Biology UPSC, Human Digestive System UPSC
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Human Respiratory System UPSC & Other Exams
The human respiratory system is a complex network of organs and tissues responsible for breathing and gas exchange.
The respiratory system brings oxygen into the body and removes carbon dioxide. This process is essential for cellular metabolism and energy production.
Most of the human respiratory system is situated within the thoracic cavity (chest cavity).
In adult humans, the normal respiratory rate (the number of breaths per minute) is typically between 12 and 15 breaths. For infants, this rate is significantly higher, averaging around 44 breaths per minute.
1. Organs of human respiratory system
External Nose
- Composed of bone (upper third) and cartilage (lower two-thirds)
- Includes the nasal bridge, nostrils (nares), and nasal tip
- Entry point for air
Nasal Cavities
- A large, air-filled space behind the nose
- Filters, warms, and humidifies incoming air
- Divided into right and left chambers by the nasal septum
- Contains mucus and tiny hairs (cilia) to trap dust and pathogens
- Lined with mucous membrane (mucosa) rich in blood vessels
- First stage of air conditioning for respiratory system
- Upper part of nasal cavity contains olfactory receptors (Smell receptors). Allows detection and identification of different odors.
Pharynx (Throat)
- Connects nasal and oral cavities to the lungs
- Shared pathway for air and food
- Divided into three sections: nasopharynx, oropharynx, and laryngopharynx
Larynx (Voice Box)
- Contains two vocal cords
- Regulates airflow
- Prevents food and liquid from entering the respiratory tract, close during eating (second protection after epiglottis)
- Plays a role in sound production
Trachea (Windpipe)
- Rigid tube lined with C-shaped cartilage rings
- Connects larynx to bronchi
- Keeps airway open
- About 4-5 inches long in adults
Bronchi
- Two main branches that split from the trachea
- Right bronchus is slightly wider and more vertical
- No cartilage support
- Divide into smaller bronchioles within the lungs
Bronchioles:
- Smaller branches of the bronchi
- No cartilage support like larger bronchi
- Further divide into smaller passages Alveoli
- Distribute air to different regions of the lungs
Alveoli:
- Tiny air sacs at the end of bronchioles
- Microscopic in size (about 0.2-0.5 mm in diameter)
- Extremely thin walls (one cell thick)
- Surrounded by dense capillary network
- Primary site of gas exchange in the lungs (The process of exchanging oxygen (O2) from inhaled air with carbon dioxide (CO2) present in the blood within the alveoli of the lungs is known as gas exchange. This crucial process occurs through the mechanism of diffusion, where molecules naturally move from areas of higher concentration to areas of lower concentration.)
- Massive number: approximately 480 million in adult lungs (provide a vast surface area for efficient gas exchange)
Gas Exchange Process in Alveoli:
- Oxygen diffuses from alveolar air into blood
- Oxygen binds to Haemoglobin in red blood cells
- Carbon dioxide diffuses from blood into alveolar air
- Carbon dioxide is expelled during exhalation
Lungs
- Primary respiratory organs
- Right lung: 3 lobes
- Left lung: 2 lobes (space for heart)
- Contain millions of alveoli for gas exchange
- Covered by pleural membranes
Diaphragm
- Not strictly a respiratory organ but crucial for breathing
- Muscle separating chest and abdominal cavities
- Contracts and relaxes to facilitate breathing
- Primary muscle of respiration
Each organ plays a specific and critical role in the respiratory process, working together to ensure efficient oxygen intake and carbon dioxide removal.
