BLOOD
Blood is the vital fluid found in humans and other animals
that provides important nourishment to all body organs and
tissues and carries away waste materials. Sometimes referred
to as “the river of life,” blood is pumped from the heart
through a network of blood vessels collectively known as the
circulatory system. Blood is a transport liquid pumped by
the heart to all parts of the body, after which it is
returned to the heart to repeat the process. Blood is both a
tissue and a fluid. It is a tissue because it is a
collection of similar cells that serve a particular
function. These cells are suspended in a liquid
matrix—called plasma—which makes the blood a fluid.
In single-celled animals and the smallest multicellular
invertebrates there is no blood system. Because of their
little size, these animals can take up oxygen and nutrients
and can release wastes directly into their surrounding
medium. In animals of increasing size and complexity,
contact with the surrounding medium is no longer enough to
provide the metabolic requirements of internal body cells.
Such animals possess a true blood system in which a
specialized fluid—i.e., blood—is pumped around the body. In
most such animals the blood passes through a respiratory
exchange membrane, which lies in the gills, lungs, or even
the skin. There the blood picks up oxygen, which is
necessary for cellular metabolism, and disposes of carbon
dioxide, which is a waste product of metabolism. The blood
also absorbs nutrients from the gastrointestinal tract or
from various storage tissues and carries them to cells
throughout the body. Another function of blood is the
transport of water-soluble toxic wastes to excretory organs
for elimination from the body.
Blood evolved from seawater, which in primitive organisms
was the source of elements essential for growth. Thus, the
basic chemical composition of plasma resembles that of
seawater, consisting of water and dissolved salts. However,
plasma also contains dissolved nutrients, wastes, and—in
many species—hormones. The cellular composition of blood
varies from group to group in the animal kingdom. Most
invertebrates have various large blood cells capable of
amoeboid movement. Some of these aid in transporting
substances; others are capable of surrounding and digesting
foreign particles or debris. Compared with vertebrate blood,
however, that of the invertebrates has relatively few cells.
Among the vertebrates, there are several classes of
amoeboid, special oxygen-carrying, and cells that help stop
bleeding. In many invertebrates the respiratory pigments are
carried in solution in the plasma, but in higher animals,
including all vertebrates, the pigments are enclosed in
cells; if the pigments were freely in solution the pigment
concentrations required would cause the blood to be so
viscous as to impede circulation.
There are two basic types of blood circulatory systems—open
and closed. In an open system, the blood flows out of
vessels and directly bathes the internal body tissues. In a
closed system, the blood never leaves its network of
vessels; exchange of materials between the blood and tissues
occurs across the walls of the capillaries, the tiniest
blood vessels.
An adult human has about 5 to 6 liters of blood, which is
roughly 7 to 8 percent of total body weight. Infants and
children have comparably lower volumes of blood, roughly
proportionate to their smaller size. The volume of blood in
an individual fluctuates. During dehydration, for example
while running a marathon, blood volume decreases. Blood
volume increases in circumstances such as pregnancy, when
the mother’s blood needs to carry extra oxygen and nutrients
to the baby.
