The Digestive System

1. Functions of the Digestive System

The digestive system performs several crucial functions that allow the body to absorb and utilize nutrients from food:

• Mechanical: Grinding, churning, and moving food through the digestive tract.

• Digestive: Breaking down food into simpler, absorbable components.

• Absorptive: Absorbing nutrients into the bloodstream and lymph.

• Excretion: Eliminating waste products and toxins from the body.

2. Organs of the Digestive Tract

The digestive tract, also known as the alimentary canal, consists of these organs:

• Mouth: The entry point for food, where it is chewed, mixed with saliva, and where starch digestion begins.

• Esophagus: A tube connecting the mouth to the stomach, transporting food through peristaltic movements.

• Stomach: A muscular sac that stores food, further grinds it, mixes it with gastric juices, and digests proteins.

• Small Intestine: The primary site for complete food digestion and absorption of most nutrients into the bloodstream.

• Large Intestine: Responsible for absorbing water, minerals, and vitamins, and forming feces.

• Anus: The exit point for waste products from the body.

3. Digestive Glands

In addition to the digestive tract, the digestive system includes the following glands:

• Salivary Glands: Secrete saliva, which moistens food and contains the enzyme amylase for starch digestion.

• Pancreas (Exocrine): Produces pancreatic juice containing digestive enzymes for proteins, lipids, and carbohydrates.

• Liver: Synthesizes bile, which aids in lipid digestion, stores glycogen, produces proteins, and detoxifies the body.

• Gallbladder: Stores and concentrates bile, releasing it into the duodenum as needed.

4. Structure of the Digestive Tract

The digestive tract has four layers, from outermost to innermost:

• Serosa (Outermost): A thin membrane that covers the outside, providing protection and reducing friction within the abdominal cavity during organ contractions.

• Muscularis: Composed of circular (inner) and longitudinal (outer) muscle layers, generating peristaltic and segmental contractions for food movement.

• Submucosa: Located between the muscularis and mucosa, containing blood vessels, lymph vessels, and the submucosal plexus (Meissner’s plexus), which controls secretions from the mucosa.

• Mucosa (Innermost): The layer directly exposed to food, containing secretory cells, absorptive cells, endocrine cells, and a lymphatic system.

Note:

• The enteric nervous system (ENS), also known as the “second brain,” is comprised of two nerve plexuses: the submucosal plexus (Meissner’s) and the myenteric plexus (Auerbach’s).

• The ENS can independently control digestion, without input from the central nervous system.

5. Structure and Functions of the Digestive Tract Organs

5.1. Mouth:

• Pharynx: Extends from the base of the skull to the sixth cervical vertebra.

• Thyroid Cartilage: More prominent in males, contributing to deeper male voices.

• Tongue: Essential for chewing, swallowing, and speech.

• Teeth: Grind food for mechanical digestion.

• Salivary Glands: Secrete saliva, which contains amylase for starch breakdown.

5.2. Esophagus:

• Located behind the trachea, slightly left of the midline of the body.

• Has three constrictions:

• Where it crosses the aortic arch.

• Where it passes behind the left bronchus.

• Where it pierces the diaphragm.

• Upper Esophageal Sphincter: Prevents food reflux from the esophagus into the throat.

• Function: Transports food down to the stomach.

5.3. Stomach:

• Divided into three regions: cardia, fundus, and pylorus.

• Secretes gastric juice containing:

• Hydrochloric Acid (HCl): Kills bacteria and activates pepsin.

• Pepsin: Digests proteins.

• Other enzymes: Gastric lipase (digests lipids).

• Function: Continues grinding food, mixes it with gastric juice, and digests proteins.

5.4. Small Intestine:

• Approximately 6-7 meters long, divided into three sections: duodenum, jejunum, and ileum.

• Major duodenal papilla: The opening for the common bile duct and main pancreatic duct.

• Function: Completes food digestion and absorbs most nutrients into the bloodstream.

• Surface structures that optimize absorption:

• Folds: Increase absorption surface area by 3 times.

• Villi: Increase absorption surface area by 10 times.

