Blood flow augmentation by intrinsic venular contraction in vivo

Abstract

Venomotion, spontaneous cyclic contractions of venules, was first observed in the bat wing 160 years ago. Of all the functional roles proposed since then, propulsion of blood by venomotion remains the most controversial. Common animal models that require anesthesia and surgery have failed to provide evidence for venular pumping of blood. To determine whether venomotion actively pumps blood in a minimally invasive, unanesthetized animal model, we reintroduced the batwing model. We evaluated the temporal and functional relationship between the venous contraction cycle and blood flow and luminal pressure. Furthermore, we determined the effect of inhibiting venomotion on blood flow. We found that the active venous contractions produced an increase in the blood flow and exhibited temporal vessel diameter-blood velocity and pressure relationships characteristic of a peristaltic pump. The presence of valves, a characteristic of reciprocating pumps, enhances the efficiency of the venular peristaltic pump by preventing retrograde flow. Instead of increasing blood flow by decreasing passive resistance, venular dilation with locally applied sodium nitroprusside decreased blood flow. Taken together, these observations provide evidence for active venular pumping of blood. Although strong venomotion may be unique to bats, venomotion has also been inferred from venous pressure oscillations in other animal models. The conventional paradigm of microvascular pressure and flow regulation assumes venules only act as passive resistors, a proposition that must be reevaluated in the presence of significant venomotion.

conventional theory concerning microvascular flow and pressure regulation assumes veins are passive resistors of flow. Veins not only return blood to the heart, but they play a critical role in regulating blood flow and pressure. Although Bayliss and Starling (2) conceptualized veins as passive resistors of flow to explain the pressure gradients measured throughout the vascular system, it was not for another 50 years that Pappenheimer and Soto-Rivera (21) explicitly formulated the relationship of arterial and venous resistances to blood pressure and flow. The elegance of their results made the formulation one of the most quoted rules in cardiovascular physiology: blood flow is inversely proportional to the sum of venous and arterial resistances, and microvascular pressure is proportional to the ratio of venous and arterial resistances. Therefore, the conventional understanding of microvascular regulation is based on the concept that dilation of the venules increases microvascular blood flow and decreases microvascular pressure (21). The only commonly accepted violation of this principle is that blood, especially in the larger veins in muscle or periodically contracting organs, can be actively pumped back to the heart by extrinsic compression (1, 11, 27).

Qualitative observations have led investigators for more than a century to suggest that intrinsic venular contraction actively pumps blood. Venomotion, the spontaneous rhythmic contraction and dilation of veins, was first observed by Jones (15) over 160 years ago in the batwing microvasculature (15). This first description of venomotion suggested that the “venous pulses” may promote blood flow in the batwing veins (15, 16). The batwing was the model of choice for studying the microvasculature, because its thin wing membrane, being almost transparent, allowed in vivo, noninvasive, nondestructive measurements with simple light microscopy (29). Batwing venules exhibit remarkable venomotion (16, 28), and the contractile activity originates locally in the periphery (17). On the basis of unpublished observations, Peristiany et al. (23) suggested that venomotion may act as a peristaltic pump that increases the efficiency of venous blood return toward the heart (23). Although suggested by qualitative observations in numerous reports (16), quantitative evidence that batwing venules can actively pump blood has been lacking.

The lack of quantitative tools has left untested the hypothesis that batwing venules pump blood. Mechanical characterization of pumps requires simultaneous measurement of diameter, velocity, and pressure. By the time such modern quantitative tools had become available, the use of the batwing model declined to the degree that we have the only extant colony of bats worldwide dedicated to cardiovascular research. Popular animal models amenable to intravital microscopy, such as rat mesentery and cremaster muscle and hamster cheek pouch, require surgical exteriorization and anesthesia, which have been shown to greatly reduce or abolish vasomotion (4). Therefore, we used the classical unanesthetized batwing model to quantitatively test the hypothesis that venules actively pump blood.

