Crocodile Respiration

On the 27.11.14 I had the great pleasure of meeting and having a seminar from Suzy Munns. She is a comparative respiratory physiologist and started off by studying a bachelor of science majoring in Zoology. Suzy struck me as a very intelligent and charismatic person and her fields of study proved to also be very interesting. 

In her research on the respiratory system in crocodiles she discovered something and was then faced with the reality of having to prove years and years of accepted knowledge wrong.

It had been believed that crocodile respiration, explained by the term hepatic piston pump was a completely new and unique mechanism for respiration only found in crocodiles.  It was thought that a muscle call the Diaphragmaticus muscle was used to pull the internal organs such as the liver backwards (caudally), which in turn pulled the bottom of the lungs backwards and expands them.

But on dissection of a crocodile Suzy noticed that this Diaphragmaticus muscle which was thought to be so crucial in the respiration of crocodile was thin and almost transparent. 

On seeing this it seems impossible that such a small muscle could be capable of pulling organs around and being the main mechanism used in respiration. 

The main reason why this Diaphragmaticus muscle was assumed to be the main mechanism for inspiration in crocodiles was because previous studies had shown that it fired during inspiration. But the issue was that technology has advanced a lot since this these studies were undertaken, and it was likely that this information was inaccurate due to the technology and information present back then. To test the impacts of the hepatic piston pump on respiration a number of test were run. 

One of the hardest things that Suzy had to undertake in this research was training crocodiles in the lab to run on a treadmill in order to be able to monitor the different parameters such as, blood gases and EMG. Breathing masked were also placed on the crocodiles, these measured pressure changes and collected expired air. Basically anything that could be measured during activity they measured. 

The tests were run under two different conditions. The crocodiles where monitored with their Diaphragmaticus muscle still intact and then they were tested again after the muscle had been severed. In order to severe the muscle the crocodiles underwent an operation under anesthesia. And of course I thought I must add that the these individuals had a couple of days to recover after their operation before the tests were run. 

The results from these tests showed that there were no significant differences in ventilation or blood gases after inactivation of the Diaphragmaticus muscle. This was tested under the conditions:

•  At rest

•   Under reduced respiratory drive (low temp)

• Under increased respiratory drive (exercise/ hypercapnia (increased carbon dioxide))

From these studies it could be concluded that the hepatic piston pump may act as a supplement in ventilation but it is not the primary ventilation mechanism. Costal ventilation was found to be the primary ventilation mechanism in crocodiles.