Private aquariums have been around for a long time. The ancient Chinese and Romans knew how to breed valued and rare aquatic species. But the first public aquarium did not open until 1853 in Regent’s Park, London. The subsequent invention of large sheets of safety glass made it possible to construct large tanks. However, not until the advent of Perspex, an extremely strong acrylic resin that is lighter than glass, was it possible to construct really huge tanks. Today it seems that the larger the tank the more visitors it attracts.

Until recently, the best that most seaside aquariums offered doughty visitors, as they groped their way along cold dark dank passages, were some ill-lit glass tanks of apparently barren seawater. Most specimens were local: the smaller tanks housed “rock-pool” creatures such as crabs, shrimps and various anemones, while the larger ones accommodated lobsters, dog-fish, wrasse and cod. Moray eels were common and if you were lucky you might see an octopus or small shark. Apart from a species name on the outside of the tank, further information was rare. Just seeing the occasional fish was considered enough to justify the entrance fee of a few pounds. At least the place provided a little shelter for the family on a wet day …

But things are changing. After visiting dozens of aquariums, both public and institutional, I reckon there are effectively three kinds. A good example of each can be found on the Pacific coast of North America. The Monterey aquarium in California is the archetype for the new British aquariums, with a mix of both education and entertainment. As usual the local species of aquatic flora and fauna predominate, but the building is light and airy. The subtle innovation is that organisms are grouped according to specific marine environments, the centrepiece being a three-storey kelp tank representing the kelp forest of Monterey Bay.

An example of the second sort of aquarium can be found in San Diego at Sea World. Here in a theme-park atmosphere, entertainment rules as dolphins and killer whales perform tricks. It’s spectacular to see a trio of three-tonne killer whales simultaneously jump into the air and funny when they drench the first three rows of spectators, but little is learnt about the marine environment.

The smaller Vancouver aquarium is by far the best I have seen. It typifies the third type of aquarium which concentrates on education. In addition to the local species of the Pacific Northwest, the aquarium has special exhibits containing aquatic life from the Amazon rainforest, an Indonesian coral reef and the Canadian Arctic. The rainforest exhibit is especially interesting because terrestrial animals are also on view, and the aquarium tanks are housed in a room full of large plants with plenty of natural light. The Arctic exhibition boasts beluga and killer whales, cruising around in large tanks. The natural lifestyle of these magnificent animals is emphasised, and watching them swim around from an underwater viewing gallery is more spectacular than seeing whales jumping out of the water in time to music by Michael Jackson. The aquarium is full of educational material and can be approached on several levels of understanding.

Interestingly, the Vancouver Aquarium, which is thriving, is surrounded by a zoo that is in serious decline. One aspect of the success of aquariums may be the public perception that the exhibited aquatic organisms are living not only in a rich environment, with several other species of plants, invertebrates and vertebrates, but also that they seem unstressed. Unlike caged zoo animals, which may often appear to the public to have unhappy faces, fish are utterly inexpressive. Some zoos exhibit animals with imprinted behaviour, a sign of stress, which only enhances the public perception of the cruelty of captivity. But a similar criticism can also be made of aquariums. In the large tanks, where one can walk through transparent Perspex tunnels, one can often see what seems to be imprinted behaviour in fish such as sharks and sunfish. They swim round and round exactly the same route, just as big cats and bears prowl around in their zoo enclosures. These big fish are ocean-going and cover many miles of open sea each day, so a tank a few metres in length must be restrictive and stress-inducing.

If fish kept in tanks are just as restricted as caged animals, does the success of public aquariums rely on the public having no affinity for aquatic organisms? Or is it that the aquatic environment is so alien that live exhibits are somehow not considered to be “captive” animals? Or maybe aquariums are more popular simply because the aquatic environment is the largest environmental zone on Earth as well as the least accessible. It’s hard to say. But whatever the reasons, anything which encourages people to appreciate wildlife – be it zoos or aquariums – should perhaps be welcomed.

Nowadays, anaesthetists have a much better understanding of their anaesthetics.
During an operation, the anaesthetist’s main functions are to ensure that
the patient gets enough oxygen and stays unconscious to avoid pain. The
patient is given oxygen together with anaesthetic gases such as nitrous
oxide. These gases are delivered to the operating theatre under high pressure,
and would seriously damage the patient’s respiratory system, or even kill
them. So the pressure of the gases must be reduced, and it is of the greatest
importance that the anaesthetist monitors pressure in the anaesthetic equipment,
and in the patient’s airways and blood.

