What is digestion?

Digestion is breaking down large, insoluble food compounds into small, water-soluble components so they can be absorbed into the blood plasma. In many organisms, these smaller substances are absorbed through the small intestine into the bloodstream. This process breaks down food into simple nutrients such as carbohydrates, fats, and proteins.

The digestive system is a complex network that breaks down food, absorbs nutrients and expels waste. It consists of both hollow and solid organs:

What is the digestive system?

1. Gastrointestinal (GI) Tract: This is a continuous tube running from the mouth to the anus and includes:

   • Mouth: Where digestion begins with chewing and saliva.

   • Esophagus: A tube that transports food from the mouth to the stomach.

   • Stomach: Where food is mixed with digestive juices and broken down further.

   • Small Intestine: Comprising the duodenum (initial segment), jejunum (middle segment), and ileum (final segment), where most nutrient absorption occurs.

   • Large Intestine: Includes the cecum (beginning), colon (main section), and rectum (end), where water is absorbed and waste is prepared for elimination.

   • Anus: The final part of the GI tract through which waste is expelled.

2. Solid Organs:

   • Liver: Produces bile, which helps digest fats, and processes nutrients from the small intestine.

   • Pancreas: Produces digestive enzymes and hormones like insulin.

   • Gallbladder: Stores and concentrates bile from the liver.

3. Other Components:

   • Gut Flora (Microbiome): Bacteria in the GI tract that assist with digestion and maintain gut health.

   • Nervous and Circulatory Systems: Nerves and hormones regulate digestive processes, and blood vessels transport nutrients absorbed from the digestive tract to various parts of the body.

Together, these components work in harmony to ensure the efficient processing of food and the removal of waste.

Importance of Digestion

Digestion is crucial because it allows our body to obtain and utilize the nutrients necessary for proper functioning and overall health. Nutrients include proteins, fats, carbohydrates (major components), vitamins, minerals, and water (minor components). The digestive system breaks these nutrients down into smaller, absorbable parts that the body can use for energy, growth, and cell repair. Specifically:

• Proteins are broken down into amino acids.
• Fats are broken down into fatty acids and glycerol.
• Carbohydrates are broken down into simple sugars.

This process ensures that our body can effectively absorb and utilize these essential nutrients.

Digestion Process

The digestive system's activities can be grouped into five main categories: ingestion, propulsion, digestion, absorption, and elimination.

1. Ingestion: This is the process of taking food into the alimentary tract, including eating and drinking.

2. Propulsion: This involves mixing the contents and moving them along the alimentary tract. It includes actions like swallowing and peristalsis (wave-like muscle contractions).

3. Digestion: This consists of:

   • Mechanical Breakdown: For example, mastication (chewing), which physically breaks down food into smaller pieces.
   • Chemical Digestion: The breakdown of food into smaller molecules by enzymes present in secretions produced by glands and accessory organs of the digestive system.

4. Absorption: This process allows the products of digestion to pass through the walls of the alimentary canal into the blood and lymph capillaries for circulation and use by body cells.

5. Elimination: Food that cannot be digested or absorbed is excreted from the alimentary canal in the form of feces through defecation.

Here’s how digestion happens in specific organs:

• Mouth: Chewing
• Esophagus: Peristalsis (movement of food through the esophagus)
• Stomach: The upper muscle relaxes to allow food entry, while the lower muscle mixes food with digestive juices
• Small Intestine: Peristalsis (movement of food through the small intestine)
• Pancreas: Produces digestive enzymes but does not have a direct mechanical role in digestion
• Liver: Produces bile but does not have a direct mechanical role in digestion
• Large Intestine: Peristalsis (movement of food through the large intestine)

The Alimentary Canal

Also known as the gastrointestinal (GI) tract, the alimentary canal is essentially a long, continuous tube through which food passes. It starts at the mouth and ends at the anus, extending approximately 5 meters in adults. The main parts of the alimentary canal are:

• Mouth
• Esophagus
• Stomach
• Small Intestine
• Large Intestine
• Rectum
• Anal Canal

Accessory Organs

Accessory organs are those that assist in digestion by secreting substances into the alimentary canal. Some of these secretions are released by glands in the lining of certain organs, such as gastric juice secreted by glands in the stomach lining. Other secretions come from glands located outside the alimentary tract, and these organs are connected to the tract via ducts. The accessory organs include:

• Three pairs of salivary glands
• The pancreas
• The liver and biliary tract

Layers of the Alimentary Canal

The walls of the alimentary canal are composed of four distinct layers:

1. Adventitia or Serosa: This is the outermost layer. In the thoracic region, it is made of loose fibrous tissue. In the abdominal region, it is covered by a serous membrane called the peritoneum, which is the largest serous membrane in the body.

2. Muscle Layer: This layer consists of two layers of smooth muscle (involuntary). The outer layer is longitudinal, while the inner layer is circular. The rhythmic contractions of these muscles, known as peristalsis, help mix digestive juices with food and move them through the canal.

