Hydrodynamics of urination and digestion

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Yang, Patricia
Hu, David L.
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The intake of food and water and its excretion is a hallmark of all life forms. However, the mechanics dictating efficient transport of wastes from the body remains poorly understood. In this thesis, we investigate the transport of urine and feces in over 40 species of mammals. We employ a combination of urine and feces collections at Zoo Atlanta and theoretical models based on those collections. We pay particular attention to the governing principles for reducing energy, time, and applied force. We discover that 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 compromising its function. As a result, large mammals such as elephants urinate in the same duration of 21 seconds as their smaller counterparts, such as cats. Our model clarifies the contributions of bladder pressure and gravity as a function of body size. The ability to excrete materials quickly regardless of body size is also exhibited in defecation, which lasts an average of 12 seconds. We measure the defecation speeds of mammals and the mucus evaporation on feces. Larger animals have thicker mucus layers that enable them to release more feces in the same time as smaller animals. Lastly, we present experiments and theory regarding the rhythmic motions within the small intestine of rats. We show that the frequency of motion is higher farther from the stomach. We rationalize this motion based on the higher occurrence of bubbles that lower the density of gut content and thus increase the resonant frequency of the intestine. This thesis sheds light on optimal transport strategies in soft tissue and provides a design principle of scalable hydrodynamic systems.
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