Hypoxia, fetal and neonatal physiology: 100 years on from Sir Joseph Barcroft

During the history of the Earth there have been dramatic changes in its oxygen (O₂) availability (Graham et al. 1995). Geochemical models suggest that in the mid‐to‐late Devonian period (ca 380 million years ago, mya), O₂ levels in the Earth's atmosphere increased from ∼18 to 20%. By the late Carboniferous period, the Earth's oxygenation had risen sharply to ∼35%, which palaeontologists refer to as ‘the oxygen pulse’. Through the Permian period within the Palaeozoic era, approximately 250 mya ago, O₂ concentrations fell to ∼11%. Thereafter, in the last 200 million years, O₂ levels steadily increased again, settling at the present day level of ∼21%.

These fluctuations in O₂ availability have shaped the evolution of animal life on Earth and are one of the earliest challenges in physiological history. When O₂ availability increased, animals harnessed this new fuel and grew larger irrespective of their phyla. Falkowski and colleagues (2005) suggest that the doubling in O₂ over the last 200 million years coincided with the evolution of physiological traits that allowed mammals to thrive. These traits include endothermy and the appearance of placentation (80–60 mya), which channelled O₂ to the growing conceptus, thereby allowing mammalian diversification. Since the Earth became better oxygenated, the greatest challenge to aerobic animals became hypoxia. Therefore, animals evolved compensatory mechanisms to withstand episodes of O₂ deprivation. How organisms consumed, distributed and utilised O₂ under normoxic and hypoxic conditions was a key focus of Sir Joseph Barcroft's research. Generations of physiologists have since delineated the mechanisms which allow fetal and adult animals to compensate for a low $P_{\mathrm{a}{\mathrm{O}}_2}$. These concepts resonate through many of the papers presented in this issue of The Journal of Physiology, which is devoted to celebrating the centenary of Barcroft's research and is appropriately entitled: Hypoxia, Fetal and Neonatal Physiology: 100 years on from Sir Joseph Barcroft. The initial motivation for this volume was the centenary celebration of the 1914 opening of the Physiological Laboratory, a purpose‐built research building for the Department of Physiology on the Downing site of the University of Cambridge, where Joseph Barcroft worked for most of his career.

Barcroft's childhood and undergraduate studies

Joseph Barcroft, born 26 July 1872, was the second of five children of a linen merchant, Henry Barcroft, and his wife Anna. As a child in Northern Ireland, Joseph enjoyed the outdoors, sports and painting, but had no formal lessons in reading, writing or arithmetic until the age of seven. He came to Cambridge for his schooling as a teenager and, whilst still at the Leys school, achieved the impressive feat of obtaining a Bachelor of Science degree from the University of London (Fig. 1). Joseph went on to study Natural Sciences at the University of Cambridge, being admitted to King's College, where he remained as a Fellow for the rest of his life. As an undergraduate, Barcroft was keenly interested in all branches of the sciences; he even made one of the first British demonstrations of an X‐ray photograph to the Natural Sciences Club. In his third undergraduate year, he specialized in Physiology, when again he excelled, obtaining a first class degree in 1897, although reports suggest that he was self‐deprecating about his successes, claiming that he had obtained his high marks only by playing off the internal against the external examiner! (Roughton, 1949 a, b; Franklin, 1953).

Figure 1. Joseph Barcroft, a month before his third birthday (left) and as Bachelor of Science of the University of London (19 years of age, right) .

From Franklin KJ (1953). Joseph Barcroft 1872–1947. Blackwell Scientific Publications Ltd. Oxford, UK. Reproduced with permission.

Barcroft's research and 100 years on

Joseph Barcroft began experimental research as soon as he graduated from Cambridge in 1897 and continued until the day of his death aged 74 (Frankin, 1953). He had no formal mentor or supervisor but his first project was suggested by John Newport Langley, the Professor of Physiology at Cambridge. Barcroft was to investigate the role of nerves in regulating the production of saliva. This involved measuring the rates of O₂ uptake and carbon dioxide output as well as saliva production by a small secretory gland found in the lower jaw of most mammals. This research showed for the first time that nerves were important in stimulating saliva production, in part, by increasing O₂ consumption (Barcroft, 1900). It also initiated Barcroft's life‐long interest in the respiratory gases and, in particular, in O₂ and its association with haemoglobin in the blood.

