Your kidneys convert about 5% of your cardiac output in to primitive urine. That's a lot urine per minute. They do it day in, day out until there is no cardiac output left, hopefully >100 years down the road. Then they stop. This is hard work. Primitive urine cannot be peed down the loo. It is absolutely loaded with stuff you need to stay alive. Water for a start. And sodium. You go on a zero salt diet and your kidneys will go in to overdrive to keep you alive despite your attentiveness to the AHA. You pour glucose in to this filtrate. You waste magnesium, potassium, bicarbonate, loads of stuff you can't do without. Then you claw it all back again. Back comes the glucose, back comes the sodium, back comes the water. Your kidneys pump 180 litres of water alone back in to your bloodstream every 24 hours. All of this is done against a concentration gradient. This takes energy. Lots of it.
While you are snoozing, your muscles are snoozing too. Same for a cow. Your kidneys are continuously working their cotton socks off, 24/7. So they must have the metabolic tools to generate serious amounts of ATP. Eating muscle tissue is a massive improvement on eating a Marsbar. Eating kidneys is the next level up. Liver (with the tubes) is the most phenomenally active organ for biochemical transformation you can imagine. You want a precursor molecule for something? Look in the liver. Or an egg yolk.
Aside: It's that age old question. Why is there so much cholesterol in an egg yolk? Because that's how much it takes to make a healthy chick. Anon (unless you know better).
Anyway, you want folate? Try some liver. B12? There too. Quite a lot of cholesterol too.
What about lung tissue? Lungs are special. Oxygen is amongst the most damaging and corrosive molecules that we all depend on to stay alive. The environment in the lung is harsh for alveolar epithelial cells. So alveolar cells know all about antioxidant defences. Staying alive in the lungs needs energy. You want useful metabolic molecules? You won't get them by eating tofu! I love haggis but can't eat the barley or oats it's made with. Now, gluten free haggis maybe. Add mashed neaps with tons of butter and just enough mashed potatoes, also with tons of butter, to stay within my carb limit. And a wee dram if I'm willing to damage my joints for the pleasure. Anyway...
I won't mention brain as it's essentially unavailable in the UK. But guess where you find a number of interesting lipids in addition to cholesterol?
I could go on. I'll shut up now. I like offal.