Model of interstitial pressure as a result of cyclical changes in the capillary wall fluid transport.

Reported interstitial pressures range from -8 to +6 mm Hg in different tissues and from <-20 mm Hg in burned tissue or more than +30 mm Hg in tumors. We have tried to link interstitial pressure to the here proposed cyclical changes in the fluid transport across the capillary wall.In the presented model interstitial pressure is considered as an average of pressures in numerous pericapillary spaces. A single pericapillary pressure is a dynamic difference between the net outward (hydraulic pressure+interstitial colloid osmotic pressure) and inward (plasma colloid oncotic pressure) forces. Hence, dominating net outward forces would result in a positive pericapillary interstitial pressure, while stronger inward forces would produce negative pressures in the pericapillary space. All interruptions of blood flow leave some blood in capillaries with a normal oncotic pressure and no hydrostatic pressure that might act as a strong absorber of interstitial fluid until the blood flow is reestablished.Model assumptions for the systemic circulation capillaries include (a) precapillary sphincters can almost entirely stop the capillary flow, (b) only a minority of sphincters are normally open in the tissue, and (c) hydrostatic pressures in unperfused capillaries are similar to the pressures at their venous ends.The key proposal is that capillaries with closed precapillary sphincters along their entire length have low hydrostatic pressure of 10 to 15 mm Hg. This pressure cannot force filtration, so these capillaries reabsorb interstitial fluid from the pericapillary space along their entire length. In the open capillaries, hydrostatic pressure filtrates fluid to the pericapillary space along most of their length. Fluid enters, moves some 20 or 30 micrometers away and back to be reabsorbed at the same point. Closed periods are periods of intense fluid reabsorption, while the short open periods refill the space with fresh fluid. It can be calculated that subcutaneous tissue interstitial pressure values might develop if the closed periods are 1.14 to 2.66 times longer than the open periods. Positive interstitial pressures observed in some organs might develop if open periods are longer than the closed periods.High interstitial colloid pressure in lungs makes both perfused and unperfused capillaries absorptive, resulting in more negative values of lung interstitial pressure. The same model is used to explain interstitial pressure values in tumors, burned tissue and intestinal villi.     

一种针刺补泻法对小型猪皮下组织液压的影响

目的研究《灵枢》卷首《九针十二原》针刺补泻法对小型猪皮下组织液压(interstitial fluid pressure,IFP)的作用,探讨其调节组织液的生物力学机制。方法在9只健康实验用小型猪腹部随机取点行补法(提/按法)和泻法(摇大针孔),观察正常状态(normal state,NS)、抽取组织液的低组织液量(low volume,LV)状态和注入生理盐水的高组织液量(high volume,HV)状态下针刺前后IFP。结果 NS下提/按法可极显著升高IFP;泻法可极显著降低IFP,针后5 min泻法组IFP降低较快。LV状态提/按法均能升高IFP,针后10 min二者IFP下降较慢。HV状态下泻法可极显著降低IFP,针后5 min与对照组变化趋势不同。结论该补泻法可升高或降低IFP,证明其对IFP有相反方向的调节作用。研究结果为针灸临床使用补泻手法提供新的科学依据。     

组织液压波在大白鼠胃经皮下组织传送规律的研究

注射０．９％的生理盐水到麻醉的大白鼠胃经的皮下，形成一个水泡，然后给予一个机械压力，由此产生一个组织液压波，测量组织液压在胃经上和胃经外的变化，发现了五种基本的组织液型及亚型，这些波型基于组织不同的粘弹性特性，组织液压波在经线上与经线外幅度差异与距离的线性回归方程为Ｙ＝－０．３５Ｘ＋１．９６，具有统计学显著性（Ｐ＜０．０１）。表明经脉线组织可能具有较好的渗透性和柔性。     

Interstitial colloid osmotic and hydrostatic pressures in human subcutaneous tissue during early stages of heart failure

