One of the most important functions of the blood circulation is O 2 delivery to the tissue. This process occurs primarily in microvessels that also regulate blood f low and are the site of many metabolic processes that require O 2 . We measured the intraluminal and perivascular pO 2 in rat mesenteric arterioles in vivo by using noninvasive phosphorescence quenching microscopy. From these measurements, we calculated the rate at which O 2 diffuses out of microvessels from the blood. The rate of O 2 eff lux and the O 2 gradients found in the immediate vicinity of arterioles indicate the presence of a large O 2 sink at the interface between blood and tissue, a region that includes smooth muscle and endothelium. Mass balance analyses show that the loss of O 2 from the arterioles in this vascular bed primarily is caused by O 2 consumption in the microvascular wall. The high metabolic rate of the vessel wall relative to parenchymal tissue in the rat mesentery suggests that in addition to serving as a conduit for the delivery of O 2 the microvasculature has other functions that require a significant amount of O 2 .

A mathematical description of blood volume restoration after hemorrhage with resuscitative fluids, particularly hyperosmotic solutions, is presented. It is based on irreversible thermodynamic transport equations and known physiological data. The model shows that after a 20% hemorrhage, the rapid addition of a hypertonic (7.5% NaCl)-hyperoncotic (6% Dextran 70) solution amounting to one-seventh of the shed blood volume reestablishes blood volume within 1 min. Measurements of systemic hematocrit, hemoglobin concentration, and plasma osmolality taken from 13 experiments on anesthetized rabbits verify this prediction. The model shows that immediately after hyperosmotic infusion, water shifts into the plasma first from red blood cells and endothelium and then from the interstitium and tissue cells. The increase in blood volume is transitory; however, it occurs in a fraction of the time compared with isoosmotic fluids at the same infusion rate and is partially sustained by Dextran 70. We theorize that the concurrent hemodilution and endothelial cell shrinkage during hyperosmotic infusion lead to a decreased capillary hydraulic resistance, an effect that is even more significant in capillaries with swollen endothelium. Our results support the significant role of an osmotic mechanism during hyperosmotic resuscitation in quickly restoring blood volume with the added benefit of improved tissue perfusion.

We recently discovered that the endothelium of skeletal muscle capillaries swells in the low-flow ischemia induced by hemorrhagic shock. The present study was undertaken to determine the Na+ transmembrane pathways involved in this swelling, since hypoxic cell swelling is attributed to an influx of Na+ and water. In an initial series of experiments, amiloride (5 mg/kg body wt), which blocks multiple Na+ pathways, was infused intravenously into anesthetized rabbits 30 min prior to shock (40% single-withdrawal hemorrhage). Intravital microscopy of treated capillaries in the rabbit tenuissimus muscle showed that after a 1-h shock period, there was no endothelial cell swelling, as evidenced by no measurable change in the width of red blood cells traversing the capillary. In contrast, the swollen endothelium of untreated capillaries reduced the luminal diameter by 20-25% with a preserved stationary abluminal membrane. The specific effects of amiloride on Na+ transport were investigated with amiloride analogues. Animal pretreatment with 5-(N,N-hexamethylene)amiloride, a selective inhibitor of Na(+)-H+ activity, in a dose of 0.5 mg/kg did not significantly mitigate shock-induced swelling; however, a dose of 1 mg/kg completely prevented it. Phenamil, a selective inhibitor of Na+ channel conductance, even at a potent dosage of 0.5 mg/kg, did not affect swelling. These results suggest a primary role for Na(+)-H+ exchange in endothelial cell swelling during hemorrhagic shock, possibly as a means to regulate cellular pH, which may become acidic during ischemia. Narrowed capillaries with elevated hydraulic resistances could delay and diminish resumption of microcirculatory flow on shock resuscitation.

Because lymphatics in skeletal muscle have no smooth muscle, they are expanded and compressed solely by stresses in the surrounding tissue. Whole organ experiments have indicated that lymph flow is significantly elevated during muscle activity, yet the underlying mechanism for lymph formation has not been identified. To investigate this mechanism, specimens of the rat spinotrapezius muscle were fixed in situ at the undeformed in vivo length, and also in the stretched and contracted states, for histological examination. Cross-sectional areas of lymphatic vessels, skeletal muscle fibers, blood vessels, and interstitial space were measured using a stereological technique. The in situ preparation with intact muscle fascia was essential for preservation of interstitial volume. The lymphatic cross-sectional areas and muscle stretch ratios from 20 rats showed that lymphatic volume increased by 57% for a 20% stretch, and decreased by 45% for a 20% contraction. Deformation of the incompressible muscle fibers appears to inversely affect surrounding tissue structures; e.g., decreased fiber cross-sectional area during stretch increases interstitial spacing between fibers, which in turn expands lymphatics.

