Key Concepts in Human Digestive System Anatomy & Physiology (Section 9.1)
Overview of Digestion
- Definition and Importance: Digestion breaks down food into absorbable nutrients (e.g., sugars, amino acids, fatty acids) for energy, growth, repair, immune support, and metabolic health. Efficient digestion prevents deficiencies and sustains energy levels.
Components of the Digestive System
- Gastrointestinal (GI) Tract: A continuous tube from mouth to anus, ~9 meters long in adults. Key parts:
- Oral cavity
- Pharynx
- Esophagus
- Stomach
- Small intestine (duodenum, jejunum, ileum – primary absorption site)
- Large intestine (colon, rectum – water reabsorption and waste formation)
- Accessory Organs: Support digestion without being part of the main tube.
Oral Cavity: Entry and Initial Processing
- Structure: Bounded by lips (vascularized muscle with sensory nerves for food retention and speech), teeth, tongue, palate, and cheeks.
- Functions:
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Food Selection: Tongue's taste buds, plus smell and sight, detect and reject unpalatable or harmful items (e.g., bones, dirt).
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Mechanical Digestion (Mastication): Teeth grind food into smaller particles, increasing surface area for enzymes.
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Chemical Digestion: Saliva from three pairs of glands mixes with food:
- Parotid glands (in front of ears)
- Submaxillary glands (behind jaws)
- Sublingual glands (below tongue)
(Figure 9.2 shows gland locations.)
Saliva Composition:
- Water and mucus: Moistens/lubricates bolus formation.
- Bicarbonate ions: Buffers oral pH (initially alkaline ~pH 8, drops to ~pH 6).
- Thiocyanate ions: Antimicrobial.
- Salivary amylase (ptyalin): Breaks polysaccharides (starch/glycogen) into disaccharides (maltose).
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- Output: Food becomes a moist bolus (small, chewed mass) ready for swallowing.
Pharynx: Transition Zone
- Structure: Muscular cavity behind the mouth; shared pathway for food (bolus) and air.
- Voluntary Phase: Tongue pushes bolus backward against raised soft palate (seals nasal passage).
- Involuntary Phase: Larynx elevates, epiglottis covers trachea (prevents aspiration), bolus enters esophagus.
Esophagus: Transport Tube
- Structure: ~25 cm muscular tube connecting pharynx to stomach; lined with stratified squamous epithelium.
- Function: Continues salivary digestion; propels bolus via peristalsis (wave-like smooth muscle contractions).
Lower Esophageal (Cardiac) Sphincter: Relaxes to allow entry into stomach; prevents acid reflux.
Motility of the Alimentary Canal
Movements ensure food progression, mixing, and absorption:
- Peristalsis: Rhythmic contractions propel food forward (e.g., esophagus to stomach/intestines). Triggered by distension or neural signals.
- Segmentation: Localized contractions in small/large intestines mix chyme (partially digested food) with enzymes, enhancing contact with absorptive surfaces.
- Antiperistalsis: Rare reversal of peristalsis; pushes contents backward (e.g., from intestines to stomach, causing vomiting).
- Hunger Contractions: Peristaltic waves in empty stomach due to low blood glucose; produce "hunger pangs" (growling sensation).
Clinical/Physiological Notes
- Disruptions (e.g., GERD from sphincter failure) can cause heartburn or aspiration risks.
- Neural control: Enteric nervous system (gut "brain") + autonomic nerves regulate motility.
- Hormonal influences: Gastrin, secretin, etc., coordinate secretions (covered in later sections).
SECTION 1: MULTIPLE CHOICE QUESTIONS
HUMAN DIGESTIVE SYSTEM
1. Digestion is the process by which the body breaks down food into: (a) Larger molecules for storage (b) Smaller, absorbable components (c) Indigestible waste (d) Energy without nutrients
Correct Answer: (b) Smaller, absorbable components Explanation: Digestion converts complex food into simple nutrients like sugars and amino acids for absorption, supporting energy, growth, and repair.
