I'm pretty sure jcot is aware of the problemmatic theoretical foundations for historic bloodletting, and the attending reputation that persists. However the interesting part is that it could have therapeudic uses, but physicians are understandably adverse to performing a procedure considered typical of medieval quackery.

Here's an excerpt from the BMJ:

>Bloodletting was done by the barbers in England at the request of
patients and must, therefore, have had some beneficial effect even if it
was just a placebo effect. It is very possible, however, that there were
very real benefits in selected patients especially those in left
ventricular or congestive cardiac failure. In withdrawing blood the
venular outflow pressure from capillaries and cardiac filling pressures
would have been immediately reduced. The pressure gradient between
capillary and interstitial fluid may have been decreased or even reversed
if it was elevated(6,7). A reversal of the pressure gradient in oedematous
patients would have promoted the mobilisation of oedema fluid and
excretion in urine provided renal function was normal. Bloodletting would
also have improved capillary blood flow, which can be severely disturbed
in septic patients, by decreasing viscosity and improving the rheological
chacteristics.

>Repeated epsiodes of bloodletting will cause anaemia which is an
independent risk factor for adverse outcome in patients with heart failure
(8). As heart failure may cause anaemia by compromising the synthesis in
the Krebs cycle of substrate needed for haemoglobin synthesis and blood
formation in the bone marrow this risk might be a consequence of the
heart failure rather than the cause. In adequately nourished and
oxygenated bone marrow haemoglobin pools should be replenished by
resynthesis unless the bloodletting was repeated too often.

>In the short term bloodletting achieves what the venodilators
glyceryl trinitrate and isosorbide mononitrate achieve, a reduction in
venular outflow pressure in capillary beds. In patients with heart failure
venodilators improve exercise performance and reduce mortality (9). Whilst
ACE inhibitors, angiotensin II receptor agonists and beta blockers may
also decrease mortality in patients in heart failure, partly or wholly
by similar mechanisms, inotropes and beta agonists may increase mortality
from heart failure. These medications have not been established to
improve disability-adjusted or un-adjusted longevity.

>Bloodletting might have been much more effective than venodilators
and as effective as diuretics for in increasing th tone of the
precapillary sphincters and establishing a pressure gradient between
interstitium and capillary the excretion of oedema fluid would have been
promoted. If in inducing a gastric intramucosal acidosis bloodletting were
also to induce preconditioning by, for example, activating anf K(atp)
channel, bloodletting might even have had extended benefits not seen with
medical therapy. [1]

I wouldn't agree that patients' requests meant that it must have been beneficial, since people willingly do all sorts of crazy things to their bodies based on the most tenuous new age theories. It also doesn't make sense that regular bloodletting would be beneficial to an already healthy subject, since we'd presumably have evolved with a mechanism to dispose of excess "bad humors". However, as an alternate mechanism for achieving results that treat a condition like congestive heart failure, it seems a lot like drilling a hole in your head to treat a head injury — ridiculous sounding, but effective in certain cases.

[1] http://www.bmj.com/rapid-response/2011/10/29/treating-heart-...