LECTURE NOTE

ON

PHYSIOLOGY OF THE INTEGUMENTARY SYSTEM

BY

MJ ADENIYI MSC

(7jimade@gmail.com, +234 08066796517)

DEPARTMENT OF PHYSIOLOGY

UNIVERSITY OF BENIN, BENIN CITY, NIGERIA

INTEGUMENTARY SYSTEM

Figure 1: Structure of the integumentary system (left diagram). The right diagram shows the structure of the skin and its blood vessels.

Integumentary system consists of the skin and its appendages. The appendages are hair follicles, sweat gland, sebaceous gland, arrector pilli muscle and nail. The functions of this system are protection from external environment, defense against insults, metabolic function (synthesis of cholecalciferol (inactive vitamin D)), body coloration, excretion of wastes, finger and toe prints investigation and cosmetic among others.Skin is the largest organ of the integumentary system and one of the largest organs in the body. The weight of the skin is about 3.4kg. The average square inch of skin has 20 blood vessels, 650 sweat glands, 6000 melanocytes and over 1000 nerve endings. Abundance of nerve endings make skins the largest sense organ in the body. The thickness of the skin varies from one body part to another. It is thicker in the palm and sole and thinner in the glans, upper limb, etc. The skin is made up of epidermis (the outer part) and dermis (the inner part). Beneath the dermis is subcutaneous layer.

The epidermis is deficient in blood vessel and lymph vessels and hence depends on diffusion of fluid from nearby dermis . The epidermis consists of: -Stratum corneum: It is the outermost layer of the epidermis made up of corneocytes (dead cells). Histologically, they are squamous epithelial cells. The process by which corneocytes are formed is called cornification. Cornification involves formation of keratin by keratinocytes of stratum germinativum and formation of Keratin cross link by keratohyaline of stratum granulosum. Stratum corneum is resistant to pressure, temperature and pain. It protects the skin against mechanical injury. This layer of the epidermis can desquamate (slough). Pressure and noxious stimuli can cause loss of this layer.

-Stratum lucidum: It is a clear and homogenous layer located between stratum corneum and stratum granulosum. It is characterized by squamous epithelial cells. Keratin meshwork can also be found in this layer. -Stratum granulosum: It is characterized by squamous epithelial cells. It is a granular layer of the skin consisting of keratohyaline, a precursor for keratin. Keratohyaline joins keratin proteins together forming keratin meshwork.-Stratum spinosum: This layer is characterized by squamous epithelial cell. It is a spine like region of the skin that overlies the stratum germinativum. Keratin proteins from stratum germinativum are found in this layer-Stratum germinativum (or stratum basale) is the deepest part of epidermis. Stratum germinativum has merkel cells which respond to tactile stimuli. This layer is characterized by cuboidal or columnar epithelial cells. Stem cells called keratinocytes are present here. These cells are capable of actively dividing forming keratin proteins which migrate to superficial layers (stratum spinosum and stratum granulosum). In the stratum granulosum, keratin proteins are joined together by keratohyaline. Apart from keratinocytes, melanocytes, factory of melanin are present. MELANINIt is a coloured protein. There are two basic types of melanin: the brown or black eumelanin and pheo melanin. The former is responsible for brown or black coloration of the skin and hair. The latter is responsible for red or yellow coloration of the palm, sole, glans, lip of vagina, among others. Melanin helps us to tolerate harsh environmental situations by absorbing ultraviolet radiation. Increase in melanin production is an important adaptive response of skin to harsh environmental conditions. An enzyme called tyrosinase is involved in the formation of melanin from tyrosine. Deficiency of this enzyme results in decreased level of melanin and a condition known as albinism. Melanin production by melanocytes is influenced by hormones such as melanocyte stimulating hormone, adrenocorticotropic hormone, testosterone and estrogen. Estrogen actually increases production of pheomelanin in pubertal female and makes skin slightly lighter. DERMISDermis is situated beneath the stratum germinativum. Blood vessels, nerve endings, lymph vessels, sweat gland, bulb of hair follicle and sebaceous gland are present in the dermis.The superficial part of the dermis is termed the papillary layer. This layer consists of projections (papilla). Blood vessel, nerve endings are present here.The reticular layer has elastic and reticular fibres, nerve endings, blood vessels and the terminal parts of appendages.Subcutaneous layerBeneath the dermis is the subcutaneous layer made of adipose tissue. Under the influence of ultraviolet radiation, 7-dehydrocholesterol is converted to vitamin D (cholecalciferol). Vitamin D is also a substrate for the production of a hormone called 1,25-dihydroxylcholecalciferol which stimulates calcium uptake by kidney and intestine and is essential for bone formation. The subcutaneous layer acts as insulator and energy reservoir.

BLOOD VESSELS AND NERVE ENDINGSCutaneous blood vessels are conduits for blood. The diameter of these vessels determines

blood flow to skin. Interestingly, the diameter of cutaneous arterioles depends on state of activity. For example, parasympathetic phenomenon (as in resting and relaxation) causes vasodilation and this increases diffusion of fluid to the sweat gland, epidermis and other structures of the skin. The reverse takes place during sympathetic stimulation.

