Invertebrate circulatory systems. Basic physicochemical
considerations.
To maintain optimum metabolism, all living cells require
a suitable environment, which must be maintained within
relatively narrow limits. An appropriate gas phase (i.e.,
suitable levels ofoxygen and other gases), an adequate
and suitable nutrient supply, and a means of disposal
of unwanted products are all essential.
Direct diffusion through the body surface supplies
the necessary gases and nutrients for small organisms,
but even some single-celled protozoa have a rudimentary
circulatory system. Cyclosis in many ciliates carries
food vacuoles—which form at the forward end of the gullet
(cytopharynx)—on a more or less fixed route around the
cell, while digestion occurs to a fixed point of discharge.
For most animal cells, the supply of oxygen is largely
independent of the animal and therefore is a limiting
factor in its metabolism and ultimately in its structure
and distribution. The nutrient supply to the tissues,
however, is controlled by the animal itself, and, because
both major catabolic end products of metabolism—ammonia
(NH3) and carbon dioxide (CO2)—are more soluble than
oxygen (O2) in water and the aqueous phase of the body
fluids, they tend not to limit metabolic rates. The
diffusion rate of CO2 is less than that of O2, but its
solubility is 30 times that of oxygen. This means that
the amount of CO2 diffusing is 26 times as high as for
oxygen at the same temperature and pressure.
The oxygen available to a cell depends on the concentration
of oxygen in the external environment and the efficiency
with which it is transported to the tissues. Dry air
at atmospheric pressure contains about 21 percent oxygen,
the percentage of which decreases with increasing altitude.
Well-aerated water has the same percentage of oxygen
as the surrounding air; however, the amount of dissolved
oxygen is governed by temperature and the presence of
other solutes. For example, seawater contains 20 percent
less oxygen than fresh water under the same conditions.
The rate of diffusion depends on the shape and size
of the diffusing molecule, the medium through which
it diffuses, the concentration gradient, and the temperature.
These physicochemical constraints imposed by gaseous
diffusion have a relationship with animal respiration.
Investigations have suggested that a spherical organism
larger than 0.5 millimetre (0.02 inch) radius would
not obtain enough oxygen for the given metabolic rate,
and so a supplementary transport mechanism would be
required. Many invertebrates are small, with direct
diffusion distances of less than 0.5 millimetre. Considerably
larger species, however, still survive without an internal
circulatory system.
