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Showing papers by "Imperial College London published in 1981"


Journal ArticleDOI
TL;DR: Three simple models embodying the essentials of the dynamical interaction between invertebrate hosts and their directly transmitted microparasites suggest that the baculovirus and microsporidian infections of many temperate forest insects will tend to produce stable cycles in host abundance and in prevalence of infection, with periods in the range 5-12 years.
Abstract: We show how directly transmitted microparasites, broadly defined to include viruses, bacteria, protozoans and fungi, may regulate natural populations of invertebrate hosts. The study combines elements of conventional epidemiology (where the host population is assumed constant) with elements of prey-predator studies (which conventionally emphasize how prey and predator populations may be regulated by their interaction). To this end, we construct simple models embodying the essentials of the dynamical interaction between invertebrate hosts and their directly transmitted microparasites. In successive refinements, these models include the effects of recovery and disease-induced mortality, castration or diminished reproduction of infected hosts, vertical transmission, latent periods of infection, stress-related pathogenicity, the interplay between disease and other density-dependent constraints on host population growth, and free-living infective stages. In analysing the dynamical behaviour of these models, we focus on: the possible regulation of the host population by the parasite; the basic reproductive rate of the parasite, and the way in which it affects the dynamics and the evolution of the host-parasite association; and the threshold host density and its implications for endemic or epidemic maintenance of the infection. These are examined in the light of synoptic compilations of field and laboratory data on: birth rates (and disease-induced reduction thereof), natural death rates and disease-induced death rates of hosts; latent periods and efficiencies of vertical transmission of pathogens; the rate of production and lifetime of free-living infective stages; and some characteristics of long-term cycles and of epidemic outbreaks of disease in forest insects. In particular, our models suggest that the baculovirus and microsporidian infections of many temperate forest insects will tend to produce stable cycles in host abundance and in prevalence of infection, with periods in the range 5-12 years. Enough is known about the European larch budmoth and an associated granulosis virus for us to undertake a detailed comparison between theory and data that strongly suggests that the observed 9-10 year cycles are driven by the host-parasite interaction. We also discuss the possible control of invertebrate pest species by pathogens, showing how our models could guide laboratory or field studies, to help estimate whether a given pathogen is capable of regulating the target pest population, and, if so, roughly what quantity is needed to effect a specific level of (local) control. Throughout, the emphasis is on the biological ingredients of the models, and on the biological conclusions to be drawn; mathematical details are given in appendixes.

1,130 citations


Journal ArticleDOI
TL;DR: A modified version of the self-tuning regulator having limited adaptability has been successfully implemented on a large-scale chemical pilot plant and the use of a variable forgetting factor with correct choice of information bound can avoid one of the major difficulties associated with constant exponential weighting of past data.

822 citations


Journal ArticleDOI
01 Apr 1981
TL;DR: By classifying programs according to their relationship to the environment in which they are executed, the paper identifies the sources of evolutionary pressure on computer applications and programs and shows why this results in a process of never ending maintenance activity.
Abstract: By classifying programs according to their relationship to the environment in which they are executed, the paper identifies the sources of evolutionary pressure on computer applications and programs and shows why this results in a process of never ending maintenance activity. The resultant life cycle processes are then briefly discussed. The paper then introduces laws of Program Evolution that have been formulated following quantitative studies of the evolution of a number of different systems. Finally an example is provided of the application of Evolution Dynamics models to program release planning.

709 citations


Journal ArticleDOI
01 Dec 1981-Brain
TL;DR: In this paper, the cerebral blood flow, oxygen extraction and oxygen utilization has been measured regionally in 22 dements, and 14 aged normal volunteers, and the results showed that a decline in cerebral flow and mean cerebral oxygen utilization was correlated with increasing severity of dementia in both degenerative and vascular dements.
Abstract: The cerebral blood flow, oxygen extraction and oxygen utilization has been measured regionally in 22 dements, and 14 aged normal volunteers. Ten demented patients were studied twice at a six-month interval from initial measurements. The use of a steady-state 15O technique and positron tomography for measuring regional cerebral blood flow, regional oxygen extraction fraction and mean cerebral oxygen utilization is discussed. The limitations of measurements are reviewed in the light of the present results and the current state of technological development in positron emission tomography is discussed. A decline in cerebral blood flow and mean cerebral oxygen utilization was correlated with increasing severity of dementia in both degenerative and vascular dements. The decline was coupled, both for the cerebral hemisphere as a whole and regionally. There was no increase in oxygen extraction ratio globally, and therefore no evidence to support the existence of a chronic ischaemic brain syndrome. Focal abnormalities in oxygen utilization were observed for both vascular and degenerative groups. In the vascular group, parietal defects were the most pronounced. Individual derangements of the regional pattern varied, reflecting the different unique patterns of ischaemic damage in these patients. In the degenerative group, parietal and temporal defects were seen in the less severe group, but a profound depression in the frontal regions with relative sparing of occipital area characterized the severe degenerative dements.

