The human digestive system is an elongated, muscular tube (gastrointestinal or GI tract) and various accessory organs. The mouth, pharynx, esophagus, stomach, small and large intestines, rectum and anus are the parts of the GI tract. The liver, gall bladder, pancreas, teeth, and tongue are other organs that also have important functions in digestion.
Digestion is more than just breaking down food. It is a coordinated process, which involves mechanical digestion, chemical digestion, absorption and elimination. The organs have specific functions and are part of a system.
In zoology the understanding of digestive system is important because digestion is a basic physiological process which is common to all the animal kingdom. The digestive organs vary in structure from one animal to another depending on the types of food they eat and how they live, but all make food into nutrients that can be absorbed by the body.
What is the digestive system?
Digestive system is a collection of organs which is used to take in food, break down food into simpler substances, absorb the useful substances and expel the undigested food.
Digestion starts with the ingestion of food in the mouth and finishes with the elimination of the waste material from the anus. Food travels from the top to the bottom through a special group of organs, each with a specific role.
The digestive system is used for:
Ingestion of food
- The breaking down of food by means of mechanical action.
- Chemical digestion
- Movement of food through the digestive tract
- Absorption of nutrients and water.
- When the materials (such as undigested food) are not digested, they are removed.
- Protection of harmful microorganisms
The digestive system also has connections with other body systems. Nutrients that are absorbed by the intestine are absorbed into the circulatory system, as are the nervous system and endocrine system, which help to control digestive activity.
There are 5 main parts of the digestive system.The digestive system has 5 main parts.
There are 2 parts of the digestive system:
- The gastrointestinal tract
- Accessory digestive organs
Gastrointestinal Tract
A muscular tube that runs from the mouth to the anus is called the gastrointestinal tract. It is several metres long in man.
It consists of the following main components:
- AccessorMouth
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anusy Organs
Accessory organs aid in digestion but the food does not enter through them.
They include:
- Teeth
- Tongue
- Salivary glands
- Liver
- Gallbladder
- Pancreas
1. Mouth
Food starts to be digested in the mouth. Teeth chew the food into small pieces when it is being eaten. This is known as mastication.
The tongue is used to move food around the mouth and to mix it with saliva which helps to form a soft blob of food called a bolus. During swallowing the tongue is also forcing the bolus towards the pharynx.
The salivary glands make saliva. It makes food more watery and has digestive enzymes. Salivary amylase is one of the important enzymes that starts the digestion of starch.
The mouth thus mechanically and chemically digests food.
- Functions of the Mouth
- Receives food
- Chews food
- Salivates food and swallows it in small sips.
- Begins carbohydrate digestion
- Makes the food bolus
- Helps initiate swallowing
2. Teeth
Teeth are specially designed structures for breaking down the food into smaller pieces. Teeth are all different and serve various purposes.
Incisors
Incisors are in the front of the mouth. They are primarily employed in cutting and bite food.
Canines
Dog's teeth point forward to tear food.
Premolars
Premolars are used to crush and grind food.
Molars
Molars have flat surfaces and are mainly used to chew food into smaller pieces.
Chewing helps to break up food and make the surface area larger for digestive enzymes to be more effective.
3. Pharynx
The pharynx is a muscular tube behind the mouth and nose. It's both a food and air channel.
After eating or drinking, food goes from the mouth to the throat. The epiglottis is a flap-like structure that stops food going down the respiratory system.
During the swallowing process, the bolus is pushed towards the oesophagus.
4. Esophagus
The esophagus is a muscular tube that connects the pharynx and the stomach. Does not do much chemical digestion. It has the primary role of transporting food that's been swallowed
Peristalsis is the rhythmic action of muscle contractions that propel food down the esophagus.
Peristalsis is the coordinated movement of the muscles of the digestive tract, in the form of waves of contraction and relaxation. These movements move the food forward even when someone is lying down.
The lower esophageal sphincter is at the bottom of the esophagus. This muscular structure is important in controlling the movement of food into the stomach and is important for preventing the passage of stomach contents back into the esophagus.
5. Stomach
The stomach is a J-shaped, muscular organ between the esophagus and small intestine. It is where food is temporarily stored and is an important digestive compartment.
The stomach has muscles in its walls which contract and mix food with gastric juices. This mechanical action turns food into a semi-liquid paste called chyme.
Gastric juice contains hydrochloric acid, enzymes and mucus that is produced by the stomach.
Hydrochloric acid is a very acidic acid. This environment aids in the activation of some of the digestive enzymes and in the destruction of many microorganisms that are ingested along with the food.
Pepsin is one of the important enzyme which starts the digestion of proteins into smaller peptides.
