The other "Golden Rule" - David L. Hu

TL;DR

Mammals use the "Other Golden Rule" to pee efficiently, with bladder pressure and gravity playing key roles.

Transcript

A cat’s bladder can only store a golf ball’s worth of urine. For humans, it’s a coffee mug and for elephants, a kitchen trash can. An elephant’s bladder is 400 times the size of a cat’s, but it doesn’t take an elephant 400 times longer to pee. So, how does this work? The answer lies in what scientists call the “Other Golden Rule.” It applies to ... Read More

Key Insights

• 🥹 Bladder size determines the amount of urine a mammal can hold, with elephants having bladders 400 times larger than cats.
• 💦 The "Other Golden Rule" explains how bladder pressure and gravity work together to achieve efficient urination across different mammal sizes.
• 🖐️ The urethra's length and diameter also play a role in urination speed, with variations among species and genders.
• 😳 The "Other Golden Rule" may have evolved to reduce vulnerability to predators and flush out pathogens.
• 💋 Different behaviors, such as marking territory or mating displays, can disrupt the efficient urination process.
• 🏋️ The "Other Golden Rule" does not apply to aquatic mammals or mammals below three kilograms in weight.
• 🥺 Surface tension affects the urination process in smaller mammals below three kilograms, leading to droplet-like urination.
• 🕵️‍♂️ Male dogs and pandas have unique urination behaviors that can impact the efficiency of urination.

Questions & Answers

Q: What is the "Other Golden Rule" in relation to mammalian urination?

The "Other Golden Rule" describes how urinary organs and forces change with body size, allowing mammals to achieve rapid relief. It explains the role of bladder pressure and gravity in efficient urination.

Q: How does gravity affect urination speed?

Gravity plays a role in urination speed through the urethra. The urethra acts as a pipe-like organ that transports urine from a higher elevation (bladder) to a lower one (exit point), using the principles of Torricelli's Law to increase flow rate.

Q: How does the length and diameter of the urethra affect urination speed?

The length and diameter of the urethra impact urination speed. A longer and wider urethra, as seen in elephants, allows for a comparable urination time to humans. Variations in urethral length and diameter exist among different species and genders.

Q: Are there any exceptions or limitations to the "Other Golden Rule"?

The "Other Golden Rule" applies only to non-aquatic mammals and those above three kilograms. Smaller mammals and different behaviors, such as marking territory or mating displays, can disrupt the efficient urination process.

Summary & Key Takeaways

• The size of a mammal's bladder determines how much urine it can hold, with a cat's bladder being the size of a golf ball and an elephant's bladder being 400 times larger.

• The speed of urination is determined by bladder pressure and gravity, with bladder pressure staying relatively constant across different body sizes and gravity playing a role in the flow rate through the urethra.

• The length and diameter of the urethra also affect urination speed, with different species having variations in length and diameter.