Blood...

[Cyberman]

Apart from the whole blood system being circulatory, meaning it all goes around and around, the blood vessels are fitted with one-way valves which prevents the blood from being pulled the wrong way when the heart readies itself for the next beat.

Honestly, this is all very reminiscent of the classic exchange from The Blood Donor:

Tony Hancock: It's a funny thing, this blood business.

Hugh Lloyd: Yes, I suppose it is.

Hancock: It all looks the same and yet... it's all different. Yes, it's a very funny stuff, blood.

Hugh: Yes. I don't know where we'd be without it.

Hancock: That's true. That's very true. Where would we be without it? Yes, it's very important, blood. Circulates right round the body you know.

Hugh: Yes, so I believe.

Hancock: Yes. It starts at the heart, it gets pumped right round, goes through the lungs, back into the heart, and round it goes again.

Hugh: What for?

Hancock: What for? Well, it speaks for itself, doesn't it? I mean, the heart's got to have something to pump round... there's no point in it banging away all day long for no reason at all.

Hugh: Well, why have a heart then?

Hancock: Well if you didn't the blood wouldn't go round, would it? It'd all stay in one place. When you stood up, it'd all sink to the bottom of your legs. It'd be very uncomfortable, wouldn't it? It'd feel like you were walking around with a bootful of water. Your heart saves you keep having to stand on your head, and jumping about to keep it moving. It does it for you.

Hugh: But I still don't see what good blood is, though.

Hancock: Well... your body's full of veins, isn't it?

Hugh: Yes.

Hancock: Well you've got to fill them up with something, haven't you?

Hugh: Ah yes, I see. Are you a doctor, then?

Hancock: Well, no not really. I never really bothered.