The lining of the stomach has a mucus layer which protects the stomach wall from the acidity of the stomach.Major Functions of the Stomach
- Stores food temporarily
- Mixes food
- Begins significant protein digestion
- Produces hydrochloric acid
- Helps destroy microorganisms
- Converts food into chyme
- Controls the release of chyme into the small intestine
Food does not remain permanently in the stomach. After sufficient mixing and digestion, small amounts of chyme are gradually released into the small intestine through the pyloric sphincter.
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6. Small Intestine
The digestion and absorption of food takes place mainly in the small intestine. It is called “small,” but is actually much longer than the large intestine.
There are three main parts to the small intestine:
- Duodenum
- Jejunum
- Ileum
- Duodenum
The first part of the small intestine is called the duodenum. It receives chyme from the stomach, and digestive secretions from the liver and pancreas.
The enzymes in the pancreas break down carbohydrate, protein and fat. The liver produces bile, which is stored in the gall bladder, which emulsifies fats into smaller fats.
Thus, the chemical digestion in the duodenum becomes very active and therefore an important site.
Jejunum
The small intestine is divided into three sections: the jejunum, the middle section. It is very specialized, for nutrient absorption.
The digested carbohydrates, amino acids, fatty acids, vitamins, minerals and other beneficial materials enter into the intestinal wall.
Ileum
The ileum is the last part of the small intestine. It remains in charge of absorbing nutrients and links up with the large intestine.
The inner lining of the small intestine is lined with finger-like projections, called villi. Microvilli are very small projections of the cells covering these villi.
Villi and microvilli combine to form an extremely large surface area available for absorption.
Blood capillaries and a lacteal (a lymphatic vessel) are present in each villus.
Capillaries are lined with water-soluble nutrients like many sugars and amino acids that are carried into the blood. Fat undergoes digestion and many of the products are taken up into the lymphatic system and thence into the blood.
The small intestine is an organ with a special structure that is extremely efficient for absorbing nutrients
8. Liver
The liver is the largest internal organ and is one of the most important accessory organs of digestion.
It plays an important role in the digestion by producing bile. The gall bladder stores and concentrates bile which is then released into the small intestine.
Digestive enzymes are not present in bile. Instead, the bile salts emulsify fat. The emulsification results in breaking the large fat droplets into smaller fat droplets, which provides more surface area for lipase enzymes.
The liver also performs numerous functions other than digestion. It breaks down the absorbed nutrients, stores glycogen, synthesizes essential blood proteins and neutralizes toxins
Digestive Functions of the Liver
- Produces bile
- Helps to process fat.
- Processes absorbed nutrients
- Reserves glycogen and some vitamins and minerals
- Participates in metabolism
9. Gallbladder
The gall bladder is a pear-shaped, small organ situated below the liver.
It's primary role is to store and concentrate the bile made by the liver.
Once the meal of fat gets to the small intestine, hormones make the gall bladder secrete bile into the duodenum.
The bile salt then helps to break down and absorb fats.
Gall bladder has its usefulness, but it is not vital to life. Even with gallbladder removal, the liver can still make bile.
10. Pancreas
The pancreas is an important accessory digestive organ located behind the stomach.
It secretes pancreatic juice which contains a number of digestive enzymes.
These include:
- Amylase from the pancreas is responsible for breaking down carbohydrates.
- Enzymes that break down proteins are called proteases.
- Fat digestion enzyme.
- Nucleases that can digest nucleic acids
Bicarbonate ions are also secreted by the pancreas. These are used to neutralize the acid chyme coming into the small intestine from the stomach.
The pancreas has both digestive and hormonal functions. It contains endocrine cells that make hormones like insulin and glucagon, which are used to maintain the level of glucose in the blood.
11. Large Intestine
The last major part of the digestive system is the large intestine. It wraps around most of the small intestine and there are multiple sections of it: the cecum, colon, and the rectum.
The colon is divided into:
- Ascending colon
- Transverse colon
- Descending colon
- Sigmoid colon
The main function of the large intestine is to absorb water and electrolytes out of the rest of the intestinal contents.
It also hosts a large community of microorganisms, called gut microbiota. These microorganisms break down undigested food particles and help in the synthesis of certain compounds including short chain fatty acids and vitamins.
As water is lost, contents of the intestine gradually toughen to make faeces.
12. Rectum and Anus
The last part of the colon is the rectum. Stores faeces temporarily until they are eliminated.
The anus is the opening that the solid waste material passes out of the body.
There are muscles called anal sphincters that regulate defecation. It is a co-ordinated function of the digestive tract, the nervous system, and the pelvic muscles.
