Why does temperature affect heart rate in exothermic (cold blooded) organisms?

Question

This relates to a Daphnia experiment. What would be the answer from a biochemical point of view?

Answer 1

Basically, the rate at which all the chemical reactions in the body proceed depends on temperature. This is because temperature is actually a measure of kinetic motion, or how fast the molecules are moving. If they move faster, they are more likely to meet, and thus be able to react. An ectotherm (or cold blooded animal) is an organism that cannot maintain its body temperature independent of that of its environment. So, when the temperature goes down, the molecules move slower, and reactions occur slower.
Specifically, the heart in Daphnia beats because of specialized cells that slowly depolarize to threshold, then fire an action potential. The cells are "leaky" to ions that diffuse in through special channels to depolarize the cell. Diffusion is also affected by temperature, so with a decrease in temperature, the cells would depolarize slower and the heart rate would go down. The University of Calgary has a web site on crustacean heats.

Answer 2

Daphnia Heart Rate Temperature

Answer 3

Unless the temperature is so hot or so cold that it is shocking or painful, temperature really doesn't affect your heart rate directly Oxygen content in the air is affected by temperature sometimes. For example, room temperature atmosphere usually holds 22% oxygen. But, if it were much colder, the oxygen content could be less by as much as four percent. When it is less, your lungs have to work harder to attain more oxygen, and therefore you heart must supply your lungs with sufficient blood through the pulmonary artery to do this. Make sense?