Page 15 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

1 Fundamentals and Cell Physiology

“... If we break up a living organism by isolating its different parts, it is only for the sake of ease in
analysis and by no means in order to conceive them separately. Indeed, when we wish to ascribe to a
physiological quality its value and true significance, we must always refer it to the whole and draw our
final conclusions only in relation to its effects on the whole.”
Claude Bernard (1865)

The Body: an Open System with an ganism is capable of eliciting motor responses
Internal Environment to signals from the environment. This is
achieved by moving its pseudopodia or
The existence of unicellular organisms is the flagella, for example, in response to changes in
epitome of life in its simplest form. Even the food concentration.
simple protists must meet two basic but essen- The evolution from unicellular organisms to
tially conflicting demands in order to survive. multicellular organisms, the transition from
A unicellular organism must, on the one hand, specialized cell groups to organs, the emer-
isolate itself from the seeming disorder of its gence of the two sexes, the coexistence of in-
inanimate surroundings, yet, as an “open sys- dividuals in social groups, and the transition
tem” (! p. 40), it is dependent on its environ- from water to land have tremendously in-
ment for the exchange of heat, oxygen, creased the efficiency, survival, radius of ac-
nutrients, waste materials, and information. tion, and independence of living organisms.
“Isolation” is mainly ensured by the cell This process required the simultaneous devel-
membrane, the hydrophobic properties of opment of a complex infrastructure within the
which prevent the potentially fatal mixing of organism. Nonetheless, the individual cells of
hydrophilic components in watery solutions the body still need a milieu like that of the
inside and outside the cell. Protein molecules primordial sea for life and survival. Today, the
within the cell membrane ensure the perme- extracellular fluid is responsible for providing
ability of the membrane barrier. They may constant environmental conditions (! B), but
exist in the form of pores (channels) or as more the volume of the fluid is no longer infinite. In
complex transport proteins known as carriers fact, it is even smaller than the intracellular
(! p. 26 ff.). Both types are selective for cer- volume (! p. 168). Because of their metabolic
tain substances, and their activity is usually activity, the cells would quickly deplete the
regulated. The cell membrane is relatively well oxygen and nutrient stores within the fluids
permeable to hydrophobic molecules such as and flood their surroundings with waste prod-
gases. This is useful for the exchange of O 2 and ucts if organs capable of maintaining a stable
CO 2 and for the uptake of lipophilic signal sub- internal environment had not developed. This
stances, yet exposes the cell to poisonous gases is achieved through homeostasis, a process by
such as carbon monoxide (CO) and lipophilic which physiologic self-regulatory mecha-
noxae such as organic solvents. The cell mem- nisms (see below) maintain steady states in
brane also contains other proteins—namely, the body through coordinated physiological
receptors and enzymes. Receptors receive sig- activity. Specialized organs ensure the con-
nals from the external environment and con- tinuous absorption of nutrients, electrolytes
vey the information to the interior of the cell and water and the excretion of waste products
(signal transduction), and enzymes enable the via the urine and feces. The circulating blood
cell to metabolize extracellular substrates. connects the organs to every inch of the body,
Let us imagine the primordial sea as the ex- and the exchange of materials between the
ternal environment of the unicellular or- blood and the intercellular spaces (interstices)
ganism (! A). This milieu remains more or less creates a stable environment for the cells. Or-
constant, although the organism absorbs gans such as the digestive tract and liver ab-
nutrients from it and excretes waste into it. In sorb nutrients and make them available by
2
spite of its simple structure, the unicellular or- processing, metabolizing and distributing
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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