8.3 The Digestive System PDF

# 8.3 The Digestive System Do you know how the nutrients from the food we consume are absorbed by our cells? This is made possible by our digestive system through the process of food digestion. **Food digestion** is the breakdown of food that are large, complex and insoluble into smaller, simple an...

# 8.3 The Digestive System Do you know how the nutrients from the food we consume are absorbed by our cells? This is made possible by our digestive system through the process of food digestion. **Food digestion** is the breakdown of food that are large, complex and insoluble into smaller, simple and soluble molecules that can be absorbed into the bloodstream and transported to different parts of the body. There are two types of digestion: **physical digestion** and **chemical digestion.** Both the processes involve the breakdown of food and occur in the digestive tract, also known as the **alimentary canal.** ## Types of Digestion ### Physical Digestion - Occurs in the **mouth**, **oesophagus**, **stomach** and **small intestine** - Does not involve any enzymes - Involves **peristalsis** #### Involves 1. Breakdown of large pieces of food into smaller pieces by the grinding action of teeth with the aid of tongue and saliva (chewing) 2. Wave-like contraction and relaxation (peristalsis) by the muscles in the walls of the oesophagus and the whole length of the digestive tract, pushes the food down the tract 3. Squeezing and mixing of food by the muscles of the stomach lining (churning) 4. Contraction and relaxation of segments of muscles in the small intestine (segmentation) ### Chemical Digestion - Occurs in the **mouth**, **stomach** and **small intestine** - Breakdown of complex food molecules into simple molecules with the aid of the enzymes - Does not involve peristalsis ## Parts of the Digestive System Food digestion takes place in the **alimentary canal** that extends from the **mouth** to the **anus.** The alimentary canal is long and muscular. It consists of the **mouth**, **oesophagus**, **stomach**, **small intestine** *(duodenum, jejunum and ileum)*, **large intestine** *(appendix, caecum, colon, rectum)* and **anus**. The total length of the alimentary canal is around 9 metres, and it is open at both ends, at one end is the **mouth** and at the other is the **anus**. The **small intestine** makes up two thirds of the total length of the alimentary canal. There are also other organs in the digestive system which are the **liver**, **gallbladder** and **pancreas.** Each part of the digestive system has its own specialised function. ## Digestion in the Mouth The digestive process begins in the **mouth**. The chewing action of teeth cuts the food into smaller pieces to increase the surface area of the food for the digestive enzymes to act on. The presence of food stimulates the **salivary glands** in the mouth to secrete **saliva**. Saliva contains the digestive enzyme, **salivary amylase**, which breaks down starch into maltose. **Salivary amylase** *Starch* + *Water* → *Maltose* The **tongue** then rolls the partially digested food into a small ball called **bolus** with the help of the saliva and pushes it into the **oesophagus**. The **oesophagus** is a long, narrow tube with strong muscles in its walls. The **bolus** is then pushed through the **oesophagus** into the **stomach** by **peristalsis**. **Peristalsis** is the rhythmic contraction and relaxation of muscles which pushes food along the alimentary canal. ## Digestion in the Stomach The **stomach** is a sac-like muscular organ at the end of the **oesophagus.** The glands in the stomach wall secrete a liquid called **gastric juice**. Gastric juice consists of **hydrochloric acid**, **mucus**, and the digestive enzyme, **pepsin**. - **Hydrochloric acid** kills bacteria in the food that enters the stomach, creating an acidic environment for the enzyme, pepsin, and stopping the action of the salivary amylase. - **Mucus** protects the stomach wall from the hydrochloric acid and digestive enzymes. - **Pepsin** breaks down protein into polypeptides. **Pepsin** *Protein* + *Water* → *Polypeptides* The **stomach** stores food for about three to four hours, enabling the food to mix with the gastric juice. From the **stomach**, the partly digested, semi-liquid mass called **chyme** moves into the **small intestine** when the sphincter muscle relaxes. There is a sphincter, ring-shaped muscles at the top and bottom of the **stomach** each. The **cardiac sphincter** regulates the food from the **oesophagus** entering the **stomach** and prevents the contents from the **stomach** re-entering the **oesophagus.** The **pyloric sphincter** controls the flow of food out of the **stomach** and allows only small amount of food into the **duodenum** *(first part of the small intestine)* at one time. ## Digestion in the Small Intestine The **small intestine** consists of the **duodenum**, **jejunum** and **ileum.