Title: The Skin

Key words: Epidermis, dermis, sweat glands, sebaceous glands, follicles, nerves, receptors, hair, mealnin, sebu, temperature regulation

Date: Aug 2000

Category: 6. The Body

Type: Article

Author: DJE Candlish

 

The Skin

Introduction

The skin is the largest organ in the body. Its functions include providing a protective covering for the other tissues in the body, regulating body temperature, retaining water, excretion and providing sensory information.

The skin consists of three layers, the epidermis, the dermis and the subcutaneous layer. Within the dermis are blood and lymph vessels, sweat and sebaceous glands, hair follicles, nerves (neurones) and nerve receptors.

The epidermis

The outer layer of the skin is called the epidermis. It consists of layers of squamous epithelium. The cells are formed in the lowest layers and gradually move up as they age, changing shape as they do so. The lowest layer or stratum basale is where most cell division takes place.

The cells here contain melanin, a pigment that gives the skin its colour and helps protect it from the sun. The daughter cells formed here are gradually pushed towards the surface. On their way, they become flattened and lose their pigmentation. By the time they reach the surface, the cells are dead and form a layer of flakes containing the tough substance keratin, which is also present in the nails.

This layer, the stratum corneum, can become very thick on the fingers, palms and soles of the feet, where it forms a callus in response to friction.

The epidermis does not contain blood vessels or lymph vessels. The only neurones present are those responsible for the sensation of pain, which is triggered by substances released from damaged cells.

The epidermis and dermis are closely interwoven where they meet, with connective tissue creating raised patterns (papillae) on the surface which form our gentically unique fingerprints.

Hair

Hairs are a unique variation of the epidermal structure. The part above the skin is the hair shaft. Within the skin is the hair root which changes into the hair bulb at its base. Here, the hair and epidermal layers merge. Hair itself is a modified form of stratum corneum. In humans it has little effect in retaining heat, in contrast with animal fur. It does however, have a sensory function, as hair is very sensitive to touch.

Different types of hair grow in different locations. The hair on the head grows fast (2mm a day) and is usually replaced when it falls out. Eyebrow hair is thicker and grows slowly, if at all. Eyelashes, underarm and pubic hair also have different characteristics.

Sweat glands

Perspiratory or sweat glands are epidermal structures, but the sweat producing part is located in the dermis. There are 2 to 3 million sweat glands over the body, mostly in the armpits, palm of the hand, the forehead, sole of the foot and back. The glands have a plentiful blood supply, as they use large amounts of fluid. Sweat consists of 99% water and the remainder is salt, acids and some waste products. The evaporation of sweat plays an important role in regulating body temperature. The heat energy used by evaporation helps to cool the body.

Sebaceous glands

These glands produce sebum, an oily substance that helps keep the skin supple and waterproof. It also protects against dehydration and microbial invasion. Most sebaceous glands are located around hair follicles. The sebum also keeps the hairs supple.

The dermis

The dermis consists of connective tissue, containing collagen and elastin fibres. The toughness and elasticity of the skin are due to this layer. Within this connective tissue are many blood and lymphatic vessels, nerve fibres and sensory nerve endings. These can detect pressure, temperature, pain and touch.

Below the dermis is a layer of subcutaneous connective tissue containing many fat cells. This acts as a thermal insulator, reducing heat loss from the body. It also acts as a food reserve and a buffer against shock and pressure. The subcutaneous layer contains blood and lymphatic vessels and nerve fibres.

Temperature regulation

The skin helps to maintain body temperature within defined limits. Although the core body temperature is usually 37oC, there are wide variations. At an external temperature of 20oC, for example, the skin of the hands may be only 28oC. In contrast, the temperature within the liver can reach 39oC.

The skin is the body's cooling organ, with an average surface area of around 1.65-1.7m2. It loses heat by different mechanisms. About 60% of body heat is lost by radiation, even in a warm environment. However, if the surrounding temperature is higher than core body temperature, the body gains heat. The capillaries in the skin can alter the amount of heat radiated. If body temperature is high, the capillaries dilate, bringing more blood closer to the surface of the skin to increase heat loss. If the surrounding environment is very cold, however, the capillaries constrict, to conserve blood and body heat.

The skin also loses heat by conduction, especially in water. Moving air over the surface of the skin helps to remove heat more quickly. Preventing air movement, by wearing clothing for example, slows down this heat loss.

The evaporation of sweat has a powerful cooling effect. However, if the surrounding air is very humid, evaporation slows down or cannot take place, so this cooling mechanism is less effective in tropical areas.

The temperature of the body is monitored in several ways. In addition to the temperature sensors in the skin, sensors in the brain monitor the temperature of the blood. These sensors determine the response of the temperature regulating centres in the brain. These control the dilation or constriction of the capillaries in the skin. If more heat is required, they can also cause shivering, where the skeletal muscles generate heat with short unco-ordinated contractions.

If body temperature appears to fall too low, the thyroid gland may be stimulated to release thyroid hormone, which increases cell metabolism and generates extra heat.