Human Digestive System: Know How The Digestive System Work?

The human digestive system comprises a collection of organs that work together to transform food into energy. The digestive system consists of the gastrointestinal tract ancillary organs, including the liver, pancreas, and gallbladder.

Knowing the Human Digestive system helps you understand the functions of your digestive system, which in turn helps you build a strong foundation in the subject of science. Read on to find out everything about the Human Digestive System, its parts, processes, functions, diagrams, etc.

What is Human Digestive System?

The human digestive system is a complex physiological process that takes place in humans. It has a long network of organs that are interdependent in function. The food ingested into the mouth goes through a series of changes and is broken down from a complex mixture to simple sugars, which eventually is given into the bloodstream. The undigested protein, carbohydrates, fats, fibre are eliminated through the anus. The digestive system is supported by certain glands like salivary glands, digestive glands which help in breaking down the food by secretion of enzymes and digestive juices.

Human Digestive System Diagram: Diagram

The human digestive system diagram is given below:

Digestive System Video

Types of Digestion In Human Digestive System

Digestion is a complex process involving multiple metabolic pathways where the complex food molecules are broken down into simpler molecules and release energy required for the survival of humans.  Digestion is classified into two types:

1)    Mechanical Digestion – Mechanical digestion primarily involves mastication in the mouth, a process by which food is broken down and semi-digested through chewing, churning of food to form chyme in the stomach, segmentation of chyme in the small intestine for bowel movement and absorption of nutrients.

2)    Chemical Digestion – Chemical digestion is the digestion of food by the gastric juices, bile juices, pancreatic juices and absorption of nutrients and water by the intestines.

Digestion takes place in 6 stages:

1)    Ingestion: It refers to the intake of food through the mouth.

2)    Propulsion: Propulsion refers to the movement of the food through the digestive tract, which involves swallowing and peristalsis. The food chewed in the mouth is swallowed through the throat and by the peristaltic movement, which is muscular contraction and relaxation of the oesophagus, the food is conducted into the stomach.

3)    Physical or mechanical digestion: This form of digestion refers to chewing of food using teeth in the mouth, churning action by the stomach, wherein the food is mixed with enzymes, juices to form chyme, segmentation by the small intestine, here, the circular muscles of the small intestine contract and relax, while absorbing water and other nutrients, further pushing the food throughout the intestine.

4)    Chemical digestion: The breakdown of complex food particles into simple molecules using digestive juices and salts, making it available for utilization in the bloodstream for energy and growth.

5)    Absorption: The absorption of nutrients and water, bile salts by the small and large intestine into blood vessels and lymph vessels.

6)    Defecation: It is the elimination of the undigested food matter post-absorption, through the anus.

Histology of Alimentary Canal

The wall of the alimentary canal is made of 4 layers of tissues:

1) Mucosa: It consists of areolar connective tissue called lamina propria, lying below epithelial cells. It contains mucosal glands and provides vascular support to epithelial cells. The products of digestion pass these layers. Muscularis mucosa, a thin, double-layered smooth muscle is present outside lamina propria, separating the mucosa layer from sub-mucosa, while helping in the movement of the mucosa layer.

2) Sub-mucosa: this layer is made of loose connective tissue, containing large blood vessels, nerves, lymph vessels, and mucus-secreting glands.

3) Muscularis propria: It is made of two layers- the inner layer is circular; the outer layer is longitudinal. They function together in the peristaltic movement of food throughout the alimentary canal.

4) Serosa: It is covered with visceral peritoneum and is the outermost loose connective tissue. It contains blood vessels, nerves, and lymph vessels.

Human Digestive System Parts and Functions

The human digestive system mainly consists of Gastro-intestinal tract (GI tract) or alimentary canal and supplemented with digestive juices by the liver, pancreas, gallbladder. The GI tract consists of a series of organs starting from the mouth, followed by the esophagus, stomach, small intestine, large intestine, and anus.

Mouth: It is the first organ of the digestive system, also known as the oral cavity. The anterior end of the oral cavity is the lips and the posterior end opens to the throat. The teeth, tongue, palates, vulva are all involved in the chewing process, also called mastication. Primary digestion takes place in the oral cavity, where the food is chewed by teeth and salivary glands surrounding the oral cavity supplies the salivary juice and the tongue helps in mixing the salivary juice with the food. Saliva contains an amylase enzyme, which breaks down complex carbohydrates such as starch in food into simpler sugars like maltose and dextrins. The palates also help in the movement of food, in the process of mastication. The partially digested food is now passed down to the throat via the vulva.

