Human nutrition

• Parts of the human digestive system

  • salivary duct
  • tongue
  • salivary gland
  • epiglottis
  • liver
  • gall bladder
  • bile duct
  • duodenum
  • caecum
  • appendix
  • salivary gland
  • buccal cavity (mouth cavity)
  • pharynx
  • oesophagus
  • diaphragm
  • stomach
  • spleen
  • pyloric sphincter
  • pancreas
  • pancreatic duct
  • colon
  • jejunum
  • ileum
  • rectum
  • anus
• Human digestion takes place in the mouth, stomach and small intestine
• Parts of the alimentary canal

  • mouth
  • oesophagus
  • stomach
  • small and large intestines
  • anus
• Other organs associated with digestion

  • liver
  • pancreas
  • gall bladder
  • salivary glands
• Buccal cavity (mouth cavity)


  Where food enters the body
• Physical and chemical digestion in the mouth

  1. Teeth break food into smaller pieces

  2. Salivary glands secrete saliva which moistens food and contains salivary amylase

  3. Tongue rolls food into a bolus which is then swallowed
• Salivary amylase


  Enzyme that breaks down starch into maltose, optimum pH is 7
• Water absorption in small intestine

  1. Passive diffusion throughout the length of the small intestine

  2. Mineral salts absorbed in the ileum
• Food leaving small intestine

  Enters the large intestine
• Large intestine


  Shaped like an inverted U and has the function of absorbing the remaining water and mineral salts that have not been absorbed by the small intestine
• Oesophagus


  Muscular tube that leads to the stomach, made up of longitudinal and circular smooth muscle layers that contract and relax to cause peristalsis
• Most of the water that was present in the small intestine had been absorbed by the small intestine
• Digestion of starch by salivary amylase continues in the oesophagus
• Movement of undigested waste matter in large intestine

  Moves along the large intestine by peristalsis, getting progressively drier
• Stomach


  Muscular bag with elastic walls, contains gastric glands that secrete mucus, gastric acid, and pepsinogen
• Digestion in the stomach

  1. Peristalsis churns the food to break it up and mix it with gastric juice

  2. Gastric acid stops salivary amylase, activates pepsin, and kills germs/bacteria

  3. Pepsin is a protease with optimum pH of 2
• Undigested waste matter comprises mainly cellulose, which is indigestible to humans
• Elimination of waste material

  Waste matter ends up at the rectum where it is stored before it can be eliminated from the body through the anus
• Chyme


  Semi-liquid mass of food that leaves the stomach and enters the small intestine through the pyloric sphincter
• The elimination of waste material is called egestion
• Parts of the small intestine

  • duodenum
  • jejunum
  • ileum
• Transport of products of digestion

  1. Blood in the capillaries of the villi becomes very rich in simple sugars and amino acids

  2. Blood capillaries of the villi converge into the hepatic portal vein, which leads to the liver

  3. Blood from the small intestine travels to the liver via the hepatic portal vein
• Digestion in the small intestine

  1. Chyme mixes with digestive juices from pancreas, liver, gall bladder, and intestinal glands

  2. Pancreatic juice contains trypsinogen, pancreatic amylase, and pancreatic lipase

  3. Intestinal juice contains lipase, enterokinase, erepsin, maltase, lactase, sucrase, and other enzymes

  4. All enzymes in small intestine have optimum pH under alkaline conditions

  5. Bile from liver and gall bladder emulsifies fats to increase surface area for lipases
• The composition of blood in the hepatic portal vein varies greatly throughout the day depending on whether absorption of nutrients is occurring in the small intestine
• Role of the liver in carbohydrate metabolism


  • Involved in carbohydrate metabolism and regulation of blood glucose concentration
  • Converts excess glucose in the blood from the hepatic portal vein into glycogen, which is stored in the liver
  • Converts stored glycogen in the liver back into glucose when blood glucose level is low
• Enzymes involved in carbohydrate digestion in small intestine

  • pancreatic amylase
  • maltase
  • sucrase
  • lactase
• Role of the liver in fat metabolism

  • Produces bile to help fat digestion by emulsifying fats
  • Oxidises fats to produce energy
  • Converts excess carbohydrates and proteins to fatty acids and glycerol which are exported and stored as fatty tissue
• Enzymes involved in fat digestion in small intestine

  • lipase
• Role of the liver in breakdown of red blood cells

  1. Aging red blood cells are removed by the spleen

  2. Haemoglobin from the red blood cells is brought to the liver, where it is broken down
• Enzymes involved in protein digestion in small intestine

  • enterokinase
  • trypsin
  • peptidases / erepsin
• The iron from the haemoglobin is stored in the liver while the other metabolic by-products of the breakdown form bile pigments
• Food is completely digested in the small intestine, jejunum and ileum function mainly to absorb nutrients and water
• Role of the liver in protein metabolism

  • Involved in the synthesis of plasma proteins and blood clotting factors
  • Responsible for the deamination of excess amino acids, converting the amino group into ammonia which is then converted to urea and removed in urine
  • Converts the remnants of the amino acid to glucose
• Adaptations of small intestine for absorption

  • Inner wall with large circular folds
  • Finger-like projections on inner wall called villi
  • Each epithelial cell on villi has smaller projections called microvilli
• Role of the liver in detoxification


  • Breaks down toxic substances for excretion in urine or bile
  • Breaks down alcohol to acetaldehyde and then to harmless acetic acid
• These adaptations increase the surface area of the small intestine, resulting in a larger surface for absorption
• Alcohol irritates oesophageal, stomach and intestinal linings, and excessive alcohol consumption can lead to inflammation and ulcers
• The villi have thin walls (one-cell thick) so that food molecules diffuse over a shorter distance
• Lacteal


  Small vessel within each villus that transports fats away from the small intestine
• Excessive alcohol consumption can also lead to inflammation, scarring and destruction of liver cells, resulting in a disease called cirrhosis of the liver and loss of liver function