Body and Tissue Water Contents

I t is of importance to know the volumes and percentages of water in the body, its compartments and tissues. Shifts of water between compartments can be as physiologically important as total volume changes. M a c h (1954) describes the different dehydrations due to changes in E C W and I C W . Tissue storage and release of water contribute to water balance. Robinson (1960) reviews the problem of intracellular and extracellular water regula-tion.

A. Methods of Measurement

Mendenhall et al. (1953) review different methods of direct measurement of water content: oven drying, lyophilization, and titration (see also Cook et al., 1952). As pointed out by Moulton (1923), water content can be re-lated reasonably only to the fat-free weight of the tissue involved. When working with the entire body, it is further important to make a distinction between the fat-free weight and the lean body mass, as the former includes the significant weight of mineral of the skeleton (see Brozek, 1961). F a t acts as a dilutant in water measurements, taking u p space and weight without providing volume for water distribution. T h e variation in a m o u n t of fat between tissues and species, and in different physiological conditions, completely confounds interpretation of d a t a expressed as water content per total weight. I n contrast, water content of normal animals is a rather constant percentage of the fat-free weight (Annegers, 1954; Babineau and Page, 1955).

Because of the constancy of composition of the fat-free weight, water content can also be estimated from Na+ and K+ (Blaxter and Rook, 1953) and nitrogen content (Bender and Miller, 1953; Dreyer, 1957).

Dilution methods are the only means of measuring water contents in living animals or of measuring separate fractions of the total water. T h e general procedure is to inject a known amount of a substance which is limited in its distribution to a particular water compartment, and, after a period of time for equilibrium, measure its concentration in the body fluid.

Commonly used substances are: E v a n s blue and radioactively tagged red blood cells for plasma volume; inulin, radioisotopes of N a⁺ and Cl~~, and thiocyanate for E C W ; D²0 , T²0 , antipyrine, and urea for total body water. I C W and interstitial fluid volumes can be calculated by difference of two measurements. Dilution methods are discussed by Elkinton and Taffel (1942), McCance and Widdowson (1951), H a r d y and Drabkin (1952), Pinson (1952), Brozek (1961), and Brozek and Henschel (1961).

130 Robert Μ. Chew Accuracy for any dilution method is limited by corrections necessary for:

(1) metabolism and excretion of the injected substance before it reaches a distribution equilibrium in the body fluids, (2) partial escape of the material from the water compartment being measured, and (3) its concentration in certain tissues. These corrections vary with species (Hix et al., 1953; White and Rolf, 1956). Erroneous estimates of E C W are obtained from substances t h a t do not penetrate the gut lumen (Cizek, 1954). Measurement of E C W is complicated in ruminants by the large amount of water t h a t m a y be in the rumen and other parts of the gut, which is u p to 2 5 % of total body water in some breeds of sheep (Budtz-Olsen et al., 1961), and the long mixing time of the test material with the water in the rumen (Till and Downes, 1962). Water compartment volume measurements should be related to total body water or fa1>free weight.

B. Total Water Content

Table X shows t h a t there are no conspicuous differences (though some are statistically significant) among the water contents of normal adult mammals; all are within the range 7 0 — 7 5 % of fat-free weight. Unfortun­

ately no d a t a are available on water contents per fat-free weight of wild mammals.

TABLE X

WATER CONTENT OF FAT-FREE BODY WEIGHT OF NORMAL ADULT MAMMALS

Water

Species content Source

(%)

White mice 74.0 Annegers (1954)

White mice 74.6 Chew and Hinegardner (1957)

White rats 73.3 Sarett and Snipper (1956)

White rats 73.7 Spray and Widdowson (1950)

White rats 72.6 Annegers (1954)

Guinea pig 74.5 Moulton (1923)

Rabbit 72.8 Moulton (1923)

Cat 72.4 Moulton (1923)

Cat 74.4 Spray and Widdowson (1950)

Dog 70.0 Moulton (1923)

Pig 75.6 Spray and Widdowson (1950)

Pig 72.5 Moulton (1923)

Cattle 70.0 Moulton (1923)

While the percentage of water in the body m a y be very similar in different species, the rate of turnover m a y differ significantly. Richmond et al.

