week 7 digestve system

Define the primary function of the digestive system and name its five core processes.

The primary function of the digestive system is to enable the body to obtain nutrients, water, and energy from food. Its five core processes are: 1. Ingestion 2. Propulsion 3. Digestion 4. Absorption 5. Defecation.

Describe the process of ingestion and its initial digestive events.

Ingestion is the intake of food and fluids into the digestive tract through the mouth. It involves placing food into the mouth, chewing (mastication), and mixing food with saliva. Saliva moistens food to make it easier to swallow and initiates the chemical digestion of carbohydrates via salivary amylase.

Describe the process of propulsion and its two main mechanisms.

Propulsion is the movement of food and digestive contents through the gastrointestinal tract. It occurs through two main mechanisms: 1. Swallowing (deglutition): Moves food from the mouth to the stomach. 2. Peristalsis: Uses coordinated waves of smooth muscle contraction to move material along the digestive tract.

Define digestion and differentiate between mechanical and chemical digestion.

Digestion is the breakdown of food into smaller molecules that can be absorbed by the body. 1. Mechanical digestion: The physical breakdown of food into smaller pieces (e.g., chewing in the mouth, churning in the stomach, segmentation in the small intestine). 2. Chemical digestion: The enzymatic breakdown of large food molecules into smaller absorbable units.

Describe the process of absorption and where it primarily occurs.

Absorption is the movement of digested nutrients, water, and electrolytes from the gastrointestinal tract into the blood or lymph. Most absorption occurs in the small intestine due to its large surface area (circular folds, villi, and microvilli). The large intestine also absorbs water and electrolytes.

Describe the process of defecation.

Defecation is the elimination of indigestible material from the body as faeces. After nutrients and most water are absorbed, remaining waste enters the large intestine, water continues to be removed, and waste is compacted into faeces for elimination.

Define macronutrients, list the three major types, and explain why digestion is required before absorption.

Macronutrients are nutrients required by the body in relatively large amounts that provide energy and building blocks for growth and maintenance. The three major macronutrients are carbohydrates, proteins, and lipids. Why digestion is required: Macronutrient molecules are too large to pass through the intestinal wall. Digestion breaks them down into smaller building blocks that can cross the intestinal lining into blood or lymph.

Identify the building blocks, dietary sources, and primary physiological functions of carbohydrates.

Building blocks: Monosaccharides (simple sugars such as glucose, fructose, and galactose). Dietary sources: Bread, rice, pasta, fruits, and vegetables. Functions: Carbohydrates are the body's preferred source of energy. Excess glucose is stored as glycogen in the liver and skeletal muscles for later use.

Identify the building blocks, dietary sources, and primary physiological functions of proteins.

Building blocks: Amino acids (joined by peptide bonds). Dietary sources: Meat, fish, eggs, dairy products, and legumes. Functions: Essential for growth, tissue repair, and the synthesis of enzymes, hormones, and cellular structures. (The body has limited capacity to store excess protein.)

Identify the building blocks, dietary sources, and primary physiological functions of lipids.

Building blocks: Fatty acids and monoglycerides (broken down from triglycerides, which consist of one glycerol attached to three fatty acids). Dietary sources: Oils, butter, nuts, avocados, fatty meats, and oily fish. Functions: Concentrated energy source, primary form of long-term energy storage, cell membrane structure, hormone synthesis, insulation, and organ protection.

The major lipid in the diet is the triglyceride

Define micronutrients and compare the major physiological roles of key vitamins and minerals.

Micronutrients are organic (vitamins) or inorganic (minerals) compounds required in much smaller quantities that do not provide energy but are essential for normal physiological function. Vitamins:

• Vitamin A: Vision and immune function

• Vitamin C: Collagen synthesis and antioxidant protection

• Vitamin D: Calcium absorption and bone health

• Vitamin K: Blood clotting.

Minerals:

• Calcium: Bone structure and muscle contraction

• Iron: Oxygen transport in haemoglobin

• Sodium: Fluid balance and nerve signalling

• Potassium: Membrane potentials and muscle function.