Barcroft's research has interested scientists for decades and many accounts of his works are available today. In this issue of The Journal of Physiology, there are additional biographical articles. Lawrence Longo describes this ‘Victorian physiologist's contributions to a half century of discovery’ with authority as a physiologist and a medical historian (Longo, 2016). John West (2016) writes a controversial piece highlighting ‘Barcroft's bold assertion’ that all dwellers at high altitudes are persons of impaired physical and mental powers (Barcroft, 1925), a statement that understandably continues to trigger much ongoing discussion today! Finally, Peter Nathanielsz (2016) adds a personal touch, reporting on the 1972 symposium on Fetal and Neonatal Physiology that he helped organise for The Physiological Society to celebrate the centenary of Joseph Bancroft's birth. Many of these biographical accounts state that from about 1900, Barcroft began working on the handling of gases in the blood more directly. He was interested in how tissues of the body received and used O₂, especially at high altitude where O₂ is limited in availability. In 1910 and 1911, he led mountain expeditions to Tenerife and Italy to study oxygenation and haemoglobin function at lower than normal partial pressures of O₂. This period of research culminated in the publication of his first book, The Respiratory Functions of Blood, in 1914. His expertise in this area, and his success as a scientist generally, were based on two main attributes: first, his technical ability in making new equipment to measure, for instance, the volume and partial pressures of gases in small amounts of blood (Fig. 2), and secondly, his intellectual ability in designing and performing experiments that others thought impossible, often carrying them out on himself. An early film on the study of the blood–oxygen equilibrium, originally made by Joseph Barcroft and found by chance in some obscure corner of Cambridge, beautifully, and somewhat amusingly, demonstrates these attributes (https://archive.org/details/Bloodandrespiration‐wellcome, Wellcome Library).

Figure 2. Joseph Barcroft's apparatus for blood gas extraction (1900) .

From Franklin (1953).

As a Quaker, Barcroft was seconded to government research at Porton Down during the First World War as the Chief Physiologist. Given his interest in respiration, his instructions were to investigate the causes, consequences and possible treatments of gas poisoning, a common weapon in trench warfare on the Western front. Again, his investigations were frequently ‘self‐experiments’, pushing himself to the limits of his own physiological compensation on several occasions (Roughton, 1949 a, b; Franklin, 1953). After the war, Barcroft returned to Cambridge and undertook more basic research on the distribution of blood to different organs and on the question of whether O₂ was secreted actively into blood from the lungs when its atmospheric concentration was low. To answer this question, Barcroft took two different approaches. First, he led another, longer expedition to Cerro de Pasco in Peru in 1920–1921 to study pulmonary gas exchange and blood chemistry at high altitude. This expedition was funded by the Royal Society and was better equipped and manned than his previous expeditions. It involved taking a train with a mobile laboratory car to an altitude of 4328 m and analysing the O₂ content of blood by direct arterial puncture of expedition volunteers and of members of non‐native and native communities who had been living at this altitude for months or several generations, respectively. An account of this expedition can be read in the transcripts of letters from Barcroft sent home to his family that are housed in the archives of the Royal Society in London and in the library of the Department of Physiology, Development and Neuroscience at Cambridge. Secondly, he built himself an air‐tight glass chamber at the Physiological Laboratory, where he could live and exercise at O₂ levels equivalent to 4877 m (Fig. 3; Barcroft, 1920). In this latter paper, Barcroft adds some colourful description of his self‐experimentation, including radial artery catheterisation, and recovery following fainting with tea and brandy! Barcroft was so enthusiastic about these experiments that he had to be extracted by his colleagues from his hypoxic ‘room’ after close to 6 days in situ (Franklin, 1953). Together, these experiments proved that O₂ was not secreted from the lungs but moved into the blood by diffusion, although there were changes in the O₂ dissociation curve of blood with acclimatisation to low O₂ levels. He, therefore, updated his book, The Respiratory Functions of Blood, and divided it into two parts, one dealing with the lessons from high altitude (1925) and the other with the contribution of haemoglobin to tissue O₂ delivery (1928).

Figure 3. Joseph Barcroft inside the ‘Glass chamber’ during his experiment in 1920 .

From Franklin KJ (1953). Joseph Barcroft 1872‐1947. Blackwell Scientific Publications Ltd. Oxford, UK. Reproduced with permission.

