System Overview

There are certain ways in which variables may relate to each other in this model; here are some of the commoner ones.

• Directly linear: where one variable is always exactly a linear function of another. An example of this is the Haematocrit and the antihaematocrit.
• Product: one variable is the product (or proportional to the product) of two other variables. An example would be the plasma volume, which is a product of blood volume and the antihaematocrit.
• Indirectly linear: where one variable approaches a linear function of another, in a manner that is rate-dependent. An common type of this is the error regulation, where the error in one variable (i.e. the deviation from the 'correct' value) determines the error in another according to a gain function. For example, the blood concentration of angiotensin II is regulated linearly to match errors in extracellular volume.
• Sum: where one variable is always simply the sum of others, for example body mass is the sum of total body water, fat, glycogen, and gut contents.
• Container relations.

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