Describe the four major functions of the stomach in digestion.

1. Storage of food: Expands to store meals temporarily so digestion and absorption can occur gradually.

2. 2. Mechanical digestion: Rhythmic muscular churning mixes food with gastric juice to produce chyme.

3. 3. Chemical digestion: Initiates protein digestion via pepsin and hydrochloric acid.

4. 4. Regulation of gastric emptying: Controls chyme release into the small intestine through the pyloric sphincter.

Describe the source, pH level, and three main physiological functions of hydrochloric acid (HCl) in the stomach.

Secreted by parietal cells in gastric glands, lowering stomach pH to approximately 1–3. Functions: 1. Denatures dietary proteins to make them easier to digest. 2. Converts inactive pepsinogen into active pepsin. 3. Destroys microorganisms ingested with food.

Describe the production, activation, and digestive function of pepsin.

Pepsin is produced in an inactive precursor form called pepsinogen by chief cells. Activation & Function: Once exposed to the acidic environment created by hydrochloric acid (HCl), pepsinogen is converted into active pepsin, which begins breaking down proteins into smaller peptide fragments.

What cell type secretes intrinsic factor, and what is its essential physiological role?

Intrinsic factor is secreted by parietal cells in the stomach lining. Role: It is essential for the absorption of vitamin B₁₂ in the small intestine, which is required for red blood cell production and nervous system function.

How does the stomach lining protect itself from acid erosion and self-digestion?

Specialised mucous neck cells and surface epithelial cells secrete a thick layer of alkaline mucus that lines the stomach wall, protecting it from hydrochloric acid and pepsin breakdown.

Identify the three segments of the small intestine and describe its overarching role in digestion.

Extending from the pyloric sphincter to the large intestine, the small intestine is the primary site of chemical digestion and nutrient absorption. Its three segments are: 1. Duodenum: Receives chyme, pancreatic juice, and bile. 2. Jejunum: Middle segment where much of nutrient digestion and absorption occurs. 3. Ileum: Final segment that continues absorption and transports contents to the large intestine.

Describe the three structural adaptations of the small intestine that increase surface area for absorption.

1. Circular folds: Permanent folds of the intestinal wall that increase surface area and slow chyme movement. 2. Villi: Finger-like projections of the mucosa that contain blood capillaries and a lacteal (lymph vessel) for absorbing nutrients. 3. Microvilli: Microscopic projections on epithelial cells forming the brush border, which contain final digestive enzymes and further amplify absorptive surface area.

Identify the regions of the large intestine and describe its primary digestive functions.

Regions: Caecum, Colon (ascending, transverse, descending, sigmoid), Rectum, and Anal canal. Functions: Absorbs water and electrolytes (such as sodium and chloride), compacts undigested material into faeces, stores faeces in the rectum, and eliminates waste via defecation.

Describe the intestinal microbiota and list four of its beneficial physiological roles.

The intestinal microbiota is a diverse community of symbiotic microorganisms residing in the large intestine. Roles: 1. Ferments undigested carbohydrates reaching the colon. 2. Synthesises essential vitamins, including vitamin K and several B vitamins. 3. Contributes to normal immune system functioning. 4. Helps prevent colonisation by harmful pathogenic microorganisms.

Identify the four main functions of saliva and the specific enzyme involved in chemical digestion in the mouth.

Saliva is produced by three pairs of exocrine glands (parotid, submandibular, sublingual). Functions: 1. Moistens and lubricates food for easier chewing and swallowing. 2. Helps form a bolus. 3. Dissolves food molecules to interact with taste receptors. 4. Initiates chemical starch digestion via salivary amylase. (Also contains bicarbonate to maintain oral pH and antimicrobial agents.)

Describe the exocrine function of the pancreas and the composition and role of bicarbonate ions in pancreatic juice.

