Digestive system

Digestive System: Detailed Overview

The digestive system, also known as the gastrointestinal (GI) system, is responsible for the ingestion, digestion, absorption, and excretion of food and nutrients. It comprises a series of organs and structures that work together to break down food, absorb nutrients, and eliminate waste.

Major Components of the Digestive System

  1. Mouth
  2. Pharynx
  3. Oesophagus
  4. Stomach
  5. Small Intestine
  6. Large Intestine
  7. Rectum and Anus

Additionally, several accessory organs aid in digestion:

  • Salivary Glands
  • Liver
  • Gallbladder
  • Pancreas

Detailed Overview of the Digestive System

1. Mouth

Function:

  • Ingestion: Intake of food and liquids.
  • Mechanical Digestion: Chewing (mastication) breaks down food into smaller pieces.
  • Chemical Digestion: Salivary amylase begins the breakdown of carbohydrates.

Key Structures:

  • Teeth: Aid in mechanical digestion.
  • Tongue: Assists in mixing food with saliva and forming the bolus for swallowing.
  • Salivary Glands: Produce saliva, which contains enzymes and mucus.

2. Pharynx

Function:

  • Passageway: Transports food from the mouth to the oesophagus.

Key Structures:

  • Nasopharynx: Upper part of the pharynx, connects with the nasal cavity.
  • Oropharynx: Middle part of the pharynx, behind the oral cavity.
  • Laryngopharynx: Lower part of the pharynx, leading to the oesophagus.

3. Oesophagus

Function:

  • Transport: Moves food from the pharynx to the stomach through peristaltic contractions.

Key Structures:

  • Upper Oesophageal Sphincter: Controls entry of food into the oesophagus.
  • Lower Oesophageal Sphincter (LES): Prevents back flow of stomach contents into the oesophagus.

4. Stomach

Function:

  • Storage: Holds food and begins protein digestion.
  • Mechanical Digestion: Churning mixes food with gastric juices to form chyme.
  • Chemical Digestion: Gastric juices (containing hydrochloric acid and pepsin) break down proteins.

Key Structures:

  • Cardia: Region where the oesophagus connects to the stomach.
  • Fundus: Upper curved part of the stomach.
  • Body: Main central region.
  • Pylorus: Lower section, leading to the small intestine.
  • Pyloric Sphincter: Controls the passage of chyme into the small intestine.

5. Small Intestine

Function:

  • Digestion: Completes the digestion of food using enzymes from the pancreas and bile from the liver.
  • Absorption: Nutrients are absorbed into the bloodstream.

Key Structures:

  • Duodenum: First part, where most chemical digestion occurs.
    • Carbohydrates and Proteins: Pancreatic amylase breaks down carbohydrates, and trypsin and chymotrypsin break down proteins.
    • Lipids: Bile emulsifies fats, and pancreatic lipase breaks them down.
  • Jejunum: Middle section, where absorption of nutrients occurs.
    • Proteins, Carbohydrates, and Lipids: Absorbed through the intestinal wall into the bloodstream.
  • Ileum: Final section, absorbs remaining nutrients and passes the residue to the large intestine.
    • Vitamin B12 and Bile Salts: Primarily absorbed here.

6. Large Intestine

Function:

  • Absorption: Absorbs water and electrolytes.
  • Formation and Storage of Faeces: Forms and stores waste material (faeces) before excretion.

Key Structures:

  • Cecum: First part, connected to the ileum.
  • Colon: Divided into ascending, transverse, descending, and sigmoid sections.
  • Rectum: Stores faeces until defecation.
  • Anus: Controls the expulsion of faeces through the anal sphincters.

Accessory Organs and Their Functions

1. Salivary Glands

Function:

  • Produce saliva that contains enzymes (e.g., amylase) which begin the digestion of carbohydrates and provide lubrication for swallowing.

Key Glands:

  • Parotid Glands: Located near the ears; produce a serous, watery secretion.
  • Submandibular Glands: Located beneath the lower jaws; produce a mixed serous and mucous secretion.
  • Sublingual Glands: Located beneath the tongue; produce a mucous secretion.

2. Liver

Function:

  • Produces bile, which emulsifies fats, aiding in their digestion and absorption.
  • Processes nutrients absorbed from the small intestine, detoxifies chemicals, metabolises drugs, and synthesises important proteins e.g. Albumin.

Key Structures:

  • Hepatocytes: Liver cells that perform most of the liver’s functions.
  • Bile Ducts: Transport bile from the liver to the gallbladder and small intestine.
  • Portal Vein: Carries nutrient-rich blood from the intestines to the liver.

3. Gallbladder

Function:

  • Stores and concentrates bile produced by the liver, releasing it into the small intestine to aid in digestion.

Key Structures:

  • Cystic Duct: Connects the gallbladder to the common bile duct.
  • Common Bile Duct: Transports bile from the liver and gallbladder to the duodenum.

4. Pancreas

Function:

  • Produces digestive enzymes (e.g., lipase, amylase, proteases) and bicarbonate, which are released into the small intestine to aid in digestion.
  • Produces insulin and glucagon, hormones that regulate blood sugar levels.

Key Structures:

  • Acinar Cells: Produce digestive enzymes.
  • Islets of Langerhans: Produce hormones like insulin and glucagon.
  • Pancreatic Ducts: Transport digestive enzymes to the small intestine.

Digestion and Absorption of Nutrients

  1. Carbohydrates:
  • Digestion: Begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase.
  • Absorption: Monosaccharides are absorbed in the small intestine (mainly in the jejunum) into the bloodstream.

2. Proteins:

  • Digestion: Begins in the stomach with pepsin and continues in the small intestine with pancreatic enzymes (trypsin and chymotrypsin).
  • Absorption: Amino acids are absorbed in the small intestine (mainly in the jejunum) into the bloodstream.

3. Lipids:

  • Digestion: Begins in the small intestine with the action of bile and pancreatic lipase.
  • Absorption: Fatty acids and monoglycerides are absorbed in the small intestine (mainly in the jejunum) into the lymphatic system via lacteals.

Absorption of Medications

Medications are primarily absorbed in the small intestine due to its large surface area and rich blood supply. The exact site of absorption can depend on the formulation and properties of the medication:

  • Stomach: Some drugs like aspirin and alcohol can be absorbed here due to the acidic environment.
  • Small Intestine: Most drugs are absorbed here due to the extensive surface area provided by villi and microvilli.
  • Duodenum and Jejunum: Absorption of drugs that require a neutral or slightly alkaline pH.
  • Ileum: Absorption of bile acids and vitamin B12 (when bound to intrinsic factor).

Conclusion

The digestive system is a complex network of organs and structures essential for breaking down food, absorbing nutrients, and eliminating waste. Understanding its components and functions is crucial for maintaining digestive health and recognising symptoms of disorders that may require medical attention. Regular check-ups, a balanced diet, and a healthy lifestyle play a significant role in supporting digestive system function.