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Digestive System By the end of this class you should understand: • The processes of physical and chemical digestion • The route of food, nutrients and waste through the body • The primary functions of the major organs of the digestive system • The essential process of nutrient absorption Digestive Organs • The digestive system is divided into two types of organs: – The alimentary organs that make up the alimentary canal – Accessory organs that do not make up the alimentary canal • The alimentary canal is the tube running through your body Alimentary Organs • • • • • • • Mouth Pharynx (throat) Esophagus Stomach Small intestine Large intestine Anus Movement • Organs are moved from one to the next through a process called peristalsis (the pushing of food through the alimentary canal by smooth muscle) – In the esophagus, swallowing is initiated by skeletal muscle but completed by smooth muscle and qualifies as peristalsis – The stomach, small and large intestine all have peristalsis but it is much slower Digestive Function • The digestive system has one goal only: to put nutrients into the bloodstream so that all our cells can access those nutrients – Many excess nutrients can be stored • The digestive system has many “preparation steps” to prepare food for this process The Most Vital Organ • Only one organ performs almost all the absorption of nutrients (excepting water): the small intestine – So called because it has a small diameter, though it is several meters long • If we drank liquids containing only single molecules of nutrients, we would need no other digestive organ! – This is what they inject into your blood when you’re in a coma Two Types of Digestion • Digestion is the process of breaking down nutrients so they can be absorbed – Undigested nutrients cannot be absorbed and pass through the small intestine to the large intestine – For more information, ask someone who is lactose-intolerant about drinking milk • Mechanical (physical) digestion is the chewing and churning of food • Chemical digestion is the breaking of chemical bonds in food Mechanical Digestion • The mouth (chewing) and stomach (churning) perform mechanical digestion of food • The mouth accomplishes this with hard calcium structures called teeth and heavy muscles attached to the mandible chewing • The stomach has layers of smooth muscles that churn the food before it moves to the small intestine Chemical Digestion • The mouth and stomach also initiate the process of chemical digestion – The mouth has salivary glands that produce saliva, which begins chemical digestion – The stomach has pepsin, an enzyme that denatures and breaks down protein • Note chemical digestion of fats does not begin until the small intestine The Stomach • The stomach produces gastric juice, which contains pepsin as well as other chemicals • This includes hydrochloric acid, which makes the stomach environment very acid • The acid itself is not for digesting the food – Kills bacteria – Enables pepsin to work Stomach Ulcers • Stomach ulcers were once thought to be caused by stress but are now known to be a bacterial infection (but stress does block the immune system) • The scientist who discovered the bacterium that causes ulcers was eventually awarded the Nobel Prize – No one had believed him initially so he drank a flask of the bacterium to gain ulcers Gastric Juice • Gastric juice is the secretion of the stomach • Gastric juice is very acidic and can damage tissues other than the stomach lining – Repeated vomiting can cause gastric juice to erode the enamel of the teeth – If gastric juice is produced in excess it can overflow into the esophagus, causing “heartburn” • Once gastric juice mixes with and digests food it is called chyme Processing of Chyme • Chyme is the partially digested food of the stomach mixed with acid • Chyme is delivered into the small intestine very slowly – Excess acid could damage the small intestine – Helps the intestine absorb all the nutrients • The small intestine has many digestive enzymes of its own that complete digestion Accessory Organs • The small intestine has enzymes on its absorptive cells but also receives digestive juices from two accessory organs • The pancreas produces a massive battery of digestive enzymes to finish all digestion of food – Most prominent enzyme is trypsin, a protease that breaks down protein into amino acids • The liver produces bile, which is stored in the gallbladder until chyme enters the small intestine Bile • Bile does not contain digestive enzymes and does not directly perform chemical digestion • Bile is instead an emulsifying agent, much like a detergent – To emulsify is to allow oil and water to mix • Bile is therefore vital for digesting and absorbing fats from food Small Intestine Absorption • Since nutrients can only be absorbed through cell membrane, the small intestine epithelial tissue has adaptations for increasing surface area – The tissue is arranged into fingerlike villi (singular villus) which create more surface area – The individual cells have a brush border, or microvilli (singular microvillus) • These cells expend ATP to actively transport nutrients into the blood Small Intestine Activity • When food enters the stomach it still consists of starches, proteins and whole fats – These cannot be absorbed since they are too large! • The small intestine can only absorb the small molecules that make these up (simple sugars, amino acids, and fat molecules) – Large molecules that are not digested cannot be absorbed! Lactose Intolerance • Lactose Intolerance is one of many digestive disorders caused by a failure to absorb nutrients – Lactose (milk sugar) is made of a molecule of glucose and galactose stuck together (too large to absorb) – Most people express a small intestine enzyme called lactase that breaks them apart • Failure to digest lactose (or other nutrients) results in the nutrients ending up in the large intestine – Bacteria metabolize it and release gas and waste acids Liver Function • Blood flows directly from the small intestine to the liver – This is called hepatic portal circulation – Protects the rest of the body from whatever you just ate • The liver has metabolic enzymes to process many nutrients after they are absorbed – Production of fat and glycogen – Storage of vitamins – Breakdown of toxins such as alcohol Large Intestine • Once all nutrients are absorbed into the small intestine, the large intestine (which is mostly composed of a tube called the colon) carries remaining indigestible materials to the anus • The large intestine is filled with colonies of bacteria that feast on our leftovers (often producing gas as a waste product) • The large intestine is capable of absorbing water and vitamins, including those produced by bacteria Hopefully that wasn’t too much to digest… • Wednesday is nutrition lesson!