• Microvilli: Increase absorption surface area by 20 times.

• Total absorption area increase is 600 times.

5.5. Large Intestine:

• Approximately 1.5 meters long, divided into six sections: cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

• Sigmoid colon: Stores feces.

• Function: Absorbs water, minerals, and vitamins, and forms feces.

Note:

• Muscle cells in the gut are connected by gap junctions, allowing for rapid transmission of nerve impulses between cells.

• The ENS plays a crucial role in controlling digestion, involving electrical waves:

• Spike Potentials: True action potentials that trigger smooth muscle contractions.

• Slow Waves: Cause slow oscillations of muscle cell membranes, regulating the frequency of spike potentials.

• Acetylcholine and digestive hormones (gastrin, cholecystokinin, secretin) depolarize smooth muscle, stimulating contraction.

• Norepinephrine, epinephrine, and sympathetic nervous system stimulation hyperpolarize smooth muscle, inhibiting contraction.

• Nitric oxide (NO) causes muscle relaxation: NO stimulates the production of cGMP from GTP, and cGMP activates MLCP, leading to muscle relaxation.

5.6. Anus:

• The point where waste exits the body.

• Has two sphincters:

• Internal sphincter: Involuntary control.

• External sphincter: Voluntary control.

6. Regulation of Digestive System Activity

• Enteric Nervous System: Independently controls the digestive tract’s activity, without input from the central nervous system.

• Peripheral Nervous System:

• Sympathetic Nervous System: Inhibits digestive activity.

• Parasympathetic Nervous System: Stimulates digestive activity.

• Digestive Hormones:

• Gastrin: Stimulates gastric juice secretion and increases stomach motility.

• Cholecystokinin (CCK): Stimulates bile and pancreatic juice secretion, decreases stomach motility, and increases small intestine motility.

• Secretin: Stimulates alkaline pancreatic juice secretion and decreases stomach motility.

• Hunger and Satiety Centers: Located in the hypothalamus, control the sensations of hunger and fullness.

7. Common Digestive System Disorders

• Peptic Ulcer Disease: Caused by the bacterium Helicobacter pylori, stress, and NSAID use.

• Colitis: Inflammation of the colon, caused by infection, stress, or an improper diet.

• Stomach Cancer, Colon Cancer: May result from an unhealthy diet, smoking, and alcohol consumption.

• Biliary Obstruction: Caused by gallstones or pancreatic head tumors.

• Hemorrhoids: Caused by constipation and increased abdominal pressure.

8. Noteworthy Points

• Maintain a balanced diet rich in nutrients, limiting spicy foods, hot foods, and stimulants.

• Drink sufficient water and engage in regular physical activity.

• Undergo regular medical checkups to detect and treat diseases early.

• Get vaccinated against infectious diseases transmitted through the digestive tract (hepatitis A, cholera, typhoid fever).

9. Exercises

Question 1: A middle-aged man with stomach cancer undergoes surgery to remove his entire stomach. What kind of supplement does he need after surgery?

Answer: C. Vitamin B12 injection.

Explanation: Vitamin B12 is primarily absorbed in the small intestine, and the stomach plays a crucial role in releasing B12 from food. Therefore, after stomach removal, the patient needs B12 supplementation through injections to ensure adequate absorption.

Question 2: Which statement is true about defecation?

Answer: A. Rectal wall distension triggers relaxation of the internal anal sphincter.

Explanation: When feces accumulate in the rectum, the rectal wall stretches, stimulating sensory receptors. These signals travel to the central nervous system, triggering a reflex that relaxes the internal anal sphincter, allowing feces to pass.

Question 3: A patient with chronic biliary obstruction due to a pancreatic head tumor is at high risk of malabsorption of which substance?

Answer: D. Vitamin D.

Explanation: Vitamin D is absorbed in the small intestine, and bile is necessary for its solubilization and absorption. When bile flow is obstructed, vitamin D absorption is impaired.

Note:

• These exercises are illustrative. Refer to your learning materials to address specific scenarios.

• Seek advice from a medical professional for personalized health guidance.