through the hepatic artery and splenic vein. The aim of the present work was

Transcription

1 60 J. Physiol. (I957) I36, THE ROLE OF THE SPLEEN AND THE HEPATIC ARTERY IN THE REGULATION OF LIVER BLOOD FLOW BY J. GRAYSON AND D. MENDEL From the Department of Physiology, University College, Ibadan (Received 30 August 1956) In previous investigations Johnson (1954) and Ginsburg & Grayson (1954) showed that equilibrium levels of liver blood flow, measured by the method of internal calorimetry, did not change when the arterial blood pressure was lowered from normal values of about 140 to 80 mm Hg. Between 80 and 60 mm Hg a fall in liver blood flow began; below the level of 60 mm Hg liver blood flow declined markedly. The mechanism whereby the liver accomplished this remarkable stability is not known. That nervous factors were not involved was shown clearly by nerve section or ganglion blockade (Johnson, 1954). Although the phenomenon was designated 'intrinsic regulation' of liver blood flow, factors outside the liver were evidently involved, since intrinsic regulation was abolished by removal of the spleen, by prolonged anoxia or by surgical shock (Grayson, 1954). Gastro-intestinal inflow has been shown to be directly related to arterial blood pressure and under conditions of haemorrhage undergoes considerable reduction. Grayson (1954) suggested that intrinsic regulation could only be effective if there was a concomitant increase in the blood reaching the liver through the hepatic artery and splenic vein. The aim of the present work was to test this hypothesis and to investigate the relations between the hepatic artery and liver blood flow and between the spleen and liver blood flow. METHODS Wistar strain albino rats ( g) of either sex were used. Anaesthesia was induced with ether and maintained with pentobarbitone sodium 60 mg/kg intraperitoneally. Polyethylene cannulae were inserted into a carotid and a femoral artery after heparinization (Liquemin, Roche, 1000 u./kg intraperitoneally). Liver blood flow. The technique of internal calorimetry, fully described elsewhere (Birnie & Grayson, 1952; Grayson, 1952) was used for the assessment of liver blood flow. The operative technique involved in the implantation of recorders has been described in detail (Grayson & Johnson, 1953). The photographic method of recording described by Carlyle & Grayson (1956), which gives automatic temperature and conductivity records at intervals of 41 sec, was used in

2 all LIVER BLOOD-FLOW REGULATION 61 experiments. The results, calculated from the photographic trace, are expressed as conductivity increment (8k), that is, the thermal conductivity in excess of that of dead liver. Blood-pressure recording. Mean arterial blood pressure was recorded optically, using a Wiggers capsule, reflecting a light beam on to the slit of the slow-moving camera which was simultaneously recording liver blood flow. The capsule was filled with 0-9 % (wfv) NaCl solution and connected to the femoral arterial cannula, and for purpose of calibration to a mercury manometer, by polyethylene tubing. Blood-pressure compensation. A carotid artery was connected to a blood reservoir containing about 10 ml. of heparinized rat blood. This was connected in turn to a pressure-stabilizing bottle. Raising or lowering the pressure inside the bottle caused blood to flow into or out of the reservoir until pressures between animals and reservoir were balanced (Grayson & Johnson, 1953). Measurement of portal pressure. A serum needle (no. 1), attached to a narrow bore saline manometer by polyethylene tubing, was passed through the transverse mesocolon and inserted into the portal vein for cm (Johnson, 1953). Measurement of liver blood flow during this procedure, both with intact circulation and after hepatic arterial ligation, showed a small fall in flow during insertion of the needle, followed by a rapid return to the original level. Consequently, the presence of the needle in the portal vein was not considered to cause serious obstruction to portal flow. Elimination of the portal blood supply. Ligation of the superior mesenteric artery followed by ligation of the portal vein, above or below the splenic vein as required, was found to be a satisfactory method of eliminating the gastro-intestinal inflow into the portal vein. Splanchnic congestion was reduced to a minimum and the animals remained in a satisfactory condition for 2-3 hr. Spleen blood flow. calorimetry was used to measure the thermal conductivity of the spleen. Anaesthesia was induced with ethyl chloride and chloroform and maintained with pentobarbitone sodium, 60 mg/kg intraperitoneally. Recorders were implanted in the spleen and liver through a mid-line In three experiments on monkeys (Papio anubis) the technique of internal abdominaliincision. A continuous record was made of spleen conductivity and 12 readings (Grayson, 1952) were made of liver blood flow. One femoral artery was used for blood-pressure recording and the other for blood-pressure compensation. RESULTS Liver blood-flow responses to changing blood pressure The techniques used in the present work enable a greater degree of accuracy, particularly in the measurement of conductivity increment corresponding to low blood flow, than was possible in the earlier investigations of relationships between liver blood flow and arterial blood pressure. Fig. 1A shows a typical result relating conductivity increment (8k) and arterial blood pressure in a rat lightly anaesthetized with pentobarbitone sodium. A standard blood-flow recorder was implanted in the liver; arterial blood pressure was controlled by means of the compensator described, and recorded independently from the femoral artery. The immediate postoperative arterial blood pressure was 110 mm Hg. Blood flow was recorded at this level, and at arterial blood-pressure levels of 80, 50, 40, 30 and 18 mm Hg. With each drop in arterial blood pressure there was an initial fall in Sk, followed by recovery complete within 5 min to a plateau which, even at low arterial blood-pressure levels, was maintained for the duration of observation,

