digestive system

• Digestive system components in order:

  Oral cavity

  Accessory organs: tonsils and salivary glands

  Pharynx

  Oesophagus

  Stomach

  Small intestine - duodenum, jejunum, ileum

  Accessory organs: liver, gallbladder, pancreas

  Large intestine - cecum, colon, rectum, anal canal

  Anus
• Digestive system functions


  Ingestion

  Mastication

  Propulsion

  Mixing

  Secretion

  Digestion

  Absorption

  Elimination
• Mastication


  Food is mechanically broken down therefore increasing the total surface area that digestive enzymes can access
• Propulsion


  The movement of food through the digestive tract. Swallowing moves the bolus from the oral cavity to the oesophagus. Peristalsis is a wave of smooth muscle relaxation which expands the tract in front of the bolus. Wave of smooth muscle contraction behind the bolus which propels the bolus forward. Occurs in the oesophagus and small intestine
• Mixing


  Segmental contractions where segments fluctuate between relaxation and contraction. It's purpose is to mix the food with digestive secretions which assist with breaking it down into smaller pieces and occurs in the small intestine.
• Secretion


  Purpose is to digest, buffer, lubricate and liquify.

  Types of secretion:

  Mucus - protect the epithelial cells from digestive enzymes, stomach acids and abrasion. Lubricates the lining of the tract and the food.

  Water - liquifies the food so it is easier to digest and absorb.

  Enzymes - secreted from pancreas, small intestine, stomach and oral cavity. Breaks down complex food molecules to smaller molecules.
• Digestion


  Mechanical: mastication and mixing

  Chemical: digestive enzymes
• Absorption


  Molecules moving out of the digestive tract and into the circulatory of lymphatic system.
• Elimination


  Waste products are eliminated from the body via defecation. Occurs in the large intestines, materials are altered from a liquified substance to a semi-solid substance by absorbing water and salts.
• Layers of the digestive tract


  Mucosa - mechanoreceptors and chemoreceptors

  Submucosa - nerves, lymphatic and blood vessels and small glands

  Muscularis - smooth muscle

  Serosa - smooth layer that covers the digestive tract
• Regulations of the digestive system


  • Nervous regulation
  • Autonomic nervous system
  • Chemical regulation
• Nervous regulation


  Enteric nervous system. A division of the autonomic nervous system located within the walls of the digestive tract. It can work independently of the ANS.
• Autonomic nervous system


  Parasympathetic nervous system:

  Rest and digest - increases motility, salivary and gastric acid secretions

  Sympathetic nervous system:

  Fight and flight - decreases motility
• Chemical regulation 


  Endocrine secretions - gastric, secretin and cholecystokinin
• Oral cavity


  Mechanical and oral digestion. Assists with the process of mastication.

  Incisors and canines: tear and cut food

  Premolars and molars: grind and crush food
• Saliva


  Where chemical digestion occurs in the mouth.

  Functions of saliva:

  • Keeps the oral cavity moist
  • Suspension of food in a solution which aids taste
  • Begins digestion - salivary amylase and lingual lipase
  • Protective functions - washes oral cavity, contains the lysozyme: antibacterial properties which assists with preventing infection from bacteria
• Pharynx


  Three parts:

  • Nasopharynx
  • Oropharynx
  • Laryngopharynx

  Only the oropharynx and laryngopharynx are associated with digestions. The uvula/soft palate prevents food material fro entering the nasopharynx. The epiglottis covers the larynx to prevent food from entering the respiratory passages.
• Oesophagus


  From pharynx to stomach. Posterior to the trachea and travels through the opening of the diaphragm. The food moves through the oesophagus and is controlled by the upper and lower oesophageal sphincters.
• Phases of swallowing


  Voluntary phase

  Pharyngeal phase

  Oesophageal phase
• Voluntary phase


  The bolus is moved by the tongue against the hard palate and towards the oropharynx.
• Pharyngeal phase


  The soft palate elevates to close the nasopharynx. The pharynx elevates and receives the bolus and moves it towards the oesophagus. The epiglottis closes over the larynx and the upper oesophageal sphincter relaxes.
• Oesophageal phase


  Peristaltic waves move food towards the stomach. Gravity is enough for liquid and the lower oesophageal sphincter relaxes.
• Stomach


  Pyloric sphincter regulates the release of food from the stomach into the small intestines.
• Stomach secretions


  Mucus: protects the stomach lining from digestive enzymes and acids.

