Respiration and Circulation
Respiration: Releasing Energy
Respiration is the process by which living organisms break down food (glucose) to release energy. This energy is stored in the form of ATP (Adenosine Triphosphate) — the energy currency of cells.
Types of Respiration
Aerobic Respiration (Requires Oxygen)
Complete breakdown of glucose into CO₂ and H₂O, releasing maximum energy (38 ATP molecules per glucose molecule).
Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) (Glucose + Oxygen → Carbon Dioxide + Water + Energy)
Stages:
- Glycolysis (in cytoplasm): Glucose (6C) → 2 Pyruvate (3C) → Net gain 2 ATP
- Krebs Cycle / Citric Acid Cycle (in mitochondria matrix): Pyruvate → CO₂, H₂O → releases some ATP
- Electron Transport Chain (inner mitochondrial membrane): Most ATP produced here (34 ATP)
Anaerobic Respiration (Without Oxygen)
Incomplete breakdown of glucose — occurs when oxygen is insufficient. Occurs in cytoplasm only. Much less energy produced (2 ATP per glucose molecule).
In plants and yeast: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + Energy (Alcoholic fermentation)
In humans and animals (muscle cells during heavy exercise): C₆H₁₂O₆ → 2C₃H₆O₃ + Energy (Lactic acid fermentation)
This causes lactic acid accumulation → muscle cramps/pain after heavy exercise (oxygen debt).
Comparison:
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required | Yes | No |
| Location | Mitochondria + cytoplasm | Cytoplasm only |
| Products | CO₂ + H₂O | Alcohol/lactic acid |
| Energy released | High (38 ATP) | Low (2 ATP) |
| Complete breakdown | Yes | No |
Breathing vs Respiration
Respiration: Biochemical process inside cells — breakdown of glucose to release energy (occurs in mitochondria).
Breathing/External Respiration: Physical process of taking in oxygen and releasing CO₂ (occurs in lungs, gills, etc.).
People often confuse the two. Plants do respire — all living things do. Breathing is specific to organisms with respiratory systems. Plants do photosynthesis only during daytime (light) but respire 24/7.
Human Respiratory System
Structure and Function
Nasal cavity: Filters dust and microbes (hair and mucus), warms and moistens air.
Pharynx: Common passage for food and air.
Larynx (Voice box):
- Contains vocal cords (vocal folds) — vibrate when air passes to produce sound
- Protected by epiglottis — a flap that closes the larynx during swallowing (prevents food entering lungs)
- Adam’s apple: Protrudes more prominently in males due to testosterone
Trachea (Windpipe):
- Tube carrying air to bronchi
- Supported by C-shaped cartilage rings (keep it open)
- Lined with ciliated epithelium (sweeps mucus and debris upward)
- Divides into two bronchi (one for each lung)
Bronchi: Branch into smaller bronchioles → end in alveolar sacs (alveoli).
Alveoli (gas exchange sites):
- 300–500 million in human lungs
- Thin membrane (~0.5 µm thick) — allows easy diffusion
- Surrounded by dense capillary network (blood vessels)
- Surface area: ~70 m² (for maximum gas exchange)
Lungs:
- Spongy, lightweight organs — air-filled
- Right lung: 3 lobes | Left lung: 2 lobes (to accommodate heart)
- Covered by pleura — double-layered membrane (visceral + parietal pleura)
- Diaphragm: Dome-shaped muscle below lungs — primary muscle of respiration
Mechanism of Breathing
Inhalation (Inspiration):
- Diaphragm contracts and flattens (moves down)
- External intercostal muscles contract → ribs move up and out
- Thoracic cavity expands → pressure inside lungs drops below atmospheric
- Air rushes into lungs (pressure gradient)
Exhalation (Expiration):
- Diaphragm relaxes → domes upward
- External intercostal muscles relax → ribs move down and in
- Thoracic cavity decreases → pressure inside lungs rises above atmospheric
- Air rushes out
Lung capacities:
- Tidal volume: Air breathed in normal breathing (~500 mL)
- Vital capacity: Maximum air that can be breathed out after maximum inhalation (~4,000 mL)
- Residual volume: Air remaining in lungs even after maximum exhalation (~1,500 mL)
Asthma: Bronchi constrict, excess mucus produced — difficulty breathing, wheezing, coughing. Bronchitis: Inflammation of bronchial tubes — common in smokers. Pneumonia: Infection causing alveoli to fill with fluid — oxygen diffusion impaired.