3 62 J. GRAYSON AND D. MENDEL a further 10 min. The points recorded on the graph represent the average values of 8k for each of these plateaux. It will be seen that the equilibrium levels of Sk were unchanged by lowering the arterial blood pressure from 110 to 80 mm Hg. A marked decline in Sk took place when the arterial blood pressure was lowered over the range 50 to 18 mm Hg, but when the arterial blood pressure was 18 mm Hg Sk was still more than 10 % of its resting value. Similar results were obtained in most experiments. Measurable values of Sk were constantly present even when the arterial blood pressure was low. Zero value of 8k were not recorded until the death of the animal. 12 A ,<6 4 2 I I B.P. (mm Hg) Fig. 1. Liver blood-flow responses to changing arterial B.P. Liver blood supply intact; (A) rat; (B) baboon. However, even in rats with liver circulation intact, liver blood flow was not always independent of arterial blood-pressure change, even at high levels of arterial blood pressure. In four out of sixty experiments liver blood flow fell markedly when the arterial blood pressure was lowered. In these exceptional instances the blood flow responses were qualitatively similar to those found after splenectomy (Fig. 10A). No consistent explanation was forthcoming. One of the animals had suffered a protracted period of anoxia, another had been accidentally exposed to the effects of haemorrhage and its blood volume restored by transfusion, but the other two were apparently in good condition throughout the experiment. The relationships between liver blood flow and arterial blood pressure were also investigated in two baboons (Papio anubis), one an adult, the other 4 weeks old. The results from the adult baboon are given in Fig. 1 B. A similar result was obtained in the other animal. When the level of blood pressure was 70 mm Hg or higher it is seen that the equilibrium levels of liver blood flow

4 LIVER BLOOD-FLOW REGULATION 63 were the same. The maximum decline in blood flow occurred between arterial blood-pressure levels of 70 and 30 mm Hg. Even below this level of arterial blood pressure there was evidence of some liver blood flow persisting until death. Arterial blood pressure and portal pressure Pressure in the portal vein is of obvious importance in the regulation of liver blood flow, and it was felt necessary to confirm the linear correlation between portal pressure and arterial blood pressure reported by Johnson (1953). Portal pressure was recorded in a rat during the experiment already shown in Fig. 1 A. The results are given in Fig. 2A; portal pressure was related to arterial blood pressure in an approximately linear fashion. In other experiments portal pressure was recorded after ligation of the hepatic artery. The findings shown in Fig. 2B were obtained during the experiment recorded in Fig. 4. The correlation between portal and arterial blood pressure was essentially the same as with the circulation intact. The experiments were repeated after removal of the spleen. Again, the results were similar. Moreover, clamping the splenic vein during an acute experiment on an animal with intact liver blood supply was not found to influence the portal pressure. Influence of the hepatic artery on liver blood flow The hepatic arterial contribution to liver blood flow was investigated at various levels of arterial blood pressure in rats lightly anaesthetized with pentobarbitone sodium. Clamping the hepatic artery. Fig. 3 shows the results of an experiment in which the arterial blood pressure was stabilized at 110 mm Hg. The hepatic artery was exposed and blood-flow recording begun. After a short period the hepatic artery was occluded for 6 min by means of a small artery clamp. During clamping there was an initial fall in 8k of about 50% followed by recovery to a level about 35 % less than the value before clamping. On releasing the clamp Sk recovered to its initial level. On lowering the arterial pressure to 80 mm Hg no change occurred in the equilibrium level of Sk. Clamping the hepatic artery now produced a drop in 8k to a level approximately 80% below the resting value. There was no tendency to recover during the period of clamping. On removing the clamp, 8k made a complete recovery. When the arterial blood pressure was lowered to 50 mm Hg there was a decline in the equilibrium level of 8k. Clamping the hepatic artery now produced a drop of about 55% initially with a recovery to approximately 27 % below initial preclamping levels. Similar results were obtained in all such experiments. In three different experiments the hepatic artery was occluded at an arterial blood pressure of 30 mm Hg without effecting any change in 8k.

