylase, or amylopsin. It is emptied into the small intestine by way of a fine tube or duct. It converts starch into maltose.

The small intestine has numerous glands along its inner wall. These glands secrete a digestive fluid called intestinal juice, which contains several enzymes. Among them are carbohydrases which help digest double sugars. For example, the enzyme sucrase changes cane sugar, or sucrose, into simple sugars. The enzyme lactase helps digest milk sugar, or lactose, into simple sugars.

3b. Chemical Digestion of Proteins

The stomach has a great number of glands along its inner wall. These glands secrete a digestive fluid called gastric juice, which contains two important substances: pepsinogen and hydrochloric acid (HCl, about 0.2% to 0.5%). In the presence of hydrochloric acid, pepsinogen is converted into the enzyme pepsin, which is a proteinase. The chemical change can be presented as follows.

Pepsinogen -> Pepsin

Pepsin changes the long protein molecules into shorter protein molecules called polypeptides. Another proteinase called trypsin, in pancreatic juice also changes proteins into polypeptides. The other proteinases called peptidases, secreted by the pancreas and intestinal glands complete the digestion of proteins by changing the polypeptide into amino acids.

Site of Digestion
Digestive Juices and Their Properties
Substrate
Products
Stomach
Gastric Juices
pepsinogen, protein, milk protein
pepsin, polypeptides
Small Intestine
Pancreatic and Intestinal Juice
protein, polypeptides
polypeptides, amino acids
Digestion of Proteins

The other protein-digesting enzyme, trypsin, is also produced by the intestinal glands as inactive trypsinogen. It is changed to trypsin when it combines with enterokinase, which is another secretion of the intestinal glands.

It has been found that another proteinase, rennin, is present in the stomach of infants. Rennin curdles the milk in preparation for the action of other proteinases. In adults, pepsin performs the function of rennin.

3c. Chemical Digestion of Fats

The large digestive gland in the body is the liver. It secretes a yellow-green liquid known as bile which is stored in the gall bladder. The gall bladder releases the bile the moment food is present in the duodenum. It empties the bile into the duodenum. It empties the bile into the duodenum by way of a fine tube or duct. Bile has no enzyme. It changes fat into tiny droplets, something like the action of soap suds on oil. In other words, fat is changed into an emulsion. The enzyme lipase can act on fats better when they are in the form of very tiny droplets.

Pancreatic juice contains several enzymes. One of these is lipase. One of the enzymes in intestinal juice is also lipase. Thus, the body has three adaptations that ensure the digestion of fats.
a. Bile, which emulsifies fats
b. Lipase in pancreatic juice
c. Lipase in the intestinal juice

In spite of these adaptations, it is not advisable, especially for elderly people to take in too much fat. This is because of a substance called cholesterol that the body manufactures from fatty foods and which, when present in great quantities, gets deposited along the inner surface of blood vessels and thereby makes the blood vessels narrower.

4. Absorption

Absorption, the fourth stage of digestion, is the process by which substances are taken in by the cells of the food tube. The final digestion of food takes place in the small intestine. It is also here, especially at the lower portion of the small intestine, that most of the digested food is absorbed.

Digested foods in the form of molecules of amino acids, simple sugars, fatty acids, and glycerol diffuse into the capillaries and reach the blood. Molecules of fatty acids and glycerol diffuse into the lacteals and reach another circulating fluid, the lymph. The process of absorption of food includes the diffusion of digested food from the food tube to the cells lining the food tube until it reaches the circulating fluids, that is, blood and lymph. Beyond this point is another process, circulation. The circulating fluids distribute the digested food to all the cells of the body.

Below is a video that shows a portion of the inner surface of the intestinal wall. It is covered by very small projections called villi. These are structures that absorb digested food from the small intestine. Each villus is provided with two kinds of vessels: capillaries and lacteals.

5. Excretion (Elimination)

The last stage of digestion is the elimination or excretion. In the elimination phase, undigested food or food molecules that cannot be absorbed by the body need to be excreted. Elimination is sometimes called defecation. This is where indigestible wastes in the form of feces, are removed from the body. The feces, before leaving the anus, are stored in the rectum, which is the last part of the large intestine.

Did you learn from the discussion?

See results

© 2020 Ray

Comments

    0 of 8192 characters used
    Post Comment
    • profile image

      Abdul Jabbar Rahber 

      3 weeks ago

      Article is good but why is farting & how to get rid of this prolem?

    working

    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, owlcation.com uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at: https://maven.io/company/pages/privacy

    Show Details
    Necessary
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the googleapis.com or gstatic.com domains, for performance and efficiency reasons. (Privacy Policy)
    Features
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Marketing
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Statistics
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
    ClickscoThis is a data management platform studying reader behavior (Privacy Policy)