METHODS

Bat preparation.

Experimental procedures and animal care were performed in compliance with the Texas A&M University Institutional Animal Care and Use Committee. We maintained Pallid bats in our chronic colony for over 2 yr before experiments. Procedures were similar to those reported previously (10). Trained bats were placed in a plastic box with their wings extended under the microscope. To minimize stress, bats were used no more than once per week, and experiments were limited to a maximum of 4 h.

Vasculature visualization.

Bat wings were held lightly against a temperature-controlled glass plate (Olympus Tokai Hit) to maintain a temperature of 27°C. One milliliter of distilled water was placed on top of the wing to immerse the water-immersion objective (×40, NA = 0.8; Olympus LUMPlanFl/IR). The vasculature was visualized with an upright microscope (Olympus BX61WI) for a total magnification of ×400. Images with a resolution of 700 × 480 pixels were recorded via digital video recorder (Panasonic KR222 S-Video Camera) at 30 frames/s.

Vessel selection.

The relative structure and position of the microvasculature were similar in all bats (Fig. 1). The venules located between the first and second major arteriolar bifurcations between the fourth and fifth digits were used for study. These first-order venules (9) allowed simultaneous visualization of both walls. The location selected for study was at least 400 μm away from either upstream or downstream valves. Consistent with previous reports (9), all venules at the selected location exhibited rhythmical venomotion.

Fig. 1.

Vascular network structure in the Pallid bat wing and location of the segment of interest. Oval represents the area where sodium nitroprusside was applied topically. Venular schematic shows the relative location of diameter, centerline blood velocity, and luminal pressure measurements in a venule. Venular diameter was measured by tracking walls of the venules in each frame of the video recorded during the experiment. Centerline blood velocity was measured with an optical Doppler velocimeter. Luminal pressure was measured by using a servo-null pressure system. D4 and D5, digits 4 and 5.

Venular diameter measurement.

The diameters of venules were measured from each video frame (resulting in 30 data points/s) from the recorded videos. Both walls of the venules were tracked, and the distance between walls was measured by manually adjusting custom video caliper software (MATLAB R2011a). Venomotion amplitude was determined as the difference between maximum (dilated) diameter and minimum (contracted) diameter. As in previous reports (8–10), venules were assumed to be cylindrical. Repeated measurements of a standard image revealed a caliper repeatability coefficient of 0.45 μm with 95% limits of agreement (0.03 ± 0.45 μm).

Venular blood flow measurement.

Centerline velocity of red blood cells in the venules was measured using Optical Doppler Velocimeter (model no. 4, Texas A&M Health Science Center, College Station, TX), as described previously (6, 7). The centerline velocity data were acquired and recorded at 30 Hz using a custom data acquisition system. Mean velocity was estimated by dividing centerline velocity by 1.6 for vessels with a diameter larger than 25 μm (6, 7). Venular cross-sectional area was calculated from the measured venular diameter, assuming a cylindrical geometry. Instantaneous blood flow was calculated as the product of mean velocity and venular cross-sectional area and was averaged each minute to yield mean blood flow. Video and velocity data recordings were synchronized at the beginning of the experiment by transiently blocking the light passing through the objective.

Venular blood pressure measurement.

A servo-null pressure system (IPM model 4A) was used to record hydrostatic pressures in the venules. Micropipettes were pulled from glass capillary tubing (1 mm OD; World Precision Instruments, Sarasota, FL) and beveled on an optical flat (model BV-10; Sutter Instrument, Novato, CA) to a final tip size of ∼1 μm. The pipettes were filled with 2 M NaCl, and the pressure system was calibrated using isotonic saline in the calibration chamber, as described previously (6, 7). The pressure data were acquired at 40 Hz rate using a data acquisition system (LabChart 7 Pro, ADInstruments, Colorado Springs, CO). Pressure measurements were accepted only if they satisfied criteria detailed previously (6, 7).