3. Submucosa: This layer is composed of loose areolar connective tissue, which includes collagen and some elastic fibers. It serves to bind the muscle layer to the mucosa and contains blood vessels, nerves, and glands.

4. Mucosa: The innermost layer, made up of columnar epithelium, performs three main functions:

   • Protection: Shields underlying tissues from mechanical damage and pathogens.
   • Secretion: Produces digestive enzymes, hormones, and mucus.
   • Absorption: Facilitates the uptake of nutrients from the digestive tract into the bloodstream.

Organ of The digestive system

Mouth

The mouth is the starting point of the digestive tract and plays a crucial role in digestion. The digestive process begins even before taking a bite, as the salivary glands become active in response to the smell of food. Once eating begins, the teeth cut, grind, and chew the food into smaller pieces, making it easier to digest. Saliva, produced by the salivary glands, mixes with the food to begin breaking it down into a form that the body can absorb and utilize. When swallowing, the tongue helps push the food into the throat and then into the esophagus.

Esophagus

The esophagus is a long, tube-like structure that transports food from the mouth to the stomach. The process begins when a bolus of food is swallowed and moves through the esophagus. This movement is facilitated by peristalsis, which are rhythmic contractions of the esophageal muscles that push the food along the canal.

At the bottom of the esophagus, a ring-like muscle called the lower esophageal sphincter (LES) plays a crucial role. It relaxes to allow food to pass into the stomach and then contracts to prevent stomach contents from flowing back into the esophagus. When the LES does not function properly and allows stomach acid to return to the esophagus, it can cause conditions such as acid reflux or heartburn.

Stomach

The stomach is a J-shaped, dilated portion of the alimentary tract located in the epigastric, umbilical, and left hypochondriac regions of the abdominal cavity. It has two curvatures: the greater curvature and the lesser curvature. The stomach is divided into three main parts:

• Fundus: The upper portion of the stomach.
• Body: The central, largest part.
• Pylorus: The lower part that connects to the small intestine.

At the end of the pylorus, the pyloric sphincter regulates the passage of stomach contents (chyme) into the duodenum, which is the first part of the small intestine.

Functions of the Stomach:

• Temporary Storage: The stomach holds food temporarily, allowing digestive enzymes and acids time to act.
• Chemical Digestion: Pepsinogen, activated by pepsin in the acidic environment, breaks down proteins into polypeptides.
• Mechanical Breakdown: The stomach's three layers of smooth muscle mix food with gastric juices, turning it into a liquid called chyme.
• Limited Absorption: The stomach absorbs small amounts of water, alcohol, and some lipid-soluble drugs.
• Non-Specific Defense Against Microbes: Hydrochloric acid in gastric juice helps kill bacteria and other pathogens. Vomiting may occur in response to irritants, such as microbes or chemicals.
• Preparation of Iron for Absorption: The acidic environment of the stomach helps dissolve iron salts, making iron available for absorption in the small intestine.
• Production and Secretion of Intrinsic Factor: This is essential for the absorption of vitamin B12 in the terminal ileum of the small intestine.
• Secretion of Gastrin: This hormone stimulates gastric acid secretion and motility.
• Regulation of Gastric Content: The pyloric sphincter controls the release of chyme into the duodenum and prevents backflow into the stomach.

Small Intestine

The small intestine is approximately 2.5 cm in diameter and around 5 meters long. It is divided into three parts:

• Duodenum: The first segment, is responsible for the continuous breakdown of food using enzymes from the pancreas and bile from the liver.
• Jejunum: The middle section, primarily involved in the absorption of nutrients.
• Ileum: The final segment, also focused on nutrient absorption, and the residual liquid food then moves to the large intestine.

Food moves through the small intestine and is mixed with digestive juices through a process called peristalsis. The breakdown of food involves several processes:

• Digestion of Carbohydrates: Polysaccharides are broken down into disaccharides by pancreatic amylase.
• Digestion of Proteins: Enzymes secreted by the microvilli in the small intestine, including enterokinase, activate trypsin and chymotrypsin. These enzymes further break down polypeptides into tripeptides, dipeptides, and amino acids.
• Digestion of Fats: Bile salts emulsify fats, allowing lipase to convert them into fatty acids and glycerol.

Bile

Bile is a digestive fluid secreted by the liver and stored in the gallbladder. It contains water, minerals, mucus, bile salts, bile pigments (like bilirubin), and cholesterol. The liver secretes between 500 and 1000 ml of bile each day. Bile functions include:

• Emulsification of Fats: Bile salts break down large fat droplets into smaller ones, enhancing the action of lipase.
• Making Cholesterol and Fatty Acids Soluble: This facilitates their absorption in the small intestine.
• Excretion of Bilirubin: Bilirubin is a waste product resulting from the breakdown of red blood cells and is excreted in bile.

Large Intestine (Colon)

The large intestine is a tube approximately 1.5 cm in diameter and 6.5 cm in length. It begins at the cecum in the right iliac fossa and ends at the rectum and anal canal. The large intestine is composed of several parts:

• Cecum: The initial segment of the large intestine.
• Ascending Colon: The portion that travels up the right side of the abdomen.
• Transverse Colon: The segment that crosses the abdomen from right to left.
• Descending Colon: The section that travels down the left side of the abdomen.
• Sigmoid Colon: The S-shaped segment that connects to the rectum.

