The history of respiratory physiology

Key points

That breathing was essential for life was clear to the ancient Egyptian civilizations 5000 years ago, but the reasons for this were unknown.
Early explanations for the function of breathing involved the air drawn into the lungs fuelling combustion in the heart and removing ‘sooty and fuliginous spirits’ from the body.
In the Renaissance, advances in knowledge of anatomy led to the discovery of the pulmonary circulation and the observation that blood changed colour on passing through the lungs.
Physiology in the 17th century involved more rigorous scientific experimentation and led to several discoveries about the mechanics and function of breathing.
Developments in fundamental sciences, particularly chemistry and physics, facilitated the elucidation of current knowledge of breathing and respiration.

The historical path along which we have gained our current knowledge of respiratory physiology is long and varied. There are periods when our understanding leapt forward in just a few years, interspersed by prolonged periods when progress was negligible, and even some periods when progress was reversed. That breathing is essential for life was clear from the beginnings of history, but the mechanism of breathing and the reasons for it remained elusive for many centuries. Progress usually occurred in parallel with understanding in other scientific disciplines, particularly chemistry, physics and anatomy. Innovative ideas on the physiology of breathing led, in more than one instance, to the premature death of the physiologist, and the history of respiratory physiology includes some of the most famous controversies seen in medical science.

This chapter is of necessity only a brief overview of the subject, and ends around 100 years ago, when the explosion of scientific progress makes the subject too large for such a short account. Significant advances in respiratory physiology in the last 100 years are reported in the other chapters of this book, and the reader interested in the history of this period is referred to more authoritative accounts. ^, For more general information on the history of respiratory physiology, numerous recent sources (by historical standards) are available. ^,

Ancient civilizations

Egyptian physiology

Ancient Egyptian civilizations existed from around 3100 to 332 BCE, when the Graeco-Roman period began. The most remarkable contribution made to history by ancient Egyptians is their writings, although knowledge of their language was mostly lost after CE 500. Approximately 1300 years later, 19th century scholars were able to use the Coptic language to assist in translating the ancient Egyptian writings. This has allowed an insight into medical knowledge from as early as 1820 BCE, the date of the earliest known medical writings in the Kahun papyrus.

Medical papyri

Many Egyptian papyri are concerned with medical topics, mostly descriptions of pragmatic ‘recipes’ for the treatment of a multitude of specific conditions.

The longest and best known of the medical papyri is the Ebers Papyrus, ^, which dates from about 1534 BCE, and is accepted as being a compilation of various earlier works. The Ebers Papyrus is unique in containing a section on physiology, including comments on respiration. The overall purpose of respiration is described thus:

As to the air that penetrates into the nose. It enters into the heart and the lung. They are those which give air to the entire body.

Further sections include detailed descriptions of specific numbers of metu conducting ‘moisture and air’ to many parts of the body. These metu seem to mostly relate to blood vessels, but also probably included such structures as tendons, muscles and the ureters. At first, this primitive view of anatomy is surprising, considering the embalming abilities of ancient Egyptians, although in practice embalming was carried out using very small inconspicuous incisions that would have revealed very little internal anatomy. Two metu are described in each ear, through which ‘ the breath of life enters into the right ear and the breath of death enters into the left ’, illustrating the ‘magical’ aspect of medicine at the time.

Ancient greece

Greek writers were primarily philosophers, but they were also outstanding physicians, with one of their number, Hippocrates, forming a school that is now widely attributed with the foundation of modern medical conduct. Early Greek philosophers such as Anaximenes (570 BCE–?) clearly stated that ‘pneuma’, or air, was essential to life, but in contrast to this correct observation, Alcmaeon reportedly claimed that goats breathed through their ears, and that some air passed from the nose directly to the brain. Empedocles (495–435 BCE) disputed many of Alcmaeon’s writings, suggesting instead that breathing occurred through the skin, and that blood flow from the heart was tidal in nature, ebbing and flowing to and from the heart. Empedocles successfully combined physiology and philosophy in his description of the ‘innate heat’ in the heart, which was closely related to the soul, and which was distributed throughout the body by the heart. This concept of heat generation within the heart gained acceptance throughout the ancient Greek period and was to remain at the centre of respiratory physiological ideas for about 1000 years.

