Gas Exchange in Organisms

All living organisms consist of cells which must exchange gases with their environment.
Common gases exchanged:
- Oxygen (O2)
- Carbon Dioxide (CO2)
Types of organisms:
- Terrestrial organisms: exchange gases with air.
- Aquatic organisms: exchange gases with water.
Viruses do not perform gas exchange.
Bacteria, fungi, animals, and plants are living organisms that engage in gas exchange.

Gas exchange mainly occurs through diffusion, where gases move from high to low concentrations across membranes.
In lungs:
- CO2 diffuses from blood into alveoli.
- O2 diffuses from alveoli into blood.
Blood flow and ventilation (breathing) maintain concentration gradients.
In plants, gas exchange occurs via stomata, where:
- O2 produced in photosynthesis diffuses out.
- CO2 diffuses in for glucose production during photosynthesis.

A larger surface area allows for greater gas exchange through more diffusion sites.
Unicellular and small multicellular organisms have a large SA:V ratio enabling gas exchange directly through their surface.
As an organism's size increases, its volume grows faster than surface area, leading to a smaller SA:V ratio and necessitating specialized gas exchange tissues (e.g. gills, lungs, stomata).
Adaptations:
- External/internal gills, lungs, leaf mesophyll, stomata—areas with increased surface area for gas exchange.

Cellular level: Respiration maintains a concentration gradient.
- Continuous O2 use lowers intracellular concentration.
- Continuous CO2 production increases intracellular concentration.
Tissue level: Blood flow through capillaries maintains concentration differences.
Organ level: Ventilation actively moves air or water over gas exchange surfaces, maintaining concentration gradients essential for gas transfer during breathing.

Respiratory system components:
- Upper respiratory tract (nose, mouth, etc.): warms and humidifies air.
- Lower respiratory tract (trachea, bronchi, bronchioles): conducts air to lungs.
- Lungs: spongy organs for gas exchange.
Diaphragm and intercostal muscles facilitate breathing by changing lung volume, enabling pressure changes that drive air movement into and out of the lungs.

A spirogram visualizes lung volume and frequency of breaths, critical for assessing lung function.
Ventilation rates (12-20 breaths/min) increase during exercise.
Key lung volume definitions:
- Tidal volume: 500 mL of air exchanged per normal breath.
- Vital capacity: maximum exhale volume; 3-5 L for adults.
- Inspiratory/Expiratory reserves: additional air volumes inhaled/exhaled forcefully.

Hemoglobin binds to oxygen in lungs and transports it to tissues.
Structure of hemoglobin:
- Contains polypeptide chains (globin) and iron-containing heme groups.
- Can carry four O2 molecules at a time.
Affinity: The strength of hemoglobin's binding to O2; it increases when O2 is abundant and decreases in low levels, stimulating oxygen release to tissues.
The Bohr effect: Increased CO2 or decreased pH lowers hemoglobin's affinity for O2, promoting oxygen release where it is most needed during exercise.
Oxygen dissociation curves illustrate how hemoglobin saturation changes with varying O2 partial pressures, revealing effective oxygen unloading at tissues with high metabolic rates.

Partial Pressure of O2 (pO2), Temperature, pH, and CO2 concentration all influence hemoglobin's oxygen binding affinity.
Higher O2 promotes loading in lungs; lower $pO2$ in tissues promotes unloading.
Exercise physiology illustrates the dynamic nature of gas exchange, emphasizing the importance of maintaining gradients for effective respiration.