5 64 J. GRAYSON AND D. MENDEL c 30 V 20 A E E10 v 90 0~~~~ 0~~~~ CL ~~~~ 0 0~~~~ 0~~~ Arterial B.P (mm Hg) Fig. 2. Relation between portal pressure and arterial B.P. (A) Liver circulation intact; (B) hepatic artery ligated On A 9.0 o Q5i 9.0 Fig Time (min) Effect on liver blood flow of clamping hepatic artery arterial B.P. of (A) 110 mm Hg, (B) 80 mm Hg, (C) 50 mm Hg. in the same rat with

6 LIVER BLOOD-FLOW REGULATION 65 Hepatic arterial ligation was also carried out in a group of seven rats in which the arterial blood pressure ranged from 130 to 150 mm Hg. At these blood-pressure levels the immediate fall in liver 8k (mean for the group) on tying the ligature was 31 % followed by a recovery to a level 21 % below the initial value. Liver blood flow and arterial blood-pressure relationships after hepatic artery ligation. In six experiments using rats the hepatic artery was ligated after implantation of a blood-flow recorder in the liver. The arterial blood pressure was immediately lowered in stages, each selected arterial blood pressure being maintained for 10 min. A typical experiment is shown in Fig. 4A. The relation between 8k and arterial blood pressure shown in Fig. 1 was no longer present. JI A do Arterial B.P. (mm Hg) -B I I I I I I I I I I I I I Portal pressure (mm saline) Fig. 4. Liver blood-flow responses after ligation of the hepatic artery. (A) Sk and arterial B.P.; (B) 8k and portal pressure. Liver 8k fell markedly with each decline in arterial blood pressure and recovered slightly to the 8k levels given in the graph. When the arterial blood pressure was 80 mm Hg the equilibrium level of 8k was about 30% of the value before haemorrhage. In this experiment portal pressure was also recorded. Fig. 4 B shows the relation between 8k and portal pressure: which, since the hepatic artery was 5 PHYSIO. CXXXVI

7 66 J. GRAYSON AND D. MENDEL ligated, was the sole effective perfusion pressure maintaining blood flow through the liver. The ak/portal pressure relations were similar to Sk/arterial bloodpressure relations. The experiments relating to arterial blood pressure andc liver blood flow were repeated in animals in which hepatic artery ligation was carried out 24 hr and 4 days before the experiment. The results (Fig. 5) were qualitatively similar to those described above. 6 4 A 2 10 * Fig B.P. (mm Hg) Liver blood-flow response to changing arterial B.P. (different rats). (A) 24 hr after hepatic artery ligation; (B) 4 days after hepatic artery ligation. Liver blood flow responses to changing arterial blood pressure with the hepatic artery as its sole blood supply. In six experiments using rats the hepatic artery was left as the sole supply to the liver by elimination of the splenic and. gastro-intestinal contributions in the manner already described (see Methods). The results of five of these experiments are shown in Fig. 6. In all these experiments blood flow in the liver ceased when the arterial blood pressure was lowered, the critical level for cessation of flow varying from 30 to 90 mm Hg. In two of the experiments initial levels of liver blood flow were maintained to a varying extent despite declining arterial blood pressure. Otherwise there was no consistent relation to arterial blood pressure. In the sixth experiment the post-ligation value of 8k was low (2.5 x 10-4), but this low level was maintained unchanged whilst the arterial blood pressure was lowered from 110 to 60 mm Hg. With a further decline in arterial blood pressure, Sk fell slowly but remained measurable even with an arterial blood. pressure of 15 mm Hg. Zero blood flow was not recorded until the animal died.

8 LIVER BLOOD-FLOW REGULATION 67 'IT-x I to I B.P. (mm Hg) Fig. 6. Liver blood-flow responses to changing arterial B.P. after elimination of the gastro-intestinal and spleen contributions (5 experiments, different rats) 10 -A ~ Splenectomy Time (min) Fig. 7. The effect of splenectomy on liver blood flow. (A) left lobe of liver; (B) right lobe of liver (different rats). The influence of the spleen on liver blood flow It has previously been inferred (Grayson, 1954) that the spleen contributes to liver blood flow in such a manner as to compensate for fluctuations in the gastro-intestinal inflow into the portal vein. This matter was investigated. Spleen contributiron to liver bloodflow. In five experiments using rats, arterial blood pressure was stabilized at the post-operative resting level ( mm Hg). 8k was recorded either from the left or from the right lobe of the liver, and after a short period the spleen was removed through the abdominal incision. The results of two experiments are shown in Fig. 7. Fig. 7 A shows blood-flow responses from the left lobe, Fig. 7 B from the right. At these levels of arterial blood pressure, acute splenectomy produced no diminution in 3k in either lobe; in some experiments a small temporary increase occurred. 5-2

9 68 J. GRAYSON AND D. MENDEL The spleen contribution to liver blood flow was investigated at different levels of arterial blood pressure. In five experiments arterial blood pressure was stabilized at 75 mm Hg by means of the compensator. The effect of acute splenectomy on Sk values was recorded. In all experiments there was an immediate fall in 8k followed by a recovery in 5-10 min to a plateau lower thani the flow before splenectomy. The immediate fall varied from 23 to 60% of the immediate presplenectomy values, the mean immediate fall being 35 %. The final level reached after the partial recovery varied from 12 to 50% below the presplenectomy levels, the mean value being 23 %. 6.0 On A Off On B 0X On Off Time (min) Fig. 8. The effect on liver blood flow of clamping the splenic pedicle in the same rat: arterial B.P. of (A) 100 mm Hg, (B) 80 mm Hg, (C) 50 mm Hg. In three experiments the effect of clamping the splenic pedicle at various levels of arterial blood pressure was investigated. In the experiment shown in Fig. 8 arterial blood pressure was stabilized at 100 mm Hg. After a short control period, the splenic pedicle was clamped for a period of 6 min. There was a decline in 8k of about 33 % complete in 2 min, folowed by a recovery to a level 14 % below the mean preclamping level. After releasing the clamp arterial blood pressure was lowered to 80 mm Hg, little change occurring in 8k. On clamping the splenic pedicle there was now a rapid fall in 8k of about 53% with no recovery during the period of clamping. After again releasing