Since blood contains coloured pigment called hemoglobin, the appearance of skin may provide information about state of health. The skin of anemic subjects appears pale. High level of carbhemoglobin gives skin bluish appearance and so on.

As stated previously, skin is endowed with abundant nerve endings. Merkel discs found in the stratum germinativum are mechanoreceptors that sense touch. Other mechanoreceptors present in dermis are not limited to pacinian corpuscle and meissner’s corpuscle. Different pain receptors and thermoreceptors are also present in the dermis. The thermoreceptors are divided into cold and heat receptors APPENDAGESSWEAT GLANDS (sudoriferous glands): These are sweat secreting structures. Sweat glands play role in water conservation. Water loss through sweat is part of sensible water loss. Structurally, they consist of the coiled portion and the tubular portion. As shown in the left diagram of figure 1, the coiled portion embeds deep in the dermis and the tubular part leads into the exterior. There are two types of sweat glands-the eccrine gland and apocrine gland. The former constitute a large proportion of sudoriferous glands in the body. The latter can be found in the axilla, groin, umbilicus and areola of the breast.

Eccrine glands produce non-odoniferous sweat. Eccrine glands are involved in temperature regulation. The composition of sweat secreted by eccrine glands is controlled by aldosterone. Aldosterone enhances reabsorption of sodium ions by eccrine glands thereby making sweat dilute. This action is an important adaptive response to prolonged heat. Eccrine glands are innervated by postganglionic sympathetic cholinergic nerve fibres.

Apocrine glands produce odoniferous sweat (although, apocrine secretion is initially odourless but upon bacterial decomposition, it acquires characteristic odour). Apocrine glands open into the hair follicle. Sweat production by apocrine glands begins at puberty. Secretions from apocrine glands occur in response to emotions and stress and it is less involved in temperature regulation. Therefore, apocrine glands are responsible for emotional sweating. Apocrine sweat contains water, carbohydrate, lipids, pyruvate, lactate, sodium chloride, waste products among others. The pH of apocrine sweat is 6-7.5. Pubertal males have more apocrine glands than female. Apocrine Apocrine secretions contain pheromones (physiological substances secreted by the body which affect the functions of other human beings). Postganglionic sympathetic adrenergic nerve fibres innervate the apocrine glands.

Sweat Sweat is a fluid secreted majorly by eccrine glands. It is a part of extracellular fluid. The

sweat is hypotonic to plasma. 0.2 to 1% of sweat is occupied by solutes while the remainder is water. The solutes of sweat are organic substances like urea, lactic acid, creatinine and uric acid

and inorganic substances like sodium, potassium, chloride, calcium, magnesium, chromium, selenium among others. Sweat also has respiratory gases. Sweat may contain alcohol, mercury and bye products of drugs depending on the level of these substances in the body. The pH of sweat varies from 4 to 7. Water loss through sweat constitutes a part of sensible water loss. Sweating means production of sweat and it is also synonymous with perspiration or hidrosis.

Sweat rate signifies sweat production per unit time. Although the level acclimatization to heat determines sweat rate, an average sweat rate is 50ml/min. Sweat helps us to excrete metabolic wastes. An unacclimatized may lose over 300mmol/liter of solutes. Acclimatization to heat involves production of profuse but dilute sweat under the influence of aldosterone. Sweat helps in regulating body temperature. For example, vaporization of 1g of sweat produces 0.6kcal of heat energy. Water loss through evaporation or vapourization is part of insensible water loss. Acclimatization to heat involves production of profuse sweat (less concentrated sweat). Increase in sweat production is termed hyperhidrosis and decrease is called hypohidrosis. Exercise may increase perspiration.

HAIR FOLLICLE AND ARRECTOR PILLIThe part of hair that extends above the epidermis is called shaft, while the underneath is root. It is not all body regions that have hair. Non-hairy skin is called glabrous skin (as in palm, sole, glans, etc) while the hairy skin is called non-glabrous. The hair plays little role in thermoregulation in human beings and is part of what is known as vestigial reflex. Arrector pilli are smooth muscles that connect the hair follicle with the skin. When arrector pilli contract, the hair become erect. This action is important in cold and stress. Contraction of arrector pilli also brings about formation of goose pimples.

SEBACEOUS GLANDIt is an ovoid gland that produces sebum, an oily substance that repels water and keeps the skin moist. It is present in the skin of the body except palm and sole. Montgomery’s gland is a sebaceous gland present in the mammary gland that keeps the nipple moist.

NAILNails are protective structures that protect the skin. It is made up of keratinized cells. BODY TEMPERATURE AND THERMOREGULATION

Under cell biology unit, the importance of thermoregulation was elucidated. Claude Bernard views the entire body as being exposed to external environment. Any change in conditions of external environment will cause changes in internal environment and these will trigger homeostatic processes. For instance, the ambient temperature varies in a day and this triggers thermoreflex bringing body temperature under control. Based on the ability to control ambient surrounding, we can divide living organism into poikilotherms (organisms whose temperatures vary with the temperature of the surrounding) and homeotherms (organisms that can control their temperature). The term body temperature reflects body heat, a culmination of metabolic processes and ambient heat.

