Interstitial Fluid Colloid Osmotic Pressure Research Articles

Interstitial Fluid Colloid Osmotic Pressure in Healthy Children

ObjectiveThe colloid osmotic pressure (COP) of plasma and interstitial fluid play important roles in transvascular fluid exchange. COP values for monitoring fluid balance in healthy and sick children have not been established. This study set out to determine reference values of COP in healthy children.Materials and MethodsCOP in plasma and interstitial fluid harvested from nylon wicks was measured in 99 healthy children from 2 to 10 years of age. Nylon wicks were implanted subcutaneously in arm and leg while patients were sedated and intubated during a minor surgical procedure. COP was analyzed in a colloid osmometer designed for small fluid samples.ResultsThe mean plasma COP in all children was 25.6 ± 3.3 mmHg. Arbitrary division of children in four different age groups, showed no significant difference in plasma or interstitial fluid COP values for patients less than 8 years, whereas patients of 8-10 years had significant higher COP both in plasma and interstitial fluid. There were no gender difference or correlation between COP in interstitial fluid sampled from arm and leg and no significant effect on interstitial COP of gravity. Prolonged implantation time did not affect interstitial COP.ConclusionPlasma and interstitial COP in healthy children are comparable to adults and COP seems to increase with age in children. Knowledge of the interaction between colloid osmotic forces can be helpful in diseases associated with fluid imbalance and may be crucial in deciding different fluid treatment options.Trial RegistrationClinicalTrials.gov NCT01044641

PO-0027 Interstitial Fluid Colloid Osmotic Pressure In Healthy Children

Background and aims Colloid osmotic pressure (COP) of plasma and interstitial fluid play important roles in transvascular fluid exchange were small pressure alterations may result in fluid shifts into or out of the capillaries optimising homeostasis. This study was conducted to determine reference values of COP in healthy children, and to evaluate the methodology of harvesting interstitial fluid. Methods COP in plasma and interstitial fluid isolated from nylon wicks implanted subcutaneously was measured in 99 healthy children from 2 to 10 years of age. Patients were sedated and intubated during wick implantation in arm and leg, and COP was analysed in a colloid osmometer. Results Mean plasma COP in all children was 25.6 ± 3.3 mmHg. Arbitrary division of children in age groups, showed no significant difference in plasma or interstitial fluid COP values for patients less than 8 years, whereas patients of 8–10 years had significant higher COP both in plasma and interstitial fluid. There was no gender difference or correlation between COP in interstitial fluid sampled from arm and leg and no significant effect on interstitial COP of gravity. Prolonged implantation time did not affect interstitial COP. Conclusions This study justifies the presumption that plasma and interstitial COP in healthy children are similar to adults. Children between 8 and 10 years had higher values than younger age groups. Knowledge of interaction between colloid osmotic forces in health and disease can be helpful in diseases associated with fluid imbalance and may be crucial in deciding different fluid treatment options.

Isolation of interstitial fluid and demonstration of local proinflammatory cytokine production and increased absorptive gradient in chronic peritoneal dialysis

In peritoneal dialysis (PD) patients, the frequent exposure to "unphysiological" dialysis fluids elicits a chronic state of a low-grade peritoneal inflammation leading to interstitial matrix remodeling and angiogenesis. Proinflammatory cytokines are important regulators involved in this inflammatory process that ultimately leads to dysfunction of the peritoneum as a dialysis membrane. We aimed to measure the local concentrations of proinflammatory cytokines in the peritoneal interstitial fluid (IF). Furthermore, we wanted to assess how the driving forces for fluid and solute exchanges are affected in a remodeled interstitial matrix and thus measured the colloid osmotic pressure (COP) gradient in rats that were exposed to chronic PD. After 8 wk of peritoneal dialysis, IF from peritoneum was isolated using a centrifugation method, and was analyzed for cytokine content and COP along with plasma. For several of the proinflammatory cytokines there were gradients from IF to plasma, showing local production. For some cytokines, the concentration in IF was increased severalfold, whereas IL-18 was increased systemically due to PD. Furthermore, the presence of the catheter per se seemed to increase cytokine levels. COP in IF was significantly decreased in the PD group, while collagen and hyaluronan content was increased. Collectively, our data suggest that the increased levels of proinflammatory cytokines after PD may be an integral component of the development of fibrosis and angiogenesis commonly seen in PD patients, and the decreased COP in IF after chronic PD may shift the Starling equilibrium across peritoneal capillaries to an absorptive state.