635 citations


Journal ArticleDOI
TL;DR: In this paper, the authors considered the problem of routing vehicles to supply customers with known demands, in such a way as to minimize the total distance traveled, which is referred to as the vehicle routing problem (VRP).
Abstract: We consider the problem of routing vehicles stationed at a central facility (depot) to supply customers with known demands, in such a way as to minimize the total distance travelled The problem is referred to as the vehicle routing problem (VRP) and is a generalization of the multiple travelling salesman problem that has many practical applications We present tree search algorithms for the exact solution of the VRP incorporating lower bounds computed from (i) shortest spanning k-degree centre tree (k-DCT), and (ii) q-routes The final algorithms also include problem reduction and dominance tests Computational results are presented for a number of problems derived from the literature The results show that the bounds derived from the q-routes are superior to those from k-DCT and that VRPs of up to about 25 customers can be solved exactly

541 citations


Journal ArticleDOI
01 Jan 1981
TL;DR: In this article, a new numerical solution method for the calculation of the thermal radiation transfer in combustors is described, which combines features and advantages of the zone, Monte Carlo, and flux model solution methods while avoiding their shortcomings.
Abstract: A new numerical solution method for the calculation of the thermal radiation transfer in combustors is described. It has been especially developed for incorporation in general combustor prediction procedures for the flow, and chemical reaction. It combines features and advantages of the zone, Monte Carlo, and flux model solution methods while avoiding their shortcomings. It is based on the solving of representatively directed beams of radiation within the enclosure between the known wall boundary conditions and on the subsequent computing of the radiation sources which arise within the finite difference control volumes of the flow procedure due to the passage of the beams. It is fast, exact applicable to complex geometries, and it retains in evidence the physics of the problem by avoiding complex mathematics.

526 citations


Journal ArticleDOI
TL;DR: In this article, the authors present an expression which gives the degree of confidence that can be assigned to the measured mean discontinuity spacing, and a reduced form of this expression is obtained for cases where the discontinuity spacings follow the negative exponential distribution.

486 citations


Book ChapterDOI
TL;DR: The effect–summation criterion may be used when the effects of all the agents in a combination are directly proportional to dose, and the key to any criterion for examining interactions between different agents lies in the definition of zero interaction.
Abstract: Publisher Summary The existence of any substantial interaction between different agents that are used clinically or to which man is exposed environmentally is potentially of great importance. One agent may affect another's absorption, metabolism, or excretion. It may alter tissue sensitivity to another agent, and may react with it physically or chemically. A variety of different criteria have been devised for deciding whether the agents in a combination interact pharmacologically. The purpose of this chapter is to examine these criteria critically for the progress in this field. To summarize, the effect–summation criterion may be used when the effects of all the agents in a combination are directly proportional to dose. The key to any criterion for examining interactions between different agents lies in the definition of zero interaction. The interactions that can be analyzed by constructing isoboles or calculating interaction indices are not restricted to cases in which all agents in a combination produce the effect under consideration. The therapeutic significance of interactions between agents requires careful consideration.

455 citations


Journal ArticleDOI
01 Jan 1981
TL;DR: In this article, the effect of the y-component of the IMF on the open model of the magnetosphere is discussed in terms of the open models of the tail and its plasma populations.
Abstract: Observed magnetospheric asymmetries which occur in response to the y-component of the IMF are discussed in terms of the open model of the magnetosphere. The torque which the IMF exerts on the magnetosphere about the Earth-Sun axis results in asymmetric addition of open flux tubes to the tail lobes about the noon-midnight meridian. In response an IMF-associated By field appears across the tail lobes. The ratio between internal and external By fields will generally be same as the ratio between internal and external electric fields. If the tail flux asymmetry is related to an asymmetric distribution of the field normal to the tail magnetopause then an asymmetry in tail lobe electric field and plasma populations will immediately result, as observed. If the flux asymmetry is associated with a twist in the tail then the By field will appear but not necessary the electric field and plasma asymmetries. Generally both effects may occur together. Simple open tail lobe models are derived which demonstrate the asymmetry effects. These represent more physically satisfactory models of the tail and its plasma populations than available hitherto, but they remain somewhat unrealistic in a number of respects. Finally, it is shown that the observed asymmetry effects on closed (auroral zone) field lines may be at least qualitatively accounted for if the cross-magnetosphere IMF-associated By field pervades not only the open but also the closed field line regime, as may be generally expected.

