Plasma Colloid Osmotic Pressure Research Articles

Effect of large volume replacement with balanced electrolyte solutions on extravascular lung water in surgical patients with sepsis syndrome.

We investigated the effect of large volume replacement with balanced electrolyte solutions on extravascular lung water (EVLW) in 16 adult surgical patients with sepsis syndrome. Patients entered the study within the 24 h period following surgical interventions for acute necrotizing pancreatitis, intra-abdominal abscesses, and/or peritonitis. Sequential measurements (n = 108) were made at intervals of 6-12 h over a 48 h period. There were no significant differences between initial and final values of thermal-dye EVLW (5.0 +/- 1.1 vs. 5.7 +/- 1.1 ml/kg), plasma colloid osmotic pressure (COP, 13.3 +/- 2.5 vs. 13.2 +/- 2.9 mmHg), pulmonary artery wedge pressure (PAWP, 9.2 +/- 3.0 vs. 10.8 +/- 3.0 mmHg), and COP-PAWP gradient (4.0 +/- 3.5 vs. 2.4 +/- 3.9 mmHg). All results expressed as (mean +/- SD). The EVLW did not correlate with plasma COP, PAWP, or COP-PAWP gradient. We conclude that large volume replacement with balanced electrolyte solutions with the secondary decrease in plasma COP and COP-PAWP gradient do not necessarily contribute to a substantial increase in EVLW. This study fails to show any causal relationship between decrease in plasma COP or COP-PAWP gradient and oedema formation in the lung.

Left atrial hypertension causes pleural effusion formation in unanesthetized sheep.

We studied the effect of left atrial pressure (LAP) elevation on the formation of pleural effusion in unanesthetized sheep. We prepared the animals by placing catheters in the left atrium, pulmonary artery, femoral artery, and vein. We also placed a balloon catheter in the left atrium. After a recovery period of at least 1 wk, we measured LAP, pulmonary artery pressure (PAP), systemic arterial pressure, systemic venous pressure, cardiac output, plasma protein concentration, and plasma colloid osmotic pressure (pi c). We calculated capillary pressure (Pc) as 0.5(PAP - LAP). We then elevated LAP such that Pc-pi c was between -10 and 19.5 mmHg for 6-24 h. At the end of the experiment, we killed the sheep and measured the volume and protein concentration of the right pleural effusion. We also determined the extravascular fluid to blood free dry weight of the right lung. We found that pleural effusions and pulmonary edema formed when Pc-pi c greater than 5 mmHg. We also found that the pleural effusion volume correlated with the amount of pulmonary edema. Our data show that elevated LAP may cause pleural effusions, but only after pulmonary edema has developed.

Colloid osmotic pressure in young analbuminemic rats.

Colloid osmotic pressure (COP) was measured in plasma and interstitial fluid (subcutaneous wick) from 8 to 75 days of age in Nagase analbuminemic rats (NAR) and control Sprague-Dawley rats (SDR). In all animals plasma COP (approximately 10 mmHg at 8 days of age) increased during growth. In the female NAR the rise in nonalbumin proteins was so large that at 75 days the plasma COP was not lower than in the SDR; whereas in male NAR a difference of approximately 4 mmHg remained. Interstitial COP increased with aging in the SDR, but not in the NAR. This resulted in equal transcapillary COP gradients in 75-day-old male and female SDR and male NAR (approximately 11-12 mmHg) but a somewhat larger gradient in the female NAR (approximately 14 mmHg). Blood pressure and plasma volume were not low in the NAR. Extracellular fluid volume (as a percentage of body weight) was similar in all groups and decreased with age. Clearances of 51Cr-labeled EDTA and 125I-labeled hippuric acid were decreased in young (45 day) NAR vs. young SDR, but not at 75 days of age. In conclusion, NAR are able to maintain a normal transcapillary COP gradient, and do not display signs of abnormal volume regulation during early development.

Plasma proteins in growing analbuminaemic rats fed on a diet of low-protein content.

