The Integumentary System

Skin

Although the skin may appear to be an inactive sheet of tissue, it's a dynamic organ with blood vessels and nerves just like any other organ. The skin is a vital organ that interacts with several other organ systems and has several functions: It provides protection from the outer environment. It acts as a barrier against foreign pathogens. It sends the pain, temperature, pressure, and touch stimuli to the brain. When skin is exposed to the sun, it helps in synthesizing vitamin D, an essential vitamin needed to carry out various body functions. The skin weighs somewhere between seven and nine pounds and takes up, on average, 1.85 square meters (20 square feet). There are about 300 million skin cells. Each square centimeter of your skin contains about four meters of small branches of the peripheral nerves (about 13 feet per 0.15 square inch). This intricate system of nerves allows you to sense very subtle changes in the immediate environment. Your skin senses touch, pressure, temperature, and pain with different types of nerve fibers. Nerve fibers then send a signal to the brain; the brain decodes the signal and sends a response to the affected body part. For example, if you accidentally touch a hot stove with your hand, you'll feel a hot, painful sensation. Nerve fibers in your hand will send the signal of heat to the brain; then the brain, in turn, sends an immediate signal to the hand muscles to retract from the hot surface. Your hand's response of removing itself from the hot surface happens so quickly that it almost seems involuntary. A single centimeter of skin also contains meter long blood vessels. In addition to supplying blood and essential nutrients to the skin, blood vessels also allow the skin to help regulate body temperature. Think of the last time you exercised. Whenever you exercise, your heart rate increases to deliver more oxygen to your working muscles. The metabolism of your muscles generates heat. To maintain a stable body temperature, your body must get rid of this extra heat. This occurs mainly through sweating. Evaporation of sweat from your skin carries away heat. The tiny blood vessels in your skin also dilate to lose heat, making your skin warm and flushed. In the opposite situation, when you're exposed to extremely cold temperatures, the blood vessels in your skin become more constricted. This shunts blood flow, along with heat, away from skin and inward to protect vital organs. Shivering also is your body's attempt to produce heat to counteract extremely cold temperatures. Skin Layers The skin has three main layers: Epidermis Dermis Subcutaneous The Epidermis The epidermis is the outermost layer of your skin. The epidermis contains five additional layers called strata (singular stratum). Each layer serves a specific function. These five epidermis layers are: Stratum corneum. The stratum corneum is the surface layer, which acts as a barrier against infections. Skin cells grow from the inside out; once cells reach the surface, they're completely dead and keratinized. These cells are closely packed together and serve as a water-resistant covering for the body. There's an extensive amount of cell turnover in the skin, which is the process in which cells continuously die and are replaced by new ones. Your skin is constantly shedding these dead cells into the environment. Stratum lucidum. The stratum lucidum is found just below the stratum corneum on the palms of the hands and feet only. This layer provides additional protection for these high-use areas. Cells in this layer appear clear. Stratum granulosum. The stratum granulosum is the epidermis layer in which cells begin the protection process and are then distributed between the layers. These cells produce melanin, a protein that gives color to hair and skin. Skin color has nothing to do with the number of melanocytes; instead, skin color is determined by the amount of melanin produced by the melanocytes. Dark-skinned people have more active melanocytes and therefore higher amounts of melanin. In contrast, light-skinned people have more sluggish melanocytes and therefore lower amounts of melanin. The stratum granulosum is the first layer not to be nourished by the dermis, so the cells start to die here, forming keratin in the process. Keratin is the substance that constitutes your skin, hair, and nails. The cells in the epidermis aren't as dense as they're on your nails, but they form a similar layer on your skin that protects it from being penetrated by bacteria. Stratum spinosum. Underneath the granulosum layer is the stratum spinosum layer, also known as the prickle layer for its spiny texture. Stratum basale. The final and deepest layer is the stratum basale, which is composed of basal cells. This layer is responsible for renewing epidermal cells. The Dermis Layer The underlying dermal layer contains the dermis and is much thicker than the epidermal layer. The dermal layer is located between the epidermis and the subcutaneous layer. This layer is sometimes considered the "true skin" because it's composed of blood and lymph vessels, as well as nerve fibers. The dermis also contains hair follicles, sweat glands, and sebaceous glands. Several types of connective tissue cells and collagen fibers support the hair follicles, sweat glands, and sebaceous glands. Fibroblast cells produce fiber that helps to repair injuries. Phagocytic cells protect the body by swallowing foreign materials such as bacteria. Mast cells contain histamine, a substance that produces allergic reactions, and heparin, a substance that prevents blood clotting. Collagen is a gluey protein found in skin, connective tissue, bones, cartilage, tendons, and ligaments. Collagen fibers provide the skin with its strength and elasticity. Because of collagen, skin can stretch—as it does during weight gains and pregnancy—and return to its normal shape again. When the collagen fibers are overstretched, they break and form linear marks called stretch marks. Whorls and loops are found on the soles of the feet, the palms of the hands, and on the fingertips. These "prints" are actually formed from the ridges and furrows found in the dermis layer. The epidermis layer is molded tightly over the dermis layer, which is why you can see whorls and loops on the epidermis. The whorls and loops in the fingerprints help you grasp objects. Receptors in the dermis layer of skin make it possible for the skin to function as a sense organ. Skin can distinguish hot from cold and pain from pleasure. The different receptors vary in depth. Touch receptors in your fingertips, for example, are closer to the epidermis so your fingers can detect very delicate surface changes. Pressure receptors are deeper, which explains why you can press on something very firmly with your hands without discomfort. The Subcutaneous Layer The subcutaneous or hypodermal layer is a connective tissue layer that attaches the skin to the muscles and to other tissues lying beneath the skin. The subcutaneous layer also specializes in the production of fat. Lipocytes, commonly known as "fat cells," are stored in the subcutaneous layer in a special tissue called adipose tissue, which is made up of singular adipocytes. The sole purpose of lipocytes is to manufacture and store large quantities of fat. We think of fat as a bad thing, but fat serves many useful purposes. You need fat to maintain good health. It helps protect your underlying organs from mechanical shock as you bounce along. It also serves as insulation and, therefore, helps to keep your body warm. When there's no food to eat, fat becomes an energy source. On the other hand, excessive fat stores are bad for your health. Technically speaking, the subcutaneous layer isn't actually part of the integumentary system because it's made of connective tissue. It's sometimes called superficial fascia. Fascia is the tissue that provides support and protection for organs. The subcutaneous layer provides a link between the integumentary system and the other body systems it covers. It serves to shape the body. SKIN GRAFTING Your skin is an organ that can grow on its own. Skin grafting is commonly used to grow and repair blood vessels, trauma wounds, deep injuries, and burns. Grafting is a process of taking a piece of your own skin or a piece of donor skin and transplanting to the affected site to grow and repair.

