Integumentary

1. LECTURE NOTE
ON
PHYSIOLOGYOF THE INTEGUMENTARY
SYSTEM
BY
MJ ADENIYI MSC
(7jimade@gmail.com, +234 08066796517)
DEPARTMENT OF PHYSIOLOGY
UNIVERSITY OF BENIN, BENIN CITY, NIGERIA

2. 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.

3. -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.
MELANIN
It 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.
DERMIS
Dermis 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 layer
Beneath 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

4. 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 ENDINGS
Cutaneous 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
APPENDAGES
SWEAT 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

5. 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 PILLI
The 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 GLAND
It 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.
NAIL
Nails 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

6. 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 TEMPERATURE
Hypothalamus 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

7. 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 MECHANISM
Thermoregulation 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;

8. 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 excretion
HEAT GAIN PROCESSES
The 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 PROCESSES
The 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%