# The Kidney’s Product Is Homeostasis

Andrew Bland, MD, FACP, FAAP · October 19, 2026

The skeleton-key illustration introduces the kidney as a regulator of the internal environment. The corrected urine-cup label uses the factory-smoke analogy, adapted from Joel Topf’s discussion.

![Diagram](https://urinenephrology.org/2025_UDPA_Lectures_Live/assets/urine-nephrology-skeleton-key-homeostasis-2026-10-19.png)

## Use the analogy to frame the lecture

Urine is an output of kidney work. The physiologic purpose is maintenance of a stable internal environment: water, electrolytes, acid–base balance, volume, and other functions.

## Then connect it to a measurement

Creatinine and eGFR describe the filtration part of kidney function. They do not summarize every homeostatic, endocrine, or tubular function, and urine production alone does not establish normal kidney function.

## Attribution

The label is an adaptation, not a verified verbatim quotation. Joel Topf used the factory-smoke analogy in his 2011 discussion of Homer Smith’s framing of renal physiology.

## Supporting evidence

- [Joel Topf: I love the smell of July 1st in the morning](https://pbfluids.com/2011/07/i-love-the-smell-of-july-1st-in-the-morning/)
- [NIDDK: Your Kidneys & How They Work](https://www.niddk.nih.gov/health-information/kidney-disease/kidneys-how-they-work)

## Continue learning

- [Opening lecture: why we have kidneys](https://urinenephrology.org/2025_UDPA_Lectures_Live/case-based-lectures/opening-lecture.html#philosophy)
- [What do the kidneys regulate?](https://urinenephrology.org/visual-reference/kidney-regulation.html)
- [Opening lecture: creatinine and GFR](https://urinenephrology.org/2025_UDPA_Lectures_Live/case-based-lectures/opening-lecture.html#creatinine-gfr)
- [Case lecture series](https://urinenephrology.org/2025_UDPA_Lectures_Live/case-based-lectures/index.html)