440 citations


Journal ArticleDOI
26 Feb 1981-Nature
TL;DR: A quantitative discussion of the possibilities of controlling rabies by culling or vaccinating foxes (or by a mixture of the two) is given.
Abstract: A simple mathematical model for the overall dynamics of the interaction between fox populations and rabies is presented. The model helps to explain epidemiological patterns observed in Europe, including the 3 to 5 year cycle in fox populations infected with rabies, threshold densities and average levels of prevalence of infection. We give a quantitative discussion of the possibilities of controlling rabies by culling or vaccinating foxes (or by a mixture of the two).

428 citations


Journal ArticleDOI
01 Jun 1981-Networks
TL;DR: This paper gives a survey of a general relaxation procedure whereby the state-space associated with a given dynamic programming recursion is relaxed in such a way that the solution to the relaxed recursion provides a bound which could be embedded in general branch and bound schemes for the solution of the problem.
Abstract: It is well-known that few combinatorial optimization problems can be solved effectively by dynamic programming alone, since the number of vertices of the state space graph is enormous. What we are proposing here is a general relaxation procedure whereby the state-space associated with a given dynamic programming recursion is relaxed in such a way that the solution to the relaxed recursion provides a bound which could be embedded in general branch and bound schemes for the solution of the problem. This state space relaxation method is analogous to Langrangian relaxation in integer programming. This paper gives a survey of this new methodology, and gives, as examples, applications to the traveling salesman problem (TSP), the timeconstrained TSP and the vehicle routing problem (VRP). Valid state space relaxations are discussed for these problems and several bounds are derived in each case. Subgradient optimization and “state space ascent” are discussed as methods of maximizing the resulting lower bounds. More details of the procedures surveyed in this paper can be found in [2 ,3 ,41.

Journal ArticleDOI
TL;DR: The law governing the experimental temperature dependence of the relaxation time of spin glasses τ −1 = ƒ o exp [ −A (T −T o ) ] is interpreted in terms of interactions between the magnetic clusters constituting them as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this article, the influence of the thickness of adjacent competent layers and also the lithology of the competent beds on fracture spacing is indicated; and possible mechanisms leading to the development of these relationships are briefly discussed.

Journal ArticleDOI
TL;DR: The composition and metabolic fate of plasma V LDL may be greatly influenced by the secretion rates of VLDL TG and apo-B, and the increased LDL production in FCHL compared to FHTG may account for a higher cardiovascular risk.
Abstract: Turnover kinetics of triglycerides (TG) and apolipoprotein-B (apo-B) of plasma very low density lipoprotein (VLDL) and their relationship to plasma VLDL composition and VLDL apo-B conversion to low density lipoprotein (LDL) were determined in age and weight-matched groups of normolipemic (NL) healthy subjects, patients with familial combined hyperlipidemia (FCHL) and patients with familial hypertriglyceridemia (FHTG). In NL subjects, a significant correlation as observed between VLDL TG or VLDL apo-B turnover rate and its circulating mass, suggesting that the plasma level of VLDL was determined by the secretion rate of VLDL TG and apo-B. The positive significant correlation between VLDL TG and apo-B also suggests that the production of these moieties was integrated at the synthetic and/or secretory sites to maintain the ratio of TG to apo-B in plasma VLDL. In moderately obese NL subjects, proportionate increases in VLDL TG and apo-B turnover rates resulted in enhanced secretion of VLDL particles. Both groups with genetic hypertriglyceridemia had increased VLDL TG and VLDL apo-B turnover rates. This increase accounted for the increase in circulating VLDL TG and apo-B mass. In patients with FCHL, turnover rates of VLDL TG and apo-B were equally increased, hence, the ratios between major VLDL constituents were within normal limits. On the other hand, the increase in VLDL TG turnover in patients with FHTG was disproportionately greater than that of apo-B resulting in a higher ratio of TG to other VLDL components. In NL subjects, approximately 72% of VLDL apo-B released into plasma was converted to LDL. This conversion correlated positively with VLDL apo-B turnover rate and inversely with VLDL TG turnover rate. Formation of LDL from VLDL was significantly greater in the obese individuals. In FCHL, conversion of VLDL to LDL represented the major pathway for VLDL apo-B catabolism. The increased VLDL apo-B load was predominantly catabolized to LDL. The greater increase in VLDL TG turnover relative to apo-B in FHTG, on the other hand, resulted in a smaller fraction of VLDL apo-B recovered in LDL, most of the VLDL apo-B being removed via a pathway that did not involve this conversion. We conclude that the composition and metabolic fate of plasma VLDL may be greatly influenced by the secretion rates of VLDL TG and apo-B. If VLDL conversion to LDL and the subsequent catabolism of the latter provides a major route for delivery of cholesterol ester to peripheral tissues, then the increased LDL production in FCHL compared to FHTG may account for a higher cardiovascular risk.