Fascination with the physiological effects of high altitude persists to this day. In this issue of The Journal of Physiology, Murray & Horscroft (2016) and Jacobs and colleagues (2016) expand on the effects of high altitude on mitochondrial function and density in muscle, recapitulating Barcroft's self‐experimentation and reporting on investigations performed on biopsies of their own vastus lateralis! Similarly, Veith et al. (2016) and Hodson et al. (2016) bring us up‐to‐date with the molecular mechanisms underlying hypoxia‐inducible factor (HIF)‐induced pulmonary hypertension and the importance of the enzyme–substrate couple of the principal HIF, prolyl hydroxylase 2 (PHD2)/HIF‐2α, in modulating ventilatory sensitivity to hypoxia. Finally, Fatemian et al. (2016) report that the peripheral chemoreflex sensitivity to CO₂ can serve as a predictor of human acclimatization to high‐altitude hypoxia.

The final phase of Barcroft's research, for which he is best remembered now, did not begin until 1932, when he was 60 and, upon the death of Langley, appointed Professor of Physiology at Cambridge.

Barcroft became intrigued by how the fetus, developing within its mother, received enough O₂ to grow so rapidly. This was one area in which he could not experiment on himself so he used pregnant animal models to determine how fetuses respire. He thought that the fetus had a lower basal arterial O₂ level than its mother and might be like the acclimatised mountaineer at high altitude. So he set about studying how fetuses survived and thrived on Everest in utero, a phrase he coined that has been used extensively since to describe the conditions in which the fetus normally develops (Barcroft, 1935). Using pregnant sheep, under the influence of anaesthesia, in acute preparations developed by Huggett (1927) where fetuses were delivered into a warm water bath, Barcroft measured placental function, fetal growth rate, fetal haemoglobin and O₂ carrying capacity, fetal respiratory movements, fetal and placental blood flow and the changes that occur in the fetal circulation and lungs at the time of birth. These studies resulted in the publication of his final book, Researches on Pre‐natal Life, in 1946, the year before he died.

Without question, this publication laid the foundation for Fetal and Neonatal Physiology as a separate branch of Physiology and opened up a new era of basic and clinical research into fetal growth and development during healthy and compromised pregnancies. Although the sheep remains the animal model of choice for studying pregnancy and fetal development, Barcroft's initial acute preparation has been refined to allow studies in the conscious state, through the efforts of Donald Barron, one of Barcroft's most influential post‐doctoral fellows, and of Barron's post‐doctoral researchers Giacomo Meschia and Frederick Battaglia. They developed the chronically catheterised fetal sheep preparation, now used worldwide, which allows the study of the physiology of the mother and fetus after full recovery from surgery without the confounding effects of pre‐operative fasting, anaesthesia and surgical stress (Meschia et al. 1965).

In this issue of The Journal of Physiology, seven papers from laboratories in North and South America, Europe, Australia and New Zealand report on findings using this type of chronically instrumented ovine preparation (Allison et al. 2016; Chang et al. 2016; Clifton et al. 2016; Galinsky et al. 2016; Giussani, 2016; Herrera et al. 2016; Lear et al. 2016). Giussani (2016) reviews the fetal cardiovascular defence to acute hypoxia, highlighting neural, endocrine and vascular redox mechanisms, and introducing the concept of the fetal brain sparing index. Herrera et al. (2016) report on cardiovascular function in term fetal sheep conceived, gestated and studied under the influence of chronic hypoxia by exploiting the natural laboratory of the high altitude of the Andean altiplano. Almost ‘keeping it in the family’, again recapitulating many of Barcroft's preoccupations, Allison et al. (2016) bring the Andean mountains back to Cambridge. Combining bespoke isobaric hypoxic chambers and a wireless data acquisition system, they introduce a new technique that is able to maintain chronically instrumented pregnant ewes and their fetuses under isobaric chronic hypoxia for most of gestation at lower fetal P_aO₂ levels than can be achieved by habitable high altitude and of direct relevance to the degree of hypoxia seen in human infants with significant intrauterine growth restriction. The technology also permits, for the first time, longitudinal wireless recording of maternal and fetal cardiovascular function in free‐moving animals, including beat‐to‐beat alterations in arterial blood pressure and blood flow signals in regional circulations. Lear and colleagues (2016) challenge the long held assumption that the sympathetic nervous system mediates fetal heart rate (FHR) variability in labour. The clinical significance of this report is important and is reviewed by Shaw et al. (2016), as this finding directly contradicts the established clinical interpretation that preserved FHR variability implies adequate fetal physiological compensation. Galinsky et al. (2016) caution on the clinical use of magnesium sulphate for the treatment for pre‐eclampsia and perinatal neuroprotection, as it alters the perfusion of several vascular beds during acute asphyxia, effects which may place the preterm fetus at greater risk of intestinal and renal compromise. Chang et al. (2016) use a transcriptomics approach to predict that hypoxia activates inflammatory pathways and reduces metabolism in the fetal kidney cortex, and show that ketamine ameliorates this response. Thus, their data suggest that ketamine may have therapeutic potential for protection from ischaemic renal damage. Clifton et al. (2016) introduce an ovine experimental model to investigate the effects on the fetus of maternal asthma in pregnancy, permitting evaluation of current as well as novel clinical interventions.