Lymphatic valves assure the forward propulsion of fluid along the lymphatic vessels. A description of valve function in skeletal muscle must be based on a knowledge of the valve morphology. To this end, histological sections of valves from lymphatic microvessels of the rat spinotrapezius muscle were examined with light microscopy. All of the approximately 50 valves studied from 20 rats had a bileaflet structure, with a buttress formed at each side of the valve by the fusion of opposing leaflets. This valve structure would allow the valve to close without inversion. There is no evidence for active smooth muscle action to open and close the valve. Since the Reynolds number of lymph flow is very small (about 0.0025), only pressure and viscous forces are available for valve closure. A particular mechanism based on the actual lymphatic valve structure is proposed.

Decreased functional capillary density (FCD) is a characteristic of low-flow ischemia which often persists on reperfusion of a tissue even though supply blood flow is reestablished. Capillary narrowing is a possible mechanism for the reduced blood reflow through an increase of the hydraulic resistance of the capillary network. We attribute the narrowing to swollen endothelial cells, a condition that could be observed directly and was found to develop with blood acidosis in both hemorrhagic shock and intentional acid infusion. A computer model of blood flow in skeletal muscle was used to predict the impact of a 21 % decrease in capillary luminal diameter on flow and leukocyte transit through the network. For hemorrhagic shock, reduction in blood flow due to capillary narrowing and lowered systemic pressure was offset by hemodilution causing a low-flow state. On reperfusion, flow could be restored if the narrowing was rectified by infusion of a hypertonic saline-dextran solution. Reinfusion with conventional Ringer’s lactate only partially restored flow because of a persistent capillary narrowing. The rheological contribution of passive leukocytes added only a few percent to network resistance even with shock-narrowed capillaries. With leukocyte activation increased cell cytoplasmic viscosity caused prolonged transit times for leukocytes in the network with a concomitant elevation in resistance. We conclude that capillary narrowing is a mechanism of FCD reduction in reperfusion after low-flow ischemia, either by a direct effect on flow or by enhancing the transient plugging of leukocytes. Therapeutic strategies should aim to ameliorate or prevent capillary endothelial cell swelling, and to minimize the level of activated circulating leukocytes.

Cells undergo activation in response to a wide range of stimuli. In vascular cells (leukocytes, endothelial cells, and platelets), the different forms of activation include degranulation, oxygen free radical formation, expression of membrane adhesion proteins, and biophysical changes such as pseudopod formation and increased cytoplasmic viscosity. Cell activation and low flow are common features of many cardiovascular diseases. There is evidence that plasma from patients contains an activating factor for neutrophils as well as other vascular cells. Activated neutrophils have the ability to impair microcirculatory transit by elevation of endothelial permeability, leukocyte adhesion to the endothelium, leukocyte capillary plugging, release of vasoactive products, and capillary deformation and compression due to oxygen-radical-mediated interstitial edema and cell dysfunction. In addition to reduced organ perfusion, cell activation can also cause cell dysfunction via release of cytotoxic mediators. A lower degree of neutrophil activation prior to acute circulatory challenge (i.e., low preactivation) correlates with improved survival rates after challenge and suggests that elevated levels of in vivo cell preactivation is a risk factor for cell injury and organ failure. Under conditions of low in-vivo cell preactivation (e.g., as is the case in endotoxin-tolerant animals), there is reduced tissue injury and lower mortality after challenge. We hypothesize that in-vivo cell preactivation due to everyday activity (infection, diet, smoking) may be a mechanism for microvascular low blood flow with leukocyte accumulation and may represent a risk factor for various cardiovascular diseases.

The combination of high-frequency oscillatory ventilation and partial liquid ventilation with perfiubron was well tolerated hemodynamically, was not associated with deterioration of gas exchange during dosing, and did not produce significant differences in either gas exchange or hemodynamic variables over a 2-hr period. There was histopathologic evidence that the combination of high-frequency oscillation and perfiubron administration produces improved recruitment in both dependent and nondependent lung regions.