Esophagus: Esophagus is a long tubular organ, which receives food from the throat and aids the transfer of food through peristalsis, but no digestion takes place in this organ. The esophagus has an upper esophageal sphincter and a lower esophageal sphincter. A bolus (ball-like) of food moves down through the upper esophageal sphincter by contraction and relaxation of oesophagal muscles creating a wave-like movement called peristalsis. The esophagus opens to the stomach via the lower esophageal sphincter.

Stomach: The stomach is a sac-like structure and one of the main organs for digestion. It opens to esophagus at cardia and chyme (the semi-digested, pulpy and acidic fluid) is sent to the small intestine post digestion into the duodenum. It has digestive juices or acids like Hydrochloric acid (HCl), Potassium Chloride (KCl) and Sodium Chloride (NaCl). The enzymes present in the stomach include gastric lipase and pepsinogen (inactive form) to pepsin in the presence of HCl. Gastric Lipase digests the fats to glycerol and fatty acids. Pepsin breaks down complex proteins into small polypeptides, thus initiating the process of protein digestion.

The stomach has four parts:

1)    Cardia: It is located towards the esophagus and conducts food into the stomach through the lower esophageal sphincter. It contains mucous secreting or cardiac glands.

2)    Fundus: It is a dome-shaped structure, present next to and above cardia.

3)    Body: It is present next to the fundus, and is the major part of the stomach, where the gastric juices are present. The pyloric antrum is connected to the body of the stomach. The fundus and body contain gastric glands.

4)    Pylorus: it is funnel-shaped and connects the stomach to the small intestine. It narrows down to the pyloric canal, the pyloric sphincter is present below this canal, which controls the opening and closing of the stomach. It produces two types of mucus and gastrin hormone.

Small Intestine

Stomach opens to the small intestine at duodenum, where the major part of digestion takes place. The small intestine has three divisions: duodenum, jejunum and ileum. The digestion gets completed in the duodenum of the small intestine. The whole of the small intestine is lined with villi and microvilli, small projections which absorb nutrients to supply it to the bloodstream. Bile juice from bile duct and pancreatic juice from pancreas gets mixed with the chyme and digestion gets completed. The pancreatic fluid contains pancreatic enzyme and bicarbonate. Bicarbonate helps neutralize the stomach acid present in the food supplemented to the small intestine. Pancreatic enzymes are lipase, amylase and protease. Lipase breaks down the triglycerides and provides the body with fatty acids and fat-soluble vitamins. Amylase breaks down the carbohydrates further to produce glucose molecules. Protease acts on the small polypeptides to release amino acids. The biomolecules are completely converted into their monomers and get absorbed into the bloodstream for various functionality of the body and its organs.

Jejunum and ileum are involved in the absorption of nutrients making it available in the bloodstream. Jejunum absorbs fatty acids, amino acids and glucose molecules and the movement of nutrients through the digestive system is rapid due to peristaltic movement of the jejunum, which remains empty post-digestion. The absorption process is completed by ileum, wherein, the bile salts and vitamins are absorbed.

Large intestine

The large intestine is the last organ of the digestive system ending at the anus. The large intestine surrounds the small intestine on the three sides. The large intestine opens to the small intestine through the cecum, which is also attached to the appendix, a vestigial organ. The large intestine completes the absorption of water & removes the undigested food matter or faeces by the process of defecation. The large intestine has four parts: Cecum, colon, rectum, and anus. Cecum connects to the small intestine and its primary function includes the reabsorption of water and salts through its highly mucous membrane, also lubricating the faecal matter using mucus. The rapid movements in the large intestine make the remaining food matter to be exposed for reabsorption of electrolytes and water. The colon is the longest component of the large intestine. It is divided into three parts: ascending colon, transverse colon and descending colon. The ascending colon and transverse colon complete the absorption of water, electrolytes, and other remaining salts causing faeces to dehydrate and lubricate the faeces to be eliminated out of the body. The faecal matter can remain within the colon for a long time and sent to the rectum about 2-3 times a day for elimination. The rectum is above the anus, where the faeces remains until defecation

Anus

Anus is that the terminal of the digestive tract, through which feces is removed from the body. The anal canal receives the fecal matter and it has separate valves to avoid removing feces and gas simultaneously. The anus is closed because of the presence of anal sphincter, made of a muscular ring.

