• Abstract and Figures
• Public Full-text

Thorax 1994;49 Supplement:S5-S8 S5 Evolution and the pulmonary circulation Thorax: first published as 10.1136/thx.49.Suppl.S5 on 1 September 1994. Downloaded from Peter Harris We were sitting, I recall, in the canteen of the their characteristic colour, was chlorophyll. Medical School at Birmingham. It must have The problem for the rest of the inhabitants been 1958. I had just returned from a year and ofthe planet was that the new process ofenergy a half at the Bellevue Hospital, New York, transformation had a highly toxic waste prod- my mind still reeling from the experience of uct. This was oxygen. Its toxicity is not due to working in the cardiopulmonary laboratory of the diatom of the molecule of oxygen, but Andre Cournand. Donald Heath had returned to other forms of oxygen known as oxygen free from the Mayo Clinic where he had coolly radicals. The free radicals are highly reactive reshaped the face of pulmonary vascular path- chemically and attack many organic molecules.' ology. And it was there, in the canteen, that They inactivate enzyme proteins by oxidising we planned to write a book on the pulmonary their sulphydryl groups, and they destroy cell circulation. It was the beginning of a col- membranes by forming lipid peroxides. laboration and friendship which was to last to The new photosynthetic process was highly this day and through which I came to ap- successful to the extent that the earth's at- preciate, among many other things, those ima- mosphere gradually accumulated oxygen. Such ginative intellectual processes whereby static pollution spelt death to most of the forms of anatomical images of dead tissue under the life then existing, and the threat to the world microscope can be transfigured into dynamic community must have been immense. concepts of living pathological processes. Doubtless, through genetic processes they During the succeeding 30 years our mutual responded by developing mechanisms ofchem- interests led us on a series ofexpeditions to high ical defence. Some were simple intracellular altitudes which, in unexpected ways, added a compounds such as glutathione which could depth of understanding and insight into com- scavenge the free radicals. Others, such as mon diseases back home. We were concerned, superoxide dismutase, were enzymes. Even not with the short term problems of ac- today these defences are still protecting the climatisation encountered by the occasional cells in our bodies, preventing - for example - http://thorax.bmj.com/ visitor to the mountains, but with the evidence our adipose tissue from going rancid inside us. for genetic adaptation in populations of humans and animals that have, for generations, been fated to spend their entire lives there. Oxygen as fuel Such studies have, inevitably, prompted The force of life is always opportunistic and, wider considerations of the way in which ge- at a certain point, some organisms learned how netic influences have fashioned the lungs and, to use the properties of oxygen as an electron in the following review, I shall try to give a donor and proton acceptor and to harness its on September 25, 2021 by guest. Protected copyright. more general background ofthe evolution ofthe reduction to the formation of ATP. An even lungs and to show how important evolutionary more astonishing step was the takeover by nuc- influences may be in understanding the pul- leated organisms of these oxygen utilising monary circulation. forms, which became incorporated in their cytoplasm as mitochondria where they have remained ever since. In this extraordinary way Pollution of the atmosphere with oxygen oxygen changed from being a lethal pollutant Life on our planet began in an atmosphere ofthe atmosphere to become the element most devoid of oxygen. The first organisms derived generally essential to life on the planet. their energy from the breakdown of complex molecules which had been formed in the im- mense heat of the creation of the world. Some Diffusion and convection of the primitive pathways of anaerobic meta- Life had its origin in water, and the first cells bolism persist to this day in every cell of our floated freely in that medium which is essential bodies. to all living organisms. The cell wall was a It may be that the supply of such complex subtle compromise between the necessity to molecules became used up. Perhaps for this maintain a barrier which would protect an reason, or more probably out of random dis- identity and the necessity to import and export covery, about a billion and a half years ago a materials. Because of their minute size the group of microorganisms discovered a way of unicellular organisms were able to exchange producing biologically useful energy from the food and excrement directly across their sur- sunlight which fell unfiltered on the earth. face. The movement of such materials to and S Polo 2171, were the