MATERIALS AND METHODS

The methods employed in this study were approved by the Texas A&M University Institutional Animal Care and Use Committee. The investigation conforms with the National Institutes of Health (NIH)Guide for the Care and Use of Laboratory Animals [DHHS Publication No. (NIH) 85–23, Revised 1985, Office of Science and Health Reports, Bethesda, MD 20892.]

Animals and surgical procedures.

Juvenile (2-mo-old, n = 6), adult (6-mo-old, n = 7), and aged (24-mo-old, n = 7) male Fischer-344 rats (National Institutes of Aging Colony and Charles River Laboratories, Raleigh, NC) were used. These ages were chosen to correspond to the normal life span and sexual development of the Fischer-344 rat; 1.5- to 2-mo-old rats are juvenile, 3- to 6-mo-old rats are young mature adults, and 24-mo-old or older rats are considered aged (11). Their life expectancy is ∼29 mo, with a maximal survival time of ∼36 mo (5). The animals were individually housed in an environmentally controlled room (23 ± 2°C) with a 12:12-h light-dark cycle and given food (commercial rat chow) and water ad libitum.

Under methoxyflurane anesthesia (Metofane), a catheter (Dow Corning, Silastic; ID 0.6 mm, OD 1.0 mm) filled with heparinized (200 U/ml) saline and connected to a pressure transducer and chart recorder was advanced into the left ventricle of the heart via the right carotid artery as previously described (9). This catheter was subsequently used for the infusion of radiolabeled microspheres to measure tissue blood flow and for the recording of intraventricular pressure. A second polyurethane catheter (Braintree Scientific, Micro-renathane; ID 0.36 mm, OD 0.84 mm), used for the withdrawal of a reference blood sample and the recording of arterial pressure, was implanted in the caudal artery of the tail and filled with heparinized saline as previously described (7). Both catheters were externalized and secured on the dorsal cervical region.

Experimental protocol.

After the animals had recovered for 2 days from the surgical procedure, all catheters and instrumentation were connected while the animals remained in their cages. The rats were allowed 20 min to stabilize after the instrumentation procedure before the microsphere infusion was performed. Pulsatile intraventricular pressures (and, hence, heart rates) were monitored during this period, which was sufficient for heart rate to stabilize. When the microsphere infusion and reference-sample withdrawal were completed, pentobarbital sodium (35 mg/kg) was infused through the carotid catheter, and the animals were euthanized by exsanguination. Tissue samples were excised, weighed, and placed in counting vials for blood flow determination. During the dissection, the carcass was kept moist to minimize evaporative weight loss. The purpose of this study was to account for total cardiac output distribution in the animals. Accordingly, all organs and all tissues from the carcass were weighed and analyzed.

Blood flow and cardiac output measurements.

Radiolabeled (⁹⁵Nb or¹⁰³Ru) microspheres (New England Nuclear) with a 15.5 ± 0.2-μm diameter were used for blood flow and cardiac output measurements as previously described (16, 27, 29). Microspheres were suspended in physiological saline with <0.5% Tween 80 and mixed before infusion by 10 min of sonication followed by 1–2 min of agitation on a vortex mixer. A reference blood sample from the caudal artery was started at a rate of 0.618 ml/min with an infusion-withdrawal pump (Harvard). Ten seconds later, ∼1.0 million spheres suspended in 0.4 ml saline were infused into the juvenile rats, and 1.5 million spheres suspended in 0.6 ml saline were infused into the left ventricular catheter of adult and aged rats over a 15- to 20-s period. One milliliter of warm (37°C) saline was infused over a 30- to 60-s period immediately after the microsphere infusion; withdrawal of the reference blood sample continued for at least 1 min after the saline flush. Fewer microspheres were infused into the juvenile rats because of their smaller body size and vascular transport capacity to minimize the possible adverse hemodynamic effects from the microspheres. After euthanasia and tissue dissection, tissue samples were counted in a gamma counter (Packard Minaxi Auto-Gamma 5000) and flows were computed (IBM personal computer) from counts per minute and tissue wet weights. Microsphere mixing with the blood was assessed by comparing bilateral kidney flows. Mixing was considered adequate if bilateral flows were within 15% of each other. No data were discarded in this study because of inadequate mixing.