** **Duodenum** receives - **chyme** from the stomach - **bile** produced by the **liver** - **pancreatic juice** secreted by the **pancreas**. The **liver** produces a greenish liquid called **bile** and stores it in the **gall bladder.** **Bile** flows into the **duodenum** through the **bile duct.** **Bile** emulsifies fat into small droplets and creates an alkaline condition for the **pancreatic enzymes** to work. The **pancreas** secretes **pancreatic juice** into the **duodenum** through the **pancreatic duct**. The **pancreatic juice** contains digestive enzymes such as **trypsin**, **lipase** and **pancreatic amylase.** Unlike **pepsin**, these enzymes require an alkaline environment to function. * **Trypsin** further breaks down polypeptides into peptides. **Trypsin** *Polypeptides* + *Water* → *Peptides* * **Lipase** breaks down fat molecules into fatty acids and glycerol. **Lipase** *Fat* + *Water* → *Fatty acids and glycerol* * **Pancreatic amylase** breaks down starch into maltose. **Pancreatic amylase** *Starch* + *Water* → *Maltose* The final breakdown of food occurs in the **ileum**. The glands on the **ileum** wall secretes **intestinal juice** which consists of the enzymes **maltase**, **sucrase**, **lactase**, **erepsin** and **lipase**. - **Maltase** breaks down maltose into glucose. **Maltase** *Maltose* + *Water* → *Glucose* ### Digestion of Carobhydrates - **Sucrase** breaks down sucrose into glucose and fructose **Sucrase** *Sucrose* + *Water* → *Glucose* + *Fructose* - **Lactase** breaks down lactose into glucose and galactose. **Lactase** *Lactose* + *Water* → *Glucose* + *Galactose* ### Digestion Of Proteins - **Erepsin** breaks down peptides into amino acids. **Erepsin** *Peptides* + *Water* → *Amino acids* ### Digestion of Fat - **Lipase** breaks down fat into fatty acids and glycerol. **Lipase** *Fat* + *Water* → *Fatty acids* + *Glycerol* Digestion ends in the **small intestine** with fatty acids, glycerol, amino acids and glucose as the final products. ## Absorption of Digested Food in the Small Intestine The **jejunum** and **ileum** are adapted to enable simple molecules produced from the digested food to diffuse through its wall. - The **small intestine** is long, enabling greater absorption of nutrients. - Its internal layer is highly folded and covered with many finger-like projections called **villi**. **Villi** play a major role in the absorption of digested food. - The **villi** wall is one-cell thick, which helps speed up nutrient absorption. - Each **villus** has many specialised epithelial cells with tiny projections called **microvilli** to increase the surface area for nutrient absorption. - The **villi** have a network of blood capillaries that ensures glucose, amino acids and water-soluble vitamins are absorbed easily. - Each **villus** contains a structure called **lacteal** to absorb fatty acids, glycerol and fat-soluble vitamins. When nutrients enter the **blood**, they are carried away to the **liver** and then throughout the body. Nutrients travel through the **bloodstream** to feed all the cells in the body. There are some substances that cannot be absorbed by the **villi**. Thus, they are left in the **small intestine.** ## Defaecation The digested food and most of the water we drink are absorbed into the body in the **small intestine**. The remaining food that cannot be digested, dead cells, fibre and water enter the **large intestine**. In the **large intestine**, some of the water is reabsorbed. The remaining semi-solid waste is called **faeces**. It moves slowly with the help of the **peristaltic action**. It takes between 12 to 24 hours to reach the **rectum**. When the **faeces** accumulates in the **rectum**, it puts pressure to the **rectum** and triggers the urge to expel the **faeces** out of the body. The **rectum** muscles will contract to pass the faeces out of the body from the **anus**. This process is called **defaecation**. The waste from a meal you consume should be expelled from your body within about 24 hours if your gut is working well. In people who suffer from **constipation**, the waste will remain in the **large intestine** for quite some time and almost all of the water will be removed. This causes the **faeces** to become dry and hard. The best way to relieve **constipation** is to add more fibre to the diet. ## 8.4 Health Issues Related to the Digestive System The digestive system is important to our wellbeing, but various health issues can impact its functioning. These include **gastroesophageal reflux disease (GERD)**, **digestive tract cancers**, **irritable bowel syndrome (IBS)**, **hiatal hernia** and more. Disorders like **diarrhoea**, **constipation**, **appendicitis** and **diabetes mellitus** further emphasise the balance required for digestive health. ### Gastroesophageal Reflux Disease (GERD) **Gastroesophageal reflux disease (GERD)**, a persistent and chronic condition, occurs when stomach acid frequently flows back into the oesophagus, causing symptoms like heartburn and discomfort. The recurrent exposure to acidic content can lead to inflammation and damage the lining of the oesophagus. In severe cases, complications