Surprisingly, during the course of an operation, an anaesthetist is
expected to cope with pressures that are given in several different units.
In the Systeme International d’Unites the appropriate unit is the pascal.
The pascal is equivalent to a newton per square metre and, ideally, should
replace older units. But at the moment, the majority of units used to express
pressure in anaesthetic practise predate the SI, and consist of units such
as millimetres of mercury (a tradition based on the mercury manometer),
atmospheres and bars (from the Greek baros meaning ‘weight’) and so on.

Oxygen is supplied to the operating theatre in cylinders or piped in
at high pressure. At this stage, the gas pressure is usually expressed in
bars, or pounds per square inch. One bar equals 100 kilopascals and one
pound per square inch equals 6.9 kilopascals. After passing through the
anaesthetic machinery and ventilator, the gas is passed on to the patient
at a greatly reduced pressure. We monitor the gas pressure in the patient’s
windpipe and lungs to ensure that the lungs are not overinflated. Airway
pressure is usually expressed in centimetres of water (1 centimetre of water
equals 0.098 kilopascals). Anaesthetists must also check carefully the proportion
of oxygen in the total volume of gas entering the patient. The oxygen pressure
in the lung airways is sometimes expressed in pascals but more often as
so many millimetres of mercury, or torrs. Now a torr is equivalent to a
millimetre of mercury, and is equal to 0.133 kilopascals. To ensure that
the oxygen is entering the blood, we measure levels of oxygen and carbon
dioxide dissolved in the blood. The amount of gas is proportional to the
gas pressure. The results from these measurements are reported in millimetres
of mercury or pascals.

By tradition, the patient’s arterial blood pressure is expressed in
millimetres of mercury. But the blood pressure in the patient’s veins is
lower than that in the arteries and does not support a sufficient column
of mercury to allow accurate assessment of the blood pressure, so the tradition
is to express venous pressure in centimetres of water. These, of course,
are not the only units of pressure that anaesthetists use and one also hears
of atmospheres, absolute atmospheres and dynes. Generally, though, high
pressures are expressed in bars, medium pressures in millimetres of mercury
and low pressures in centimetres of water.

The problem is compounded when unfamiliar units are used. To express
the pressure in the gas cylinders in pascals would leave most people looking
blank, while expressing tensions of blood gas in bars would result in people
reaching for their calculators. It would be equally unconventional to express
the pressure of blood in the arteries in pounds per square inch.

History is to blame for the use of units of pressure which rely on units
of length. It all began with Galileo’s pupil Evangelista Torricelli, who
is thought to have made the first mercury barometer in 1643. In his most
famous experiment he placed a glass tube in a vessel of mercury and saw
that the mercury rose in the tube to a steady level, and suggested it was
the weight of the atmosphere that supported the column of mercury. But it
was the French philosopher and physicist Blaise Pascal who was able to prove
that it was actually the pressure of the atmosphere that supported the mercury.
Pascal postulated that the height of a mercury column would decrease with
increasing altitude. In 1648 he persuaded his brother-in-law Florin Perier
to carry a mercury column up the 1485 metres of the Puy de Dome mountain
in central France. The results confirmed Pascal’s theory. From this experiment
the pressure unit of an atmosphere evolved.

One atmosphere is the pressure exerted by the atmosphere at sea level,
and is equivalent to 14.7 pounds per square inch, which is sufficient to
support a column of mercury 760 millimetres high and a column of water 10.33
metres high. A bar is equivalent to 1 atmosphere. The pascal is derived
from the newton, so it is a very small unit representing 100 000th of an
atmosphere. One kilopascal is equal to about 1 per cent of an atmosphere
and 10 centimetres of water. Despite being such a small unit it can still
replace all the other units, so the pressures of an oxygen cylinder or pipeline
would be expressed as 14 megapascals and patient airway pressures as 2 kilopascals.

The main objection to using only pascals as units of pressure is the
need for a range covering several orders of magnitude, from pascals to megapascals.
But this objection is strange. Most people quite easily understand the difference
between 10p and £1 million, a range of eight orders of magnitude.
Another objection to change is the effort of having to relearn all the normal
pressure values and safety limits – a traumatic process, especially when
patients’ lives depend upon it.

This situation is unlikely to change if textbooks continue to use old
units such as millimetres of mercury. Textbooks from the US remain the
biggest culprits of this practice, and most European textbooks now give
both old and new units. Manufacturers also label anaesthetic equipment both
ways, and monitors can display both pascals and old units. Let’s hope that
publishers will soon insist that their authors use only SI units. Newly
trained anaesthetists will then be able to talk about pressure in only one
unit, the pascal. We shall all sleep peacefully then.

Mark Williams is a research fellow at the Nuffield Department of Anaesthetics,
University of Oxford, in the Radcliffe Infirmary, Oxford.

