digestive system

• the main function of the digestive system is to digest and absorb nutrients in food, and to eliminate wastes
• food gives us the nutrients that we need to move, grow and repair our bodies
• some important nutrients from food:

  • carbohydrates
  • proteins
  • lipids (aka fats)
• most of the nutrients we consume are too large and complex for our bodies to absorb, so the digestive system breaks down these molecules into simple, soluble chemicals that can be absorbed and used by our cells
• the two types of digestion:

  • mechanical digestion
  • chemical digestion
• mechanical digestion:

  • involves the physical breakdown of food into smaller pieces
  • increases surface area of food particles (increases effectiveness of digestive chemicals)
  • food is NOT chemically changed
  • e.g. using teeth to chew on a food to break it into smaller pieces
• chemical digestion:

  • special chemicals (enzymes) chemically break down molecules in food into different, simpler molecules that the body is able to absorb
  • e.g. enzymes in saliva begin to break down complex carbohydrates found in bread (starch) into the simpler and smaller molecule (glucose)
• organs of the digestive system:

  • mouth
  • oesophagus
  • stomach
  • pancreas (side attachment)
  • liver (side attachment)
  • gall bladder (side attachment)
  • small intestine
  • large intestine
• digestion occurs all along the digestive tract (the tube starting at mouth and ending at the anus)

  there are multiple organs that are attached that assist with the process of digestion
• the mouth's role in digestion:

  • digestion begins here
  • both mechanical and chemical digestion occurs here
  • mechanical: involves action of teeth
  • chemical : enzyme called amylase found in saliva helps break down complex carbs into glucose
  • saliva: produced by salivary glands, contains h20, mucus and amylase, helping lubricate food
  • tongue: taste buds determine if food is ok to eat, helps roll chewed food into a ball (bolus) and then pushes it down into oesophagus
  • swallowing: the epiglottis (flap of skin) covers the trachea (windpipe)
  • oesophageal sphincter (circular muscle) relaxes to receive food
• role of oesophagus in digestion:

  • is 25-35cm long
  • made up of ring of muscles
  • pushes food into stomach via peristalsis
  • peristalsis: an orderly sequence of wave-like contractions
  • lots of mucus produced for lubrication
  • no enzymes produced
  • once food enters the stomach, a sphincter closes to make sure food doesn't come back up
• stomach's role in digestion:

  • food stays here for 1-6 hours
  • large food storage sack in abdomen
  • capacity: empty> 1/5 cups, full> 8 cups (2L)
  • both mechanical and chemical digestion occurs here
  • mechanical: stomach muscles churn food
  • chemical: gastric juices including enzyme pepsin (breaks down proteins into amino acids) and HCl (kills harmful bacteria and helps digestion)
  • lining of stomach produces digestive enzymes and HCl
  • entrance and exit to stomach is controlled by sphincters
• pancreas' role in digestion:

  • produces pancreatic juices: trypsin (breaks down proteins), lipase (breaks down fats) and amylase (breaks down carbohydrates)
  • produces liquid called alkali (opposite of acid) to help neutralise acid in food as it leaves stomach and enters duodenum (first part of small intestine)
  • produces insulin to control amount of sugar in blood
• liver's role in digestion:

  • largest internal organ (~1.5kg)
  • known as chemical factory
  • produces heat
  • involved in detoxification by breaking down poisons (e.g. alcohol)
  • produces bile (~700-1000mL per day)
  • bile: works as detergent to break down fats into smaller pieces > involved in mechanical digestion, NOT chemical digestion
• gall bladder's role in digestion:

  • ~8cm long
  • stores bile made by liver (~50mL)
• small intestine's role in digestion:

  • the longest part of the digestive tract (~4-6m)
  • called small due to its narrowness (~3-4cm wide)
  • composed of three parts:

  duodenum: stomach + tubes (coming from pancreas, liver, gall bladder) attach to this section

  jejunum: coiled midsection

  ileum: final section that leads into large intestine

  • digested food takes 1-4 hrs to pass through small intestine
  • most of digested nutrients are absorbed here
• the small intestine is lined with many villi to help absorb nutrients more effeciently
• villi are tiny finger-like projections lining the small intestine, and are covered in microvilli. these increase the surface area of the small intestine drastically to allow nutrients to pass into the bloodstream much quicker
• large intestine's role in digestion:

  • bulkiest part of digestive system (~1.5 m long, ~6-7 cm wide)
  • only h20 and a few minerals are absorbed here
  • undigested waste material pass here, and stools are formed to be expelled via the anus
  • ~1/3 of solid part of faeces is made up of intestinal bacteria, which break down fibre and contributes to smell
• it takes ~10 hrs up to a few days for material to pass through the large intestine