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Digestive System When we eat such things as bread, meat, and vegetables, they are not in a form that the body can use as nourishment. Our food and drink must be changed into smaller molecules of nutrients before they can be absorbed into the blood and carried to cells throughout the body. Digestion is the process by which food and drink are broken down into their smallest parts so that the body can use them to build and nourish cells and to provide energy. The main purpose of the Digestive system is to break down food and absorb nutrients. There are two basic divisions to the digestive system; these are the gastrointestinal (GI) tract, also known as the alimentary canal, and the accessory digestive organs. Digestion is the process by which food substances are changed into forms that can be absorbed through cell membranes. It allows the nutrients and energy of the plant and animal cells that we eat to be released and absorbed. Structure of the Digestive system Foods must be broken down to their building blocks to be absorbed. Building blocks of carbohydrates are simple sugars, or monosaccharides; Building blocks of proteins are amino acids. Building blocks of fats, or lipids, are fatty acids and glycerol. Gastrointestinal Tract Mouth: Food is ingested into the gastrointestinal tract through the mouth, or oral cavity. Both mechanical (chewing) and chemical food breakdown begin in the mouth. Saliva contains mucus, which helps bind food together into a bolus, and salivary amylase, which begins the chemical breakdown of starch. Saliva is secreted in response to food in the mouth, mechanical pressure, and psychic stimuli. Essentially no food absorption occurs in the mouth. Pharynx: From the mouth, food passes posteriorly into the pharynx, which is the common path for food, fluids, and air. The pharynx is divided into three sections: the nasopharynx (air from the Fit College Digestive System v1.1 Mar 2012 1 nose passes here), the oropharynx (food and air from the mouth passes here), and the laryngopharynx (air going to the lungs passes here). The walls of the pharynx contain two skeletal muscle layers. The cells of the inner layer run longitudinally; those of the outer layer (the constrictor muscles) run around the wall in a circular fashion. Alternating contractions of these muscle layers propels food through the pharynx into the oesophagus below. This propelling mechanism is called peristalsis. Oesophagus: The Oesophagus is a straight, soft tube that connects the mouth to the stomach. It has a layer of muscle running up and down its length. This allows the oesophagus to ‘push’ mouthfuls of food down into the stomach immediately after swallowing. This action is called peristalsis. Stomach: When food enters the stomach, gastric secretion is stimulated by vagus nerves and by gastrin (a local hormone). Hydrochloric acid activates the protein-digesting enzyme pepsin, and chemical digestion of proteins begins. Food is also mechanically broken down by the churning activity of stomach muscles. The enterogastric reflex controls movement of chyme into the small intestine. Small Intestine: Chemical digestion of fats, proteins, and carbohydrates is completed in the small intestine by intestinal enzymes and, more importantly, pancreatic enzymes. Alkaline pancreatic juice neutralizes acid chyme and provides the proper environment for the operation of enzymes. Both pancreatic Juice (the only source of lipases) and bile (formed by the liver) are necessary for normal fat breakdown and absorption. Bile acts as a fat emulsifier. Secretin and cholecystokinin, hormones produced by the small intestine, stimulate release of bile and pancreatic juice. Segmental movements mix foods; peristaltic movements move foodstuffs along the small intestine. Most nutrient absorption occurs by active transport into the capillary blood of the villi. Fats are absorbed by diffusion into both capillary blood and lacteals in the villi. Large Intestine: The large intestine receives bacteria-laden indigestible food residue. Activities of the large intestine are absorption of water and salts and of vitamins made by resident bacteria. Rectum / anus: The rectum and anus are the final place for storage and elimination of faeces from the body during defecation. When faeces are delivered to the rectum by peristalsis and the defecation reflex is initiated. Fit College Digestive System v1.1 Mar 2012 2 Accessory Organs Salivary Glands: Three pairs of major salivary glands (parotid, submandibular, and sublingual glands) and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during mastication. Saliva contains water, mucus, and enzyme amylase. Functions of saliva include the following: It has a cleansing action on the teeth. It moistens and lubricates food during mastication and swallowing. It dissolves certain molecules so that food can be tasted. It begins the chemical digestion of starches through the action of amylase, which breaks down starch and polysaccharides into disaccharides. Liver: The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into two major lobes and two smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule. The liver receives blood from two sources. Freshly oxygenated blood is brought to the liver by the common hepatic artery, a branch of the celiac trunk from the abdominal aorta. Blood that is rich in nutrients from the digestive tract is carried to the liver by the hepatic portal vein. The liver has a wide variety of functions and many of these are vital to life. Hepatocytes perform most of the functions attributed to the liver, but the phagocytic Kupffer cells that line the sinusoids are responsible for cleansing the blood. Liver functions include the following: Secretion Synthesis of bile salts Synthesis of plasma protein Storage Detoxification Excretion Carbohyrate metabolism Lipid metabolism Protein metabolism Filtering Gallbladder: The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol. Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of haemoglobin are excreted from the body in the bile. Fit College Digestive System v1.1 Mar 2012 3 Pancreas: The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include amylase, trypsin, peptidase, and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin. Food undergoes three types of processes in the body: Digestion Absorption Metabolism Digestion and absorption occur in the digestive tract. After the nutrients are absorbed, they are available to all cells in the body and are utilized by these cells in metabolism. Metabolism is the sum of all the chemical reactions that take place in the body including the utilization of nutrients. The digestive system prepares nutrients for utilization by body cells through: Ingestion: The first activity of the digestive system is to take in food. This needs to take place before anything else can happen. Mechanical digestion: Large pieces of food need to be broken down into smaller ones so that various enzymes can begin another activity in the chain of events. Mechanical digestion begins in the mouth with chewing or mastication of the food and continues with the churning and mixing actions within the stomach. Chemical digestion: Complex molecules are transformed by chemical digestion into smaller molecules that can be easily absorbed and utilized by the cells. Movements: After ingestion and mastication, the food particles move from the mouth into the pharynx, then into the oesophagus. This movement is deglutition or swallowing. The movements that propel the food particles through the digestive tract are called peristalsis. These are rhythmic waves of contractions that move the food particles through the various regions in which the mechanical and chemical digestion takes place. Absorption: The simple molecules that result from chemical digestion pass through cell membrane of the lining in the small intestine into the blood or lymph capillaries. This process is called absorption. Elimination: The food molecules that cannot be digested need to be eliminated from the body. The removal of indigestible wastes through the anus, in the form of faeces, is called defecation. Fit College Digestive System v1.1 Mar 2012 4 The functions of digestive system organs are summarised below: Organ Mouth Function Chewing, Salivating Oesophagus Stomach Swallowing Breakdown proteins Some absorption Small Intestine Breakdown all food Some absorption Large Intestine Rectum/Anus Absorption of water and electrolytes Formation of faeces Storage of faeces and defecation Forms of digestion Mechanical (chewing) Chemical (Saliva) Movements Chemical Movements Chemical: enzymes, gastric secretions Absorption Mechanical Energy Expenditure The amount of energy, measured in calories that a person uses. Calories are used by people to breath, circulate blood, digest food, and be physically active.Daily energy expenditure consists of three components: Resting Metabolic Rate (RMR): Resting metabolic rate (RMR) is the minimum number of calories your body needs to support its basic physiological functions, including breathing, circulating blood and all of the numerous biochemical reactions required to keep you alive. Your RMR is generally 60-75% of your total daily energy expenditure. Thermic effect of food: Eating requires energy for the ingestion and digestion of food, and for the absorption, transport, interconversion, oxidation and deposition of nutrients. These metabolic processes increase heat production and oxygen consumption, and are known by terms such as dietary-induced thermogenesis, specific dynamic action of food and thermic effect of feeding. The metabolic response to food increases total energy expenditure by about 10 percent of the BMR over a 24-hour period in individuals eating a mixed diet. Thermic effect of exercise: The amount of energy the body burns during daily activities such as exercise, recreation, work, and housework influences daily energy expenditure. Daily physical activities account for 20 - 40% of calories burned each day. This part will vary depending on the individual and how active they are each day. A sedentary person will require fewer calories to maintain weight than a busy worker in a construction site. The intensity, frequency and duration of any activity all have an effect on metabolism. Adaptive thermogenesis: Adaptive thermogenesis (AT) represents the way the body responds to stress whether it is emotional or environmental (for example, changes in temperature). Typically, stressful situations result in an increase in calories burned. Scientists are still not sure about how much it contributes to total daily