2.1 Human Respiratory System
The human respiratory system is responsible for taking in oxygen from the surroundings for cellular respiration and removing carbon dioxide and water vapor from the body.
Structure of the Human Respiratory System
Air passes through the following respiratory tract:
Nasal cavity → Pharynx → Larynx → Trachea → Bronchus → Bronchiole → Alveolus
Breathing Mechanism
Breathing involves two main processes: inhalation (breathing in) and exhalation (breathing out).
- Inhalation:
- Intercostal muscles contract, pulling the ribcage upwards and outwards.
- Diaphragm muscle contracts and flattens.
- Volume of the thoracic cavity increases, causing air pressure in the thoracic cavity to decrease below atmospheric pressure.
- Air is forced into the lungs.
- Exhalation:
- Intercostal muscles relax, moving the ribcage downwards and inwards.
- Diaphragm muscle relaxes and curves upwards (dome-shaped).
- Volume of the thoracic cavity decreases, causing air pressure in the thoracic cavity to increase above atmospheric pressure.
- Air is pushed out of the lungs.
2.2 Movement and Exchange of Gases in the Human Body
Diffusion of Oxygen and Carbon Dioxide in the Alveoli
Gaseous exchange occurs in the alveoli through diffusion down a concentration gradient:
- Oxygen: Inhaled air in the alveolus has a higher partial pressure of oxygen than the blood capillaries. Oxygen dissolves in the moist lining of the alveolus and diffuses into blood capillaries, binding with hemoglobin in red blood cells to form oxyhemoglobin.
- Carbon Dioxide: Deoxygenated blood arriving at capillaries has a higher partial pressure of carbon dioxide than the alveolus. Carbon dioxide diffuses from blood into alveoli to be exhaled.
Adaptations of Alveoli for Efficient Gas Exchange
- Large Surface Area: Millions of tiny alveoli provide a massive surface area for gas exchange.
- Very Thin Wall: Alveolar and capillary walls are only one cell thick, shortening diffusion distance.
- Moist Inner Lining: Allows gases (oxygen and carbon dioxide) to dissolve before diffusing across membranes.
- Dense Network of Blood Capillaries: Maintains a steep concentration gradient for rapid gas exchange.
2.3 Health of the Human Respiratory System
Harmful Substances to the Respiratory System
- Cigarette Tar: Black, sticky substance that damages cilia, coats alveoli, and causes lung cancer.
- Carbon Monoxide: Colorless, odorless gas found in tobacco smoke and vehicle exhaust. Binds preferentially with hemoglobin to form carboxyhemoglobin, reducing oxygen transport in blood.
- Sulfur Dioxide & Nitrogen Dioxide: Acidic gases from factories and vehicles that irritate the respiratory tract, causing coughs, asthma, and bronchitis.
- Haze, Dust, and Particulate Matter (PM2.5): Physical pollutants that irritate air passages and cause breathing difficulties.
Respiratory Diseases
- Asthma: Inflammation and narrowing of bronchioles triggered by allergens (dust, pollen, cold air). Symptoms: wheezing, shortness of breath.
- Bronchitis: Inflammation of the bronchial tubes, usually caused by infection or smoking, resulting in persistent coughing and excessive mucus production.
- Emphysema: Destruction and damage of alveolar walls, reducing the surface area for gas exchange. Usually caused by long-term smoking. Symptoms: extreme breathlessness.
- Lung Cancer: Uncontrolled cell division in lung tissue caused by carcinogens like cigarette tar.
2.4 Adaptations for Gas Exchange in Other Animals
Gaseous Exchange Structures in Different Animals
- Amphibians (e.g., Frogs):
- Lungs: Used during active periods on land (simple lung structure).
- Moist Outer Skin: Used in water and moist terrestrial environments. Skin is thin, highly permeable to gases, and richly supplied with blood capillaries underneath.
- Fish:
- Gills: Composed of numerous gill filaments covered in thin plates called lamellae.
- Provides a large surface area and dense capillary network for absorbing dissolved oxygen from water.
- Insects (e.g., Grasshopper):
- Tracheal System: Consists of tiny pores called spiracles on the body surface leading to a network of tracheae and smaller tracheoles.
- Oxygen diffuses directly from tracheoles into body cells without requiring a blood circulatory system.
2.5 Gas Exchange in Plants
Mechanism of Opening and Closing of Stomata
Stomata (singular: stoma) are microscopic pores mostly found on the lower epidermis of leaves, bounded by a pair of guard cells.
- Daytime / Light (Stoma Opens):
- Guard cells undergo photosynthesis and produce glucose (or absorb potassium ions), causing water to enter guard cells via osmosis.
- Guard cells become turgid and curve outwards, opening the stoma to allow $CO_2$ intake for photosynthesis and $O_2$ release.
- Nighttime / Darkness / Water Shortage (Stoma Closes):
- Guard cells lose water via osmosis and become flaccid, causing the stoma to close to prevent excessive water loss (transpiration).
Importance of Environmental Pollution Control on Plants
Pollutants such as soot, dust, and acidic haze clog leaf stomata, reducing the rate of photosynthesis, gas exchange, and transpiration, ultimately stunting plant growth.