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Digestion is a type of catabolism which involves the breakdown of food into tiny molecules so that the body may absorb and use it for energy, development and repair. Food gets digested as it travels through the digestive tract of our body. The process of digestion starts in the mouth itself and ends in the large intestine. There are mainly two types of digestion, namely, Mechanical and Chemical digestion. When digestion is complete, numerous simple nutrient molecules are produced, which must be absorbed from the gastrointestinal tract (GI) by blood or lymph before being utilised by cells throughout the body. In this article, we will study more about the histology and structure of human alimentary canal, process of digestion, digestive glands and their secretions etc.
Table of Content
| Table of Content |
Key Terms: Digestion, Absorption, Macromolecules, Carbohydrates, Protein, Digestive Glands, Circulation, Alimentary Canal, Food, Enzymes
What is Digestion?
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Digestion can be defined as the process of breakdown of large, insoluble and complex food molecules into smaller ones for its absorption and circulation in the body. This process involves the usage of a variety of digestive fluids and enzymes, including saliva, mucus, bile and hydrochloric acid, among others.
Types of Digestion
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Digestion is a sort of catabolism that is frequently separated into two categories based on how food is broken down. Mentioned below are the types:
- Mechanical Digestion: Mechanical digestion is the physical breakdown of large portions of food into smaller particles that digestive enzymes can access. The mouth does mastication, whereas the small intestine performs segmentation contractions. Food is broken down into little molecules by enzymes, which the body may use for chemical digestion. It ignites with food inserted in the mouth to chew it with teeth.
- Chemical Digestion: Chemical digestion is a biological process that breaks down macromolecules in food into smaller molecules that may be absorbed into bodily fluids and delivered to cells all throughout the body. Carbohydrates, proteins etc are required to be chemically digested, thus, conversion of proteins into amino acids, lipids into fatty acids etc are forms of chemical digestion. Chemical digestion cannot be successful without mechanical digestion as it greatly increases the surface area of food particles, allowing digestive enzymes to function more efficiently on them.
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Structure of Alimentary Canal
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The alimentary canal is a long, continuous muscular tube that runs through the body, measuring approximately 8 to 10 metres in length. It is open on both ends, with the mouth on the front and the anus on the back. The Alimentary Canal consists of following parts:
- Mouth: The mouth is where food enters the gastrointestinal tract. The upper lip and lower lip block the upper and lower openings of the alimentary canal. This is the point at which food is consumed.
Anatomy of Mouth
- Salivary Glands: When we eat, smell, or even think about food, saliva is created by the salivary glands and discharged into the buccal cavity. The submandibular, parotid, and sublingual salivary glands are the three primary pairs.
- Teeth: There are 32 teeth in the buccal cavity of an adult that help in breaking and crushing food particles. The canines and incisors (front teeth) cut and rip food, while the molars (back teeth) & bicuspids smash and grind it.
Teeth
- Tongue: The tongue is a triangular-shaped organ with a meaty and muscular appearance.
Tongue
- Pharynx: The pharynx is a structure located near the back of the mouth. It is the place where the mouth and the oesophagus meet. It is a common pathway for both the food we eat and the air we inhale. The epiglottis seals the windpipe during food ingestion.
Pharynx
- Oesophagus: The oesophagus is a muscular, tubular tube that, in the presence of peristaltic motions, transports the bolus to the stomach from the pharynx. In a healthy adult, it’s about 25 cm long. It contains epithelial cells that secrete mucus, which helps the bolus travel smoothly.
Oesophagus
- Stomach: The stomach is a pouch-like structure with extremely muscular walls that sits directly beneath the diaphragm. The stomach is divided into four sections, namely, fundus stomach, body stomach, antrum stomach, and pyloric stomach.
Stomach
- Small Intestine: The small intestine is the longest portion of the alimentary canal. The stomach and small intestine are connected by the pyloric sphincter. It is divided into three sections, namely, Duodenum, Jejunum and Ileum.
Small Intestine
- Large Intestine: It is also divided into three sections:
- Caecum: The caecum is a blind sac that houses bacteria and is connected to the vermiform appendix (a vestigial organ).
- Colon: It is divided into four sections: ascending, transverse, sigmoid, and descending.
- Rectum: The anus exits the body through the rectum.
