# The Mammalian Gas Exchange System ## Overview - Gas exchange takes place in the human thorax, a collection of organs and tissues in the chest cavity - The mammalian gas exchange system contains multiple tissue types, each structurally adapted for specific functions - Different components work together to ensure efficient oxygen delivery and carbon dioxide removal ## Tissues of the Gas Exchange System ### Cartilage - Strong and flexible tissue found throughout the respiratory system - Present as C-shaped rings along the trachea (tracheal rings) - **Function**: Supports the trachea and ensures it stays open while allowing movement and flexing during breathing - In bronchi, cartilage does not form C-shapes but can form full rings or irregular blocks

Cartilage rings are C-shaped in the trachea to prevent friction with the oesophagus located directly behind

### Ciliated Epithelium - Specialised tissue found along the trachea, bronchi and larger bronchioles - Each cell has small projections called cilia - **Function**: Cilia sweep mucus, dust and bacteria upwards and away from the lungs and epithelium itself - Works in coordination with goblet cells and mucous glands

Ciliated epithelium is tissue composed of cells with hair-like projections (cilia) that move rhythmically to transport materials

### Goblet Cells - Mucus-producing cells scattered throughout ciliated epithelium in the trachea and bronchi - Secrete viscous mucus that traps dust, bacteria and other microorganisms - Prevents pathogens from reaching the lungs where they could cause infection - Mucus is swept upward by cilia and eventually swallowed - Any microorganisms are destroyed by stomach acid ### Mucous Glands - Work with goblet cells in the trachea and bronchi to produce mucus - Located deeper in the tissue layers - **Function**: Provide additional mucus production to trap inhaled particles ### Squamous Epithelium - Lines the alveoli with thin, flattened cells - Forms the structure of the alveolar wall - **Function**: Very thin and permeable to allow easy diffusion of gases - Single layer of cells minimises diffusion distance between air and blood

Squamous epithelium consists of flat, thin cells arranged in a single layer to facilitate rapid diffusion

### Smooth Muscle - Found throughout the walls of the bronchi and bronchioles - Not under conscious control (involuntary muscle) - **Function**: Regulates the flow of air into the lungs - Dilates (relaxes) when more air is needed (e.g., during exercise) - Constricts (contracts) when less air is needed - Helps to adjust airway diameter based on oxygen demands ### Elastic Fibres - Present in all lung tissues, especially around alveoli - **Function**: Enable the lungs to stretch during inhalation and recoil during exhalation - The ability to recoil makes expiration a **passive process** (requires no energy at rest) - Critical for maintaining lung function and preventing alveolar collapse

In conditions like emphysema, elastic fibres are damaged, reducing the lungs' ability to recoil and making breathing difficult

### Capillaries - Extensive network surrounds each alveolus - Diameter of approximately 3-4 μm (wide enough for only one red blood cell at a time) - Walls are one cell thick (flattened endothelial cells) - **Function**: Gas exchange between alveoli and blood - Oxygen diffuses from alveoli into capillaries - Carbon dioxide diffuses from capillaries into alveoli - Narrow diameter ensures sufficient time for gas exchange to occur ## Components of the Gas Exchange System ### Trachea (Windpipe) - Main airway leading from the mouth/nose to the lungs - Approximately 12 cm long and 2 cm in diameter in adults **Key features:** - C-shaped rings of cartilage keep the airway open at all times - Lined with ciliated epithelium - Contains numerous goblet cells and mucous glands producing mucus - Wall contains smooth muscle and elastic fibres connecting the ends of cartilage rings **Function:** - Provides a clear passage for air to reach the lungs - Mucus traps inhaled particles preventing lung contamination ### Bronchi (singular: Bronchus) - The trachea divides into two bronchi (left and right) - Each bronchus leads to one lung - Similar structure to trachea but with: - Thinner walls - Smaller diameter - Cartilage forming irregular blocks or complete rings (not C-shaped) **Function:** - Conduct air from trachea into each lung - Branch into progressively smaller airways (bronchioles) ### Bronchioles - Narrow, self-supporting tubes branching from bronchi - Walls progressively get thinner closer to alveoli - Most bronchioles have **no cartilage support** (a few larger ones may contain some) - Lined with ciliated epithelium but usually **no goblet cells** in smaller bronchioles - Diameter less than 1 mm **Key features:** - Larger bronchioles contain elastic fibres and smooth muscle - Adjust airway size to increase/decrease airflow based on oxygen demand - Smallest bronchioles have elastic fibres but no smooth muscle - Very large number present in the respiratory system **Function:** - Distribute air throughout the lungs to the alveoli - Allow fine control of air distribution ### Alveoli (singular: Alveolus) - Tiny air sacs located at the ends of bronchioles - Site of gas exchange in the lungs - Approximately 480-500 million alveoli in adult human lungs - Total surface area of 40-75 m² **Structure:** - Wall consists of a single layer of squamous epithelium - Elastic fibres in extracellular matrix allow stretching and recoiling - Surrounded by extensive capillary network - Lined with thin watery fluid that facilitates gas diffusion **Adaptations for efficient gas exchange:** | Feature | How it enhances gas exchange | |---------|------------------------------| | Large number (millions) | Provides enormous surface area for diffusion | | Thin walls (one cell thick) | Short diffusion distance for rapid gas exchange | | Rich blood supply | Maintains concentration gradient by continuously removing oxygenated blood and bringing deoxygenated blood | | Moist lining | Gases dissolve before diffusing across membrane | | Elastic fibres | Allow alveoli to expand and recoil, maintaining optimal surface area |