Journal ArticleDOI
TL;DR: The presence of predeposited or surface-accumulated Sn during MBE growth modifies the reconstruction of the (001)GaAs surface, as observed by RHEED, at coverages as low as 0.025 monolayer.

Journal ArticleDOI
TL;DR: Progress in the application of numerical models, and in the refinement of the physical models, could be greatly accelerated if practitioners were enabled to employ a reliable and economical standard computer program, incorporating the well-established balance laws of fluid mechanics, heat transfer and mass transfer.

Journal ArticleDOI
TL;DR: In this paper, a new formulation of N = 1 Poincare supergravity is presented with an axial vector A μ and an antisymmetric tensor a μν as auxiliary fields.

Journal ArticleDOI
26 Feb 1981-Nature
TL;DR: Here it is shown that for insects in natural conditions, strongly asymmetrical competition (amensalism or near amensalistism) is the norm rather than the exception by a ratio of at least 2:1.
Abstract: It is widely believed that when populations of two species of animals compete, each has an adverse effect on the other1,2. In other words, each zero growth isocline (dNi/dt = 0 where Ni is the population density of species i) is assumed to be some negative function of the population density of the other species. Cases where one species has a marked effect on the other, but there is no detectable reciprocal effect are sometimes distinguished as ‘amensalism’2,3. This is regarded as unusual. Standard textbooks of ecology either do not mention it1,4–6, define it but do not discuss it2, or occasionally give a brief account3. Ricklefs7 states that “competition can manifest itself by reducing the numbers of one or both competing species” without saying which is the more usual. Here we show that for insects in natural conditions, strongly asymmetrical competition (amensalism or near amensalism) is the norm rather than the exception by a ratio of at least 2:1.

Journal ArticleDOI
TL;DR: The fragmentation pathways observed are defined and the utility in sequence analysis is commented on and a simple acetylation experiment is shown to aid rapid sequence assignment.

Journal ArticleDOI
TL;DR: In this paper, fundamental aspects of the wear of organic polymers are reviewed and a personal view of the state of our current understanding of this subject is presented. But this is not a comprehensive review.

Journal ArticleDOI
TL;DR: In this paper, it was shown that the maximally extended supersymmetric Yang-Mills theory is conformally invariant, under the assumption that the theory is O(4)-invariant and that the structure of anomalies is given by the breakdown of conformal invariance in its coupling to supergravity.

Journal ArticleDOI
TL;DR: In this paper, an ion velocity distribution function f(v) extending from solar wind energies (-1 keV) to 1.6 MeV during the postshock phase of an energetic storm particle (ESP) event was obtained.
Abstract: Data from the Los Alamos Scientific Laboratory/Max-Planck-Institut fast plasma experiment on Isee 2 have been combined with data from the European Space Agency/Imperial College/Space Research Laboratory low-energy proton experiment on Isee 3 to obtain for the first time an ion velocity distribution function f(v) extending from solar wind energies (-1 keV) to 1.6 MeV during the postshock phase of an energetic storm particle (ESP) event. This study reveals that f(v) of the ESP population is roughly isotropic in the solar wind frame from solar wind thermal energies out to 1.6 MeV. Emerging smoothly out of the solar wind thermal distribution, the ESP f(v) initially falls with increasing energy as E/sup -2.4/ in the solar wind frame. Above about 40 keV no single power law exponent adequately describes the energy dependence of f(v) in the solar wind frame. Above approx.200 keV in both the spacecraft frame and the solar wind frame, f(v) can be described by an exponential in speed (f(v)proportionale/sup -v/v//sub o/) with v/sub o/ = 1.05 x 10/sup 8/ cm s /sup -1/. The ESP event studied (August 27, 1978) was superposed upon a more energetic particle event which was predominantly field-aligned and which was probably of solar origin.more » Our observations suggest that the ESP population is accelerated directly out of the solar wind thermal population or its quiescent suprathermal tail by a stochastic process associated with the shock wave disturbance. The acceleration mechanism is sufficiently efficient that approx.1% of the solar wind population is accelerated to suprathermal energies. These suprathermal particles have an energy density of approx.290 eV cm/sup -3/.« less

