Regulation of Body Temperature
This chapter primarily focuses on skin and its accessory organs. Skin is also called the cutaneous membrane. This chapter also covers different types of membranes of the body.
Membranes are organs as they are composed of at least two different tissues. There are four major types of membranes of the body.
1) Serous Membranes - These membranes secret a slippery fluid called serous fluid. They line the thoracic cavity as well as the abdominopelvic cavity. These membranes also cover the organs of the thoracic cavity and the abdominopelvic cavity. Examples of this type of membrane includes the parietal pleura, the visceral pleura, the parietal pericardium, the visceral pericardium, the parietal peritoneum, and the visceral peritoneum. The serous fluid allows the organs of these cavities to move against the body wall and each other without creating friction.
2) Mucous Membranes - Mucous membranes secrete mucus. They line tubes and organs that open to the outside world. For example, mucous membranes line the organs of the digestive tract, respiratory tract, urinary tract, and genital tract. Therefore, they line the mouth, stomach, nose, throat, bladder, etc.
3) Synovial Membranes - These membranes secrete a slippery fluid called synovial fluid. They line joint cavities of bones that are freely moveable. For example, they line the joint cavities of the elbows, knees, shoulders, fingers, etc. The synovial fluid allows the bones to move against each other easily.
4) Cutaneous Membrane - This is the membrane that covers the body and is more commonly called skin.
The skin is a complex organ consisting of three layers: 1) epidermis 2) dermis and 3) hypodermis (subcutaneous layer). Skin and its accessory organs such as hair, nails, glands make up the integumentary system.
Click here to see a picture of skin
Click here to see structures found in skin and layers of skin
The epidermis is the most superficial layer of skin. It is composed of keratinized stratified squamous epithelium. The epidermis can be divided into two layers: 1) stratum corneum and 2) stratum basale.
The stratum corneum is the most superficial layer of the epidermis. Most of the cells in this layer are dead and very flat. Because they have accumulated keratin, the cells in this layer stick together and form an impermeable layer for skin. Most bacteria, viruses, toxins, and water cannot penetrate an intact stratum corneum.
The stratum basale is the deepest layer of the epidermis. The cells in this layer are constantly dividing. The old cells are pushed up toward the stratum corneum.
The most numerous cell type in this layer is the keratinocyte. It is a cell that accumulates the protein keratin. Keratin is a durable protein that makes the epidermis water-proof and not easily permeable to bacteria, viruses, and toxins.
Another less numerous cell type found in this layer is the melanocyte. Melanocytes make the pigment melanin. Melanin is deposited throughout the layers of the epidermis. This pigment traps UV radiation from sunlight and prevents the radiation from harming structures in the underlying dermis of skin.
Skin color is primarily determined by the amount of melanin in the epidermis of skin. Melanin can range in color from yellowish to brownish. The more melanin a person has in their skin, the darker or more pigmented the skin color. All people have about the same number of melanocytes regardless of skin color. What varies from person to person is how active melanocytes are in producing melanin. A person with dark skin has very active melanocytes.
Another factor that determines skin color is the amount of oxygenated blood in the dermis of skin. Hemoglobin is a pigment in blood that is bright red when it is oxygenated. When hemoglobin is not oxygenated it is a dark red color. If a person has a rich supply of oxygenated blood in their skin, the skin will have a pinkish hue. When the supply of oxygen is low in the blood, the skin looks rather pale or bluish.
The dermis is the middle layer of skin and is the most complex layer. The dermis contains all major tissue types. That is, it contains epithelial tissue, connective tissues, muscle tissue, and nervous tissue. Within the dermis you find sweat glands, sebaceous glands, hair follicles, arrector pili muscles, collagen fibers, elastic fibers, nerve fibers, and lots of blood vessels. The dermis binds the epidermis to the hypodermis.
The subcutaneous layer is also called the hypodermis as it lies beneath the dermis. It is largely made of adipose tissue. It also contains blood vessels and nerve fibers.
Hair follicles are tube-like depressions in the dermis of skin. Hair follicles are made of epithelial tissue and function to generate hairs. Cells called keratinocytes make up most of the hair follicle. As new keratinocytes are produced in the base of the hair follicles, old ones are pushed toward the surface of skin. The old keratinocytes stick together to produce a hair. The portion of the hair embedded in skin is called the root while the portion of the hair extending from the surface of skin is called the shaft. When a hair follicle dies, the hair falls out.
Arrector pili muscles are attached to most hair follicles. When a person is cold or nervous, these muscles pull on hair follicles causing hairs to stand erect. These muscles also pull on fibers in the dermis of skin causing "goose bumps" to form.
Click here to view a hair follicle
Sebaceous glands are more commonly called oil glands. They produce an oily substance called sebum. Sebum is secreted onto hairs to keep it soft and pliable. Sebum eventually is deposited onto skin to keep it soft as well.
Nails function to protect the ends of the toes and fingers. The portion of a nail that is embedded in skin is called the nail root. The nail root contains active keratinocytes that constantly divide to produce nail growth. Your nails are nothing more than dead, hard keratinocytes! The white half-moon-shaped area at the base of a nail is called a lunula. The lunula also contains very active keratinocytes. Beneath each nail is a layer called the nail bed. The nail bed holds the nail down to underlying skin.
Most sweat glands are located in the dermis of skin. However their ducts open onto the epidermis of skin. There are two types of sweat glands: 1) eccrine and 2) apocrine.
Eccrine sweat glands are the most numerous type. They produce a watery type of sweat and are activated primarily by heat. Once sweat is deposited onto skin, it evaporates and carries away heat from the body. Eccrine sweat glands are most concentrate on the forehead, neck, and back.
Apocrine sweat glands produce a thicker type of sweat and are most concentrated in areas of skin with course hair such as the armpit and groin areas. They are primarily activated by nervousness or stress. They are responsible for the "cold sweat".
Click here for a table that summarizes different types of glands associated with skin
Skin plays a major role in regulating body temperature. When a person is hot, dermal blood vessels dilate which is why a person's skin becomes pinkish. Since the dermal blood vessels dilate, more blood passes through skin than normal. This is a good thing as blood carries a lot of the heat in the body. When the blood gets close to the surface of the body (to skin), the heat can escape. Conversely, if a person is cold, the dermal blood vessels constrict preventing the heat in blood from escaping.
When skin is injured it becomes inflammed. An inflammed area looks red due to dilated blood vessels. The inflammed area also becomes swollen due to the leakiness of the blood vessels in the area. Inflammation promotes healing as more blood is delivered to the area. The blood carries nutrients needed for skin repair as well as defensive cells to clear up the cause of inflammation.
When the epidermis and dermis are injured a blood clot initially forms. The blood clot is eventually replaced by a scab which is basically clotted blood and other dried tissue fluids. The scab is normally replaced by collagen fibers that act to bind the edges of the wound together. Collagen fibers are whitish and are the major components of scars. Sometimes scars are replaced with the original tissue that was damaged but if the wound is extensive, the scar will persist.