Subcutaneous oedema is a common finding in heart failure. However, some patients have reduced cardiac pump function without oedema. The aim of this study was to investigate whether local mechanisms in subcutaneous tissue contribute to oedema prevention. A reduction in interstitial colloid osmotic pressure (pii) and a rise in interstitial fluid hydrostatic pressure (Pi) will both counteract a rise in capillary filtration caused by heart failure. Cardiac catheterization was done in 22 angina pectoris patients without visible oedema. Two days later pii was measured with a wick method and Pi was measured with a wick-in-needle method. Both parameters were measured in subcutaneous tissue on thorax at heart level and on the ankle. Plasma volume was determined by 125I-albumin and extracellular volume measured with 35SO4. Parameters of cardiac pump function ranged from normal to clearly pathological values. Mean pii was 13.0 mmHg on thorax and 8.3 mmHg on the ankle. Pi averaged -2.1 mmHg on thorax and -1.5 mmHg on the ankle. Statistically significant (P less than 0.05) correlations were found between pii on thorax and left ventricular end diastolic pressure (rs -0.40) and pii on thorax and cardiac index (rs 0.42). Pi was positively correlated to right atrial pressure (rs 0.50). Body fluid volumes were normal or moderately reduced. The study shows that a reduction in cardiac pump function is associated with a reduction in pii and a rise in Pi. These changes may help to prevent oedema formation in the early stages of heart failure.     

Transcapillary fluid movement in human calf after drinking hypertonic saline

1. Transcapillary absorption of interstitial fluid was demonstrated with a pressure plethysmograph applied to the human calf after the ingestion of 200 ml. hypertonic (5.1%) saline. Capillary absorption began within 15 min after ingestion and lasted for about 2 hr. The maximum rate of absorption (0.019 ml./min. 100 ml. tissue) was attained 30-75 min after ingestion.2. The total amount of fluid absorbed into capillary blood vessels in the calf was 1.11 ml./100 ml. tissue. The amount of fluid thus absorbed in the whole body was estimated to be 677 ml.3. The capillary filtration coefficient (CFC) of the calf was also measured by the pressure plethysmograph. This was 0.0038 ml./min. mm Hg. 100 ml. tissue.4. The peak value of capillary absorption pressure was 5.2 mm Hg.5. The total osmotic pressure of the plasma rose by 12.6 m-osmole/kg H(2)O after ingestion. This rise was accompanied by transcapillary fluid absorption.6. The plasma protein concentration and packed cell volume were almost unchanged by ingestion, indicating that the plasma volume was unaltered.7. It was estimated that the net shift of fluid between intracellular and interstitial compartments during the period of transcapillary fluid absorption was very small.8. It is concluded that the volume of fluid moving from plasma into intestinal lumen is the same as that flowing from interstitial fluid into plasma, and that the transcapillary absorption is caused by a difference in osmotic pressure between the plasma and the interstitial fluid.     

Interstitial fluid pressure in the isolated perfused rabbit lung

WIIG, H., OPDAHL, H., NICOLAYSEN, A. & NICOLAYSEN, G. 1985. Interstitial fluid pressure in the isolated perfused rabbit lung. Acta Physiol Scand 125 , 601–607. Received 10 March 1985, accepted 9 May 1985. ISSN 0001–6772. Department of Physiology, University of Bergen and Institute of Physiology, University of Oslo, Norway. Interstitial fluid pressure was measured in nine isolated perfused rabbit lungs with the servonull micropipette method. Bevelled glass micropipettes, with tip diameter 2–6 μm (o.d.) were inserted 2–6 mm into the left lung. At alveolar pressures of 3 to 5 cm H₂O we found mean interstitial fluid pressures of 1.0 (SD 1.0) and 1.6 (SD 1.0) cm H₂O relative to pleural pressure in the upper (n = 19) and lower (n = 21) lobes respectively. The vertical distance between the measuring sites in the upper and lower lobes was about 3 cm. Net filtration caused by elevated left atrial pressure caused practically no change in interstitial fluid pressure. Increased alveolar pressure either increased or decreased interstitial fluid pressure. The measured pressures probably represent interstitial fluid pressure in alveolar junctions or in the interstitium around small pulmonary arteries or veins. We conclude that interstitial fluid pressure in these sites is between alveolar and pleural pressure, and that it is only moderately affected by changes in alveolar pressure. The interstitial compliance appears to be high and there seem to be little or no vertical gradients in interstitial fluid pressure within the lung.     