The huge surface area of alveoli (equivalent to a tennis court) ensures efficient gas exchange despite the relatively small size of the lungs

**Gas exchange mechanism:** 1. Air in alveoli contains high concentration of oxygen 2. Oxygen diffuses from alveoli into blood capillaries 3. Blood transports oxygen around the body for aerobic respiration 4. Blood in capillaries has high concentration of carbon dioxide 5. Carbon dioxide diffuses from blood into alveoli 6. Carbon dioxide is then exhaled ## Structures & Functions Summary | Structure | Function | |-----------|----------| | **Ciliated epithelial cells** | Line the trachea to bronchi with tiny cilia projections that sweep mucus, dust, bacteria and microorganisms upward to the mouth where they are swallowed and digested by stomach acid | | **Goblet cells** | Embedded throughout ciliated epithelium; mucus-producing cells that trap dust, bacteria and microorganisms | | **Mucous glands** | Work with goblet cells in trachea and bronchi to produce mucus | | **Cartilage** | Rings line the trachea or windpipe; strong but flexible tissue essential for keeping the trachea open due to its flexibility; allows movement and flexing during breathing | | **Smooth muscle** | Not under conscious control; found in bronchi and bronchioles walls; regulates airflow by dilating and widening when more air needed | | **Elastic fibres** | Found in squamous epithelium of alveolar walls; critical component allowing tiny alveoli to stretch as they fill with air and recoil to force air out; enables large surface area and in serious cases like COPD/emphysema, alveoli lose these elastic properties | | **Squamous epithelium** | Consists of flat, thin cells in alveolar walls; thinness means oxygen and carbon dioxide have a very short diffusion pathway during gas exchange | ## Thorax Structures | Structure | Description | |-----------|-------------| | **Trachea** | Airway leading from mouth and nose to bronchi; lined with mucus-secreting goblet cells and cilia that sweep microorganisms and dust away from lungs | | **Lungs** | Two lungs form central part of respiratory system where gas exchange takes place | | **Bronchi** | Plural of "Bronchus"; left and right bronchi at bottom of trachea are similar in structure but narrower; bronchi lead to bronchioles | | **Bronchioles** | Narrow tubes (less than 1mm) carrying air from bronchi to alveoli; so narrow they have no supporting cartilage and can collapse | | **Alveoli** | Main site of gas exchange in lungs; tiny sacs with many structural adaptations enabling efficient gas exchange, such as thin walls and large surface area to volume ratio | | **Capillary network** | Extensive network of capillaries surrounds alveoli and provides exchange surface between lungs and blood; during gas exchange, oxygen diffuses from alveoli into capillaries while carbon dioxide diffuses in opposite direction and is exhaled | ## Comparison with Other Gas Exchange Systems **Key differences from single-celled organisms:** - Single-celled organisms can rely on diffusion alone due to high SA:V ratio - Example: *Chlamydomonas* (20 μm diameter) - oxygen only needs to diffuse 10 μm to reach centre (takes 100 milliseconds) - In contrast, if diffusion distance increased to 15 cm, diffusion time would increase to 7 hours - This demonstrates why larger multicellular organisms need specialised gas exchange systems **Mammalian lungs compared to other systems:** - Unlike fish gills, mammalian lungs use tidal ventilation (air flows in and out same route) - Unlike insect tracheal systems, mammals use blood to transport gases rather than direct air delivery to tissues - Mammalian system allows for much larger body size than insect tracheal system

Remember the hierarchy: Trachea → Bronchi → Bronchioles → Alveoli. Each level has progressively less cartilage support but more emphasis on gas exchange efficiency

## Clinical Relevance **Common respiratory conditions affecting gas exchange:** - **Asthma**: Smooth muscle in bronchioles contracts excessively, narrowing airways - **Emphysema**: Elastic fibres around alveoli are destroyed, reducing recoil ability - **Bronchitis**: Excessive mucus production and inflammation of bronchi - **Cystic fibrosis**: Abnormally thick mucus that cilia cannot clear effectively **Healthy lung maintenance:** - Avoiding smoking (damages cilia and increases mucus production) - Regular exercise (improves lung capacity and efficiency) - Minimising exposure to air pollutants (reduces inflammation and damage)