Digestive System
Components of the Human Digestive System
Alimentary canal: Mouth -> Pharynx -> Oesophagus -> Stomach -> Small intestine -> Large intestine -> Rectum -> Anus.
Associated glands: Salivary glands, Liver, Pancreas.
Mouth and Oral Cavity
- Teeth: Mechanical digestion (mastication). 32 teeth in adults (incisors, canines, premolars, molars).
- Tongue: Taste buds, mixes food, pushes bolus.
- Salivary glands: Parotid, Submandibular, Sublingual. Secrete saliva (amylase for starch digestion, lysozyme for antibacterial).
Pharynx and Oesophagus
- Pharynx: Common passage for food and air.
- Epiglottis: Prevents food entry into trachea.
- Oesophagus: Peristalsis (rhythmic muscle contractions) moves food to stomach.
Stomach
- J-shaped, muscular.
- Gastric glands secrete: HCl (kills bacteria, activates pepsinogen), Pepsinogen -> Pepsin (protein digestion), Mucus (protects stomach lining).
- Partial protein digestion (chyme formation).
- Rennin (in infants): Curdles milk.
Small Intestine
- Duodenum: First part. Bile duct (from liver) and pancreatic duct open here.
- Jejunum and Ileum: Main sites of digestion and absorption.
- Intestinal wall has villi and microvilli (increase surface area for absorption).
Intestinal enzymes:
- Peptidases: Complete protein digestion (peptides -> amino acids).
- Disaccharidases (sucrase, maltase, lactase): Carbohydrates -> monosaccharides.
- Nucleotidases: Nucleic acids -> nucleotides.
Large Intestine
- Caecum, Colon, Rectum.
- Absorbs water, salts, vitamins (B and K from gut bacteria).
- Undigested material -> faeces.
Accessory Organs
Liver: Largest gland.
- Produces bile (stored in gall bladder).
- Bile emulsifies fats (physical digestion only, no enzymes).
- Functions: Detoxification, glucose storage (glycogen), protein synthesis, urea production.
Pancreas: Mixed gland (exocrine and endocrine).
- Pancreatic juice: Trypsin (proteins), Lipase (fats), Amylase (starches), Nucleases.
- Endocrine: Islets of Langerhans secrete insulin and glucagon.
Digestion Summary
| Food Type | Initial Site | Enzymes | Final Product |
|---|---|---|---|
| Carbohydrates | Mouth, Small intestine | Amylase, Maltase, Sucrase, Lactase | Glucose, Fructose, Galactose |
| Proteins | Stomach, Small intestine | Pepsin, Trypsin, Peptidases | Amino acids |
| Fats | Small intestine | Lipase (bile emulsifies) | Fatty acids + Glycerol |
| Nucleic acids | Small intestine | Nucleases | Nucleotides |
Absorption
- Monosaccharides and amino acids: Active transport/ facilitated diffusion into blood capillaries.
- Fatty acids and glycerol: Absorbed into lacteals (lymphatic system) as chylomicrons.
- Water: Passive absorption by osmosis.
- Vitamins: Fat-soluble (A, D, E, K) absorbed with fats. Water-soluble (B, C) absorbed directly.
Respiratory System
Components
- Nostrils -> Nasal cavity -> Pharynx -> Larynx -> Trachea -> Bronchi -> Bronchioles -> Alveoli.
Nasal cavity: Warms, moistens, filters air. Mucus and cilia trap particles.
Pharynx: Common passage.
Larynx (voice box): Contains vocal cords.
Trachea (windpipe): C-shaped cartilage rings keep it open. Ciliated epithelium and mucus for cleaning.
Bronchi and bronchioles: Trachea divides into two bronchi (each lung), further branching into bronchioles.
Alveoli: Tiny air sacs. Site of gas exchange. Thin walls (one cell thick), large surface area, rich blood supply.
Mechanism of Breathing
| Process | Diaphragm | Rib Cage | Air Flow |
|---|---|---|---|
| Inhalation | Contracts (flattens) | Moves up and out | Air enters |
| Exhalation | Relaxes (domes up) | Moves down and in | Air exits |
Intrapleural pressure: Always slightly negative (less than atmospheric).