Measurement of interstitial fluid pressure: Comparison of methods

Interstitial fluid pressure (IFP), i.e., the pressure in a saline-filled tube brought into contact with the interstitium, has been measured in cats with two “acute” [micropipettes and wick-in-needle (WIN)] and two chronic (perforated and porous capsules) methods. In a control situation, similar pressures of −1.5 and −1.6 mm Hg were recorded in skin with micropipettes and both types of capsules, respectively, while WIN pressure in subcutis was −1.2 mm Hg. IFP in skeletal muscle was −0.5, −0.5, and −1.1 mm Hg as recorded with micropipettes, WIN, and porous capsules, respectively. During infusion of Ringer's solution, pressures in both types of capsules rose by 4 to 6 mm Hg, while pressure recorded with the acute methods increased by 1 to 1.5 mm Hg only. Two hours after infusion all techniques gave similar pressures. Peritoneal dialysis for 2 hours reduced micropipette and WIN pressures by 3 to 4 mm Hg. Pressure in perforated capsules fell by 10 mm Hg during dialysis and remained low for an additional 2 hours, while porous capsule pressure fell by 7 mm Hg during dialysis but thereafter increased and reached the pressure recorded with micropipettes and WIN 2 hours after ended dialysis. In both overhydration and dehydration, capsules probably react to changes in interstitial fluid colloid osmotic pressure; in overhydration the capsules react also to changes in capillary pressure, resulting in the discrepancy between chronic and acute methods during non-steady-state conditions. In conclusion, acute and chronic methods record similar pressures during steady-state conditions, but the chronic methods are sensitive to changes in vascular pressure and interstitial fluid colloid osmotic pressure and are therefore not suitable for measuring the changes that occur in IFP within a few hours.     

Changes in interstitial pressure during and after blood volume expansion in rats

Summary Interstitial fluid pressure was measured via a chronically implanted capsule before, during and after acute isotonic, iso-oncotic blood volume expansion in normal or in 48-h dehydrated rats. At the same time, the patterns of body fluid distribution, of selected renal responses and of mean arterial and mean central venous pressure responses were studied. Dry tissue weight (DTW) was subsequently determined by freeze drying of the shaved carcass.Dehydration decreased plasma volume and interstitial fluid volume significantly below normal values. The initial intracapsular pressure in dehydrated animals (–3.7±0.6 mm Hg) was not significantly different from that in normal rats (–2.5±0.5), but dehydrated rats showed initially a very significantly lower effective interstitial compliance (0.0005 ml/mm Hg per gram DTW) than did the normal group (0.0704).In the course of the renal response to the volume load, effective interstitial complicance increased to 0.0350 in dehydrated rats but showed no change in normal rats. Neither group completely corrected its elevated blood volume; both returned their central venous pressures to pre-infusion levels; both decreased their interstitial fluid volumes below preinfusion levels and both decreased their intracapsular fluid pressures 1 mm Hg below the level prevailing in non-infused animals at that time.It is concluded that a reduction in interstitial, hydrostatic pressure can be a functionally important influence in the apparent control of central venous pressure following acute blood volume expansion.This study was supported by the Canadian Heart Foundation.     

Oncotic pressures opposing filtration across non-fenestrated rat microvessels

We hypothesized that ultrafiltrate crossing the luminal endothelial glycocalyx through infrequent discontinuities (gaps) in the tight junction (TJ) strand of endothelial clefts reduces albumin diffusive flux from tissue into the 'protected region' of the cleft on the luminal side of the TJ. Thus, the effective oncotic pressure difference (σπ) opposing filtration is greater than that measured between lumen and interstitial fluid. To test this we measured σπ across rat mesenteric microvessels perfused with albumin (50 mg ml⁻¹) with and without interstitial albumin at the same concentration within a few micrometres of the endothelium as demonstrated by confocal microscopy. We found σπ was near 70% of luminal oncotic pressure when the tissue concentration equalled that in the lumen. We determined size and frequency of TJ strand gaps in endothelial clefts using serial section electron microscopy. We found nine gaps in the reconstructed clefts having mean spacing of 3.59 μm and mean length of 315 nm. The mean depth of the TJ strand near gaps was 67 nm and the mean cleft path length from lumen to interstitium was 411 nm. With these parameters our three-dimensional hydrodynamic model confirmed that fluid velocity was high at gaps in the TJ strand so that even at relatively low hydraulic pressures the albumin concentration on the tissue side of the glycocalyx was significantly lower than in the interstitium. The results conform to the hypothesis that colloid osmotic forces opposing filtration across non-fenestrated continuous capillaries are developed across the endothelial glycocalyx and that the oncotic pressure of interstitial fluid does not directly determine fluid balance across microvascular endothelium.     