Sebaceous Glands

Flask-shaped sebaceous glands are also located in the dermis. Commonly attached to hair follicles, the sebaceous glands secrete an oily substance called sebum, which is released through ducts into the hair follicles. Sebum oils the hair, lubricates the skin, and prevents water loss.

Hair and Nails

Hair and nails are modified forms of the epidermis. In both, the horny cells, filled with keratin, are pushed through the epidermis. Because hair and nails are composed of dead cells, no pain is felt when they're cut. Like the skin, hair and nails provide protection. Hair keeps heads warm and serves as a filter system for ears, eyes, and nose. Nails protect the ends of fingers and toes. We tend to think of hair simply as long structures growing out of our skin. Yet each strand of hair on your body is a complex entity with a full support system. Hair fibers are composed of a tightly fused meshwork of horny cells. A single hair is composed of four basic parts—the hair shaft, hair root, hair follicle, and the papilla. The hair shaft is the visible part of the hair, while the hair root is embedded in the dermis. If you pluck a hair from the root, you'll see the hair follicle, a bulblike attachment at the end of the hair. The follicle consists of the root and its coverings. The papilla is a loop of capillaries enclosed within connective tissue. Some parts of the human body have no hair, such as the palms of the hands and the soles of the feet. But other parts have an abundance of hair. Your hair is constantly shed and replaced as more horny cells move upward from hair follicles through the hair shaft. Nails provide physical support to the fingertips to allow you to pick up small objects. Nails are hard, keratin-filled plates that are composed of tightly cemented horny cells. Just as a hair grows from the follicle, a nail grows from the lunula, the white crescent at the base of the nail. Unlike hair, which sheds at intervals, fingernails grow indefinitely at the rate of about one millimeter a week unless they're cut or broken. Toenails also grow indefinitely, but at a slower rate than fingernails. The cuticle is the narrow band of the epidermis that extends from the nail wall onto the surface. The pinkish color seen beneath the nail is produced by underlying capillaries.

Sweat Glands

Sweat glands are located deep in the dermal layer, very close to the subcutaneous layer of the skin. There are two types of sweat glands: eccrine and apocrine. Eccrine sweat glands are numerous, important, and widespread throughout your body. These tiny coiled tubes reach the epidermis through crooked ducts called pores. Apocrine sweat glands are found primarily in the axillae (armpits) and in the pigmented skin areas around the genitals. Eccrine sweat glands produce a watery sweat, whereas the apocrine sweat glands produce a thicker, milky secretion that can be odorous. Approximately two million sweat glands work together to help your body maintain its proper temperature. As muscle movement and physical activity generate body heat, the sweat glands secrete water, more commonly known as perspiration. As the perspiration evaporates, the body becomes cooler. On average, a person sweats approximately one quart of water each day. That's why it's important to drink enough water to replenish body fluids. The milk-producing mammary gland is actually a modified sweat gland, but it doesn't work full time like the sweat gland. The mammary gland secretes milk only after the birth of a child. Other modified sweat glands are the ceruminous glands, located in the skin that lines the external auditory canals (your ears). Instead of sweat, these glands secrete earwax, or cerumen.

Overview

The integumentary system is the body system that provides protective coverage for your internal organs. The integumentary (pronounced in-teg-yuh-MEN-tuh-ree) system comprises of the skin, hair, nails, sweat glands, and sebaceous glands. Nails protect the ends of fingers and toes. The skin, hair, sebaceous (oil) glands, and sweat glands enable the body to regulate temperature. In this section, you'll learn the basic structure and functions of the integumentary system organs.