Protocol I: characterizing the effect of reducing venomotion on venular blood flow.

In the first set of experiments, the effect of minimizing venomotion on venular blood flow was evaluated in one venule in each of eight bats. After visualizing the bat wing vasculature, a venule segment away from valves was selected, as described in Vessel selection. The video of the selected venule and the recording of centerline blood velocity were synchronized and recorded at baseline for at least 10 min. Sodium nitroprusside, a NO donor, was used to dilate venules at selected locations to minimize venomotion. Following baseline measurements, a 0.5-ml aqueous solution of sodium nitroprusside (SNP, sodium nitroprusside anhydrate, 4% wt/vol; Sigma-Aldrich, St. Louis, MO) was added to the water placed between the water-immersion objective and the wing. The final concentration of SNP was 2% wt/vol. To minimize dilation of a significant portion of the bat wing vasculature, care was taken to restrict the spread of water containing SNP to less than 1 cm². Venomotion frequency, amplitude of venomotion, mean venous diameter, mean centerline blood velocity, and mean blood flow were determined for the 2-min period immediately before SNP application. Values of these variables were determined again 3 min after SNP application for another 2-min period. The location of diameter and centerline blood velocity measurements is depicted in Fig. 1.

Protocol II: characterizing venular dynamics away from valves.

In the second set of experiments, measurement of luminal blood flow and pressure was used to characterize venular dynamics in one venule in each of four bats. After visualizing the bat wing vasculature, a venule was selected as described in Vessel selection. Simultaneous measurements of venular diameter, centerline blood velocity, and luminal pressure were analyzed. Venular diameter was determined from the video recording as described in Venular diameter measurement. Data were analyzed in each venule for 2 min. The relative locations of diameter, centerline blood velocity, and pressure measurements are depicted in Fig. 1.

Protocol III: characterizing venular dynamics at a confluence.

A third set of experiments was conducted to characterize venular dynamics at a confluence in 4 bats. After visualizing the bat wing vasculature, a venular confluence from the batwing region used in the previous two protocols was selected. Only venular confluences that exhibited synchronous, as well as asynchronous venomotion at different times during the experiment, were chosen. At these confluences, during asynchronous venomotion, the contraction waves from only one of the upstream venules continued in the downstream venule, while the contraction waves in the other upstream venule ended at the confluence (Fig. 6) (5). Furthermore, the two upstream converging venules at these confluences each had a set of valves that was located at the end of each segment. The two upstream converging venules, as well as the downstream venule at each confluence, were visible in the video frame. The relative timing of the contraction waves in the upstream converging venules and the downstream venule was evaluated. Furthermore, the “open” or “closed” status of the valves was derived from the recorded video. Data for 2 min were analyzed at each selected venular confluence.

Fig. 6.

The two hypothesized temporal relationships of contraction waves (indicated by arrow) in venules and relative “Open” or “Closed” status of venular valves at a confluence. A, Type 1: contractions in the upstream converging venules were synchronous and the contraction wave continued in the downstream venular segment. Valves in both upstream converging venules remained open, allowing blood flow into the downstream venule. B, Type 2: contractions in the upstream converging venules were asynchronous. However, the contraction wave from one of the upstream venules continued in the downstream segment, while the valves in the other asynchronous venule remained closed.

Data analysis.

To determine the effect of SNP, values of venomotion frequency, amplitude of venomotion, mean venous diameter, mean centerline blood velocity, and mean blood flow before SNP application were compared with those after SNP application. Paired t-tests were used to identify significant changes in these variables. A one-sample t-test was used to compare a hypothesized mean of zero to the difference between the time at which blood velocity and luminal pressure peaked and the time at which venular diameter was minimum. A P value less than 0.05 was considered significant. All data are reported as means ± SE.

RESULTS

Effect of minimizing venomotion on venular blood flow (protocol I).

