Digestive System

Diagram

The five stages of food processing

1. Ingestion, the taking of substances (e.g. food and drink, into the body).
2. Digestion, the breakdown of food. This involves both mechanical breakdown and chemical breakdown by enzymes).
3. Absorption, the movement of nutrients from the intestines into the blood.
4. Assimilation, the uptake and use of nutrients by cells.
5. Egestion, the removal of undigested food from the body as feces.

Structures and functions of the digestive system

Alimentary canal

• Mouth
  • Function: Site of ingestion.
  • Mechanical digestion: Teeth chew and break food into smaller pieces (increasing surface area)
  • Chemical digestion: Saliva containing the enzyme amylase begins breaking down starch into maltose.
• Oesophagus
  • Function: A muscular tube connecting the mouth to the stomach. It moves the bolus downward via peristalsis.
• Stomach
  • Function: A muscular bag where about 1L of food (but can stretch to accommodate more) is held for a few hours.
  • Mechanical digestion: Churns and squeezes food into a liquid called chyme.
  • Chemical digestion: Secretes gastric juice containing protease (pepsin) to break down proteins into peptides. It also secrets hydrochloric acid which kills harmful bacteria and provides the optimum acidic pH (pH 1.5-2.0) for pepsin to work.
• Small intestine, divided into two main parts: duodenum and ileum.
  • Function of duodenum (first part): Main site of chemical digestion. Enzymes from the pancrease and bile from the liver are released here to complete the breakdown of large molecules.
  • Function of ileum (second part): Main site of absorption. Highly adapted for moving digested nutrients (glucose, amino acids, fatty acids) out of the intestine and into the bloodstream. It has a massive surface area covered in microscopic, finger-like projections called villi.
• Large intestine, divided into the colon, rectum, and anus.
  • The colon is responsible for the absorption of remaining water from undigested material, turning liquid waste into semi-solid feces.
  • The rectum stores the feces temporarily until they are ready to be expelled.
  • The anus is a sphincter muscle controlling egestion; it relaxes to remove undigested food from the body as feces.

Associated organs

• Salivary glands secrete saliva into the mouth to lubricate food and release amylase.
• The pancreas secretes pancreatic juice into the duodenum. This juice contains a cocktail of digestive enzymes:
  • amylase for carb. digestion
  • protease/trypsin for protein digestion
  • lipase for fat digestion.
  • NaHCO to neutralize stomach acid.
• The liver produces bile, which neutralizes stomach acid and emulsifies fats. The liver is also heavily involved in assimilation, where it sorts, processes, and stores nutrients arriving from the blood via the hepatic portal vein (e.g. converting excess glucose to glycogen, or deaminating excess amino acids).
• The gall bladder is a small storage sac that stores bile produced by the liver before releasing it into the duodenum via the bile duct.