anatomy and physiology of the digestive system

• Digestion


  The process by which foods are broken down for the body to use in growth, development, healing and prevention of disease
• Absorption


  The process by which digested proteins, fats, carbohydrates, minerals and water are actively and passively transported through the intestinal mucosa into the blood or lymphatic circulation
• Metabolism


  The complex chemical process that occurs in the cells to allow for energy use and for cellular growth and repair
• Nutritional Utilization

  • Digestion
  • Absorption
  • Metabolism
  • Excretion
  • Role of the Liver
• Peristalsis


  The waves of involuntary muscle contractions that transport food and waste matter
• Food bolus

  Food in small pieces and well mixed with saliva
• Food Bolus


  A thick fluid mass of partially digested food and gastric secretions passed from the stomach to the small intestine
• Chyme


  A semiliquid mixture of food
• the mouth (Oral or Buccal Cavity)

  1. Mechanical digestion through mastication (Chewing)

  2. Chemical digestion through the action of amylase (ptyalin) which breaks down starches into maltose

  3. Deglutition (swallowing) once the food is broken down into small pieces and well mixed with saliva (food bolus)
• Esophagus


  • Serves as passage for food bolus from mouth to stomach by peristalsis
  • Distal end is guarded by lower esophageal sphincter (LES) which prevents gastric reflux
• Stomach


  • Located in the left upper quadrant of the abdomen
  • Has a capacity of approximately 1500 ml
  • Has regions: cardiac, fundus, body & antrum or pyloric
  • Has a lesser or greater curvature
• Functions of the Stomach

  1. Mechanical digestion: storage, mixing & liquefaction of bolus of food into chyme

  2. Secretion: 300 ml of gastric juice composed of mucus, HCl, pepsinogen and water

  3. Chemical digestion: digestion of protein starts through pepsin, amylase inactivated by acidity, pepsinogen converted to pepsin by HCl, milk curdled by rennin, emulsified fats digested by gastric lipase

  4. Protection: acid medium reduces activity of harmful bacteria, provides medium for absorption of calcium and other minerals

  5. Absorption: minimal water, alcohol, glucose & some drugs absorbed through gastric mucosa

  6. Control passage of chyme into duodenum through peristaltic waves
• Small Intestine


  • About 6 m long (20 - 22 ft)
  • Extends from pyloric sphincter to ileocecal valve
  • Divided into duodenum, jejunum & ileum
• Functions of the Small Intestine

  1. Mucus secretion

  2. Enzyme secretion

  3. Hormone secretion

  4. Chemical digestion

  5. Absorption

  6. Motor activities
• Large Intestine

  • Approximately 15 m (5 - 6 ft) long
  • Divided into cecum, colon, rectum
  • Vermiform appendix attached to cecum
  • Colon divided into ascending, transverse, descending & sigmoid sections
  • Final segments are rectum & anus with internal & external sphincters
• Functions of the Large Intestine

  1. Motor activities

  2. Secretion

  3. Absorption

  4. Vitamin synthesis

  5. Formation of feces

  6. Defecation
• Common Conditions Affecting the Digestive System

  • Gastrointestinal Reflux Disease (GERD)
  • Celiac Disease
  • Irritable Bowel Syndrome (IBS)
  • Ulcerative Colitis
  • Crohn's Disease
• Function of the stomach

  • mechanical digestion
  • secretion
  • chemical digestion
  • protection
  • absorption
  • control passage of chyme into duodenum
• The stomach has the following regions: cardiac region, fundus, body & antrum or pyloric region.
• Mechanical digestion

  Storage, mixing & liquefaction of bolus of food into a semisolid mixture called chyme

  The rugae liquefy solid food particles
• a.   Chemical digestion

  ü  Digestions of protein starts from the stomach through the action of pepsin, which converts pepsin into polypeptides

  ü  Amylase from the salivary glands is inactivated by the acidity in the stomach so carbohydrates digestions stops

  ü  Pepsinogen is converted into pepsin in the presence of HCL

  ü  Milk is curdled & casein is set free through the action of rennin

  ü  Digestion of emulsified fats also start in the stomach due to the presence of small amounts of gastric lipase
• a.   Protection

  ü  The acid medium is also responsible for the reduced activity of harmful bacteria that may have been taken in with food.

  ü  It also provides a favourable medium for the absorption of calcium and other minerals
• a.   Absorption

  ü  Minimal water, alcohol, glucose & some drugs are absorbed through the gastric mucosa
• a.   Control passage of chyme into duodenum

  ü  Through peristaltic waves, carbohydrates are emptied within 1 – 2 hours, proteins with 3 – 4 hours, fats within 4 – 6 hours.

  ü  Once acidic chyme is formed, slow peristaltic waves travel from the fundus to the pylorus,

  ü  Pressure builds & pyloric sphincter opens
•                 LIVER – produces bile
•  

          The gallbladder stores the liver’s digestive juices until they are needed in the intestine.
•  

                  PANCREAS – produces a juice that contains enzymes to break down the carbohydrates, fat and protein in food.
• 1. The Small Intestine

  Ø  The small intestine is about 6 m long (20 – 22 ft)

  Ø  It extends from the pyloric sphincter to ileocecal valve

  Ø  It is divided into parts:  Duodenum, Jejunum & Ileum

  Ø  Majority of the digestive process is completed in the duodenum & the rest function primarily in the absorption of the nutrients
• Functions of the small intestine

  • Mucus secretion
  • Secretion of enzymes
  • Secretion of hormones
  • Chemical digestion
  • Absorption
  • Motor activities
• Mucus secretion


  Goblet cells & duodenal (Brunner's) glands secrete mucus to protect the mucosa
• Secretion of enzymes


  • Brush border cells secrete sucrose, maltase & enterokinase which act on disaccharides to form monosaccharides
  • Enterokinase activates trypsinogen from the pancreas
• Secretion of hormones


  Endocrine cells secrete cholecystokinin, secretina and enterogastone that regulate the secretion of bile, pancreatic juice and gastric juice
• Chemical digestion


  1. Presence of acidic chyme in the duodenum stimulates secretion of pancreozymin

  2. Pancreozymin stimulates enzyme secretion of pancreas amylase, lipase & trypsin

  3. Amylase completes digestion of carbohydrates

  4. Lipase completes digestion of fats

  5. Trypsin completes digestion of protein

  6. Presence of fats in acidic chyme causes duodenum to secrete cholecystokinin which causes contraction of the gallbladder and relaxation of the sphincter

  7. Bile emulsifies fats, enabling pancreatic lipase to complete digestion of fats

  8. Bile & pancreatic juice are alkaline, neutralizing the acidic chyme
• Absorption


  Nutrients & water move from the lumen of the small intestine into the blood capillaries & in the villi
• Motor activities


  1. Mixing (segmental) movement and Peristalsis propel the chyme through the small intestine

  3. Chyme remains in the small intestine for 3 - 10 hours
• Motor activities of the intestine


  1. Haustral churning

  2. Peristalsis
• Secretion

  The mucus protects the mucosa from injury, binds fecal particles into a formed mass, lubricate & allows passage of fecal residue & counteracts the effects of acid – forming bacteria
• Approximately 800 – 1000 ml of water absorbed in the large intestine
• Fecal material ¾ water & ¼ solid material
• Defecation


  Act of expelling feces from the body