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Vertebrate Digestion – SBI 3U1
Overview The digestive system is comprised of two main parts: the alimentary canal, and the accessory organs. The alimentary canal is basically a long muscular tube open at both ends, as in the earthworm digestive system that you studied in grade 10. Vertebrates however have several important specialized sections and organs along their alimentary canal. alimentary canal: mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, anus. accessory organs: tongue, salivary glands, liver, gall bladder, pancreas The four roles of the digestive system include ingestion, digestion, absorption and egestion Ingestion: the physical act of eating Digestion: the breakdown of organic molecules into smaller particles. chemical – involving the use of enzymes (lipases break down lipids) mechanical – physical breakdown, (e.g.: mastication by teeth) Absorption: food particles are taken into the blood and distributed to all parts of the body Egestion: unused food material is eliminated from the alimentary canal The Mouth The sight or scent of food can activate your salivary glands. (You can guess why a wise restaurant owner will provide a free appetizer such as peanuts, pretzels, or nacho chips while you are looking over the menu.) Saliva is composed of water, mucous, and an enzyme, amylase. Amylase immediately begins chemical digestion of the starches in the food, breaking it down into smaller carbohydrates. The water and mucous also dissolve food particles, allowing our taste buds to identify the food. Food can only be tasted when the flavour molecules are dissolved in solution. The teeth also masticate the partially dissolved food. Together with the tongue, this aids in mechanical digestion. A semi‐solid ball of food, called a bolus, is pushed to the back of the throat by the tongue. The bolus moves past the pharynx, and across the opening of the trachea. It does this without passing into the trachea because a small flap of skin covers the trachea in a reflex action during swallowing. This flap of skin is called the epiglottis. Once past the pharynx, the bolus moves down the esophagus by a series of rhythmic, wavelike contractions. This is called peristalsis. The Stomach The stomach is a J‐shaped muscular organ. The cardiac sphincter controls the opening to the stomach. This is a muscular ring that must relax to allow food to enter the stomach. At the end of the stomach, the pyloric sphincter controls the movement of foods out of the stomach, into the small intestine. The stomach is lined inside with millions of secretory cells. These release gastric juices into the stomach that aid in the chemical digestion of the food. The gastric cells begin secretion shortly after food enters the mouth. Hormones control this early release of gastric juice. Gastric juices contain water, hydrochloric acid, and pepsinogen. Proteins begin chemical digestion in the stomach. The precursor protein pepsinogen is inactive, and is changed into the active form, pepsin by the acidic environment. Rennin is another enzyme released in the gastric juices. It is important in infants because it slows down the movement of milk, allowing increased breakdown and absorption of nutrients. It is less important in adults because it is only effective in a neutral or alkaline environment. The mucous lining of the stomach protects it from the hydrochloric acid. Very little absorption of nutrients occurs in the stomach. Ulcers Stress and tension can affect the digestive system. Stress can cause the gastric cells to release too much gastric juice, or at the wrong times. Without food in the stomach, this excess acid in the can burn through the mucous layer and cause an ulcer. Recently evidence has also shown that certain types of ulcers can be cause by a bacterial infection, Hylobacter pylori. If the cardiac sphincter opens unnecessarily during digestion, acid can move up the esophagus, causing acid indigestion or “heartburn”. The Small Intestine (hey who’re you calling small?!) Food leaving the stomach is a soupy mixture of food particles, enzymes, and acid. It is referred to as chyme. The chyme leaves the stomach via the pyloric sphincter, and enters the small intestine, which is actually about 7 metres long and 4 centimetre in diameter. There are three sections to the small intestine: duodenum – chemical digestion (first 25‐30 cm) jejunum – chemical digestion and absorption ileum – absorption The inner membrane of the small intestine (especially in areas important for absorption) is lined with many small finger‐like projections called villi. The villi increase the surface area to maximize absorption. Chemical digestion is completed in the small intestine. The pancreas, one of the accessory organs, secretes most of the enzymes necessary for final digestion. carbohydrates – more amylase is released to continue breakdown of starches that began in the mouth – a series of disaccharide enzymes complete the breakdown of carbohydrates. proteins – trypsinogen is released by the pancreas, and is converted to trypsin by enterokinase, an enzyme of the small intestine – erepsins break small chain polypeptides into individual amino acids lipids – lipases released by the pancreas begin the digestion of lipids – there are several kinds of lipases, each acting on