Skin: Functions

00:01 Welcome. I'm really pleased to be joining you again to talk about pathology, this time having to do with pathology of the skin or dermatopathology.

00:10 The skin has a lot of jokes made about it.

00:14 Are you thick-skinned or thin-skinned? Do you have skin in the game? Even the old Washington Redskins.

00:22 But today we're really going to talk about the organ that has the greatest total surface area in the body. Nice thing about skin, too, is that it's right there before your eyes. You can see everything that has to do with it. Important before we talk about the pathology, is to give you a basic organization for the physiology and structure of the skin.

00:43 If you understand that, you'll understand a lot of what goes on when there is pathophysiology.

00:50 So let's begin. Let's talk about what the skin is good for.

00:53 And it's not good just for a substrate for adding makeup.

00:58 But in fact skin is really important obviously for as a barrier function for protection, and it is somewhat of a resistance force against mechanical or thermal or other physical injury. Clearly, knives and other sharp things can pierce through the skin, but the skin does provide some mechanical barrier.

01:18 It's also a barrier against harmful agents up to a point.

01:22 So very cold, very warm things.

01:25 Electrical injury, various chemicals.

01:28 There is a dead layer on top of the skin, the stratum corneum, that will be protective to the underlying viable tissue.

01:36 It's not only protection against things coming from the outside, it's protection against loss of things coming from the inside.

01:43 So the skin, that barrier, the stratum corneum in particular, prevents the loss of moisture and proteins, unless you're trying to have thermoregulation, which we'll talk about in just a second.

01:56 If you have a full thickness loss of skin, as in a burn injury, then in fact, besides losing a barrier to infection and being at risk for a variety of microbiologic invasion, you are also going to be at risk for losing fluid. You're going to be at risk for hypotension due to dehydration.

02:18 So the skin is also important for the loss of moisture and proteins.

02:23 It's also protective against UV radiation.

02:25 Yes, we have ozone in the atmosphere, but it does not screen out all of the ultraviolet light. And that high-frequency radiation is a major source of injury to the underlying viable keratinocytes.

02:39 We also have other mechanisms that we'll talk about that's protective, such as the production of melanin.

02:45 But the upper layers of the skin are protection against UV.

02:49 It's also clearly a barrier protection against infection.

02:54 And that whole layer of relatively dead keratinocytes, just keratin layer in the stratum corneum, is going to be very good to protect against the onslaught of bugs that we live in, in our environment.

03:07 I already previously mentioned that the skin is important for thermoregulation.

03:12 Yes, we don't want too much fluid to go out, but if we get too warm in our core, we need to be able to have evaporative radiation, which will allow us to cool.

03:22 So the skin regulates blood flow through capillaries at the base of the epidermis and also regulates sweat production.

03:31 And sweat is not just the production by sebaceous glands of fat and other things, but you also have eccrine glands that are going to be really important for releasing water and electrolyte.

03:44 That will allow you to have evaporative cooling.

03:47 Sensation. Yes. We have to be able to feel that touch, know when our body is in contact with something else in the environment, and be able to sense whether it's rough or whether it's smooth or whether it is hot, whether it's cold, and other forms of sensation that include pain sensation.

04:04 So the skin is a very important organ in that regard.

04:08 It's also important for being our first line of resistance to infection.

04:14 So there's a lot of immunologic surveillance.

04:16 The skin, all that surface area is in contact with an incredible wealth of microorganisms out there in the environment.

04:23 And you need to be able to say friend or foe.

04:26 And there are cells specifically within the skin, Langerhans cells that we'll talk about in a moment that are important for that process, for constantly surveilling what's out there and alerting the immune system to what's going on. Finally, not finally, but importantly, too, we're also the skin because it sees ultraviolet light, which can be used to produce vitamin D.

04:48 The skin is the major source for vitamin D production in the body.

04:54 Yes, you can get vitamin D from exogenous sources, from fish, from milk products, etc.

04:59 but vitamin D from light exposure, UV change of the precursor that we synthesize is really important. And vitamin D, as you know, is important for regulation of calcium.