Lowering of interstitial fluid pressure in rat submandibular gland: a novel mechanism in saliva secretion

The submandibular gland transports fluid at a high rate through the interstitial space during salivation, but the exact level of all forces governing transcapillary fluid transport has not been established. In this study, our aim was to measure the relation between interstitial fluid volume (V(i)) and interstitial fluid pressure (P(if)) in salivary glands during active secretion and after systemically induced passive changes in gland hydration. We tested whether interstitial fluid could be isolated by tissue centrifugation to enable measurement of interstitial fluid colloid osmotic pressure. During control conditions, V(i) averaged 0.23 ml/g wet wt (SD 0.014), with a corresponding mean P(if) measured with micropipettes of 3.0 mmHg (SD 1.3). After induction of secretion by pilocarpine, P(if) dropped by 3.8 mmHg (SD 1.5) whereas V(i) was unchanged. During dehydration and overhydration of up to 20% increase of V(i) above control, a linear relation was found between volume and pressure, resulting in a compliance (DeltaV(i)/DeltaP(if)) of 0.012 ml.g wet wt(-1).mmHg(-1). Interstitial fluid was isolated, and interstitial fluid colloid osmotic pressure averaged 10.4 mmHg (SD 1.2), which is 64% of the corresponding level in plasma. We conclude that P(if) drops during secretion and, thereby, increases the net transcapillary pressure gradient, a condition that favors fluid filtration and increases the amount of fluid available for secretion. The reduction in P(if) is most likely induced by contraction of myoepithelial cells and suggests an active and new role for these cells in salivary secretion. The relatively low interstitial compliance of the organ will enhance the effect of the myoepithelial cells on P(if) during reduced V(i).

Protein washdown as a defense mechanism against myocardial edema.

Myocardial edema occurs in many pathological conditions. We hypothesized that protein washdown at the myocardial microvascular exchange barrier would change the distribution of interstitial proteins from large to small molecules and diminish the effect of washdown on the colloid osmotic pressure (COP) of interstitial fluid and lymph. Dogs were instrumented with coronary sinus balloon-tipped catheters and myocardial lymphatic cannulas to manipulate myocardial lymph flow and to collect lymph. Myocardial venous pressure was elevated by balloon inflation to increase transmicrovascular fluid flux and myocardial lymph flow. COP of lymph was measured directly and was also calculated from protein concentration. Decreases occurred in both protein concentration and COP of lymph. The proportion of lymph protein accounted for by albumin increased significantly, whereas that accounted for by beta-lipoprotein decreased significantly. The change in the calculated plasma-to-lymph COP gradient was significantly greater than the change in the measured COP gradient. We conclude that the change in the distribution of interstitial fluid protein species decreases the effect of protein washdown on interstitial fluid COP and limits its effectiveness as a defense mechanism against myocardial edema formation.

Transcapillary fluid shifts in tissues of the head and neck during and after simulated microgravity

To understand the mechanism, magnitude, and time course of facial puffiness that occurs in microgravity, seven male subjects were tilted 6 degrees head-down for 8 h, and all four Starling transcapillary pressures were directly measured before, during, and after tilt. Head-down tilt (HDT) caused facial edema and a significant elevation of microvascular pressures measured in the lower lip: capillary pressures increased from 27.7 +/- 1.5 mmHg (mean +/- SE) pre-HDT to 33.9 +/- 1.7 mmHg by the end of tilt. Subcutaneous and intramuscular interstitial fluid pressures in the neck also increased as a result of HDT, whereas interstitial fluid colloid osmotic pressures remained unchanged. Plasma colloid osmotic pressure dropped significantly by 4 h of HDT (21.5 +/- 1.5 mmHg pre-HDT to 18.2 +/- 1.9 mmHg), suggesting a transition from fluid filtration to absorption in capillary beds between the heart and feet during HDT. After 4 h of seated recovery from HDT, microvascular pressures in the lip (capillary and venule pressures) remained significantly elevated by 5-8 mmHg above baseline values. During HDT, urine output was 126.5 ml/h compared with 46.7 ml/h during the control baseline period. These results suggest that facial edema resulting from HDT is caused primarily by elevated capillary pressures and decreased plasma colloid osmotic pressures. The negativity of interstitial fluid pressures above heart level also has implications for maintenance of tissue fluid balance in upright posture.

