SUMMARY

Inside the body, mammals maintain the flow of nutrition by various forms of fluid such as urine, feces, and processed food. In this thesis, we investigate the fluid transportation in the urinary and digestive systems. The urethra serves as a siphon to accelerate urine flow by gravity, enabling the urinary system to be enlarged by a factor of 3,600 without com- promising its function. As a result, large mammals such as elephants urinate as fast as their smaller counterparts, such as cats. Regardless of body size, the acceleration of fluid is also discovered in the process of defecation, which begins in the large intestine. Larger animals have thicker and more slippery mucus layers to excrete more feces without compromising the efficiency. Before feces are formed in the gastrointestinal tracts, processed food in the small intestine is full of bubbles. These bubbles reduce the load of the intestinal wall in segmental contraction. The small intestine contracts faster with higher gas fraction near the large intestine than near the stomach. The study sheds light on optimal transport strategies in soft tissue and provides a design principle of scalable hydrodynamic systems.