a specific type of lipid. ‐ pancreatic lipase – breaks down triglycerides into glycerol and fatty acids. ‐ cholesterol lipase – removes a fatty acid from a cholesterol molecule. ‐ phospholipase – breaks down components of a plasma membrane When the contents of the stomach enter the small intestine, the acidic nature of the contents would destroy the small intestine if not for the secretions of the pancreas. A chemical in the small intestine called prosecretin is converted into secretin. Secretin is carried by the bloodstream back to the pancreas. This stimulates the pancreas to release a flood of bicarbonate ions through the pancreatic duct to the duodedum, which buffer the HCl, raising the pH to approximately 9. This also serves to inactivate the enzyme pepsin, which could digest the walls of the small intestine if it were active. The pancreas also secretes insulin, a hormone that acts in the liver to regulate the amount of sugar in the blood. Absorption Refer to diagram of villi, showing the lacteals. Folds in small intestine, villi contain a rich supply of blood vessels. This network of capillaries allow the absorption of carbohydrates (glucose) and proteins (amino acids) directly into the bloodstream. Some of these nutrients are absorbed by diffusion, but a lot of active transport is also used (use of ATP). Lipids are not absorbed by the blood vessels, but rather by the lymphatic system. Surrounded by the blood vessels in the villi are lacteals, vessels of the lymphatic system. The Liver The liver has several hundred functions. 1. Produces bile – bile is not an enzyme – it is stored in the gall bladder ‐ it is released into the duodenum to break fat into fat droplets – it is an emulsifier ‐ this increases the surface area of the fat droplets to allow lipase to act on it. 2. Regulates glucose levels in the bloodstream. With the help of insulin, the liver converts excess glucose into glycogen. This is an easily accessible supply of energy. The hormone glucagon converts glycogen back into glucose. 3. Excess glucose (beyond storage capacity of liver) is converted into fat, and stored in adipose tissue. 4. Removes poisons from body – ammonia (product of amino acid breakdown) ‐ liver turns ammonia in to urea – urea is mixed with water and eliminated as urine ‐ alcohol – taken to the liver from by the blood directly from the stomach ‐ if there is too much for the liver to handle, damage can occur – cirrhosis (irreversible scar tissue) 5. Produces cholesterol – used to rebuild cells – liver cannot cope with excess dietary cholesterol – ends up in bloodstream, and can clog arteries. Large Intestine The large intestine’s function is to extract water and salt from the chyme. Wastes and undigested material forms feces. The large intestine (also known as the colon) also stores some feces before defecation. Anatomy of the Colon: ‐ 3 straight portions (ascending, transverse, and descending) ‐ no villi, no enzymes (chemical digestion is already complete) ‐ smooth muscle layer inside ‐ small blind sac called the appendix attached to the inferior end of the ascending colon, which apparently has no function. Muscular contractions are slower than in the small intestine, which is good for absorption of water. About 30 minutes between contractions, compared to 9‐12 minutes for the small intestine. Massive contractions occur about 2‐4 times per day (after meals). These move feces into the descending colon for storage. A reflex action is initiated about one hour following a meal, producing the urge to defecate. This makes room for incoming food. Stretch receptors in the rectum initiate the reflex. This opens the internal anal sphincter, and the rectum contracts. This sphincter is under involuntary control. The external anal sphincter (voluntary) must relax to release wastes. If it does not, it remains closed, and the rectum will relax until another mass movement occurs. Defecation frequency varies among individuals (1 per day or 1 per week). Long delays can result in constipation. This is often caused by the absorption of too much water. Movements can be increased with a diet high in fibre (e.g.: bran, or other forms of roughage). This can also help remove the potentially toxic wastes out of the colon. A toxic waste in the colon for too long is potentially a contributing factor to cancer of the colon. Wastes moving through the colon too rapidly results in diarrhea. Too little water is removed. Foods that slow down contractions (increasing water absorption) include bananas, apple juice, rice and tea. Diarrhea is often caused by a viral infection such as influenza. Bacteria normally reside in the colon. Contractions are slow enough to allow them to survive. Bacteria feed on undigested food (fermentation) which causes gases to be produced (CO2, CH4) Gases leaving the colon are referred to as flatus. Some food (e.g.: beans) contain carbohydrates for which humans lack enzymes. This provides abundant food for bacteria, so lots of gas is formed. Swallowed air may also enter the colon, and released as flatus or burped out from stomach. Bacteria can also make some vitamins that humans can absorb and use, such as vitamin K. The colour of feces is brown due to the action of bacteria on the bile (yellow‐green), constipation (long delay) – dark brown diarrhea (too fast) lighter (less bacterial action