Large Intestine
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Histology of Alimentary Canal
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The GI tract’s organs are made up of the inner lining, or mucosa, the submucosa, which contains blood vessels and lymphatics, the muscularis externa, or smooth muscle layer, and the outermost layer, or serosa/adventitia. Each tissue layer has a specific function in the digestive system, ranging from forming a protective barrier against the stomach’s highly acidic contents to delivering hormones, creating muscle contractions, and draining lymph. Supporting cells, for example, are present in the epithelium lining the inside of the stomach For digestion, a layer of defence of stomach acid & mucus is formed.
Other supporting cells, such as the stomach’s mucosal cells and the pancreas’ acinar cells, also generate inactive versions of digestive enzymes known as zymogens that are stimulated to become active enzymes.
Read More: Human Body
Human Alimentary Canal
Digestive Glands
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Digestive Glands are present inside our digestive tract which secrete digestive juices that help in the process of digestion, i.e, breakdown of complex food particles into simpler ones. Some of them are:
- Salivary Glands: These produce saliva, which improves digestion, keeps your mouth moist, and promotes tooth health. The parotid, sublingual, and submandibular salivary glands are located beneath and behind your jaw.
- Liver: The liver is a big, meaty organ located on the right side of the abdomen. It has two lobes, namely, the left lobe and right lobe. Bile juice is secreted by the hepatic cells and stored in the gallbladder. The cystic duct (which comes from the gallbladder) and the hepatic duct combine to form the bile duct.
Liver
- Pancreas: The pancreas has two primary functions: exocrine digestion and endocrine blood sugar regulation.
Pancreas
- Sphincter of Oddi: The hepato-pancreatic duct enters into the duodenum through the Sphincter of Oddi. The smooth muscle that surrounds the terminal section of the common bile duct and pancreatic duct is known as the sphincter of Oddi. During a meal, this muscle relaxes, allowing bile and pancreatic juice to flow into the intestines.
Sphincter of Oddi
Read More: Digestive Enzymes
Process of Digestion
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Digestion may be broken down into several phases, such as:
- Digestion in Cavity of Mouth: Chewing and mixing of the meal occurs when food is taken in through the mouth. Chemical breakdown of carbohydrates is catalysed by Saliva. Amylase, a salivary enzyme, is responsible for the hydrolysis of 30% of the starch. Lysozyme, the other enzyme, is an antibacterial agent that helps to prevent infections. The pharynx aids in the passage of the bolus into the oesophagus, from where it travels to the stomach via the oesophagus’ peristaltic movements.
- Digestion in Stomach: Food stays in the stomach for about 4 to 5 hours once it reaches it. The mucosa lining of the stomach contains a number of gastric glands. Mucus is secreted by the mucous neck cells. Peptic Cells generate pepsinogen. Parietal or Oxyntic Cells release HCl which is necessary for vitamin B12 absorption. The churning actions of the stomach muscle thoroughly mix the food in the stomach with the gastric juices. The chyme refers to a semi-digested, acidic, and pulpy mass of food. In the stomach, the proteins are primarily digested.
- Digestion in Small Intestine: The chyme is further mixed and churned as a result of the organ’s different motions. In the small intestine, a variety of enzymes are released. All of them interact with food particles, breaking them down into smaller pieces that can be taken into the bloodstream. The duodenum is where macromolecules are broken down. In the jejunum and ileum, all of the simpler forms of the digested food are absorbed. The large intestine receives any remaining undigested, unabsorbed food particles.
- Digestion in Large Intestine: The activity of digesting in the large intestine is greatly reduced. Bacterial action on residual food particles takes place here. The large intestine absorbs minerals, water, and some medications. Apart from lubricating the large intestine, the mucus released by it aids in the retention of waste particles. The faecal matter, or undigested and unabsorbed waste particles, is subsequently transferred to the rectum, where it is expelled through the anus.
Read More: Polysaccharides
What is Absorption?
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Absorption is the process of digested food molecules being absorbed into the circulation and delivered to the GI of the body. Below mentioned are the details:
- The small intestine aids food in getting absorbed. The molecules of digested food flow past the small intestinal walls and into the circulation.
- When food particles enter the circulation, they are carried to various sections of the body where they are needed. Only smaller, soluble food molecules may pass through the small intestine’s walls.