Typically, in a 350-g rat, blood flow distribution is measured by infusing 0.5 million microspheres per experimental treatment, and up to three to four treatments can be made in a single animal (e.g., Refs. 7,9, 29). In the present study, the infusion of 1.0 million microspheres in juvenile rats and 1.5 million microspheres in the two groups of adult rats for a single blood flow measurement was done to ensure that small-mass tissue samples contained sufficient numbers of spheres for accurate flow determinations. To determine whether the infusion of these numbers of microspheres disturbed the cardiovascular system or altered the distribution of blood flow within or among tissues, we made two sequential blood flow measurements by using two different radiolabeled-microsphere species in a preliminary group of juvenile (n = 6) and 6-mo-old adult (n = 6) animals. In the juvenile rats, blood flow was sequentially measured with two infusions of 0.5 million microspheres per infusion; in the adult rats, flow was measured with one infusion containing 1.0 million microspheres and a second containing 0.5 million microspheres. In both the juvenile and adult animals, heart rate and blood pressure were similar before and ∼1 min after each of the two microsphere infusions (Table1). In addition, cardiac output (Table 1) and tissue blood flows (data are not shown but are similar to that in Tables 6 and 7) were similar between the first and second infusions, indicating there were no hemodynamic disturbances induced by the infusion of 1.0 and 1.5 million microspheres in juvenile and adult rats, respectively.

Cardiac output was assessed in two ways in the experimental animals. The first was a direct measurement of cardiac output with the method described by Ishise et al. (27), by using the following equation: cardiac output (ml/min) = [reference-sample withdrawal rate (ml/min) ⋅ injected isotope counts (counts/min)] / reference blood sample counts (counts/min). The second was to sum individual tissue flows (ml/min) as previously described (9). Most tissue flows were measured from the entire organ, e.g., brain, heart, and viscera; flow to bilateral organs, such as skeletal muscle, feet, skin, and bone, were measured unilaterally and doubled.

Because of the difficulty in separating some bones from muscle and connective tissue with blunt dissection (e.g., vertebrae and feet), the bones from a separate group of rats (juvenile,n = 3; adult,n = 3; aged,n = 3) were dissected identically to the groups of rats used to measure blood flow. The bone samples were then weighed and placed in a boiling 2% KOH solution for several minutes to dissolve excess muscle and connective tissue. The bones were then blotted and reweighed to determine the proportion of bone in the original samples. Because of the similarity in the perfusion rates per unit mass of bone and muscle, flow (ml/min) to bone-muscle samples was partitioned in proportion to the bone and muscle composition of the sample.

Arterial pressure, heart rate, systemic vascular resistance, cardiac index, and stroke volume measurements.

Mean arterial pressure was electronically averaged from pulsatile pressure measurements from the caudal catheter. Heart rate was estimated from pulsatile left intraventricular pressure tracings. Pressure recordings, made with pressure transducers (Electromedical) and recorded on a polygraph (Gould 2800), were made immediately before and after the microsphere infusion and averaged, because simultaneous pressure measurements and withdrawal of a reference blood sample were not possible. Systemic vascular resistance (mmHg ⋅ ml⁻¹ ⋅ min) was calculated by dividing mean arterial pressure (mmHg) by the cardiac output (ml/min) derived from summing individual tissue flows. Cardiac index (ml ⋅ min⁻¹ ⋅ kg⁻¹) was calculated by using total body mass (kg) and cardiac output (ml/min) derived from individual tissue flows. Stroke volume (ml/beat) was calculated from cardiac output (ml/min) and heart rate (beats/min) measurements.

Statistical analysis.

To determine whether the infusion of 1.0 and 1.5 million microspheres induced hemodynamic disturbances in juvenile and young adult rats, respectively, a one-way analysis of variance with repeated measures was used to compare mean arterial pressure and heart rate means across conditions (preinfusion vs. infusion 1vs. infusion 2), and a pairedt-test was used to compare cardiac output and tissue blood flow means between conditions (infusion 1 vs.infusion 2). To determine the effects of maturation and aging, a one-way analysis of variance was used to compare variable means (cardiac output, arterial pressure, systemic vascular resistance, heart rate, cardiac index, stroke volume, tissue blood flows, and body and tissue masses) across groups (juvenile vs. adult vs. aged). Duncan’s new multiple-range test was used to determine the significance of differences among means. For all analyses, the 0.05 level was used to indicate statistical significance.

