The skin system is a key topic in the MCAT and forms part of the Biological and Biochemical Foundations of Living Systems section. The MCAT syllabus breaks down what you need to know about the skin system for the exam.
To help you to revise effectively, this guide provides an overview of everything included in the MCAT syllabus, which is as follows:
The skin serves as a barrier between the body and the environment outside it – the skin's structure is illustrated in figure 1.
The skin has three main layers:
The epidermis is the top section of the skin with layers of corneocytes (cornified keratinocytes) arranged in a loose scaled structure. These cells are dead in the upper part of the epidermis and in a tightly compacted structure with normal corenocytes towards the bottom of the epidermis. The keratin-packed corneocytes are what make the skin relatively impermeable to water. Between these corneocytes are fat-abundant cells that make up the intercellular matrix, which prevents water from entering. The epidermis also has melanocytes which give the skin pigment by producing melanin and dendritic cells that help protect against pathogens.
The dermis is the thickest layer of the skin and has both an upper layer of thin, loose connective tissue known as the papillary layer as well as the reticular layer that is thicker and consists of more dense connective tissue (including bundles of collagen fibers). In addition, the dermis is where blood vessels, hair follicles, sebaceous glands, sweat glands and muscles are found.
The hypodermis Is the deepest layer of the skin and is where the adipose tissue is found, along with blood vessels, sensory neurons and hair follicle receptors. It is the connection between the skin and the underlying fascia.
Homeostasis is a self-regulating process that allows organisms to maintain a steady internal environment. Homeostasis can be broken down into different categories including osmoregulation and thermoregulation, two regulative processes aided by the skin.
Osmoregulation is the process by which bodily fluids' water balance and electrolyte concentration are regulated. The skin specifically maintains osmotic pressure (the tendency of water to move to a more concentrated solution) by excreting excess water or excess electrolytes/waste through sweating.
The skin also plays a major role in thermoregulation – maintaining an optimal body temperature.
The arrector pili muscles control whether the hairs on your skin lie flat or erect. When the muscles contract, the hairs are erect – creating a layer of still air between the hairs and preventing major heat loss from the body. When hairs lie flat, heat is no longer getting trapped in that layer of still air, and is therefore free to leave the body – reducing the internal body temperature.
The hypodermis layer of the skin, where adipose tissue is, stores a lot of fat cells which act as an insulator – keeping your body warm, particularly in cold environments.
As mentioned in the previous section, sweat is used to excrete excess water and electrolytes. When sweat is released from the sweat glands in the dermis and pools on your skin, it evaporates and takes heat away. This provides a cooling effect that allows the body temperature to reduce. The higher your sweat rate (also known as perspiration), the larger the decrease in body temperature.
When a blood vessel dilates (vasodilation), particularly capillaries near the skin's surface, more heat is brought with the blood to the skin where the heat can dissipate into the air. This important process allows the body temperature to lower back to a normal range. When the body needs to reduce heat loss, blood vessels constrict – reducing the amount of blood (and therefore heat) running near the skin surface, thus reducing heat loss. This allows the body to retain as much heat as possible – important in cold climates. It must be noted that vasoconstriction also majorly reduces perspiration, again minimizing heat loss.
The human epidermis is relatively thick and strong compared to that of other mammals. It therefore provides a layer of protection against damage such as abrasion (the wearing away or scraping of the body). The keratinocytes of the skin produce inflammatory mediators that regulate the skin’s immune responses and therefore aid in the skin’s protection against disease. The skin acts as the first barrier that a pathogen comes into contact with and the release of mediators alerts the whole body that immune defenses need to be put up.
There are also specialized structures of the skin that can provide further physical protection. The nail is a hard structure with the nail plate made out of keratin – that is what is commonly known as the nail. The nail also includes the nail bed, where the nail plate attaches for it to be anchored to the finger/toe. The nail’s function is to protect the fingers and toes as well as serve as a tool (for scratching for example).
Calluses are thickened pieces of skin on places on the body that experience constant abrasion. They are formed by the overactive division of the keratinocytes – leading to a thicker skin layer. Calluses are a normal response to any excessive pressure/damage to the skin in aid to try and protect that skin in the future. The process of forming calluses can also be known as ‘hyperkeratosis’.
Finally, the hair can also be a form of physical protection. Some examples of this include: eyelashes prevent dust from entering the eye; ear hairs prevent germs or foreign objects from damaging the middle ear and beyond; and head hair protects the scalp from the sun’s UV radiation. Overall, hair provides an additional layer between the outside environment and the internal body.
Some functions in thermoregulation relating to the skin are controlled by hormones secreted due to the sympathetic nervous system (the fight or flight response). Sweat glands (specifically eccrine glands) are stimulated during a time of stress and the hypothalamus releases epinephrine and norepinephrine also. These hormones control many aspects of the fight or flight response including vasoconstriction of blood supply from non-essential organs like the skin and vasodilation of blood supply to essential organs like the heart and skeletal muscles. This process is also known as vascular shunting, where blood flow is redirected to more important organs.
Revising these key features of the skin system and ensuring you can answer questions related to them will help you to prepare effectively for the MCAT. You can find more guidance on MCAT preparation in our MCAT Guide and Checklist. And to support your revision of other key MCAT topics, we have a range of blogs covering everything from the skeletal system to the growth and physiology of prokaryotic cells.