The Process Of Digestion In Humans Explained

The process of digestion in humans follows an amazingly well organized  sequence. The various parts and organs found along the digestive tract play different but important roles. Any disruption in this normal process can cause abdominal pain.  See a step by step account of how this marvelous process works to keep us healthy.

Organs involved in the process of digestion.

When we put that favorite meal of ours into our mouth, it would not be useful to the body without the food getting digested and absorbed.

Digestion is the process where the complex chemical make up in the food we eat is broken down into simpler components that our body can absorb and use to maintain health and proper functions.

Any significant disruption in this process will manifest as a cause of abdominal pain and often cause many other symptoms too like nausea, vomiting, bloating, diarrhea or constipation.

Most food types we eat contain carbohydrates, proteins or lipids in varying proportions.  Indigestible components of plants called fibres may also be present in our diet.

As food is transported from our mouth through the gullet or esophagus into the stomach and then to the duodenum (first part of the small intestines), jejunum (second part of the small intestines), ileum (third part of the small intestines), into the colon or large intestines, various organs and glands are encountered along the way. Each of these glands or organs produce unique types of chemicals called enzymes and emulsifiers and lubricants to aid digestion.

To help us understand the human digestive process more clearly, let us take a look at the different organs involved in this process, including fluids, enzymes, emulsifiers and lubricants that are released and involved in digestion. We shall then put together and follow the sequence in which they act and bring about digestion.

Organs Involved In The Process Of Digestion In Humans

Picture of the human digestive tract, from the teeth to the rectum

Digestion in humans is achieved by the mechanical breakdown of large food components so that the chemical process of digestion can proceed efficiently and effectively.

There is no clear separation in real time between the physical and chemical process of digestion. These two processes are brought about by the organs involved in food digestion.

Do you know that the process of digestion begins in the mouth? Yes. Not in the stomach or bowels.

Let's see each of the organs that bring about digestion and exactly what each of these organs does.

The Teeth

  • There are 32 teeth in the adult human mouth.
  • They come in four different kinds - the incisors, canine, premolar and molars.
  • The incisors help to cut food items that does not require significant jaw power, like cutting off banana or biscuit or potatoes. The canine teeth helps to tear meat off bones, or cut through tough material that the incisors may not be able to handle. The premolar and molars help in grinding ingested food. 
  • The teeth thus, plays an important role in the mechanical break down of ingested meals into small particulate materials, increasing the surface area for chemical digestion to take place and for a smoother onward transmission of the just crushed or grounded meal into the stomach.

The Tongue.

  • The tongue is one of the most important muscles in the body.
  • Without it, food cannot be transferred from the mouth through the throat into the esophagus and stomach.
  • It does more than help in the rolling up of meals into bolus for transfer.
  • The tongue has powerful sensors that helps identify parts of a mouthful of food that requires further chewing and positioning it for further chewing.
  • It is also covered with wonderful taste buds that helps add pleasure to life, helping us relish different tastes and flavors. 
  • Inappropriate chewing of our food can lead to poor digestion and abdominal pain and bloating.

Salivary Glands

  • Salivary glands are a group of cells found in 3 different parts of the mouth cavity that produce saliva.
  • Saliva helps in cleaning our mouth, lubricate the food we eat to help with easy transfer as well as contains vital enzymes that kicks off the chemical digestive process in humans. 
  • Salivary glands are another marvel of design inside our mouth, producing about 5 to 8 cupfuls of saliva every day in each person and something most of us do not pay attention to. Eating and digestion would be very very difficult without these silent lubrication and digestive factory in our mouth.

 The Pharynx And Muscles In The Back Of The Throat (Pharyngeal Muscles)

  • This is an often ignored part of the digestive system.
  • The pharynx is the part behind the nose and mouth.
  • Food from the mouth is rolled and pushed through the pharynx into the gullet.
  • Though this part of the digestive tract acts mainly as a conduit, it also help protects the airways from being clogged with food.
  • If the muscles of the pharynx are weak (like following a strike), it leads to choking when eating or drinking. This is because the pharynx can no longer close effectively and protect the wind pipe or trachea from food going down the throat.

Esophagus (Gullet)

  • The esophagus (oesophagus) is a narrow collapsible muscular tube that connects the pharynx and the stomach.
  • It measures about the size of a ruler (30cm long).
  •  The lining of the inner wall of the esophagus is made up of acid resistant cells at the bottom of the tube.
  • It also has a sphincter at its junction with the stomach, which closes most of the time to prevent acid flowing backwards from the stomach into the mouth.
  • Heavy smoking or heavy drinking of alcohol can erode the protective cell lining the esophagus, leading to Barret esophagus and eventually cancer of the esophagus.
  • Weakness of the sphincter in the lower oesphagus can lead to acid reflux.

