panting has more significance in furry animals than it has in people. It is their main mechanism for losing excess heat, whereas we can lose it through the skin in a regulated fashion by bringing heat to the surface with boosted blood flow, and by the evaporation of secreted sweat. The hart may pant ‘for cooling streams’, but it is the panting which will cool it more effectively than a cold dip.

When breathing increases for whatever reason, the greater the volume breathed in and out, the more heat is lost from the body. The inhaled air is automatically warmed and moistened on its way into the lungs (provided that the atmosphere is cooler than body temperature and is not already fully saturated with water vapour) because it passes over moist and warm surfaces, particularly in the nose. On its way out, some of the heat and water in the exhaled air is retrieved by the surfaces which were cooled and dried during inhalation, but a greater fraction is lost to the atmosphere.

Panting apart, the depth and rate of breathing normally vary according to the body's changing requirement for intake of oxygen and excretion of carbon dioxide. An increase in this gas exchange during exercise, for example, is achieved by greater ventilation of the whole of the lungs: breathing more deeply as well as breathing faster. Such an increase causes a greater rate of heat loss than when the body is at rest, and assists disposal of the extra heat generated by the working muscles. But if a body needs to lose excess heat by this route when not exercising — when no additional oxygen–carbon dioxide transfer is needed — it would be not only unnecessary but also undesirable to over-ventilate the depths of the lungs, because of the deleterious effects of excessive wash-out of carbon dioxide. Panting is therefore different: it is rapid and shallow, whereas breathing in exercise may be rapid, but is also deep. Because it is shallow, not much of the extra to-and-fro of air actually reaches the depths of the lungs (the alveoli), instead it shuttles in and out of the upper air passages and the bronchial tree (known as the ‘dead-space’ because no gas exchange takes place here).

This route of heat loss is reflexively called into play in humans if body temperature keeps rising because the mechanisms for losing heat from the skin are already fully active, or ineffective. Curiously, and with no apparent physiological utility, immersion in cold water also stimulates rapid breathing of the panting variety.

Very heavy and sometimes distressed breathing during or after a bout of heavy work or exercise may sometimes be described colloquially as ‘panting’, though in the physiological sense it may be related more to the metabolic requirement of the muscular activity than to heat loss. Rapid shallow breathing may also occur in states of anxiety; although there may be a predominance of ‘dead-space’ breathing, hyperventilation is likely, with washout of carbon dioxide and its consequences.

In the literary sense, panting is also associated with all kinds of yearnings, as in the analogy of the dog with its tongue hanging out, panting and eager with desires and thirsts. Because water loss accompanies heat loss, causing concentration of the body fluids, panting does indeed lead to thirst; so the hart that was heated in the chase may well be needing the cooling streams for a drink.

Sheila Jennett

See also breathing; hyperventilation; sweating; temperature regulation.