Week 11: Digestive System

Question 1

List the two main groups of the digestive system

Answer 1

1. alimentary canal
2. accessory digestive organs

Question 2

Describe alimentary canal

Answer 2

• Also known as gastrointestinal tract or GI tract
• The continuous hollow tube extending from the mouth to the anus
• its walls are constructed by the oral cavity (mouth), pharynx, esophagus, stomach, small and large intestines, leadig to the anus (terminal opening).

Question 3

Describe accessory organs

Answer 3

• Organs that contribute to the digestive process but are not part of the alimentary canal
• include the tongue, teeth, salivary glands, pancreas, liver.

Question 4

Label image

Answer 4

Question 5

List and describe the major processes occurring during digestive system activity

Answer 5

1. Ingestion: Involves the oral cavity (lips, tongue), the entry point of food into the digestive system.
2. Propulsion: The movement of food through the tract by swallowing (voluntary) and peristalsis (involuntary).
3. Mechanical digestion: The break down of food into small fragments for chemical digestion.
4. Chemical digestion: The breakdown of complex food molecules by enzymes into smaller chemical building blocks.
5. Absorption: The passage of digested end products from the digestive tract lumen into the blood or lymph via passive or active transport.
6. Defecation: Elimination of indigestible substances from the body.

Question 6

Describe Propulsion (5 points)

Answer 6

• The propulsion of food through the digestive tract involves two different processes.
• swallowing, a voluntary act
• and peristalsis, an involuntary act that sees an alternate wave of muscle contraction and relaxation push food down the tract.
• In this process, the bolus (foodstuff) is pushed down the tract by the circular muscles that contract behind the bolus.
• Longitudinal muscles ahead of the bolus also relax to force the bolus ahead. This process of contraction and relaxation physically pushes the bolus down the tract.

Question 7

Describe mechanical digestions

Answer 7

• Mechanical digestion occurs in three major sites:
  1. The mouth: here the teeth mechanically churn, grind and mix the food with saliva.
  2. The stomach: here food is churned and mixed with gastric juices.
  3. The small intestine: here segmentation occurs. This is where the food mixes with the digestive juices which aids the process of nutrient absorption.
• Segmentation occurs similarly to propulsion however this process breaks the food into smaller “lumps” while simultaneously moving them through the small intestine.

Question 8

Label this image

Answer 8

Question 9

Describe mucosa

Answer 9

• The mucosa is the innermost layer of the alimentary canal. It is composed of three sub-layers:
  1. Epithelium (simple columnar cells) form the internal layer. It is rich in secretory cells.
  2. The lamina propria is composed of loose connective tissue containing capillaries and lymphoid follicles.
  3. The muscularis muscosae, a thin layer of smooth muscle that moves the mucosa

Question 10

List each of mucosa layers’ functions

Answer 10

1. Epithelium
   - Secrete mucus which protects the mucosa and helps food move along the canal
   - Secrete digestive enzymes which aide in chemical digestion
   - Secrete hormone which control motility and secretion of digestive juices
   - Absorb nutrients
2. Lamina propria
   - Capillaries are used in the absorption of end products of digestion
   - Lymphoid follicles present here act to protect against infectious diseases
3. Muscularis mucosa
   - Produces folds in the mucosa of the small intestine to increase the surface area available for absorption

Question 11

Describe submucosa

Answer 11

• The submucosa is external to the mucosa meaning it sits behind the mucosal layer of tissue in the alimentary canal.
• It is composed of loose connective tissue which allows it the ability to stretch and recoil.
• The submucosa is richly supplied with blood vessels (for nutrient absorption), lymphatic vessels (transport of lipids), lymphoid tissue (immune system) and nerves (control of motility and secretion).

Question 12

Describe the muscularis externa

Answer 12

• The muscularis externa surrounds the submucosa tissue.
• It is responsible for peristalsis (propulsion) and segmentation (mechanical digestion).
• The tissue is composed of two layers of smooth muscle (the inner circular and outer longitudinal layers).

Question 13

Describe the serosa

Answer 13

• The serosa is the outermost layer of tissue.
• This layer is composed of connective tissue with a thin layer of squamous epithelium. - It functions to protect the alimentary canal and anchors the alimentary tissues within the peritoneal cavity.

