Digestive System Notes Structure Function Relation Mouth cavity Mechanical digestion by teeth; chemical digestion of starch by saliva. Salivary glands Three pairs of glands which secrete saliva containing mucus that lubricates the mouth and food assisting in forming a bolus for swallowing. Salivary amylase is produced. Pharynx (Throat) joins the mouth cavity to the oesophagus and larynx. Tongue Oesophagus Muscle for tasting, molding and pushing food (bolus) down the oesophagus to the stomach for digestion. Muscular tube that carries food (bolus) from the throat to the stomach. Double layer of muscles allow for waves of constriction and contraction pushing food (bolus) along the oesophagus. This is known as peristalsis.
Liver Produces bile, which is stored and concentrated in the gall bladder. Bile emulsifies lipids in the small intestine. Mechanical digestion by churning action; chemical digestion by pepsin, which begins protein digestion. Waves of muscular constriction and contraction churn the stomach which agitates food and hydrochloric acid catabolism. Pyloric sphincter Band of circular muscle that regulates flow of material (such as chyme) from stomach to duodenum. Pancreas Produces pancreatic juices containing enzymes for digesting proteins, lipids and nucleic acids. Gall bladder Stores bile and releases it into the small intestine, where the bile emulsifies lipids. Duodenum The first part of the small intestine approximately 25cm long. Small intestine Its lining secretes intestinal juice, which contains many enzymes. The internal surface is lined with villi for absorption of digested food. Large internal surface area, 6m long, mucosa has folds to increase internal surface area, villi lines mucosa and microvilli covers villi to assist with absorption. Caecum First part of the large intestine which joins the end of the small intestine. Large Intestine Consists of the transverse, ascending and descending colon, which absorbs water, minerals and vitamins. Faeces are solidified and stored here. Bacteria catabolise any remaining compounds. Large diameter, mucous lining and 1.5m in length assist in final reabsorption and excretion. Rectum Final part of the large intestine in which faeces are formed and defaecated. Anus Opening surrounded by the anal sphincter a muscle that can be voluntarily controlled.
Purpose of Digestion 1. Ingestion of food and water 2. Mechanical digestion of food 3. Chemical digestion of food 4. Movement of food along the alimentary canal 5. Absorption of digested food and water into the blood and lymph 6. Elimination of material that is not absorbed The Mouth The intake of food (ingestion) occurs at the mouth. The jaw and teeth begin mechanical digestion whereby food is broken up into smaller pieces. As the food is masticated (chewed), salivary amylase is produced by the three salivary glands. This is an enzyme which catabolise starch (a form of carbohydrate) into smaller molecules. Tooth Incisors Canines Premolars Molars Function Chisel-shaped teeth used for biting and cutting Conical teeth used for tearing Broad, rounded teeth for crushing and grinding Canine Molar Incisor Premolar
The Alimentary Canal Region Mouth cavity and pharynx Function Ingestion of food Mastication by teeth, food forms a bolus and pushed into oesophagus by tongue and check muscles Saliva added, containing salivary amylase to catabolise carbohydrates Food tasted by tongue Oesophagus Transports food to stomach by peristalsis Duodenum Small Intestine Churns food Secretes hydrochloric acid chemically digests food, kills pathogens and activates pepsin Secretes pepsin catabolises proteins into polypeptides Secretes mucous protection from acidic environment Receives bile Receives pancreatic juices Secretes intestinal juices catabolise carbohydrates, proteins and lipids Absorb nutrients via villi from digested food Large Intestine Absorbs water, minerals and vitamins Rectum Stores faeces prior to defaecation via anus Peristalsis This is the automatic contraction and relaxation of muscles along the alimentary canal responsible for the movement of food. The Liver Formation of bile bile is stored in the gall bladder and is used to emulsify (break down into smaller droplets) fats before their digestion by lipase in the duodenum.
Chemical Digestion Enzymes of the alimentary canal and their functions: Region Enzyme Substrate Product Mouth cavity Salivary amylase Carbohydrates Polysaccharides (gastric juices) Pepsin Proteins Polypeptides Pancreatic amylase Polysaccharides Disaccharides Duodenum (pancreatic juices) Pancreatic protease Polypeptides Peptides Pancreatic lipase Lipids Fatty acids & glycerol Intestinal amylase Disaccharides Monosaccharides Small intestine (intestinal juices) Intestinal protease Peptides Amino acids Intestinal lipase Fatty acids & glycerol Fats Absorption The folded internal surface of the small intestine is covered with small projections called villi. These are lined with cells which themselves also have smaller projections called microvilli. There is, consequently, an enormous surface area presented to the contents of the small intestine for absorption of the products of digestion. Structure of the inner surface of the small intestine: Lacteal Capillaries Simple Diffusion Fatty acids & glycerol and fat soluble vitamins Water and water soluble vitamins Active Transport N/A Glucose Amino acids
Mechanical and Chemical Digestion Organ Mechanical Digestion Chemical Digestion Other functions Food is dissolved Breaks food into small Saliva, which contains salivary Mouth cavity in saliva so that it particles by mastication amylase, begins starch digestion can be tasted Carries food from Oesophagus the mouth to the Small intestine Large intestine Waves of contraction churn food Muscular contractions church food; bile salts emulsify lipids Gastric juice, which contains pepsin, breaks down proteins into polypeptides Pancreatic juice contains: pancreatic amylase, which breaks starch into disaccharides; pancreatic protease, which breaks proteins and polypeptides into peptides; pancreatic lipase, which breaks lipids into fatty acids and glycerol stomach Stores large quantities of food as it is eaten, absorbs certain drugs, including some alcohol Absorbs simple sugars, amino acids, fatty acids, glycerol, vitamins, minerals, nutrients and water Absorbs water and vitamins; stores faeces; defaecation Digestive Complications Constipation Occurs if the movements of the large intestine are reduced and the contents remain there for long periods of time. As water is absorbed, the faeces become drier and harder than usual. Defaecation becomes difficult and possibly painful, a condition known as constipation. Constipation may be cause by a lack of fibre in the diet. Fibre is a form of cellulose, which cannot be digested by enzymes in the human body but it is important because it stimulates the movements of the alimentary canal. Diarrhoea Characterised by frequent defaecation of watery faeces. It is caused by irritation of the small or large intestine, which increases peristalsis so that the contents of the intestines move through before there is adequate absorption of water. Such irritation may be the result of a bacterial or viral infection. Bacteria that produce food poising may cause diarrhoea. Cholera bacteria cause such severe diarrhoea that death often results from dehydration through loss of water from the intestines. Bowel Cancer Colorectal cancer is an uncontrolled growth of cells in the wall of the large intestine. Research suggests that bowel cancer may be linked to diet, high alcohol consumption and smoking. A diet high in red and processed meat, and low in fibre (fruit and vegetables), may increase the risk of developing bowel cancer. Being overweight or obese and physical inactivity are also risk factors. Coeliac Disease Those with coeliac disease are unable to tolerate the protein, gluten, which is found in wheat, rye and barley. If sufferers eat food containing gluten, their immune system responds by damaging or destroying the villi in the small intestine. Without healthy villi, nutrients cannot be absorbed and the person becomes malnourished no matter how much food they eat. Coeliac disease is inherited and there is no current cure the only treatment is a gluten-free diet.