RESULTS

Body mass increased as a function of age (Table2). In mature rats, this resulted from increases in all tissue masses, whereas, with old age, the increase in body mass was primarily associated with increases in adipose tissue, bone, brain and spinal tissue, heart, kidney, splanchnic organ, and lung masses (Table 2). In comparison to juvenile rats (Table 3), the two groups of adult rats had a body composition with a higher percent mass of adipose tissue and salivary gland (young adult only), and a lower percent mass of brain and spinal tissue, myocardium, kidney, liver, lung (young adult only), skeletal muscle, skin (aged only), and thyroid and parathyroid glands (aged only). The percent masses of the adrenal glands, skeleton, splanchnic tissue, reproductive tissue, and tissue contents (which consisted of the contents within the gastrointestinal tract, urinary bladder, and seminal vesicles) were not different among groups.

Dissected tissue masses accounted for 88.5 ± 1.2% of total body mass in the juvenile rats, 84.4 ± 1.6% in the adult rats, and 85.1 ± 1.3% in the aged rats; this was not different among groups. The contents within the gastrointestinal tract, urinary bladder, and seminal vesicles (Table 3) accounted for 5.6 ± 0.5, 4.7 ± 0.4, and 4.6 ± 0.5% of total body mass in the juvenile, adult, and aged rats, respectively. The remaining 5.9, 10.7, and 10.3% body mass of the juvenile, adult, and aged rats, respectively, presumably consisted of blood lost during euthanasia and water lost through tissue dehydration during dissection. The blood has been reported to represent ∼7.4% of body mass in an adult rat (4).

Cardiac output, calculated as the sum of all body tissue blood flows (Table 4), was lower in juvenile rats relative to the two adult groups; there was no difference in cardiac output between these latter two groups. By using the method of Ishise et al. (27), cardiac output was 55 ± 4 ml/min in juvenile rats, 112 ± 9 ml/min in adult rats, and 125 ± 14 ml/min in aged animals. Thus there was only an 8, 6, and 5% difference between the two methods of assessing cardiac output in the juvenile, adult, and aged rats, respectively. Differences of this magnitude are similar to those previously reported (9).

Cardiac index was different among groups (Table 4). However, stroke volume was lower and systemic vascular resistance was higher in the juvenile group compared with the two adult groups.

Heart rate and mean arterial pressure were also not different among groups (Table 4). In addition, preinfusion heart rates (pre- vs. postinfusion: juvenile, 395 ± 7 vs. 390 ± 7 beats/min; adult, 397 ± 6 vs. 394 ± 8 beats/min; aged, 371 ± 16 vs. 365 ± 12 beats/min) and arterial pressures (pre- vs. postinfusion: juvenile, 115 ± 3 vs. 114 ± 4 mmHg; adult, 118 ± 6 vs. 115 ± 6 mmHg; aged, 116 ± 3 vs. 114 ± 3 mmHg) were not different from that measured ∼1 min after the infusion of the microspheres.

Adipose tissue, which made up 8, 11, and 15% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), received 5, 8, and 11% of cardiac output, respectively (Table5). Mass-specific blood flow (ml ⋅ min⁻¹ ⋅ 100 g⁻¹) to visceral abdominal, subcutaneous, and epididymal fat was not different among groups (Table 6).

The adrenal glands, which made up ∼0.02% of body mass in all groups (Table 3), received between 0.25 and 0.30% of cardiac output (Table5). Mass-specific blood flows to the adrenal cortex and the adrenal medulla were not different among groups (Table 6).

The skeletal system, which made up ∼7% of body mass in all groups (Table 3), received ∼7% of cardiac output (Table 5). The brain and spinal cord made up ∼1% of body mass in all groups (Table 3) and received ∼4, 3, and 3% of cardiac output in juvenile, adult, and aged rats, respectively (Table 5). Blood flow distribution patterns within the skeletal system and brain will be published separately.

The heart made up 0.33, 0.26, and 0.26% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), and received ∼5% of cardiac output (Table 5). Mass-specific blood flows to the heart were not different among groups (Table 6).

The kidneys made up ∼1.0, 0.8, and 0.8% of the total body mass in juvenile, adult and aged rats, respectively (Table 3), and received 13–14% of cardiac output (Table 5). Renal flow in the juvenile rats was less than that in adult rats but was similar to that in aged rats (Table 6).