10 LIVER BLOOD-FLOW REGULATION 69 the clamp, arterial blood pressure was lowered to 50 mm Hg. Sk declined from 4-8 x 10-4 to a mean value of 1-75 x 1o-4. Clamping the splenic pedicle now produced a drop of about 30 % in Sk, with some recovery to a level about 10% below the mean preclamping level. A similar result was obtained in all these experiments. Thus clamping the splenic vein had little effect at high levels of arterial blood pressure, a marked effect at blood pressure levels of about 100 mm Hg, a maximum effect at blood-pressure levels of about 75 mm Hg and less effect when the blood pressure was about 50 mm Hg Splenectomy xo ~ ~ ~ ~~~~~~3 2 II I I ly l l l l l l l l l l l l l2i Time (min) B.P. (mm Hg) Fig. 9 Fig. 10 Fig. 9. The effect of splenectomy 5 days after hepatic artery ligation. Arterial B.P. 120 mm Hg. Fig. 10. Liver blood-flow responses to changing arterial B.P.: (A) 24 hr after splenectomy; (B) 7 days after splenectomy. Spleen contributions to liver bloodflow after hepatic artery ligation. In order to determine whether there was an increase in the contribution of the spleen to liver blood flow after hepatic artery ligation, splenectomy was carried out 1-4 days after hepatic artery ligation in four experiments using rats. The arterial blood pressure was stabilized at the immediate post-operative levels ( mm Hg). The result of a typical experiment is shown in Fig. 9. There was a big fall in Sk within 2 min of splenectomy followed by a return to presplenectomy levels of Sk in a further 3 min. The fall in Sk to very low levels was not a result of kinking of the portal vein, as it occurred after the spleen had been removed and not during the operative procedure. Similar results were obtained in all four experiments.- Liver blood flow and arterial blood pressure after splenectomy. Four experiments were performed using rats in which arterial blood pressure was lowered

11 70 J. GRAYSON AND D. MENDEL in stages immediately or 24 hr after removing the spleen. Fig. 10A shows the results of an experiment performed 24 hr after removing the spleen. In this experiment there was a modification of the result obtained from experiments in which the blood supply to the liver was intact (Fig. 1 A), in that there was a marked change in the equilibrium levels of liver blood flow with each change in arterial blood pressure over the range mm Hg. The fall in liver blood flow, however, was never as great as that which occurred after hepatic artery ligation (Fig. 4A). Similar results were obtained in all such experiments. The relation between liver blood flow and arterial blood pressure was further investigated in rats in which splenectomy had been performed 1 week previously. A typical result is shown in Fig. lob. It will be seen that little change occurred in liver blood flow between arterial B.P. of 140 and 60 mm Hg. The response was essentially similar to that recorded from rats with intact liver blood supply (Fig. 1). Spleen responses during changing arterial blood pressure. The above results pointed to the necessity for the direct measurement of blood flow in the spleen. TABLE 1. Blood-flow responses to lowering the arterial B.P. in liver and spleen of two baboons (unit =8k[ x 10-4])* B.P. (mm Hg) Adult baboon Liver Spleen Young baboon Liver Spleeni * I.e. numbers given are 104 x actual values Accordingly the method of internal calorimetry was applied to this organ. In the isolated, perfused spleen a linear relation between rate of perfusion and thermal conductivity has already been demonstrated (Grayson, 1952). Although it does not necessarily follow that measurements of &k in the intact spleen are similarly related to arterial inflow, Sk is a measure of fluid movement within an organ and it seems reasonable to regard such determinations as related to splenic venous outflow. Accordingly in two baboons, one an adult, one 4 weeks old, standard blood-flow recorders were implanted in the spleen and the effect of lowering the arterial blood pressure in stages was investigated. The results are given in Table 1. In the adult baboon, maximum values of Sk were recorded from the spleen when the blood pressure was 80 mm Hg, at which level there was little difference in this respect between spleen and liver. When the arterial blood pressure was lowered further, however, spleen 8k values declined rapidly. The results obtained in the young baboon were less satisfactory, low values of Sk being obtained throughout the experiment. Maximum values were obtained when the blood pressure was about 40 mm Hg.

