Digestive system

• Functions:

  • Ingestion of food and water
  • Mechanical digestion of food
  • Chemical digestion food
  • Movement of food along the alimentary canal
  • Absorption of digested food and water into the blood and lymph
  • Elimination of material that is not absorbed.
• The Alimentary Canal:

  • The continuous tube that runs from the mouth to the anus
  • Together with associated organs like the pancreas and the gall bladder, the alimentary canal makes up the digestive system.
  • The digestive system extracts nutrients from the food we eat and absorbs them into the body for use by the cells.
• The Mouth

  • Food is formed by the tongue into a rounded lump (bolus)
  • To swallow, the tongue pushes the bolus into the back of the mouth.
• Oesophagus

  • Double layer of muscle: circular and longitudinal muscle
  • Food enters the oesophagus and the circular muscle contracts to form constriction.
  • Successive bands of circular muscle contract forming a wave that pushes the food down known as peristalsis
  • Movement of food is lubricated by mucus.
• The Stomach

  • Three layers of muscle: circular, longitudinal and oblique
  • Enables the stomach to contact in a variety of ways to churn the food and mix it to form a soupy liquid.
  • At the lower end of the stomach there is a thickening of muscle called the Pyloric sphincter.
  • This sphincter opens to let stomach connects pass through to the small intestine.
• Small Intestine

  • 6m long
  • Duodenum - jejunum - ileum
  • Contains Villi
• Small Intestine - Duodenum

  • First 15cm
  • Bile duct and pancreatic duct are connected
  • C-shaped
• Small Intestine - Jejunum and Ileum

  • Contains glands
• Villi

  • Finger-like projections
  • Increase the surface area of the small intestine to ensure effective absorption of nutrients.
• Liver

  • Produce bile for fat  emulsification
  • Bile is stored in the gallbladder and carried to the duodenum via common bile duct.
• Pancreas

  • Lies between stomach and duodenum
  • Secretes pancreatic juice (containing amylase, protease and lipases) into the duodenum via the common bile duct.
  • Also controls blood insulin levels
• Large Intestine

  • 1.5m long
  • No villi
  • Large amount of mucus.
  • Movement is slow and water is absorbed so contents become solid.
  • Bacteria in the large intestine break down majority of the remaining organic compounds.
  • Caecum -> ascending colon -> transverse colon -> descending colon -> rectum -> anus  
  • The appendix is attached to the caecum. (has 0 functions in human)
• Absorption:

  • The products of digestion are absorbed through the wall of the small intestine into the blood.
  • Efficient absorption requires a large surface area. This happens by

  1. Length (6m long)
  2. Folded inner lining called mucosa
  3. Villi extends from the folded mucosa
  4. Microvilli projecting from the villi.
• Absorption:

  • Inside the villi are small lymph capillaries called lacteals surrounded by a network of blood capillaries.
  • Nutrients are absorbed either by simple diffusion or active transport.
• Absorption into the bloodstream.

  • Glucose and amino acids pass into the blood capillary of the villi
  • From here they go to the live in the hepatic portal vein and continue in the circulatory system to the body.
• Absorption into Lymph

  • The fatty acids and glycerol go into the lymphatic vessels in the villi called lacteals.
  • Lymph drains into the lymphatic vessels (carrying the fatty acids and glycerol with it) before joining the blood again in the superior vena cava.
• Faeces

  • The semi-solid material left after water absorption and bacterial action makes up faeces
  • Contain water, undigested food material, bacteria, bile pigments (giving their colour) and dead cells of the alimentary canal.
• Defecation/Elimination

  • Around the anus is a circular muscle called the anal sphincter.
  • As the walls of the rectum are stretched by faeces, the anal sphincter relaxes and faeces are expelled.
• • Constipation - happens if the movements of the large intestine are reduced and the contents stay there for a long time.
  • As water is absorbed, the faeces become drier and harder than usual.
  • Causes: Lack of roughage (cellulose or insoluble fibre) in the diet.
• • Diarrhea -  Watery faeces.
  • Caused by irritation of the small or large intestine.
  • Increases peristalsis
  • Content moves through before there is absorption of water.
• • Bowel Cancer - uncontrolled growth in cells in the wall of the large intestine.
  • May be linked to diet, lots of alcohol and smoking.
• • Coeliac disease - unable to tolerate gluten
  • If gluten is eaten - immune system responds by damaging the villi in the small intestine.
  • Without healthy villi, nutrients cannot be absorbed and a person becomes malnourished.
  • Symptoms: muscle cramps, joint pain, tingling in legs.
  • Is inherited
  • No cure - only treatment is a gluten-free diet.