This issue of The Journal of Physiology also focuses on continuing studies on the placenta, the interface between mother and fetus. In placentae obtained from normotensive and pre‐eclamptic women at sea level and at high altitude, Kurlak et al. (2016) report that the placental renin–angiotensin system is responsive to changes in tissue oxygenation. They introduce the concept that this could be important in the interplay between reactive oxygen species as cell‐signalling molecules for angiogenesis and, hence, placental development and function. Three studies that model hypoxic pregnancy in mice highlight placental adaptations to decreased oxygenation at functional, cellular and molecular levels. Higgins et al. (2016) report that hypoxia modifies the placental transport phenotype and resource allocation to fetal growth. The study highlights that there appears to be a threshold between 13% and 10% of maternal inspired O₂, corresponding to altitudes between ca 3700 m and 5800 m, at which the mouse placenta can no longer adapt to support fetal resource allocation with implications for human pregnancies at higher altitudes. Using the same mouse model of hypoxic pregnancy, Skeffington and colleagues (2016) report that adenosine monophosphate‐activated protein kinase (AMPK) is an uterine artery vasodilator and may provide a key molecular link between maternal uterine vascular responses, placental function and fetal growth in normal and complicated pregnancy. They show that manipulation of AMPK may be a novel mechanism for developing new therapies in pregnancies complicated by chronic hypoxia. Finally, Matheson et al. (2016) report further placental adaptations to chronic hypoxia at the molecular level, showing activation of endoplasmic reticulum stress, a conserved homeostatic response that mediates translational arrest through phosphorylation of the eukaryotic initiation factor 2 subunit α (eIF2α), which may underlie fetal growth restriction. They also report sexually dimorphic morphological and molecular changes in the murine placenta exposed to normobaric hypoxia throughout pregnancy.

Further reports in this issue of The Journal of Physiology focus on the transition to postnatal life and potential therapy to ameliorate the problems that face the vulnerable infant. In a beautiful imaging study using simultaneous phase‐contrast X‐ray and angiography in rabbits, Lang et al. (2016) discuss previously unknown mechanisms responsible for mediating the increase in pulmonary blood flow at birth. They conclude that mechanisms unrelated to oxygenation or to the spatial relationships that match ventilation to perfusion initiate the large increase in pulmonary blood flow at birth. McGillick et al. (2016) provide evidence for stimulatory effects of vascular endothelial growth factor (VEGF) administration on structural maturation in the lung of both the normally grown and placentally restricted IUGR sheep fetus, raising VEGF as an interesting potential candidate for therapy against respiratory distress syndrome of the preterm infant. Additional candidate protective therapy for newborn life is provided by Aridas and colleagues (2016), who report that umbilical cord blood stem cells administered after perinatal asphyxia can convey neuro‐protection in newborn lambs, with translational potential for the treatment of human infants following hypoxic ischaemic encephalopathy. In contrast, Barton et al. (2016) caution on the potential adverse side‐effects of erythropoietin administration as a neuroprotective therapy. They report that erythropoietin administration within minutes of the onset of injurious ventilation in newborn lambs can amplify pro‐inflammatory cytokine gene expression in both the periventricular and subcortical white matter.