- The small intestine then absorbs around 95% of nutritional molecules. The bulk of these molecules are absorbed in the jejunum, which is the second section of the small intestine.
- Iron is absorbed in the duodenum, while vitamin B12 is absorbed in the ileum. Nutrient molecules are transferred to other body areas after being absorbed in the small intestine.
- The epithelial epithelium that lines the small intestine is designed to absorb certain nutrients. It has a lot of wrinkles and is coated in villi and microvilli providing large surface area for absorption. The villi’s thin epithelial cell surface layer carries nutrients from the small intestine lumen into these capillaries and lacteals.
- The majority of compounds, such as simple sugars, amino acids, glycerol, salts, and water-soluble vitamins, are absorbed by the blood in the capillaries (vitamin C and the many B vitamins). Fatty acids and fat-soluble vitamins are absorbed via the lacteals’ lymph (vitamins A, D, E, and K).
Process of Absorption
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During absorption of digested products, a network of mucous membranes aids in the transport of digested, soluble food molecules into the circulation or lymph. The following are the steps in the absorption process:
- Diffusion
- Active Transportation
- Facilitated Transportation
- Passive Transportation
Diffusion: Simple diffusion is the process of passage of solute particles through a permeable membrane from an area of greater concentration to lower concentration. Monosaccharides like glucose, ions like fluoride and chlorides, and amino acids are all delivered into the circulation by this mechanism. This is the initial stage in the digestion and absorption process.
Active Transport: Active transport is the process by which solute particles move from a location of lower concentration to that of greater concentration by using energy, such as ATP. Active transport aids the diffusion of electrolytes like Na+ ions into the circulation against a concentration gradient.
Facilitated Transport: It is the active transport of solute particles across a permeable membrane, which is facilitated by certain carrier proteins. Amino acids, carbohydrates, and glucose are all transported into the circulation by facilitated transport.
Passive Transport: Passive transport is the passage of solute particles across a permeable cell membrane without the use of energy. This is the last step in the digestion process. Passive transport is the process through which simple and soluble food particles are transported into the circulation after digestion.
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Things to Remember
- Mechanical digestion is a straightforward procedure which entails the physical disintegration of food without changing its chemical composition. Chemical digestion, on the other hand, is a multi-step process that breaks down food into its chemical components, which are subsequently absorbed to feed the body's cells.
- Larger sized food molecules (such as proteins etc) must be converted into tiny enough subunits for easy absorption by alimentary canal's lining.
- Food is converted into molecules small enough to be absorbed by the epithelial cells of the intestinal villi via mechanical and digestive processes.
- The small intestine absorbs almost all of the food we eat, as well as 80% of electrolytes and 90% of water. Although water and lipids are absorbed throughout the small intestine, the jejunum is where the majority of carbs and proteins are absorbed.
- Mouth is the place where the first step of digestion takes place. Three pairs of salivary glands in the skull secrete mucus-containing saliva as well as the enzyme amylase. Food is moistened by mucus, and carbohydrates are partly digested by amylase (starches). Food then passes via the throat and oesophagus to reach the stomach.
Sample Questions
Ques. What exactly does digestion entail? (3 marks)
Ans. The mechanical and enzymatic breakdown of food into molecules that may be taken into the circulation is referred to as digestion. Fats, carbs, and proteins are the three macronutrients that must be digested before they can be absorbed. These macronutrients are broken down into molecules that can pass past the intestinal epithelium and into the circulation, where they may be absorbed by the body and utilised during digestion. Both of these processes are necessary for proper digestion, and deficiencies in either mechanical or chemical digestion can lead to nutritional deficiencies and gastrointestinal illnesses.
The lower intestine's jejunum and ileum are primarily responsible for nutrition absorption into the circulation. The contents of the small intestine start off semi-solid and end up liquid after travelling through it. The change in consistency is caused by water, bile, enzymes, and mucus.
Ques. How are Lipids digested in our body? (3 marks)
Ans. Lipid consumption is limited to 35% of total calorie intake in a balanced diet. Triglycerides, which are made up of a glycerol molecule coupled to three fatty acid chains, are the most frequent dietary lipids. Dietary cholesterol and phospholipids are also eaten in small amounts.
