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What is the Human digestive system?
The human digestive system is that system where we process our food so that it can be easily absorbed and used by our body for our day to day life. This system consists of the gastrointestinal tract (GI tract or digestive tract) and the liver, pancreas, and gallbladder.
The GI tract is basically a series of hollow long pipe-like organs joined back to back in a twisted manner from the mouth to the anus including the mouth, esophagus, stomach, small intestine, large intestine, and anus. The liver, pancreas, and gallbladder are the solid organs of the digestive system attached to the digestive tract in various locations.
Why is digestion so important?
Now one important question generally comes to your mind that is this digestion very much important? and if so then why? Digestion of foods is necessary because to stay healthy and work properly our body needs essential nutrients that come from only after thoroughly digested and absorbed food and drink we take. Essential nutrients include various proteins, fats, carbohydrates, vitamins, minerals, and water. In several stages, our digestive system breaks these nutrients into parts that are small enough for our bodies to absorb and use for energy, growth, and cell repair.
- Carbohydrates break into simple sugars
- Proteins break into amino acids
- Fats break into fatty acids and glycerol
How does the human digestive system work (the digestive process)?
The human digestive system consists of five basic stages – Ingestion, Digestion, Absorption, Assimilation, and Defecation or Egestion. Each part or stage has its various roles that help to move foods and liquids through our GI tract, break food and liquid into smaller parts, or both. Below, let’s discuss those stages in detail.
Ingestion
Mouth
Ingestion is the consumption of substances or food materials by an organism. Here in this case of human beings or in animals, generally, it is accomplished by taking in substances (foods and drinks) through the mouth into the gastrointestinal tract such as through eating or drinking. But the single-celled organisms generally do ingestion by absorbing desired substances through their cell membrane. So in simple words, we can say ingestion is to take foods and drinks through the mouth.
Digestion in the Human Digestive System
Mouth
The digestive process starts in the mouth when we chew. Besides ingestion digestion also starts in the mouth as our mouth has various organs to break the foods. These various organs include basically two types of organs one is mechanical and the other one is chemical.
The mechanical part includes two organs which are following –
- Teeth – Teeth helps us to break the foods we take to various small parts so that those parts get the maximum surface area to attach with our chemical substances and the digestion process will occur smoothly and effectively.
- Tongue – Besides teeth tongue is also a very essential and important mechanical organ that helps to mix up and stir the foods in our mouth and makes big food particles available to teeth for more break down. Also, There are various taste buds on our tongue to sense the food taste. We sense sweet on the tip of the tongue, salt on either side of the front of the tongue, sour taste buds are available behind this, bitter taste in the way in the end towards the throat, and umami in the middle-upper section.
Now come to the chemical part in the mouth which helps digestion. This part includes saliva which comes from various salivary glands located in our mouth. Saliva is a digestive juice that moistens food so it moves more easily through our esophagus into our stomach also it has an enzyme that begins to break down starches in the foods we take.
There are three salivary glands located in our mouth –
- Submaxillary gland – Its located in the upper/top part of the mouth.
- Submandibular/ Sublingual – It is in the lower part beneath the tongue.
- Parotid – It located in the cheeks. Some notes: Due to a virus infection parotid gland expands and called mumps. Snake’s poison comes from its parotid gland.
Our saliva has mainly two enzymes – one is Salivary Amylase or Ptyalin and another one is Maltase. These enzymes help to digest various carbohydrates by breaking them into more simple sugars. Salivary amylase breaks starch-based carbohydrates by catalyzing the hydrolysis of polysaccharides into disaccharides, α-limit dextrins, maltotriose, isomaltose maltose. This salivary amylase gets inactivated in our stomach when mixes with gastric acid. Maltase converts maltose to glucose.
However, the main site or location for carbohydrate digestion in the small intestine. A healthy human being excretes approximately 0.75 to 1.5 liters of saliva in a day. Though there is much debate about this amount, it is generally accepted that during sleep the amount drops significantly. Saliva’s ph. value is 6.8 thus considered acidic in nature. It also has lysozyme (antibacterial) to fight external bacteria which presents in the foods we take. This final mixture in the mouth is called bolus which includes food particles and saliva.
Esophagus
After digestion of food in the mouth, when we start to swallow, our tongue pushes the bolus into our throat. Here comes epiglottis which is a small flap of tissue that blocks the paths of our windpipe and the lungs through folding over it and prevents choking. Thus the food passes into our esophagus without any difficulty and finally reaches the stomach. Epiglottis stands open during breathing and helps in airflow into the larynx. One important point to remember that in this esophagus no digestion occurs.
The esophagus is a muscular tube of length 25-cm that connects the pharynx to the stomach. It is a large hollow organ of our GI tract contain a layer of muscle that enables their walls to move bolus. It is a very important part while we are eating and drinking. Once we begin swallowing, the process becomes automatic as our brain takes over the process and sends signals to the muscles of the esophagus, and peristalsis begins.
In this movement process, the muscles that are behind the food (means in the direction to our mouth) contracts and squeeze the food forward, and at the same time muscles that are in front of the food (means in the direction to our stomach) relaxes to allow the food to move forward. This movement of food in the esophagus is called peristalsis and the action or process is called deglutition.