algae. The essential was 30125 Venice, These blue-green from the cell primarily by diffusion, al- Italy substance through which the conversion of en- though the random currents of the watery en- P Harris ergy could be effected, and which gave them vironment must also have provided a fortuitous S6 Hamis help. As multicellular organisms developed and of 90-150 ml/min/kg, while that of fishes3 is grew larger, however, simple diffusion became about 10-40 ml/min/kg. Thorax: first published as 10.1136/thx.49.Suppl.S5 on 1 September 1994. Downloaded from inadequate and they needed to impose their A visit to the local butchers and fishmongers own convective streams on the surrounding will illustrate the reason for the increase in water in order to maintain an adequate ex- oxygen consumption. Mammalian and avian change. Many simple organisms, such as the flesh is mostly red, while fish flesh is white. coelenterates and sponges, use the surrounding The white skeletal muscle of the fish derives water as their sole means of convective trans- its energy mainly from an anaerobic glycolytic port, which serves the combined functions of breakdown, while the red skeletal muscle of ingestion, respiration, and excretion. the mammals and birds is rich in mitochondria As animals grew ever larger and more com- and the carrier pigment myoglobin which per- plex, an internal convective transport system mits an increased use of oxygen. Glycolysis can also became essential. The external convective provide energy only for short intense periods system was retained and, in primitive chordates of activity. The great advantage of oxidative such as amphioxus or the tunicates, provides metabolism is that it permits the prolonged a means for filtering food at the same time activity that has made the warm blooded ani- as for respiratory exchange. Even in such an mals so dominating a force on the planet. advanced form of life as the fishes the function of the external convective system is still mixed. The passage of water through the mouth is The cardiovascular system used for ingestion, while its movement across In the crustaceans and molluscs the internal the gills serves to exchange, not only respiratory convective system is an open one. The cardiac gases, but also water and salts and ammonia, pump moves blood through the arteries directly the chief end product of nitrogenous meta- into the intercellular spaces, whence it returns bolism. to the heart through ostia. In the vertebrates The migration of the animals on to dry land the cardiovascular system has become - for the imposed great restrictions on the external con- most part - a closed one, the blood being vective system. In amphibia considerable ex- separated from the interstitial liquid by change of water, salt, urea, and respiratory capillaries. There are points in the circulation, gases could still take place across the moist however, where this separation is not complete skin. Exploration far from the water's edge - in the liver, spleen, marrow, and renal glom- required an impervious skin and so those func- eruli. In addition the lymphatic system provides tions of the skin largely became discarded. a direct communication between the blood and Instead the skin took over the fuinction of re- the interstitial liquid. gulation of temperature which was required The fish heart4 comprises a single atrium by the evolution of warm blood. By now the and ventricle. The blood is expelled from the exchange of respiratory gases was restricted ventricle into the ventral aorta, then passes http://thorax.bmj.com/ to the newly evolved lungs where a certain through the gills before reaching the dorsal obligatory loss of water and heat also took aorta. The supply of blood to the tissues is place, while the excretion of nitrogenous waste from the dorsal aorta through a second capillary was restricted to the kidneys. bed in series with the exchange capillaries of Motion of the body as a whole is also a form the gills. of external convection. The ambivalence of The amphibian heart is composed of two the function of movement is expressed at its atria and one ventricle. Systemic venous blood simplest in the Paramecium. Does it swim? Or returns to the right atrium, while blood from on September 25, 2021 by guest. Protected copyright. is it simply renewing the water around it? Many the primitive lungs passes to the left atrium. fish use swimming to force water across the The presence of a spiral valve in the conus gills and to trap food in the pharynx. The arteriosus and the positions of the atrio- movement of any predator is only an extension ventricular orifices are believed to direct most of the same principle. And we all walk away of the systemic return to the lungs, and most from our own excrement. In these ways move- of the pulmonary venous blood to the rest of ment of the body takes its place alongside the the body.

Load more

  4

Copy Link