Lingual Lipase, Gastric Lipase and Pancreatic Lipase are the three lipases that aid in the digestion of the thing called lipid. However, because the pancreas is the sole significant generator of lipase, the small intestine is the site of almost all lipid breakdown. Pancreatic lipase degrades triglycerides into two free fatty acids and a monoglyceride for each one. Short-chain (less than 10 to 12 carbons) and long-chain fatty acids are both present.
Ques. What is the Vitamin absorption process? (3 marks)
Ans. The vitamins included in food and supplements are absorbed through the small intestine. Simple diffusion allows fat-soluble vitamins (A, D, E, and K) to be absorbed alongside dietary lipids in micelles. When using fat-soluble vitamin supplements, it is recommended that you eat some fatty meals. Simple diffusion is also used to absorb most water-soluble vitamins (including most B vitamins and vitamin C). Vitamin B12, which is a relatively massive molecule, is an exception. Intrinsic factor binds to vitamin B12 in the stomach, delaying digestion and forming a complex that attaches to mucosal receptors in the terminal ileum, where it is taken up by endocytosis.
Ques. Explain the structure of the GI Tract. (4 marks)
Ans. The structure of Gastrointestinal Tract is mentioned below:
- A single layer of epithelium with exocrine and endocrine cells covers the luminar surface.
- Exocrine cells degrade and dissolve, releasing their enzymes into the lumen.
- The mucosa is a layer of epithelia with an underlying layer of connective tissue (lamina propria) and muscle (muscularis mucosa).
- The muscularis externa, a layer of inner circular and outer longitudinal smooth muscle underneath the mucosa, produces the forces that move and mix the GI contents.
- The tube's outermost layer is made up of connective tissue known as serosa. The Gastrointestinal tube's luminar surface is excessively convoluted.
- A lacteal is a single blunt-ended lymphatic vessel that runs through the middle of each villus.
- The hepatic portal vein carries absorbed materials to the liver for processing via venous drainage from the colon.
Ques. What is a bolus? (2 marks)
Ans. Bolus is the chewed food that has been combined with saliva in the mouth. Chewing aids in the reduction of food particles to a size that can be swallowed; saliva contributes digestive enzymes, water, and mucus to aid in chemical reduction, hydration for flavour, and lubrication for easy swallowing.
Ques. What is the significance of digestion? (3 marks)
Ans. Digestion is crucial because our body needs nutrients from food and drink in order to function correctly and stay healthy. Nutrients include proteins, lipids, carbs, vitamins, minerals and water. Our digestive system breaks down nutrients into small enough pieces for your body to absorb and use for energy, development, and cell repair.
- Proteins are broken down and converted to amino acids.
- Fats are broken down and converted to Fatty acids and glycerol.
- Carbohydrates are broken down and converted to simple sugars.
Ques. How are enzymes, proteins and nucleic acid digested by our body? (5 marks)
Ans. Mentioned below is the process:
- Carbohydrate Digestion: In a normal Western diet, the plant polysaccharide amylose accounts for around 80% of digestible carbs. The amylase breaks amylose and glycogen. With the aid of amylase released by the pancreas, carbohydrate digestion is finished in the small intestine. Polysaccharides are lengthened throughout the digestion process. Shorter polysaccharides and disaccharides are formed from the macromolecules. Enzymes of the small intestine help in digestion of carbohydrates too. Sucrose, or table sugar is broken down by the enzyme sucrase into glucose and fructose. Lactase converts lactose into glucose and galactose for absorption in the intestine.
- Protein Digestion: Polypeptides must get converted into amino acids before absorption begins. The action of three major enzymes in the stomach and small intestine is responsible for protein digestion: pepsin, released by the stomach, and trypsin and chymotrypsin, secreted by the pancreas. The stomach also secretes hydrochloric acid, which makes the contents very acidic, allowing pepsin to function. In order to function, trypsin and chymotrypsin in the small intestine need an alkaline environment. As the acidic chyme empties into the small intestine, bile from the liver and bicarbonate from the pancreas neutralise it. After pepsin, trypsin, and chymotrypsin break down proteins into peptides, peptidases, which are also released by the pancreas, break them down further into amino acids.
- Nucleic Acid Digestion: Ribonuclease and deoxyribonuclease are pancreatic enzymes that break down Ribonucleic acid and Deoxyribonucleic acid into smaller nucleic acids. Small intestinal enzymes called nucleases then break them down further into nitrogen bases and sugars.
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