An extreme form of this is seen in cases of humans infected with a parasitic roundworm, Wuchereria bancrofti . The worms live in the lymph nodes, blocking the flow of lymph and leading to accumulation of fluids. Extreme swelling of the lower limbs, a condition called elephantiasis, can result. Consideration of this pathology will convince you of the importance of this path for fluid movement in the body. You know that cells require oxygen for cellular respiration.
All oxygen uptake by animals occurs across a respiratory structure.
These same features are seen in both animals that breathe in water and animals that breathe in air. Note that there are specific disadvantages to each way of life. Four general types of respiratory surfaces are found in animals. The first is simply the body surface. This will work if it is thin and moist enough.