On their trip to Utah, Rose received some quite interesting information about Time Lord biology: The Doctor had two hearts! Not one, two. Most people would be surprised by this, heck, Rose knew, if the circumstances were different, she'd probably spend a few hours screaming "what?!" inside her head a few times. This was not the case however, as this was not the first she had seen of his Time Lord biology. In fact, she would see one particular part a few times, while they they were travelling.
The first time was in Cardiff 1869. Rose had just been saved from some Gelth-possessed bodies, and was sitting in Sneed's living room with Gwyneth, and Charles Dickens. As you do, when with the Doctor. She noticed the Doctor seemed a bit off. He wasn't joining in the conversation, but he didn't have his thinking face on, either. He also seemed a bit restless, or squirmy. He clenched his thighs, rubbing them against each other a couple of times. Then he crossed his legs. That's when she realized what was wrong. She tried asking him: "You alright?". "Yeah I'm fine," he replied, not looking straight at her. Yep. She was right. He needed the loo. Made sense, since they'd had tea. She didn't bring it up again, though she didn't need to. Gwyneth interrupted the Doctor's attempt at ignoring his bladder:
"It's just down the end of the hall," she said.
"What is?" asked the Doctor.
"Where you need to go, but won't ask where is, out loud," she clarified.
Having a stranger know he needed to pee would normally be quite embarrassing, as it would mean he was far too obvious. Luckily he could dismiss this as being purely due to her psychic abilities.
He thanked her and swanned off to the loo.
The second time was during the Slitheen invasion. Getting them locked in the Cabinet room was not one of his better ideas, but then, it got even worse. He felt it. The oh, so familiar pressure in his bladder. He needed the loo, and not just a little bit. It was very clear that he needed to get out very SOON! He started pacing around the room, trying to keep his mind of the issue. Rose once again noticed he wasn't thinking of a solution.
"Are you alright?" Rose asked.
"No. I'm not. It's been 3 days, 11 hours,49 minutes and 33 seconds, the Slitheen are doing god knows what, and I'm stuck in here with a member of parliament, Fantastic! Just Fantastic!" moaned the Doctor nervously.
"What do you mean 3 days? 3 days since what?" asked Rose.
The Doctor was suddenly very aware of his audience.
"Since Cardiff." answered the Doctor.
"So?" Rose asked, not getting it.
"What happened in Cardiff? At Sneed's place?" he explained.
Rose remembered. She did some quick math based on the numbers the Doctor mentioned.
"So, it takes what, 3 days for..?" Rose asked, the Doctor nodded.
"I've got 1 hours 10 minutes 15 seconds left of my limit, then.." said the Doctor, not wanting to voice the last part in the slightest. Certainly not in front of Harriet.
At this point Harriet got really curious.
"What are you talking about. An hour till what?!" she demanded.
The Doctor looked to Rose for help. She whispered the truth to Harriet. The Doctor blushed.
"Oh. That's not good. And you're absolutely sure of this?" she asked sympathetically.
"I'm a Time Lord. Punctuality's in my DNA. I've kept track of my limit for 900 years, it's been this long for the last 100 of 'em. It can't have changed," he confirmed.
The two women looked frantically around the room for a toilet, whilst the Last of the Time Lords did a mix of a pogo stick impression and the chicken dance with crossed legs. They found a bathroom in the corner, at which point the Doctor ran in as fast as he could and locked the door. He did not soundproof however so Rose got an explanation for the lengthy bladder retention. He had a really big bladder, taking 2 minutes, with both women giggling silently to the noise, to empty.
"If I were his mother I'd hide the wine as a preventive measure" Harriet joked, before both women slid their glasses to the edge of the table, smiling.
The third time was at Satellite Five. The Doctor had drunk a Jumbo Sized smoothie thing, with some kind of alien berry, the Doctor apparently loved. Just a few minutes later, as they were looking at the Earth from above, his bladder decided to bombard him with the reminder of where his drink went, and where it would like to go now. Specifically, out of him. He tried to ignore it. It was when he scratched the back of his thigh, that Rose noticed.
"Got an itch?" she asked, amused.
"Not exactly" he admitted.
"What does that mean?"
"Really shouldn't have had that extra large smoothie." said the Doctor, now openly wriggling.
"You know, there is solution to that," Rose said sarcastically, with a chuckle.
"How am I supposed to find a toilet in this massive place? Computer's no help, I tried that thanks," he explained.
"You could try asking," Rose suggested, not letting the Doctor be that stupid about something so natural.
"No way."
"Why not?"
"The fact I need the loo's none of their business. That's private."
"Come on, Doctor. It's not a big deal, everyone needs the loo, nothing's wrong with asking, it's completely normal," Rose insisted. "And.. If you're not asking, I'm doing it for you," she said, walking determinedly from the room. The Doctor caved, and left instead.
He managed to ask someone, and went to the loo, feeling much better after, if a bit embarrassed.