12 LIVER BLOOD-FLOW REGULATION 71 The influence of gastro-intestinal venous outflow on liver blood-flow responses to changing arterial blood pressure Gastro-intestinal outflow as the sole hepatic blood supply. In four experiments using rats the hepatic artery was tied and the spleen removed, producing a mean drop in liver Sk of 33 %. In the experiment shown in Fig. 1 A the immediate post-operative value of 3k was 4-5 x 1O-4. Arterial blood pressure was lowered in steps. With each decline in arterial blood pressure there was an immediate fall in Sk followed by a partial recovery to a stable plateau. It will be seen that the relation between plateau levels of 8k (shown in the graph) and arterial blood pressure is similar to that already recorded when the hepatic artery was ligated (Fig. 4A), a drop in liver blood flow occurring with each arterial blood-pressure decrement. 60 A 4.5 S B.P. (mm'hg) Fig. 11. Liver blood-flow responses to changing arterial B.P. (A) hepatic artery and splenic pedicle ligated, portal vein intact; (B) gastro-intestinal contribution to portal flow eliminated (hepatic artery and splenic vein intact). Gastro-intestinal inflow into the portal vein eliminated. In these experiments using rats the hepatic artery and the splenic vein were left intact but the portal vein was tied proximal to the splenic vein after previous ligation of the superior mesenteric artery. In the experiment shown in Fig. 11B, when the blood pressure was stabilized at 108 mmhg the mean value for Sk was 5-1 x 104. Although transient falls in 8k occured with each fall in arterial blood pressure, the equilibrium levels remained elevated. Thus, when the

13 72 J. GRAYSON AND D. MENDEL arterial blood pressure was 60 mm Hg, Sk was still 4-5 x 1O-4. Below this level 8k declined but there was still evidence of measurable blood flow (8k = 1-5 x 1o-4) when the arterial blood pressure was 15 mm Hg. Changes in vascular resistance to blood flow within the liver The above investigations reveal differences in the response to changing arterial blood pressure as between the portal contribution to liver blood flow and the hepatic arterial contribution. The results shown in Table 2 were calculated from the experiments shown in Fig. 3, using the expression Resistance - arterial blood blood flow pressure (8k) TABLE 2. Resistance to flow of hepatic artery and portal venous blood, calculated in arbitary units as arterial pressure/sk: (portal pressure is assumed to be a linear function of arterial pressure). Portal resistance calculated from immediate residual Sk value after clamping hepatic artery. Hepatic artery Hepatic artery contribution to Resistance B.P. Intact blood clamped liver flow Resistance to to hepatic (mm Hg) supply (Sk[ x 10-4]) (Sk[ x 10-4]) portal flow* artery flow* x (Sk[ X 10-4]) y z X/Y X/Z 110 9s * Arbitrary units. TABLE 3. Resistance to portal inflow after ligation of the hepatic artery (from Fig. 4) Resistance to Portal portal inflow B.P. pressure (portal pressure/ (mm Hg) (mm H20) Sk( x 10-4) Sk[ x 10-4]) It is clear that resistance to hepatic arterial inflow fell when the arterial blood pressure was lowered from 110 to 80 mm Hg and rose again when it was further lowered to 50 mm Hg. Clamping the hepatic artery when the arterial blood pressure was 30 mm Hg had no effect on blood flow, indicating an infinite resistance to blood flow at this level of blood pressure: resistance to portal inflow, however, rose when the blood pressure was lowered from 110 to 80 mm Hg and fell again when the blood pressure was further lowered to 50 mm Hg. Table 3 shows the results of resistance calculations on the experiment shown in Fig. 4B. In this experiment the hepatic artery was ligated and the portal pressure measured. Resistance was calculated as portal pressure/flow (8k). It will be seen that there was a progressive increase in resistance with each

14 LIVER BLOOD-FLOW REGULATION 73 decline in blood pressure, up to 50 mm Hg; thereafter resistance fell. This finding was typical of all results with direct measurement of portal pressure after hepatic arterial ligation. The peak resistance usually occurred between arterial blood pressures of 80 and 50 mm Hg. The experiments shown in Fig. 6, where the hepatic artery was the sole source of blood supplying the liver, were not sufficiently consistent to justify similar calculations with respect to the hepatic artery alone, although with one exception resistance to hepatic arterial inflow rose to infinity below arterial blood pressures of 30 mm Hg. The effect on survival time of combined splenectomy and hepatic ligation In a series of twenty rats the hepatic artery alone was ligated and the animal permitted to recover. Four of these animals died within 36 hr: the remainder survived indefinitely. In a series of thirty experiments splenectomy was performed and the animals allowed to recover. None of these animals died. In five experiments, however, splenectomy was performed after previous hepatic arterial ligation. All these animals died within 24 hr. DISCUSSION The relation between liver blood flow and arterial blood pressure The concept of intrinsic regulation which forms the main subject of the present work is at first sight hard to reconcile with the findings of Wakim & Mann (1942), who maintained that at any one time large portions of the liver are closed off from the circulation at large. They visualized, so to speak, a patchy distribution of active blood flow, and regarded the liver circulation as highly variable and unstable. This view received some confirmation from the findings of Grindlay, Herrick & Mann (1941) who, using thermostromuhr methods, found variations in flow along each of the main afferent vessels. In contrast, most bromsulphthalein investigations have shown great steadiness of blood flow (Bradley, Inglefinger, Bradley & Curry, 1945). All the work reported to date, in which internal calorimetry was the technique of investigation, confirmed that blood flow in the region of the heated thermocouple was remarkably steady under a variety of conditions. Moreover, in such experiments, the liver possessed the ability to preserve a constant circulation despite big changes in arterial blood pressure. These findings were made mainly in the rat. Johnson (1953) has deduced some evidence to suggest that the same applies to anaesthetized man and the present work has shown intrinsic regulation in the monkey's liver. So many experiments have now been performed using this method that, although each indicates local changes only, it must be concluded that the findings are representative of the whole liver of several mammalian species. In confirmation of this conclusion, Heinemann Smythe & Marks (1953), using the bromsulphthalein clearance method, showed

