杏吧原创

Gut feeling

How does the stomach repair itself after injury? I would guess that the normal repair proteins and enzymes in a human body鈥檚 toolbox would not function well, if at all, in the warm, strongly acid bath inside the stomach. So what is involved in this case?

鈥� The walls of the stomach are protected from the highly acidic contents of its lumen by a very effective mechanism. An outer mucous layer and bicarbonate secreted from specialised cells in the wall provide a buffer zone preventing contact with gastric acid. In the event of injury the tissue adjacent to the site provides a buffer zone for the injured cells and allows the body鈥檚 natural defence and repair mechanisms to kick in and heal the wound.

Mark Dunleavy

Department of Physiology,

Royal College of Surgeons in Ireland, Dublin

鈥� Healing in the stomach occurs in much the same manner as in other parts of the body, starting with local inflammation and leading hopefully to eventual tissue regeneration. The normal stomach lining protects itself from autodigestion by rapid cellular turnover and by secreting neutralising mucus onto its lining. However, the low pH of stomach juices does hinder the process when damage occurs.

The protective mechanisms are impaired by damage and therefore chronic ulcers or even perforation of the stomach wall can occur. Whether an injury heals or not ultimately depends on many factors, such as the extent of the injury and the patient鈥檚 eating habits. However, drugs such as ranitidine and omeprazole, which block acid secretion by the stomach, can speed up recovery. Also, antibiotics are often used now to treat stomach ulcers, since a bacterium called Helicobacter pylori has been found to play a role in causing stomach ulcers.

Adrian Root

London, UK

Wrap up well

Recently I was flying at 12,000 metres and 800 kilometres per hour through air at a temperature of 鈭�50 掳C. The wind chill should have been horrendous, but luckily I was in an airliner. However, the cabin walls were only 10 centimetres thick. What insulation is used, and can I have some for my sea-level home?

鈥� The wind-chill factors one commonly sees are for turbulent flow, usually over exposed human skin, which loses heat by evaporation and convection. Laminar flow over smooth dry metal, which is what aircraft designers aim for, is much less efficient at transferring heat. At 9000 metres, air density is about one-third of its value at sea level: it is as if the plane were flying in a vacuum flask.

At speeds above 500 kilometres per hour, there is significant frictional heating of the outer surface of any aircraft. Parts of Concorde get 200 掳C warmer in flight, and the skin of a returning spacecraft of course gets red hot.

The human metabolic power density in watts per cubic metre inside an aircraft packed with passengers is hundreds of times higher than in even the smallest of houses, and the surface-to-volume ratio of a smooth cylinder is much less than that of an irregularly shaped house.

The air inside a pressurised cabin is temperature-conditioned and circulated. Several megawatts of surplus heat is available from the engines to drive the air conditioning system, so there鈥檚 no problem maintaining a comfortable cabin air temperature in flight. The trick is to line the cabin with just enough plastic so you can鈥檛 touch any cold bits of metal, and to fill the cavity between the skins with fairly ordinary foam or fibre insulation, with properties very similar to the walls of a house. So it鈥檚 pleasant in the cabin, but your correspondent is right when he assumes that parts of the aircraft do chill. The tail cone and rear baggage compartment do indeed get very cold on a long flight.

It鈥檚 worth noting that a plane on the ground with the engines switched off is no warmer than an unheated caravan.

Alan Calverd

Bishops Stortford, Hertfordshire, UK

鈥� The temperature outside a plane at high altitude is very low but the aircraft鈥檚 skin can become very hot. These extreme temperatures require thermal insulation but, apart from a couple of areas such as those above and below the passenger compartment, the main insulation consideration is acoustic.

To keep the inside of the plane protected from engine and wind noise, the insulation is thicker than that required for thermal considerations. Fibreglass batting, the material used, has the very small fibre diameter necessary to provide the best acoustic insulation. The insulation usually varies between 12 centimetres in the roof to 8 centimetres in the sides and 3 centimetres in the floor, with obvious variations between different types of plane. The type of fibreglass used in planes is especially light, but similar products are widely available to the public and commonly used in buildings for thermal insulation.

David Kettle

Dalkeith, Midlothian, UK

Can do

How are non-alcoholic fizzy drinks sealed in a can? Is it done in a pressurised environment, or is there some additive that produces the carbon dioxide bubbles later?

鈥� This is a victory for kinetics over equilibrium. CO₂ gas is dissolved in drinks by injecting it under pressure. As long as the pressure is maintained, a corresponding quantity of gas will be kept in solution 鈥� the two are in equilibrium. When the pressure is released, gas comes out of solution by escaping through the surface until an equilibrium is reached again.

This is very slow. However, if bubbles can form in the drink at nucleation sites, then gas loss is greatly accelerated. These sites could be microscopic irregularities on the inner surface of the can, or crystals of salt or sugar added to the drink.

So when canning fizzy drinks, the trick is to keep the drink under pressure for as long as possible. This is easy while the can is being filled, but once it leaves the filler it must be transferred very quickly to the 鈥渟eamer鈥� that seals the lid. These systems operate at high speed, filling more than a thousand cans a minute. Hence the triumph of kinetics over equilibrium.

Jonathan Brown

Edinburgh, UK

This week鈥檚 question

The morning after

Why have the whites of your eyes gone red when you wake with a hangover?

Sally Goble

London, UK