Stomach

  • The stomach is a big bean-shaped muscular bag that seats in the upper left side of our abdomen.
  • It does not only act as a storage sac for the food we consume, it also works as a powerful mixer of the food.
  • The inner lining of the wall of the stomach contains glands that produce different chemicals including strong acids that help sterilize our food as well as aid digestion.
  • The enzyme pepsin is produced in the stomach and it helps with protein breakdown, as well as several other chemicals like gastrin, cholecystokinins, substance P etc that helps in both digestion and fine regulation of other parts of our digestive system.
  • The stomach also acts as a surface for the absorption of certain food items and alcohol into the blood stream.
  • Certain drugs (like aspirin, ibuprofen, diclofenac, almodipine), and chemicals like alcohol can cause the irritation of the wall of the stomach leading to upper abdominal pain, bloating, feeling of nausea and vomiting.

Liver and Gallbladder

  • The liver produces bile - that greenish bitter liquid that gives a greenish colour to the content of a vomit.
  • Bile helps to break fats in our diet into smaller water soluble substance in a process known as emulsification.
  • Bile is not an enzyme.
  • It is stored in the gallbladder, a small pear shaped sac attached to the under surface of the liver.
  • When the gallbladder gets clogged (by stones or sludge) it can lead to right upper abdominal pain or biliary colic.

Pancreas

  • The pancreas is a j-shaped gland that lies within the curve of the duodenum, just behind the stomach.
  • It is one of the most important glands in the body, producing vital chemicals like insulin and glucagon that helps in the regulation of blood glucose.
  • It is very essential of the digestion of carbohydrates, proteins and lips. It produces the pancreatic juice that contains pancreatic amylase (responsible for the breakdown of starch), protease (for the break down of proteins and pancreatic lipase (for the breakdown of lipids or fats).
  • Poor functioning of the pancreas would lead to diabetes and or bulky, floating strong odorous feces.

Duodenum

  • This is the first part of the small intestine.
  • It is here that the pancreas releases the pancreatic juice that helps digestion.
  • Irritation of the wall of the duodenum leads to ulcer forming here - duodenal ulcer and can cause significant abdominal pain.

Jejunum & Ileum

  • The jejunum is the second part of the small intestines.
  • Ileum is the third part of the small intestines.
  • The small intestines produces the intestinal juice called succus entericus
  • Succus entericus contains sodium bicarbonate that helps create the right environment for enzyme function, mucus that helps in further lubrication of the food material as it passes down the gut and a host of powerful enzymes including trypsin, chymotrypsin, maltase, lactase, sucrase and enteropeptidases.
  • These enzymes help in further digestion and break down of milk and other forms of protein and smaller molecules of sugars .
  • The small intestine also provide a very large surface area for the absorption of digested food material. The indigestible component of food is then passed down to the colon or large intestines.

Colon

  • The colon is the large bowels or large intestines.
  • It is divided into four parts - the ascending colon on the right side of the abdomen; the transverse colon lies across the upper part of the abdomen; the descending colon on the left side of the abdomen, and the sigmoid colon  on the lower side of the lower abdomen.
  • Once the process of digestion is complete, the indigestible remnant of the ingested material passes on to the colon.
  • Here, the food is held for a while to allow anaerobic digestion take place by the activities of the good bacteria of gut. These good bacteria ferment plant parts that would otherwise be impossible to breakdown by human enzymes.
  • Some of the breakdown products are then quickly absorbed, and what is left mixes with mucus produced by the colon. Water is absorbed from this mix and the remnant is passed down the rectum and anus as feces.

Chemicals Involved In Digestion In Humans

The following are the main enzymes and chemicals known to be involved in the digestive process in humans. They include:

