The Digestive System: Functions and Mechanisms

Study Notes

The main functions of the digestive system are to ingest, break down eaten material into nutrient molecules, and absorb the nutrients, water, and ions from our food.
The digestive system also eliminates the wastes (anything not absorbed) as feces.

Ingestion: Food must be placed into the mouth before it can be acted on, which is an active, voluntary process.
Propulsion: Food is propelled from one organ to the next through peristalsis (involuntary, alternating waves of contraction and relaxation of the muscles in the organ wall).
Food Breakdown: Mechanical Digestion: Mechanical digestion prepares food for further degradation by enzymes by physically fragmenting the foods into smaller pieces.
- Examples of mechanical digestion include:
  - Mixing of food in the mouth by the tongue (mastication or chewing)
  - Churning of food in the stomach
  - Segmentation in the small intestine
- Segmentation: Activates circular muscles in the intestines that contract to move food back and forth, mixing it with gastric juices and helping break it down into smaller pieces for digestion.
Food Breakdown: Chemical Digestion: Chemical digestion involves breaking down large food molecules into their building blocks by enzymes.
- Proteins are broken down into amino acids.
- Carbohydrates are broken down into simple sugars.
- Fats are broken down into molecules like fatty acids and cholesterol.
Absorption: Transport of digested end products from the lumen of the GI tract to the blood or lymph occurs through active or passive transport processes.
Defecation: Elimination of indigestible residues from the GI tract via the anus in the form of feces.

The organs of the digestive system can be separated into two main groups: those forming the alimentary canal and the accessory digestive organs.
The alimentary canal is a continuous, hollow muscular tube that winds through the ventral body cavity and is open at both ends, also known as the gastrointestinal (GI) tract.