An organism’s metabolic rate can be measured by monitoring its oxygen consumption, carbon dioxide production, or heat production.
→ The higher the oxygen consumption the higher the metabolic rate
→ The higher the carbon dioxide production the higher the metabolic rate
→ The higher the heat production the higher the higher the metabolic rate
Metabolic rates must always be controlled for the body mass of an organism (and time), e.g. if the total oxygen consumed by an organism in one hour is $x$ $mm^3$ $h^{-1}$, then the metabolic rate will be $\dfrac{x}{body \space mass}$ $mm^3$ $h^{-1}$ $g^{-1}$. This is to ensure a fair comparison.
Metabolic rate can be measured using respirometers, oxygen probes, carbon dioxide probes and calorimeters.
respirometers measure oxygen consumption per unit time
oxygen probes can be used to monitor oxygen levels in water → changes detected give an indication of metabolic rates of fish
carbon dioxide probes monitor carbon dioxide levels → changes under different conditions can be used to compare metabolic rates
calorimeters measure the increase in temperature over a known period in time → well insulated containers
Organisms with high metabolic rates require more efficient delivery of oxygen to cells.
Birds and mammals have higher metabolic rates than reptiles and amphibians, which in turn have higher metabolic rates than fish.
There is different anatomy and physiology of heart chambers and circulatory systems in amphibians, reptiles, mammals and birds, and fish.
Birds and mammals have a complete double circulatory system consisting of two atria and two ventricles.
— In a complete double circulatory system, there are four heart chambers — two atria and two ventricles.
— In a complete double circulatory system, blood passes through the heart twice during each complete circuit of the body.
— Complete double circulatory systems enable higher metabolic rates to be maintained.
There is no mixing of oxygenated and deoxygenated blood and the oxygenated blood can be pumped out at a higher pressure. This enables more efficient oxygen delivery to cells.
Amphibians and most reptiles have an incomplete double circulatory system consisting of two atria and one ventricle.
— Incomplete double circulatory systems have three heart chambers — two atria and one ventricle.
— In an incomplete double circulatory system blood passes through the heart twice during each complete circuit of the body.
— In an incomplete double circulatory system, oxygenated and deoxygenated blood mixes in the ventricle of the heart.
— In an incomplete double circulatory system, cells receive incompletely oxygenated blood at high pressure.