Question 14

Describe the basic functions of the mouth

Answer 14

• The mouth plays several key roles in the digestive system, primarily focused on the mechanical and chemical breakdown of food. Here are the basic functions:
  1. Ingestion: The mouth is the entry point for food, where digestion begins. Ingestion only occurs at the mouth
  2. Mechanical digestion (chewing or mastication):
  The teeth break down food into smaller pieces, increasing the surface area for enzymes to work. The tongue helps move and mix the food during chewing.
  3. Chemical digestion:
  Saliva, produced by salivary glands, contains enzymes like amylase that begin breaking down carbohydrates (starch into sugars).
  4. Formation of the bolus:
  The tongue and saliva help form the chewed food into a soft mass called a bolus, which is easier to swallow.
  5. Swallowing (deglutition):
  The tongue pushes the bolus to the back of the mouth, triggering swallowing, which moves the food into the esophagus.
• These functions prepare food for further digestion and absorption in the stomach and intestines.

Question 15

Label the salivary glands

Answer 15

Question 16

Describe saliva

Answer 16

• Secretion of the salivary glands
• Cleanses the mouth
• Dissolves food chemicals so they can be tasted
• Moistens food and helps compact it into a bolus
• Contains amylase, digestive system enzyme that breaks down starchy foods.
• Consists of:
  1. Mainly water (97 -99.5%)
  2. mucin protein which acts as a lubricant
  3. enzymes (salivary amylase, lingual lipase) which begin to help break down food
  4. antimicrobial proteins (IgA antibodies, lysozyme, defensins)
  4. Electrolytes
  5. Metabolic wastes

Question 17

Describe the basic functions of the pharynx

Answer 17

• For digestion, the pharynx has two main basic functions:
  1. Swallowing (Deglutition):
  The pharynx helps move the bolus (chewed food) from the mouth to the esophagus. This is done through coordinated muscle contractions that push the food downward.
  2. Preventing food from entering the airway:
  During swallowing, the epiglottis (a flap in the pharynx) closes over the trachea to direct food into the esophagus and prevent choking or aspiration.
• While the pharynx does not digest food itself, it plays a critical transport and protective role in the digestive process.

Question 18

Describe the basic functions of the oesophagus

Answer 18

• The oesophagus is a muscular tube that connects the pharynx (throat) to the stomach. Its basic functions in digestion are:
  1. Transport of food and liquids: The oesophagus moves the bolus (chewed and swallowed food) from the pharynx to the stomach.
  2. Peristalsis: This is a series of wave-like muscle contractions that push food down the oesophagus. It ensures that food moves efficiently, even when lying down.
  3. Lower oesophageal sphincter (LES) control:
  At the bottom of the oesophagus, the LES opens to let food enter the stomach and then closes to prevent stomach acid and contents from flowing back up (reflux).
• In summary, the oesophagus acts as a muscular passageway that safely delivers food and liquids to the stomach for further digestion.

Question 19

Describe the basic functions of the stomach

Answer 19

• The stomach plays several vital roles in digestion. Its basic functions include:
  1. Temporary storage of food: The stomach holds food after it’s swallowed, allowing controlled release into the small intestine.
  2. Mechanical digestion: The muscular walls of the stomach churn and mix food with gastric juices to form a semi-liquid mixture called chyme.
  3. Chemical digestion: A. Hydrochloric acid (HCl) helps break down food, denature proteins, and kill bacteria.
  B. Pepsin, an enzyme activated by HCl, begins protein digestion.
  4. Secretion of digestive substances:The stomach produces (1) Mucus (protects the stomach lining), (2) Intrinsic factor (needed for vitamin B12 absorption in the small intestine), (3) Gastric enzymes and acid (aid digestion)
  4. Absorption of fat soluble substances such as alcohol
  5. Regulation of food release: The pyloric sphincter controls the passage of chyme into the duodenum (first part of the small intestine), ensuring gradual digestion.
• In summary, the stomach’s main functions are storage, mixing, chemical breakdown, and controlled delivery of food to the next stage of digestion.

Question 20

Describe the microscopic stomach anatomy

Answer 20

• The stomach mucosais is composed of columnar epithelial cells and produces an alkaline mucus.
• This lining is spotted with gastric pits which produce mucus and are connected to gastric glands that produce up to 3L of gastric juice each day.
• Gastric glands are composed of (1) chief cells (secret pepsinogen), (2) parietal cells (secrete hydrochloric acid) and (3) enteroendocrine cells (secrete hormones that control gut motility and secretion).