Which was right: 3 million or 300 million? Both are large numbers but
we must get our facts right. I immediately consulted another modern physiology
textbook. It stated clearly that human lungs contain roughly 300 million
alveoli, the air sacs in the lungs through which oxygen enters the blood.
So obviously the 3 million was a typescript error.

I wondered why I did not spot the error when I read page 12 the first
time. It was probably the reference to the surface area and the tennis court
that made me accept the 3 million, as I knew the lungs had a large surface
area. I became curious: what did other textbooks say? Several more modern
textbooks were all agreed that the lungs contain 300 million alveoli. And
then I became suspicious. How was it that these authors gave the same estimate?

I looked deeper, into older textbooks. One said: ‘In man the two lungs
contain approximately 700 million alveoli giving a total surface area of
over 70 square metres; ie, if the lungs were opened out into a continuous
sheet they would cover the surface of a tennis court.’ Now, the area of
a singles tennis court is 196 square metres, not 70 or 140, so this estimate
is inaccurate.

Another text went even further: ‘In practice the gas-blood interface
is some 100 m ² in area with a mean thickness of less than 1
mu m. If its thickness were increased to 1 cm and its dimensions remained
the same, the membrane would cover the whole of Wales (or Connecticut) so
that its shape is well suited to its gas exchange function.’

Only one older textbook and two modern specialist texts referenced the
source of the estimates for the number of alveoli. The references could
be traced to two papers: one in 1945 by F. J. W. Roughton (where the estimates
of 700 million originate) and another in 1972 by G. E. Angus and W. M. Thurlbeck,
who estimated that the number varied with body size and ranged from 200
to 600 million with a mean of 375 million.

Would exam students be marked right if they wrote: ‘The surface area
of the lungs covers the area of a tennis court’ or ‘The lungs contain around
500 million alveoli’? The first statement is wrong and the second right,
but according to the textbooks it is the other way around. So we must be
careful of generalisations when summarising information for textbooks so
that myth does not become fact.

In historical terms this is a minor inconsistency, but the past is full
of what seem to us today quite major misconceptions. Take, for instance,
the belief in spontaneous generation or abiogenesis. In the 17th century,
Alexander Ross wrote about a Sir Thomas Browne’s doubt as to ‘whether mice
may be bred by putrification’.

Ross wrote: ‘So may he (Browne) doubt whether in cheese and timber worms
are generated; or if beetles and wasps in cow’s dung; or if butterflies,
locusts grasshoppers, shell-fish snails, eels, and such like, be procreated
of putrefied matter, which is apt to receive the form of that creature to
which it is by formative power disposed. To question this is to question
reason, sense and experience. If he doubts of this let him go to Egypt,
and there he will find the fields swarming with mice begot of the mud of
Nylus, to the great calamity of the inhabitants.’ Clearly, only a brave
student of those times would dare question spontaneous generation when defended
in such strong terms.

Even further back into the past, the ancient Greek physicians thought
that female hysteria resulted from the movement of the womb (Greek hystera
uterus or womb, whence the name), which during an attack of hysteria rose
from the lower abdomen into the throat. Expressed in Plato’s Timaeus: ‘The
womb is an animal which longs to generate children. When it remains barren
too long after puberty, it is distressed and sorely disturbed, and straying
in the body and cutting off passages of breath, it impedes respiration and
brings the sufferer into the extremist anguish and provokes all manner of
diseases beside.’

The Greeks also thought that the ‘animal’ womb was sensitive to aromatic
smells. Aretaeus of Cappadocia, a Greek physician, who lived in Rome in
the latter half of the 2nd century stated: ‘It delights also, in fragrant
smells and advances towards them; and it has an aversion to fetid smells,
and flees from them, and, on the whole, the womb is like an animal within
an animal.’ Thus by applying pungent smelling salts at the nose the womb
would be driven back to its rightful place in the lower abdomen and no longer
impede breathing.

It was not until relatively modern times when physicians performed human
dissection that the idea of a womb moving around of its own will became
unacceptable. Likewise, it was not until the invention of microscopy that
the theory of spontaneous generation fell by the wayside and the maxim omne
vivum e vivo, that every living organism came from a pre-existing living
thing, was coined. But what is important is not that these widely accepted
theories were wrong. It is that they stimulated people to make new measurements
which ultimately led to the technology we see all around us today.

Technological advances are needed before the number of lung alveoli
can be determined accurately, but does it matter if we do not know the number
of lung alveoli to the nearest dozen or the lungs’ surface area to the nearest
square centimetre? Only time will tell!

Mark Williams is a research fellow in the Department of Anaesthetics
at the John Radcliffe Infirmary, Oxford.