Thermoregulation in dogs is of great importance. It has been experimentally established that dogs can withstand temperatures down to -40 ° C without damage to health. But in the heat, the animal has a more difficult time, since the mechanisms that ensure the removal of excess heat from the body are much less developed in them. The process that ensures a decrease in body temperature in humans is due to the evaporation of the secretion of the sweat glands. Do dogs sweat if their thermoregulation is based on completely different principles?
Physiology of thermoregulation
One of the characteristics of the canine organism is its high metabolic rate. It is this characteristic that is responsible for body temperature. In dogs, depending on the size of the body, the temperature normally varies from 37.5-39.2 ° С in representatives of large breeds, to 38.5-39.4 ° С in small ones. Therefore, the risk of overheating in small dogs is higher than in large ones.
The widespread opinion among ordinary people that dogs do not sweat is a mistake. A dog’s skin contains very few sweat glands. The main type of sweat glands in dogs is apocrine. They are located on the paw pads, on the tip of the nose mirror. They are larger in diameter than the main human eccrine sweat glands, have a different structure, and the secreted secretion differs from human sweat.
Sweat glands are found on the entire surface of the body – most of all on the back, less on the chest and sides, and very few on the lower body. The myth that dogs don’t sweat arose from the fact that sweating is not as important in thermoregulation in dogs as it is in humans.
Sweating in canines is not regulated by the central nervous system, but responds to local exposure to high temperatures. This mechanism is obviously provided to protect the skin from local overheating. General thermoregulation is carried out due to the evaporation of saliva from the tongue and mucous membranes of the oropharynx, as well as heat transfer during rapid breathing.
A significant part of endogenous and exogenous heat is transported by blood to the respiratory organs. A noticeable secretion of secretion by the cutaneous sweat glands is noted only when, as a result of intense heat exposure, there is a threat of damage to the epithelium.
Heat transfer mechanism in dogs
A dog covered with thick fur does not need a mechanism to evaporate sweat from the surface of the body. The main heat transfer in dogs is carried out through the respiratory organs, but part of the heat loss occurs from the body surface by contact.
The long tongue of the dog is abundantly permeated with blood vessels that bring warmth. On the tongue, the mucous membrane of the palate and cheeks, the ducts of the small salivary glands open, secreting a liquid secret. When you breathe, moisture evaporates, taking with it the heat from the blood. But when breathing, the air does not fully come into contact with the mucous membranes, but where it comes into contact it is quickly saturated with water vapor.
For heat exchange, it is necessary that the masses of air passing through the respiratory tract are significant. In the process of intensive breathing, a lot of physical work is done, in which a lot of energy is expended and a lot of heat is released.
In order to ensure rational air rotation, nature has provided a counterflow mechanism – the dog inhales air through his nose and exhales it through his mouth. In hot climates, evaporation and heat loss increase significantly, but it is not enough to maintain thermal homeostasis of the dog’s body. Moreover, the inhaled hot air is not very capable of absorbing heat from the blood.
In a healthy dog, the tip of the nose is cool and moist, which is provided by the evaporation of sweat glands and mucous membrane secretions. Warm air, passing through the winding, folded nasal passages, abundantly covered with moisture, cools and enters the lungs. At the border of the warm flow from the inner “core” of the body and cool air, condensation of water vapor from the atmosphere occurs, due to which the mucous membrane does not dry out, and the level of evaporation is maintained with each act of breathing.
In addition, the large nasal gland provides moisture to the nasal cavity. A person does not have such a gland, so its functions have remained unexplored for a long time. Its sole purpose is to maintain moisture in the dog’s nose, which increases the cooling of the inhaled air stream. In a sense, this gland performs the same function as the human sweat glands.
If the ambient temperature is comfortable for the dog, it breathes through its nose. Measurements of the temperature of air inhaled and exhaled through the nose showed that its t = 29 ° С – slightly higher than the body temperature of the animal. And when air is exhaled through the mouth, it is + 38 ° C. At the same time, from 1 liter of air when exhaled by the mouth, 27.7 calories of heat are “carried away” from the body, which is two times higher than when exhaled through the nose! By redirecting the flow of exhaled air, the dog can regulate heat transfer without changing the breathing rate.
With an increase in ambient temperature, the dog has heat dyspnea, in which the respiratory rate increases approximately 100 times per minute. With such work, it would seem that such an amount of heat should be released, which would negate the operation of the counterflow mechanism.
But in the middle of the last century, Y. Crawford described the process of resonant oscillations, in which the respiratory system oscillates with its own frequency, requiring minimal muscular efforts. This explains the insignificant heat production during intense breathing.
Contact or gemological
Heat transfer during contact exchange occurs from hot blood to cooler air. The amount of heat released into the atmosphere is influenced by the speed of blood flow and its volume. The central nervous system affects the regulation of the diameter of the vessels. Thermal exchange by this mechanism is carried out in places where thermal protection is less – on the stomach, in the anal-genital area, on the auricles, on the pads. As the temperature rises, vasodilation occurs, blood flow increases, and heat transfer increases.
The diameter of the blood vessels is affected by both hormone levels and the activity of the hypothalamus. In the skin and mucous membranes are α- and β-adrenergic receptors, sensitive to the level of adrenaline and norepinephrine. When α-receptors are activated, the vessels constrict and heat transfer decreases, while when β-receptors are stimulated, the release of heat through the skin increases. Thermal stimulation of the hypothalamus in dogs causes increased blood flow, vasodilation, and increased heat transfer.
Overheating prevention techniques
The well-coordinated and efficient heat exchange system described above works no less successfully than “sweating” in humans. However, this does not mean that at high temperatures the dog can be safely walked or kept in the sun.
Under the influence of dry superheated air and heating by direct sunlight of wool, especially of a dark color, the animal not only increases the frequency of breathing, but also deepens it, which leads to hyperventilation of the lungs. In this case, the blood loses a significant amount of carbon dioxide, its pH shifts to the acidic side, manifestations of respiratory alkalosis are noted:
- deterioration of microcirculation in tissues;
- the occurrence of ventricular and atrial tachycardia;
- the appearance of muscle cramps, tetany;
- mental disorders – anxiety, agitation, aggression;
- electrolyte imbalance.
To prevent the dog from suffering from hyperthermia, the following rules must be observed during the hot period:
- walk in the morning or evening;
- avoid intense physical activity;
- have a supply of water with you in order to water the dog or moisten the coat at any time;
- after a walk with cool water, rinse the pads, muzzle and hair on the back, give a drink of cool fresh water.
In a normal state, a dog is able to withstand high temperatures for a long time, but dehydration of the body leads to a rapid increase in body temperature, a decrease in the respiratory rate, a violation of blood flow, which causes a “failure” of all thermoregulation systems and can lead to the death of the animal.
Evolution very rationally “provided” living organisms with thermoregulation mechanisms, and in dogs it is one of the most interesting and effective, given the peculiarities of the structure and physiology of these animals.