Digestive System Notes PDF

**The Digestive System: Anatomy and Physiology** The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. This unit covers the anatomy and physiology of the gastrointestinal (GI) tract, the accessory organs, the processes involved in digestion and abso...

**The Digestive System: Anatomy and Physiology** The digestive system is responsible for breaking down food, absorbing nutrients, and eliminating waste. This unit covers the anatomy and physiology of the gastrointestinal (GI) tract, the accessory organs, the processes involved in digestion and absorption, and the regulation of food intake and energy balance. **1. Components of the Gastrointestinal Tract** The gastrointestinal tract is a continuous tube extending from the mouth to the anus, where digestion and absorption occur. **Oral Cavity:** - **Functions:** The oral cavity (mouth) is the entry point for food. It is where mechanical digestion begins through chewing (mastication) and chemical digestion starts with the secretion of saliva, which contains the enzyme amylase to break down starch. - **Saliva:** Produced by the salivary glands, saliva lubricates food and begins the process of carbohydrate digestion. **Pharynx and Oesophagus:** - **Pharynx:** A muscular tube that serves as a pathway for the movement of food from the mouth to the oesophagus. It also plays a role in the respiratory system by directing air to the larynx. - **Oesophagus:** A muscular tube that connects the pharynx to the stomach. It uses coordinated muscle contractions known as **peristalsis** to move food down towards the stomach. **Peristalsis:** - **Definition:** Peristalsis is the rhythmic contraction of the smooth muscles in the wall of the gastrointestinal tract that propels food along the digestive system. It occurs throughout the entire GI tract, from the oesophagus to the rectum. **Small Intestine:** - **Anatomy:** The small intestine is divided into three parts: - **Duodenum:** The first part of the small intestine, where most chemical digestion occurs. It receives bile from the liver and gallbladder and digestive enzymes from the pancreas. - **Jejunum:** The middle section, primarily involved in the absorption of nutrients. - **Ileum:** The final section, which absorbs remaining nutrients and bile acids. - **Differentiated Cells:** - **Enterocytes:** Absorptive cells that line the small intestine and are responsible for nutrient absorption. - **Goblet Cells:** Secrete mucus to protect the intestinal lining. - **Paneth Cells:** Located at the base of the crypts, they secrete antimicrobial peptides to protect the gut from infection. - **Enteroendocrine Cells:** Release hormones that regulate digestion and appetite. **Large Intestine:** - **Anatomy:** The large intestine is composed of the caecum, colon (ascending, transverse, descending, and sigmoid), rectum, and anus. - **Functions:** The primary function of the large intestine is to absorb water and electrolytes from the remaining indigestible food matter and to form and store feces until defecation. - **Specialised Cells:** - **Colonocytes:** Absorptive cells in the large intestine that reabsorb water and electrolytes. - **Goblet Cells:** Secrete mucus to lubricate the passage of feces. **2. Accessory Organs of Digestion** The accessory organs of digestion include the pancreas, liver, and gallbladder, which play essential roles in producing digestive enzymes and bile. **Pancreas:** - **Functions:** The pancreas has both endocrine and exocrine functions. It produces digestive enzymes (such as amylase, lipase, and proteases) that are released into the duodenum to aid in the digestion of carbohydrates, fats, and proteins. The pancreas also secretes bicarbonate to neutralise stomach acid in the duodenum. **Liver:** - **Functions:** The liver produces bile, which is essential for the emulsification and digestion of fats. The liver also plays a vital role in metabolism, detoxification, and storage of nutrients like glycogen, vitamins, and minerals. **Bile:** - **Production and Storage:** Bile is produced by the liver and stored in the gallbladder. It is released into the duodenum in response to the presence of fats in the chyme, aiding in their digestion and absorption. **3. The Process of Absorption** Absorption occurs primarily in the small intestine, where nutrients pass through the intestinal wall and into the bloodstream or lymphatic system. **Carbohydrate Absorption:** - **Process:** Simple sugars (monosaccharides) are absorbed directly into the bloodstream through the enterocytes in the small intestine. **Protein Absorption:** - **Process:** Amino acids and small peptides are absorbed into the bloodstream via active transport mechanisms in the enterocytes. **Fat Absorption:** - **Process:** Fatty acids and monoglycerides are absorbed into the enterocytes, where they are reassembled into triglycerides, packaged into chylomicrons, and then transported into the lymphatic system before entering the bloodstream. **Vitamins and Minerals:** - **Fat-Soluble Vitamins (A, D, E, K):** Absorbed along with dietary fats in the small intestine. - **Water-Soluble Vitamins (B, C):** Absorbed directly into the bloodstream. - **Minerals:** Absorbed through active or passive