Understanding Gas Exchange

Gas exchange refers to the process where oxygen is taken up from the environment and carbon dioxide is released as a by-product of cellular respiration.
This bi-directional exchange happens via diffusion, where gases move from high to low concentration.
The efficiency of gas exchange is determined by several factors including concentration gradients, surface area for exchange, and the distance over which exchange occurs.

Mechanism of Gas Exchange

In the lungs of mammals, oxygen diffuses from the alveoli (sac-like structure with a large surface area) into the blood, while carbon dioxide diffuses from the blood into the alveoli.
The walls of alveoli and the surrounding capillaries are very thin, allowing for a short distance for diffusion, thus enabling efficient gas exchange.
For optimal gas exchange, blood and air have to flow in opposite directions, known as counter-current flow. This maintains a concentration gradient along the entire length of the exchange surface.
Oxygenated blood and deoxygenated blood are kept separate, preventing any dilution of oxygen concentration in blood leaving the lungs — this is known as a double circulatory system.

Fish extract oxygen from water through their gills. The water flows over the gills in one direction while the blood flows in another (counter-current flow), allowing for efficient extraction.
For insects, spiracles (small openings on their bodies) act as entry/exit points for gases. Oxygen travels down the tracheae and tracheoles by diffusion and reaches cells, while carbon dioxide is removed via the same route.
In plants, gas exchange happens through small pores called stomata in the leaves. They allow carbon dioxide in for photosynthesis, and let oxygen and water vapour out.

Haemoglobin, a protein in red blood cells, plays a pivotal role in gas exchange.
It has a high affinity for oxygen and binds with it to form oxyhaemoglobin when oxygen concentration is high in the lungs.
In body tissues where oxygen concentration is low, haemoglobin releases oxygen, resupplying cells involved in metabolic activities.
The haemoglobin-oxygen binding affinity is influenced by the concentration of carbon dioxide, pH, and temperature — this is termed the Bohr effect.