  • Salivary Amylase in the mouth. This is an enzyme produced by the salivary glands that helps break down starch  into less complex carbohydrate. They are found in the saliva.
  • Hydrochloric Acid. This is a strong acid produced by the cells lining the wall of the stomach. They help in killing off bacteria in the food we eat. They also help to provide the right working environment for the chemicals and enzymes in the gastric juice produced by the stomach.
  • Pepsinogen. This is the inactive form of an enzyme called pepsin. It is secreted by the stomach and activated to pepsin by the release of hydrochloric acid. Pepsin helps in the breakdown of protein in the food material within the stomach.
  • Cholecystokinin (also called pancreozymin) itself is not a digestive enzyme. However, it is produced within the stomach cells and it causes the release of the pancreatic enzymes in response to the presence of food in the stomach. It also causes a strong contraction of the gallbladder and release of bile for the emulsification of fats in our food.
  • Bile. This is a green bitter liquid produced in the liver in response to the hormone hepatocrinin and  stored in the gallbladder. It contains bile salts.  It acts like a detergent, emulsifying fat -  exactly what happens when you mix washing up liquid with oil in a dinner plate. Without this action of reducing fat into tiny small globules, it will be impossible for enzymes - which are water soluble, to mix with and digest fat in our food. Bile is released into the duodenum by the contraction of the gallbladder in response to stimulation by the hormone, cholecystokinin.
  • Pancreatic Juice. This is produced in the pancreas. It contains enzymes that helps in the breakdown of carbohydrates (pancreatic amylase), Proteins (proteases) and fats (lipases). It also contains sodium bicarbonate, which helps to create the right working environment for the pancreatic enzymes.
  • Intestinal juice. It is also called Succus entericus. It is a pale yellow liquid produced from within the glands in the small intestines. It is released in response to the enzyme enterokinase. It contains a number of enzymes (trypsin, chymotrypsin, maltase, lactase, sucrase and enteropeptidases) as mentioned above, as well as lubricating mucus and bicarbonate.

The Complete Process of Food Digestion In Humans

The full process of digestion of food in humans involves a step by step sequence which we will explain here. For clarity, we shall use an example of ingestion of a meal of fried chicken and roasted potato. You could chose a meal of white rice and chicken or any favorite meal of yours.

The following are the steps that are involved in the process of digestion of such a meal:

  • As the chicken and potato is seen and smelt, it causes a level of stimulation leading to the release of saliva, anxiously waiting to immerse the meal and absorb the taste into the raving taste buds. 
  • Once the chicken and potato touches the floor of the mouth, the teeth goes into action, grinding the meat and potato into smaller smooth bits. The salivary glands wastes no time in pouring out volumes of saliva into the mix.  Salivary Amylases are unleashed. They swing into action, breaking down complex starch (polysaccharides) found in the potato into less complex carbohydrates (disaccharides). 
  • The tongue, like a faithful miller churns the food and vigorously searches for any part of the bolus that needs further grinding by the tireless teeth.  salivary mucus see to it that the food is well lubricated for seamless transmission down the gullet.  Once the smoothness of the bolus is ensured, the tongue rolls the bolus through to the next part of the conduit, the pharynx.
  • Within a split second, the muscles of the pharynx ensures that the upper connecting tube (larynx) to our lungs in front is closed, and the bolus is delivered through powerful contraction of the pharyngeal muscles which creates a negative pressure, into the oesophagus or gullet. 
  • Aided by gravity and the strong contractile action of the muscle of the oesophagus, the food bolus is sent down to the stomach. 
  • The stomach, which must have been contracting and warming up for the arrival of the bolus before now, then releases a gush of fluid, about 20 to 100ml in volume which bathes the food.  The stomach also contracts in powerful rhythm, churning the food and mixing it into a smooth paste. 
  • Hydrochloric acid is released which helps to kill bugs in the food, as well as stimulate or activates pepsinogen and convert it to pepsin. Pepsin helps in the breakdown of protein in the chicken to oligo-petides and amino acids (the smaller components which make up proteins). 
  • The stomach also senses the presence of fat in the chicken, leading to the release of cholecystokinin that causes the contraction of the gallbladder for the release of bile.
  • Food normally stay in the stomach for half an hour to four hours, depending on the consistency of the food ingested to allow time for effective digestion.  Here, small amount of glucose already broken down and released from the ingested food is absorbed, as is alcohol, if a glass of wine was taken to help wash down the meal.  
  • The fully processed ingested food in the stomach is now turned into a pasty semifluid mass of partly digested food, mixed with enzymes called chyme. It is now ready to leave the stomach for onwards journey into the duodenum, the first part of the small intestine. 
  • It is in the duodenum that most of the chemical digestion of food happens. It is the shortest part of the small intestines. It is here that the bile released from the gallbladder gets secreted through an opening to mix with the semi digested food material or chyme. 
  • It is also here that the pancreas release its secretions of powerful enzymes and bicarbonate, breaking down the semi-digested molecules of carbohydrates - disaccharides, polypeptides of proteins and components of fat. 
  • As the chyme passes through the duodenum, absorption of digested material  starts in earnest. 
  • The duodenum has millions of finger-like structures that pops up from its wall called villi, increasing the overall surface area for absorption by more than 60,000 percent. What a marvel of ingenious design!
  • The process of digestion continues with the chyme passing down the remaining length of the small intestines, where over 90% of absorption occurs. Intestinal juice or succus entericus is released, and mixes with the partially digested food, bringing about complete digestion. 
  • Undigested food passes down the colon. Water is absorbed from it and what is left is passed out as feces. 

That is the step by step account of the process of digestion in humans.

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