Good day everyone, you are welcome to my blog. Today we are discussing about the skin:
Learning outcomes
After studying this section, you should be able to:
- describe the structure of the skin
- explain the principal functions of the skin
- compare and contrast the processes of primary and secondary wound healing.
The first part of this chapter explores the structure and functions of the skin, which is also known as the integumentary system. The effects of ageing on the skin are discussed in the following section. The chapter concludes with a review of common skin conditions.
The skin completely covers the body and is continuous with the membranes lining the body orifices. It:
- protects the underlying structures from injury and from invasion by microbes
- contains sensory nerve endings that enable discrimination of pain, temperature and touch
- is involved in the regulation of body temperature.
Structure of the skin
The skin is the largest organ in the body and has a surface area of about 1.5-2 m² in adults. In certain areas, it contains accessory structures: glands, hair and nails. There are two main layers; the epidermis, which covers the dermis. Between the skin and underlying structures is a subcutaneous layer composed of areolar tissue and adipose (fat) tissue.
Epidermis
This is the most superficial layer and is composed of stratified keratinised squamous epithelium. It varies in thickness, being thickest on the palms of the hands and soles of the feet. There are no blood vessels or nerve endings in the epidermis, but its deeper layers are bathed in interstitial fluid from the dermis, which provides oxygen and nutrients, and drains away as lymph.
There are several layers (strata) of cells in the epidermis which extend from the deepest germinative layer the to the most superficial stratum corneum (a thick horny layer) Epidermal cells originate in the germinative and undergo gradual change they progress the surface. The cells the surface flat, non-nucleated, dead squames, in which the cytoplasm has been replaced by the fibrous protein keratin. The surface cells constantly rubbed off and replaced by those beneath. Complete replacement of the epidermis takes about a month.
Health epidermis depends upon three processes being synchronised:
- desquamation (shedding) of the keratinised cells from the surface
- effective keratinisation of cells approaching the surface
- continual cell division in the deeper layers with newly formed cells being pushed upwards to the surface.
Hairs, secretions from sebaceous glands and ducts of sweat glands pass through the epidermis to reach the surface.
Upward projections of the dermal layer, the dermal papillae, anchor this securely to the more super. ficial epidermis and allow passage and exchange of nutri ents and wastes to the lower part of the epidermis. This arrangement stabilises the two layers preventing damage due to shearing forces. Blisters develop when trauma causes separation of the dermis and epidermis, and serous fluid collects between the two layers.
In areas where the skin is subject to greater wear and tear, e.g. the palms and fingers of the hands and soles of the feet, the epidermis is thicker and hairs are absent. In these areas the dermal papillae are arranged in parallel lines giving the skin surface a ridged appearance. The pattern of ridges on the fingertips is unique to every individual and the impression made by them is the ‘fingerprint’.
Skin colour is affected by various factors.
- Melanin, a dark pigment derived from the amino acid tyrosine and secreted by melanocytes in the deep germinative layer, is absorbed by surrounding epithelial cells. The amount is genetically determined and varies between different parts of the body, Between people of the same ethnic origin and between ethnic groups. The number of melanocytes is fairly constant so the differences in colour depend on the amount of melanin secreted. It protects the skin from the harmful effects of ultraviolet rays in sunlight. Exposure to sunlight promotes synthesis of melanin.
- Normal saturation of haemoglobin and the amount of blood circulating in the dermis give white skin its pink colour. When oxygen saturation is very low, the skin may appear bluish (cyanosis).
- Excessive levels of bile pigments in blood and carotenes in subcutaneous fat give the skin a yellowish colour.
Dermis
The dermis is tough and elastic. It is formed from con- nective tissue and the matrix contains collagen fibres interlaced with elastic fibres. Rupture of elastic fibres occurs when the skin is overstretched, result- ing in permanent striae, or stretch marks, that may be found in pregnancy and obesity. Collagen fibres bind water and give the skin its tensile strength, but as this ability declines with age, wrinkles develop. Fibroblasts, macrophages and mast cells are the main cells found in the dermis. Underlying its deepest layer is the subcutaneous layer containing areolar tissue and varying amounts of adipose (fat) tissue. The structures in the dermis are:
Also read: the liver
- blood and lymph vessels sensory nerve endings
- sweat glands and their ducts
- hairs, arrector pili muscles and sebaceous glands.
Blood and lymph vessels. Arterioles form a fine network with capillary branches supplying sweat glands, sebaceous glands, hair follicles and the dermis. Lymph vessels form a network throughout the dermis.
Also read: the concepts associated with pains
Sensory nerve endings. Sensory receptors (specialised nerve endings) sensitive to touch, temperature, pressure and pain are widely distributed in the dermis. Incomin stimuli activate different types of sensory receptors; for example, the Pacinian corpuscle is sensitive to deep pressure. The skin is an important sensory organ through which individuals receive information about their environment. Nerve impulses, generated in the sensory receptors in the dermis, are transmitted to the spinal cord by sensory nerves. From there impulses are conducted to the sensory area of the cerebrum where the sensations are perceived.