such as **Barrett's oesophagus** may develop, increasing the risk of oesophageal cancer. Lifestyle modifications, medications to reduce acid production, and, in some cases, surgical interventions aim to manage **GERD** and prevent complications. ### Digestive Tract Cancers **Digestive tract cancers**, encompassing the oesophagus, stomach and intestines, present various symptoms, including weight loss, abdominal pain and changes in bowel habits. Early detection plays an important role in treatment success. Risk factors such as age, genetic predisposition and lifestyle choices like smoking and diet contribute to the development of these cancers. Screening tests, such as colonoscopies, aid in identifying precancerous lesions. Integrated approaches, combining surgery, chemotherapy, and radiation therapy, are often used to treat digestive tract cancers. ### Irritable Bowel Syndrome (IBS) **Irritable bowel syndrome (IBS)** is a functional gastrointestinal disorder characterised by abdominal pain, bloating and changes in bowel habits. The symptoms of **IBS** are influenced by factors like stress and dietary patterns. Treating **IBS** involves making changes in lifestyle and diet. Try to manage stress, eat regular and balanced meals and drink plenty of water. Adding fibre to the diet through fruits, vegetables and whole grains can also help. If needed, the doctor may recommend medications to ease specific symptoms like pain or diarrhoea. ### Lactose Intolerance **Lactose intolerance** occurs when the body has difficulty digesting lactose, the sugar in milk, due to an inability to produce enough lactase, the enzyme that breaks down lactose. Symptoms like bloating, diarrhoea and abdominal pain can arise after consuming dairy products. Controlling lactose intake is important for symptom relief. This can involve choosing lactose-free alternatives, taking lactase supplements, and introducing dairy gradually to assess tolerance levels. ### Hiatal Hernia In a **hiatal hernia**, the stomach protrudes into the chest through the diaphragm, leading to symptoms like acid reflux, chest pain and difficulty swallowing. Treatment options may include making lifestyle changes such as dietary adjustments and elevating the head during sleep. In severe cases, surgery might be recommended to repair the hernia and relieve symptoms. ### Stomach Ulcer **Stomach ulcers**, also known as **gastric ulcers,** are sores that develop on the lining of the stomach. Contributing factors can include *Helicobacter pylori* infection or prolonged use of nonsteroidal anti-inflammatory drugs (NSAIDs). The symptoms of stomach ulcers often involve burning stomach pain, bloating and indigestion. Treatment for stomach ulcers depends on the cause of the ulcer. The treatments include taking antibiotics and acid-suppressing medications, limiting the use of NSAID pain relievers, managing stress, boosting immunity and controlling inflammation, and eating a low-processed, nutrient-dense diet. ### Diarrhoea **Diarrhoea** is characterised by loose or watery stools. Most cases of diarrhoea are due to bacterial, viral, or parasitic infections. It can also result from certain medications or underlying health issues. Diarrhoea is treated through various approaches, depending on the underlying cause. Antibiotics or antiparasitic medications are administered to eliminate infection-causing germs. Additionally, probiotics, which are beneficial bacteria, are sometimes recommended to restore a healthy balance in the gut. ### Constipation **Constipation** involves infrequent bowel movements and difficulty passing stools, often resulting from factors such as inadequate fibre intake, dehydration and the use of certain medications. Conditions like **irritable bowel syndrome (IBS)** and **hypothyroidism** can also contribute to constipation. Making lifestyle changes such as exercising regularly, drinking more water, and eating more high-fibre foods is important in relief. In persistent cases, over-the-counter or prescription medications may be recommended to promote bowel regularity and ease discomfort. ### Appendicitis **Appendicitis**, characterised by inflammation of the appendix, presents symptoms like abdominal pain, nausea and fever. Surgical removal of the appendix is typically required to prevent complications, including the risk of rupture. ### Diabetes Mellitus **Diabetes mellitus** is a chronic metabolic disorder characterised by elevated blood glucose levels. It occurs due to either insufficient production of insulin by the pancreas or ineffective utilisation of insulin by the body's cells. Digestive issues can arise, affecting nutrient absorption. Symptoms include frequent urination, thirst and unintentional weight loss. The treatment for diabetes mellitus typically involves a combination of lifestyle changes, medication and insulin therapy. Lifestyle changes include a balanced diet, regular exercise and weight management. In some cases, insulin injections are necessary to regulate blood sugar levels. Maintaining blood sugar levels is important in diabetes care.