1. Analbuminaemic and Sprague-Dawley (control) rats were fed on low- (60 g/kg) protein and control (200 g protein/kg) diets ad lib. from weaning. Males and females were studied separately. Body-weight and plasma protein concentrations were determined at 10 d intervals from 25 to 75 d of age. Electrophoresis of plasma proteins was performed in samples from day 75. Extracellular fluid volume was measured at 10 d intervals from day 45 onwards. Colloid osmotic pressure was measured in plasma and interstitial fluid (wick technique) at the start and end of the trial. 2. Body-weight increased much less on the low-protein diet than on the normal diet in both strains and sexes. The growth retardation was slightly more pronounced in the male analbuminaemic rats than in the male Sprague-Dawley controls. 3. Plasma protein concentration increased during normal growth in all groups, particularly in the female analbuminaemic rats. This increase was reduced by the 60 g protein/kg diet in all groups, with the exception of the male analbuminaemic rats. 4. Differences in plasma colloid osmotic pressure were similar to those seen in plasma protein concentration. Interstitial colloid osmotic pressure was higher in the control rats than in the analbuminaemic ones. The interstitial colloid osmotic pressure increased during growth in the control but not in the analbuminaemic rats. The difference in interstitial colloid osmotic pressure between the strains was maintained during low-protein intake, but at a lower level than during normal protein intake. 5. Subtracting interstitial from plasma colloid osmotic pressure, resulted in a rather similar transcapillary oncotic gradient in the various groups at 75 d, both on the control protein diet (11-14 mmHg), and on the low-protein diet (9-11 mmHg). 6. All protein fractions were reduced to a similar extent by the low-protein diet in the control rats, whereas in the analbuminaemic rats protein fractions produced in the liver were more severely depressed. 7. Extracellular fluid volume as a percentage of body-weight was similar in all groups, and decreased with increasing age. 8. In conclusion, the analbuminaemic rats were able to maintain the transcapillary oncotic gradient on both diets by reducing the interstitial colloid osmotic pressure. Oedema was not observed. 9. Despite the absence of albumin, the protein-malnourished analbuminaemic rat is no more susceptible to hypoproteinaemia and oedema than its normal counterpart.

Effect of high frequency oscillatory ventilation on fluid filtration rate in isolated perfused rabbit lungs.

We have studied the effect of changing ventilatory frequency (VF) of high frequency oscillatory ventilation (HFOV) on fluid filtration rate (FFR) in twelve isolated rabbit lungs perfused at constant blood flow. Mean pulmonary artery pressure (Ppa), mean left atrial pressure (Pla), airway pressure (Paw), pulmonary vascular resistance (PVR), pH, O2 and CO2 partial arterial pressures (PaO2 and PaCO2) and plasma colloid osmotic pressure (COP), were measured. We ventilated the lungs with a modified Bird Mark 7 ventilator which could achieve HFOV (range 5-30 Hertz). In each experiment VF was randomly varied on ten different occasions, maintaining each variation for ten minutes. The first group of six rabbits was ventilated under normal haemodynamic conditions. The other six rabbits were ventilated after the production of hydrostatic lung oedema. Blood gas exchange in both groups of rabbits was satisfactory. There was no statistically significant correlation between VF and FFR. We conclude that variations in VF using HFOV does not alter lung fluid balance in normal and in hydrostatic oedema rabbit lungs.

Permeability studies in a hypovolemic traumatic shock model: Comparison of ringer's lactate and albumin as volume replacement fluids

In order to shed light on the controversy surrounding the choice of resuscitative fluids in shock, we used a canine model which we feel to be a superior mimic of human traumatic shock, combining hemorrhage (to a mean arterial pressure of 50 mmHg), fracture of both femora, and soft tissue crush. After 90 min, animals were resuscitated by reinfusion of shed blood, supplemented by 5% albumin (n = 8) or lactated Ringer's solution (n = 8). Plasma colloid osmotic pressure (COP), transcapillary escape rate for albumin (TER), total lung water and extravascular lung water (EVLW) were measured. COP fell in both groups, but remained above 9 mmHg in the albumin recipients, while falling below 7 in those receiving crystalloid (P less than 0.05). Overall, the increase in EVLW averaged 20%; albumin recipients fared better (9.7%) than Ringer's recipients (31.1%), but wide inter-animal variation precluded statistical significance (P = 0.095). TER rose 30% per hour, without difference between groups. Quality of resuscitation (achieved blood pressure and cardiac output) was somewhat better in the albumin group. We conclude that this model allows study of the early microvascular leakage seen in shock; within the time-frame studied (maximum 4.5 h following shock), colloid and crystalloid resuscitation were approximately equivalent.