The writings of Plato, Aristotle and the Hippocratic school only rarely directed their attention to respiration, but their contribution to scientific method and thinking was enormous. Subsequent philosopher-physicians adopted a more scientific approach to investigating physiology. At this time, dissection became widely practised, sometimes in public, and on both animals and humans. Animal vivisection also took place, and there are even disputed reports of human vivisection of criminals. Herophilus (circa 325 BCE) distinguished between arteries and veins, and, along with Aristotle, asserted that they contained air. Erasistratus (304–250 BCE), more widely renowned as the father of philosophy, was the first to apply scientific principles to explain breathing. His view was that air was taken into the lungs and passed to the heart along the pulmonary artery. In the heart, air was converted into a ‘vital spirit’ that was distributed to all parts of the body by the arteries, whilst the brain further converted the vital spirit into ‘animal spirit’ which travelled down the hollow nerves to activate muscles. Erasistratus seemed to understand that heart valves only allowed flow to occur in one direction but failed to apply this knowledge to elucidate the transport of vital spirit around the body. After Erasistratus, Greek interest moved away from medicine to philosophy and the physical sciences, and the progression of physiological knowledge halted for about 400 years.

Roman medicine and galen (129–199 BCE)

By the age of 28 years Claudius Galen was physician to the gladiators of Pergamun, and 12 years later became physician to the Roman emperor Marcus Aurelius. He wrote numerous works on anatomy and physiology, many of which still exist in modern form, including two with much material on respiration: On the usefulness of the parts of the body and On the use of breathing . ^, Galen’s work provides the first direct evidence of experimentation and the application of clinical observations to explain physiology.

Galen’s system of physiology and anatomy

In Galen’s descriptions, food was processed in the gut before being used by the liver to produce blood, which passed to the right heart. Much of this blood flowed into the pulmonary artery to nourish the lung, whilst the remainder passed across invisible pores in the interventricular septum, to be combined with ‘pneuma’ brought from the lung via the pulmonary vein ( Fig. 35.1 ). In the left heart, the pneuma instilled the blood with vital spirit that was circulated to the body and brain, as described by Erasistratus.

• Fig. 35.1, Illustration reconstructing Galen’s scheme of cardiovascular and respiratory physiology as described in the text. Galen did not use illustrations in his writings; this diagram is taken from reference 12.

Anatomically Galen regarded the lungs as having three types of intertwining vessels, the pulmonary artery, pulmonary vein and the ‘rough artery’ (trachea). On respiratory mechanics, Galen regards the ribs as primarily providing protection for the intrathoracic organs, particularly the heart, but he also clearly describes the role of the intercostal muscles and diaphragm in effecting both inspiratory and expiratory movements. He understood the potential problems of diaphragmatic splinting, describing respiration as ‘little and fast’ in such conditions as pregnancy and ‘water or phlegm in the liver’.

Experiments on respiration

Galen’s experiments provided mixed results:

1.
For the first time he proved that arteries contained no air, but only blood, by ligating an animal’s artery in two places before opening the vessel under water. He wrote at length about blood flow, realizing that tidal blood flow to and from the lungs with each breath was ‘ in no way suitable for the blood ’. He suggested the existence of capillaries 1500 years before they were discovered by stating:

All over the body the arteries and veins communicate with one another by common openings and exchange blood and pneuma through certain invisible and extremely narrow passages.

2.
Galen ligated both carotid arteries of a dog, an intervention that he observed caused the animal no detectable harm. He concluded that the brain could therefore derive pneuma directly from the nose, to make the animal spirit earlier described by the Greeks (termed ‘psychic pneuma’ by Galen).
3.
During his time in the gladiator arena he observed that the level of neck injury sustained by gladiators affected their breathing, so proving that respiration originated in the brain. He did many animal experiments to ascertain more precisely the spinal level at which the nerves responsible for respiration originated and went on to describe the nerve roots and destination of the phrenic nerves.
4.
On the necessity for breathing via the mouth and nose Galen was unclear, writing in earlier works that pneuma could enter arteries via the pharynx, heart or skin as well as the lungs. An experiment to attempt to demonstrate this was carried out:

Covering the mouth and nostrils of a boy with a large ox-bladder, or any such vessel, so that he was unable to draw breath at all outside it, we saw him breathing unhindered through a whole day.
- Galen’s conclusion from this study is contradictory: ‘ Hence it is clear that the arteries all through the animal draw in the outer air very little or not at all ’. Modern views of this experiment are that the ox-bladder was unlikely to be airtight, or that Galen’s assistants must have removed the bladder to allow the boy to breathe easily when their master was not directly supervising the experiment.