Transcapillary forces in muscle compartments of lower limbs with deep venous thrombosis

Interstitial fluid pressure (Pif), plasma- and interstitial fluid colloid osmotic pressures (COPpl, COPif) were measured in muscle compartments of the calf in lower limbs of patients with deep venous thrombosis (DVT) and in contralateral limbs. Pif was measured with the 'wick-in-needle' technique in the anterior and deep posterior compartments in three periods of time following start of treatment for DVT. The first period was from 0-24 h, the second from 24-48 h and the third later than 48 h. We found a significant increase in both muscular compartments in legs with DVT compared to the contralateral in the three periods except for Pif in the anterior compartment in the third period. A significant reduction of Pif in both muscular compartments of limbs with DVT was found when comparing the first and second periods but not from the second to the third period. No difference was found in Pif in the muscular compartments of the contralateral limbs between the three periods. Interstitial fluid for COPif measurements was obtained by inserting intravenous cannulas in the deep posterior muscle compartment of the calf of limbs with DVT and contralateral limbs. In oedematous tissue it was possible to collect some fluid, but this was almost impossible in the non-oedematous tissues of the contralateral limbs. COPif was 4.2(2.4) mmHg (mean(SD)) in the deep posterior compartment of limbs with DVT, which is probably significantly reduced. Increased Pif and reduced COPif indicate increased transcapillary fluid filtration and reduced reabsorption as the main cause of oedema in muscular compartments in DVT of the lower limbs.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms behind increased dermal imbibition pressure in acute burn edema.

We have measured tissue pressures in excised rat skin subjected to in vitro burn injury and investigated the mechanisms behind the increased imbibition (swelling) pressure in burned skin. Skin pieces wrapped in aluminum paper were immersed into boiling hot water for 10, 30, or 60 s. Dermal imbibition pressure was measured with micropipettes and tissue osmometry as interstitial fluid hydrostatic pressure (Pif) and/or interstitial fluid colloid osmotic pressure (COPif). COPif was also measured in interstitial fluid sampled with intradermal wicks. Control values of Pif (micropipettes) and of COPif (wick fluid) averaged -1.5 mmHg and -17.5 mmHg, respectively. An increase in imbibition pressure was seen after thermal injury. After 10 s of heat exposure, the imbibition pressure gain was mainly due to a strongly negative hydrostatic pressure (Pif mean value -33.3 mmHg). Pif became slightly positive and COPif increasingly negative after longer exposure (mean Pif 0.3 and mean COPif -133 mmHg after 60-s exposure). Collagen degradation and water solubility increased with extension of the heat exposure time. Thermal degradation of collagen seems to be the main mechanism responsible for the generation of increased imbibition pressure.

The influence of thoracic epidural analgesia on transcapillary fluid balance in subcutaneous tissue

Clinical observation and calculation of fluid balance have shown that patients undergoing aortocoronary bypass surgery with thoracic epidural analgesia (TEA) in addition to general anesthesia retain less fluid than patients having general anesthesia only. The present study was designed to investigate whether this effect could be explained by thoracic epidural analgesia influencing the transcapillary fluid balance, i.e. the transcapillary forces (COPpl, COPif, Pif). Interstitial fluid colloid osmotic pressure (COPif) and interstitial fluid pressure (Pif) were measured subcutaneously at heart level by the blister suction technique and the wick-in-needle technique, respectively. Simultaneously plasma colloid osmotic pressure (COPpl) was recorded. Sixteen male patients were allocated to two groups, one having general anesthesia only (controls, n = 8). The other group (TEA, n = 8) received, at the induction of anesthesia, bupivacaine 50 mg via an epidural catheter as an adjunct to general anesthesia. TEA was maintained by continuous infusion for 24 h postoperatively. Preoperatively no intergroup differences were observed in "the Starling forces" (COPpl, COPif, Pif). At the start of extracorporeal circulation COPpl was significantly lower in the TEA-group than in controls. During extracorporeal circulation the transcapillary COP-gradient (COPpl-COPif) was reversed in both groups. At the end of extracorporeal circulation Pif increased to a minor degree in the TEA-group and remained significantly lower than in controls from 3 to 24 h postoperatively. The subcutaneous interstitial tissue could be less expanded postoperatively in the TEA-group, also reflected by a lower increase in Pif.

