ORGAN FUNCTION
Organ function is assessed by measuring how
well an organ is doing what it is supposed to be
doing. For example, the pump function of the heart
is studied by measuring the cardiac output and the
blood pressure; the cognitive function of the brain is
investigated using intelligence tests. The functional
status of synthetic organs, organs of cell generation
(such as the marrow), organs of elimination, and
organs of absorption can also be evaluated. The
laboratory studies used to make these measurements
may assay the organ's function directly, such as
analyzing semen to determine testicular function.
More often, though, the studies assess function only
indirectly, using the blood or plasma concentration
of some secreted, eliminated or absorbed substance
as a quantitative marker of the functional status.
SYNTHESIS AND CLEARANCE
The indirect assessment of an organ's synthetic
function is based upon the logical notion that the
functional status of an organ is reflected in the rate
of synthesis of the product secreted by the organ.
Given an understanding of the relationship between
the plasma concentration of the product and the
synthetic rate of the product, a measurement of the
product's plasma concentration can be used to infer
the organ's functional status. In a similar fashion,
the functional status of an organ of elimination is
assumed to be revealed by the rate of removal of a
substance by the organ. Given the relationship
between the plasma concentration of the removed
substance and the rate of its removal, the functional
status of the organ can be inferred from the plasma
concentration of the substance.
Under steady-state conditions, the relationship
between the rate of synthesis of a substance secreted
into the plasma and its plasma concentration and the
relationship between the removal rate of a substance
and its plasma concentration are expressed in a
single simple equation (DiStefano 1976),
plasma concentration
=
synthetic rate
clearance rate
Synthetic rate quantifies the entry of a substance into
the plasma. Its dimensions are amount per unit time
(e.g., millimoles per hour). It appears in the
numerator because the plasma concentration of a
substance is directly related to its rate of entry into
the plasma; the more rapid the entry, the greater the
concentration. Figure 7.1 shows the direct relation-
ship between plasma concentration and synthetic rate
at various clearance rates. Clearance rate quantifies
the removal of substance from the plasma. It has the
dimensions volume per unit time (e.g., milliliters per
minute). Substance concentration is inversely
related to its rate of removal, so this term appears in
the denominator. Figure 7.2 illustrates the hyper-
bolic relationship between substance concentration
and clearance rate at different synthetic rates.
The volume of distribution is the constant of
proportionality between the concentration of a
substance and the amount of the substance. The
substance concentration in the steady state does not
depend upon the volume of distribution of the
substance; it is determined solely by the synthetic
rate and clearance rate. On the other hand, the time
needed to attain the steady state (as a rule of thumb,
five half-lives of the substance) depends upon both
the clearance rate and the volume of distribution.
Organ Function
7-1
Chapter 7
ORGAN FUNCTION
© 2001 Dennis A. Noe
Figure 7.1
Steady-state relationship between substance
concentration and synthetic rate for four different clearance
rates.
Synthetic rate
Steady-state plasma concentration