15 74 J. GRAYSON AND D. MENDEL in dogs that a drop in blood pressure of up to 60 mm Hg produced by haemorrhage caused an immediate drop in liver blood flow but was followed by spontaneous recovery to near control levels. The observations of Wakim & Mann (1942) may well be dependent on the small vascular territories they were observing. Random variations in capillary beds are a feature of the circulation in many parts of the body (Chambers & Zweifach, 1944) and do not necessarily produce random variations in total flow in a bigger area. Phenomena similar to intrinsic regulation of liver blood flow have been demonstrated in the kidney (Shipley & Study, 1951) in skeletal muscle (Folkow, 1953) and in the brain (Carlyle & Grayson, 1956). In the latter organ blood flow was maintained constant at lower levels of arterial blood pressure than is the case in the liver. However, as the blood pressure fell towards 20 mm Hg there was a sudden rise in vascular resistance so that brain blood flow ceased even though the blood pressure was still 20 mm Hg. From the present work, this sudden closure of the circulation does not occur in the liver with intact blood supply. There was evidence of some blood flow even at very low levels of blood pressure, zero flow only being recorded on the death of the animal. Contribution of the hepatic artery to liver blood flow The flexibility of hepatic arterial contributions to total liver blood flow has been noted by Burton-Opitz (1911) after clamping the portal vein, and emphasized by Soskin, Essex, Herrick & Mann (1938) who found that the hepatic artery could supply 10-90% of total liver blood flow. It is therefore difficult without strict definition of conditions to compute the contribution of the hepatic artery to total liver blood flow. Nevertheless, this computation has been attempted by a number of workers. Burton-Opitz (1911) and Barcroft & Shore (1912) estimated the mean contribution of this vessel to total hepatic flow to be between 30 and 34% in the dog and cat. Thermostrohmuhr methods suggested even lower figures; Grindlay et al. (1941) claim that the hepatic artery of the dog only carried 14-3% of total liver blood. The findings of Ginsburg & Grayson (1954) using 'internal calorimetry', suggested that the coeliac axis contributed about 40% of total hepatic blood. In these analyses, however, the effect of arterial blood pressure was largely ignored, but there can be no doubt from the present work that the extent of the hepatic arterial contribution to total liver blood flow depends very greatly on this factor. The results of clamping the hepatic artery are themselves highly suggestive. This procedure had a larger lowering effect on liver blood flow when the arterial blood pressure was at 80 mm Hg than it had either at 110 mm Hg or at lower levels. Indeed, at arterial blood pressures as low as 30 mm Hg, clamping the hepatic artery had no effect on liver blood flow. Quantitative interpretation of these results is complicated by the fact that clamping the

16 LIVER BLOOD-FLOW REGULATION 75 hepatic artery produced an immediate drop in flow followed by some recovery. Ginsburg & Grayson (1954) have previously suggested that the immediate drop was more representative of the hepatic arterial contribution than the net drop after recovery to a plateau. Assuming this to be so, the hepatic arterial contribution to total liver flow varied from 50 % at arterial blood pressures of 110 mm Hg to 80% at arterial blood pressures of 80 mm Hg, and to nil at arterial blood pressures of 30 mm Hg. From the results summarized in Table 2 it is clear that this change could only have been accomplished by a process of active vasodilatation in the hepatic vascular bed as the arterial blood pressure fell to 80 mm Hg, followed by vasoconstriction as it fell further. In experiments where the hepatic artery was the sole blood supply to the liver it was sometimes possible, despite the severity of the operation, to demonstrate vasodilatation with the arterial blood pressures dropping to 30 mm Hg. In all such experiments, however, except one, hepatic blood flow ceased whilst arterial blood pressure was still positive. The evidence strongly suggests therefore that in the presence of low arterial blood pressure spasm usually occurred in the hepatic artery or in the vascular territory it supplied. It may well be that this apparent closure of the arterial supply to the liver results from a drop in blood pressure to below the critical pressure consistent with patency in an elastic or muscular vessel (Burton, 1952). Contribution of the portal vein to liver blood flow The present experiments confirm the variability of portal supply to the liver previously reported (Grindlay et al. 1941; Grayson, 1954). It must be concluded from the effect of clamping, that the portal contribution to liver blood flow was far less when the arterial blood pressure was 80 mm Hg, than when the arterial blood pressure was 100 mm Hg. When the arterial blood pressure was low, however, clamping the hepatic artery had no effect on flow and the portal vein was probably the only source of blood supply remaining to the liver. Calculations of resistance (Table 2) to portal inflow showed that lowering the arterial blood pressure increased the portal resistance to a maximum at blood pressure levels between 80 and 60 mm Hg. However, when the blood pressure was lowered further, resistance to portal flow declined. Further to this it may be deduced from the persistence of flow in the portal vein up to the moment of death that at extreme low blood pressure resistance in the portal vein was small. The results of all experiments where the sole source of blood supply to the liver was the portal vein were consistent and showed the same pattern of response, namely resistance increasing as the blood pressure was lowered to a maximum between 50 and 80 mm Hg followed by a decline in resistance as zero blood pressures were approached. The mechanism of build up of resistance to portal flow at blood pressures

