The matter of the bladder was discussed in the Laws-Karam debate.  Attached is the part of the debate where this occurred (just click on the file bladder2.mp3 and it will load in a player).  Karam denies that 400 ml is a full bladder and he asserts that you can carry around that much without even noticing.  Apparently there was a urologist at the retrial who attested that a person can hold up to 3-4 litres of urine. Now I am not a urologist but this is a very simple matter of fact and I know how to read reference books and they are probably the same reference books read by urologists.  Nowhere in any reference book can I find mention of a full bladder being any more than 1 litre and most cite 600-800 as being a suitable maximum.  Depending on the source, the amount necessary to elicit the desire to pee is 200-400 ml.  If Robin had 400 ml then he would most certainly have been aware of it and ready to relieve himself at any time.

I would like to know what this urologist said at the retrial, if there was one, and whether it bears any resemblance to what Karam said in the debate. 

I found this report in the ODT

Another Wellington specialist, consultant urologist Grant Russell was asked to consider the significance of Robin Bain having 400ml of urine in his bladder when he was shot.

The Crown says that meant it was unlikely Robin could be the murderer, given the lengthy struggle with Stephen and the time he would have spent cleaning up, that he would have needed to go to the toilet.

But Mr Russell said an ageing male was likely to have a slightly enlarged prostate gland which would cause some obstruction of the bladder opening, inhibiting the squeezing out of urine and increasing its retention in the bladder.

He could not draw any conclusions as to whether someone with 400ml of urine in their bladder could get up and murder someone without having to empty their bladder.

To Mr Bates, the witness agreed 400ml could be the normal overnight collection of urine in a man's bladder.

And while a man would normally want to empty his bladder when he got up, that desire could decrease with age.

Mr Russell agreed it would not necessarily be normal to wait for an hour or two before going to the toilet, but it could be normal for a man of Robin Bain's age to get the paper, perhaps say a quick prayer, then go downstairs to cook breakfast and, if the toilet was in the downstairs area, go to the toilet then.

That could be normal "as could a lot of other scenarios", Mr Russell said.

There's no mention there of 3-4 litres and I am sure that if that was said on the stand then this would have been reported because from what I can see it is incorrect and it would have been seriously questioned.  What this witness is saying is that under normal circumstances a person Robin's age would not have felt much compunction to pee with 400ml in his bladder.  Note that this scenario of a 'normal' morning does not include the intense activity of killing 5 people taking place between getting up and arriving at the toilet.

I can only conclude that Karam's mention of 3-4 litres is an irresponsible and gross misrepresentation of the facts or of what Karam must have been told, because no competent urologist would have told him that a person can hold 3-4 litres at any one time.  Such a condition  is likely either fatal or chronic and in need of immediate medical attention.  It is possible that Karam heard someone say that the average human produces up to 3 litres of urine every 24 hours and that information may have transmutated in his mind. Given the amount of time that Karam spends on the subject of Bain he needs to get his facts right.  The audience that would have been listening to the debate would not have been informed enough to understand that those facts are quite incorrect and would have been taken in. While Laws refuted what he said, sensibly, understanding that 400 ml was a considerable amount of urine, he isn't a health professional and didn't have the reference material on hand to question the validity of Karam's claim regarding the 3-4 litres. Should Karam ever be brought to task on this by someone who does know what they are talking about, no doubt he would profess that it was an honest mistake.

Next time you go to the kitchen check out your measuring jug.  A litre of water is a lot of liquid.  One litre of water weighs about the equivalent of 1kg so 3-4 litres weights 3-4 kg which is the same as the average new born baby.  Having 3-4 litres of urine in the bladder would make you look like you were pregnant!