Lesson

The integumentary system includes the skin and its accessory organs, the hair and the nails. Skin is the largest organ in the human body, and in the average adult covers about 22 square feet. It has weight (8–10 pounds), texture, and color. It changes over the years, losing much of its elasticity and suppleness as we age, due to hardening of collagen.

The skin has four important functions. First, it serves as a protective membrane that covers the entire body, preventing the loss of important substances, such as water and salts. The secretions of the skin are slightly acidic in composition and help to prevent the invasion of pathogens, such as bacteria. The skin also secretes vitamin D, which aids the body in its absorption of calcium. Specialized cells in the skin also have an immune function.

In addition to protection, there are glands in the middle portion of the skin, the dermis, that secrete important substances. Sebaceous glands produce an oily secretion called sebum that goes through pores, or openings, to the surface of the skin. Sebum lubricates the skin and prevents excessive dryness. Sweat glands, on the other hand, produce a watery substance that is mostly composed of water. When sweat goes to the surface of the skin, it evaporates and keeps the body cool.

In addition to covering and protecting, nerve endings in the skin allow sensory messages to be transported to the brain and spinal cord. As a result, an individual has the sensation of touch, pressure, pain, and temperature. With changes in the external environment, the individual is able to adapt.

Lastly, the skin keeps heat in the deeper tissues of the body. Nerves from the skin conduct impulses to areas of the brain that regulate increases or decreases in body temperature.

Three major regions of the skin are the:

1. Epidermis - outermost region composed of squamous epithelium. Its nourishment comes from the middle layer, the dermis. The deepest layer of the epidermis is called the basal layer; new cells are constantly being produced there. The older cells are pushed toward the stratum corneum, which is the most superficial layer of the epidermis. Once the older cells are flattened, they lose their nuclei and die. They are then filled with a protein material called keratin. In a few weeks, the keratinized cells, called horny cells, are sloughed off the surface of the skin. Each day we lose thousands of dead skin cells, whether we realize it or not (estimated at 30,000 to 40,000 cells).
2. Dermis - middle region consisting of connective tissue. The dermis contains blood vessels, lymph vessels, and nerve fibers, as well as the connective tissue that gives skin its elasticity, called collagen. (From your study of the musculoskeletal system, you recall that collagen is the same protein material that is found in the bones, cartilage, tendons, and ligaments.) It is in the dermis layer that sebaceous glands, sweat glands, and hair follicles are found.
3. Subcutaneous - the deepest region. It produces lipocytes, which are fat cells that remain in this layer. The lipocytes store fat. The subcutaneous layer of the skin is instrumental in preserving body heat and energy.

Some related combining forms are:

• cutane/o
• dermat/o, derm/o
• adip/o
• lip/o
• seb/o
• kerat/o

The endocrine system, like the nervous system, is involved in conducting information and regulating organisms. It accomplishes this, however, through blood vessels rather than through nerves. The system is made up of glands, which are located throughout the body and which secrete chemical substances called hormones. Hormones regulate many functions and produce their effects by binding to receptor sites, which are found on the different target tissues. For every hormone, there is one receptor to which the hormone binds, very similar to the way in which a key and a lock fit together.

Although the exocrine glands secrete substances into ducts, or tubes, that lead to the outside of the body, the endocrine glands secrete hormones directly into the bloodstream.

The endocrine glands include the following:

1. Pituitary gland - Located in the sella turcica at the base of the brain, the pituitary gland contains two parts, an anterior lobe and a posterior lobe. The hypothalamus is a structure that is located close to the pituitary gland. Signals from the hypothalamus regulate secretions of the pituitary gland. Hormones from the anterior pituitary gland include growth hormone, thyroid-stimulating hormone, adrenocorticotropin, gonadotropic hormones, and prolactin. Hormones from the posterior pituitary gland include antidiuretic hormone and oxytocin.
2. Pineal gland - Located in the center of the brain, this gland produces melatonin, a substance that induces sleepfulness and regulates wake/sleep patterns.
3. Thyroid gland - Located on either side of the trachea, the thyroid gland secretes two hormones that are involved in maintaining normal levels of metabolism in the body. The two hormones are thyroxine (T4 or tetraiodothyronine) and triiodothyronine (T3).
4. Parathyroid gland (four glands) - The parathyroids are adjacent to the thyroid gland. They secrete parathyroid hormone (PTH), which mobilizes calcium that is stored in the bones to enter the bloodstream. Calcium is vital for normal muscle and tissue function.
5. Thymus gland - Located behind the sternum, the thymus gland produces thymosin, a hormone that promotes the immune response in newborns. The gland is larger in childhood, but in adulthood, it shrinks.
6. Pancreas (islets of Langerhans) - Located near the stomach, the pancreas is both an exocrine gland that secretes digestive enzymes, and an endocrine gland that secretes insulin and glucagon. The specialized cells of the pancreas that function as an endocrine gland are called islets of Langerhans, composed of alpha and beta cells. Insulin and glucogon produced by these cells help to metabolize sugars and starches.
7. Adrenal glands - Located on top of each kidney, there are two pairs, and each pair has a cortex and a medulla portion. The adrenal cortex produces glucocorticoids, which affect metabolism of fat, proteins, and sugars. Mineralcorticoids and gonadocorticoids are also secreted.
8. Testes in male (one pair) - Located in the inguinal region in the scrotal sac of the male, the testes produce spermatozoa, which are gametes, and testosterone, a male hormone.
9. Ovaries in the female (one pair) - Located in the lower portion of the abdomen in the female, the ovaries produce ovum, which are gametes, and estrogens and progesterone, female hormones. Hypersecretions or hyposecretions of the endrocrine glands result in abnormal, pathological conditions.

Some related combining forms are:

• adren/o
• aden/o
• gonad/o
• pancreat/o
• parathyroid/o
• pituitar/o
• thyr/o
• thyroid/o
• andr/o
• estr/o
• glyc/o
• kal/i
• insulin/o
• hormon/o