MadSci Network: Zoology

Re: Do all animals with kidneys have hearts and vice versa?

Date: Wed May 19 10:54:13 2004
Posted By: Mike Klymkowsky, Professor
Area of science: Zoology
ID: 1084847385.Zo

Kidney and Heart


The answer to your question has much to do with what you mean by heart and kidney and how you define an animal. 

The smallest metazoans (animals), such as the hydra can survive without either a heart or a kidney, because their functions are not needed.

These organisms are two cell layers thick.  O2, nutrients and waste products (such as CO2) diffuse into and out of all of its cells.

As metazoans grow larger, simple diffusion is no longer adequate.  To bring O2 and nutrients to all of the cells of the body, they must develop a circulatory system.  

To drive cells and fluid through this circulatory system, they need a pump -- a heart.  

The circulatory system also accumulates wastes from the cells of the body.  These wastes have to be removed. 

In vertebrates, the organs that remove these wastes are the lungs, which exchange waste CO2 for O2, and the kidneys, which removes non-gaseous wastes.  

Other organisms have analagous organs, such as the nephridium of flatworms and the malphigian tubules of insects.  Like the kidnesy, these are used to collect and remove wastes from the body. 

So once you reach a certain size you need organs that perform circulatory, pumping and waste removal functions.  So Quine appears to be correct.

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