  Hydrochloride acid: produces low pH in the stomach, its primary role is to kill bacteria. Inactivates salivary amylase and assists with the functioning and activation of pepsin.

  Hormones: gastrin increases gastric secretions.

  Enzymes: gastric lipase breaks down fats/lipids. Pepsin breaks down proteins. Pepsinogen is converted to pepsin once in the stomach and comes into contact with hydrochloride acid.
• Regulation of stomach secretions

  • Cephalic phase
  • Gastric phase
  • Intestinal phase
• Cephalic phase


  Stimuli to cause stomach secretions to get started are:

  The smell and taste of food

  Chewing and swallowing

  Pleasant thoughts about food
• Gastric phase


  Stimuli to cause stomach to go for it are:

  • Enlargement of the stomach as food enters - activation of mechanoreceptors
  • Presence of proteins
• Intestinal phase


  Low pH chyme entering the duodenum simulates chemoreceptors in the duodenum. Chyme containing lipids and fatty acids and digested protein products entering the duodenum resulting in inhibiting gastric secretions and the increase secretion of secretin and cholecystokinin.
• Small intestine


  About 6m long, small in diameter. Accessory organs - liver, pancreas and gallbladder. Its function is to continue digestion an absorption and is the site of the greatest amount of absorption.

  Three parts:

  • Duodenum
  • Jejunum
  • Ileum
• Duodenum


  Starts at the pyloric sphincter and ends at the jejunum. The entry point of ducts from the liver and pancreas. The surface area and thus absorption capability is greatly increased by the presence of circular folds, villi and microvilli.
• Jejunum


  Similar in structure to the duodenum and is becoming smaller. Is a major site of absorption.
• Ileum


  Smaller in - diameter, wall thickness, number of folds and microvilli. Less absorption and ileocacal sphincter and valve allowing for one way valve flow to the large intestine.
• Small intestine secretions


  Mucus, electrolytes and water:

  • Lubricates and protects the wall from acidic chyme
  • Keeps the chyme in liquid form to assist with continued digestion

  Hormones - cholecystokinin and secretin:

  • Decreases gastric secretions
  • Stimulates liver and pancreas secretion

  Enzymes - disaccharides and peptidases:

  • Assists with the breakdown of foods

  Segmental and peristaltic contractions mix and move chyme in the small intestine.
• Liver


  The largest gland int he body and second to largest organ. Situated int he upper right quadrant of the abdomen and has two lobes - a right and left separated by a ligament.

  • Hepatic artery: carries oxygenated blood from the heart to the liver
  • Hepatic portal vein: carries nutrient rich, deoxygenated blood from the small intestine to the liver
  • Right and left hepatic ducts: carries bile from the liver to the gallbladder and duodenum
• Bile


  Bile is produced by the hepatocytes. From the hepatic cords, bile is collected by the bile canaliculus. Flow towards the hepatic triad and exits via the hepatic ducts.
• Liver functions


  • Bile production
  • Storage
  • Inter-conversion of nutrients
  • Detoxification
  • Synthesis of molecules
• Bile production


  Bile enters from the duodenum and neutralises and dilutes stomach acids as a low pH renders the effectiveness of pancreatic enzymes. It contains bile salts which emulsifies lipids:

  • Lipids are hydrophobic, therefore they clump together
  • Bile helps breaks down the clumps apart
  • Enzymes can then breakdown lipids
  • Bile contains no digestive enzymes

  Stimuli: parasympathetic stimulations, secretin secretion, bile salts

  Secretion continues until the duodenum empties.
• Storage


  Sugar as glycogen, lipid and vitamins. Storage fluctuates depending on the bodies needs.
• Inter-conversion of nutrients


  The correct nutrients are not always present in the required amounts thus the liver can convert nutrients to others if needed.
• Detoxification


  By-products produced during metabolism and ingested toxins are further broken down and thus altered to less harmful toxins in the liver. The liver is the primary organ to break down alcohol.