The liver, which made up ∼5, 4, and 4% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), received ∼1% of cardiac output through the hepatic artery (Table 5). There was no difference in hepatic arterial flow among the different lobes of the liver. The splanchnic tissues made up ∼5% of the total body mass in all groups (Table 3) and received 20–23% of cardiac output (Table5). Therefore, the liver received a total of 22, 24, and 21% of cardiac output in juvenile, adult, and aged rats, respectively, via the hepatic artery and splanchnic circulations. Mass-specific blood flow to splanchnic tissues tended to be lower in juvenile animals (Table 6). In addition, flow to the spleen in aged animals was less than that in adult rats.

The lung made up ∼0.6, 0.4, and 0.6% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), and received 1–2% of cardiac output (Table 5).

Skeletal muscle, which composed ∼39, 36, and 32% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), received 33, 26, and 27% of cardiac output, respectively (Table 5). Blood flow to the superficial gluteal muscle was lower in the aged rats than in juvenile rats (Table 7), and flow to the white portion of rectus femoris muscle was lower in the old rats than in young adult animals.

Male reproductive tissues made up ∼1% of body mass in all groups (Table 3) and received ∼0.9, 1.1, and 0.7% of cardiac output in juvenile, adult, and aged rats, respectively (Table 5). Blood flow to the seminal vesicles and testes was not different among groups, but flow to the prostate gland was lower in aged rats than in adult animals.

The skin made up ∼17, 16, and 15% of the total body mass in juvenile, adult, and aged rats, respectively (Table 3), and received 7, 8, and 5% of cardiac output, respectively (Table 5). Mass-specific blood flow to skin tissue was not different among groups (Table 6).

The salivary gland, thyroid and parathyroid glands, eyes, urethra, urinary bladder, and other miscellaneous tissues made up ∼3% of body mass among groups (Table 3) and collectively received 3–4% of cardiac output (Table 5). Blood flows to the salivary and thyroid and parathyroid glands were lower in juvenile animals than young adult rats (Table 6), and flow to the thyroid and parathyroid glands in the aged rats was lower than that in the adult rats.

DISCUSSION

The purpose of the present study was to test the hypothesis that maturation, and in particular old age, does not alter tissue blood flows per unit mass in conscious Fischer-344 rats at rest but that alterations in body composition may result in changes in the fractional distribution of cardiac output. With few exceptions, the data support the hypothesis that old age does not alter tissue perfusion rates per unit mass. Furthermore, there were relatively few changes in body composition among the organ systems associated with old age. In some of those tissues where proportional changes in body mass did occur, such as with adipose and cutaneous tissues, there were corresponding changes in the fractional distribution of cardiac output. However, a change in the proportion of body mass did not always translate into a proportional change in cardiac output.

Five patterns emerged that illustrate the various relationships between changes in body composition and alterations in the fractional distribution of cardiac output in major organ systems of adult rats. The first is a pattern that occurred in most organ systems, such as the skeleton, brain and spinal cord, heart, kidneys, liver, and splanchnic tissues, and it is that neither the proportion of body mass (Table 3) nor the fraction of cardiac output received by these tissues (Table 5) is altered by age in adult animals. The second pattern illustrates changes that occurred in adipose tissue (Fig.1). The percent body mass of fat pads increased as a function of age, and there was a corresponding increase in the fraction of cardiac output going to adipose tissue. The third pattern, an inverse of the second, occurred in cutaneous tissue (Fig.2). The proportion of body mass made up of skin decreased with age, and the fraction of cardiac output directed to cutaneous tissue was lower in aged rats relative to their young adult peers. The fourth pattern reflected changes that occurred in skeletal muscle (Fig. 3). The proportion of body mass made up of skeletal muscle was lower with increasing age, but the relative proportion of cardiac output directed to skeletal muscle was not different between young and old adult animals. And fifth, in the male reproductive tissues, there was no change in the proportion of body mass made up by these tissues, but the fraction of cardiac output going to the reproductive tissues was lower in aged compared with adult rats (Fig. 4). Unlike the other organ systems, this change in the fractional distribution of cardiac output resulted from age-related changes in mass-specific tissue blood flow, i.e., a decrease in flow to the prostate gland.

