I'm really sorry if this seems obvious to most of you, but it's all a mystery to me...

How does lactation work?  By that I mean:  Sometimes the puppies get really angry-sounding and pummel away at the nipples, then I can tell when the milk comes in because they go quiet and make big gulping noises.  So I get that much.  So then there's a break and mum leaves the box, and during the time more milk is produced, and then it's there when she next gets back in?  So it's not like *immediate* supply meeting demand...?

Questions:

If some puppies nurse first, and drink all the milk available, and other puppies come along next who were sleeping, is it possible that there is no milk left in a nipple for those?  Just because sometimes they don't want a particular nipple - they sniff it and might do one suck, but then they dismiss it and they want to move on to another one.  Whereas another puppy might have been nursing vigorously at that nipple just before... If I try again to put a puppy on that particular one, it's clear they don't want it now.   Is that how it works? 

My girl's lowest two boobs on each side get really big and swollen looking and I guess I worry about mastitis - they do milk right down each time though...

It's actually a really interesting subject and I think it is a situation of supply and demand, it would seem to me that the milk is there in the gland and the puppy suckling stimulates the bitch to release or let down the milk, strong healthy puppies sucking well will increase the production of milk. My observation would be that once the puppies have had most of the milk they try other nipples and a recently sucked one is not going to have much left so they fuss about and leave it, if puppies are sleeping when the bitch comes to feed them then generally they will wake up pretty quickly but probably if they don't then they are not very hungry.  If one puppy is always late to feed then keep any eye on it and maybe wake it up quick when mum gets in the box, the lowest boobs are usually the ones with the most milk so tend to look bigger but if you are worried then latch on one of the strongest pups when the milk is let down and check for "hot"  or red boobs which might indicate the beginning of mastitis. I hope others come along and have more to say about this miracle.

I always wonder what it is exactly which tells Mum when to return to the box and feed the babies.  Because as time goes on, Mum stays away longer and longer, yet she will suddenly arise and announce in no uncertain terms that IT IS TIME!  Is it that the milk has replenished, and that's how she knows when it is time to feed again?  At the beginning, nursing is basically constant, and the babies can only take a little at one go.  Then as they get older and stronger, they nurse less often and less long at a time but take in a lot each time.

It's no different to when I breastfed my children. You just 'know' that it's time for a feed. It's a very natural, instinctive bond between mum and baby or mum and puppies.
When breeding kittens everything is much more hands off, letting nature get on with it. I often wonder if, even though I love my cats, it's because I don't have the same bond with them.

That's what I've always assumed.  That it's like breast-feeding your own children.  You are often just glad to get the milk out of there!  It's a relief!

Yep.supply and demand and hormones

So when weaning starts, I've read I should feed pups some food first and then let them top up from mum.  Presumably they will naturally want more food and less milk as time goes on?

I get how to introduce food to the pups for the first time, but not sure about how to help wean them off mum.

YOU don't wean them off the bitch, she does.

Oh ok then, that makes it easier!

Fully agree with Goldmali, and all my bitches physically are fit (tight undercarriage) and dry by the time pups are 10 - 12 weeks old. 

If it wasn't for the post partum moult they would be fit to be shown, so instead we have to wait 6 months for a half decent coat.

>Oh ok then, that makes it easier!

But you should know that the pups take forever to recognise food as something which they can, well, EAT!  They want Mum's milk, thank you very much.  So for what seems like ages they kind of make a pass at the food while waiting for you to allow Mum in with them.  Naturally if you leave the food down, Mum will eat it all up herself.  But the pups start to imitate her and eat right along with her, and I find this one of the best ways to really get the idea across to them.  When Mum runs out of milk big time, then hunger sets in, and the puppies start to realise that the next best thing to Mum is:  FOOD!  At least this is what happens with MY puppies! 

My other girl is still sagging around the last two teats 6 months after whelping. She is raw fed, lean otherwise and goes running etc all the time. She is actually back at work but I just do not know how to help tighten her undercarriage. Any suggestions?

The nipples stay larger but I have never had any issues with undercarriage with any of mine, I suppose time , and a naturally elastic skin are what make a difference.

I have always found puppies to be very keen to start eating but I don't generally start very young, I offer the chosen food at room temperature at approx 3 weeks and if they don't jump straight in then I offer it the following day (fresh of course) and every day until they want it. Mum will soon get to the stage where she wants to go in the pen for 60 seconds and then leap out having eaten the puppy food if she can get it and mine also have the habit of going up to the pen checking the puppies and then jumping out leaving them awake and hungry so I try them with food then.

Swimming is good for tightening up the undercarriage, depending if she likes it naturally