Additionally, there is a worm-like structure called the vermiform appendix or simply the appendix, which does not have a known digestive function.

Functions of the Large Intestine:

• Absorption: The large intestine absorbs large amounts of water through osmosis, concentrating the waste material into semi-solid feces. It also absorbs minerals, salts, and vitamins.
• Microbial Synthesis: The large intestine hosts a variety of bacteria, such as E. coli, Enterobacter aerogenes, Streptococcus faecalis, and Clostridium perfringens. These bacteria are involved in synthesizing vitamin K and folic acid. While generally harmless, these bacteria can sometimes become pathogenic and cause disease.
• Constituents of Feces: Feces are typically brown and solid, with the color attributed to stercobilin. They consist of fibers, microbes, epithelial cells, fatty acids, and mucus, which helps lubricate the feces for easier elimination.

Rectum & Anal Canal

The rectum and anal canal are the final sections of the gastrointestinal (GI) tract.

• Rectum: This is the lower end of the large intestine where stool is stored until it is ready to be expelled. It functions as a temporary holding area for feces before elimination.

• Anal Canal: This is the terminal part of the GI tract that opens to the outside through the anus. It consists of the internal and external anal sphincters that control the release of stool.

Function: When stool or gas enters the rectum, sensory signals are sent to the brain, which triggers the urge to defecate. The rectum then contracts to push the stool toward the anal canal, while the anal sphincters (internal and external) relax to allow the passage of stool or gas through the anus and out of the body.

Salivary Glands

The salivary glands are responsible for producing saliva, which is secreted into the mouth through ducts. There are six salivary glands, organized into three pairs:

1. Parotid Glands: Located on each side of the face, the parotid glands secrete saliva through the parotid ducts, which open into the mouth near the second upper molar tooth.

2. Submandibular Glands: Found on each side of the face under the angle of the jaw, the submandibular glands have ducts that open into the floor of the mouth, one on each side of the frenulum of the tongue.

3. Sublingual Glands: Positioned under the mucous membrane of the floor of the mouth, in front of the submandibular glands, the sublingual glands have numerous small ducts that open into the floor of the mouth.

Together, these glands produce approximately 1.5 liters of saliva per day. Saliva consists of water, the enzyme amylase, mineral salts, mucus, the enzyme lysozyme, and some antibodies.

Functions of Saliva:

• Lubrication: Saliva mixes with food to form a bolus, making it easier to swallow.
• Enzymatic Digestion: Enzymes in saliva, such as amylase, break down complex sugars into simpler forms that are more easily digestible.
• Taste Stimulation: Saliva helps dissolve food particles, stimulating taste buds and enhancing the sense of taste.
• Oral Hygiene: Saliva cleans the mouth, prevents damage to the mucous membrane, and keeps the mouth moist.
• Microbial Defense: Saliva contains antibodies and other antimicrobial agents that help protect against microbes.

Pancreas

The pancreas is a large gland, weighing about 60 grams and measuring 12-15 cm in length. It is located in the epigastric and left hypochondriac regions of the abdomen. The pancreas functions as both an endocrine and exocrine gland:

• Exocrine Function: The pancreas produces digestive enzymes that are released into the small intestine to aid in digestion.
• Endocrine Function: It secretes hormones such as insulin and glucagon, which regulate blood sugar levels.

Liver

The liver is the largest gland in the body, characterized by its reddish-brown color. It is situated primarily in the right hypochondriac region, extends into the epigastric region, and partially into the left hypochondriac region. The liver has four lobes:

• Right Lobe: The largest lobe.
• Left Lobe: Smaller and wedge-shaped.
• Caudate Lobe: Located on the posterior surface.
• Quadrate Lobe: Also on the posterior surface.

A key feature of the liver is the portal fissure on its posterior surface, where structures such as the portal vein, hepatic artery, nerve fibers, and lymph vessels enter and leave the gland.

Functions of the Liver:

• Metabolism of carbohydrates, fats, and proteins.
• Production, secretion, storage, and excretion of bile.
• Detoxification of chemicals and drugs.
• Production of heat.

Biliary Tract

The biliary tract comprises:

• Hepatic Ducts: Right and left hepatic ducts merge to form the common hepatic duct.
• Cystic Duct: This duct connects the gallbladder to the common hepatic duct.
• Common Bile Duct: Formed by the union of the common hepatic duct and the cystic duct. It is joined by the main pancreatic duct at the hepatopancreatic ampulla, which opens into the duodenum.

Gallbladder

The gallbladder is a pear-shaped sac located on the posterior surface of the liver, attached by connective tissues. It consists of three parts:

• Fundus: The rounded, lower portion.
• Body: The central part.
• Neck: The tapered end that connects to the cystic duct.

Functions of the Gallbladder:

• Storage of Bile: It stores bile produced by the liver.
• Release of Bile: It releases stored bile into the duodenum through the bile ducts.