2. Which of the following is NOT part of the gastrointestinal (GI) tract? (a) Oral cavity (b) Stomach (c) Liver (d) Small intestine
Correct Answer: (c) Liver Explanation: The liver is an accessory organ that produces bile; the GI tract is the continuous tube from mouth to anus, including oral cavity, stomach, and small intestine.
3. The accessory digestive organs include all EXCEPT: (a) Salivary glands (b) Pancreas (c) Gallbladder (d) Pharynx
Correct Answer: (d) Pharynx Explanation: Pharynx is part of the GI tract; accessory organs like salivary glands, pancreas, and gallbladder aid digestion without forming the main tube.
4. The primary function of lips in the oral cavity is to: (a) Produce saliva (b) Retain food during chewing and aid phonation (c) Digest starch (d) Absorb nutrients
Correct Answer: (b) Retain food during chewing and aid phonation Explanation: Lips, made of vascularized muscle with sensory nerves, hold food in place while chewing and modify sound for speech.
5. The tongue's role in food selection involves: (a) Mechanical breakdown (b) Taste buds for detecting unpleasant tastes or objects (c) Producing mucus (d) Peristaltic movement
Correct Answer: (b) Taste buds for detecting unpleasant tastes or objects Explanation: The muscular tongue uses taste buds to sense flavor, texture, and reject harmful items, aided by smell and sight.
6. Mastication refers to: (a) Chemical breakdown by enzymes (b) Physical breakdown of food by teeth (c) Swallowing of bolus (d) Absorption in intestines
Correct Answer: (b) Physical breakdown of food by teeth Explanation: Chewing (mastication) reduces food size, increasing surface area for enzymatic action in the oral cavity.
7. Which salivary gland is located in front of the ears? (a) Sublingual (b) Submaxillary (c) Parotid (d) Pancreatic
Correct Answer: (c) Parotid Explanation: Parotid glands are anterior to ears; submaxillary behind jaws, sublingual below tongue; they all secrete saliva into the oral cavity.
8. Saliva primarily contains: (a) Hydrochloric acid (b) Water, mucus, bicarbonate, thiocyanate, and salivary amylase (c) Bile salts (d) Pepsin
Correct Answer: (b) Water, mucus, bicarbonate, thiocyanate, and salivary amylase Explanation: These components moisten food, buffer pH, kill microbes, and initiate starch digestion; fresh saliva is alkaline (pH 8), dropping to pH 6.
9. Salivary amylase digests polysaccharides into: (a) Proteins (b) Disaccharides like maltose (c) Fatty acids (d) Vitamins
Correct Answer: (b) Disaccharides like maltose Explanation: It partially breaks starch and glycogen into maltose during oral chemical digestion.
10. After oral processing, the food mass is called: (a) Chyme (b) Bolus (c) Feces (d) Mucus
Correct Answer: (b) Bolus Explanation: The moist, chewed mixture of food and saliva forms a small, swallowable bolus.
11. The pharynx serves as a common passageway for: (a) Only digestive tract (b) Only respiratory tract (c) Both digestive and respiratory tracts (d) Urinary and digestive tracts
Correct Answer: (c) Both digestive and respiratory tracts Explanation: It allows bolus passage to esophagus while preventing entry to trachea during swallowing.
12. The voluntary phase of swallowing involves: (a) Larynx movement (b) Tongue pushing bolus backward against soft palate (c) Peristalsis in esophagus (d) Gastric churning
Correct Answer: (b) Tongue pushing bolus backward against soft palate Explanation: This seals the nasal passage; the involuntary phase follows with epiglottis closing the trachea.
13. During swallowing, the epiglottis: (a) Opens the trachea (b) Lowers to cover trachea opening (c) Produces saliva (d) Mixes food with enzymes
Correct Answer: (b) Lowers to cover trachea opening Explanation: Larynx elevation lowers the epiglottis, preventing aspiration as bolus enters esophagus.
