Digestive system

• Ingestion


  Eating food through the mouth cavity into the alimentary canal
• Digestion


  Breaking down of large insoluble food molecules to small water soluble molecules using mechanical and chemical processes
• Mechanical/physical digestion


  Chopping and grinding food with teeth result in breaking large complicated insoluble particles into small particles with no chemical change, without using an enzyme to increase the surface area
• Chemical digestion


  Breaking down large particles into small simple soluble units using an enzyme so that digested food will be absorbed by the wall of small intestine
• Absorption


  Uptake of small simple units and ions such as glucose, amino acid and sodium ions by the wall of small intestine to be passed into blood through capillaries and fatty acid and glycerol through lacteals
• Assimilation


  Uptake and use of food molecules by cells
• Egestion


  Removal of undigested food in the form of faeces out of our body through anus
• Parts of alimentary canal (digestive system) and their role

  • Mouth (mechanically digest food, secrete saliva with amylase enzyme)
  • Esophagus (muscular tube that uses peristalsis to push food bolus to stomach)
  • Stomach (churns food, secretes gastric juice with pepsin and HCl)
  • Duodenum (receives food from stomach, receives bile and pancreatic juice)
  • Small intestine (completes digestion and absorbs nutrients)
  • Large intestine (absorbs water, stores undigested waste)
• Mouth


  • Mechanically digest the food by cutting, chewing and grinding
  • Secrete saliva which contain amylase enzyme to digest starch into maltose at optimum temperature 37°C and pH 7.5
  • Roll the food into balls and push them into the back of the cavity for swallowing
• Esophagus


  • Muscular tube down which waves of muscle contraction followed by relaxation (peristalsis) pushing each food bolus towards the stomach
• Peristalsis


  A wave of contraction followed by a wave of relaxation that moves food through the digestive tract
• Dietary fibers are mainly needed to keep the digestive system healthy
• Stomach


  • Muscular bag that churns the food up to around 4 hours
  • Wall secretes gastric juice from gastric glands which contains inactive pepsinogen, hydrochloric acid (pH 2), and mucus
  • HCl activates pepsinogen to pepsin which breaks down proteins into amino acids
  • HCl provides the correct pH for pepsin to work
  • HCl kills potentially harmful bacteria in the food
  • Mucus protects the wall of the stomach from HCl and from being digested by pepsin
• Cardiac sphincter


  • Circular muscle that joins between esophagus and stomach
  • Relaxes and opens to allow food bolus to enter the stomach
  • Contracts as food passes to prevent backflow of stomach contents into esophagus
• Pyloric sphincter


  • Ring of muscle that relaxes to open and allow food to enter the duodenum
• Duodenum


  • Small C-shaped tube that receives food from the stomach, bile juice from the liver, and pancreatic juice from the pancreas
• Bile juice


  • Produced by the liver, stored in the gall bladder, and passed to the duodenum via the bile duct
  • Contains bile pigments from breakdown of red blood cells
  • Contains bile salts used for fat emulsification and neutralization of stomach acid
• Pancreatic juice


  • Produced by the pancreas and passed to the duodenum via the pancreatic duct
  • Contains digestive enzymes like amylase, protease, and lipase
  • Contains sodium hydrogen carbonate to provide an alkaline environment for enzyme activity and neutralize acidic food from the stomach
• Liver


  • Largest internal organ, not part of the alimentary canal but closely associated with digestion
  • Produces bile juice
  • Performs deamination of excess amino acids, assimilation of digested food, storage of iron and vitamins, breakdown of hormones, and detoxification
• Pancreas


  • Lies between the stomach and duodenum, not part of the alimentary canal
  • Secretes hormones like insulin and glucagon
  • Secretes pancreatic juice that passes to the duodenum for digestion of food
• Small intestine


  • Very long (5-6 meters)
  • Inner surface covered with villi and microvilli to increase surface area
  • Contains blood capillaries and lacteals to absorb digested nutrients
  • Completes digestion and absorption of nutrients
• Large food molecules are almost ready for absorption but first maltose must be changed to glucose (by maltase enzyme in intestine juice secreted by epithelial lining)
• Fibers remain undigested to add bulk to the food and maintain peristalsis
• Absorbed particles


  • Glucose
  • Amino acids
  • Fatty acids and glycerol
  • Minerals
• Unabsorbed particles


  • Fibers
  • Vitamins
• Importance of many blood capillaries and lacteals around the epithelial lining

  • In contact with more villi so more digested food can be absorbed
  • Remove the absorbed food as blood flows in capillaries
  • Increase the diffusion gradient
  • Allow more food to be reabsorbed
• Importance of water


  • Dissolve food particles (mechanical digestion)
  • Activate enzymes for chemical digestion
  • Help in peristalsis
  • Help in absorption of digested food
  • Plasma mainly consists of water that allows transporting of digested food
• Maltose


  Must be changed to glucose (by maltase enzyme in intestine juice secreted by epithelial lining)
• Fibers


  Remain undigested to add bulk to the food and maintain peristalsis
• Absorbed particles


  • Starch
  • Protein
  • Fats
  • Minerals
• Absorbed


  • Glucose
  • Amino acids
  • Fatty acid and glycerol
  • Minerals
  • Fibers (undigested)
  • Vitamins
• Not absorbed


  • Vitamins
• Importance of many blood capillaries and lacteals around the epithelial lining

  • In contact to more villus so more digested food can be absorbed
  • Removed the absorbed food as blood flows in capillaries
  • Increase the diffusion gradient
  • More food can be reabsorbed by many capillaries and lacteals
• Importance of water


  • Dissolve food particles (mechanical digestion)
  • Activate enzymes for chemical digestion
  • Helps in peristalsis
  • Helps in Absorption of digested food
• Plasma


  Mainly consists of water that allows the transporting of digested food
• Large Intestines


  • Undigested food pass through it (90% fibers) as its main function
  • Absorption of water from undigested food to be carried by the blood
  • Then the undigested food will be stored in rectum and egested through anus
• Assimilation


  1. The main food substance, absorbed as small soluble molecules, which will be needed by the body

  2. All capillaries are collected forming hepatic portal vein transport blood with all digested food from small intestine to liver where assimilation takes place

  3. After meal the blood in the vein contains vey high concentration of glucose, amino acids, vitamins, minerals and carbon dioxide
• Liver assimilation of glucose

  1. Glucose may be used as it is a substrate for respiration, to release energy

  2. Liver use some glucose for respiration

  3. Then all body parts will use some glucose for respiration

  4. The Excess is needed to be stored. Hence, it is built up into large insoluble molecule called glycogen. It is stored in the cells of liver and muscle under the effect of insulin hormone from the pancreas

  5. When the blood sugar concentration fails below the required level (such as during fasting) glycogen can be converted again to glucose by the effect of glucagon hormone (released from the pancreas) and released into blood
• Liver assimilation of amino acids

  1. Amino acids will be used by liver to form proteins for tissue repair and its enzyme synthesis

  2. Then to all body parts as amino acids are needed for building up protein growth and tissue repair, enzymes and hormones, hair and nails, blood protein (fibrinogen)

  3. Amino acids are never stored as any excess to our needs are broken down by deamination as toxic urea made in liver
• Deamination


  The removal of the nitrogen-containing part of amino acid in excess to our need and be changed to urea while the remaining part of amino acid is used to release energy