Thread by @nickmmark

Here’s a #tweetorial about extremes of #physiology that are very much "compatible with life".

Case #1. You are called about a critical drop in platelets.
Platelet Count (x10^3/uL)
Previous 475
Current 45

It’s nothing to worry about. This sample came from a ___.

Answer 🐿
To avoid VTE while hibernating, squirrels drop their Plt counts 10x! They also ⬇️vWF & Factors VIII/IX

Red tailed 🦌 have Plt counts ~750 as fawns that drop to ~250 as adults (over yrs)

Stepping on a Lonomia🐛is a medical emergency. It *CAUSES* thrombocytopenia/DIC!

Case #2: You get a call from bedside nurse about an abnormal blood pressure:
BP 330/240

You reassure him, that's a totally OK blood pressure because it was measured in a ___.

Answer: 🦒
🦒have the highest BP of any extant species, an adaptation to perfuse their 🧠 atop 2m necks.

🐙🩸is very viscous (no RBCs, just protein) so their BPs run high for invertebrates (70s/30s) *BUT* they go into CARDIAC ARREST w/ exercise

🐇can develop HTN w/ BPs170s/150s

Bonus answer: 🦕
If you think that’s a high BP, long-necked dinosaurs (Barosaurus & Brachiosaurus) 🦕 were once thought to have SBP >700mmHg with head raised. This would have required a 1000+ kg heart. More likely it didn’t raise its head but used it to forage while swimming.

🥡 Point: “normal” BP depends on position. Because humans walk upright our BP must be higher.
This is why orthostatic BP is so useful.

Also, in case you were wondering, taller people *do* have slightly higher BP (in the NHANES dataset at least).
ncbi.nlm.nih.gov/pmc/articles/P…

Case 3: Now you are called by the hematology lab about a critical value on a CBC:
WBC 5
Hb 0
Hct 0
RBC 0
Platelet 150

You tell the lab "It’s cool. Nothing to worry about because the sample came from ____."

Answer: 🐟
Icefish (channichthyidae literally means white blood fish) are the only vertebrates w/o hemoglobin

They survive by combining high SV (5x larger heart), big capillaries (low SVR), & increased O2 solubility in the arctic ocean. They also don’t do very much (lower VO2).

Other answers:
The 🐙has hemocyanin, an O2 binding metalloprotein w/ Cu instead of Fe

The Opah is the only warm blooded🐠; it actually uses more O2

The vampire🦑 is adapted to the O2 minimum zone (O2 sat 3%); w/ large gills & O2 avid hemocyanin it extracts >90% of dissolved O2

Turns out FiO2 is pretty important if you don’t have lungs

300 million years ago, insects were *much* bigger. Imagine a dragonfly the size of a seagull (2.5 foot wingspan).
Why? Atmospheric FiO2 was ⬆️ in the Paleozoic period: 35% compared to 21% today.
eurekalert.org/pub_releases/2…

🥡Rarely people survive very low hemoglobin too:
One person survived a Hb of 1.8 g/dL w/o transfusion & another survived Hb of 0.7 g/dL.

Like the animal kingdom examples, this is possible due to ⬆️ FiO2, ⬆️ SV, ⬇️ SVR, ⬆️ O2 extraction, & ⬇️ O2 consumption (& also a lot of luck)

Case 4: Your 📟 just keeps on going off! Now you are getting paged about an abnormal urinalysis result.

Urine Osm 9000
SG > 1.060

That’s expected because it came from a ____.

Answer: 🐹

Humans can concentrate our urine to ~1200 mOsm/L
Dolphins reach ~1700 but contrary to belief they can’t drink seawater
Camels reach 2800 mOsm/L but dessert rodents are the champs. Chinchilla go without💧for weeks & concentrate urine >7500 mOsm/L. That’s a SG > 1.060!

🥡: Don’t underestimate dehydration in people “found down"
We lose >1.2L/day insensibly (& more if sweating). Life-threatening dehydration occurs w/i DAYS at rest or HOURS with activity

Thanks to @robertpdickson for recommended this great📖; it even discusses survival on Arrakis

I hope you’ve enjoyed this extremes of #physiology #tweetorial part deux. If so, you may also enjoy my prior🧵on the topic.

https://twitter.com/nickmmark/status/1346589425938010112

I’ve tried to sneak in some useful clinical take-home (🥡) learning points too. If you have ideas for future topics please comment.