Circadian Variations of Plasma Colloid Osmotic Pressure During the Third Trimester in Normal and Preeclamptic Pregnancies

The present study was performed to assess the changes of plasma colloid osmotic pressure (COP) in preeclampsia, and to clarify the character of circadian variations of plasma COP in preeclampsia. During the third trimester of a normal pregnancy, plasma COP values were 21.0 ± 0.2 at 28-31 weeks' gestation, 19.3 ± 0.5 at 32-35 weeks' gestation and 20.4 ± 0.5 mmHg at 36 weeks or more of gestation. These values were significantly lower than those in normal non-pregnant subjects (23.5 ± 0.6). In preeclampsia, plasma COP values (14.7 ± 0.4) were significantly lower than those in normal pregnancy. A circadian variation of plasma COP confirmed a clear circadian rhythm with the acrophase at afternoon in all subjects, and there were no differences among normal non-pregnant, pregnant and severe preeclamptic women. These results indicate that circadian variations of plasma COP are not influenced by either pregnancy or preeclampsia, though plasma COP in preeclampsia significantly decreases.

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.

The role of albumin in nutritional support.

Hypoalbuminemia is considered one of the hallmarks of protein-calorie malnutrition and chronic liver disease. Recently, serum albumin has also been proposed as a critical predictor of the response to nutritional support and tolerance to enteral feeding in critically ill patients. Albumin is essential for maintenance of plasma colloidal osmotic pressure, prevention of edema, and transport of certain drugs and nutrients. Experimental studies have shown that rapid plasma expansion and reduced plasma protein concentration and osmotic pressure induce a net secretion of sodium and water into the small intestinal lumen. However, the effects of chronic hypoalbuminemia per se on intestinal absorption, independent of malnutrition, have not been fully studied. It is documented that both chronic illness and malnutrition may profoundly affect intestinal anatomical structure and function, inducing some degree of malabsorption. In the last few years, some have advocated albumin infusion to improve clinical response to patients with hypoalbuminemia receiving parenteral nutritional support or to reduce intestinal intolerance and diarrhea in patients receiving enteral tube feeding. A review of the literature shows that both clinical and experimental data to support these suggestions are scarce and further investigations are needed. Hypoalbuminemia is one of many parameters of malnutrition, and it is unlikely that correction of a single parameter for a short time would lead to major clinical benefits.

Fluid resuscitation in shock

Blood volume deficits are common in most forms of circulatory shock. Fluid repletion is a vital therapy in patients with circulatory shock because it increases venous return to the heart (preload) and cardiac output, and thereby increases oxygen delivery to tissues. Both colloid and crystalloid plasma volume expanders may be used as resuscitative fluids. Colloid-containing fluids maintain or augment plasma colloid osmotic pressure (COP), while large volume infusions of crystalloid fluids decrease COP. Critical reduction of COP has been associated with the development of systemic, cerebral, and pulmonary edema during fluid resuscitation of patients with circulatory shock.

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.

Colloid osmotic pressure of cat brain homogenate separated from autogenous CSF by a copper ferrocyanide membrane

The colloid osmotic pressure (oncotic pressure) of cat brain homogenate against cerebrospinal fluid (CSF) was determined using an electronic osmometer with a rigid selectively permeable copper ferrocyanide membrane. First, the basic characteristics of the membrane were examined. The relative value of the reflexion coefficient (σ) for polyethylene glycol (PEG) became rather stable above a molecular weight of 6000 Da, below which σ declined. The osmometer showed a linear response to sucrose concentration, whereas it revealed a non-idealistic change with respect to concentration of high molecules (PEG). The temperature effect on σ for albumin solution was found to be large. The membrane could thus be used for measuring the relative value of the colloid osmotic pressure of protein solutions without dilution and at the same temperature as the calibrating solution. Samples of CSF and brain cortex were freshly obtained from 5 cats. The cortex was immediately homogenized and the colloid osmotic pressure of the homogenate against the CSF was measured at room temperature within 3 min after cortex removal. The mean value was213±11(S.D.) mm Hg with the osmometer calibrated with respect to the colloid osmotic pressure of autogenous plasma.