Now, when the food mixture or bolus reaches the end of the esophagus, there is a ringlike muscle (lower esophageal sphincter) that relaxes and lets the mixture pass into our stomach. In normal time this lower esophageal sphincter stays closed that also helps to keep what’s in our stomach from flowing back into the esophagus.
Stomach
After food passes through the esophagus it reaches the stomach and then the stomach starts acting on it. The human stomach is basically an expanded section of our digestive tube and located between our esophagus and small intestine.
The inner layer of the stomach is full of wrinkles known as rugae (or gastric folds). Rugae allow our stomach to stretch in order to accommodate large meals and also helps to grip and move food during digestion.
Glands in our stomach lining produce stomach acid which is HCL and enzymes that break down food. Muscles of our stomach mix the food with these digestive juices (acid and enzymes). In our stomach food remains stored for approximately 4 hours and that is why our stomach is also called a warehouse.
If we do a cross-section of a human stomach, we see various cells in which epithelial cells (the outer cells of the stomach) and oxyntic cells (the inner cells of the stomach) are most important to discuss. The stomach also has a gland called the gastric gland that produces gastric juice (in a day approx 2 liters of gastric juice produces) which includes two categories of enzymes to digest protein and fat. But these enzymes only get activated and works in an acid environment or medium.
Now oxyntic cells excrete HCl (Hydrochloric Acid) which makes ph value between 1.5 to 2.5 and protein digestion takes place. After digestion of protein, it is essential to neutralize the acid medium otherwise this acid will burn stomach walls and develop peptic ulcers.
Now the epithelial cells produce mucus which neutralizes excess HCL after its requirement for the acid environment and protects our stomach from damages. Food in the stomach after a mixture with HCl is called chime. The stomach slowly empties its contents (chyme), into our small intestine.
Absorption in the Human Digestive System
Small Intestine (Final Digestion and Maximum Absorption)
The human small intestine or small bowel is approx a 6 meters or 20 feet long pipe-like organ or pipe in the gastrointestinal tract that connects our stomach and the large intestine. Villi (singular is villus which also has microvilli) are small, finger-like structures in the small intestine that increases the surface area of the small intestinal wall to a great extent.
This extension of the area helps to quickly absorb more nutrients from our digested food. The small intestine has three parts. The first part is called the duodenum, the middle part is the jejunum and the ileum is at the last or ends part of our small intestine.
After the stomach, the next part occurs in our small intestine. Here in this small intestine the final digestion and maximum absorption of nutrients and minerals from food take place. The small intestine produces digestive juice which mixes with bile and pancreatic juice through the pancreatic duct to complete the final breakdown of proteins, carbohydrates, and fats of coming semi-digested foods from the stomach.
Some helpful bacterias in our small intestine make some of the enzymes we need to digest carbohydrates. This small intestine moves water from our bloodstream into our GI tract to help break down food and also absorbs water with other nutrients.
Along with digestion maximum absorption also occurs in the small intestine. The walls of the small intestine absorb digested nutrients and water and send it to our bloodstream. And finally, as peristalsis continues, the waste products of the digestive process move from the small intestine into the large intestine.
Liver
Our liver makes a digestive juice which is called bile and helps in fat digestion and also some vitamins. There are bile ducts which carry bile from our liver to our gallbladder for storage purpose or to the small intestine for usage depending on the situation.
Gallbladder
The main function of the human gallbladder to store bile between meals. Whenever we eat, the gallbladder squeezes bile through the bile ducts into our small intestine and this is how the bile requirement is fulfilled.
Pancreas
The human pancreas makes digestive juice that has various enzymes that can break down carbohydrates, fats, and proteins. Like the gallbladder, the pancreas also delivers the digestive juice to the small intestine through small tubes which are called ducts.
Large Intestine
The large intestine in humans includes various parts like the cecum, colon, appendix, and rectum. In the first part of the large intestine we find the cecum, then comes the colon and in the last part, the rectum is there. There is also we can see a finger-shaped pouch-like organ attached to the cecum is also known as the appendix.
After a major part of digestion completed in the small intestine, there remains some waste part that includes undigested parts of food, fluid, and older cells from the lining of your digestive system tract. There are also some beneficial bacterias in our large intestine that help to break down of remaining nutrients and produce vitamin K.
Though maximum absorption occurs in the small intestine, the human large intestine mainly absorbs water and sends it to the bloodstream. Here in this part of the digestive system, a liquid form of waste becomes stool and peristalsis helps this stool to move into our rectum.
Assimilation in Human Digestive System
In the next phase of the digestive system, the absorbed food materials are transported by blood and lymph and this lymph finally transfer it to the blood circulation. The blood transports absorbed food nutrients to different body cells and tissues in different locations where these food molecules become an integral component of the living protoplasm.
This process is used for various health functions like the supply of energy, development, and new cell production cell growth, cell repair, etc. This whole process of synthesizing the biological compounds (macromolecules) from the absorbed simple molecules is called assimilation of food.
Assimilation is a very essential and mandatory function of a powerful and healthy digestive system. It basically means the food and drinks consumed become one with the living body and ready to provide nutrients and energy required for our body cells for proper functioning. One can say, it is that transition process where cells can make use of the digested food.