The way food is broken down.
Digestion may be broken down into several steps.
1. Ingestion
Ingestion is the ingestion of food via the mouth.
2. Propulsion
Propulsion propels food along the digestive tract. This is mediated by swallowing and peristalsis.
3. Mechanical Digestion
Mechanical digestion is physical digestion, which is the process in which food is broken down into smaller pieces without changing its chemical makeup.
Examples include:
- Chewing
- Stomach contractions
- Intestinal mixing
4. Chemical Digestion
Chemical digestion is the process in which digestive enzymes or other substances break down large molecules of food into smaller molecules.
For example:
Carbohydrates → simple sugars
Amino acids and small peptides are the products of the hydrolysis of proteins.
Proteins → amino acids and polypeptides
5. Absorption
Absorption: Passage of digested food from the digestive tract into the blood or lymph
Most nutrient absorption occurs in the small intestine.
6. Elimination
Any materials left undigested and wastes are excreted as faeces.
Digestion of Carbohydrates
The initial step in digestion of carbohydrates occurs in the mouth.
Salivary Amylase begins to convert starch into smaller carbohydrate molecules. Carbohydrate digestion is continued in the small intestine by pancreatic amylase.
Carbohydrates are broken down into simple sugars, glucose, galactose or fructose, at the intestinal surface by enzymes.
The sugars are then absorbed into the blood stream.
Glucose is especially significant since many cells rely on it as a primary source of energy.
Digestion of Proteins
The digestion of protein starts primarily in the stomach.
Hydrochloric acid helps to make the environment favourable to the enzyme pepsin. The enzyme pepsin is used to digest proteins into smaller peptides.
Protein digestion continues in the small intestine, as a result of pancreatic proteases and small intestinal enzymes.
The end-products are amino acids and small peptides which are absorbed through the intestinal wall.
Amino acids are the building blocks and repair for tissues and are used in the production of many important molecules in the body.
Digestion of Fats
The small intestine is the main site for the digestion of fat
Bile salts break up fats and make them more surface area. Pancreatic lipase then releases the triglycerides to smaller molecules which are absorbed.
Fats being digested combine with bile salts to form micelles which are used to carry lipids to the surface of the intestine.
Lots of absorbed lipids are then loaded into particles called chylomicrons that carry them via the lymphatic system into the bloodstream.
The enzymes that break down Nucleic Acids.
Food also contains other compounds called nucleic acids, including DNA and RNA.
Nucleic acids are degraded by enzymes in the pancreas and in the intestines to smaller molecules such as nucleotides and derivatives.
These chemicals can then be absorbed and utilized or metabolized by the body.
Enzymes play an important role in digestion.
The digestive enzymes are biological catalysts which help to break down the food molecules faster.
Some important digestive enzymes include:
Enzyme Main Function
Salivary amylase Affects starch digestion (starts it)
Pepsin Starts the digestion of protein
Pancreatic amylase Digests carbohydrates
Trypsin Digests proteins
Chymotrypsin Digests proteins
The enzyme pancreatic lipase breaks down fats.
eases Digests nucleic acids
The enzymes that break peptides down into amino acids.
Each enzyme works under particular conditions. In the stomach, for instance, pepsin is able to break
proteins down, whereas many pancreatic enzymes break down proteins in the less acidic environment of the small intestine.
The passage of food within the digestive system.
The digestive tract has smooth muscles to move and mix food.
Peristalsis is one of the most important movements. It is a series of wavelike movements that move food.
The other is segmentation in the small intestine. In the segmentation, the gut contents are mixed with the digestive fluid and the nutrients are placed in contact with the gut wall.
These movements help digestion and absorption to take place properly.
The digestive system is controlled by hormones.
Both the nervous system and hormones can regulate the digestive system.
Gastrin
The stomach makes the most of gastrin. It facilitates gastric activity and secretion of acid.Secretin
The small intestine releases secretin into the blood when chyme enters the duodenum and is acidic. It stimulates bicarbonate excretion by the pancreas and lowers the excess acidity.
Cholecystokinin
CCK, or cholecystokinin, is secreted after the intake of fats and proteins. It stimulates secretion of pancreatic enzymes and contracts the gall bladder.
These hormones aid in the regulation of digestive processes as appropriate to the kind and quantity of food available.
The Gut Microbiome
There are trillions of microorganisms in the digestive tract, mostly bacteria. These microorganisms are collectively known as the gut microbiota.
The gut microbiota can participate in the following:
Fermentation of some indigested carbohydrates
The production of some metabolites and vitamins
A disease's interaction with the immune system.The disease's effect on the immune system.