Body temperature can be measured using clinical thermometer. Body temperature varies from one region of the body to another. Core temperature represents temperature of viscera and it is fairly stable. Rectal or vaginal temperature is used as index of core body temperature. The

peripheral temperature varies with the environment. For example skin temperature depends on whether the weather is hot or cold, oral temperature is affected by the temperature of ingesta and so on. Despite this, the average body temperature is 36.70C-37.50 C.

Figure 2: Relationship between head temperature and calorie per second (adapted from textbook of medical physiology)

Body temperature is affected by:-Sex: The body temperature of pubertal male is higher than that of pubertal female.-Circadian rhythm: Temperature is lower in the morning than afternoon-State of activity: When metabolism increases, heat production increases and body temperature rises. Muscular activities like shivering and exercise increase body temperature. Heat produced due to these processes is termed mechanical thermogenesis. -Reproductive stage: In women, temperature increases by 0.5-10c during postovulatory phase. This rise is due to progesterone. Pregnancy and lactation increase body temperature. Menopause is associated with an increase in temperature (hot flash).-Ingestion of food: Body temperature increases postprandial because of increase in metabolic rate. Heat production as a result of food ingestion is called specific dynamic action (SDA) or diet induced thermogenesis. -Body surface area: The higher the body surface area, the more the metabolic rate and the greater the temperature. In neonates, the temperature regulatory mechanism is not well developed and therefore their temperatures vary with the ambient temperature. -Hyperthyroidism, fever and other disease conditions affect body temperature

HYPOTHALAMUS AND BODY TEMPERATUREHypothalamus plays important role in regulation of body temperature. The anterior

hypothalamus contains heat sensitive area (preoptic nuclei). This area is stimulated by heat. Posterior hypothalamus, apart from controlling shivering, has connection with sympathetic chains of the spinal cord. Usually, when preoptic nuclei are stimulated, posterior hypothalamus become inhibited and vice versa.

The preoptic nuclei monitor the temperature of blood flowing through them. The temperature that the nuclei are used to is called temperature set point. Temperature set point is the standard temperature of the body and it is 37.10c. Although, increase in peripheral and core

temperatures excite the preoptic nuclei and result in stimulation of heat loss processes in body temperature, preoptic nuclei are not compromised. In fever, proinflammatory cytokines affect the nuclei and this makes them insensitive to heat (standard temperature is reset) resulting in cold sensation (chill) and later on crisis.

Figure 3: Thermoregulatory mechanism

THERMOREGULATORY MECHANISMThermoregulation occurs through a reflex. The reflex consists of thermoreceptors, afferent (or sensory) nerve fibres, control centers and skeletal, cardiac and smooth muscles.

Affarent nerve fibres conduct information mediating heat in form of action potential to the preoptic nuclei of hypothalamus (the heat loss centre) from heat receptors in skins and peripheral and deep tissues of the body. This results in inhibition of posterior hypothalamus (check figure 3). Inhibition of posterior hypothalamus results in -Decrease in muscular activities- behavioural changes like ingestion of cold food and water, staying in cold or perhaps avoidance of hot food and -Cutaneous vasodilation.Due to increase in blood flow, perspiration increases and cooling effect is achieved. As mentioned earlier, vaporization of 1g of water produces 0.6kcal of heat energy. The heat loss through this process is called evaporation.

Cold brings about stimulation of posterior hypothalamus (heat gain centre) and results in; Heat generation processes like shivering, increase in voluntary muscular activity, increase in appetite, behavioural changes like keeping warm and Heat -loss- prevention process like vasoconstriction. Vasoconstriction reduces blood flow to the cutaneous arterioles and this decreases sweat formation and excretionHEAT GAIN PROCESSESThe body gains heat through:

-Radiation: Heat energy can be transferred to the body from distant sources.-Conduction: Heat transfer between a warmer solid and the body in contact with each other. For example you can gain heat when you sit on a warmer chair.-Food ingestion: Due to increase in metabolism, heat is produced -Chemical thermogenesis: Production of heat by brown adipose tissue (baby fat) is called chemical thermogenesis. Thyroid hormones and catecholamine elicit an increase in metabolic rate.-Mechanical thermogenesis: As earlier mentioned, shivering, exercise and voluntary muscular activities HEAT LOSS PROCESSESThe body loses heat through:-Radiation: It accounts for 60% of heat loss. Heat is transferred from the body to other objects far off. -Evaporative cooling: Heat energy is lost in sweat producing a cooling effect. When 1g of water is vaporized, 0.6kcalorie of heat energy is lost. Evaporative cooling accounts for 30% of heat loss in the body. -Convection: This is heat loss from the body to the circulating fluid which can be air or water. For example, after getting exposed to heat, you stay under fan or take cold bath so that you can dissipate the heat. It accounts for 7% of heat loss-Conduction: Heat loss from the body to a colder solid in close contact with each other. For example, you can lose heat to that chair you sit on. This form of loss takes 3%