Chapter 6 Digestive system

• All cells require nutrients in order to provide energy for cells activities and materials for cell growth, reproduction, secretion and other metabolic processes
• Digestive system


  Extracts nutrients from food we eat and absorbs them into the body for use by cells
• Digestive system functions


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


  Chemicals break down large, complicated molecules into smaller, simpler molecules that are small enough to be absorbed in the bloodstream
• Products of chemical digestion


  • Monosaccharides (glucose, fructose, galactose)
  • Peptides and amino acids
  • Fatty acids and glycerol
  • Nucleotides
• Enzymes


  Biological catalysts that are able to increase the rate of reaction without being consumed
• Body cells require simple sugars, amino acids, fatty acids, vitamins, minerals and water to function normally
• Mechanical digestion


  Physical breakdown of food particles in the mouth, stomach and small intestine
• Mechanical digestion processes


  • Teeth cut, grind, tear food
  • Churning action in stomach breaks food down further
  • Gall bladder releases bile into small intestine, bile salts act as emulsifying agents, breaking fat down into smaller droplets
• Alimentary canal


  Continuous tube that runs from the mouth to the anus, makes up the digestive system along with associated organs
• Indigestion


  Intake of food, occurs in the mouth
• Mastication


  Process where food is chewed in the mouth
• Saliva


  Fluid secreted into mouth cavity by three pairs of salivary glands, contains mucus and the digestive enzyme salivary amylase
• Types of teeth


  • Incisors
  • Canines
  • Premolars
  • Molars
• Bolus


  Rounded lump of chewed food
• Oesophagus


  Tube that connects pharynx to stomach, has double layer of muscle that enables peristalsis
• Stomach


  Enlarged section of alimentary canal, mechanical digestion achieved by muscular contractions that churn food and mix it with gastric juice
• Gastric juice


  Digestive juice containing hydrochloric acid, mucus and digestive enzymes
• Pyloric sphincter


  Constriction at lower end of stomach that prevents stomach contents moving through unless pushed along by peristalsis
• Small intestine


  Longest part of alimentary canal, approx. 6-7m long, where most chemical digestion and absorption of nutrients occurs
• Regions of small intestine

  • Duodenum
  • Jejunum
  • Ileum
• Pancreatic juice


  Secreted by pancreas, helps neutralise acid and contains enzymes for digestion of carbohydrates, proteins and lipids
• Bile


  Secreted by liver, does not contain digestive enzymes but bile salts that emulsify fats
• Intestinal juice


  Secreted by glands in lining of small intestine, contains enzymes that complete digestion of carbohydrates, proteins and lipids
• Segmentation


  Contractions of circular muscles in small intestine that help break up the bolus and mix it with digestive juices
• Absorption of nutrients


  Products of digestion, along with vitamins, minerals and water, are absorbed through the wall of the small intestine into the blood and lymph
• Mechanisms of nutrient absorption


  • Simple diffusion
  • Active transport
• Hepatic portal vein


  Carries substances absorbed into blood capillaries in villi to the liver
• Lacteals


  Lymph capillaries in villi that absorb fatty acids, glycerol and fat-soluble vitamins
• Constipation


  Occurs if movements of large intestine is reduced and contents remain there for long period of time, causing faeces to become drier and harder
• Roughage


  Cellulose or insoluble fibre, a major component of plant foods that stimulates movements of alimentary canal
• Bowel cancer


  Uncontrolled growth of cells in wall of large intestine, may be linked to diet, alcohol consumption and smoking
• Diarrhoea


  Frequent defecation of watery faeces, caused by irritation of small or large intestine that increases peristalsis
• Causes of diarrhoea


  • Bacteria
  • Virus
  • Parasite
  • Cancer
  • Coeliac disease
  • Lactose intolerance
• Soluble fibre


  Includes pectin, gums and mucilage, linked to lower cholesterol, decreased risk of heart disease and cancer, and beneficial effects on blood glucose levels
• Coeliac disease


  Inability to tolerate the protein gluten, which damages villi in small intestine and prevents nutrient absorption
• Large internal surface area is achieved in number of way:

  • Small intestine is very long- 6-7m long
  • Inner lining, known as mucosa, has folds that extend into interior of small intestine 
  • Mucosa has small, finger-like projections called villi that extend from folded surface 
  • Cells covering outside of villi have tiny microscopic projections from their external surfaces, called microvilli 
• Each villus is how long?

  1mm long

  although villi in jejunum is longer than in the duodenum and ileum 
• Each villus is covered by

   single layer of cells 
• lacteal

  Inside villus is lymph capillary

  surrounded by network of blood capillaries