Nutrition in Humans

• Parts of the Alimentary Canal


  Mouth, Oesophagus, Stomach, Small Intestine (Duodenum, Jejunum, Ileum), Large Intestine, Rectum, Anus
• Digestion in the Mouth

  1. Chemical Digestion - Amylase on Starch

  2. Physical Digestion - Teeth cut and grind food into smaller pieces for more effective digestion
• Digestion in the Stomach

  1. Chemical Digestion - Gastric Juice from gastric glands (pepsin on proteins to polypeptides)

  2. Physical Digestion - Stomach contractions
• Digestion in Small Intestine

  1. Chemical Digestion

  - Pancreatic Juice from Pancreas (trypsin on proteins to polypeptides + amylase on starch to maltose + lipase on fats to fatty acids and glycerol)

  - Intestinal Enzymes from Epithelial Cells (sucrase on sucrose to glucose and fructose + lactase on lactose to glucose and galactose + lipase on fats to fatty acids and glycerol + peptidases on polypeptides to amino acids + maltase on maltose to glucose)

  
  

  2. Physical Digestion

  - Bile from Liver (gall bladder), bile on fats emulsifies, smaller fat globules
• Absorption


  The process whereby digested food substances are absorbed into the body cells.
• Peristalsis


  The rhythmic, wave-like muscular contractions in the wall of the alimentary canal.
• Assimilation


  The process whereby some of the absorbed food substances are converted into new protoplasm or used to provide energy.
• Functions of the Liver

  1. Regulation of blood glucose concentration

  - Too much glucose, insulin from pancreas is secreted, liver converts glucose to glycogen, blood glucose concentration decreases.

  - Too little glucose, glucagon from pancreas is secreted, liver converts glycogen to glucose, blood glucose concentration increases.

  [ Insulin + Glucagon from Pancreas, Islets of Langerhans ]

  
  

  2. Production of bile

  
  

  3. Iron storage

  - RBC destroyed in spleen, haemoglobin transported to liver, liver breaks it down and stores iron that is released.

  
  

  4. Protein synthesis

  - Liver synthesises proteins found in blood plasma such as prothrombin and fibrinogen

  
  

  5. Deamination of amino acids

  - Excess amino acids brought to liver, amino groups removed and converted to urea through deamination.

  - Carbon residue from amino acid is converted into glucose and then into glycogen for storage.

  
  

  6. Detoxification

  - Harmful substances made harmless in liver

  - Alcohol broken down in the liver
• Effects of Excessive Alcohol Consumption

  1. Digestive Effects

  - Cirrhosis: liver cells are destroyed and replaced with fibrous tissue, liver less able to function, bleeding in liver, liver failure, death.

  
  

  2. Nervous Systemic Effects

  - Depressant: Slows down brain functions

  - Reduced self-control: takes away personal and social liberties,

  - Reduced reaction times: slurred speech, blurred vision, poor muscle coordination, may cause traffic accidents

  
  

  3. Social implications

  - Alcohol addiction

  - Neglect work and/or families

  - Violent behaviour

  - Criminal activity
• The Hepatic Portal vein


  Transports blood from the Small Intestine to the Liver
• Intestinal villi


  Finger-like projections that line the walls of the small intestine
• Villi increase the surface area of the wall of the small intestine to maximize the rate of absorption of digested food
• Flat line between points A and B


  Material has to transfer across it
• Measuring the flat line between points A and B gives a length of 10 centimeters
• Line between points A and B with finger-like projections


  Surface area is far longer than 10 centimeters
• Larger surface area means more material can be transferred in the same amount of time
• Food molecule transfer in the small intestine

  1. Transferred from central space into the villi

  2. Epithelial cells of villi lining have many microvilli

  3. Transferred to blood capillaries

  4. Lipid digestion products absorbed into lacteal
• Blood supply constantly flowing in and out through many blood capillaries in the villi
• Most materials are transferred to the blood capillaries
• Lipid digestion products are absorbed into the lacteal and eventually reach the bloodstream at a different point