Human blood. Introduction
liquid medium (plasma) containing several types of
specialized cells in suspension. The circulatory system
provides the mechanism by which the blood transports
substances to and from the organs and tissues. The circulating
blood continuously supplies oxygen, nutrient substances,
and other materials necessary for the viability and
activity of all the cellsof the body and carries away
cell products, including carbon dioxide and other waste
materials. If blood flow ceases, death occurs within
minutes because of the effects of an unfavourable environment
on highly susceptible cells.
The constancy of the composition of the blood is made
possible by the circulation, which conveys blood through
the organs that regulate the concentrations of its components.
In the lungs blood acquires oxygen and releases carbon
dioxide transported from the tissues.The kidneys remove
excess water and dissolved waste products. Nutrient
substances derived from food reach the bloodstream after
absorption by the intestinal tract. Endocrine glands
release their secretions into the blood, which transports
these hormones to the tissues in which they exert their
effects. Many substances are recycled through the blood;
for example, iron released during the destruction of
old red cells is conveyed by the plasma to sites of
new red cell production where it is reused. Each of
the numerous components of the blood is kept within
appropriate concentration limits by an effective regulatory
mechanism. In many instances feedback control systems
are operative; thus a declining level of blood sugar
leads to accelerated release of sugar into the blood
so that a potentially hazardous depletion of blood sugar
does not occur.
Unicellular organisms, primitive multicellular animals,
and the early embryos of higher forms of life lack a
circulatory system, and exchange of substances between
cell and environment is accomplished by simple diffusion.
In larger and more complex animals transport of adequate
amounts of oxygen and other substances requires some
type of blood circulation. The diffusion process then
occurs between the body cells and the fluid derived
from the blood, which by its constant motion maintains
the constancy of the internal environment. Some simple
animals, including small worms and mollusks, have blood
that lacks an oxygen-binding substance analogous to
hemoglobin; others are provided with pigments capable
of transporting relatively large amounts of oxygen.
In many invertebrates the blood pigment is dissolved
in the plasma. Hemocyanin, a copper-containing protein
chemically unlike hemoglobin, is found in certain crabs
and other lower animals. Hemocyanin is blue in colour
when oxygenated and colourless when oxygen is removed.
Some invertebrates have hemoglobin in solution in the
plasma. In almost all vertebrates, including humans,
hemoglobin is contained exclusively within the red cells
(erythrocytes) of the blood. The red cells of the lower
vertebrates (e.g., birds) have a nucleus, whereas mammalian
red cells lack a nucleus. Red cells vary markedly in
size among mammals; those of the goat are much smaller
than those of humans, but the goat compensates by having
many more red cells per unit volume of blood. The concentration
of hemoglobin inside the red cell varies little between
species.
For additional information on blood in general and
comparison of the blood and lymph of diverse organisms,
see tissues and fluids: The tissues and fluids of animals:
Blood and lymph and circulation.