Human Circulatory System
Blood
Blood is the fluid that circulates in the blood vessels. It has two main components:
Plasma (~55% of blood):
- 90% water
- Dissolved nutrients (glucose, amino acids), waste products (urea), hormones, proteins (albumin, fibrinogen, globulins)
- Fibrinogen helps in clotting
Blood cells (~45% of blood):
-
Red Blood Cells (RBCs) — Erythrocytes:
- Carry oxygen (hemoglobin protein binds O₂)
- Biconcave shape — increases surface area
- No nucleus (in mammals) — more space for hemoglobin
- Produced in bone marrow (red marrow)
- Lifespan: ~120 days
- Hemoglobin: Iron-containing protein, gives blood its red color. Each hemoglobin can carry 4 O₂ molecules.
- Low RBC count → Anaemia (fatigue, pallor)
- RBC count: ~5 million/mm³
-
White Blood Cells (WBCs) — Leukocytes:
- Part of immune system — fight infection
- Larger than RBCs, but fewer in number
- Can cross blood vessel walls (diapedesis) to reach infection site
- Types:
- Phagocytes/Neutrophils (60–65%): Engulf and destroy pathogens
- Lymphocytes (25–35%): Produce antibodies — T cells (cellular immunity) and B cells (humoral immunity)
- Monocytes (3–8%): Transform into macrophages — large phagocytes
- WBC count: 4,000–11,000/mm³
- Leukopenia: Low WBC (immunosuppression) | Leukocytosis: High WBC (infection)
-
Platelets — Thrombocytes:
- Cell fragments, not complete cells
- Essential for blood clotting
- Count: ~150,000–400,000/mm³
- Release thromboplastin → converts prothrombin to thrombin → converts fibrinogen to fibrin → fibrin mesh traps blood cells → clot
Blood Groups
ABO System
| Blood Group | Antigens on RBC | Antibodies in Plasma | Can Receive From | Can Donate To |
|---|---|---|---|---|
| A | A antigen | Anti-B antibodies | A, O | A, AB |
| B | B antigen | Anti-A antibodies | B, O | B, AB |
| AB | Both A and B | Neither | A, B, AB, O (universal receiver) | AB only |
| O | Neither | Both Anti-A and Anti-B | O only | A, B, AB, O (universal donor) |
Rh Factor (Rhesus Factor)
- Some people have an additional antigen called Rh factor (Discovered in rhesus monkey)
- Rh+: Has Rh antigen on RBCs (~85% of Indians)
- Rh−: No Rh antigen
- During first pregnancy with Rh+ baby, Rh− mother may produce anti-Rh antibodies
- Erythroblastosis fetalis (Hemolytic disease of newborn): In second pregnancy, mother’s antibodies can attack baby’s RBCs. Prevented by RhoGAM injection after first delivery.
Blood Vessels
| Type | Function | Structure |
|---|---|---|
| Arteries | Carry blood away from heart | Thick, elastic walls, narrow lumen, no valves |
| Veins | Carry blood back to heart | Thin walls, valves present (to prevent backflow), wider lumen |
| Capillaries | Exchange of gases and nutrients | Single cell thick wall, very narrow (connects arteries to veins) |
The Heart
- Size: Roughly your fist; located slightly left of center in chest
- Weight: ~300 g
- Four chambers: 2 atria (upper), 2 ventricles (lower)
- Heart sounds: “Lubb-dubb” — “Lub” = tricuspid and mitral valves closing; “Dub” = pulmonary and aortic valves closing
Double Circulation:
- Pulmonary circuit: Right ventricle → Pulmonary artery → Lungs → Pulmonary vein → Left atrium (deoxygenated → oxygenated)
- Systemic circuit: Left ventricle → Aorta → Body → Superior/inferior vena cava → Right atrium (oxygenated → deoxygenated)
The oxygenated and deoxygenated blood NEVER mix — this is a complete (double) circulatory system.
Cardiac cycle: Heart beats ~70–75 times per minute at rest. Each beat involves:
- Diastole (relaxation) → ventricles fill with blood
- Atrial systole (atria contract) → pushes blood into ventricles
- Ventricular systole (ventricles contract) → blood pumped out
Blood pressure: Measured using sphygmomanometer
- Normal: 120/80 mmHg (systolic/diastolic)
- Systolic: Pressure when ventricles contract
- Diastolic: Pressure when ventricles relax
- Hypertension: High BP (>140/90) — strain on heart, risk of stroke
CTET Exam Focus
- Aerobic vs anaerobic respiration: Products, ATP yield, location
- Breathing vs respiration: Internal vs external process
- Human respiratory system: Nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli
- Alveoli: Gas exchange, 300-500 million, surface area 70 m²
- Blood: Plasma (water + dissolved substances), RBC (hemoglobin, carry O₂), WBC (immune function), platelets (clotting)
- ABO blood groups: A, B, AB, O — antigens and antibodies
- Rh factor: Rh+ vs Rh-, erythroblastosis fetalis
- Double circulation: Pulmonary and systemic circuits, oxygenated/deoxygenated never mix
- Heart sounds: Lubb (valve closure atria), dub (valve closure ventricles)
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