Other Organs involved in Digestion

Liver: The liver is a bi-lobed organ present on the right side of the human body, besides the stomach. The primary function of the liver is the secretion of bile juice, composed of bile acids, phospholipids, traces of metals and fats. It has a pH of about 7-8. Apart from this, the liver is involved in the detoxification of blood which is supplied by the vein in digestive organ, before it is conducted to other regions of the body and synthesizes protein for blood clotting, albumin, cholesterol & triglycerides. It also converts glucose to glycogen to be stored in liver and muscle cells. It catabolizes drugs, medications, insulin and other hormones in the body.

Pancreas: Pancreas is situated below the stomach towards the left side of the body and above the small intestine. The pancreas is chiefly involved in the synthesis of insulin and glucagon hormones; insulin acts to reduce the blood sugar level and glucagon acts to increase the blood sugar level. This activity of liver is called an endocrine function and pancreatic enzymes such a protease, amylase, lipases are synthesized in the pancreas, which is released into the duodenum when food reaches the stomach. This is described as an exocrine function.

Gall bladder: it is a small organ which is connected to the liver, where the bile juice is secreted and concentrated before being used for digestion in the small intestine. When the food reaches the stomach and converted into chyme. The bile juice from gall bladder is released into the bile duct and further into the duodenum, where it combines with pancreatic juice and chyme to be digested by the small intestine.

Human Digestive System: Common Digestive Disorders

1) GERD (Gastro-Esophageal Reflux Disease) – Prolonged and recurrent heartburn is associated with GERD. The uncontrolled heartburn can cause erosion of the inner lining of the esophagus and lead to bleeding. GERD can be clinically diagnosed by the practitioner by the study of symptoms. Change is food styles and habits can control GERD along with OTC medications.

2) Chronic Diarrhea – Diarrhea is a condition in which a person passes loose stools more than 4-5 times a day, repetitively for a week or more. This causes dehydration and loss of appetite. Chronic diarrhea goes on for a few weeks and is difficult to diagnose the cause for it. It can be due to bacterial or viral infections, improper food, fewer roughages or disorders like Crohn’s disease, Irritable bowel syndrome (IBS) or Ulcerative Colitis.

3) Chronic Constipation – It refers to 2 or 3 bowel movements in a week or few. It causes tightening of stools and becomes hard to pass out. Chronic constipation is often treated initially with stool softeners and fiber supplements, Over-the-counter medications.  Certain exercises are recommended.

4) Gastroenteritis – It is often associated with bacterial or viral infections. It has symptoms like vomiting, fever, diarrhea or headache. The best way to overcome this disorder is by drinking plenty of fluids and good hygiene practices.

5) Ulcers – Ulcers are often associated with food styles and stress, but it was found that the common cause is the bacterial infection like H. pylori and OTC non-steroidal anti-inflammatory drugs like ibuprofen and naproxen. The inner lining of the stomach is affected and often difficult to treat, due to constant exposure to stomach acids. Antibiotics are given to get rid of the bacterial infection or a proton pump inhibitor may be prescribed to reduce the acid in the stomach.

6) Hemorrhoids – It is the swelling of the rectal veins, causing pain and itching around the anus. It is often linked with long duration of sitting in the toilet, pregnancy, chronic constipation or family history. It can be treated by using OTC ointments, hot water soaking or softener, in adverse cases, rubber band ligation or sclerotherapy are performed to reduce inflammation.

FAQs

Here are some of the frequently asked questions and their answers:

Q.1. Deduce the function of the human digestive system.
Ans: The human digestive system consists of the alimentary canal and accessory organs. The digestive system breaks down the food into its components and produces the necessary nutrients required for the living body to function and sustain life.

Q.2. Outline the process of digestion, step by step.
Ans: The process of digestion involves six steps, namely:

• a. Ingestion
• b. Mixing and Movement
• c. Secretion
• d. Digestion
• e. Absorption
• f. Excretion

Q.3. What is Diarrhea and why is it caused?
Ans: Diarrhea is a condition in which a person passes loose stools more than 4-5 times a day, repetitively for a week or more. This causes dehydration and loss of appetite. Chronic diarrhea goes on for a few weeks and is difficult to diagnose the cause for it. It can be due to bacterial or viral infections, improper food, fewer roughages or disorders like Crohn’s disease, Irritable bowel syndrome (IBS) or Ulcerative Colitis.

Q.4. Name the common digestive disorders.
Ans: The 6 common digestive disorders are as follows:
a. GERD (Gastro-Esophageal Reflux Disease)
b. Chronic Diarrhea
c. Chronic Constipation
d. Gastroenteritis
e. Ulcers
f. Hemorrhoids

Q.5. What are the types of digestion in the human digestive system?
Ans: The types of digestion in the human digestive system is classified into two types. The types of the human digestive system are divided into Mechanical digestion and chemical digestion.

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