Journal ArticleDOI
16 May 1981-BMJ
TL;DR: A significance association was shown between serious neurological illness and pertussis vaccine, though cases were few and most children recovered completely.
Abstract: OBJECTIVE--To determine long term outcome in children who had a severe acute neurological illness in early childhood associated with pertussis immunisation. DESIGN--Follow up study of cases and matched controls. SETTING--Assessment of children at home and at school throughout Britain. SUBJECTS--Children recruited into the national childhood encephalopathy study in 1976-9 were followed up, with one of their two original matched controls, in 1986-9. MAIN OUTCOME MEASURES--Performance in educational attainment tests; behaviour problems reported by teachers and parents; continuing convulsions; evidence of other neurological or physical dysfunction. RESULTS--Over 80% of cases and controls were traced. Case children were significantly more likely than controls to have died or to have some form of educational, behavioural, neurological, or physical dysfunction a decade after their illness. The prevalence of one or more of these adverse outcomes in case children who had been immunised with diphtheria, tetanus, and pertussis vaccine within seven days before onset of their original illness was similar to that in case children who had not been immunised recently. The relative risk for recent diphtheria, tetanus, and pertussis immunisation in children who had died or had any dysfunction in comparison with controls was 5.5 (95% confidence interval 1.6 to 23.7). However, the number of cases associated with vaccine (12) was extremely small and statistically vulnerable, and other possible agents or predisposing factors could not be excluded. CONCLUSIONS--Diphtheria, tetanus, and pertussis vaccine may on rare occasions be associated with the development of severe acute neurological illnesses that can have serious sequelae. Some cases may occur by chance or have other causes. The role of pertussis vaccine as a prime or concomitant factor in the aetiology of these illnesses cannot be determined in any individual case. The balance of possible risk against known benefits from pertussis immunisation supports continued use of the vaccine.

Journal ArticleDOI
01 Oct 1981-Nature
TL;DR: A library of 50,000 recombinants representative of the human X chromosome has been constructed for the investigation of genetic diseases such as Duchenne muscular dystrophy and of neoplasia, where several specific chromosomal anomalies have been identified.
Abstract: A library of 50,000 recombinants representative of the human X chromosome has been constructed. Human X chromosomes were physically separated using a fluorescence-activated cell sorter. The DNA was purified from the chromosomes, digested to completion with the restriction enzyme EcoRI and cloned into the phage lambda gtWES.lambda B. The X-derived nature of the recombinants was confirmed by hybridization to rodent/human cell line DNA containing only the human X chromosome. Such libraries will be particularly useful for the investigation of genetic diseases such as Duchenne muscular dystrophy, where the basic defect has not been elucidated, and of neoplasia, where several specific chromosomal anomalies, particularly for the leukaemias, have been identified.