Venular diameter and centerline blood velocity in a representative venule before and after SNP application are depicted in Fig. 2, A and B, respectively. Before and after SNP, flow periodically stopped in diastole. Because the region of interest was not near valves, the cessation of flow was not related to valve closure. Topical application of SNP decreased the amplitude of venomotion from 33.2 ± 4 to 6.2 ± 1.8 μm and decreased the frequency of venomotion from 9.94 ± 0.47 to 4.19 ± 1.31 min⁻¹, thus effectively reducing venomotion in all venules studied (n = 8). After SNP application (Fig. 2C), mean venous diameter increased by 26% (81.3 ± 4.1 to 102.6 ± 3.7 μm), mean centerline blood velocity decreased by 56% (1.14 ± 0.12 to 0.52 ± 0.07 ml/min), and calculated mean blood flow decreased by 35% (0.26 ± 0.05 to 0.17 ± 0.03 μl/min) (Fig. 2C). Despite significant reduction in venomotion, fluctuations in flow were still pronounced, since venomotion in upstream and downstream venules was not inhibited by SNP.

Fig. 2.

Venular diameter and centerline blood velocity before (A) and after (B) Sodium nitroprusside (SNP) application in a representative venule. C: SNP application resulted in decreased venomotion frequency and venomotion amplitude and increased mean venular diameter. Reduced venomotion was accompanied by a decrease in mean blood velocity and mean blood flow (n = 8). *Significant change, P < 0.05.

Establishing the venular diameter and centerline blood velocity relationship (protocol II).

Figure 3 depicts the temporal relationship between venular diameter and centerline blood velocity in a representative venule. In the four venules studied, there was a one-to-one relationship between the diameter change and the centerline blood velocity (i.e., they had the same frequency). During each venomotion cycle, centerline velocity peaked before the venular diameter was minimum. In a total of 75 contractions analyzed in the four venules, the centerline velocity peaked 2.3 ± 0.33 s before venular diameter reached minimum level. Furthermore, centerline blood velocity was zero during venular diastole in each venomotion cycle.

Fig. 3.

Simultaneous recording of diameter and centerline blood velocity in a representative venule. There was a one-to-one relationship between diameter change and centerline blood velocity change; however, centerline velocity peaked before the venular diameter was at a minimum level. Instead, increase in velocity ahead of contraction was consistent with peristaltic pumping.

Establishing the venular diameter and luminal pressure relationship away from valves (protocol II).

Figure 4 depicts the temporal relationship between venular diameter and luminal pressure in a representative vessel. In the four venules studied, the diameter change and the luminal pressure change had the same frequency. In a total of 75 venular contraction cycles studied, the venular luminal pressure peaked 0.97 ± 0.23 s before venular diameter reached a minimum level.

Fig. 4.

Temporal relationship between venular diameter and luminal pressure in a representative venule. There was a one-to-one relationship between diameter change and luminal pressure change. Venular luminal pressure peaked 0.97 ± 0.23 s before venular diameter reached minimum level. 100% of the venular contractions studied are in accord with an active venular peristaltic contraction wave.

Establishing work done by venular contraction (protocol II).

During venular diastole in each venomotion cycle, venular centerline blood velocity was zero and blood flow stopped, and luminal pressure reached a minimum. Each venular contraction led to an increase in blood flow. In a total of 75 contractions, the blood flow averaged 0.03 ± 0.01 μl/contraction. Furthermore, these venular contractions led to a 27.8 ± 9.1 cmH₂O increase in luminal pressure downstream from contraction. Figure 5 depicts the change in luminal pressure and flow as a result of one contraction cycle in a representative venule. The average energy provided by venular contractions, calculated as a product of change in luminal pressure and resulting flow, was 75.5 ± 36.3 nJ/contraction.

Fig. 5.

Change in luminal pressure and flow as a result of an active venous contraction in a representative venule. The energy deposition by venular contractions (product of change in luminal pressure and resulting flow) averaged to 75.5 ± 36.3 nJ/contraction.

