Digestive system & homeostasis

• Parts of the digestive system

  • Gastrointestinal tract
  • Accessory organs

  
• Gastrointestinal tract


  • Series of hollow organs joined by a long, muscular tube
  • Mouth (oral cavity)
  • Pharynx (throat)
  • Stomach
  • Small intestine
  • Large intestine (includes the rectum & anus)
• Accessory organs


  • Salivary glands (3 pairs)
  • Liver
  • Gall bladder
  • Pancreas

  
• Principle functions of the digestive system

  • Digestion (mechanical/ chemical)
  • Absorption & circulation of nutrients
  • Elimination of waste

  
• Ingested food in the mouth

  1. Ingestion: Food/fluid intake through the mouth

  2. Mastication (Mechanical digestion): Chewing of ingested food, physically breaking it down to increase surface area for exposure to digestive enzymes, aided by saliva

  3. Chemical digestion: Begins with the action of enzymes in saliva on carbohydrates

  
• Saliva


  • 98% water, mucus, proteins, ions and enzymes (salivary amylase & lysozyme)
  • Moistens food which allows the tongue to taste food, & helps in the creation of a food bolus
  • Salivary amylase begins chemical digestion of starch into maltose & dextrin
  • Lysozyme is a mild, antibacterial enzyme that prevents bacterial infections in the mouth
• Fate of swallowed food

  1. The bolus is pushed into the pharynx by the tongue

  2. Pharynx links the oral cavity to the esophagus, delivers food/fluids to the esophagus, aided by alternate contractions of muscles located in the walls of the pharynx

  3. Esophagus: Muscular tubing linking the pharynx to the stomach, Peristalsis: alternate involuntary contractions of circular muscle in the esophageal walls

  4. Cardiac sphincter (lower esophageal sphincter): Located at the base of the esophagus, regulates the movement of food into the stomach, prevents backward movement (regurgitation) of stomach contents into the esophagus

  
• Stomach


  • Temporary storage of food
  • Mechanical digestion: alternate contraction & relaxation of muscle layers in the stomach walls
  • Chemical digestion: enzymatic breakdown of proteins by pepsin
  • Delivers chyme to the small intestine

  
• Fate of bolus in the stomach

  Chyme: food processed in the stomach, enters small intestine through the pyloric sphincter

  
• Small intestine


  • Measures 15-25 cm in length, holds about 4L of food
  • Inner surface (mucosa) has many large folds (rugae)
  • Stomach wall is lined with millions of gastric pits, each leading into 3-5 gastric glands which release gastric juice into the stomach lumen

  
• Liver


  • Largest gland in the body, consists of 4 lobes, located below the diaphragm and almost completely covers the stomach
  • Many metabolic & regulatory functions, key function is to produce bile
• Bile


  • Made in the liver, transported to the gall bladder where it is stored and concentrated
  • Contains water, mucus, bile salts, phospholipids, small percentage of bilirubin (from breakdown of hemoglobin), cholesterol, electrolytes etc.
  • Released into the duodenum when chyme enters the small intestine from the stomach

  
• Gall bladder


  • Pear-shaped, muscular sac
  • Store & concentrate bile that is produced in the liver
  • Relaxed when digestion is not taking place, bile flows in through the cystic duct
  • Bile is concentrated in the gall bladder through removal of water
  • With the entry of fatty food in the duodenum, hormonal situation causes it to contract, releasing bile into the common bile duct
  • The common bile duct carries bile & pancreatic duct carries pancreatic juice meet at the duodenal ampulla
  • Sphincter of Oddi: ring of smooth muscle that controls the movement of bile & pancreatic juice from the pancreas into the duodenum
• Pancreas


  • Able to perform as an exocrine gland and an endocrine gland
  • Secretes pancreatic juice which consists of digestive enzymes into network of ducts -> pancreatic duct -> duodenum of small intestine
  • Acinar cells produce bicarbonate and digestive enzymes
  • Contain cells that releases hormones into the bloodstream
  • Islet of Langerhans: beta cell produce insulin, alpha cell produce glucagon

  
• Digestion & absorption in the small intestine

  1. Peristalsis & segmentation contractions (ring-like contractions): Move chyme back & forth, increases its exposure to intestinal secretions & mucosal surface

  2. Intestinal mucosa cells secrete mainly mucus, ions & water. This lubricates and protect the intestinal wall from the acidic chyme & action of digestive enzymes

  3. Brush border enzymes (BBEs): peptidase (digest proteins), disaccharides (digest disaccharide)

  4. Absorption: Surface area of small intestine is greatly increased by the presence of permanent circular folds, finger-like projections (villi) & microvilli

  5. Digested food can be absorbed directly into the bloodstream or via the lacteal

  
• Villi


  Covered with a single layer of epithelial cells, involved in absorption only
• Microvilli (brush border)