Subcutaneous and liver tissue oxygen tension in hemorrhagic shock

Because it is difficult to verify the efficacy of hemorrhagic shock treatment, we compared subcutaneous O2 tension (PscO2) with liver oxygenation in efforts to correct shock in piglets with two different colloids, hydroxyethyl starch (HES-120) and dextran-70. Nineteen animals were bled to shock and the shed blood was retransfused in the control group. Liver oxygenation was measured directly by means of a silicone tube used as a tonometer, and indirectly by calculating liver O2 consumption (VO2). PscO2 was monitored with a needle electrode. The two colloid groups were compared by measuring plasma lactate, and the plasma colloid osmotic pressure (COPp). PscO2 followed closely the changes in liver tissue PO2 during the experiment; it seems to be a useful tool in estimating volume filling during the treatment of hemorrhagic shock. A wider variation was noted in calculated liver VO2 compared with hepatic venous PO2 or liver tissue PO2. Despite the fact that COPp increased to a higher level after the administration of dextran, HES proved to be at least as effective as dextran in restoring mean arterial pressure, cardiac output, liver oxygenation, PscO2, arterial pH, arterial plasma lactate, and liver lactate uptake.

Plasma protein concentration and colloid osmotic pressure in nephrotic rats

Colloid osmotic pressure (COP) was related to plasma total protein concentration in rats with adriamycin nephrosis. Nephrotic rats were divided into three groups on the basis of plasma albumin concentration. Measured values for COP were 6 to 8 mm Hg below those predicted by the Landis-Pappenheimer equation in group 3 plasma samples with albumin concentrations 0.5 to 1.0 g/dl. In contrast, measured values for COP were only slightly below those predicted by the Landis-Pappenheimer equation in group 2 plasma samples with albumin concentrations 1.0 to 1.5 g/dl and in group 1 plasma samples with albumin concentrations 1.5 to 2.0 g/dl. The reduction in concentration of albumin was accompanied by an increase in the concentration of non-albumin proteins in each group of nephrotic rats. COP exerted by these non-albumin proteins partially offset the reduction in COP attributable to reduced albumin concentration. Results show that the Landis-Pappenheimer equation significantly overestimates COP only in nephrotic rats whose plasma albumin concentration is very markedly reduced.

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.

Hypoproteinemia and recovery from edema in dogs.

We studied the effects of hypoproteinemia following 12 days of repeated plasmapheresis and low-protein diet on sodium balance, fluid volumes, and renal hemodynamics in six conscious dogs on 50 mmol sodium intake. Measurements during hypoproteinemia were obtained during a 5-day recovery period starting 20 h after the final plasmapheresis session, with continued low-protein diet. During the plasmapheresis period sodium was retained. Sodium balance became negative on the first recovery day when plasma protein was 29 +/- 1 g/l (control 60 +/- 2 g/l, P less than 0.01), and plasma colloid osmotic pressure (COP) was 9 +/- 1 mmHg (control 22 +/- 1 mmHg, P less than 0.01). Subcutaneous fluid COP was lowered from 14 +/- 1 to 4 +/- 1 mmHg (P less than 0.01). Blood volume, plasma renin activity, and aldosterone were unchanged. Glomerular filtration rate and effective renal plasma flow were slightly reduced (NS), and filtration fraction was unchanged. After a second plasmapheresis period in three of the dogs, plasma protein fell to 26 +/- 1 g/l and COP to 7 +/- 1 mmHg. Now sodium was retained on the first day after stopping plasmapheresis, and renin and aldosterone were high. The next day, when plasma protein was again 29 +/- 1 g/l and COP 8 +/- 1 mmHg, these three dogs were able to completely excrete an infusion of 130 mmol sodium. These data suggest that the level of plasma COP below which dogs on a medium-sodium intake would retain sodium averages 8 mmHg, which is considerably lower than generally thought.

Relation between trans-synovial flow and plasma osmotic pressure, with an estimation of the albumin reflection coefficient in the rabbit knee.

The volume of synovial fluid in a joint correlates inversely with plasma colloid osmotic pressure (COP). The inferred influence of plasma osmotic forces on trans-synovial flow was investigated directly here, in isolated perfused hindquarters of sixteen rabbits. Flow of intra-articularly infused Krebs solution across the synovial lining of the cannulated knee was recorded at controlled intra-articular pressure (18 cmH2O). Colloid osmotic pressure in the synovial microcirculation was varied by perfusion with oxygenated red cells resuspended in albumin solution or plasma from an extra-corporeal system at constant perfusion pressure. Studies in vitro showed that the COP versus concentration curve for commercial bovine albumin samples was variable and not reliably described by a widely used polynomial. The rate of trans-synovial absorption Qs was a positive linear function of intravascular COP pi p (r = 0.936, P less than 0.001, n = 83). The average slope dQs/d pi p was 0.20 microliter min-1 cmH2O-1 (S.E. +/- 0.01 microliter min-1 cmH2O-1), the slope depending on hydraulic conductance and osmotic reflection coefficient. Trans-synovial flow was a negative linear function of synovial capillary pressure (Pc). Absolute slope dQs/d pi p was on average only 78% of dQs/dPc in the same joint. The osmotic reflection coefficient of the blood-joint barrier to serum albumin was estimated from these slopes as 0.78-0.81 (S.E.M. +/- 0.06). Vascular perfusion with a hyperosmolar solution of glucose, sucrose or NaCl generated a transient, rapidly decaying osmotic absorption from the joint cavity, with a half-life of 17-60 s. A reversed osmotic transient occurred on reperfusion with isotonic fluid. It was concluded that the blood-joint barrier, which comprises fenestrated endothelium and synovial intima, approximates to an imperfect semipermeable membrane for albumin solutions, justifying the application of Starling's hypothesis to trans-synovial flow. For small solutes the tissues form a highly permeable but nevertheless slightly osmotically reflective membrane.

