How has efficiency of respiratory surface increased from fishes to human being

In humans, certain specialised structures are present which fulfils the increased demand for oxygen in the body. Human beings need more oxygen than fishes because simple diffusion is not enough for their body's oxygen needs. These structures are lungs in human. Lungs are more efficient than gills (respiratory surface) in fish. lungs have alveoli which are the site of respiratory exchange of gases and have a more dense network of blood capillaries than gills.