Short answer: yes, blood cells, specifically, can be bigger in bigger animals. The obvious reason behind this is that the larger the animal the more oxygen it would need to get to various parts of it’s body and thus would need bigger and more numerous blood cells. But I am no biologist and apparently there is division in the scientific community over the exact reason for the difference in sizes:
Although scientists generally agree on the theory’s fundamentals, they disagree on the reasons for the scaling. One camp thinks metabolic rate is driven by cell size; another thinks it corresponds to the size and geometry of physiological supply networks, such as the circulatory system.
The question that specifically lead me to this topic was “are blood cells bigger in whales?” to which I found a very interesting, though extremely short, 1953 paper from the Medical Department of St. Josephs Hospital, Porsgrunn, Norway, written by A Harboe and A Sghrumpf (don’t ask me how to pronounce that) detailing the answer to that specific question:
In the blue whale a mean diameter (M) of 7.7 µ was found with a standard deviation (σ) of 0.7 µ. The mean difference between the largest and the smallest diameter (f) was 0.4 µ.
In the humpback whale, the corresponding figures were as follows: M 8.2 µ, σ 0.7 µ and f 0.7 µ.
The corresponding figures for the human red cell according to Larsen arc: M 7.80 µ, a 0.46 µ. The upper normal limit for M is 8.35 µ and the lower limit 7.20 µ.
They show, however, that the red cell diameter of the whale is not very different from that found in other big mammals. Like the human red cell, those of the whale are not far from circular.
So, yes, the cells are bigger but not by much, which is amazing considering the size difference.
This has been corroborated in other studies, including a more recent study using geckos of similar morphology but differing sizes due to their respective species.
Sure enough, the team found that the larger geckos had bigger red blood cells and a lower metabolic rate relative to body size than small geckos did. Their work supports the idea that cell size helps determine metabolic rate—which, in turn, underlies much of life’s patterning.
An interesting aside to the above quote: “Their work supports the idea that cell size helps determine metabolic rate” is this paper on the evolutionary genomics of birds and dinosaurs which supports the idea that, for dinosaur-to-bird evolution at least, metabolic demands had no or limited impact on genome size which raises questions on metabolic impact over time, evolutionarily speaking, and how this changes the demands on cell types, sizes and structures. As can bee seen in the below illustration, the genomic size does change depending on species.
Another 2001 paper by TR Gregory states that, though there is a large amount of variation, generally speaking the larger the genome, the larger the red blood cell and that there is a correlation between some metabolisms in vertebrates and red blood cell size. Have a read if you are interested!
But I digress – back to blood cells and oxygen! Blood cells transport oxygen around the body in a system called “gaseous exchange” which helps an organism take in oxygen and expel carbon dioxide.
It would make sense then, that some organisms would need more oxygen than others, but the larger the organism the slower their metabolic rate and therefore the reduction in need for oxygen, even if it is more than what a smaller creature would require. This is why such large animals, such as elephants and whales need more oxygen but have a more meandering lifestyle. Though where elephants are largely herbivores, blue whales are, in fact, predators and enjoy the status of the worlds largest carnivore, eating up to 3,600 kg’s of krill a day! This and their largely underwater lifestyle raise questions on their circulatory system and it’s efficiency as they would need large amounts of oxygen to be passed through their systems to maintain their diving habits.
Moving on, an interesting question on insects arises in terms of their blood cells. I found a brilliant summary online courtesy of Heena Jain which answers this question nicely (though English might not be her first language, it’s still easily understandable):
Ant, like all other insects, do not have an arteries or vein system, but they do have an open circular system. Their blood is called haemolymph, it is almost colorless and it does contain only 10 % blood cells, most of its volume is plasma. This haemolymph is used for the transport of hormones, nutrients and metabolic products, but not for the exchange of oxygen and carbon dioxide.
To enable the circulation of this haemolymph, ants have a very simple heart which is located at the abdomen of ants. Their heart is like an arteria which is surrounded by some small muscles. When this heart contacts by these muscles (going from the back to the head), the haemolymph is pressed into the different body parts, a significant part is directed to the head of the ant. Insects may also increase their haemolymph circulation by pressing their abdominal parts.
For their oxygen supply, they have small openings called spiracles at each body segment which supply their body directly with fresh air. When the oxygen enters the body, it goes via tracheal trunks and the smaller tracheal tubes to the different body parts and organs. As this system works mostly by passive air exchange, the body size of insects is limited to the dimension we know, so huge monster insects know from certain movies could not exist as they simply could not breathe.
It should be noted that insects were huge in prehistoric times specifically due to the high oxygen content available at the time which has diminished over the millennia.
And to answer a question that has probably arisen from this: cells, such as skin cells and those in organs etc, are all more or less similar in size across various species, including insects. Kind of weird to think that the skins cells of a human are similar in size to both a blue whales and an ants’ exoskeleton, but there you have it! Here’s a handy cross-sectional diagram of an insect exoskeleton for your convenience:
And let’s end this off with some images of giant insects just for you. Enjoy ;)
And some modern insects!
Sleep well ;)