CT analysis of the use of the electrical impedance technique to estimate local oedema in the extremities in patients with lymphatic obstruction

The aim of the study is to establish the validity of the use of the electrical impedance technique for estimating local oedema in extremities through comparison with computed-tomography (CT) analysis. Eight healthy women (group 1) and nine women with unilateral leg oedema following chronic lymphatic obstruction (group 2) are examined sequentially utilising both methods. ‘Equivalent resistivities’ of extra- and intracellular fluid (RE and RI) are measured in the leg's upper portion with a multi-frequency impedance meter, and seven CT slices are taken in the same leg segments. In each slice, the ratios of interstitial fluid to subcutaneous tissue and to muscle are calculated using the CT number of plasma for interstitial fluid, and CT numbers of subcutaneous tissue and muscle of group 1 for ‘normal’ tissues. A decrease in RE and an increase in RI/RE, indicating extracellular fluid increase, are observed in oedematous legs in group 2 when compared with the left legs in group 1. Correspondingly, an increase in the ratios of interstitial fluid to subcutaneous tissue area and to total cross-sectional area is observed in the oedematous legs when compared with those of normal legs in CT analysis. Thus the multi-frequency impedance technique is effective in detecting local oedema in extremities.     

Role of tissue fluid hyperosmolality in the pathogenesis of post-traumatic fluid loss.

The changes in interstitial fluid osmolality in the injured tissues during and after bilateral hind-limb ischaemia have been studied in young rabbits. There was a marked tissue fluid hyperosmolality which was directly related to the period of ischaemia. The increase in osmolality could be attributed to increases in lactate and potassium concentration. The interstitial fluid hyperosmolality disappeared rapidly after restoration of the circulation to the hind-limbs and would therefore only be of importance in the earliest stages of the pathogenesis of post-ischaemic fluid loss. Although the tissue fluid hyperosmolality is so short-lasting 56% of the fluid loss, measured 1 h after a 4-h period of ischaemia, occurs during the first 10 min following removal of the tourniquets. The lactate concentration in ischaemic skeletal muscle of the newborn rabbit was higher than that in the older animal and if this is reflected in an increased tissue fluid hyperosmolality it may be a factor in the greater post-ischaemic fluid loss characteristic of the neonate.     

Lymph capillary pressure of rat intestinal villi during fluid absorption.

A newly developed intestinal preparation is described for determining lymph capillary pressure (PL) in the villi in vivo and in vitro. Determination of PL provided an estimate of tissue fluid pressure in the villi. PL was related to the fluid absorption rate and increased by lymphatic obstruction. During fluid absorption from isotonic mucosal fluid, PL was 1.4 +/- 0.5 or 1.1 +/- 0.4 cmH2O determined in vivo or in vitro, respectively. Both pressures were essentially in the same range as that (0.7 +/- 0.3--1.3 +/- 0.5 cmH2O) in which the mucosal fluid was isotonic Na2SO4 solution or Na-free solutions from which little fluid absorption occurred. This range of pressures may be taken as the normal tissue fluid pressure in the villi. At a high rate of fluid absorption from hypotonic mucosal fluid, PL increased to 5.2 +/- 1.4 cmH2O and tissue fluid pressure was also similarly increased. It is concluded that the fluid absorptive process by the epithelium could not develop an appreciable hydrostatic pressure in the villus tissue space or in the lymphatics.     

Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure.

Cardiopulmonary baroreceptor unloading in humans comparably increases sympathetic discharge to skeletal muscle in the forearm and calf, but blood flow studies have disclosed differential rather than uniform vasomotor responses in the extremities. The aim of the present study was to address the issue of differential effects of orthostatic stress on forearm and calf vascular adjustment and to extend previous studies by determining changes in vascular responses separately in various vascular beds of the limbs. The local [133Xenon] washout method was used for recording blood flow rates in subcutaneous tissue and skeletal muscle. Simultaneous recordings from the forearm and calf were performed in 11 healthy young males during lower body negative pressure at -10 mmHg. Heart rate, arterial mean and pulse pressures did not change during lower body negative pressure. In the forearm blood flow rates decreased significantly, in subcutaneous tissue by 16 +/- 2% (mean +/- SEM) and in skeletal muscle by 16 +/- 1%. In the calf lower body negative pressure induced a significant decrease in blood flow rates of 17 +/- 3% in subcutaneous tissue and of 30 +/- 2% in skeletal muscle. This vasoconstriction in calf skeletal muscle was consistently disclosed in both legs and was about the same magnitude in each calf when studied with the one leg exposed to lower body negative pressure and the other outside the lower body negative pressure chamber. These findings suggest that during unloading of cardiopulmonary afferents, reflex sympathetic activation as an important autonomic adjustment to orthostatic stress is accompanied by uniform vasoconstriction in subcutaneous and skeletal muscle vascular beds of human limbs.     