Respiratory Volumes and Capacities
- Tidal Volume (TV): Normal breath volume (~500 mL).
- Inspiratory Reserve Volume (IRV): Extra air inhaled (~3000 mL).
- Expiratory Reserve Volume (ERV): Extra air exhaled (~1200 mL).
- Residual Volume (RV): Air remaining after forced exhalation (~1200 mL).
- Vital Capacity (VC): TV + IRV + ERV = ~4700 mL.
- Total Lung Capacity (TLC): VC + RV = ~5900 mL.
Gas Exchange
In alveoli (external respiration):
- O2 diffuses from alveoli (high pO2) to blood (low pO2).
- CO2 diffuses from blood (high pCO2) to alveoli (low pCO2).
In tissues (internal respiration):
- O2 diffuses from blood to tissues.
- CO2 diffuses from tissues to blood.
Transport of Gases
Oxygen transport (97% by RBCs, 3% dissolved in plasma):
- Haemoglobin + O2 -> Oxyhaemoglobin (reversible).
- Binding is cooperative: Binding of first O2 increases affinity.
CO2 transport (three ways):
- Dissolved in plasma (7%).
- As bicarbonate ions (70%):
CO2 + H2O -> H2CO3 -> H+ + HCO3-(in RBC, catalysed by carbonic anhydrase). - As carbamino compounds (23%): Hb + CO2 -> Carbaminohaemoglobin.
Respiratory Centre
- Medulla oblongata: Primary respiratory centre (rhythmic breathing).
- Pons: Pneumotaxic and apneustic centres (regulate rate and depth).
Disorders
- Asthma: Bronchial inflammation and constriction.
- Emphysema: Alveoli damage (often due to smoking).
- Pneumonia: Infection of lungs (fluid accumulation).
Worked Examples
Example 1: Calculate the vital capacity of a person with TV = 500 mL, IRV = 3000 mL, ERV = 1200 mL. Solution: VC = 500 + 3000 + 1200 = 4700 mL.
Example 2: How is CO2 transported in the blood? Solution: Mainly as bicarbonate ions (70%) formed by carbonic anhydrase in RBCs. Also as carbamino compounds (23%) and dissolved in plasma (7%).
Common Mistakes
- Bile has no enzymes: Bile only emulsifies fats physically. Digestion is by pancreatic lipase.
- Alveoli are the site of gas exchange: Bronchioles are passageways, not for exchange.
- Digestion of starch starts in the mouth: Continues briefly in stomach, then pancreas produces more amylase.
- CO2 transport vs O2 transport: CO2 is mainly transported as bicarbonate; O2 is mainly bound to haemoglobin.
ISC Exam Focus
- Theory (70%): Organ functions, enzyme actions, digestion summary, breathing mechanism, gas transport.
- Application (30%): Calculating lung capacities, tracing food/air pathway, enzyme tables.
- ISC frequently asks: "Describe the process of digestion of ..." and "Explain the transport of O2/CO2."
Self-Test Questions
Q1: Name the enzymes present in gastric juice and their functions. Answer: Pepsin (protein -> peptides), Rennin (curdles milk), HCl (activates pepsinogen, kills bacteria).
Q2: What is the role of bile in digestion? Answer: Emulsifies fats (breaks into small droplets, increasing surface area for lipase action). No enzymes.
Q3: Draw and label the human respiratory system. Answer: Nostrils, pharynx, larynx, trachea, bronchi, bronchioles, alveoli, lungs, diaphragm.
Q4: What is tidal volume? Answer: Volume of air inhaled or exhaled in a normal breath (~500 mL).
Q5: How is oxygen transported in blood? Answer: 97% as oxyhaemoglobin (HbO2), 3% dissolved in plasma.
Q6: Differentiate between external and internal respiration. Answer: External: gas exchange between alveoli and blood. Internal: gas exchange between blood and tissues.