Our hypothesis that tissue blood flows per unit mass are not altered by old age was based on the observation that age-induced changes in whole body oxygen consumption result from changes in body composition rather than from cellular metabolism (17, 39). Therefore, if cellular metabolic rate is unaltered, we postulated that tissue perfusion would be unaltered. This is based on the assumption that metabolic rate will be the primary determinant of tissue perfusion and that other factors that regulate organ perfusion will not be altered during normal aging. This is clearly not the case. For example, in skeletal muscle and other tissues, endothelium-derived relaxing factors are important determinants of resting perfusion rates (30). Aging has been shown to diminish endothelium-dependent relaxation in the abdominal aorta (8), mesenteric conduit and resistance arteries (12, 34), carotid arteries (24), and cerebral arterioles (32) of the rat, as well as in coronary (13) and forearm muscle (19) resistance arteries of humans. In addition, aging in rats has been shown to result in rarefaction (36), reduced arteriolar cross-sectional area and distensibility (22), and diminished vasodilation elicited by metabolites (6). In light of the seemingly global reduction in endothelium-mediated dilation, as well as other vascular alterations that would serve to increase vascular resistance, it was surprising that blood flow was reduced to so few tissues, i.e., the spleen, prostate gland, thyroid and parathyroid glands, and 2 of 41 muscle and muscle parts examined in the aged rats. The maintenance of blood flow through old age is perhaps attributable to the redundancy of vascular control mechanisms in tissues (30).

There are few other animals studies describing hemodynamic alterations induced by old age in conscious animals. For example, of the studies reporting regional blood flows in mature young and old rats (23, 33,38, 41) and dogs (20, 21), one-half used anesthetized animals (20, 23,41), and most measured blood flow to only a relatively few tissues. In the conscious rat, McDonald et al. (33) found that blood flow to the heart, kidneys, liver, pancreas, spleen, tibia, four muscle samples, and four fat deposits was not different between 12-mo-old and 24-mo-old male Fischer-344 rats. In addition, Tuma and co-workers (38) reported that flow to the heart, kidneys, liver, duodenum, stomach, lungs, brain, and five muscle samples (grouped as a single observation) was not different between 12-mo-old and 24-mo-old female Fischer-344 rats but that splenic blood flow was lower in aged animals. Therefore, the preponderance of evidence from conscious rats is that, despite vascular alterations that occur with advancing age (31), blood flow is well maintained in most tissues at rest. However, it is possible that these apparent age-related vascular “deficits” may not impact tissue perfusion when the animal is at rest but could adversely affect blood flow and cardiac output distribution during periods of altered metabolism, such as during exercise (40). In support of this hypothesis, Irion et al. (25) reported that skeletal muscle blood flow of anesthetized aged male rats was lower than that of young adult rats during stimulation-induced tetanic contractions.

We are unaware of any studies documenting the effect of gender differences on central and peripheral hemodynamics in association with body composition in aged rats. However, the results of the present study of conscious 24-mo-old male Fischer-344 rats and that of Tuma et al. (38) of conscious 24-mo-old female Fischer-344 rats afford an opportunity to make several comparisons. First, cardiac output in male rats is almost twice that of the females, although cardiac index is virtually identical. Mean blood flows to skeletal muscle and visceral tissues are also similar between genders. However, myocardial perfusion rates may be different. Mean ventricular flow in aged female rats is 57% higher than that in aged male rats. Support for this being a true gender difference comes from the observation that there is a similar blood flow pattern in active skeletal muscle of aged rats. Irion and co-workers (25) demonstrated that muscle blood flow elicited during tetanic contractions was lower in 24-mo-old male rats than in 12-mo-old male rats but that muscle blood flow in 24-mo-old female rats was similar to that of 12-mo-old females (26). In addition, questions regarding gender-specific differences in body composition remain unresolved. For example, we know that, in the aged male rat, the proportion of total body fat increases with age (Fig. 1), as does the percentage of visceral abdominal fat (juvenile: 1.5 ± 0.2%, adult: 2.1 ± 0.3%, aged: 3.9 ± 0.5%). This may be biologically significant because, in humans, men accumulate greater deposits of visceral abdominal fat than women do (14), and the level of visceral adipose tissue is an important correlate for several prevalent diseases, such as diabetes (28) and cardiovascular disease (10).