Interstitial fluid pressure in soft tissue as a result of an externally applied contact pressure

Manipulation of interstitial fluid pressure (IFP) has a clinical potential when used in conjunction with near-infrared spectroscopy for the detection of breast cancer. In order to better interpret how the applied pressure alters the vascular space and interstitial water volumes in breast tissue, a study on tissue-mimicking, gelatin phantoms was carried out to mimic the translation of external force into internal pressures. A complete set of three-dimensional (3D) pressure maps were obtained for the interior volumes of phantoms as an external force of 10 mmHg was applied, using mixtures of elastic moduli 19 and 33 kPa to simulate adipose and fibroglandular values of breast tissue. Corresponding linear elastic finite element analysis (FEA) cases were formulated. Shear stress, nonlinear mechanical properties, gravity and tissue geometry were all observed to contribute to internal pressure distribution, with surface shear stresses increasing internal pressures near the surface to greater than twice the applied external pressure. Average pressures by depth were predicted by the linear elastic FEA models. FEA models were run for cases mimicking a 93 kPa tumor inclusion within regions of adipose, fibroglandular tissue, and a composite of the two tissue types to illustrate the localized high fluid pressures caused by a tumor when an external force is applied. The conclusion was that external contact forces can generate potentially clinically useful fluid pressure magnitudes in regions of sharp effective elastic modulus gradients, such as tumor boundaries.     

Biocompatibility of electroactive polymers in tissues

The biocompatibility of ethylene-vinyl acetate copolymer (EVAc), polyethylene (PE), and polyaniline (PANi) films in the emeraldine (EM), nigraniline (NA) and leucoemeraldine (LM) intrinsic oxidation states were assessed through subcutaneous implantation into male Sprague-Dawley rats beneath the dorsal skin, for a period ranging from 19 to 90 weeks. Histological examination, interstitial pressure measurement, and X-ray photoelectron spectroscopy (XPS) were employed to determine the biocompatibility of the polymers. The polymers did not provoke inflammatory responses in the subcutaneous tissues over the entire implantation period. Characteristics features associated with tissue-implant incompatibility were not evident near the implantation. Interstitial pressure was measured to evaluate the development of tissue. Low interstitial pressure readings on the region of implantation confirmed the biocompatibility of these polymer types. The surface composition of the electroactive aniline polymers before and after the implantation was characterized by XPS.     

An analysis of interstitial fluid pressure in the web of the bat wing.

A mathematical theory of interstitial fluid motion and capillary exchange is developed in order to understand and explain measurements of capillary and interstitial fluid pressure in the bat wing, in which a high degree of vasomotor activity occurs. As a result of precapillary sphincter activity, fluid is alternately filtered and reabsorbed from the capillary, and previous studies of related steady problems by the author suggest that the interstitial fluid pressure should oscillate with time. Wiederhielm and Weston did not observe such fluctuations when they measured tissue pressures near the capillaries. This apparent paradox is resolved in the present paper by including unsteady effects. It is shown that pressure oscillations are predicted, but because of the very high vasomotor frequency they die out within a very short distance from the capillary wall. The capillary pressure measurements of Wiederhielm and Weston are also explained in terms of precapillary sphincter activity.     

Effect of age on transvascular fluid movement.

Swellings of the mouse tail and ear were produced by subjecting them to subatmospheric pressures of minus 40 to minus 80 mmHg for 15-60 min. Increase in volume was measured volumetrically in the tail and gravimetrically in the ear. Blood volume increases in the tail, as measured with 51Cr erythrocytes, contributed a minor part of the fluid increase. Comparison of mice from 3 to 36 wk in age showed a large decrease of fluid movement with age, with major changes during the growth period. Study of permeability of the ear under decreased pressure,to intravenously administred Evans blue, showed no influence of age on permeability to the protein-bound dye. Measurement of transmission of the applied negative pressure through the skin, and of compliance of the tissues of the ear and tail in mice of different age groups, indicated that these factors were not responsible for the observed changes with age.     

