Introduction to the Digestive System SBI3U Why Do We Eat Food??? ? To obtain the raw materials for growth and maintenance from nutrients in food ? Nutrients provide energy (chemical energy) that fuels all biological processes and physical activities such as growth and movement What Do We Eat? ? Carbohydrates ? Proteins ? Lipids ? Water ? Vitamins and Minerals What Do We Eat? Vitamins and Minerals ? A vitamin is a compound that an organism needs as a nutrient only in small amounts ? Regulate cell functions, growth, and development ? Classified as fat soluble or water soluble ? Minerals are naturally occurring elements that the body uses to carry out metabolic processes and to build or repair tissues ? Necessary for bone formation, nerve impulse transmission, and muscle contractions Digestion ? There are 4 main steps ? Ingestion – taking in of nutrients ? Digestion – physical and chemical breakdown of complex food molecules ? Absorption – transfer of digested nutrients into the bloodstream ? Egestion – the removal of waste food materials from the body Human Digestive System ? Often referred to as the gastrointestinal tract (GI tract) ? Is approximately 7-9 meters long depending on size and age of the individual Digestion SBI3U Digestion in the Mouth � Physical digestion Food is broken down into smaller pieces by the teeth (mastication) The front teeth are used to grab and cut food, back teeth are used for grinding and crushing food � Chemical Digestion Saliva contains enzymes (chemicals that increase the rate of chemical reactions in living things) which breaks down food into simpler particles. � Amylase – breaks down starch into smaller disaccharides and monosaccharides � Mucus – protective secretion that acts as a lubricant to aid swallowing Swallowing ? At this point the food is now called bolus and is pushed to the back of the mouth where it is swallowed ? As the food is pushed into the pharynx ? The soft palate is raised to prevent food from entering the nasal passages ? The larynx is raised against a flap of soft tissue called the epiglottis, preventing food from entering the trachea and lungs Swallowing Going from the Mouth to the Stomach ? After swallowing, the bolus moves to the stomach by way of the esophagus, a long muscular tube ? When bolus enters the esophagus it activates the smooth muscles to undergo rhythmic, wave-like contractions called peristalsis The Stomach ? Is a J-shaped, muscular organ where physical and chemical digestion occurs ? The entry and exit inside the stomach is controlled by circular muscles called sphincters ? The gastroesophageal sphincter is located between the esophagus and the stomach ? When relaxed it allows food to enter the stomach, when contracted it prevents food from re-entering the esophagus ? The pyloric sphincter is located between the stomach and small intestine The Stomach Has four layers each with a special structure and function ? The innermost layer is the mucosa and is extensively folded and produces gastric juices (mixture of digestive enzymes, acid and mucus) ? The next is the submucosa- made of connective tissue containing networks of nerves and blood vessels ? The third layer is the muscularis and consists of smooth muscles which contract frequently, churning the mixing the food with gastric juices to produce a mixture called chyme ? The outermost layer is the serosa and holds the stomach in place and secretes a lubricating fluid eliminating friction between organs Chemical Digestion in the Stomach ? Digestion is controlled by enzymes, hormones and nerves ? When nerves detect the presence of food, the hormone gastrin is released ? Gastrin stimulates the release of gastric juice ? Gastric juice is mostly made of mucus but also contains acid and digestive enzymes ? The mucus coats and protects the lining of the stomach from acid and digestive enzymes Chemical Digestion in the Stomach ? The acid in gastric juice is very strong, ranges from pH of 2 – 3 ? The acid kills many harmful microorganisms ingested with food ? The acid activates other digestive enzymes such as pepsinogen and converts it to its active form, pepsin which breaks down proteins into amino acids Stomach Disorders Acid Reflux ? When the gastroesophageal sphincter does not close properly, acid can enter the esophagus ? Causes a burning sensation known as heartburn or acid reflux Stomach Ulcers ? One species of bacteria is able to survive by secreting acid-neutralizing enzymes and burrows into the mucosa preventing mucus from forming and protecting the stomach lining ? When the stomach lining is exposed to the acid an open sore, or ulcer, is formed ? Can bleed and be quite painful and can also be dangerous Anatomy & Physiology of Digestive System Organ Physical / Mechanical Digestion Chemical Digestion Mouth (Buccal or Oral cavity) Mastication- breakdown of food by the action of teeth (food now becomes bolus) Salivary glands secretes amylase (present in saliva) that breakdowns carbohydrates. Fats are partially broken down here. Esophagus Peristalsis (rhythmic, wave-like contractions) Partial chemical breakdown of proteins & fats Stoamch Churning Action (Propulsion and Retropulsion) Hormone called Gastrin stimulates Gastric juice & Pepsin which together bring about protein breakdown.
