In the episode titled "No Appointment" of the 4th season of the show, Dr. Joan Watson (played by Lucy Liu) finds Sherlock bouncing on a mini trampoline and asks the obvious question, "What's with the trampoline?". The following dialog ensues:
Watson: What's with the trampoline?
Sherlock: It really is shocking what they don't teach you in medical school.
Watson: Okay, forget I asked.
Sherlock: The lymphatic system, unlike the cardiovascular system, has no pumping capability; it relies on muscular contraction.
Sherlock: Bouncing up and down is effective, because it stimulates the system's one-way valves to open and close simultaneously.
Watson: You're increasing lymph flow to boost your immunity to keep your sinuses clear-- got it.
Sherlock: Seemed like a wise choice.
Watson: Why? Are you getting a cold?
Sherlock: Not yet, but we live in close quarters and I detected a phlegmy rattle in your voice last night.
Watson: I feel fine.
Sherlock: Do you?