ndividual, the organism replaces it to some degree within an hour or so.
However, it takes weeks for the hemoglobin (the oxygen-bearing substance
in the red blood cells) to be brought up to normal.

If blood loss is great (more than 10 percent of the total blood volume)
there occurs a sudden, systemic fall in blood pressure. This is a
well-known protective mechanism to aid blood clotting. If the volume of
blood lost does not exceed 30 to 40 percent, systolic, disastolic, and
pulse pressures rise again after approximately 30 minutes as a result of
various compensatory mechanisms.[23]

[Illustration: FIGURE 2.--Venesection manikin, 16th century. Numbers
indicate locations where in certain diseases venesection should be
undertaken. (From Stoeffler, 1518, as illustrated in Heinrich Stern,
_Theory and Practice of Bloodletting_, New York, 1915. Photo courtesy of
NLM.)]

If larger volumes than this are removed, the organism is usually unable to
survive unless the loss is promptly replaced. Repeated smaller bleedings
may produce a state of chronic anemia when the total amount of blood and
hemoglobin removed is in excess of the natural recuperative powers.

_When to Bleed_

Selecting a time for bleeding usually depended on the nature of the
disease and the patient's ability to withstand the process. Galen's
scheme, in contrast to the Hippocratic doctrine, recommended no specific
days.[24] Hippocrates worked out an elaborate schedule, based on the onset
and type of disease, to which the physician was instructed to adhere
regardless of the patient's condition.

Natural events outside the body served as indicators for selecting the
time, site, and frequency of bloodletting during the Middle Ages when
astrological influences dominated diagnostic and therapeutic thought. This
is illustrated by the fact that the earliest printed document relating to
medicine was the "Calendar for Bloodletting" issued in Mainz in 1457. This
type of calendar, also used for purgation, was known as an
_Aderlasskalender_, and was printed in other German cities such as
Augsburg, Nuremberg, Strassburg, and Leipzig. During the fifteenth century
these calendars and _Pestblatter_, or plague warnings, were the most
popular medical literature. Sir William Osler and Karl Sudhoff studied
hundreds of these calendars.[25] They consisted of a single sheet with
some astronomical figures and a diagram of a man (_Aderlassmann_)
depicting the influence of the