(1960, 1962) found t h a t the biological half-times for turnover of body water v a r y with the 0.80 power of body weight for six species of domesti-cated mammals, ranging from 1.1 days for white mice to 8.4 days for horses. T h e desert kangaroo r a t (Dipodomys deserti), however, has a much slower turnover of 12 days, which is an obvious adaptation to an arid environment.

1. Factors that Influence Water Content

a. Age. Variation of water content with age in different species is re-viewed by Moulton (1923), Spray and Widdowson (1950), and McCance and Widdowson (1951, 1956). T h e water content of the fat-free weight decreases from 9 5 - 9 7 % shortly after conception to 7 6 - 8 8 % at birth, to a constant 7 0 - 7 5 % in mature animals. Moulton (1923) first proposed t h e idea of chemical maturity at the point where water, protein, and total mineral contents become stabilized. At birth different species have water contents inversely related to their degree of development: rat 8 8 % , mouse 8 6 - 8 7 % , rabbit 8 4 % , cat 8 3 % , dog, man, and swine 8 2 % , guinea pig 77.8%, and cattle 7 6 . 3 % . Although chemical m a t u r i t y occurs before or after sexual m a t u r i t y in different species, t h e conceptual age a t chemical m a t u r i t y is a relatively constant 4 . 6 % of the total life span.

Schreiber (1950) found an abrupt decrease in the rate of decline of body water content and a drop in Cl~ content in rats at the time of opening of the eyes. H e suggests this m a y be due to suddenly altered water exchanges following development of t h e hypothalamic-pituitary system under the stimulus of light received by the eyes.

b. Starvation and water deprivation. Starvation does not significantly alter the water content of the fat-free weight of the white rat until it is prolonged for 168 hours. Starvation with water deprivation, however causes a significant reduction in 24 hours (Annegers, 1954). One rat dropped as low as 6 4 % water without appearing moribund, so t h e minimum water content compatible with life is less t h a n this.

For rodents moribund after chronic water restriction, water contents were: white mice, 68.8-69.7% fat-free weight (Chew and Hinegardner, 1957); Peromyscus leucopus noveboracensis, 66.6% total weight (Chew, 1951); Dipodomys, 67.2%, and white rats, 64.2%, total weight (B. Schmidt-Nielsen et al., 1948).

W a t e r contents are abnormal in strains of white mice in which metabo-lism is abnormal, ranging from 8 3 . 2 % fat-free weight in an obese strain

(61 gm.) to 7 2 . 6 % in a thin strain (7.8 gm.) (Benedict and Lee, 1936).

132 Robert Μ. Chew c. Water loads. Transient increases of up to 4 % of fat-free water content can be produced in white rats by water loading (Eversole et al, 1942);

Adolph and Parmington, 1948). R a t s repeatedly given water by stomach tube increase their resistance to intoxication, possibly by improved distrib­

ution of water excesses, but adrenalectomized individuals show very little adaptation (Liling and Gaunt, 1945).

C. Tissue Water Contents and Exchanges

Table X I gives a summary of d a t a on normal tissue water contents.

Additional detailed d a t a on the tissues of rats, guinea pigs, and rabbits are given by Allen et al (1959). Only some of the connective tissues are con­

spicuously low. The range in skin water content m a y be partly due to the fact t h a t it varies (at least in rats) with the cycle of hair growth (Butcher and Grokoest, 1941). Water content of connective tissue also varies with the concentration of hydrophilic hexosamine colloids, which decreases with age (Boas and Foley, 1954). Work on white mice (Hvidberg, 1960) suggests t h a t the skin can make a slow, long-term adjustment of its water-binding capacity to the prevailing conditions of hydration, by changes in the quantity of acid mucopolysaccharides; short-term, rapid adjustments m a y be made by changes in t h e physicochemical condition of the mucopolysac­

charides. Tissues of desert rodents possibly have lower water contents (Khalil and Abdel-Messeih, 1954).

Pandazi et al (1960) have made a specific study of the water contents of adipose tissues of rats. The water present in these tissues is associated with the nonfat solids, and when neutral fat is added, it is added without further water. Therefore, the percentage water content of the total tissue varies inversely with the amount of fat deposited, from 5 to 4 0 % water. The ratio of water to nonfat solids remains constant at 76:42. Allen et al. (1959) also found t h a t fat from various regions of the body has the same water-density system as other tissues.

