Chapter 5: Digestion in humans

• Nutrition is the process by which organisms obtain food and energy for growth, repair and maintenance of the body
• Nutrition


  1. Feeding or ingestion

  2. Digestion

  3. Absorption

  4. Assimilation

  5. Egestion
• Feeding or ingestion


  Food is taken into the body
• Digestion


  Large food molecules are broken down into smaller, soluble molecules that can be absorbed into the body cells
• Absorption


  Nutrients move from the small intestine into the bloodstream
• Assimilation


  Nutrients are used by cells to provide energy or to make new cytoplasm for growth
• Egestion


  Undigested matter is removed from the body
• Food needs to be digested
• Human digestive system


  • Mouth
  • Salivary glands
  • Oesophagus
  • Stomach
  • Duodenum
  • Pancreas
  • Gall bladder
  • Liver
  • Ileum
  • Colon
  • Rectum
  • Anus
• Peristalsis


  Rhythmic wave-like contractions of the muscles to mix and propel the contents of the alimentary canal
• People choke when they talk while eating because the epiglottis cannot fully close to prevent food from reaching the larynx and trachea
• Pharynx


  • Connects the buccal cavity to the oesophagus, larynx (voice-box) and trachea (windpipe)
  • The glottis is a slit located at the larynx
  • Both food and air pass through the pharynx
  • Air passes through the trachea, while food passes through the oesophagus
  • The epiglottis closes over the glottis to prevent food from reaching the larynx and trachea when swallowing
• Oesophagus


  • Narrow, muscular tube that passes through the thorax (chest) and diaphragm
  • Connects the mouth and stomach
  • Consists of a layer of longitudinal muscles and a layer of circular muscles
• Stomach


  • Lies beneath the diaphragm, to the left side of the abdomen, and is partly covered by the liver
  • Has numerous pits and gastric glands found in the stomach lining
  • Has gastric glands which secrete gastric juice that play an important part in digestion
• Small intestine


  • Consists of a U-shaped duodenum and the much-coiled ileum
  • Carries out most of the digestive processes
  • Contains glands, in the wall lining, that secrete digestive enzymes
  • Enzymes in the small intestine speed up the digestion of food
  • Absorbs water and nutrients from food, which usually takes place in the ileum
• Large intestine


  • Is shorter and broader than the small intestine
  • Consists of the colon, rectum and anus
  • Absorbs remaining water and mineral salts from undigested food
  • Temporarily stores faeces in the rectum
• Liver


  • Largest gland in the body, located below the diaphragm
  • Three blood vessels are attached to the lower surface of the liver: the hepatic portal vein, the hepatic vein, and the hepatic artery
  • Liver cells produce and secrete bile, which contain bile salts that aid in the emulsification of fats
• Gall bladder


  • Attached to the liver
  • Bile, produced by the liver, is stored temporarily in the gall bladder
  • When the gall bladder contracts, bile flows via the bile duct into the duodenum, which is part of the small intestine
• Pancreas


  • Connected to the duodenum by the pancreatic duct
  • Produces pancreatic juice, which contains digestive enzymes such as amylase, lipase and protease
  • Secretes the hormones insulin and glucagon, which help regulate the blood sugar concentration in the body
• Physical digestion increases the surface area-to-volume ratio of ingested food so that digestive enzymes can act of the food particles more effectively
• Digestion in the mouth


  1. Salivary glands secrete saliva

  2. Saliva mixes with the food and softens it

  3. Salivary amylase in saliva digests starch to maltose

  4. Optimum pH for salivary amylase is at pH 7

  5. Food is broken down into smaller pieces by chewing

  6. Food is rolled into small round masses called boli

  7. Boli are swallowed and passed down into the oesophagus via the pharynx
• Digestion in the stomach


  1. Gastric glands secrete gastric juice

  2. Peristalsis churns and breaks up the food and mixes the food with gastric juice

  3. Gastric juice contains hydrochloric acid, mucus and protease

  4. Hydrochloric acid stops the action of salivary amylase, provides a low pH environment for protease, and kills microorganisms

  5. Protease digests proteins into polypeptides

  6. Food remains in the stomach for 3-4 hours, becoming partially digested substance called chyme

  7. Chyme passes into the duodenum in small amounts
• Digestion in the small intestine


  1. Chyme entering the duodenum stimulates the pancreas to secrete pancreatic juice

  2. Pancreatic juice contains the enzymes pancreatic amylase, protease and pancreatic lipase

  3. Gall bladder releases bile into the duodenum, which speeds up the digestion of fats

  4. Small intestines produce enzymes maltase, protease and lipase

  5. Acidic chyme from the stomach is neutralised by the alkaline fluids from the pancreas, gall bladder and small intestine, achieving a pH of 8
• Carbohydrate digestion


  • Examples of carbohydrates include starch, sucrose and cellulose
  • Carbohydrates are digested by carbohydrases such as amylase and maltase
  • Starch is broken down into maltose by amylase
• Enzymes are found in gastric/intestinal juice and they aid in digestion
• Hydrochloric acid in the stomach stops the action of salivary amylase by denaturing it
• Hydrochloric acid in the stomach provides a low pH environment for protease to digest proteins
• Hydrochloric acid in the stomach kills microorganisms in food
• Protease in the stomach digests proteins into polypeptides
• Pancreatic juice contains the enzymes pancreatic amylase, protease and pancreatic lipase
• Bile from the gall bladder speeds up the digestion of fats by breaking up large fat molecules into smaller fat molecules
• The acidic chyme from the stomach is neutralised by the alkaline fluids from the pancreas, gall bladder and small intestine, achieving a pH of 8 which is suitable for digestion in the small intestine
• Gall bladder


  Releases bile (produced by the liver and stored in the gall bladder) into the duodenum via the bile duct
• Bile does not contain enzymes, so it cannot digest food
• Bile


  Speeds up the digestion of fats by breaking up large fat molecules into smaller fat molecules
• Small intestine


  Produces enzymes maltase, protease and lipase
• Acidic chyme from the stomach


  Is neutralised by the alkaline fluids from the pancreas, gall bladder and small intestine, thus achieving a pH of 8, which is suitable for digestion in the small intestine
• Carbohydrates


  Examples include starch, sucrose and cellulose
• Carbohydrate digestion

  1. Carbohydrates are digested by carbohydrases such as amylase and maltase

  2. Starch is broken down into maltose by amylase in the mouth and small intestine

  3. Maltose is broken down into glucose by maltase in the small intestine
• Protein digestion


  1. Proteins are digested by proteases

  2. Protein digestion starts in the stomach, where stomach protease digests proteins to polypeptides

  3. The undigested proteins that enter the small intestine are digested by intestinal protease to polypeptides

  4. The polypeptides produced are further digested to amino acids by intestinal protease