which is to cause the proteids to dissolve in the plasma.

2. _The wastes_ are formed at the cells, whence they are passed by the
lymph into the blood plasma. They are carried by the blood until removed
by the organs of excretion. The two waste products found in greatest
abundance in the plasma are carbon dioxide and urea.

The substances dissolved in the plasma form about 10 per cent of the whole
amount. The remaining 90 per cent is water. Practically all the
constituents of the plasma, except the wastes, enter the blood from the
digestive organs.

*Purposes of Water in the Blood.*—Not only is water the most abundant
constituent of the blood; it is, in some respects, the most important. It
is the liquefying portion of the blood, holding in solution the
constituents of the plasma and floating the corpuscles. Deprived of its
water, the blood becomes a solid substance. Through the movements of the
blood the water also serves the purpose of a transporting agent in the
body. The cells in all parts of the body require water and this is
supplied to them from the blood. Water is present in the corpuscles as
well as in the plasma and forms about 80 per cent of the entire volume of
the blood.

*Coagulation of the Blood.*—If the blood is exposed to some unnatural
condition, such as occurs when it escapes from the blood vessels, it
undergoes a peculiar change known as _coagulation_.(11) In this change the
corpuscles are collected into a solid mass, known as the _clot_, thereby
separating from a liquid called the _serum_. The serum, which is similar
in appearance to the blood plasma, differs from that liquid in one
important respect as explained below.