Clinical Applications of Human Anatomy and Physiology for Healthcare Professionals. Jassin M. Jouria

plate – also known as the “body” of the nail, is the overall surface area of the nail.

      •Proximal root or proximal nail fold-this is the portion of the nail located at the juncture of the skin of the finger or toe where it disappears under the skin. This area is often called the nail bed.

      The root of the nail evolves into the nail matrix. The matrix is the portion of the fingernail or toenail that is responsible for its growth.

      In some conditions, the appearance of the fingernails and toenails can indicate an illness or nutritional deficiency. For example, thick, ridged, and yellowish-looking nails can be an indication of a fungal infection (onychomycosis).

      A concave nail might indicate iron deficiency, while horizontal ridges across the nail surface may be a sign of malnutrition. Students should note however, that vertical (longitudinal) lines (onychorrhexis) on the nail body are relatively normal and are often caused by general aging processes.

       ■Regulation of Body Temperature

      Regulating body temperature is one of the main functions of our skin. Skin helps our body maintain a regular or homeostatic temperature, neither too hot nor too cold. Homeostasis provides the optimal environment for body functions, and the integumentary system is a vital component in how the body functions in this way.

      Our temperature is considered normal at roughly 98.6°F. Some people can run a bit higher or lower than that, which is perfectly normal. However, in cases where body temperature increases, the nervous system kicks into action, stimulating blood vessels in the dermis to dilate. This triggers secretion of the sweat glands.

      Figure 3-10 Toenail with fungus.

On hot days, this sweat is visible. Depending on environment, a body may sweat up to 3 gallons of body water daily.17 As water evaporates from the skin’s surface, body heat is dissipated, promoting cooling. Insensible water loss (water lost through respiration, skin, and water in feces) also plays a role in water balances in the body.

      When the outside air temperature is cooler than that of our bodies, the blood vessels in the dermis constrict, reducing sweating. This constriction activity restricts flow of warm blood to the surface of the skin and away from the skin’s surface, allowing skin temperature to drop. This activity reduces passive heat loss and is effective in helping the body conserve heat.

      Figure 3-11 Sweat depends on environment.

      The hypothalamus, located in the brain, is considered the body’s thermostat. A constant and optimal body temperature may remain consistent as long as heat loss and heat production are balanced. Every tissue in the body does produce heat. At rest, most of the body’s heat is produced metabolically by the brain, the endocrine organs, the kidneys, the brain, and the liver.

      When it’s cold and we shiver, that active shivering helps to warm the body. Shivering is actually defined as involuntary contractions or shuddering-like contractions of muscle that are triggered by the hypothalamus. This involuntary contraction activity increases muscle tone. As this occurs, stretch receptors in the skin respond, as do antagonistic muscles. This response raises body temperature because the activity of the muscle produces heat.

      Likewise, when our environment is colder than that of our body, our metabolism increases.

      For example, if you go outside and walk in a wintry environment where the temperature is at freezing or below, your body automatically kicks into action to stay warm.

      Cold weather effectively stimulates the adrenal medulla. This inner part of the adrenal gland manufactures adrenaline (epinephrine) and norepinephrine. This activity occurs in response to stimuli of the sympathetic nervous system. So too does the endocrine system; for example, thyroid levels increase in cold weather.

      During exercise or intense physical labor, our skeletal muscles produce more heat than other portions of the body. This is why engaging in activity when it’s cold can actually help warm your body and increase body temperature.

      Body temperature also affects enzyme activities in the body. Normal body temperature (96° to 101°F) is considered optimal for such activities. As the body heats up, enzyme catalysis also increases. However, if body temperature gets too high or elevates above optimal homeostatic levels (hovering around 106°F), proteins denature and neurons are depressed. This can lead to a life-threatening situation.

      Lower body temperatures are effective in reducing not only metabolism, but nutrient and oxygen requirements to the heart and other vital body organs, as well as body tissues. It’s for this reason that some people who succumb to sustained immersion in an icy lake don’t experience any physical damage after their body is warmed. Of course, the length of time in cold water and other factors also influence negative outcomes.

Different areas of the body maintain different “resting” temperatures. Our core (skull, thoracic, and abdominal regions) typically has a higher temperature than the skin, which has the lowest.18

      When the skin is warmer than the outside or external environment, the body loses heat as blood floods into the capillaries found in the skin.

      However, in colder environments, when heat is preserved, blood bypasses the capillaries in the skin. In this way, our skin helps our body’s core temperature to remain constant, even while withstanding drastic variations in external temperature.

       ■Common Skin Conditions

      Our skin is incredibly durable and tough, but a number of threats can interfere with this protective layer for the body. Depending on the integrity of the skin, the body is susceptible to hundreds of skin conditions and infections caused by yeast, bacteria, and viruses. Skin lesions and conditions like dermatitis not only cause damage to the skin, but in some situations, have a negative influence and impact on our psychological state of well-being, our appearance, and our level of confidence.

      Common skin problems afflict many of us. Some skin problems are caused by bacterial, fungal, or viral infection. Others are caused by autoimmune disorders or hereditary ‘glitches’ in our development.

      At regular doctor office visits, whether you notice or not, the doctor is likely assessing your skin for signs of skin issues. Certain demographics are more likely to undergo a more thorough skin assessment, especially the elderly, where their skin is evaluated for color, temperature, moisture, elasticity (turgor and mobility) as well as texture and thickness.

      Figure 3-12 Sensory stimulus on the skin.

      Skin lesions

      Skin lesions are fairly common but can look different in regard to shape, color, and texture. The most common skin lesions include:

      •Macule – Typically called freckles, a macule can also be a flat mole, or non-palpable such as rubella.

      •Plaque – Common with psoriasis, a plaque lesion defines a lesion that is slightly elevated though flat-topped, rough, and has a firm surface.

      •Nodule – Most often palpable, elevated, and firm, typically one to two cm in diameter.

      •Scale – This type of skin lesion is defined as keratinized cells of regular shape (thick or thin) and often silvery-white in color. These lesions often present with flaky exfoliation such as with psoriasis or exfoliative dermatitis.

      •Papule – An elevated lesion that is firm yet palpable, and typically measures less than 1 cm in diameter, such as a wart.

      •Wheal – This type of lesion is often irregular shaped, elevated, and can be solid or transient with variable diameters. It’s typical with insect bites or urticaria.