In the human body, why is there redundancy with some organs, but not others?

Well, ignoring the complexity it would take to have two hearts working in unison and not mess up the flow of blood….. quite frankly, we don’t know why not.

It’s just the way we evolved.

I don’t think it’s a redundancy issue for every duplicate organ. Two lungs is more of an issue with increasing the amount of oxygen absorbed per respiration (thus feeding our brains more), but I do agree that two ovaries/testicles is good redundancy for our primary animalistic purpose.

As to why we have two kidneys, I would have to say it’s because that’s the way we evolved. Natural selection favored certain creatures with two kidneys over one. It’d be interesting to see when exactly this occurred in the development of different creatures.

Interesting question. Here’s what I found: http://health.howstuffworks.com/two-lungs-one-heart.htm

The heart and the brain are protected. The liver is regenerative.

Our hearts do have two symmetrical sides with one independent pump in each, though – why call it one heart, but two lungs? You could also say we have only one brain, but we do have two brain hemispheres.

There’s lots of organs that aren’t bilaterally symmetrical and don’t have back-ups.

ummm, about the two genitalia . . . I’m pretty sure that’s not normal . . .

Very interesting link. That our heart is actually two fused, makes sense.

When we talk about redundancy, I also think we might have to take into account that we evolved in an environment, where we most likely got poisoned in one way or another, much more often than today. Pertaining liver and kidneys that is.

I am not sure redundant is the right word. I would maybe say that in today’s world, our body has an over-capacity in some senses.

ummm, I’m pretty sure we only have one liver as well

@Kayak8 Yes, but we have excess functionality when it comes to the liver and kidneys. Living like we do today anyway, in the western world.
But I wouldn’t be surprised if the liver was two livers fused, like the heart.

WAIT. WHO HAS TWO DICKS AND WHERE CAN I MEET THEM?

@oratio

Excessive functionality . . . hmmmmm I wish my single (two-hemisphere) brain had that . . .

@Kayak8: In a way you have that already.
There are unfortunate people who need to have either hemisphere surgically removed in order to survive, and continue to live with just one hemisphere. The remaining hemisphere can take over most tasks formerly carried out by the removed one.
Of course, taking one hemisphere away can impair certain cognitive functions, but some patients are even reported not to be significantly affected. The Wikipedia article says that

“One case, demonstrated by Smith & Sugar, 1975; A. Smith 1987, demonstrated that one patient with this procedure had completed college, attended graduate school and scored above average on intelligence tests.”

and that

“Studies have found no significant long-term effects on memory, personality, or humor after the procedure, and minimal changes in cognitive function overall.”

Long story short, there’s functional redundancy in the brain too.

As a side note, if we define “excessive” as “more than necessary in today’s world”, then brains cannot be excessively functional, even if they end you up with an IQ of over nine thousand. We can always put exceptionally high intelligence to good use.

@Fyrius Yes. You are right there. Interesting that, about the example of lobotomy.

What!? Nine thousand? There is no way that could be right!