Xu Yi shook his head: "Boss, you are wrong this time, in fact, dogs do not have sweat glands, but sweat glands are very rich, so to speak, mammals all have sweat glands, even dolphins and whales living in the sea have sweat glands! The difference is only in the strength of the sweat gland secretion function, not to mention summer, even if you are wrapped in a fur coat in this weather, I think your tongue is not enough to cool down!"

Hu Yifei asked in surprise: "Yes, but I see how the pet forum says that dogs have no sweat glands?"

Xu Yi smiled and said: "This is just a miscommunication, just like someone who sees pigs like to roll in the mud pond and says that pigs have no sweat glands, in fact, they are all self-righteous and self-righteous." ”

Hu Yifei pursed his lips and asked, "Is this really the case?"

Xu Yi nodded and said: "Actually, pigs like to roll in the mud pond, it's not because pigs don't have sweat glands, I can say this with certainty." ”

Xu Yi remembered what happened when he was a child, and couldn't help but curl the corners of his mouth, and said: "People are thinking about whether they can knock down a cow, but even if it is an unarmed child, they can easily push down a fat pig." No matter whose pig it is, if you can stand next to it and reach out and scratch its belly and tickle it, it won't be a minute before it will be lying on the ground enjoying it and both eyes will be squinting. I have done this kind of thing since I was a child, and in the summer this pig is still wet, and it sweats a lot more than a human body. Therefore, in the summer, the pigs have to be fed water and salt, and in the winter, they don't need to be fed so diligently. Therefore, it is certainly not reliable to say that pigs do not have sweat glands on their bodies and can only be cooled by a layer of mud. ”

"Of course, the sweat glands on the pig are not enough to cool it down enough in the hot environment. will run into the pond and roll in the possibility. However, the reason why they are covered in mud is partly for the purpose of enhancing cooling. In fact, it is mainly related to the living habits of their ancestors. ”

"Pigs were domesticated from wild boars, and mosquito bites and parasite breeding are inevitable in the wild. Therefore, wild boars are more accustomed to rolling in mud pools, and even domestic pigs like to rub their bodies on things that protrude from the ground to relieve itching. It can be seen that the main purpose of their mud coating is to avoid parasites and mosquitoes. ”

After saying this, Xu Yi turned the topic back: "Of course, this sweat gland must have a function that is rarely mentioned in our books, that is, the function of hydrating dead stratum corneum cells. ”

"Really?"

"If you think about it, more beauty websites won't talk about this, they just say that you have dry skin. It's because of the reduced function of the sebaceous glands, which is good for selling moisturizers and lotions after all, so naturally I have to write about it, but what can sweat glands say, at most, they will sell you some masks or something. In fact, sweat glands reduce secretion due to low temperature, coupled with drier air and large evaporation, the stratum corneum is more prone to dehydration, which is the main reason for dry skin in winter. ”

Liu Liping asked, "According to what you say." Doesn't this skin still have a secretory function?"

Xu Yi nodded: "In a narrow sense, it is true, and it is certainly correct to say that sweat glands have secretion functions, but it is not appropriate to amplify this function to the skin." ”

Liu Liping asked, "Why?"

"Just like the human eye appendages naturally have tear glands, which play the main purpose of sterilizing and lubricating the eyeball. But in general, no one would say that the eye has a secretory function. Another example is that the lungs are naturally the place where gas is exchanged, but the secretory cells of the alveoli still secrete surfactant. But everyone thinks of the lungs as a place for gas exchange. Its secretory function is not emphasized. Although this secretory function is important for the elimination of foreign bodies and the prevention of lung infection, as well as for avoiding alveolar collapse, even in moistening alveolar cells. There is no substitute for ensuring gas exchange. But the lungs are still only used as respiratory organs, and even the immune function that it also has is rarely mentioned. ”

"Sweat glands and sebaceous glands are likewise nothing more than appendages of the skin. Relying on their synergistic effect, they can prevent the stratum corneum from dehydrating and drying, so as not to destroy the integrity of the stratum corneum because of light activity, and at the same time prevent the growth of microorganisms and reduce the harmful substances affecting the tissues and organs under the skin. ”

"So why are these features listed separately?"

"Compared to these two organs with more powerful secretory functions, the secretory function of the skin is really not on the table. I guess the reason why it is singled out is not because there is nothing written about the bricks and beasts of dermatology, but because the demand of cosmetics and other industries has affected the dissemination of medical knowledge. ”

In the same way, in fact, the remaining sensory functions and immune functions are actually just the same, and they can only enhance the effect of skin defense and avoid damage to the body. ”

Liu Liping asked relentlessly: "What about the absorption function?"

Xu Yi smiled: "The so-called absorption function of the skin includes the respiratory function you mentioned, after all, the absorption and exchange of gas is only an absorption, and this function is not worth mentioning for the skin." ”

"First of all, we have talked about the characteristics of the skin before, let's talk about the most unreliable function you mentioned - the respiratory function of the skin. As he spoke, Xu Yi looked up at a few people and asked, "The skin must be breathable from a certain distance, but this is not enough to become the basis for the skin to have a breathing function." Do any of you remember how much surface area the human skin has?"

