True Skin (Dermis)

The dermis is the layer that gives the skin its elasticity, and it contains the blood vessels, the adnexae of the skin (sebaceous glands, sweat and scent glands, hair and nails), and the nerves and sensory organs of the skin. The dermis is located underneath the epidermis and the connecting basement membrane, and provides the foundation for the outer skin or epidermis. It consists primarily of ground substance and a dense network of bundles of connective tissue (collagen fibrils) and elastic fibres. Collagen accounts for about 18 to 30 % of the volume of the dermis. The collagen fibrils allow the skin to stretch, and the elastic fibres give it its resilience. The skin is elastic and, depending on region, can stretch by between 10 and 50 %. When the skin is pressed in, the ground substance is displaced between the collagen fibrils.

The papillary dermis (stratum papillare) forms the upper zone of the dermis and consists of loose connective tissue. It forms finger-like ridges, connecting with the basement membrane and the epidermis. These ridges (papillae of connective tissue) increase the contact surface between the epidermis and the dermis and improve nutrition to the epidermis, which has no blood vessels. The papillae of connective tissue do not serve to connect the epidermis with the dermis. This is done only by the basement membrane and the fibres of connective tissue (collagen) of the dermis.

The deeper and thicker of the two layers of the dermis is called the reticular dermis (stratum reticulare) and consists of bundles of strong collagen fibrils that interweave with each other and cross each other at certain angles. Changes in this angle allow the skin to stretch without any loss of tensile strength. The resilience of the skin is given by the elastic fibres that return the collagen fibrils to their former position after stretching.

The ground substance is of great importance for the skin’s ability to store water. It is able to store fluid and electrolytes, and is a vital depot for water. Almost all the water in the connective tissue of the skin is held in unfree form and originates from the blood.

Blood and lymphatic system

To maintain the functions of tissues and organs, a constant exchange of nutrients, metabolism products and cell regulation substances is necessary betweeABB.5n the bloo

d vessels and the cells. The blood circulates in a closed system of elastic tubes, the blood vessel system. Blood which is rich in oxygen and nutrients is transported from the heart through the arteries and then distributed through ever narrower vessels into the most minute capillaries. The walls of the capillaries are semi-pervious, allowing oxygen and nutrients to be released one way and carbon dioxide and waste products to be absorbed the other. These are then transported in the blood through increasingly bigger blood vessels (veins) to the heart.

The blood vessel system of the skin consists of two network-like structures, located between the papillary and reticular layers of the dermis above and at the junction to the fatty tissue of the subcutis below. The deep network and the superficial network of vessels are linked by vertical vessels. From the upper network, very small vessels (capillaries) penetrate to immediately below the epidermis. The capillaries ensure the supply of nutrients to the tissue. The total length of the capillaries in the skin is about 240 km, while all the blood vessels of the body together have a length of 1440 km.

The blood vessel system not only guarantees the supply of nutrition to the cells and tissue but is also important for heat regulation and assists blood pressure regulation. In cold weather, the surface vessels contract, reducing the amount of heat released through them. In hot surroundings, the superficial vessels dilate, allowing more body heat to be released. 75 % of excess heat is given off through the blood vessels. Heat released via the sweat glands through evaporation accounts for about 25 %. The body’s heat is regulated in a complex process of interaction between the heat receptors of the skin, the brain, the blood vessels and the sweat glands.

The fluid which, among other things, enters the tissue through the semi-pervious walls of the capillaries, is transported away through the vessels of the lymphatic system. The lymph vessels mostly run parallel to the blood vessels. Unlike the capillaries of the blood system, the lymphatic system starts with dead-end lymph capillaries. The capillaries carry away the fluid from the spaces between the tissue. Many small capillaries join to form bigger lymph vessels, which gradually increase in diameter and wall thickness until emptying into the large veins near the heart. Apart from draining the fluid in the tissue (normally approx. 2 litres per day), the lymphatic system with the lymph vessels, lymph nodes, lymph organs, lymphocytes and other specific cells is also part of the body’s immune defence system.

Nervous system

Human response to the external environment is based, among other things, on the interaction between the nervous system of the skin and certain regions of the brain. Specific areas of the skin can trigger feelings of pleasure or defensive responses. The first impressions received by a baby on being born are transmitted through the skin. In the womb, the foetus already develops a sense of touch when still only 2.5 cm in size. Also after birth, the signals from the skin are of extreme importance for the sense of well-being and even the survival of an infant.

The skin performs its task as a sensory organ with a dense network of nerves, sensitive nerve fibres with the finest ramifications, and specially formed nerve end apparatuses located in all layers of the dermis, and to some extent also the epidermis. Pressure, vibration, temperature, pain and itching are transmitted via special receptory organs and nerves to the spinal cord and brain stem and then to the cerebral cortex of the brain. One square centimetre of skin contains up to 2 receptors for heat, 12 for cold, 50 for pressure and 200 for pain.

Merkel’s cells are located in the germ layer of the epidermis and serve as pressure points when touching. Pressure is transmitted by Meissner’s corpuscles, located directly under the epidermis, and vibration by the Pacinian corpuscles in the lower dermis and upper subcutis. In the case of infants and small children, there are specially numerous touch points on the lips and tongue, and in adults on the finger tips. The touch receptors enable us to recognise objects. The touch corpuscles, for example, are able to recognise depressions in the skin of only 0.01 mm and weight differences of 4 mg. This is what allows blind people to read braille and “see” with their finger tips.

Just under the surface of the epidermis lie the cold receptors (Krause’s end bulbs). These reach maximum stimulation at between 17° and 36° C. In the middle of the dermis lie the heat receptors (Ruffini’s corpuscles) with maximum stimulation at between 40° and 47° C. At temperatures above 45° C, the heat receptors signal pain instead of a feeling of warmth. The temperature stimuli of the skin are compared in the brain with the body temperature, and the brain then regulates the blood system via the nerves. The blood vessels dilate in heat and contract in cold.

Pain and itching, which can be closely related to pain, are transmitted via free nerve endings by medullated nerve fibres (type A) and non-medullated nerve fibres (type C). The thick, fast nerves of type A generate a sudden pain that normally triggers a reflex. The thin, slow nerves of type C transmit a dull, chronic pain and signal disturbances of the body. Most functions of the skin are regulated in precise interaction with the nerves of the vegetative nervous system.

The hairs can transmit contact through the fine nerve fibres of the hair sheath, while the finger and toe nails transmit pressure through touch corpuscles. Hairs, finger nails and toe nails also serve as organs of touch.

Gas exchange through the skin, i.e. “cutaneous respiration”, with oxygen being absorbed one way and carbon dioxide released the other, is very small. No more than 2 % of the body’s gas exchange takes place through the skin.

By producing vitamin D, the skin also has functions as a metabolistic organ. Vitamin D, an important component for calcium and bone metabolism, is produced from pre-stages of cholesterol. With sufficient exposure to sunlight, a healthy adult is able to produce his or her own needs of this vitamin (5 mg per day). To do this, only 20 to 30 minutes of daylight on the face and hands are sufficient