Journal ArticleDOI
TL;DR: In this paper, two procedures for detecting observations with outlying values either in the response variable or in the explanatory variables in multiple regression are presented as half normal plots with envelopes derived from simulation in order to avoid overinterpretation of the data.
Abstract: SUMMARY The paper describes two procedures for detecting observations with outlying values either in the response variable or in the explanatory variables in multiple regression. These procedures are presented as half normal plots with envelopes derived from simulation in order to avoid overinterpretation of the data. Analysis of a well-known data set leads to the use of a data transformation, a simple test for which is commended, and to some comments on the relationship with robust regression. The widespread availability of sophisticated computer software has made the fitting of multiple regression equations painless, but the very ease of these procedures may cause insufficient care to be given to checking and scrutiny of the data. Transcription, punching and data manipulation errors may lead to bad values both of the observations y and of the independent or explanatory variables x. It is the purpose of the present paper to describe and exemplify two plots which provide checks against such bad values. It is argued that these plots and a test for transformations should accompany any thorough regression analysis. Bad values of the response, or outliers, have long been detected by a variety of plots of residuals. Bad values of the explanatory variables are less easily detected even when they lead to an extreme point in the design space. Since the fitted equation will pass close to such an influential observation the residual may not be especially large even after allowance has been made for the small variance of the fitted value at such a point. To detect such behaviour a quantity is needed which exhibits the dependence of the fitted model on each point or group of points. Discussions of such quantities are given by Hoaglin & Welsch (1978) and, more recently, by Belsley, Kuh & Welsch (1980, ? 2 1), by Cook & Weisberg (1980) and by Pregibon (1981). The quantity used here is a modification of a statistic proposed by Cook (1977), which is derived in ? 2. Two half normal plots are presented in ? 3. As an aid to interpretation envelopes to the plots are generated by simulation. Examples of the plots are given in ? 4, both for simulated data and for the stack loss data given by Brownlee (1965, p. 454). Transformations of this data set are discussed in ? 5. The paper concludes with some comments on the relationship with robust

Journal ArticleDOI
TL;DR: In this paper, the effects of stream turbulence on two-dimensional, separated and reattaching flows were investigated and it was found that the mean flow field responds strongly to turbulence intensity but with little effect of integral scale.

Journal ArticleDOI
TL;DR: In this article, hot-wire measurements of second-and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step.
Abstract: Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.

Journal ArticleDOI
01 Oct 1981-Zeolites
TL;DR: A review of some aspects of zeolitization giving it, firstly, its setting among other crystal growing procedures and its relation to pH and temperature among other minerals which can be made hydrothermally.

Journal ArticleDOI
28 May 1981-Nature
TL;DR: The results demonstrate that the renin–angiotensin system maintains blood pressure in these conditions and that the system is an important controller of aldosterone secretion as well as sodium excretion on a normal sodium intake.
Abstract: The view that the renin–angiotensin–aldosterone system is a defence mechanism that maintains blood pressure only in conditions of volume depletion or sodium restriction1 has been supported by studies with saralasin2,3, a competitive inhibitor of angiotensin II, and teprotide1, an inhibitor of angiotensin converting enzyme that blocks the formation of angiotensin II. However, both saralasin and teprotide will underestimate the role of the renin–angiotensin system because: (1) they have to be given intravenously, thus inhibiting only the short-term effects of circulating angiotensin II; (2) all subjects in these studies were rested for some time before infusion1–3; and (3) saralasin is an agonist4–7 which only inhibits the action of circulating angiotensin II when its level is high. Niarchos et al.8 demonstrated a drop in blood pressure with teprotide in normal subjects on a sodium intake of 150 mmol per day, but this was partly due to one subject fainting. Therefore, none of these studies clearly shows at what stage of volume depletion the system actively maintains blood pressure. By analogy with other biological systems, a gradual increase in the importance of the renin–angiotensin system as sodium is lost is more likely than an all-or-none phenomenon. Using the recently developed oral inhibitors of angiotensin converting enzyme9–11, it is now possible to inhibit the formation of angiotensin II long-term and study human subjects during normal activity. We have now investigated the effect of one such inhibitor, captopril, in normotensive, healthy, male subjects on a normal sodium intake. Our results demonstrate that the renin–angiotensin system maintains blood pressure in these conditions and that the system is an important controller of aldosterone secretion as well as sodium excretion on a normal sodium intake.

Book ChapterDOI
01 Jan 1981
TL;DR: In this article, the authors provide an introduction to nonlinear filtering from two points of view: the innovations approach and the approach based upon an unnormalized conditional density, which concerns the estimation of an unobserved stochastic process given observations of a related process.
Abstract: In this paper we provide an introduction to nonlinear filtering from two points of view: the innovations approach and the approach based upon an unnormalized conditional density. The filtering problem concerns the estimation of an unobserved stochastic process xt given observations of a related process yt; the classic problem is to calculate, for each t, the conditional distribution of xt given ys, 0 ≤ s ≤ t. First, a brief review of key results on martingales and markov and diffusion processes is presented. Using the innovations approach, stochastic differential equations for the evolution of conditional statistics and of the conditional measure of xt given ys, 0 ≤ s ≤ t are given; these equations are the analogs for the filtering problem of the kolmogorov forward equations. Several examples are discussed. Finally, a less complicated evolution equation is derived by considering an “unnormalized” conditional measure.