Time-related changes in the Starling forces following extracorporeal circulation.

The intra- and postoperative variations of the transcapillary forces [colloid osmotic pressure of plasma (COPpl), colloid osmotic pressure of interstitial fluid (COPif), average hydrostatic pressure in the interstitium (Pif)] were studied in the subcutaneous tissue as a function of time in 13 patients operated on for coronary artery disease using extra-corporeal circulation (ECC). The measurements were performed before operation, during ECC, and during the first 24 hours postoperatively. COPif was measured subcutaneously on the chest both by the wick method and by a noninvasive blister suction method. The latter technique allowed several consecutive measurements in the same individual during the postoperative period. Pif was measured by "wick-in-needle" technique in the same area as the COPif measurements. COPpl was measured in a blood sample collected from a cubital vein. COPpl was reduced about 50% during ECC returned to pre-ECC level within the first 6 hours postoperatively. During ECC COPif was higher than COPpl, reaching its minimum level 4 to 5 hours postoperatively. Measurements performed following ECC showed return of the transcapillary COP-gradient to the normal direction (COPpl greater than COPif). Pre-ECC level of COPif was not entirely obtained during the first postoperative day. Pif increased gradually during ECC and continued to increase the first 2 to 3 hours following ECC. Pre-ECC level was reached within 24 hours postoperatively. The present investigation has demonstrated major dynamic variations in the transcapillary forces in patients undergoing open heart surgery with ECC. There was an increased net capillary filtration (F) intraoperatively predisposing to interstitial edema formation in subcutaneous tissue until several hours following the termination of ECC.

Subcutaneous interstitial fluid colloid osmotic pressure in dehydrated rats.

We collected interstitial fluid for measurement of colloid osmotic pressure (COPi) of normally hydrated and dehydrated rats with two methods: (1) saline-soaked nylon-wicks implanted in vivo for 1 h; and (2) the crossover method, i.e. titration with wicks pre-loaded with serial dilutions of rat plasma implanted post mortem for 15-20 min. All wicks were implanted in subcutis on the back. Dehydration was induced by injection of 40 mg furosemide followed by 24 h water deprivation. The saline-soaked wick method gave an average control COPi of 10.8 mmHg, 2-3 mmHg lower than the corresponding pressure obtained with the crossover method. This discrepancy between the methods increased to 10-11 mmHg in dehydrated animals, indicating that the saline-soaked wick method underestimates COPi even more during dehydration than during control conditions. The mean COPi of 22.8 mmHg (SD = 3.7, n = 9) obtained in dehydrated rats using the crossover method corresponds well with what is to be expected from the reduction in interstitial fluid volume. The present study suggests that the frequently used implantation period for saline-soaked wicks of 60 min should be prolonged to 90-120 min. It further shows that the rise in COPi is about 3 times more important than the fall in interstitial fluid hydrostatic pressure in restricting mobilization of interstitial fluid during dehydration.

Transcapillary forces of the subcutaneous tissue in patients with coronary artery disease: a comparison between pulsatile and nonpulsatile flow during extracorporeal circulation

The aim of this study was to evaluate subcutaneous transcapillary fluid balance during pulsatile and nonpulsatile extracorporeal circulation (ECC). Changes in the transcapillary fluid balance were studied by measuring subcutaneous interstitial fluid pressure (Pif) using the 'wick in needle' (WIN) method, as well as measuring subcutaneous interstitial fluid colloid osmotic pressure (COPif), using either the wick technique or the blister suction technique. The measurements were performed on the chest wall at the heart level. Simultaneous recordings of plasma colloid osmotic pressure (COPpl) were carried out. Nineteen male patients undergoing aortocoronary bypass grafting were subjected to nonpulsatile flow (group I, n = 11) or pulsatile flow (group II, n = 8) during ECC. Preoperatively there was no difference in the Starling forces between the two groups. During ECC the COP gradient (COPpl-COP if) was reversed to the same extent in the two groups. At three hours and six hours following ECC, COPif in the pulsatile group (12.1 mmHg and 11.4mmHg respectively) was significantly higher than in the nonpulsatile group (10.8mmHg and 10.3mmHg respectively). When weaning from ECC as well as three hours and six hours following ECC, Pif in the pulsatile group was significantly lower than in the nonpulsatile group. Conclusion: in the early postoperative period there is less dilution of the subcutaneous interstitial tissue following application of pulsatile flow during ECC compared to nonpulsatile flow.