17 76 J. GRAYSON AND D. MENDEL between 50 and 80 mm Hg is far from clear. Since there was no such build up when the sole blood supply to the liver was the hepatic artery, it may be concluded that the location of the main resistance, at least, was not in the hepatic veins draining the liver, but probably in the terminal branches of the portal vein itself. This conclusion is consistent with the observations of Friedman, Frank & Fine (1951), which showed constriction of branches of the portal vein during haemorrhage. A further point of interest is that there was no evidence of any phenomenon which could be attributed to critical closing pressure in the portal vein or its branches. Relation of the spleen to liver blood flow It was first suggested by Hargis & Mann (1925) that the spleen might serve anastomotic functions linking aorta and portal vein, neutralizing variations in gastro-intestinal inflow and thus ensuring a continued blood supply to the liver. This view of splenic function, overshadowed in the intervening years by concentration on the spleen's storage function, must be seriously reconsidered in view of the present findings. Grayson (1954) has already demonstrated that intrinsic regulation is abolished by removal of the spleen, but returns after a period of about one week. This finding was amply confirmed in the present work. The immediate effect on liver blood flow of ligating the splenic pedicle of a rat with a systemic blood pressure of mm Hg was surprisingly small. Usually no change occurred, sometimes liver blood flow increased. It is not suggested that under these conditions spleen blood flow was, in fact, nil, but that a rapidly acting compensatory mechanism, possibly involving the hepatic artery, might have been operative. That this was partially true was shown after hepatic artery ligation when even at high arterial blood pressure levels ligation of the splenic pedicle produced a marked drop in liver blood flow. When the arterial blood pressure was 75 mm Hg and the hepatic artery patent, clamping the splenic pedicle caused an immediate drop in liver blood flow varying between 23 and 60%, followed by a recovery to 12-50% of the original flow. The changes produced by clamping were less pronounced at lower blood-pressure levels. It seems likely, therefore, that the splenic contribution to liver flow rises to a maximum as the blood pressure falls to about 75 mm Hg, thereafter diminishing to zero as the blood pressure falls further. In the present work measurements of thermal conductivity were made in the spleens of baboons in an attempt to confirm directly the presence of a compensatory increase in spleen blood flow during haemorrhage, but although in the isolated spleen Sk is a linear function of outflow, the present findings must be viewed with caution pending further justification of the method. Nevertheless, the results were suggestive of an increase in spleen blood flow accompanying a fall in arterial blood pressure. Moreover, since the blood flow

18 LIVER BLOOD-FLOW REGULATION 77 was increasing whilst the arterial blood pressure was falling, the conclusion seems justified that pronounced vasodilatation occurred within the spleen. From the above considerations the spleen may justifiably be regarded as a compensatory arteriovenous anastomosis in the manner suggested by Hargis & Mann (1925). The present work has shown that the relation of portal to systemic arterial pressure is not affected by splenectomy. It is unlikely, therefore, that purely mechanical factors can be responsible for the dilatation which occurs at low arterial blood pressures. General considerations concerning intrinsic regulation of liver blood flow It is evident that intrinsic regulation of liver blood flow is a phenomenon normally depending on the integrity of the liver's blood supply. The hepatic artery is particularly important in this respect. The present findings differ from Johnson's suggestion (1954) that intrinsic regulation was still present after ligation of the hepatic artery. As he himself recognized, his technique was not fully reliable with low blood flows and his published results did not justify any dogmatic conclusion. In the present work the position was quite clear. Intrinsic regulation of liver blood flow was never present after ligation of the hepatic artery, nor did it return even after several days. In contrast, a liver whose sole blood supply was the hepatic artery could still show intrinsic regulation. It may be concluded, then, that intrinsic regulation is the result of resistance changes in the hepatic arterial bed, but that such changes cannot occur in the portal vascular bed. This conclusion must be amplified, for the spleen, whose contribution is to the portal vein, also possesses an important relation to liver blood flow. Thus liver blood flow regulation during a fall in blood pressure probably depends on a combination of splenic vasodilatation and compensatory changes occurring in the hepatic arterial bed. It may be significant in this respect that either splenectomy or hepatic arterial ligation alone was rarely lethal, whereas the combination of these procedures produced in every case death within 24 hr. Probably the importance of these mechanisms arises from the fact that the gastro-intestinal contribution to liver blood flow varies considerably with blood pressure. At high levels most of the liver blood probably derives from this course, at intermediate levels probably very little, but at very low levels of blood pressure the gastro-intestinal contribution is probably the only source of blood available to the liver. SUMMARY 1. Intrinsic regulation of liver blood flow, a mechanism whereby liver blood flow is maintained constant despite decline in blood pressure over the range mm Hg, was investigated in the rat and the baboon.