Question 21

Label gross anatomy of stomach

Answer 21

Question 22

Label the microscopic anatomy of stomach

Answer 22

Question 23

describe the basic function of small intestine

Answer 23

• The small intestine is the main site of digestion and absorption in the digestive system. Its basic functions include:
  1. Digestion of food: The small intestine continues the chemical digestion of carbohydrates, proteins, and fats.
• It receives:
• Enzymes from the pancreas (like lipase, amylase, proteases)
• Bile from the liver/gallbladder (to emulsify fats)
• Brush border enzymes on the lining that finish breaking down nutrients.
  2. Absorption of nutrients: 90% of nutrient absorption occurs in the small intestine. Most nutrients (sugars, amino acids, fatty acids, vitamins, minerals) are absorbed into the blood or lymph.
• Its large surface area (thanks to villi and microvilli) maximizes absorption efficiency.
  3. Mixing and Movement of Contents: Two types of muscle movements occur:
  A. Segmentation: Rhythmic contractions that mix chyme with digestive juices and ensure it contacts the walls for maximum absorption.
  B. Peristalsis: Wave-like motions that move chyme forward toward the large intestine.

Question 24

Describe the gross anatomy of the small intestine

Answer 24

• longest part of the alimentary canal. Its length provides a huge surface area alone for nutrient absorption
• The small intestine has three subdivisions

Question 25

List and briefly describe the sections of the small intestine

Answer 25

1. Duodenum: First part; main site of chemical digestion 2. Jejunum: Middle part; major site of nutrient absorption 3. Ileum: Final part; absorbs remaining nutrients, especially vitamin B12 and bile salts

Question 26

Describe the microscopic anatomy of the small intestine

Answer 26

- has three structural modifications, Circular folds, villi and microvilli, which further increase this surface area and increasing the efficiency of nutrient absorption. - Circular folds (plicae circulares) - are deep folds of the mucosa and submucosa and act to slow down the movement of chyme, increasing the time for nutrient absorption to occur. - Villi - are small projections of the mucosa and are composed of epithelial cells that are absorptive and columnar and bound by tight junctions (for nutrient and electrolyte absorption). The core contains a capillary bed and a wide lymph capillary (lacteal). - Microvilli - are very small, densely packed villi on the surface, forming a "brush boarder". The brush boarder enzymes complete carbohydrate, protein and nucleic acid digestion. - Along with these structures, the small intestine also display tubular glands located in the pits between villi. - These are known as intestinal crypts and they secrete intestinal juice (pH 7.4-7.8), a watery mucus that facilitates nutrient absorption. - They also contain enteroendocrine cells which secrete hormones that stimulate bile and pancreatic juice secretion, lymphoid tissue for defence and stem cells that renew villus epithelium as they migrate from the crypt and up the villi.

Question 27

Label image of small intestine

Answer 27

Question 29

Describe nutrient absorption

Answer 29

- That 90% of nutrient absorption occurs in the small intestine. - These nutrients, as well as water, vitamins and electrolytes are absorbed across the single layer of epithelial cells and into the capillaries within the villi. - Water soluble nutrients are absorbed into the blood and travel via the hepatic portal vein to the liver. - Lipid soluble nutrients are absorbed into the lymphatic system and enter the bloodstream at the thoracic duct. - The remaining 10% occurs in the stomach (lipid soluble substances such as alcohol) and the large intestine (water, electrolytes and some vitamins).

Question 30

Describe the basic functions of the liver

Answer 30

- The liver is responsible for numerous roles: 1. Its main function in the digestive system is the production of bile for fat digestion 2. Synthesis of plasma proteins (albumin, clotting factors) 3. Metabolic functions: A. Processing of blood-bone nutrients - this includes the removal and storage of excess nutrients (glucose to glycogen), breakdown of fatty acids (energy production), use of amino acids to build glucose or fat, vitamin and mineral storage B. Regulating levels of circulating nutrients - this includes the regulation of glucose, triglycerides, fatty acids and cholesterol, drug and hormone metabolism and detoxification 4. Heat production via chemical reactions - Blood supply to the liver is dual (two sources): 1. The hepatic artery: supplies oxygenated blood from the abdominal aorta (20%) 2. The hepatic portal vein: supplies nutrient rich blood from the small intestine (80%)

Question 31

Label the gross anatomy of liver

Answer 31

Question 32

Label the microscopic anatomy of liver

Answer 32

Question 33

Describe the gallbladder

Answer 33

- The gall bladder is a thin walled muscular sac that stores and concentrates unused bile. - Bile is secreted into the duodenum from the gall bladder or the liver. - Bile is produced by hepatocytes and is yellow-green in colour. This pigmentation is a result of bilirubin, a by product of the breakdown of haemoglobin from recycled RBCs. - Bile also contains bile salts and phospholipids which emulsify fats (break them down into small droplets, increasing surface area for digestive enzymes) and facilitates fat and cholesterol absorption. - The release of bile is triggered by intestinal hormones secreted when chyme enters the duodenum. It is then reabsorbed in the ilium and recycled.

