Why are pig's hearts used for dissection?
First of all, a pig heart is fairly easy to acquire. Pig hearts are also fairly similar to human hearts. The are approximately the same size, they both have four chambers, and they both have the same approximate pumping capacity.
Since students (unless they are medical students) do not generally have access to a human heart, the next best thing is used, the pig heart. Examining a pigs heart can give students the opportunity to get a good look at how the heart actually works. Some students do refuse to participate in these experiments because they feel it is unethical. Usually an alternate assignment is given.
During the study of any organ in it very useful to be able to see the internal anatomy and functioning of the actual organ for improved understanding. Dissections are a very good way to do this.
The heart of the pig is used for dissection as it is very similar to the human heart in size, structure and function. This does not mean that a pig's heart can be used for transplant in a human but this does make it an ideal candidate for the study of the human heart without actually having to use the heart from a human for the dissection.