To our knowledge, the present study represents the most thorough description in the literature of the body composition and distribution of cardiac output among tissues in an immature and aged animal. Knowledge of total organ and/or tissue masses and their respective blood flows is necessary for a variety of biological modeling procedures that include tissue mass and perfusion as variables. For example, knowledge of tissue volumes and the respective fractional distribution of cardiac output is critical as input variables for pharmacokinetic models (2, 4). We have previously published these variables in young adult male Sprague-Dawley rats (9). However, the paucity of data for immature and aged rats make these models unreliable for estimating toxicity in these age groups. In addition, there appear to be animal strain differences that could be important for modelers. For example, in regard to body composition, young adult male Sprague-Dawley and Fischer-344 rats are similar in most respects. However, the percent body fat and bone are quite different between the two rat strains. The Fischer-344 rats have over twice as much adipose tissue, and the Sprague-Dawley rats have over twice as much bone. In our previous study of Sprague-Dawley rats (9), we did not determine the extent of muscle and other soft tissue on bone via chemical means. Therefore, on the basis of our experience in the present study, we believe the skeletal mass of the Sprague-Dawley rats was previously overestimated by 10–20%.

One technical concern when microspheres are used to measure regional distribution of blood flow is the possible hemodynamic disturbances that large numbers of microsphere can induce. For example, Stanek et al. (37) reported that the net infusion of 1.44 million microspheres did not alter cardiac output, blood pressure or total peripheral resistance, but it did result in a decrease in heart rate. These investigators also reported that the infusion of 0.72 million microspheres in a single dose resulted in transient reductions in cardiac output and heart rate that persisted 3–5 min. In designing the present study, we wanted to infuse a large number of microspheres into the rats to maximize our ability to get sufficient numbers of spheres into the small tissue samples but not so many as to induce those hemodynamic disturbances reported by Stanek et al. Therefore, we conducted a preliminary study to determine whether the infusion of 1.0 million spheres in juvenile rats and 1.5 million spheres in adult rats induced hemodynamic disturbances. We found that infusions of this number of microspheres did not alter heart rate, mean arterial pressure, cardiac output (Table 1), or tissue blood flows (data not shown). Although the cardiac output and heart rate results appear to be at odds with those of Stanek et al., there are several important differences between the two studies that may explain the discrepancies. First, the adult animals used by Stanek et al. were ∼100 g lighter than the adult animals in the present study. This is a potentially significant difference because body size is an important factor for limiting the total number of microspheres that can be infused. A second, and perhaps more important, difference is that Stanek et al. suspended their microspheres in a dextrose solution. Stanek et al. reported that infusion of dextrose solution without microspheres resulted in no hemodynamic disturbances. However, in our experience, we have found that microsphere suspensions containing dextrose result in feet and facial swelling in rats, and in some cases the animals temporarily collapse during infusion. Elimination of dextrose from the microsphere suspension has eradicated swelling and collapse. Flaim et al. (15) have also reported microsphere suspensions containing dextran to have adverse effects. Thus the hemodynamic disturbances reported by Stanek et al. (37) may be due to the combined effects of the microspheres and dextrose. Other factors may also contribute to the different cardiac output and heart rate responses between studies, such as the density of the microsphere suspension infused into the heart [2.5 million spheres/ml in present study vs. an unknown density in the study of Stanek et al. (37)] or perhaps the severity of surgical instrumentation (carotid and caudal catheters in present study vs. ascending aortic flow probe and carotid, femoral, and left atrial catheters in the study of Stanek et al.), which might affect the animals’ ability to maintain a stable cardiac output and heart rate. Regardless of the factor(s) responsible, the results of the present study indicate that the infusion of 1.0 million spheres in juvenile rats and 1.5 million spheres in adult rats does not disrupt heart rate, arterial pressure, cardiac output, or tissue blood flow.

In conclusion, the present study demonstrates that old age does not alter mass specific perfusion rates in most tissues. Blood flows to the spleen, prostate gland, thyroid and parathyroid glands, and the superficial gluteal and white portion of the rectus femoris muscles were, however, lower in aged rats. In addition, the proportion of body mass and the fraction of cardiac output going to most tissues does not change with old age. However, adipose and lung tissue made up a greater proportion of body mass in the old animals, whereas skeletal muscle and skin tissue accounted for a lower percent of body mass. Finally, old-age-related alterations in the proportional distribution of cardiac output occurred in adipose, cutaneous, and reproductive tissues. The age-related alterations in cardiac output distribution to adipose and cutaneous tissues appear to be associated with the increases in percent body fat and the decreases in the fraction of skin mass, whereas the decrease in the proportion of cardiac output directed to reproductive tissue of aged rats was unrelated to a change in total mass but resulted from a decrease in blood flow per unit mass.