Digestive system Revision Notes

The human digestive system is a complex series of organs responsible for processing food, extracting nutrients, and eliminating waste. It is divided into two main groups: the alimentary canal and the associated accessory organs. The alimentary canal is a continuous muscular tube through which food passes and is digested, while the accessory organs produce and store substances that aid digestion but do not form part of the food pathway itself.

Understanding the location and structure of these organs is essential for recognising their functions and how they work together to maintain nutrition and health.

• The alimentary canal is a continuous tube from mouth to anus.
• Accessory organs assist digestion but food does not pass through them directly.

The alimentary canal includes the mouth, oesophagus, stomach, small intestine, and large intestine. Each organ has a specific role and structure adapted to its function in digestion and absorption.

The mouth is the entry point where food is ingested. Teeth mechanically break down food by chewing, and saliva from salivary glands begins chemical digestion.

The oesophagus is a muscular tube that transports food from the mouth to the stomach using rhythmic contractions called peristalsis, ensuring food moves efficiently even against gravity if necessary.

• Mouth: mechanical digestion by chewing; saliva moistens food.
• Oesophagus: muscular tube transporting food to stomach via peristalsis.
• Stomach: muscular sac that mixes food with gastric juices for chemical digestion.

The small intestine is the primary site for digestion and absorption of nutrients. It is divided into two main parts: the duodenum and the ileum.

The duodenum receives partially digested food (chyme) from the stomach along with digestive enzymes and bile from accessory organs. This is where most chemical digestion occurs.

The ileum continues the process, mainly absorbing nutrients into the bloodstream through its highly folded lining, which increases surface area for efficient absorption.

• Duodenum: first part of small intestine; mixes chyme with enzymes and bile.
• Ileum: absorbs nutrients through villi into blood.
• Small intestine length and folds maximise nutrient absorption.

The large intestine absorbs water and minerals from the remaining indigestible food matter, forming solid waste (faeces).

The colon is the longest part, where water absorption occurs and bacteria help break down some materials.

The rectum stores faeces until they are expelled through the anus during egestion, the final stage of digestion.

• Colon: absorbs water, compacts waste.
• Rectum: stores faeces before elimination.
• Anus: muscular opening controlling faeces release.

These organs do not form part of the alimentary canal but produce and store substances essential for digestion.

Salivary glands produce saliva containing enzymes that begin starch digestion in the mouth.

The pancreas produces digestive enzymes released into the duodenum to break down carbohydrates, proteins, and fats further. It also secretes alkaline fluids to neutralise stomach acid entering the small intestine, protecting its lining and optimising enzyme activity.

• Salivary glands: produce saliva with amylase enzyme.
• Pancreas: secretes enzymes and alkaline fluid into duodenum.
• Liver: produces bile, which emulsifies fats to aid digestion.
• Gall bladder: stores and concentrates bile, releasing it into the duodenum when needed.

The alimentary canal forms a continuous path from mouth to anus, with accessory organs connected via ducts to the small intestine, mainly the duodenum.

For example, bile from the liver and gall bladder enters the duodenum through the bile duct, while pancreatic enzymes enter via the pancreatic duct.

This arrangement ensures that digestive secretions mix with food at the correct stage for efficient digestion and absorption.

• Alimentary canal organs form a continuous tube.
• Accessory organs connect to the duodenum via ducts.
• Coordinated secretion of enzymes and bile facilitates digestion.

Ingestion is the first step in the digestive process, involving the intake of food and drink through the mouth. This is where food enters the body and is prepared for digestion. For example, biting into an apple introduces the food into the digestive system. The mouth also begins mechanical digestion by chewing, breaking food into smaller pieces to increase surface area for enzymes later. Saliva, produced by salivary glands, moistens food to aid swallowing and contains the enzyme amylase, which starts chemical digestion of starch.

• Food enters the mouth during ingestion.
• Chewing mechanically breaks down food.
• Saliva moistens food and contains amylase for starch digestion.

Digestion involves both mechanical and chemical processes that convert large, complex food molecules into smaller, absorbable units. Mechanical digestion continues in the stomach through muscular contractions that churn food, mixing it with gastric juices. Chemical digestion uses enzymes to break down carbohydrates, proteins, and lipids into their monomers: sugars, amino acids, and fatty acids respectively.

For example, in the stomach, the enzyme pepsin breaks down proteins into smaller peptides. In the small intestine, pancreatic enzymes further digest carbohydrates, proteins, and fats. Bile from the liver emulsifies fats, increasing the surface area for lipase to act efficiently.

• Mechanical digestion includes chewing and stomach churning.
• Chemical digestion uses enzymes to break down macromolecules.
• Bile emulsifies fats to aid lipase action.

Absorption mainly occurs in the small intestine, especially the ileum, where digested nutrients pass through the intestinal wall into the bloodstream. The small intestine has a large surface area due to villi and microvilli, which are finger-like projections that increase absorption efficiency.

For example, glucose molecules produced from starch digestion are absorbed by active transport into blood capillaries within villi. Fatty acids and glycerol enter lacteals (lymph vessels) before eventually reaching the bloodstream.

• Absorption occurs primarily in the small intestine.
• Villi and microvilli increase surface area for absorption.
• Nutrients enter blood capillaries or lymph vessels.

Assimilation is the process where absorbed nutrients are taken up by body cells and used for energy, growth, and repair. For instance, glucose absorbed into the blood is transported to muscle cells, where it is used in respiration to release energy.

Proteins provide amino acids for building new proteins in cells, such as enzymes and structural components. Lipids are used in cell membranes or stored as energy reserves. Assimilation ensures that nutrients are incorporated into the body's metabolism and structure.

• Assimilation involves nutrient uptake by cells.
• Glucose is used in cellular respiration for energy.
• Amino acids build new proteins in cells.
• Lipids contribute to membranes and energy storage.

Egestion is the final stage of the digestive process, involving the removal of undigested food as faeces. The large intestine absorbs water from the remaining material, forming solid faeces.

For example, dietary fibre cannot be digested and passes through the large intestine to the rectum. When the rectum is full, faeces are expelled through the anus during defecation. This process prevents the build-up of waste and maintains digestive health.

• Egestion removes undigested food as faeces.
• Large intestine absorbs water to form faeces.
• Faeces are stored in the rectum and expelled via the anus.