Digestive system in humans

The human digestive system involves a fascinating journey of food through what's known as the alimentary canal, or gut. This long, hollow tube stretches from the mouth all the way to the anus, and along the way, food undergoes several key processes:

1. Ingestion: This is simply the act of taking food into the mouth – the starting point of the digestive adventure!
2. Digestion: Once inside, the food needs to be broken down into much smaller particles. This is digestion, a crucial step that allows our bodies to absorb the necessary nutrients.
3. Absorption: With food broken down into soluble end products, the next step is absorption. This is where these tiny nutrient particles pass through the walls of the alimentary canal and enter the bloodstream, ready to be transported throughout the body.
4. Assimilation: Now in the cells, these absorbed nutrients are put to work in the body's metabolism. This incorporation of digested products into cell metabolism is called assimilation.
5. Egestion: Finally, whatever food particles our bodies can't digest or absorb need to be eliminated. This process of removing indigested food from the body through the anus is egestion.

The digestive system isn't just the alimentary canal itself. It also includes associated organs like the pancreas and liver, which play vital roles in producing enzymes and other substances that aid in digestion.

The alimentary canal itself is comprised of several distinct parts, each with its own specialized function:

1. Mouth: Where ingestion and the initial stages of digestion begin.
2. Pharynx: A passageway for both food and air.
3. Gullet (Esophagus): The tube that carries food from the pharynx to the stomach.
4. Stomach: A muscular organ that further breaks down food.
5. Duodenum: The first part of the small intestine, where a significant amount of digestion occurs.
6. Ileum: The final section of the small intestine, primarily responsible for absorbing nutrients.
7. Caecum: A pouch connected to the junction of the small and large intestines.
8. Colon: The main part of the large intestine, involved in water absorption and waste formation.
9. Rectum: The final section of the large intestine, where feces are stored before elimination.

Digestive system of a human being

Digestion process

Digestion is the process by which food is broken down into forms that can be absorbed and used by the body. It involves two main types of breakdown:

• Mechanical Breakdown: Physical breakdown of food into smaller pieces to increase surface area.
• Chemical Breakdown: Breakdown of food by digestive enzymes into simpler molecules.

Major regions of digestion in the alimentary canal include the mouth, stomach, duodenum, and ileum.

Mechanical digestion

Occurs mainly in the mouth where teeth chew food, breaking it into smaller pieces to increase surface area for enzymes to act.

Chemical digestion

Done by digestive enzymes secreted at various points in the alimentary canal.

Mouth

• Food mixes with saliva, forming a bolus (a soft ball of chewed food).
• Saliva is alkaline and contains salivary amylase, which starts breaking down starch into maltose.
• Mucus in saliva lubricates the food for easier swallowing.
• Water in saliva acts as a solvent, dissolving food substances.
• Saliva secretion is controlled by the nervous system, triggered by smell, taste, sight, or thoughts of food.
• The tongue helps roll the food into a bolus and pushes it down the oesophagus.

Pharynx

A region where the air passage (glottis) and food passage cross. The epiglottis prevents food from entering the trachea (windpipe). No digestive enzymes are active here.

Oesophagus (Gullet)

A muscular tube connecting the pharynx to the stomach. Food moves down by peristalsis, which is rhythmic contractions pushing the bolus. Muscle valves called sphincters control peristalsis. No digestive enzymes present here.

Stomach

Food mixes with hydrochloric acid (HCl) and gastric juice, forming a semi-solid called chyme. The stomach walls contain gastric glands that secrete gastric juice.

Structure of a human stomach

Components of Gastric Juice:

1. Water
2. Hydrochloric Acid (HCl):
   • Provides an acidic environment for enzyme action.
   • Breaks down food particles.
   • Kills bacteria in food.
3. Mucus: Protects the stomach lining from being damaged by acid.
4. Enzymes:
   • Pepsin: Breaks down proteins into smaller peptides.
   • Rennin: Coagulates milk protein casein (mainly in young mammals).

Functions of the Stomach:

1. Temporary storage of food.
2. Beginning protein digestion.
3. Mixing food by churning.
4. Absorbing some substances like water and alcohol.
5. The pyloric sphincter controls the passage of chyme to the duodenum.

Duodenum

• The first part of the small intestine.
• Closely associated with the liver and pancreas.

Liver: Produces bile, stored in the gall bladder and released via the bile duct.

Functions of Bile:

• Emulsifies fats into tiny droplets, increasing surface area for lipase.
• Provides an alkaline medium for enzymes to work.
• Neutralizes acidic chyme from the stomach.

Pancreas: Produces pancreatic juice containing:

• Pancreatic amylase: breaks starch into maltose.
• Pancreatic lipase: breaks fats into fatty acids and glycerol.
• Sodium hydrogen carbonate ($NaHCO_3$): neutralizes acid, providing basic conditions.
• Trypsin: breaks proteins into peptides.

The mixture in the duodenum remains chyme.

Ileum (Small Intestine)

• The longest section of the alimentary canal.
• Contains cells that secrete intestinal juice (succus entericus) and mucus.

Enzymes in Intestinal Juice:

• Erepsin (peptidase): digests peptides into amino acids.
• Maltase: breaks maltose into glucose.
• Lipase: breaks fats into fatty acids and glycerol.
• Sucrase: breaks sucrose into glucose and fructose.

Main Functions:

• Completes digestion of carbohydrates, proteins, and fats.
• Absorbs end products of digestion (amino acids, glucose, fatty acids, glycerol) into the bloodstream.

Other important processes

• Absorption: Movement of soluble end products of digestion from the ileum into the bloodstream.
• Assimilation: Incorporation of absorbed nutrients into body cells for metabolism.
• Egestion: Removal of undigested food waste (faeces) through the anus.