2. Functions of the human respiratory system
Gas Exchange
- Brings oxygen into the body
- Removes carbon dioxide from the body
- Occurs primarily in the alveoli
- Enables cellular respiration and metabolism
- Supports energy production in cells
Oxygen Transportation
- Delivers oxygen to blood through Haemoglobin in red blood cells
- Blood transports oxygen to all body tissues and organs
- Supports cellular function and survival
- Enables energy production in mitochondria
Removal of Waste Gases
- Eliminates carbon dioxide produced by cellular metabolism
- Helps maintain body’s acid-base balance
- Prevents accumulation of toxic metabolic waste
pH Regulation
- Helps control blood pH through carbon dioxide elimination
- Balances acid-base equilibrium in the body
- Critical for maintaining cellular and systemic homeostasis
Temperature Regulation
- Helps cool the body through exhalation
- Assists in heat exchange
- Contributes to maintaining body temperature
Defense Mechanism
- Filters incoming air
- Traps dust, pathogens, and foreign particles
- Uses mucus and cilia to protect against infections
- First line of immune defense in respiratory tract
Sound Production
- Enables speaking and vocalization
- Air passing through vocal cords creates sound
- Supports communication
Endocrine Function
- Produces some hormones and biochemical substances
- Converts angiotensin I to angiotensin II
3. Breathing vs Respiration
Breathing (Ventilation):
- A physical process of moving air in and out of the lungs
- Involves mechanical movements of the diaphragm and chest muscles
- Visible and external process
- Includes inhalation (taking air in) and exhalation (pushing air out)
- Inhalation (Inspiration): Diaphragm contracts, expanding chest cavity and drawing air in
- Exhalation (Expiration): Diaphragm relaxes, pushing air out of the lungs
- Primarily occurs in the respiratory tract (nose, trachea, lungs)
Respiration (Cellular Respiration):
- A biochemical process that occurs at the cellular level
- Involves breaking down glucose to produce energy (ATP)
- Invisible process that happens inside cells
- Occurs in the mitochondria of cells
- Requires oxygen and produces carbon dioxide as a byproduct
- Converts chemical energy from food into usable energy for cell functions
4. Aerobic Respiration vs Anaerobic Respiration
Aerobic respiration is a metabolic process that uses oxygen to break down glucose into energy (ATP), carbon dioxide, and water. It’s more efficient than anaerobic respiration, yielding significantly more ATP.
Anaerobic respiration, on the other hand, doesn’t require oxygen. It’s a less efficient process that produces less ATP and often results in byproducts like lactic acid or ethanol.
Key difference between Aerobic Respiration and Anaerobic Respiration:
| Aerobic Respiration | Anaerobic Respiration | |
| Oxygen Requirement | Requires oxygen to produce energy Occurs in mitochondria in cell Complete breakdown of glucose | Does not require oxygen Occurs in cytoplasm Partial breakdown of glucose Emergency energy production |
| Energy Production | Highly efficient Produces 38 ATP molecules per glucose molecule (Use glucose and oxygen) Occurs in most human cells Primary energy production method | Less efficient Produces 2 ATP molecules per glucose (Use glucose only) Occurs during intense physical activity Short-term energy source |
| Process Steps | Krebs Cycle Glycolysis Electron Transport Chain | Glycolysis only Occurs in muscle cells during extreme exertion |
| End Products | Carbon dioxide Water Large amount of energy (ATP) | Lactic acid (Humans/Mammals), Ethanol (Yeast/Fungi) Small amount of energy (ATP) Causes muscle fatigue |
| Equation |
5. Photosynthesis vs Respiration in Plants
Respiration is the metabolic process where plants break down glucose to release energy for cellular activities.
Photosynthesis is an essential process by which plants convert light energy into chemical energy, producing oxygen and glucose.
Key differences between photosynthesis and respiration:
| Photosynthesis | Respiration | |
| Purpose | Creates energy and organic compounds It is a constructive process The weight of the plant increase. | Breaks down energy for cellular use It is a disruptive process. The weight of the plant decrease. |
| Processes | Light Absorption Occurs in chloroplasts using chlorophyll pigments Captures solar energy from sunlight Takes place primarily in green parts of the plant, especially leaves | Energy Production Occurs in mitochondria Converts stored glucose into usable ATP (adenosine triphosphate) Happens continuously in all living plant cells |
| Chemical Transformation | Carbon dioxide from the air is combined with water Using light energy, plants create glucose (sugar) Oxygen is released as a byproduct Overall Chemical Equation 6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ (Glucose) + 6O₂ | Glucose is broken down using oxygen Releases carbon dioxide and water Generates energy for growth, reproduction, and maintenance Overall Chemical Equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP) |
| Energy Flow | Converts light to chemical energy | Converts chemical energy to usable cellular energy |
| Gas Exchange | Absorbs CO₂, releases O₂ | Absorbs O₂, releases CO₂ |
6. Composition of Atmospheric Air and Breathing
Nitrogen (N₂)
- Percentage in Inhalation: 78%
- Percentage in Exhalation: Remains approximately 78%