Question 34

Describe the pancreas (12 points)

Answer 34

- largely composed of exocrine tissue. - It produces and secretes pancreatic juice into the ducts that deliver it to the duodenum. - This pancreatic juice has a pH of 8.0 and is composed of: 1. Water 2. Electrolytes (bicarbonate ions to neutralise stomach acids and aide enzyme activity) 3. Enzymes: A. Proteases - digest proteins B. Amylases - digest carbohydrates C. Lipases - digest lipids D. Nucleases - digest nucleic acids - As well as exocrine tissue, some endocrine tissue is also structurally part of the pancreas. - This tissue contains islets which secrete hormones into the blood to regulate glucose levels.

Question 35

Label the image of microscopic anatomy of pancreas

Answer 35

Question 36

Describe the basic function of large intestine (10 points)

Answer 36

- The large intestine plays a key role in the final stages of digestion. Its main functions include: 1. Water and Electrolyte Absorption: The large intestine absorbs water, sodium, and other electrolytes from the remaining undigested food. This helps to form solid feces and maintain the body's fluid balance. 2. Formation and Storage of Feces: As water is removed, the waste material becomes more solid. Feces are stored in the rectum until they are eliminated through defecation. 3. Chemical Digestion by Bacteria: It is important to remember that chemical digestion in the large intestine occurs only by bacteria, not by the body’s enzymes. Approximately 500 species of bacteria are enteric (naturally present) to the large intestine. * Some bacteria survive the stomach and small intestine to reach the colon. * These normal flora: A. Prevent colonization by harmful bacteria B. Ferment undigested carbohydrates, producing gases C. Synthesize vitamins, such as vitamin K and some B vitamins 4. Mucus Secretion and Lubrication: Goblet cells in the intestinal lining secrete mucus to protect the intestinal wall from damage and lubricate fecal matter for easier movement

Question 37

Describe the gross anatomy of large intestine (14 points)

Answer 37

- main sections and structural features are: 1. Cecum: The first part of the large intestine. Receives chyme from the ileum of the small intestine via the ileocecal valve. Attached to the cecum is the vermiform appendix, a narrow, worm-like tube that contains lymphoid tissue. 2. Colon: Divided into four main regions: A. Ascending colon: Travels upward on the right side of the abdomen from the cecum to the liver. B. Transverse colon: Crosses the abdomen horizontally from right to left under the stomach. C. Descending colon: Travels down the left side of the abdomen. D. Sigmoid colon: An S-shaped segment that connects the descending colon to the rectum. Located in the lower left quadrant. 3. Rectum: A straight, muscular tube that stores feces before defecation. Begins at the end of the sigmoid colon and ends at the anal canal. 4. Anal Canal: The final portion of the large intestine. Passes through the pelvic floor muscles and ends at the anus. * Controlled by two sphincters: Internal anal sphincter (involuntary) and External anal sphincter (voluntary) - Special Structural Features 1. Teniae coli: Three distinct bands of longitudinal smooth muscle running along the colon. 2. Haustra: Pouch-like sacs formed by the contraction of the teniae coli, giving the colon a segmented appearance. 3. Epiploic appendages: Small, fat-filled sacs of peritoneum attached to the outer surface of the colon.

Question 38

Describe the microscopic anatomy of large intestine (4 points)

Answer 38

- The mucosa of the large intestine is composed of simple columnar epithelial cells. - It is largely abundant with crypts and large numbers of goblet cells. - These act to produce mucus that ease passage of faeces and protect the intestinal wall from bacterial waste products. - No villi

Question 39

Label the gross anatomy of large intestine

Answer 39

Question 40

Label the microscopic anatomy of large intestine

Answer 40

Question 41

Describe chemical digestion

Answer 41

- Chemical digestion is a catabolic process, this means that it breaks down larger molecules (polymers) into smaller units (monomers) and in the process releases energy. - These smaller molecules (monomers) are then absorbed from the gastrointestinal tract into the blood or lymph. - This break down of polymers is catalysed by specific enzymes.