The exocrine pancreas produces pancreatic juice and secretes it into the duodenum via the pancreatic duct. Bicarbonate ions: Secreted in pancreatic juice to neutralise acidic chyme arriving from the stomach (raising intestinal pH), protecting the intestinal mucosa and creating an optimal alkaline pH for digestive enzymes.

Identify the major digestive enzymes in pancreatic juice and their specific target substrates.

1. Pancreatic amylase: Continues carbohydrate digestion by breaking down starch into smaller sugars.

2. 2. Pancreatic proteases (e.g., trypsin, chymotrypsin): Continue protein digestion by breaking polypeptides into smaller peptides.

3. 3. Pancreatic lipase: Breaks down triglycerides into fatty acids and monoglycerides.

Describe the digestive roles of the liver and gallbladder, and contrast the storage and secretion of bile.

Liver: The largest internal organ - continuously produces bile (alongside nutrient processing, glycogen storage, plasma protein synthesis, and detoxification).

Gallbladder: A small sac beneath the liver that stores and concentrates bile by removing water, contracting during digestion to release bile into the duodenum via the bile duct.

Describe the composition of bile and explain the process and importance of emulsification by bile salts.

Composition: Water, bile salts, phospholipids, and bilirubin (contains no digestive enzymes).

Emulsification: Bile salts break large fat droplets into many smaller droplets.

Importance: Does not chemically digest lipids, but dramatically increases the surface area of lipids exposed to pancreatic lipase, improving lipid digestion and absorption efficiency.

Describe the site of release, trigger, and physiological actions of Gastrin.

Site of release: Enteroendocrine cells (G cells) in the stomach lining.

Trigger: Entry of food into the stomach.

Actions: Stimulates secretion of gastric juice (including hydrochloric acid) and increases stomach motility/churning to enhance protein digestion.

Describe the site of release, trigger, and physiological actions of Secretin.

Site of release: Enteroendocrine cells in the duodenum.

Trigger: Arrival of acidic chyme from the stomach.

Actions: Stimulates the pancreas to secrete bicarbonate-rich pancreatic juice to neutralise stomach acid in the duodenum.

Describe the site of release, trigger, and physiological actions of Cholecystokinin (CCK).

Site of release: Enteroendocrine cells in the duodenum.

Trigger: Presence of fats and proteins in chyme.

Actions: Stimulates gallbladder contraction to release bile, and stimulates the pancreas to release enzyme-rich pancreatic juice.

These actions promote the digestion of lipids, proteins and carbohydrates within the small intestine.

Describe the smooth muscle layers and neural regulation involved in gastrointestinal motility.

Gastrointestinal motility is produced by coordinated contractions of two smooth muscle layers:

1. Inner circular layer: Muscle fibres encircle the GI tract.

2. Outer longitudinal layer: Muscle fibres run along the length of the GI tract.

Neural regulation: Coordinated by the enteric nervous system (ENS), an intrinsic neural network within the GI tract wall.

Describe the mechanism and primary function of peristalsis.

Mechanism: Smooth muscle contracts behind a bolus/chyme and relaxes ahead of it, creating a wave-like contraction that pushes material forward. Function: One-way propulsion of digestive contents along the oesophagus, stomach, small intestine, and large intestine.

Describe the mechanism and primary functions of segmentation, and compare it with peristalsis.

Mechanism: Localised, rhythmic contractions that repeatedly divide and mix chyme in the small intestine.

Functions: Mixes chyme with digestive enzymes, brings nutrients into contact with the brush border for absorption and enhances chemical digestion

Comparison: Unlike peristalsis (which propels material forward), segmentation moves contents back and forth to maximize mixing and absorption efficiency in the small intestine.

Gastrointestinal hormones

Gastrointestinal hormones are chemical messengers released by specialised enteroendocrine cells within the lining of the gastrointestinal tract. These hormones help coordinate digestive activities by regulating the secretion of digestive juices, the release of bile, and the movement of food through the digestive system

promote the digestion of lipids, proteins and carbohydrates within the small intestine.