  Brush like appearance found on the surface of each epithelial cell extending into the intestinal lumen, involved in absorption and digestion due to the secretion of brush border enzymes
• Large intestine


  • Food residue passes from the small intestine into the large intestine through the ileocecal valve
  • Consists of: caceum, appendix, colon, rectum
  • Does not participate in digestion of food
  • Goblet cells which produce mucus helps to bind dehydrated waste & act as lubricant can be found on its mucosal lining
  • Key functions: absorption of water and ions, formation & elimination of indigestible food from the body as feces

  
• Metabolism


  Sum of all life-sustaining chemical reactions that take place within a living organism
• Main purpose of metabolism

  • Catabolism: Conversion of energy within food into energy (ATP) to run cellular processes
  • Anabolism: Conversion of food into building blocks for the manufacture of proteins, lipids, nucleic acids and some carbohydrates
  • Elimination of metabolic waste
• Basal metabolic rate (BMR)


  • Defined as the amount of heat produced by your body per unit time to accomplish its most basic (basal) life-sustaining functions (breathing, circulation, nutrient processing, cell production)
  • BMR= rate of energy consumption (Kcal) per hour per m^2 of surface area
• Total metabolic rate (TMR)


  Sum of your BMR + added energy needed for voluntary activities (muscle contraction & other physical activities)
• Homeostasis


  • Regulation by an organism of the chemical composition of its internal environment
  • Describes the mechanism to achieve constancy of the internal environment
  • It is the state of steady internal, physical and chemical condition maintained by living systems
  • Essential for the survival of all living things
• Internal environment in the human body

  Refers to the tissue fluid environment surrounding cells in a tissue which must be kept constant at all times
• Factors that are kept constant by homeostasis

  • Homeostasis does not involve keeping conditions static, instead involved in keeping internal conditions within tightly-regulated physiological tolerance limits (normal range)
  • It is a dynamic equilibrium: a state of balance achieved within an environment
  • It is the result of internal control mechanisms that continuously oppose outside forces that tend to change that environment
• Set point


  A physiological value around which the normal range fluctuates
• Normal range


  Physiological tolerance limits (range defined by upper and lower values), e.g. body temperature, set point= 37°C, normal range (36.5 to 37.5)
• Components of a homeostatic control system

  • Stimulus (physiological variable): Requires regulation, e.g. body temperature, blood pressure
  • Receptor (sensor): Physiological early-warning system, sensory receptor that detects the change in the level of a variable
  • Control centre (regulator): Coordinating center that determines the set point, receives info from detector about changes in the level of a variable, then activates appropriate effectors, located in the brain and spinal cord, e.g. hypothalamus (body temperature)

  
• Equilibrium


  A state of balance achieved within an environment
• Normal range


  Physiological tolerance limits (range defined by upper and lower values)
• Components of a homeostatic control system

  • Stimulus (physiological variable)
  • Receptor (sensor)
  • Control centre (regulator)
  • Effector

  
• Stimulus


  Requires regulation, e.g. body temperature, blood pressure
• Receptor (sensor)


  Physiological early-warning system, sensory receptor that detects the change in the level of a variable
• Control centre (regulator)


  Coordinating center that determines the set point, receives info from detector about changes in the level of a variable, then activates appropriate effectors, located in the brain and spinal cord
• Effector


  Adjusts level of the parameter and either reduces the amount of change (negative feedback) so the system returns to set-point; or increases the amount of change (positive feedback) so that reaction continues at an even faster rate until the end-point is reached
• Feedback


  Biological mechanism that aids in maintaining homeostasis
• Positive feedback


  A loop whereby a physiological change leads to even greater change (self-amplification) in the same direction, increases a stimulus, rarely occurs, less associated with stability of the system, can be either beneficial or harmful
• Negative feedback


  A loop whereby a deviation from set point brings about a sequence of events that counteract the disturbance (e.g. slowing down/inhibition of a process) in order to restore the system to its original state, reduces a stimulus, frequently occurs, more associated with stability of the system, always beneficial
• Body temperature regulation


  1. Increased body temp: Receptor (blood warmer than hypothalamic set point), Regulator (activates heat loss center in hypothalamus), Effector (skin blood vessels dilate, sweat glands activated)

  2. Decreased body temp: Receptor (blood cooler than hypothalamic set point), Regulator (activates heat loss center in hypothalamus), Effector (skin blood vessels constrict, skeletal muscles shiver)
• Blood glucose regulation


  1. Blood glucose level rises: Receptor (Islet of Langerhans detects rise), Regulator (Beta cells of pancreas release insulin), Effector (Body cells take up more glucose)

  2. Blood glucose level falls: Receptor (Islet of Langerhans detects drop), Regulator (Alpha cells of pancreas release glucagon), Effector (Liver and muscles break down stored glycogen and release glucose)