transport, depending on the mineral. **4. Phases of Digestion** Digestion occurs in three distinct phases: cephalic, gastric, and intestinal. **Cephalic Phase:** - **Trigger:** Initiated by the sight, smell, taste, or thought of food, this phase prepares the digestive system for food intake. - **Process:** The brain signals the stomach to begin producing gastric juices (including hydrochloric acid and pepsin) and increases salivation in anticipation of food. **Gastric Phase:** - **Trigger:** Begins when food enters the stomach. - **Process:** The stomach distends, and gastric juices are secreted to break down proteins and fats. The hormone gastrin is released, stimulating further acid production and gastric motility. **Intestinal Phase:** - **Trigger:** Starts when chyme enters the small intestine. - **Process:** Hormones like secretin and cholecystokinin (CCK) are released, slowing gastric emptying and stimulating the release of pancreatic enzymes and bile. This phase ensures that nutrients are properly digested and absorbed in the small intestine. **5. Hormones Involved in Digestion** Several hormones regulate digestion and appetite. **Leptin:** - **Function:** Produced by adipose tissue, leptin signals the brain to reduce appetite and increase energy expenditure, helping regulate body weight. **Ghrelin:** - **Function:** Produced primarily by the stomach, ghrelin stimulates appetite by signaling the brain when the stomach is empty. **Gastrin:** - **Function:** Secreted by the stomach, gastrin stimulates the production of gastric acid, promoting digestion in the stomach. **Secretin:** - **Function:** Released by the small intestine in response to acidic chyme, secretin stimulates the pancreas to release bicarbonate to neutralise the acid and protect the intestinal lining. **Cholecystokinin (CCK):** - **Function:** Also released by the small intestine, CCK stimulates the gallbladder to release bile and the pancreas to release digestive enzymes, aiding in the digestion of fats and proteins. **6. Concept of Basal Metabolic Rate (BMR), Calories, and Body Mass Index (BMI)** **Basal Metabolic Rate (BMR):** - **Definition:** BMR is the amount of energy expended while at rest in a neutrally temperate environment. It represents the minimum amount of energy required to maintain basic physiological functions, such as breathing, circulation, and cell production. **Calories:** - **Definition:** A calorie is a unit of energy. In nutrition, calories refer to the amount of energy provided by food and drinks. The body uses calories from food for energy to perform all activities, from basic functions like breathing to physical activity. **Body Mass Index (BMI):** - **Definition:** BMI is a measure of body fat based on height and weight that applies to adult men and women. It is calculated by dividing a person's weight in kilograms by the square of their height in meters (kg/m²). - **Classification:** - Underweight: BMI \< 18.5 - Normal weight: BMI 18.5 -- 24.9 - Overweight: BMI 25 -- 29.9 - Obese: BMI ≥ 30 **Multiple Choice Questions (MCQs)** 1. **What is the primary function of bile in digestion?** - a\) Neutralising stomach acid - b\) Breaking down proteins - c\) Emulsifying fats - d\) Stimulating saliva production 2. **Which phase of digestion is triggered by the presence of food in the stomach?** - a\) Cephalic phase - b\) Gastric phase - c\) Intestinal phase - d\) Absorptive phase 3. **Where does most nutrient absorption occur in the digestive tract?** - a\) Stomach - b\) Large intestine - c\) Small intestine - d\) Oesophagus 4. **Which hormone is known as the "hunger hormone" because it stimulates appetite?** - a\) Leptin - b\) Ghrelin - c\) Gastrin - d\) Secretin 5. **What is the role of the pancreas in digestion?** - a\) Producing bile - b\) Storing nutrients - c\) Secreting digestive enzymes - d\) Absorbing nutrients **Clinical Cases** **Case 1: Peptic Ulcer Disease** **Presentation:**\ A 45-year-old man presents with burning abdominal pain, particularly after eating, along with nausea and occasional vomiting. He reports a history of NSAID use for chronic back pain. **Discussion:** - **Question:** Discuss the role of gastric acid and the mucosal barrier in the development of peptic ulcers. How does NSAID use contribute to this condition? - **Answer:** Peptic ulcers develop when the mucosal barrier is compromised, allowing gastric acid to erode the stomach or duodenal lining. NSAIDs inhibit prostaglandin production, which is crucial for maintaining the protective mucosal layer, leading to increased susceptibility to ulcers. **Case 2: Gallstones** **Presentation:**\ A 60-year-old woman presents with severe right upper quadrant pain, especially after eating fatty meals. She also reports nausea and vomiting. **Discussion:** - **Question:** Explain how gallstones form and the role of bile in this process. What dietary modifications might help prevent gallstone formation? - **Answer:** Gallstones form when there is an imbalance in the substances that make up bile, such as cholesterol, bilirubin, and bile salts. This can lead to the crystallisation of these substances in the gallbladder. Reducing the intake of fatty foods and maintaining a healthy weight can help prevent gallstone formation.