The functions of breathing

Apart from providing pneuma to the heart, Galen described other functions for breathing:

1.
Regulation of heat . Galen’s writings strengthened the analogy between the heart and a flame, and several pages of On the use of breathing are concerned with the similarities between the two. For example, the observation that flames were extinguished when deprived of air or that an oil lamp burns out when its sustenance, the oil, is used up, were seen as analogous to humans seen ‘perishing when deprived of air’ or who lacked sufficient nourishment. Galen was concerned about the contradictory requirements for the idea of the heart and lungs generating innate heat, realizing that a fine balance must be drawn between ‘ fanning the source of the innate heat and from cooling in due proportion ’, citing examples such as fever, with increased breathing, when the balance was disturbed.
2.
Voice . Galen described in detail the anatomy of the laryngeal cartilages and muscles, and wrote a whole treatise on the voice, clearly recognizing the importance of the lungs. The rough artery (trachea) provided preliminary regulation of the voice, which was produced in the larynx and amplified off the roof of the mouth with the uvula acting as a plectrum. The purpose of having such a large volume of air in the lung was to allow continuous use of the voice.
3.
Removal of sooty and fuliginous spirits . Waste products from the blood were discharged from the lung, and this was the function of expiration. Without doing so, the heart would have become stifled by its own ‘ smoky vapours ’, once again like a burning flame. Explanations by Galen as to how the body separated the fuliginous spirits from the pneuma have become uncertain with the passage of time, one explanation being that the fuliginous spirits were regurgitated through the incompetent mitral valve and passed back along the pulmonary vein to the lung.
4.
Physical protection of the heart . The spongy nature of lung tissue, and the position of the heart in the centre of the chest, led Galen to suggest that the lung served to cushion the heart from the effects of body movements.

Galen’s legacy

Galen was the first physician to apply the Hippocratic method of scientific thinking to physiology, and he ingeniously combined the knowledge of his predecessors with his own thinking to produce an impressive treatise on the workings of the human body. Also, it is from the writings of Galen that we have obtained our knowledge of many of his predecessors: most of what is known of Erasistratus’s views on physiology is derived from Galen’s comments on it. Galen’s work also deserves a place in history as the longest unchallenged scientific work. The physiology described in this section was taught in medical schools throughout the world, and scientifically mostly unchallenged, for around 1400 years.

There was also a darker and more controversial side to Galen. He is widely believed to have been conceited, dogmatic and abusive of those criticising him. On the usefulness of the parts of the body contains several prolonged and personal refutations of the ideas of his predecessors, for example, accusing ‘ Asclepiades, wisest of men ’ of making errors ‘ no child would fail to recognise, not to mention a man so full of his own importance ’.

After galen

When Galen died, the study of physiology and anatomy effectively ceased. The Roman Empire was in decline, and in CE 389 Christian fanatics burned down the library in Alexandria, which contained many writings by the Greek philosopher-physicians.

Preservation of knowledge now fell to scholars of the Byzantine and Arabic empires. The latter embraced Galen’s ideas with enthusiasm and translated many Greek works into Arabic, almost certainly adding their own refinements as they did so. The greatest of these Arabic scholars was Avicenna (circa 980–1037), whose canon was an impressive document pulling together and classifying all the available medical knowledge of the time, creating what has been described as a popular medical encyclopaedia of the medieval period. Some years later, Ibn Al Nafis (1210–1288), a prolific Arabic writer on many subjects, studied Avicenna’s writings and wrote his own treatise Sharh Tashirh Al-Qanun (Commentary on the Anatomy of the Canon of Avicenna). In this he challenged Galen’s scheme of pores in the interventricular septum through which blood passed, and instead suggested that blood passed through the lung substance where it permeated with the air. ^, This was an early breakthrough in explaining the true nature of the pulmonary circulation, but Ibn Al Nafis’ work did not become well known for many more centuries.

The renaissance

In the 12th and 13th centuries, scholastic pursuits began again with the foundation of many European universities, firstly Oxford, Cambridge and Bologna, closely followed by Paris, Naples and Padua. Soon, many of the ancient documents were translated from Greek or Arabic into Latin, and human dissection began to be performed after many centuries of interdiction by the Pope. Knowledge of anatomy again began to advance, although interest in the function of the body only began again with Leonardo da Vinci in the 15th century.

Leonardo da Vinci (1452–1519) ^,

Leonardo da Vinci exemplified the Renaissance trend for combining art with science. His anatomical drawings are both extensive and ingenious, being mostly surrounded by extensive explanatory notes. ^, These notes are written in Latin and in mirror writing, possibly simply because da Vinci was left-handed and received no formal schooling to correct this, or possibly to make his notes harder to read by uneducated persons described by him as ‘ bad company ’.

Although da Vinci is known to have dissected over 30 human cadavers, most of his drawings of the respiratory system are based on dissections of animals, including Figure 35.2 , showing in beautiful detail the structure of the pig lung. In the commentary on this drawing, da Vinci considers the use of the ‘ substance ’ of the lung and extends Galen’s protective function of the lung parenchyma when he states that ‘ the substance is interposed between these ramifications [of the trachea] and the ribs of the chest to act as a soft covering ’. Structures entering the chest cavity are labelled a–e, and their functions described:

a.
trachea, whence the voice passes
b.
oesophagus, whence the food passes
c.
apoplectic [carotid] arteries, whence the vital spirit passes
d.
dorsal spine, whence the ribs arise
e. spondyles [spinous processes of the vertebrae], whence the muscles arise which end in the nape of the neck and elevate the face towards the sky.