Enhanced slow caudad fluid shifts in orthostatic intolerance after 24-h bed-rest

To evaluate mechanisms of late orthostatic intolerance, slow fluid shifts along the body axis were studied during deconditioning by 24-h bed-rest and during 13-min upright tilts before and after this manoeuvre. In 11 healthy male subjects the fluid volumes of a thorax and a calf segment (impedance plethysmography) as well as tissue thickness at the forehead and the tibia (miniature ultrasonic plethysmograph) were recorded. Cardiovascular performance was monitored by recording heart rate (electrocardiogram), brachial and finger arterial pressure (by the Riva Rocci method and by the Finapres technique) as well as stroke volume (by impedance cardiography). Bed-rest led to a cephalad fluid shift with a mean interstitial leg dehydration of 2.2 ml·-100 ml^–1 with no changes in body mass and plasma volume. No syncope during the tilt occurred before bed-rest, while after bed-rest 8 subjects fainted between min 2.1 and 9.0 of the tilt. Bed-rest resulted in an augmented initial heart rate response to tilting which was similar in all subjects. In later orthostasis, bed-rest caused two- to threefold faster caudad fluid shifts with higher calf filtration rates in fainters (prior to hypotension) than in nonfainters. Through bed-rest the estimated extravasation within 10 min into general lower body tissue spaces increased by 192 ml in (late) fainters as opposed to only 23 ml in nonfainters. It was concluded that contributing factors to orthostatic intolerance may be slow transcapillary fluid shifts which are easily underestimated and whose quantity and time course call for further investigation after various deconditioning manoeuvres. In particular, the postflight fluid shifts in astronauts who will have markedly dehydrated legs, may impose a circulatory stress which needs to be evaluated. In general, the filtration rate in relevant areas appears to be an integrative and easily determined parameter, reflecting hormonal and neurogenic vascular as well as local interstitial control of the Starling forces.     

Renal interstitial pressure and tubuloglomerular feedback control in rats during infusion of atrial natriuretic peptide (ANP)

Atrial natriuretic peptide (ANP), injected at physiological concentrations, is known to induce both natriuresis and diuresis. It has been suggested by some investigators that these changes result from an increasing glomerular filtration rate (GFR), but others have been unable to demonstrate an increased GFR. The tubuloglomerular feedback (TGF) mechanism is an important regulator of GFR, and the sensitivity of TGF is decreased during ANP administration. Furthermore, resetting of TGF is, in most instances, related to changes in renal interstitial hydrostatic and oncotic pressures. It is also known that ANP may increase capillary permeability which may change renal interstitial pressure. The present study was performed to examine renal interstitial pressures and the TGF mechanism during ANP infusion. In accordance with previous studies, TGF sensitivity was found to be decreased. The tubular flow rate which elicited half the maximal drop in stop-flow pressure (P_sf) was increased from 18.5 to 25.7 nl min^-1. In contrast, ANP infusion resulted in a decreased interstitial hydrostatic pressure and an increased interstitial oncotic pressure. From previous experiments, such changes in interstitial pressures would be expected to increase TGF sensitivity. The changes in interstitial pressure cannot, therefore, directly explain the resetting of the feedback mechanism. In conclusion, the present paper shows a decreased renal net interstial pressure after intravenous administration of ANP.     

On the edema-preventing effect of the calf muscle pump

Summary During motionless standing an increased hydrostatic pressure leads to increased transcapillary fluid filtration into the interstitial space of the tissues of the lower extremities. The resulting changes in calf volume were measured using a mercury-in-silastic strain gauge. Following a change in body posture from lying to standing or sitting a two-stage change in calf volume was observed. A fast initial filling of the capacitance vessels was followed by a slow but continuous increase in calf volume during motionless standing and sitting with the legs dependent passively. The mean rates of this slow increase were about 0.17%·min⁻¹ during standing and 0.12%·min⁻¹ during sitting, respectively. During cycle ergometer exercise the plethysmographic recordings were highly influenced by movement artifacts. These artifacts, however, were removed from the recordings by low-pass filtering. As a result the slow volume changes, i.e. changes of the extravascular fluid were selected from the recorded signal. Contrary to the increases during standing and sitting the calf volumes of all 30 subjects decreased during cycle ergometer exercise. The mean decrease during 18 min of cycling (2–20 min) was −1.6% at 50 W work load and −1.9% at 100 W, respectively. This difference was statistically significant (p≤0.01). The factors which may counteract the development of an interstitial edema, even during quiet standing and sitting, are discussed in detail. During cycling, however, three factors are most likely to contribute to the observed reduction in calf volume: (1) The decrease in venous pressure, which in turn reduces the effective filtration pressure. (2) An increased lymph flow, which removes fluid and osmotically active colloid proteins from the interstitial space. (3) An increase in muscle tissue pressure, which counteracts the intravascular pressure during the muscle contraction thus playing an important role as an edema-preventing factor, which has not been considered to date.