Establishing the temporal relationship of contraction waves and valve closure in venules at a confluence (protocol III).

Figure 6 depicts the two hypothesized temporal relationships of contraction waves in venules and relative “Open” or “Closed” status of venular valves at a confluence. From a total of 239 contractions studied at four confluences (one in each of four bats), 36 contractions were of type 1, where contractions in the upstream converging venules were synchronous, and the contraction wave continued in the downstream venular segment. Under this condition, valves in both upstream converging venules remained open, allowing blood flow into the downstream venule. The remaining 203 contractions were of type 2, where contractions in the upstream converging venules were asynchronous. The contraction wave from one of the upstream venules continued in the downstream segment, while the valves in the other asynchronous venule remained closed. Therefore, 100% of contraction waves in the downstream venule continued from either one or both of the upstream venules. Furthermore, all of the valves closed during the trailing end of the contraction cycle, preventing backflow of blood into the upstream venular segment.

DISCUSSION

The present work demonstrates that batwing venules act as pumps and augment blood flow. First, there was a one-to-one relationship of venular contraction and local blood velocity and pressure pulsations (Figs. 3 and 4). Second, venular contractions increased blood pressure downstream from contraction (Fig. 4). Third, venular contractions provided energy to blood flow (Fig. 5). Fourth, venular contraction closed upstream valves and displaced blood in the forward direction (Fig. 6). Fifth, inhibiting venular contractions decreased blood flow (Fig. 2). On the basis of these five pieces of complementary evidence, we conclude that batwing venules pump blood.

Reducing venomotion decreases blood flow.

The strongest of the five pieces of evidence that batwing venules actively pump blood is the observation that minimizing venular contraction with sodium nitroprusside caused a significant decrease in blood flow. The area treated was limited to 1 cm² to ensure that there was no unintentional dilation of collateral pathways, allowing for a potential “vascular steal”. Inspection of venular segments 1-cm proximal and distal to the application of SNP revealed that venular segments exhibited venomotion and closure of valves. Because of the proximity of arterioles, it was not possible to minimize venomotion without affecting arteriolar tone. However, because these proximal arterioles feed the vascular beds drained by the venules of interest, their dilation would increase flow in the venule of interest. Because of the possibility that arteriolar dilation may have ameliorated the decrease in flow, the observed 35% decrease in venular blood flow that we recorded may actually underestimate the contribution of venular pumping.

Identifying the venular pumping mechanism.

The present work provides evidence that batwing venules act as pumps with characteristics of both peristaltic and reciprocating pumps. The entire segment of venule between valves does not contract simultaneously. Consistent with previous reports (17, 23), we observed a propagation of the contraction wave away from the periphery toward the heart. Because we could not record venular diameter and pressure at two separate locations simultaneously, the relative spatial relationship between luminal pressure and venular contraction was inferred from the temporal relationship. That is, at the location of measurement, increases in blood pressure preceded the arrival of venular contraction wave (Fig. 4). Assuming a pressure wave velocity the same as venular contraction velocity [0.33–3.5 mm/s, (5)] and a 0.97 ± 0.23-s time difference between maximum luminal pressure and minimum venular diameter (Fig. 4), we can infer that pressures increased 0.25–4.2 mm downstream from a contraction. Such an increase in luminal pressure downstream from a contraction is characteristic of peristaltic pumps (14, 25). Similar to the heart valves, venular valves open and close synchronously with venular dilation and contraction. These functional valves, characteristic of reciprocating pumps, enhance the efficiency of the venular peristaltic pump by preventing retrograde flow. Furthermore, the presence of valves obviates the necessity of coordination between converging venular segments at a confluence, and prevents flow into a noncontracting venule.

Revisiting the role of veins in microvascular pressure and flow regulation when venules are not passive resistors.

