Topic 9
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Rapid summary for last-minute revision before your exam.
Topic 9 — Key Facts for DU Admission (Bangladesh) Core concept: Human Digestive and Respiratory Systems — structure, function, and physiology High-yield point: Mechanism of breathing, gas exchange, enzyme action in digestion ⚡ Exam tip: Diagrams are frequently asked — practice labeling alimentary canal and respiratory tract
🟡 Standard — Regular Study (2d–2mo)
Standard content for students with a few days to months.
Topic 9 — DU Admission (Bangladesh) Study Guide Overview: Human physiology is a major component of DU admission biology Core principles: Organs, functions, and integrated system working Key points: Peristalsis, lung volumes, enzyme specificity Study strategy: Draw diagrams, connect structure to function
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Human Digestive and Respiratory Systems — Complete Study Notes
Part A: Digestive System
Overview
The digestive system breaks down complex food into absorbable nutrients through mechanical and chemical processes. The human alimentary canal is approximately 9 meters long, running from mouth to anus.
Alimentary Canal — Structure and Functions
1. Mouth/Buccal Cavity
- Teeth: 32 in adult (incisors, canines, premolars, molars)
- Tongue: 5 taste zones, helps in swallowing, speech
- Salivary glands: 3 pairs (parotid, submandibular, sublingual)
- Saliva: Contains ptyalin (amylase) — digests starch to maltose
- Optimal pH: 6.8 (slightly acidic)
- Ptyalin acts on cooked starch only
2. Pharynx and Oesophagus
- Pharynx: Common passage for food and air
- Epiglottis: Cartilaginous flap that covers glottis during swallowing
- Oesophagus: ~25 cm long, connects pharynx to stomach
- Peristalsis: Wave of contraction (involuntary) — food moves even if you eat upside down
- No digestion occurs here; mucus lubricates food passage
3. Stomach
- J-shaped muscular sac
- Capacity: ~1.5 liters
- Regions: Cardiac, Fundus, Body, Pyloric
- Gastric glands secrete:
- HCl: Kills bacteria, activates pepsinogen → pepsin
- Pepsinogen: Inactive enzyme (zymogen) → Pepsin (protein digestion)
- Mucus: Protects stomach wall (bicarbonate layer)
- Intrinsic Factor: Essential for Vitamin B₁₂ absorption
- Gastrin: Hormone controlling HCl secretion
- Optimal pH: 1.5–2 (highly acidic)
- Food remains 1–4 hours
4. Small Intestine (~6 meters)
- Three parts: Duodenum (25 cm), Jejunum (2.5 m), Ileum (3.5 m)
- Villi: Finger-like projections (1 mm) for absorption
- Each villus has capillaries and lacteal (lymphatic)
- Microvilli: “Brush border” increases surface area 600×
- Intestinal glands (Crypts of Lieberkühn): Secrete intestinal juice (succus entericus)
- Wallace’s patch: Peyer’s patches (lymphoid tissue) in ileum
5. Large Intestine (~1.5 meters)
- Regions: Caecum → Ascending → Transverse → Descending → Sigmoid → Rectum
- Appendix: Vestigial organ, can cause appendicitis
- Functions:
- Water absorption
- Vitamin K and B synthesis by bacteria
- Formation of feces (semi-solid)
- No digestion of cellulose in humans
Digestive Enzymes — Complete Table
| Enzyme | Site | Substrate | Product | Optimal pH |
|---|---|---|---|---|
| Ptyalin | Mouth | Starch | Maltose | 6.8 |
| Pepsin | Stomach | Protein | Peptones | 1.5–2 |
| Rennin | Stomach | Milk protein | Curds | 1.5–2 |
| HCl | Stomach | — | — | 1.5–2 |
| Trypsin | Duodenum | Protein | Peptides | 8 |
| Amylase | Duodenum | Starch | Maltose | 8 |
| Lipase | Duodenum | Fats | Fatty acids + Glycerol | 8 |
| Maltase | Small intestine | Maltose | Glucose | 8 |
| Sucrase | Small intestine | Sucrose | Glucose + Fructose | 8 |
| Lactase | Small intestine | Lactose | Glucose + Galactose | 8 |
Absorption
| Substance | Absorbed By | Mechanism |
|---|---|---|
| Glucose | Small intestine | Active transport (Na⁺ dependent) |
| Amino acids | Small intestine | Active transport |
| Fatty acids + Glycerol | Small intestine | Micelle formation → lacteal |
| Water | Large intestine | Osmosis |
| Vitamins | Small intestine | Passive diffusion/carrier |
| Iron | Duodenum | Active transport |
| Calcium | Duodenum | Active transport (Vitamin D dependent) |
Large Intestine Absorption Summary
- Bile pigments: Give brown color to feces
- Bacteria: E. coli produces Vitamin K and B12
- Water absorption: 1–1.5 liters per day
Disorder
| Disorder | Cause/Symptoms |
|---|---|
| Peptic ulcer | HCl damages stomach lining |
| Gall stones | Cholesterol precipitation in bile |
| Constipation | Reduced water absorption in colon |
| Diarrhea | Reduced water absorption, increased secretion |
| Jaundice | Liver dysfunction, yellowing of skin/eyes |
Part B: Respiratory System
Overview
The respiratory system facilitates gas exchange — O₂ intake and CO₂ removal. Respiration includes external respiration (gas exchange in lungs) and internal respiration (gas exchange at tissue level).
