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HUMAN DIGESTIVE SYSTEM (Exocrine Glands [D.2.U2: Exocrine glands secrete…

- - - - The medulla oblongata then responds using the parasympathetic ("rest and digest") division of the autonomic nervous system.
        
        The Digestive Phases
        
        The Gastric Phase
        
        The gastric phase begins once the food arrives in the stomach - it builds on the stimulation provided during the cephalic phase.
        
        Walls of the stomach become distended (expanded as a result of internal pressure) due to the ingestion of food.
        
        Autonomic nervous system signals are sent by the vagus nerve to the medulla oblongata if:
        
        Stretch receptors detect the stomach distension.
        
        Chemoreceptors in the stomach wall detect peptides in the stomach contents.
        
        The medulla oblongata then sends impulses back to the glandular parietal cells of the stomach, with the aid of: acetylcholine, histamine, and gastrin.
        
        These impulses cause the continuation and increase of production of hydrochloric acid and pepsinogen.
        
        The Intestinal Phase
        
        In this phase, a valve at the lower end of the stomach opens up and releases the partially digested food (chyme) into the duodenum of the small intestine.
        
        Hormones secretin and somatostatin are released into the bloodstream, which: inhibit gastrin secretion if the pH in the bloodstream falls too low (secretion of HCl lowers the pH of the stomach)
        
        Inhibition of gastrin secretion leads to lower gastric pit activity, referred to as "gastric empyting".
        
        The Cephalic Phase
        
        Action potentials are sent from the medulla oblongata to the stomach by the vagus nerve, which stimulate the production and secretion of hydrochloric acid and pepsinogen.
        
        The inactive pepsinogen, on contact with the hydrochloric acid, converts into its active enzymatic form: pepsin.
        
        The hormone gastrin is also secreted from the action potential stimulation of the endocrine cells in the lower portion of the stomach.
        
        Gastrin enters the bloodstream and is carried to other cells elsewhere in the stomach - resulting in an even higher secretion of hydrochloric acid and pepsinogen.
- - - - Rennin, an example of a protease, is also a part of the composition of gastric juice, however, it is only present in the fourth stomach of cud-chewing animals such as cows. The function of rennin is to coagulate milk, which is an action carried by pepsin in animals that lack rennin, such as humans.
- - - - Assists in the digestion of food by dissolving chemical bonds within food molecules.
      - Activates stomach proteases such as pepsin is activated when pepsinogen is proteolytically cleaved in acid conditions.
      - Prevents pathogenic infection as stomach acids destroy microorganisms in ingested food.
    - - The pancreas releases bicarbonate ions into the duodenum which neutralizes the stomach pH (intestinal pH ~7.0 - 8.0).
      - Certain foods such as antacids may also neutralize stomach acids, impairing digestion and increasing chances of infection.
- - - - These substances continue into the large intestine and become part of the solid waste. The excretory products, the unabsorbed water and undigested dietary fibre are egested as faeces.
        
        The substances which become a part of the solid waste are:
        
        Lignin, another component of plant cell walls
        
        Bile pigments (from bile), which give the characteristic color to faeces
        
        Cellulose, from the cell walls of ingested plant material
        
        Bacteria, because a few survive the low pH in the stomach and become a constantly regenerating population of billions of mutualistic inhabitants of our digestive tract
        
        Bilirubin, from the breakdown of red blood cells
- - - - A healthy balanced diet contains fibre as it increases the bulk of material passing through the intestines and helps to prevent constipation as it draws water into the intestine.
        
        In fact, there is a positive correlation between the amount of fibre in a person’s diet and the rate of movement of material through his or her large intestine.
        
        In order for peristalsis (smooth muscle contractions that propel material through the alimentary canal) to function optimally, the muscles that ‘push’ food along the intestines need to have a sufficient volume of material to apply pressure to.
        
        High-fibre diets also help people manage their body mass better - fibre provides a feeling of satiety without introducing excess energy.
        
        Absorption of sugars may be slowed down, helping to prevent the development of type II diabetes.
        
        Fibre also reduces the risk of various diseases of the large intestine such as bowel cancer, hemorrhoids, and appendicitis.
        
        It is recommended to eat at least 5 servings of fruit or vegetables (sources of fibre)each day.
- - - - Bacteria releases a toxin that binds to the receptors found on intestinal cells.
      - Through endocytosis the toxin is brought into the cell.
      - A domino/cascade reaction is triggered which causes an efflux of Cl- and HCO3+ ions from the cell to the intestine.
      - Through osmosis, water flows into the intestines as well, causing watery diarrhoea.
      - Loss of water from the intestinal cells is replaced with water that is drawn from the blood.
      - This leads to severe dehydration, which can lead to death if the patient is not rehydrated.
- - - - Individuals taking protein pump inhibitors may have increased susceptibility to gastric infections due to the reduction of acid secretion, which usually has one of the functions of preventing pathogenic infection.
      - This effectively raises the pH in the stomach to prevent gastric discomfort which is caused by high acidity, an example being acid reflux.
      - The effect on the overall acid production system is not permanent because the pumps are normally recycled and replaced with new pumps.
      - The protein pump inhibitors are consumed in an inactive form. Then, the acid conditions in the vicinity of the parietal cells convert them to the active form close to their target.
- - - - Ulcers occur when the mucus membrane layer of the stomach, known as the gastric mucosa, is worn away and stops functioning effectively.
        
        Stomach (peptic) ulcers were formerly thought to be caused by lifestyle stress, coffee consumption, or spicy foods.
        
        However, it is now known that about 90% of peptic ulcers are actually caused by a bacterial infection (which can be cured).
        
        This was discovered in the early 1980s, when two researchers isolated living bacterial cells Helicobacter pylori, from the stomach lining of patients suffering from peptic ulcers.
        
        Process of Bacteria Infection leading to Stomach Ulcers:
        
        H. pylori survives when introduced into the stomach - it burrows beneath the mucus layer and infects the stomach lining cells.
        
        H. pylori employs the enzyme urease to create ammonia (urease reacts with urea), and this helps to neutralize stomach acid (allowing the bacteria to survive in the harsh conditions of the stomach)
        
        H. pylori enters the body through the mouth - it is transmitted person-to-person by saliva (but can also be spread by fecal contamination of food or water).
        
        Ammonia is toxic to the human body, and depending on where the infection occurs in the stomach, H. pylori can also cause an overproduction of stomach acid (HCl)
        
        H. pylori infection of the stomach lining then leads to peptic ulcers, gastritis and the development of stomach cancers.
- - - - control the volume and content of gastric secretions required for digestion
      - control the secretion of digestive juices in the body to ensure that digestion only takes place when needed, thus conserving energy.
- - - - 1878: Claude Bernard discovered the role of the pancreas and liver in digestion and established that the small intestine was the main organ involved in digestion.
        
        1882: Theodor Schwann- discovery of Pepsin as the main enzyme for digestion in the stomach.