Hu Yifei thought for a while, and then said: "If you are an adult, it is about two square meters." ”

"What about the area of the lungs?"

"Close to a hundred square meters. ”

Xu Yi nodded: "Thirty percent of the lung function utilization is enough to meet the physiological needs of normal people, and this is based on the special structure of the lungs, the alveoli are only a thin layer, and the capillaries are only one layer, which undoubtedly makes the permeability very good." Oxygen dissolves in mucus on the surface of the alveolar cell membrane, is then absorbed by the cells and exchanged with capillaries. It is then recirculated throughout the body. ”

Seeing a few people nodding, Xu Yi continued: "But even if there is no sebaceous film on the surface. The stratum corneum cells are dead, and there is a lot of filler material in the cells and intercellular spaces. Do you want to have more than ten or twenty layers of dead cells, how much penetration power of the gas can be left, and can this maintain enough partial pressure of oxygen?"

"And skin cells don't have the function of alveolar cells, so how can they absorb too much oxygen? Of course, I haven't seen any information about this, so I can't say more definitely. If it is further down, if the gas really reaches the dermis, then it is indeed because of the difference between the partial pressure of oxygen and the partial pressure of carbon dioxide that there is a certain difference, which is enough for the capillaries of the dermis to exchange gas with the outside world. However, the amount of capillaries in the skin itself is far more abundant than in the lungs, and there are fewer that are usually open, and the difference in partial pressure of oxygen is certainly not large, so the process of gas exchange that can occur is quite limited. ”

"Even a frog that can really breathe through the skin can only replace one-third of its lung function when the epidermis is moist, which is also a compensatory ability based on the incomplete development of the frog's lungs, and even the frog's metabolism is reduced to a low enough level during hibernation to completely use the lungs for gas exchange. But I believe that if in the summer. Soak the frog in water and don't let it come up to breathe, I'm afraid it won't be long before this thing hangs up. ”

"It's only one hundredth the size of a lung, plus the structural differences. I'm afraid that the respiratory function of this skin is not even 1/10,000 of the lungs, and such a small breathing effect can only be ignored. Therefore, Chu Liuxiang in the novel is destined to be a function made up by Gu Long. And then from the point of view of professional books. I haven't seen any books that say that the skin has the ability to breathe, which indirectly confirms that this statement is definitely inappropriate. ”

Xu Yi said firmly: "So." I'm afraid that the claim that your skin has a breathing function is destined to be some rumor with ulterior motives or subjective assumptions. Even if there is any pore breathing or something like that, there is no reliable evidence!"

Liu Liping asked: "Even if you can't breathe on the skin, at least this absorption function is always there, just like cosmetics have all kinds of nutrients, and there are various ointments and other drugs in use." ”

Xu Yi smiled and said, "Then let's divide the substance into water-soluble and fat-soluble substances and explain them separately, okay?"

"Well, you say. ”

"Let's start with water-soluble substances. You think the results of our previous analysis have proved that it is difficult for water to enter the stratum corneum outside the normal skin, and this small molecule naturally has to pass through the guò concentration gradient to enter the stratum corneum and affect the skin tissue below. In this way, it is difficult to penetrate the stratum corneum, and it is naturally difficult for those water-soluble substances to establish an effective concentration gradient, so it is undoubtedly difficult for these substances that can theoretically penetrate the skin to enter the subcutaneous substance. In order for these small molecules of water-soluble substances to enter the subcutaneous substance, the first thing is that they need to be exposed for a long enough time to achieve the purpose of real absorption into the subcutaneous skin. ”

"Isn't that still absorbable?"

Xu Yi nodded, then shook his head slowly. "But after all, people are not fish or frogs, they will not be exposed to water for a long time, and even in order to maintain the strength of the stratum corneum, they must avoid contact with water for too long, so it is actually a very far-fetched statement to say that the skin can absorb these water-soluble substances. ”

"Actually, there are a lot of things that I don't need to talk about, we have already talked about them in class, and you still feel unfamiliar, maybe it's just because you're not good at summarizing, so you can't think of this. ”

"As for the large molecules of water-soluble substances, let's take collagen as an example, the elasticity and smoothness of the skin, which can only be achieved by relying on the connective tissue of the dermis. As we said before, even if it is a living cell, a long-chain protein like collagen, even if it is a living cell, it is difficult to ingest, let alone pass through these micropores on the surface of dead cells. ”

Xu Yi concluded: "As for the collagen with a molecular weight of tens of thousands, if it wants to enter the dermis through the skin, there is no possibility at all, so the collagen in any mask and cosmetics must not be absorbed by the skin!"

"So, why do you use this in cosmetics?" (To be continued......)