Respiratory Tract — Structure
1. Nasal Cavity
- Filter, warm, and humidify air
- Hairs and mucus: Trap dust and pathogens
- Conchae: Increase surface area for air conditioning
2. Pharynx
- Common passage for food and air
- Larynx opens into pharynx
3. Larynx (Voice Box)
- Made of cartilage (Thyroid, Cricoid, Arytenoid)
- Glottis: Opening between vocal cords
- Epiglottis: Covers glottis during swallowing
- Vocal cords: Produce sound (vibration of ligaments)
- Pitch controlled by tension of vocal cords
4. Trachea (Windpipe)
- ~11 cm long, anterior to oesophagus
- C-shaped cartilage rings: Keep airway open
- Lined with ciliated epithelium
- Bifurcates at Carina (level of 5th thoracic vertebra)
5. Bronchi and Bronchioles
- Right bronchus: Wider, shorter, more vertical (more aspiration pneumonia)
- Bronchi: Primary, secondary, tertiary branches
- Bronchioles: No cartilage, smooth muscle control
- Bronchioles → Alveolar ducts → Alveoli
6. Lungs
- Right lung: 3 lobes (Upper, Middle, Lower)
- Left lung: 2 lobes (Upper, Lower) — due to cardiac notch
- Pleura: Double membrane covering (visceral + parietal)
- Pleural fluid: Reduces friction during breathing
- Alveoli: ~300–400 million, total surface area ~70 m²
Alveolar Structure
| Feature | Description |
|---|---|
| Type I pneumocytes | Thin squamous cells for gas exchange |
| Type II pneumocytes | Secrete surfactant (reduces surface tension) |
| Surfactant | Prevents alveolar collapse ( Lecithin/Sphingomyelin ratio) |
| Blood supply | Pulmonary artery (deoxygenated) + Pulmonary veins (oxygenated) |
Mechanism of Breathing
Inspiration (Inhalation)
- Diaphragm contracts and flattens
- External intercostal muscles contract (ribs lift up and out)
- Intrapulmonary pressure drops below atmospheric
- Air rushes in
- Active process — requires muscle contraction
Expiration (Exhalation)
- Diaphragm relaxes (domes up)
- Internal intercostal muscles contract
- Intrapulmonary pressure rises above atmospheric
- Air rushes out
- At rest: Passive process (no muscle energy needed)
Lung Volumes and Capacities
| Volume | Description | Normal Value |
|---|---|---|
| Tidal Volume (TV) | Air during normal breathing | 500 mL |
| Inspiratory Reserve Volume (IRV) | Max air inspired after normal inspiration | 2500–3000 mL |
| Expiratory Reserve Volume (ERV) | Max air expired after normal expiration | 1000–1200 mL |
| Residual Volume (RV) | Air remaining after max expiration | 1100–1200 mL |
| Vital Capacity (VC) | TV + IRV + ERV | 3500–4500 mL |
| Total Lung Capacity (TLC) | VC + RV | 5500–6000 mL |
Gas Exchange
- Occurs by diffusion across alveolar membrane
- Partial pressures: PO₂ in alveoli ~100 mmHg; venous blood ~40 mmHg
- Diffusion follows pressure gradient
- Henry’s Law: Gas dissolves in liquid proportional to partial pressure
Transport of Gases
Oxygen Transport
- 98.5%: Bound to hemoglobin (Hb) as oxyhemoglobin (HbO₂)
- 1.5%: Dissolved in plasma
- Each Hb binds 4 O₂ molecules
- Oxyhemoglobin dissociation curve: Right shift = ↓ affinity (Bohr effect)
Carbon Dioxide Transport
- 70%: As bicarbonate ions (HCO₃⁻) in plasma
- 23%: Bound to Hb as carbaminohemoglobin
- 7%: Dissolved in plasma
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
- Enzyme: Carbonic anhydrase (present in RBCs)
Control of Respiration
- Respiratory center: Medulla oblongata + Pons
- Medulla: Sets basic rhythm (dorsal — inspiration; ventral — expiration)
- Pons: Smooth transition between inspiration and expiration
- Hering-Breuer reflex: Stretch receptors prevent overinflation
- Chemoreceptors: Respond to [H⁺] and CO₂ levels
Respiratory Quotient (RQ)
RQ = CO₂ produced / O₂ consumed
| Substrate | RQ |
|---|---|
| Carbohydrates | 1.0 |
| Fats | 0.7 |
| Proteins | 0.8 |
Disorders
| Disorder | Description |
|---|---|
| Asthma | Bronchospasm, wheezing, dyspnea |
| Emphysema | Alveolar destruction (smoking) |
| Pneumonia | Alveolar infection and inflammation |
| Tuberculosis | Mycobacterium tuberculosis infection |
| Pleurisy | Inflammation of pleural membranes |
| Hypoxia | Low oxygen in tissues |
Must-Remember Facts
- Vital capacity = TV + IRV + ERV
- Total lung capacity = VC + RV
- Right lung has 3 lobes, left has 2 lobes (cardiac notch)
- Surfactant prevents alveolar collapse (premature babies lack it → RDS)
- Hemoglobin carries 98.5% of O₂
- CO₂ mainly transported as bicarbonate (70%)
- Pneumonia affects alveoli; Bronchitis affects bronchi
- Henry’s Law: Gas solubility proportional to partial pressure
Common DU Admission Questions
- Label alimentary canal diagram
- Difference between small and large intestine
- Gastric juice composition and function
- Mechanism of breathing (inspiration/expiration)
- Lung volumes and capacities calculation
- Gas exchange at alveoli — partial pressure gradient
- Oxygen and CO₂ transport
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