Plasma exchange using dextran 40‐electrolyte solution as the sole replacement fluid in malignant paraproteinemia

The relative safety of 3 percent Dextran 40-electrolyte solution alone as replacement fluid during plasma exchange is described. Using an automated blood cell separator, the authors exchanged 3.21 of plasma 64 times in 11 patients with paraproteinemia secondary either to Waldenström's macroglobulinemia or multiple myeloma (malignant paraproteinemia). Then they replaced the plasma with an equal volume of 3 percent Dextran 40-electrolyte solution. They administered no protein-containing solutions during or after the plasma exchanges. Despite marked hypoalbuminemia immediately after plasma exchange, the plasma colloid osmotic pressure level fell only slightly because of the high concentration of plasma low-molecular dextran. The hemostatic abnormalities were similar to those seen in patients whose plasma was replaced with albumin solution. The exchanges were tolerated well clinically and no serious complications occurred during or after exchange. The authors believe that the 3 percent Dextran 40-electrolyte solution can be used safely as the sole replacement fluid in intensive plasma exchange programs for patients with malignant paraproteinemia.

Transcapillary fluid balance in subcutaneous tissue of patients undergoing aortocoronary bypass with extracorporeal circulation.

Colloid osmotic pressure in plasma (COPpl) from a cubital vein and in interstitial fluid (COPif) in the subcutaneous tissue at heart level, and interstitial fluid pressure (Pif) at the same level, were measured in 18 healthy subjects and in 28 patients requiring aortocoronary bypass. Interstitial fluid was collected via subcutaneously implanted double nylon wicks and Pif was measured with the 'wick-in-needle' technique. Measurements were made preoperatively and 1 1/2 to 8 hours (mean 4 hours) after termination of extracorporeal circulation. Pif rose to 2.3 mmHg above the pre-bypass level. COPpl concomitantly fell from 22.2 to 14.4 and COPif from 12.4 to 10.1 mmHg. These changes were statistically significant. Although the relatively large COPpl drop resulted in a net rise of 3.2 mmHg in filtration pressure, the incidence of pulmonary complications was low and no subcutaneous edema was discernible. The fall in COPif and rise in Pif may be regarded as important edema-preventing mechanisms.

Interaction of plasma colloid osmotic pressure and joint fluid pressure across the endothelium-synovium layer: Significance of extravascular resistance

Fluid exchanges between blood and a synovial joint cavity across two membranes in series—synovial capillary wall (fenestrated) and synovial intima (modified connective tissue). The relation between transsynovial absorption of intraarticular Krebs solution (flow Q ̇ s ) and plasma colloid osmotic pressure π p was investigated in rabbit knees perfused at constant blood pressure. Intraarticular pressure P j was independently controlled. Linear relations between transsynovial flow and plasma colloid osmotic pressure established that transsynovial flow obeys the Starling hypothesis. However osmotic conductance, d Q ̇ s dπ p , increased 3.9 times when P j was raised from 6 cm H 2O or subatmospheric pressure to 18 cm H 2O—the “yield phenomenon.” Comparison of the effects of π p and capillary pressure revealed no major change in the osmotic reflection coefficient of the blood—joint barrier to albumin upon raising P j. The large increase in osmotic conductance was predicted quantitatively by a previous model (prediction 3.8 ×) based on increases in extravascular (intimal) conductance as a function of extravascular pressure. It is argued that capillary endothelium is not the sole significant hydraulic resistance in this pathway. In the terminology of Intaglietta and de Plomb (1973) synovial capillaries are functionally intermediate between “tubes” and “tunnels.”