The different tissues play different roles in providing water to circulating blood volume during dehydration and in storing water excesses. I n the cat during acute dehydration the visceral organs lose water most quickly and have the greatest relative losses, but muscle and skin make the greatest absolute contributions to blood volume (Skelton, 1927). Normal viscera exchange D2O rapidly with the blood, while muscle, skin, and connective tissues equilibrate slowly (Edelman, 1952), possibly because of different capillary permeabilities (Skelton, 1927). I n chronic dehydration t h e skin loses the most water, both in relative and absolute terms, in the cat (Skelton, 1927), young dog (Hamilton and Schwartz, 1935), and rabbit (Flemister, 1941). Although muscle shows the least proportionate loss in all the above

TABLE XI WATER CONTENTS OF TISSUES OF ADULT MAMMALS" ·6 Tissue Gerbillus Jaculus White White White White Guinea Dog6 Dog7 pyramidum jaculus mouse rat1 rat2 rat pig Skeletal muscle 74.9* 74.8* 75.9 76.5 74.4 75.73 74.7 87.3 79.2 Liver 70.1* 69.9* 71.5 71.1 72.3 71.43 69.1 76.0 — Skin 39.3* 48.7* — 62.0 — — — 73.3 77.3 Smooth muscle 72.5* 74.5* — — — — — — — Kidney 77.8* 77.7*

—

— — — — — — Heart — —

—

79.2 — — — — — Brain — —

—

75.8 77.9 — — — — Gut — — — 79.5 — — — 83.0 — Lung — —

—

— — — 80.25 — — Adrenal — —

—

— — — 65.8 — — Nerve — — — — — — 62.7 — — Tendon — — — — — — 55.7 — — Bone 27.1* 26.8*

—

— — — — — — Adipose tissues Retroperitoneal — —

—

— — 25.04 — — — Subcutaneous — — — — — 31.54 — — — Perineal — —

—

— — — 8.1 — — ° Data are water contents as percentages of fat-free weight, except tissues marked with asterisk (*). 6 Sources: Gerbillus and Jaculus, Khalil and Abdel-Messeih (1954); white mouse, Chew and Hinegardner (1957); white rat: 1. Groll- man (1954), 2. Cook et al. (1952), 3. Dicker (1949), 4. Lepkovsky et al. (1957); guinea pig: 5. Mendenhall et al. (1953), Other data, Pace and Rathbun (1945); dog. 6. Harrison et al. (1936), 7. De Boer (1946).

134 Robert Μ. Chew cases, because of its total bulk it provides a major part of the total water.

There is a 6 % loss of water from the skin and 1.4% from muscles in adult dogs during a 6-7-day fast without water (De Boer, 1946). The skin water content was still low 5 days after "recovery" from fast. Chloride con­

centration d a t a show t h a t the skin loses water without electrolyte, whereas muscle loses isotonic fluid. Apparently it is easier to recover from the latter type of loss.

In water-loaded cats, the viscera show the greatest proportional increase of water, but skin and muscle provide the bulk of storage. The skin takes up even more water when the fluid load is isotonic or hypertonic saline (Skelton, 1927). I n the rabbit, the skin stores about half the water load t h a t is not immediately excreted (Flemister, 1941); similar skin storage occurs in overhydrated newborn rats (Capek et al., 1956).

I n rats there is a decrease in the water content of the skin, muscle, stomach, and intestine immediately after eating, and, if water is denied during and for 9 hours after feeding, the decrease is even greater. I t is suggested t h a t these tissue losses provide fluid for digestive juices (Lepkov­

sky et al, 1957).

The functioning of the skin as a dynamic reservoir for water depends on its connective tissue components. I n fully hydrated h u m a n dermis, 2 0 % of the total water content is chemically bound, 2 0 % is trapped in minute capillary pores, and 6 0 % is free (Herrmann and March, 1959). This 6 0 % can be removed or replaced readily in adjustment of water balance without systemic disturbance. The 6 0 % is reciprocally associated with the collagen fibers and ground substance of the dermis; when one of these swells, in response to changes in water content, electrolyte content, or p H of the tissue fluid, the other usually shows dehydration.

Though the water in the lumen of the gut m a y be 6.7% of total water content in rats and 1 7 - 2 0 % in guinea pigs (Cizek, 1954), there is no actual storage of water in the digestive tract, even in the fabled camel (K. Schmidt-Nielsen et al, 1956).

X. Dehydration

In document Water Metabolism of Mammals (Pldal 87-92)