Digestive System PDF

# Digestive System ## 9.1 Structure of the Human Digestive System The human digestive system is made up of a long and muscular alimentary canal that starts from the mouth to the anus. The parts of the alimentary canal include the mouth, oesophagus, stomach, small intestine, large intestine and an...

# Digestive System ## 9.1 Structure of the Human Digestive System The human digestive system is made up of a long and muscular alimentary canal that starts from the mouth to the anus. The parts of the alimentary canal include the mouth, oesophagus, stomach, small intestine, large intestine and anus. The other organs in the digestive system are liver, gallbladder and pancreas. Salivary, gastric and intestinal glands secrete digestive juices into the alimentary canal. ## 9.2 Digestion ### Types of Digestion Digestion is the process that breaks down large and complex pieces of food into smaller and simple pieces that can be dissolved for easy absorption. Digestion is made up of two parts, that is, physical digestion and chemical digestion. | Type | Description | |---|---| | Physical Digestion | The mechanical breakdown of food to form small particles. Involves chewing and peristalsis. | | Chemical Digestion | The decomposition process of complex molecules into simple molecules. Involves enzymes reaction. | ### Digestion of Carbohydrates in the Mouth The digestive process begins in the mouth. The presence of food in the mouth stimulates the secretion of saliva from the salivary glands. Saliva contains salivary amylase that hydrolyses starch to maltose. The pH of the saliva ranges between 6.5-7.5, which is suitable for salivary amylase to act at its optimum. ## 9.3 Absorption ### The Adaptations of Ileum and Villus in the Absorption of Digested Food Simple molecules produced from the digested food are absorbed in the ileum of the small intestine. ### Ileum The long ileum is adapted to absorb nutrients because its internal layer is folded and covered by tiny projections called villi. ### Villus Villus has the following adaptations to absorb nutrients: - The epithelial layer of the villus is one cell thick. This helps accelerate nutrient absorption. - Goblet cells secrete mucus to aid digestion. - The network of blood capillaries helps to transport digestive products to the whole body. - Lacteal carries droplets of fatty acids and glycerol. - The intestinal glands secrete intestinal juices that have digestive enzymes. - On the surface of the villus epithelium, there are many tiny projections called microvillus. Microvillus provides a large surface area to increase the rate of nutrient absorption. ## 9.4 Assimilation ### The Role of the Circulatory System The human circulatory system consists of the blood circulation system and the lymphatic system to help transport nutrients to be assimilated. In the assimilation process that occurs in cells, nutrients are used to form complex compounds or structures of components. The blood capillaries in the small intestine combine to form the **hepatic portal vein** that transports blood to the liver. Lacteals combine to form bigger lymph vessels in the lymphatic system. Then, the contents of the lymph vessels enter the **thoracic duct** that flows into the left subclavian vein. This lipid is then transported by blood throughout the body. ### Functions of Liver in the Assimilation of Digested Food The liver is the regulator that controls the quantity of nutrients that enter the blood circulatory system. The liver carries out the following functions: **Metabolism of Digested Food** - Glucose is used for cellular respiration. Amino acids are used for synthesising plasma proteins and enzymes. - Through the deamination process, excess amino acids are turned into urea to be excreted through the urine. **Detoxification** - Liver cells expel toxic substances from the blood. - Toxic substances are expelled through the urine. **Storage of Nutrients** Excess glucose is converted to glycogen to be stored. ## 9.5 Defaecation ### Functions of the Large Intestine After the absorption of nutrients is completed in the ileum, undigested food, dead cells, epithelial cells, fibre and water enter the large intestine and move slowly through peristaltic action. Fibre consists of cellulose walls of plant cells. The large intestine carries out two main functions: - Absorption of water and vitamins. - Formation of faeces. ### Absorption of Water and Vitamins Substances absorbed are: - Water and mineral salts. - Metabolic byproducts of some bacteria such as vitamin B, vitamin K and folic acid. ### Formation of Faeces After the water is absorbed, the remaining waste is a semisolid called faeces. Faeces contains dead cells