19 78 J. GRAYSON AND D. MENDEL 2. The relative contributions to liver blood flow of the hepatic artery and the spleen were investigated at various blood-pressure levels. 3. At blood-pressure levels of mm Hg the hepatic artery contributed about 30% of the total liver flow. The contribution increased at lower blood-pressure levels, being maximum at arterial pressures of 80 mm Hg. When the arterial pressure was lower than this, the hepatic contribution declined, being nil at blood pressures lower than 30 mm Hg. 4. No splenic contribution to liver blood flow could be demonstrated at blood pressures of mm Hg. There was an increase in splenic contribution with falling blood pressure to maximum values of 23-60% of the total liver flow at arterial pressures of about 75 mm Hg. Below these levels the splenic contribution declined. 5. Gastro-intestinal inflow was the main blood supply of the liver at bloodpressure levels of mm Hg. The gastro-intestinal contribution was small at intermediate blood pressures, but at blood pressures below 30 mm Hg it was probably the only blood supply to the liver. 6. Intrinsic regulation of liver flow was abolished permanently by hepatic arterial ligation and temporarily by splenectomy. This work was aided by a grant from the Medical Research Council. REFERENCES BARORoFT, J. & SHORE, L. E. (1912). The gaseous metabolism in the liver. J. Phy8iol. 45, BIRNIE, J. H. & GRAYSON, J. (1952). Observations on temperature distribution and liver blood flow in the rat. J. Phy8iol. 116, BRADLEY, S. E., INGLEFINGER, F. J., BRADLEY, G. P. & CURRY, J. J. (1945). Estimation of hepatic blood flow in man. J. Clin. Invest. 24, BURTON, A. C. (1952). Laws of physics and flow in blood vessels. The Visceral Circulation, Ciba Foundation Symposium. London: Churchill. BuRTON-OPITZ, R. (1911). The vascularity of the liver. IV. The magnitude of the portal inflow. Quart. J. exp. Phy8iol. 4, CARLYLE, A. & GRAYSON, J. (1956). Factors involved in the control of cerebral blood flow. J. Physiol. 133, CHAMBERS, R. & ZWEIFACH, B. W. (1944). Topography and function of the mesenteric capillary circulation. Amer. J. Anat. 75, FOLKow, B. (1953). A study of factors influencing the tone of denervated blood vessels perfused at various pressures. Acta phy8iol. 8cand. 27, FRIEDMAN, E. W., FRANK, H. A. & FiNE, J. (1951). Portal circulation in experimental haemorrhagic shock. Ann. Surg. 134, GINSIBURG, H. & GRAYSON, J. (1954). Factors controlling liver blood flow in the rat. J. Phy8iol. 123, GRAYSON, J. (1952). Internal calorimetry in the determination of thermal conductivity and blood flow. J. Physiol. 118, GRAYSON, J. (1954). The role of the portal vein in the interpretation of splanchnic blood flow. L'hypertension portale. Le 'dumping syndrom', IV Congres de Gastro-Enterologie, pp Paris: Masson et Cie. GRAYSON, J. & JOHNSON, D. H. (1953). The action of adrenaline and noradrenaline on liver blood flow. J. Physiol. 120, GRiNDLAY, J. H., HERRIcK, J. F. & MANN, F. C. (1941). Measurement of the blood flow of the liver. Amer. J. Physiol. 132,

20 LIVER BLOOD-FLOW REGULATION 79 H.ois, E. H. & MANN, F. C. (1925). A plethysmographic study of the changes in the volume of the spleen in the intact animal. Amer. J. Phy8iol. 75, HBINEMANN, H. O., SMYTHE, C. M. & MARKS, P. A. (1953). Effect of haemorrhage on estimated hepatic blood flow and renal blood flow in dogs. Amer. J. Phy8iol. 174, JoHNsoN, D. H. (1953). The Liver in Hypoten8ion. M.D. Thesis, University of Bristol. JOiNSON, D. H. (1954). The effect of haemorrhage and hypertension on the liver blood flow. J. Phy8iol. 126, SHIPLEY, R. E. & STUDY, R. S. (1951). Changes in renal blood flow, extraction ofinulin, glomerular filtration rate, tissue pressure and urine flow with acute alterations of renal artery blood pressure. Amer. J. Physiol. 167, SosKNm, S., ESSEX, H. E., HERRICK, J. F. & MANN, F. C. (1938). Mechanism of regulation of blood sugar by the liver. Amer. J. Phy8iol. 124, WArmn, K. G. & MANN, F. C. (1942). Intrahepatic circulation of the blood. Anat. Rec. 82,