Interstitial fluid colloid osmotic pressure of the subcutaneous tissue in controls and patients before and after open-heart surgery: A comparison between the wick technique and the blister suction technique

Interstitial fluid colloid osmotic pressure of the subcutaneous tissue in controls and patients before and after open-heart surgery: A comparison between the wick technique and the blister suction technique

Microvascular exchange and interstitial volume regulation in the rat: model validation.

A dynamic mathematical model is formulated and used to describe the distribution and transport of fluid and plasma proteins between the circulation, interstitial space of skin and muscle, and the lymphatics in the rat. Two descriptions of transcapillary exchange are investigated: a homoporous "Starling model" and a heteroporous "plasma leak model." Parameters used in the two hypothetical transport mechanisms are determined based on statistical fitting procedures between simulation predictions and selected experimental data. These data consist of interstitial fluid volume and colloid osmotic pressure measurements as a function of venous pressure for muscle and interstitial colloid osmotic pressure vs. venous pressure for skin. The values determined for the transport parameters compare well with data in the literature. The fully determined model is used to simulate steady-state conditions of hypoproteinemia, overhydration, and dehydration, as well as the dynamic response to changes in venous pressure and intravascularly administered protein tracers. Comparisons between the simulation predictions and experimental data for these various perturbations are made. The plasma leak model appears to provide a better description of microvascular exchange.

Calculated capillary hydrostatic pressure in normal pregnancy and preeclampsia

Capillary hydrostatic pressure has been calculated in normal pregnancy and preeclampsia. In humans, capillary hydrostatic pressure cannot be measured directly but may be calculated when the colloid osmotic pressure in plasma and interstitial fluid and interstitial fluid hydrostatic pressure are known (Starling equation). New methods have made it possible to measure the interstitial fluid colloid osmotic pressure and interstitial fluid hydrostatic pressure. In the present study interstitial fluid was collected from the subcutaneous tissue by implanted wicks (wick method), and interstitial fluid colloid osmotic pressure was determined. Interstitial fluid hydrostatic pressure was recorded by the wick-in-needle technique. Capillary hydrostatic pressure was calculated in 10 women in the first trimester and 10 in the third trimester of normal pregnancy, in 15 patients with mild preeclampsia, and in 13 women with severe preeclampsia. In normal pregnancy, capillary hydrostatic pressure increased by about 30% between the first and third trimesters. In mild preeclampsia, capillary hydrostatic pressure values did not differ significantly from those in the third trimester of normal pregnancy. However, in severe preeclampsia capillary hydrostatic pressure was significantly lower (40%) than in mild preeclampsia. Whether the low capillary hydrostatic pressure is caused by the severe general vasospasm seen in this condition or is a secondary event is unknown.

Transcapillary colloid osmotic gradient and body fluid volumes in renal failure

The aim was to study the transcapillary fluid balance in dialysis patients during and after ultrafiltration. Plasma and subcutaneous interstitial fluid (wick technique) colloid osmotic pressure, plasma volume (I125-albumin space) and extracellular fluid volume (radiosulfate space) were measured in nine patients on maintenance hemodialysis before (pre-dialysis state) and after (dry-weight state) ultrafiltration. In the pre-dialysis state, interstitial colloid osmotic pressure was reduced compared to normal controls (12.7 +/- 3.5 versus 15.8 +/- 2.7 mmHg, mean values +/- SD) and transcapillary colloid osmotic gradient increased (15.3 +/- 3.0 versus 12.8 +/- 2.7 mmHg). Ultrafiltration resulted in a parallel decrease of plasma volume and interstitial fluid volume of 19 to 20%, and an increase in mean interstitial colloid osmotic pressure of 3.4 mmHg and in mean transcapillary colloid osmotic gradient of 1.9 mmHg. The mean ultrafiltration rate was 21.9 +/- 1.9 ml/min and the plasma refilling rate was 16.5 +/- 2.7 ml/min. It is concluded that the changes in plasma and interstitial fluid colloid osmotic pressure tend to preserve plasma volume and limit the interdialytic increase in interstitial fluid volume.

Transcapillary forces in subcutaneous tissue of lower limbs with deep venous thrombosis.