Microbial flora and their effects on the gut environment.Intestinal flora and its effects on the intestinal environment
Diet, age, lifestyle, medications, and many other factors can affect the composition of the gut microbiota.
A healthy gut flora is seen as a crucial element in healthy gastrointestinal function.
The most prevalent digestive system ailments are:The most frequent digestive systems ailments are:
There are numerous diseases that may impact the digestive system.
Acid Reflux
Acid reflux is a condition in which the food and fluids that are in the stomach travel back up into the esophagus. It may cause heartburn and irritation.
Gastritis
Gastritis is an inflammation of the lining of the stomach. May be linked to several causes, such as: infection, medications, etc.
Peptic Ulcers
Peptic ulcers are open sores that form in the lining of the upper small intestine or stomach.
Diarrhea
Diarrhoea is the passage of loose stools or watery stools, more than usual. It can be caused by infections, food issues, medications and other factors.
Constipation
Constipation is when it becomes hard or hard not to pass stool. It can be caused by low fiber consumption, insufficient fluid intake, lack of exercise and more.
Inflammatory Bowel Disease
Crohn's and ulcerative colitis are examples of inflammatory bowel disease. All these disorders are characterized by long-lasting inflammation in the gastrointestinal tract.
If a person has severe, ongoing or warning symptoms, it is important they are assessed by a knowledgeable healthcare provider.
The role of the digestive system in overall health.How a healthy digestive system contributes to good health.
One of the body's needs is a healthy digestive system, as they need to obtain nutrients from the food they eat.
Healthy digestion is promoted by:
- Eating a balanced diet
- Eating sufficient amounts of dietary fibre
- Drinking enough fluids
- Eating a variety of fruits and vegetables
- Having the correct protein sources is important.
- Reducing the consumption of foods that have been highly processed
- Regular exercise is the key to looking and feeling better.
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Adopting good food hygiene
- Trying to prevent the unnecessary use of medicines
- Dietary fibre is also of great importance for normal bowel function and can also act as a substrate for beneficial gut microbes
- The digestive system of other animals.Other animals' digestive system.
- The digestive system is quite diverse from a zoological point of view.
- The digestive structure of some simple animals is relatively simple. Some have very specialized digestive systems that are suited to specific types of food.
- Many herbivores have specific digestive systems that enable them to extract nutrients from plants
- . For instance, ruminant animals or cattle have a complicated stomach system which enables the fermentation of plant fibre by microbes.
- Carnivores tend to have digestive systems that are suited to an animal diet, while omnivores eat and digest both animal and plant tissue.
- The digestive organs of birds consist of crop, proventriculus, and gizzard. The gizzard is a muscular digestive organ that aids in mechanical digestion of food
- These differences illustrate what digestive systems are adapted for in order to meet the needs of their diet and environment.
- The digestive system is interdependent with other systems of the body.
- The digestive system is not a completely separate system.
Digestive and Circulatory Systems
The digestive tract absorbs nutrients which are carried by the circulatory system to tissues throughout the body.Digestive and Nervous Systems
The digestive movements and secretions are regulated by the nervous system. The gastrointestinal tract is home to extensive networks of neurons (the enteric nervous system).The hormones regulate many digestive functions such as secretion, movements and appetite.
Digestive and Immune Systems
The gastrointestinal tract has a high proportion of immune activity since it continually encounters food particles and microorganisms.
The cooperation shows that digestion is an integrated part of the physiologic system.
Conclusion
The digestive system is a highly organized and complex system that allows the human body to obtain essential nutrients from food. It begins with the mouth, where food is mechanically broken down and mixed with saliva, and continues through the pharynx, esophagus, stomach, small intestine, and large intestine before waste is eliminated through the rectum and anus.
Each organ has a specialized role. The stomach begins substantial protein digestion, the small intestine completes most chemical digestion and absorbs nutrients, while the large intestine absorbs water and contributes to feces formation. Accessory organs such as the liver, gallbladder, and pancreas provide important substances that make digestion possible.
Digestion involves mechanical processes, chemical reactions, enzyme activity, muscular movements, hormonal regulation, and microbial interactions. These processes work together to transform food into substances that cells can use for energy, growth, repair, and normal physiological functions.
From a zoological perspective, the digestive system also provides an excellent example of adaptation. Different animals have evolved different digestive structures according to their diets and environments. Despite these differences, the fundamental goal remains similar: acquiring nutrients necessary for survival.
A properly functioning digestive system is therefore essential for overall health. Understanding its structure and functions not only helps explain how humans obtain nutrition but also provides insight into the remarkable diversity of digestive adaptations throughout the animal kingdom.
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