The conventional assumption that venules passively resist blood flow leads to the prediction that venular dilation increases flow. Figure 7A illustrates an electrical analog depicting arterial and venous resistances and represents the implicit assumptions embedded in the classical formulation of the relationship of arterial and venous resistances to microvascular pressure and flow (21). The resulting formula for microvascular flow and pressure arises from two simple equations. Flow into the microvasculature (Q_a) is equal to the difference between arterial pressure (P_a) and microvascular pressure (P_c), divided by arterial resistance (R_a).

Fig. 7.

A: electrical analog of the conventional microvascular pressure (P_c) and flow (Q_a and Q_v) regulation as a function of arterial (R_a) and venous (R_v) resistances. P_a , arterial pressure, P_v, venous pressure. B: graphical balance point representation of the conventional microvascular pressure and flow regulation. Microvascular pressure and flow at equilibrium are determined by the intersection of inflow (solid line) and outflow (dashed line) lines representing flow into the microvasculature (Q_a, Eq. 1) and flow out of the microvasculature (Q_v, Eq. 2), respectively. Venular dilation i.e., decrease in R_v (dash-dotted line) increases equilibrium flow (Q_a' and Q_v') and decreases equilibrium pressure (P_c'). C: flow out of the microvasculature is characterized using a parameter (α) related to contractility of the venules (Eq. 5). Using this new paradigm, we show that relaxation of contractile venules decreases the slope of the outflow line (dotted line) resulting in decreased equilibrium flow (Q_a” and Q_v”) and increased equilibrium pressure (P_c”).

$$Q_a=\frac{(P_a-P_c)}{R_a}$$ (1)

Flow out of the microvasculature (Q_v) is equal to the difference between microvascular pressure (P_c) and venous pressure (P_v), divided by venous resistance (R_v).

$$Q_v=\frac{(P_c-P_v)}{R_v}$$ (2)

In equilibrium, Q_a is equal to Q_v. Simultaneously solving Eqs. 1 and 2 for Q_a and P_c yields standard formulation relating venous resistance to microvascular flow and pressure.

$$Q_a=\frac{(P_a-P_v)}{(R_a+R_v)}$$ (3)

$$P_c=\frac{\frac{R_v}{R_a}\cdot P_a+P_v}{\left(1+\frac{R_v}{R_a}\right)}$$ (4)

Dilating the venules (and thus decreasing venous resistance, R_v) will increase microvascular flow (Eq. 3) and decrease microvascular pressure (Eq. 4). However, instead of solving Eqs. 1 and 2 algebraically, they can be solved graphically to provide additional insight. Figure 7B (left) illustrates the results of plotting Eqs. 1 (solid line) and 2 (dashed line) on the same graph. Flow into the microvasculature has a negative slope equal to −1/R_a, and flow out of the microvasculature has a positive slope equal to 1/R_v. The intersection represents equilibrium flow and pressure. Venular dilation can then be represented graphically (Fig. 7B, right) as increasing the slope of the outflow line (dash-dot line), which increases equilibrium flow (Q_a' and Q_v') and decreases the equilibrium microvascular pressure (P_c^′).

Reevaluating role of venules in microvascular pressure and flow regulation when venules actively pump blood.

Microvascular pressure and flow regulation have not been characterized previously using the balance-point characterization. When venules are assumed to act as passive conduit vessels in a manner consistent with conventional belief, the resulting balance point characterization of microvascular pressure and flow is represented by Fig. 7B. The fact that batwing venules actively pump blood necessitates the modified characterization depicted in Fig. 7C. It is, therefore, very similar to Guyton's classical cardiac output-venous return balance point (12). Davis et al. (9) reported that the stroke volume × contraction frequency of batwing venules in vitro increased with transmural pressure, up to a critical value, much like Starling's Law of the heart (26). Approximating this behavior with a simple linear relationship:

$$Q_v=Q_o+\alpha \cdot P_c,$$ (5)

where α and Q_o are simple empirical parameters determined by the slope and the intercept of the flow-pressure relationship (9). The values of α and Q_o are related to contractility of the venule and characterize the effect of luminal pressure on flow. Using this new characterization, one can calculate flow and microvascular pressure by simultaneously solving Eqs. 1 and 5.