Starling forces (interstitial fluid pressure, plasma- and interstitial fluid colloid osmotic pressures) were measured in the subcutaneous tissue of legs in 22 patients with deep venous thrombosis (DVT). For comparison, measurements were obtained in the contralateral leg and in healthy controls as well. Interstitial fluid pressure (Pif) was measured with the 'wick-in-needle' technique and we found a significantly higher pressure in limbs with DVT compared to contralateral limbs and controls. Interstitial fluid was collected from subcutaneous tissue with the 'wick technique'. Colloid osmotic pressure was measured in wick interstitial fluid (COPif) and plasma (COPpl) with an osmometer. A significant reduction of COPif was found in limbs with DVT compared to the contralateral side and controls. The COPpl was also significantly reduced in patients compared to controls. Saphenous vein pressure (Pv) measured at the ankle was significantly increased in the thrombosed limb compared to the contralateral. We conclude that the main cause of subcutaneous oedema in DVT of the lower limbs is increased transcapillary filtration caused by a rise in Pv. This leads to a fall in COPif and rise in Pif which act as oedema-preventing mechanisms.

Transcapillary forces in normal pregnant women.

Plasma and interstitial fluid colloid osmotic pressure (COP) and interstitial fluid hydrostatic pressure (Pi) were measured in 10 normal pregnant women in first and third trimester. Interstitial fluid COP was measured by a "wick" technique and Pi by a "wick-in-needle" technique. The COP was significantly lower both in plasma and interstitial fluid in the third trimester compared to that in the first trimester. However, the fall in COP was more marked in the interstitial fluid than in plasma. Only small changes in Pi were observed. Our results support the view that reduction of COP in interstitial fluid is a major local edema-preventing mechanism. This compensates for the increased capillary pressure and the hypoproteinemia in late pregnancy. The relatively low interstitial fluid COP in third trimester women without edema implies a reduced safety margin against edema formation and may explain why 40-80% of normal pregnant women experience edema.

Interstitial fluid volume, plasma volume and colloid osmotic pressure in patients with nephrotic syndrome

Colloid osmotic pressure in plasma (IIp) and in subcutaneous interstitial fluid (IIi) (wick technique), plasma volume (PV) and interstitial fluid volume (IFV) were measured in nephrotic patients (n = 11) and in healthy controls. Six of the patients were treated with ultrafiltration and the parameters were measured before and after withdrawal of mean 7460 ml (total ultrafiltrate in two sessions). In the nephrotic patients mean IIp was 11.6 mmHg and IIi was 3.9 mmHg compared to 28.6 mmHg and 15.8 mmHg, respectively, in healthy controls. PV was in the normal-to-high range and IFV was increased to 150% of the valued in controls. No statistically significant change in PV, IIp or IIi was found after ultrafiltration, and IFV was reduced by 20%. The results indicate that reduction in IIi is an important oedema-preventing factor and that the reduction in interstitial protein mass is more than proportional compared to the degree of hypoproteinaemia. Ultrafiltration in the rate and magnitude we have used, leads to reduction in IFV, but small changes in PV, IIp or IIi.

Fluid shifts and muscle function in humans during acute simulated weightlessness.

Head-down tilt is considered an effective experimental model to simulate weightlessness. To determine the acute effects of simulated weightlessness on transcapillary fluid balance, tissue fluid shifts, muscle function, and triceps surae reflex time, eight supine subjects were tilted 5 degrees head down for 8 h. A cephalic fluid shift from the legs was indicated by facial edema, nasal congestion, increased urine flow, decreased creatinine excretion, reduced calf girth, and decreased lower leg volume. As measured by wick catheters inserted under local anesthesia, interstitial fluid pressure in the tibialis anterior muscle (4.6 +/- 0.6 mmHg) and subcutaneous tissue (0.6 +/- 0.5 mmHg) of the lower leg fell significantly to -2.8 +/- 0.5 and -3.8 +/- 0.4 mmHg, respectively. Other transcapillary pressures (capillary and interstitial fluid colloid osmotic pressures) were relatively unchanged. Needle-biopsy specimens, obtained just before and after tilt, indicated that total water content of soleus muscle was unchanged during 8 h of head-down tilt. After head-down tilt, isometric strength and isokinetic strength of the plantar flexors were unchanged. Triceps surae reflex time associated with plantar flexion movement slowed slightly after the tilt maneuver. Collectively these results demonstrated a dehydration effect of head-down tilt on muscle and subcutaneous tissues of the lower leg that may affect muscle function.