MODULE SUPPLEMENT: RENAL SYSTEM Body Composition and Fluid Balance
Mr. W.
is an 85 year old Caucasian male who comes to clinic
complaining of palpitations and an irregular heart beat for
the past several days. During the history, he notes that he
feels he is very healthy and that his weight of 175 is the
same as when he was in college at age 20. Mr. W. remains
active, walking 4 miles per day, gardening, and enjoying
weekend hikes in the Marin Headlands with his wife of 60
years.
How
does the lean/adipose tissue ratio influence fluid balance?
Adipose tissue is 'drier'--it contains less water--than dense
lean tissue. Total body water declines with age, decreasing
from about 55-60% of body weight at age 20 to about
45-55% at age 80 (references vary slightly in percentages given).
Because women, in general, have proportionately
more adipose tissue than men, they also have lower total
body water throughout adulthood
Proportion of Water in Men versus Women
across Adulthood (Morley, 1998).
Newborn:
73-77%
Adult:
Male 60%
Female 52%
40-59 yo:
Male 55%
Female 47%
>70 yo:
Male 52%
Female 46%
Mr. W.
is diagnosed with atrial fibrillation and is going to be
prescribed digoxin, a water soluble drug.
Although decreased total body water has important
implications for fluid status and how drugs are distributed in
the body, changes that occur in renal function may be of
even greater importance. These changes influence how
well fluid status is maintained under conditions of stress
(such as illness), and how many drugs are eliminated. For
example, impaired renal function rather than altered
distribution is the primary reason digoxin may accumulate to
toxic levels in older adults, especially those over the age of
75.