$$Q_a=\frac{\alpha \cdot (P_a-P_v)}{(1+R_a\cdot \alpha )}$$ (6)

$$P_c=\frac{P_a-R_a\cdot Q_o}{(1+R_a\cdot \alpha )}$$ (7)

Figure 7C graphically illustrates a new balance point assuming Eqs. 1 and 5, with the intersection representing the equilibrium flow and pressure. Venular relaxation and consequent decreased contractility can then be represented graphically (Fig. 7C, right) as “decreasing” the slope of the outflow line (dotted line), which “decreases” equilibrium flow (Q_a” and Q_v”) and “increases” equilibrium pressure (P_c”).

Perspectives and Significance

The batwing model has a unique structure that allows unique measurements that cannot be made in other animal models. Devoid of surrounding muscle, venules are only loosely tethered to the wing membrane (25–45 μm thick). Although the focus of the present work is the ability of venules to actively pump blood, this unique animal model has also been used to demonstrate that venomotion can initiate intrinsic contraction of adjacent lymphatic vessels, as well as pump lymph via extrinsic contraction (10). The effects on lymph flow and venous blood flow could be appreciated in the bat wing because it can be studied without the confounding effects of surgery and anesthesia. Similar to arterioles (4) and the portal vein (33, 34), Ca²⁺ and K⁺ are critical for phasic contractions of batwing venules (24). Since anesthetic agents have been shown to affect vasomotion (23, 33, 34) by affecting Ca²⁺ and K⁺ currents, this may suggest that venomotion in the bat wing is similarly affected. In fact, in preliminary studies, we found that inhaled isoflurane dilated arterioles and venules and reduced frequency of venomotion and lymphatic contractions in the bat wing (22). Although the rather pronounced venomotion of the bat wing may be unique, in many other aspects, the batwing vasculature behaves similar to other common animal models. For example, vascular responses of the bat wing, such as shear-induced dilation (8, 30), the myogenic response (3), sympathetic innervation (31), and responses to common vasoactive agents (10), are similar to those in other tissues. Furthermore, the hemodynamic measurements made in the veins from rabbit ear, dog limbs, and human feet suggest a venular origin of the observed oscillations in blood flow and pressure (13, 18–20, 32). Perhaps venomotion at rest in other mammals would not only aid in return of blood to heart, but also have a substantial impact on the regulation of peripheral resistance. Whether this new interpretation of regulation of microvascular blood pressure and flow will supplement the standard model represented by Fig. 7B hinges on whether venules in other (unanesthetized) mammals can be demonstrated to pump. If such behavior can be confirmed, it is more likely to occur during pathological conditions, such as reduced perfusion (6, 8). Under these circumstances, venomotion may play a critical role in propulsion of blood and, thus, in microvascular pressure and flow regulation.

GRANTS

Portions of this work were supported by the American Heart Association (10GRNT4320043 to C. M. Quick), the National Science Foundation (CMMI-1063954 to C. M. Quick), and the National Institutes of Health (R01 HL092916 to R. H. Stewart).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

Author contributions: R.M.D., C.M.Q., J.K.M., G.A.L., M.J.D., and R.H.S. conception and design of research; R.M.D. and J.C.V. performed experiments; R.M.D. and J.C.V. analyzed data; R.M.D., C.M.Q., G.A.L., M.J.D., and R.H.S. interpreted results of experiments; R.M.D. and J.C.V. prepared figures; R.M.D., C.M.Q., and R.H.S. drafted manuscript; R.M.D., C.M.Q., J.K.M., G.A.L., M.J.D., and R.H.S. edited and revised manuscript; R.M.D., C.M.Q., J.C.V., J.K.M., G.A.L., M.J.D., and R.H.S. approved final version of manuscript.