- Nitrogen is an inert gas that passes through the respiratory system without significant changes
Oxygen (O₂)
- Percentage in Inhalation: 21%
- Percentage in Exhalation: 17%
- Difference: About 4% of oxygen is absorbed by the body during each breath
- Oxygen is actively consumed for cellular respiration
Carbon Dioxide (CO₂)
- Percentage in Inhalation: 0.03% (0.04% in recent measurements)
- Percentage in Exhalation: 4.6%
- Significant increase due to metabolic processes producing CO₂ as a waste product
7. Respiratory Mechanisms Across Different Organisms
Unicellular Organisms
- Breathe through simple diffusion
- Gas exchange occurs directly through cell membrane
- No specialized respiratory organ required
- Oxygen and carbon dioxide pass through cell surface
Aquatic Animals
- Primary Respiratory Organ: Gills
- Specialized for extracting dissolved oxygen from water
- Thin, highly vascularized membranes
- Efficient in extracting oxygen from water through countercurrent exchange
- Examples: Fish, some amphibians, aquatic invertebrates
Insects
- Respiratory Organ: Tracheal System
- Network of air tubes (tracheae) directly delivering oxygen to tissues
- Small openings called spiracles on body surface
- Air moves through passive diffusion and body movements
- Highly efficient for their body size
Humans and Mammals
- Respiratory Organ: Lungs
- Complex respiratory system with bronchi, bronchioles, and alveoli
- Active breathing mechanism using diaphragm and intercostal muscles
- Allows for efficient oxygen intake and carbon dioxide removal
- Highly developed respiratory surface area
Frogs (Amphibians)
- Multiple Respiratory Organs:
- Gills (in larval/tadpole stage)
- Lungs (in adult stage)
- Skin (continuous respiratory surface)
- Unique ability to breathe through multiple methods
- Skin is moist and highly permeable, allowing gas exchange
- Demonstrates evolutionary adaptation to different life stages and environments
Key Principles of Respiratory Mechanisms
- Maximize surface area for gas exchange
- Maintain thin membranes for efficient diffusion
- Adapt to specific environmental conditions
- Evolve mechanisms suitable for organism’s size and habitat
8. Role of Haemoglobin in the Respiratory System
Haemoglobin (respiratory pigment) is a crucial protein found within red blood cells that plays a vital role in the respiratory system. Its primary function is to transport oxygen from the lungs to the body’s tissues and to carry carbon dioxide back to the lungs for exhalation.
Deoxygenated Haemoglobin (bound with CO2) appears purple or bluish, while oxygenated Haemoglobin (bound with O2) is red.
Haemoglobin (Hb) + O2 → HbO2 (Oxyhaemoglobin – Red)
Haemoglobin (Hb) + CO2 → HbCO2 (Deoxyhaemoglobin – Purple)
Haemoglobin has a significantly higher affinity for carbon monoxide (CO) than for oxygen (O2), approximately 250 times greater. This means that in the presence of CO, Haemoglobin preferentially binds with CO instead of O2, reducing its oxygen-carrying capacity.
Increased CO in the atmosphere leads to decreased oxygen levels in the blood, triggering an increase in the respiratory rate.
9. Common Respiratory (Pulmonary) Defects
Asthma
A chronic inflammatory respiratory condition that causes inflammation and narrowing of the airways (bronchioles), making breathing difficult. Bronchioles become narrowed, leading to difficulty breathing, wheezing, and chest tightness. Environmental triggers like allergens, pollutants, stress, or respiratory infections can precipitate asthma attacks. Symptoms may vary in severity and frequency among individuals.
Pneumonia
An acute infectious respiratory disease that causes inflammation of the lung’s air sacs (alveoli). Primarily caused by bacterial, viral, or fungal pathogens, pneumonia results in alveoli filling with fluid or pus, significantly impairing respiratory function.
Tuberculosis (TB)
Tuberculosis (TB) is an infectious disease caused by bacteria (Mycobacterium tuberculosis). It primarily affects the lungs, but can spread to other parts of the body. the BCG vaccine is available to help prevent TB infection.
Lung Cancer
Uncontrolled growth of abnormal cells in the lungs, which can spread to other parts of the body.
Cystic Fibrosis
An inherited disorder that causes thick, sticky mucus to build up in the lungs, making it difficult to breathe and increasing the risk of infection.
Pulmonary Embolism
A blockage in one of the pulmonary arteries in the lungs, usually caused by a blood clot.
COVID-19 (Corona)
A viral respiratory disease caused by the SARS-CoV-2 coronavirus, known for its significant impact on lung function. Inflammation and potential scarring of lung alveoli. Reduced oxygen exchange capacity.
Hypoxia
Hypoxia is a condition in which the body or a region of the body is deprived of adequate oxygen supply at the tissue level.
- Zymase is a complex enzyme system that plays a crucial role in fermentation, particularly in yeast cells.
- Zymase is not a single enzyme, but a complex mixture of enzymes
- It is found in yeast cells
- Catalyzes the breakdown of glucose into ethanol and carbon dioxide
- C6H12O6 (glucose) → 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)
- Essential in the process of alcoholic fermentation
- Converts sugar into alcohol during processes like beer and wine production
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