Gas Exchange

In a biological sense, it's the transfer of gases across cell membranes to enter or leave an organism. In humans, this refers to O2 in and CO2 out, occurring first in the lungs and then in individual cells.
Gas exchange is necessary to capture and use energy with vital life processes:
- Respiration and photosynthesis
- Metabolism
- Waste removal
- Homeostasis
- Survival in various environments
- Nutrient transport
- Ecosystem balance
As an organism increases in size, the distance from the center to its exterior increases, creating an upper limit of animal size.
Metabolic rate declines as organisms increase in size towards a lower limit needed for survival.
Examples of gas exchange mechanisms include:
- Small unicellular organisms (bacteria, protozoa) with high surface-area to volume ratio: gas exchange occurs across the cell membrane.
- Insects: tracheal system delivers oxygen directly to cells and tissues.
- Larger organisms: specialized structures (gills, lungs, spongy mesophylls) provide a large surface area for effective gas exchange.

Gas exchange surface refers to the surface tissue or membrane at the site of gas exchange in an organism (e.g., alveoli + capillaries in lungs).
Gas exchange surfaces have unique features for efficient gas movement:
- Permeability: Cell membranes or specialized tissues must be selectively permeable to allow gases to pass through while preventing the loss of essential fluids. Nonpolar molecules (O2 and CO2) can freely diffuse across cell membranes.
- Thickness: Thin tissues minimize the barrier that gases must cross, ensuring that diffusion occurs quickly and efficiently. The barrier between alveoli in the lungs and the capillaries of the blood vessels are each just one cell thick (2 cell membranes to cross to get into the bloodstream)
- Moisture content: Gases are more soluble in moist substances than in dry ones, which can also help reduce surface tension, which allows gasses to move at a higher rate.
- Surface area: Structures that increase surface area (folds, projections, specialized structures like alveoli or gill filaments) allow for more gas molecules to be exchanged simultaneously, increasing overall efficiency. An adult human lung contains between 300 - 500 million alveoli.

Gas exchange is typically a passive process in living organisms.
Concentration gradients must be maintained at gas exchange surfaces using:
- Dense networks of blood vessels
- Continuous blood flow
- Ventilation (for lungs)
- Water movement (for gills)
Dense blood vessel networks:
- The average adult has approximately 100,000 kilometers of blood vessels.
Continuous blood flow:
- Constant blood flow replenishes the concentration gradient by removing substances that have diffused across the exchange surface, maintaining a higher concentration in the external environment.
Ventilation (for lungs):
- Movement of air across an exchange surface replenishes the supply of gases and maintains a concentration gradient.
- Lungs, along with muscles (diaphragm and intercostal muscles), enable ventilation, ensuring a continuous supply of oxygen-rich air to the alveoli and facilitates the removal of CO2.
Water movement (for gills):
- Gases exchanged by aquatic organisms are typically dissolved in the water.
- Many fish and aquatic animals actively move water over the gills using movements of their mouths and opercula (bony covers protecting the gills).
Countercurrent exchange system (fish):
- Water flows over the gills in a direction opposite to the flow of blood within the gill filaments.

Mammalian lungs demonstrate adaptations to enhance gas exchange efficiency:
- Presence of surfactant
- High surface area
- Diaphragm and intercostal muscles
- Thin respiratory membrane
Flow of air into the lungs:
- trachea
- bronchus
- bronchiole
- alveolus
Surfactant:
- Alveoli are covered with a thin layer of pulmonary surfactant- a protein/lipid liquid that reduces surface tension in the alveoli.
- This reduction makes it easier for O2 and CO2 to diffuse in/out of the lungs, which in turn makes it easier for the lungs to inflate.
High surface area:
- Mammalian lungs have ≈30,000 bronchioles, providing a very large surface area for gas exchange.
- An extensive capillary bed surrounds each of the ~500 million alveoli found in adult human lungs.
Diaphragm and intercostal muscles:
- The movement of air in/out of the lungs is controlled the diaphragm and the intercostal muscles - which expand and contract to create the pressure gradients needed to bring air in and out.
Thin respiratory membrane:
- O2 and CO2 only need to pass through a 2-cell barrier - one thin cell lines the alveoli, and one thin cell lines the capillaries.

When the pressure in the chest is less than the atmospheric pressure, air will move into the lungs (inspiration/inhalation).
When the pressure in the chest is greater than the atmospheric pressure, air will move out of the lungs (expiration/exhalation).
Bones, cartilage, muscle, tendons, and ligaments work together to make inspiration and expiration possible.
Different groups of muscles are required to expand and contract chest volume, altering the pressure in the chest.
Like all antagonistic muscle pairs, when one muscle contracts, the other relaxes.
During inhalation, the diaphragm AND external intercostal muscles both contract to increase the volume of the lungs. Pressure drops, and air rushes in.
The diaphragm contracts to increase volume and cause the inflow of air.
During exhalation, the diaphragm and external intercostal muscles relax.
During long or forced exhalations, the internal intercostal muscles also contract. Additional abdominal muscles also contract to push the diaphragm upward.

Ventilation rate: number of breaths/minute.
Tidal volume: total volume of air that moves in or out of the lungs with each breath.
Vital capacity: maximum amount of air you can inhale and exhale in one breath.
Expiratory reserve: volume of air you can breathe out after normal inhalation and exhalation
Techniques for measuring ventilation rate or lung tidal volume:
- Via simple observation (counting number of breaths per minute)
- Chest belt and pressure meter (recording rise / fall of the chest)
- Spirometer (recording the volume of gas expelled per breath)
Spirometer:
- A spirometer is a device that detects the changes in ventilation and presents the data on a display.
- Spirometry involves measuring the amount (volume) and / or speed (flow) at which air can be inhaled or exhaled.
A more simplistic method of spirometry involves breathing into a balloon and measuring the volume of air in a single breath.
Effects of exercise on ventilation:
- Increased ventilation rate
- Increased tidal volume
- Over time, consistent training will also increase vital capacity and expiratory reserve.