• Fig. 35.2, da Vinci’s drawing of the thoracic organs of a pig ( c. 1508). The organs are labelled in mirror writing in Latin: polmone , lung; feghato , liver; milza , spleen; stommacco , stomach; djaflamma , diaphragm; spina , spine. See text for an explanation of labels a–e above the drawing on the right.

da Vinci adhered to other Galenic ideas such as the presence of air in the pleural space but was unsure how the air entered or left this space, and in his later drawings he was clearly beginning to doubt that air was always present. da Vinci’s adherence to Galen’s ideas was in some areas unshakable, in particular his depiction of the nonexistent interventricular pores in several drawings of the cardiovascular system.

He did however challenge some Galenic ideas by applying his engineering expertise. For example, he did not accept that the heart generated innate heat, instead writing that heat generation in the heart resulted from mechanical friction between the blood and the walls of the heart. Similarly, his engineering knowledge made him intrigued by the actions of the chest wall and respiratory muscles, including the complexities of defining the different function of internal and external intercostal muscles. For the diaphragm, da Vinci described four functions—dilating the lung for inspiration; pressing the stomach to drive food into the intestine; contracting with the abdominal muscles to drive out abdominal superfluities; and separating the spiritual (thoracic) organs from the natural (abdominal) ones. Finally, he considered in detail the movements of the trachea and bronchi on breathing, showing them to dilate and open wider at branches on inspiration as shown to the right of Figure 35.3 .

• Fig. 35.3, da Vinci’s drawing of the pulmonary circulation in relation to the bronchi ( c . 1513). Pulmonary vessels arise from several parts of the heart, leading da Vinci to propose a dual blood supply to the lung. Coronary arteries and veins can be clearly seen on the heart. At the lower end of the main drawing, da Vinci has drawn a small circle containing the letter N. The notes describe the structure as having ‘ a crust, like a nutshell’ containing a ‘ dust and watery humour ’, possibly representing his discovery of a lung cyst 17 or a tuberculous cavity. 15

da Vinci and the bronchial circulation

In Figure 35.3 , da Vinci depicts in detail the relationship of the pulmonary circulation to a bronchus. Much of the commentary in the drawing is concerned with the superiority of drawings rather than words to describe such anatomical configurations. The figure clearly shows a dual blood supply to the lung, and suggests that the smaller of these two supplies is to ‘ nourish and vivify the trachea ’. From this drawing, many writers have credited da Vinci with discovering the bronchial circulation, although this claim is disputed. ^, The drawing is believed to be based on an ox, a species recently shown to have distinct small pulmonary veins draining directly into the left atrium, which may be those found by da Vinci.

The possibility of artistic license in his drawings has caused disputes that will never be resolved, such as that of the bronchial circulation. For example, in Figure 35.2 the perfectly branching pattern of the bronchi on the lung surface is clearly not based on true observation of pig lungs. In Figure 35.3 of ox lungs, the right upper lobe bronchus that arises directly from the trachea in this species is absent. However, in spite of these misgivings regarding his drawings, da Vinci’s genius in combining art, science and engineering in the study of physiology is undisputed.

Anatomy in the renaissance

After da Vinci, the pursuit of medical knowledge in the universities continued, with anatomy in particular aided by the continuing resurgence of dissection and vivisection. Andreas Vesalius (1514–1564) is primarily remembered as the founder of modern anatomy, his dissections culminating in the publication in 1543 of De Humanis Corporis Fabrica , a book of seven volumes including over 250 anatomical illustrations ( Fig. 35.4 ). His ideas met with resistance from his contemporaries whenever his views were at odds with those of Galen, and this eventually forced Vesalius to cease his study of anatomy and to return to work as a physician. Nevertheless, the Fabrica continued to gain acceptance, and became the foundation for future anatomy texts. Vesalius was also a skilled physiologist. He was the first to describe an experiment reproduced much later, in which a section of the chest wall of an animal was carefully removed without breaching the pleura beneath, so enabling direct observation of lung movements through the transparent pleura.

• Fig. 35.4, Figure from Book VI of Vesalius’s Fabrica , 19 showing an anterior view of the lungs after removal of the heart. A, Oesophagus; B, trachea; C, pulmonary artery; D, pulmonary vein; I, diaphragm. E–H refer to the lobes of the lun—Vesalius’s illustrations always showed each lung to have four lobes.

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