from the inner layer of the intestine, waste products such as bile pigments, bacteria and toxic substances. The walls of the large intestine secrete mucus to smoothen the movement of faeces until the anus. The movement of faeces takes about 12 to 24 hours before entering the rectum. The faeces will accumulate in the rectum until the pressure in the rectum increases and triggers the need to expel faeces from the body. The rectum muscles will contract to expel faeces from the anus. This process is called defaecation. ## 9.6 Balanced Diet ### Energy value in a food sample A balanced diet refers to a diet that consists of all seven food classes (carbohydrates, lipids, proteins, vitamins, mineral salts, fibre and water) in the correct proportion and balanced quantity according to individual needs so that optimal health can be maintained. #### Energy Value - Energy value is the total amount of energy released when one gram of food is oxidised completely. - The energy value in food is measured in the form of heat energy, that is, in kilojoule per gram (kJ g^-1). - Another unit of heat energy is calorie. - 1 calorie or 4.2 joule is defined as the quantity of heat energy needed to raise the temperature of 1 gram water by 1 degree Celsius (°C) at a pressure of 1 standard atmosphere. - Energy value of food (kJ g^-1) = Water mass (g) x 4.2 J g^-1 °C^-1 x Increase in water temperature (°C) / Mass of food sample (g) x 1000. ### The contents of vitamin C in fruit or vegetables juices The nutrient content in various types of food is different. For example, the vitamin C content in fruits and vegetables are different. ## 9.7 Health Issues Related to the Digestive System and Eating Habits ### Adaptation of digestive organs Obesity is a health issue on the rise throughout the world. Although obesity can be controlled through diet management and routine exercise programmes, at times, obesity requires medical treatment. Specialist docters may suggest surgical procedures to reduce body weight such as **gastric bypass**. A gastric bypass involves a reduction of the stomach size using various methods of surgery. Among the short-term side effects of this surgery are acid reflux, nausea, vomiting, expanded oesophagus, certain food prohibitions and risk of infection.. The long-term side effects are dizziness, low blood sugar level, malnutrition, stomach ulcer and defaecation problems. ### Health issues related to defaecation The food class that is most important in the defaecation process is fibre. Intake of diet that is high in fibre such as fruits and vegetables can smoothen bowel movements. This can prevent health problems such as constipation, colon cancer, rectum cancer and haemorrhoid. Some of the functions of fibre are to: - Stimulate peristalsis. - Absorb and expel toxic substances. - Regulate the absorption of glucose especially for diabetes mellitus patients. - Increase the population of beneficial bacteria in the large intestine. Besides, the intake of a large amount of water can ensure that the faeces stay soft and move easily along the large intestine to aid the process of defaecation. ### Health issues related to eating habits Apart from a balanced diet, eating habits also play an important role in fulfilling our energy requirements and maintaining our health. Poor eating habits and an imbalanced diet can cause numerous health problems such as gastritis, muscle dysmorphia, anorexia nervosa and bulimia nervosa. **Gastritis** Gastritis refers to the inflammation and corrosion of the stomach epithelial layer by gastric juice when there is no food in the stomach. Untreated gastritis can result in gastric ulcers. The causes of gastritis include eating irregular quantities of food at irregular hours and excessive intake of alcohol or painkillers. **Anorexia Nervosa** Anorexia nervosa is quite common amongst teenage girls who are obsessed with their body weight. Anorexics will avoid food to achieve their ideal body weight. They also suffer from psychological problems and nutrient deficiency because of their normal digestive system is affected **Bulimia Nervosa** For people with bulimia nervosa who are also obsessed with controlling their body weight, they will eat a lot and vomit out the food that they have eaten or take laxatives that cause diarrhoea. In the long run, the patient may suffer from dehydration, nutritional problems and eventually cardiovascular disease or kidney failure. **Muscle Dysmorphia** Some individuals feel that their size is small with not enough growth. So, they subject themselves to extreme weightlifting training and exercise. Sometimes, they consume steroids or muscle building supplements. This health issue is called muscle dysmorphia.