caloric expenditure. Many of them think it may be an important factor in explaining why two people can be leading the same type of lifestyle in terms of exercise and diet and yet, have two completely different body types. Fit College Digestive System v1.1 Mar 2012 5 Medical Conditions Diabetes Diabetes is a condition in which there is an inability of insulin, a hormone produced by the pancreas, to regulate the amount of glucose (sugars) in the blood. Type 1 diabetes typically occurs in childhood and is characterized by an inability of the pancreas to produce insulin; this type of diabetes often requires injections of insulin. Type 2 diabetes, or adult onset diabetes, occurs when the body "resists" insulin and glucose levels remain increased. Gallstones Gallstones form in the gallbladder when cholesterol or pigments crystallize out of bile. They can vary in size from as small as grains of sand to as large as golf balls. The tendency to form gallstones is partially inherited and is more common in people who are obese, overeat or lose weight rapidly through extreme diets. Cancer Cancer is a general term for about 100 diseases characterized by abnormal and uncontrolled growth of cells. Diet is thought to explain much of the variation in incidence of some cancers, around the world and over time. This is a complex area of research because of the long delays in cancer development after exposure to dietary causes. Major difficulties arise when researchers try to estimate accurately what individuals ate many years ago. Nonetheless progress in understanding the role of diet is being made. It is now estimated that about 35% of deaths from cancer in economically developed countries may be prevented if everyone had a healthy diet. The following are nutritional issues associated with cancer. Obesity increases the risk of colon, breast, gallbladder, ovary, prostate and uterine cancer. High fat diets may contribute to the development of certain cancers, particularly those of the breast, colon and prostate. High fibre foods may help reduce the risk of colon cancer. A varied diet containing plenty of vegetables and fruit may reduce the risk for many cancers. Salt-cured, smoked and nitrate-cured foods have been linked to oesophageal and stomach cancers. Gout Gout (old name: podagra) is a form of arthritis caused by the accumulation of uric acid crystals (due to hyperuricemia) in joints. It is an immensely painful disease, which in most cases affects only one joint, most commonly the big toe. The patient usually suffers from two sources of pain. The crystals inside the joint can cause immense pain whenever the affected area is moved. Fit College Digestive System v1.1 Mar 2012 6 Generally thought to be a hereditary disease, there are many causes: increased purine intake, sudden weight loss due to dieting, and/or diuretics can cause elevated uric acid and mimic the symptoms of gout. Nutritional deficiencies Research has shown that optimum nutrient levels are required to maintain good health. Nutrient deficiencies are associated with poor health such as iron deficiency in anaemia, iodine imbalance with thyroid function, Vitamin D & Calcium with osteoporosis, Oxygen deficiency associated with fatigue. Essential nutrients are involved in the formation of: Antioxidants, blood supply, immune and nervous systems, brain function, digestion, skin growth, bone building and energy levels. Whilst nutrients are supplied by food, air & water modern farming & processing techniques are depleting the essential nutrients found in these foods. Stress, poor diets and allergies also have a contribution to nutritional deficiencies. Iron deficiency: Iron deficiency is the most common known form of nutritional deficiency. In the human body, iron is present in all cells and has several vital functions. Iron acts as a carrier of oxygen to the tissues from the lungs in the form of haemoglobin, as a facilitator of oxygen use and storage in the muscles as myoglobin, as a transport medium for electrons within the cells in the form of cytochromes, and as an integral part of enzyme reactions in various tissues. Too little iron can interfere with these vital functions and lead to morbidity and mortality. Calcium deficiency: One of the major symptoms of a deficiency of calcium is skeletal abnormalities. Osteopenia, osteomalacia, osteoporosis and rickets may all be caused by calcium deficiency. A constant supply of calcium is necessary throughout life, but is especially important during phases of growth, pregnancy, and lactation (breast feeding). Protein Energy Malnutrition (PEM): The lack of sufficient energy or protein to meet the body's metabolic demands, as a result of either an inadequate dietary intake of protein, intake of poor quality dietary protein, increased demands due to disease, or increased nutrient losses. Vitamin A deficiency: Vitamin A deficiency (VAD) is the leading cause of preventable blindness in children of developing countries, and raises the risk of disease and death from severe infections. In pregnant women VAD causes night blindness and may increase the risk of maternal mortality. It exists when the chronic failure to eat sufficient amounts of vitamin A or beta-carotene results in levels of bloodstream Vitamin A that is below a defined range. Fit College Digestive System v1.1 Mar 2012 7