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 Internal Environment

The existence of unicellular organisms is the epitome of life in its simplest form. Even simple protists must meet two basic but essentially conflicting demands in order to survive. A unicellular organism must, on the one hand, isolate itself from the seeming disorder of its inanimate surroundings, yet, as an "open system" (^ p. 40), it is dependent on its environment for the exchange of heat, oxygen, nutrients, waste materials, and information.

"Isolation" is mainly ensured by the cell membrane, the hydrophobic properties of which prevent the potentially fatal mixing of hydrophilic components in watery solutions inside and outside the cell. Protein molecules within the cell membrane ensure the permeability of the membrane barrier. They may exist in the form of pores (channels) or as more complex transport proteins known as carriers (^ p.26ff.). Both types are selective for certain substances, and their activity is usually regulated. The cell membrane is relatively well permeable to hydrophobic molecules such as gases. This is useful for the exchange of O2 and CO2 and for the uptake of lipophilic signal substances, yet exposes the cell to poisonous gases such as carbon monoxide (CO) and lipophilic noxae such as organic solvents. The cell membrane also contains other proteins—namely, receptors and enzymes. Receptors receive signals from the external environment and convey the information to the interior of the cell (signal transduction), and enzymes enable the cell to metabolize extracellular substrates.

Let us imagine the primordial sea as the external environment of the unicellular organism (^ A). This milieu remains more or less constant, although the organism absorbs nutrients from it and excretes waste into it. In spite of its simple structure, the unicellular or ganism is capable of eliciting motor responses to signals from the environment. This is achieved by moving its pseudopodia or flagella, for example, in response to changes in the food concentration.

The evolution from unicellular organisms to multicellular organisms, the transition from specialized cell groups to organs, the emergence of the two sexes, the coexistence of individuals in social groups, and the transition from water to land have tremendously increased the efficiency, survival, radius of action, and independence of living organisms. This process required the simultaneous development of a complex infrastructure within the organism. Nonetheless, the individual cells of the body still need a milieu like that of the primordial sea for life and survival. Today, the extracellular fluid is responsible for providing constant environmental conditions (^ B), but the volume of the fluid is no longer infinite. In fact, it is even smaller than the intracellular volume (^ p. 168). Because of their metabolic activity, the cells would quickly deplete the oxygen and nutrient stores within the fluids and flood their surroundings with waste products if organs capable of maintaining a stable internal environment had not developed. This is achieved through homeostasis, a process by which physiologic self-regulatory mechanisms (see below) maintain steady states in the body through coordinated physiological activity. Specialized organs ensure the continuous absorption of nutrients, electrolytes and water and the excretion of waste products via the urine and feces. The circulating blood connects the organs to every inch of the body, and the exchange of materials between the blood and the intercellular spaces (interstices) creates a stable environment for the cells. Organs such as the digestive tract and liver absorb nutrients and make them available by processing, metabolizing and distributing

A. Unicellular organism in the constant external environment of the primordial sea —|

Signal reception

Substance absorption and excretion

Signal reception

Substance absorption and excretion

Ion exchange

Exchange of gases

Excretion

Ion exchange

Exchange of gases

Excretion

B. Maintenance of a stable internal environment in humans

External signals

Integration through nervous system and hormones

Emission of heat

Exchange of gases

External signals

Integration through nervous system and hormones

Emission of heat

interstice

02 CO2

Exchange of gases

Extracellular space

Intracellular space interstice

Kidney

Excretion

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