HUMAN NUTRITION

• Mammalian digestive systems


  • Advanced and complex
  • Various nutritional requirements to support biological functions
• Orders of mammals


  • Primata
  • Proboscidea
  • Pinnipedia
  • Cetacea
• Digestive system


  1. Ingestion

  2. Digestion

  3. Secretion

  4. Absorption

  
• Ingestion


  The process of taking in food usually at the point where the gastrointestinal tract begins
• Digestion


  The process of breaking down the ingested food into smaller molecules for an efficient absorption of nutrients
• Digestion


  • Mechanical digestion - physical breakdown of food molecules, increases surface area
  • Chemical digestion - utilizes enzymes specific for food molecules, makes absorption more efficient
• Secretion


  Digestive organs synthesize and release enzymes that facilitate chemical digestion
• Absorption


  The ingested food is completely digested for transport to other tissues of the body
• Capillaries


  Usually transport water-soluble nutrients
• Lacteals


  Usually in-charge of glycerol and fatty acids
• Human digestive system


  1. Mouth cavity

  2. Pharynx

  3. Epiglottis

  4. Esophagus

  5. Stomach

  6. Small intestine

  7. Large intestine

  8. Rectum & anus
• Mouth cavity


  • Site for both mechanical and chemical digestion
  • Hard and soft palates separate oral and nasal cavities
  • Tongue is a muscular organ for tactility and gustation
  • Teeth physically break down food
• Pharynx


  • Laryngopharynx is the passageway for food
  • Nasopharynx is the passageway for air
  • Oropharynx is a food and air passage
• Epiglottis


  A flap of tissue found in the throat behind the tongue, moves down during deglutition to block the trachea and prevent entry of food into the respiratory tract
• Esophagus


  • A long tube that connects the mouth with the stomach
  • Sphincters prevent food backflow
  • Peristalsis pushes food
• Stomach


  • A hollow, curved, muscular organ that serves as a site for both storage and digestion of food
  • Has rugae that allow stretching to accommodate food
  • Consists of the cardia, fundus, and pylorus
• Gastric secretions


  • Gastric amylase - digests complex carbohydrates
  • Gastric lipase - initiates digestion of dietary fats
  • Hydrochloric acid - kills bacteria, activates pepsinogen
  • Pepsin - initiates protein digestion
• Small intestine

  • 20 foot-long, site of most chemical digestion
  • Duodenum is the site of most chemical digestion, receives enzymes from liver and pancreas
  • Jejunum and ileum are sites of digestion and nutrient absorption

  
• Duodenal secretions


  • Maltase - digests maltose
  • Lactase - digests lactose
  • Sucrase - digests sucrose
  • Peptidase - digests peptide chains
  • Nucleosidase - digests nucleotides
• Large intestine


  • 3 foot-long, little to no digestive function
  • Site of absorption of vitamins and water
  • Primary secretion is mucus
  • Has crypts similar to small intestine
• Rectum and anus


  • Rectum is the storage site for feces before defecation, can also absorb water
  • Anal columns and sphincters help regulate defecation
• Accessory organs

  • Salivary glands - secrete saliva that lubricates and moistens food, contain amylase
  • Liver - synthesizes bile
  • Gallbladder - stores and regulates bile secretion
  • Pancreas - releases pancreatic juice with amylase, trypsin, and lipase
• Functions of accessory organ secretions

  • Bile - responsible for fat emulsification
  • Amylase - responsible for starch digestion
  • Trypsin - responsible for protein digestion
  • Lipase - responsible for lipid digestion
  • Nuclease - performs nucleic acid digestion
• Ruminant digestive system


  1. Ingestion of cellulose-rich food

  2. Mechanical digestion and exposure to cellulase in rumen

  3. Reticulum helps in regurgitation

  4. Omasum performs further mechanical digestion

  5. Abomasum, the true stomach, performs final digestion
• Ruminant animals


  • Hog deer
  • Cattle
  • Sheep
  • Camel
• Fish digestive system

  1. Bony fishes engulf water and gill rakers trap particles

  2. Food moves to esophagus and stomach

  3. Pylorus leads to poorly-understood pyloric cecum

  4. Pancreas secretes proteases, lipases, and carbohydrases

  5. Nutrient absorption in intestines
• Avian digestive system


  1. Bird does not chew food during ingestion

  2. Food moves to crop where it can be regurgitated

  3. Proventriculus facilitates chemical digestion

  4. Gizzard facilitates mechanical digestion

  5. Food wastes and urine have common exit point - the cloaca
• Examples of birds


  • Hummingbirds
  • Penguins
• Parent birds are able to gather food from distances and store them in their crop for regurgitation upon return to their nests