14. The human esophagus is approximately: (a) 10 cm long (b) 25 cm long (c) 50 cm long (d) 100 cm long
Correct Answer: (b) 25 cm long Explanation: It connects pharynx to stomach, using peristalsis to transport bolus.
15. Peristalsis in the alimentary canal is: (a) Back-and-forth mixing (b) Rhythmic waves of contraction propelling food forward (c) Reversal for vomiting (d) Hunger-induced contractions
Correct Answer: (b) Rhythmic waves of contraction propelling food forward Explanation: Smooth muscle waves squeeze food along the tube, from esophagus to intestines.
16. The lower esophageal sphincter prevents: (a) Food entry into stomach (b) Backflow of stomach contents into esophagus (c) Saliva production (d) Bolus formation
Correct Answer: (b) Backflow of stomach contents into esophagus Explanation: Also called cardiac sphincter, it opens under food pressure to allow entry while blocking reflux.
17. Segmentation occurs primarily in the: (a) Oral cavity (b) Esophagus (c) Small and large intestines (d) Stomach only
Correct Answer: (c) Small and large intestines Explanation: Ring-like contractions mix food with secretions, enhancing absorption without propulsion.
18. Antiperistalsis is associated with: (a) Normal propulsion (b) Mixing in intestines (c) Reversal leading to vomiting (d) Hunger pangs
Correct Answer: (c) Reversal leading to vomiting Explanation: It pushes intestinal contents back to stomach, unlike forward peristalsis.
19. Hunger contractions are triggered by: (a) High blood glucose (b) Low blood glucose levels (c) Excess saliva (d) Bile release
Correct Answer: (b) Low blood glucose levels Explanation: These peristaltic waves in empty stomach cause discomfort known as hunger pangs.
20. The initial pH of fresh saliva is: (a) Acidic (pH 4) (b) Neutral (pH 7) (c) Alkaline (pH 8) (d) Variable (pH 10)
Correct Answer: (c) Alkaline (pH 8) Explanation: It buffers oral contents; pH drops to 6 upon CO2 loss.
SECTION 2: SHORT QUESTIONS
HUMAN DIGESTIVE SYSTEM
1. What is the gastrointestinal (GI) tract? A continuous tube from mouth to anus, including oral cavity, pharynx, esophagus, stomach, small intestine, and large intestine.
2. List the accessory digestive organs. Salivary glands, liver, gallbladder, and pancreas.
3. What are the main functions of the oral cavity? Food selection via taste buds, mechanical digestion by mastication, and chemical digestion by saliva.
4. Describe the composition and roles of saliva. Water and mucus for lubrication; bicarbonate for buffering (pH 8 to 6); thiocyanate for antimicrobial action; salivary amylase for starch digestion.
5. What is mastication and its purpose? Chewing by teeth to break food into smaller pieces, increasing surface area for enzymes.
6. Define bolus and how it forms. A small, moist mass of chewed food mixed with saliva, formed in the oral cavity.
7. Outline the key steps in swallowing. Tongue pushes bolus back (voluntary); soft palate raises to seal nasal passage; epiglottis covers trachea (involuntary); bolus enters esophagus.
8. What is peristalsis? Rhythmic smooth muscle contractions that propel food forward along the alimentary canal.
9. Differentiate between peristalsis and segmentation. Peristalsis propels food forward; segmentation mixes food locally in intestines without net movement.
10. What causes hunger pangs? Peristaltic contractions in an empty stomach triggered by low blood glucose.
SECTION 3: LONG QUESTIONS
1. Describe the anatomy of the oral cavity and pharynx, including their roles in initial digestion and swallowing.
2. Explain the structure and motility mechanisms of the esophagus, including the role of sphincters and types of movements in the alimentary canal.-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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