In 1946, Barcroft wrote: ‘never losing sight of the fact that one day the call will come and the fetus will be born. Not only has the fetus to develop a fundamental life which will suffice for intrauterine conditions, but at the same time it has to develop an economy which will understand the shock of birth, and will suffice, nay more than suffice, for its new environment.’ This statement hints that Barcroft begun to think like many developmental physiologists do today, agreeing that sometimes compensation carries a price. Now we know that compensatory or adaptive responses to adverse intrauterine conditions, while beneficial in terms of survival in utero, may sometimes trigger unwanted adverse side‐effects in the offspring, increasing the risk of pathology in later life (Barker, 1998; Fowden et al. 2006; Gluckman et al. 2008). In fact, programmed cardiovascular and metabolic disease in later life specifically linked to chronic fetal hypoxia has been recently reviewed by Giussani & Davidge (2013). While chronic fetal hypoxia is known to increase the risk of cardiac and endothelial dysfunction in later life (Giussani et al. 2012), recent focus has been on the effects of combined pre‐ and postnatal challenges. In this issue of The Journal of Physiology, Walton et al. (2016) show that late gestational hypoxia combined with a postnatal high‐salt diet exacerbates the programming of endothelial dysfunction and arterial stiffness in adult mouse offspring. Shah and colleagues (2016) report that resveratrol, a natural polyphenol found in grape skin, improves cardiovascular and metabolic health in adult rat offspring exposed to prenatal hypoxia and a postnatal high‐fat diet, and that this protective effect is independent of the sex of the offspring. Vega et al. (2016) conclude that resveratrol also partially prevents increased indices of oxidative stress in the mother, placenta and offspring in pregnancy complicated by maternal under‐nutrition and that some of these effects protect the mother and offspring against metabolic dysfunction.

Barcroft: the person, his final days and his legacy

Joseph Barcroft became much loved for his enthusiasm, kindness and attention to detail as a mentor and teacher as well as being a great communicator, often lecturing without notes (Fig. 4). In the decade preceding World War I, he had supervised almost every Cambridge physiologist, many of whom became successful scientists themselves. According to his biographer, Francis Roughton (1949 a): ‘one of his most charming characteristics was that he always took particular trouble to get to know and to help those working on all rungs of the ladder, from top to bottom, of any institution or concern with which he was connected.’ During his career, Barcroft published > 300 papers, books and research reports, and travelled widely within the UK and abroad to scientific and other educational meetings. He talked ‘in that simple, always exciting and slightly breathless way he had, making all he had discovered seem so self‐evident, poking fun at himself and paying generous tribute to his collaborators’ (Roughton, 1949 a,b). Barcroft also had an ability to attract bright young people to work with him, many of whom continued with research into hypoxia, and fetal and neonatal physiology after his death. It was common knowledge that Barcroft attributed his success to only one thing – a daily afternoon nap! Although Joseph Barcroft retired as Professor in 1937, he remained scientifically active until his sudden death of a heart attack 10 years later. According to his colleague Roughton, that morning, Barcroft had been his usual energetic, shrewd and good‐humoured self, departing to catch a bus with a quip and a smile. When they later heard the news of his death, his colleagues remarked that Joseph Barcroft had gone on doing first class work right up to the last moment of his life, and for him ‘physiology was the greatest sport in the world!’ (Fig. 5) This special issue of The Journal of Physiology is a testament to Barcroft's continuing legacy and to the fascination many of us still find in the unanswered physiological questions that drove his research more than 100 years ago.

Figure 4. Joseph Barcroft lecturing in Lecture Theatre 1 at the Physiological Laboratory, University of Cambridge in 1935 (left) and discussing science with August Krogh in 1929 (right) .

From Franklin KJ (1953). Joseph Barcroft 1872–1947. Blackwell Scientific Publications Ltd. Oxford, UK. Reproduced with permission.

Figure 5. Joseph Barcroft driving off (1929, left) and in the preface to his book The Respiratory Function of the Blood (1914, right) .

In the preface to his book The Respiratory Function of the Blood he stated: ‘At one time, which seems too long ago, most of my leisure was spent in boats. In them I learned what little I know of research, not of technique or of physiology, but of the qualities essential to those who would venture beyond the visible horizon.’ From Franklin KJ (1953). Joseph Barcroft 1872–1947. Blackwell Scientific Publications Ltd. Oxford, UK. Reproduced with permission.

Additional information

Competing interests

The authors declare no conflicts of interest.

Author contributions

All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Funding

D.G. is supported by the British Heart Foundation, The Biotechnology and Biological Sciences Research Council, The Royal Society, The Wellcome Trust, Action Medical Research and the Isaac Newton Trust.