Human RBC's
Frog RBC's (missing graphic)
Compare these two views of red blood cells (RBC's) one taken from a mammal and the other from a frog. RBC's in all vertebrates serve the same purpose: to carry oxygen from a source to a sink. To aid in this process RBC's contain many molecules of hemoglobin. Each molecule of hemoglobin can temporarily hold as many as four oxygen molecules. Since each red blood cell carries a load of millions of hemoglobin molecules (about 250 million in human RBC's) it is important that oxygen is able to enter quickly during the short period that a given RBC is passing through a lung, gill or other organ exposed to a high concentration of oxygen. In short, the surface-to-volume ratio must be high.
Although human blood and frog blood are both red, since they both contain hemoglobin--which is red--you may notice that there are distinct differences in the appearance of frog blood and human blood on the cellular level.
Q: Which of the differences visible here do you think might be most important?
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It has been asserted that form follows function. Let us look more closely at the work of a red blood cell. It should be clear that an important feature of a red blood cell is its surface-to-volume ratio. It is through the surface of the cell's membrane that oxygen must pass. The amount of that surface is proportional to (related to) the size of the cell. The cell's volume determines how much hemoglobin it contains and how much oxygen it will load, hold and unload. (Volume is related to how much oxygen it needs (to be fully loaded), while surface area is related to its ability to meet that need.) If a cell gets bigger and keeps the same shape, its demand grows faster than its ability to supply that demand.