Anatomy and physiology

Lecturer:
Dr-Abdirizak Yusuf Olow
MBBS
Functions of the skin
 Functions of the skin
• 1. Protection
• The skin forms a relatively waterproof layer,
provided mainly by its keratinized epithelium, which
protects the deeper, most delicate structures.
• As an important nonspecific defense mechanism it
acts as a barrier against:
– invasion by micro-organisms
– chemicals
– physical agents, e.g. mild trauma, ultraviolet light
– dehydration.
 Cont
• The epidermis contains specialized immune
cells called dendritic (Langerhans) cells, which
are a type of macrophage.
• They phagocytose intruding antigens and
travel to lymphoid tissue, where they present
antigen to T-lymphocytes, thus stimulating an
immune response.
 Cont
• Abundant sensory nerve endings in the dermis
enable perception, discrimination and location
of internal and external stimuli.
• This allows responses to changes in the
environment, e.g. by reflex action
(withdrawal) to unpleasant or painful stimuli,
protecting it from further injury.
• The pigment melanin protects against harmful
ultraviolet rays in sunlight.
 Cont
• 2. Regulation of body temperature
• Body temperature remains fairly constant
around 36.8°C across a wide range of
environmental temperatures ensuring that the
optimal range for enzyme activity required for
metabolism is maintained. In health, variations
are usually limited to between 0.5 and 0.75°C,
although it rises slightly in the evening, during
exercise and in women just after ovulation.
 Cont
• Heat production
• When metabolic rate increases, body
temperature rises, and when it decreases
body temperature falls.
• Some of the energy released during metabolic
activity is in the form of heat; the most active
organs produce most heat.
 Cont
• The principal organs involved are:
• Skeletal muscles – contraction of skeletal
muscles produces a large amount of heat and
the more strenuous the muscular exercise, the
greater the heat produced. Shivering also
involves skeletal muscle contraction, which
increases heat production when there is a risk
of body temperature falling below normal.
 Cont
• The liver is very metabolically active, which
produces heat as a by-product. Metabolic rate
and heat production are increased after
eating.
• The digestive organs that generate heat
during peristalsis and the chemical reactions
involved in digestion.
 Cont
• Heat loss
• Most heat loss from the body occurs through the
skin. Small amounts are lost in expired air, urine and
faeces.
• Only heat loss through the skin can be regulated;
heat lost by the other routes cannot be controlled.
• Heat loss through the skin is affected by the
difference between body and environmental
temperatures, the amount of the body surface
exposed and the type of clothes worn.
 Cont
• Control of body temperature
• The temperature regulating centre in the
hypothalamus is sensitive to the temperature
of circulating blood. This centre responds to
decreasing temperature by sending nerve
impulses to:
– Arterioles in the dermis, which constrict
decreasing blood flow to the skin
– Skeletal muscles stimulating shivering.
 Cont
• Activity of the sweat glands. When body
temperature is increased by 0.25 to 0.5°C the
sweat glands secrete sweat onto the skin surface.
• Evaporation of sweat cools the body.
• Loss of heat from the body by evaporation of water
through the skin and expired air still occurs even
when the environmental temperature is low. This is
called insensible water loss (around 500 mL per
day) and is accompanied by insensible heat loss.
 Cont
• Regulation of blood flow through the skin.
• The amount of heat lost from the skin
depends largely on blood flow through dermal
capillaries.
• As body temperature rises, the arterioles
dilate and more blood enters the capillary
network in the skin. The skin is warm and pink
in colour.
 Cont
• If the environmental temperature is low or if
heat production is decreased, the arterioles in
the dermis are constricted.
• This reduces blood flow to the body surface,
conserving heat. The skin appears paler and
feels cool.
• Fever
• This is often the result of infection and is
caused by release of chemicals (pyrogens)
from inflammatory cells and invading bacteria.
Pyrogens, e.g. interleukin 1 , act on the
hypothalamus, which releases prostaglandins
that reset the hypothalamic thermostat to a
higher temperature.
• The body responds by activating heat-
promoting mechanisms, e.g. shivering and
vasoconstriction, until the new higher
temperature is reached.
 Cont
• When the thermostat is reset to the normal
level, heat-loss mechanisms are activated.
There is profuse sweating and vasodilatation
accompanied by warm, pink skin until body
temperature falls to the normal range again.
 Cont
• 3. Formation of vitamin D
• 7-Dehydrocholesterol is a lipid-based
substance in the skin and is converted to
vitamin D by sunlight.
• This vitamin is used with calcium and
phosphate in the formation and maintenance
of bone.
 Cont
• 4. Cutaneous sensation
• Sensory receptors are nerve endings in the
dermis that are sensitive to touch, pressure,
temperature or pain. Stimulation generates
nerve impulses in sensory nerves that are
transmitted to the cerebral cortex.
• Some areas have more sensory receptors than
others causing them to be especially sensitive,
e.g. the lips and fingertips.
 Cont
• 5. Absorption
• This property is limited but substances that
can be absorbed include:
– Some drugs, in transdermal patches, e.g. hormone
replacement therapy during the menopause,
nicotine as an aid to smoking cessation
– Some toxic chemicals, e.g. mercury.
 Cont
• 6. Excretion
• The skin is a minor excretory organ for some
substances including:
– sodium chloride in sweat; excess sweating may
lead to low blood sodium levels (hyponatremia)
– urea, especially when kidney function is impaired
 Cont
• 7. Wound healing
• Conditions required for wound healing
• Systemic factors. These include good
nutritional status and general health.
Infection, impaired immunity, poor blood
supply and systemic conditions, e.g. diabetes
mellitus and cancer, reduce the rate of wound
healing.
 Cont
• Local factors. Local factors that facilitate
wound healing include a good blood supply to
provide oxygen and nutrients and remove
waste products, and freedom from
contamination by, e.g., microbes, foreign
bodies or toxic chemicals.
 Cont
• Primary healing (healing by first intention)
• This type of healing follows minimal destruction
of tissue when the damaged edges of a wound
are in close apposition, e.g. a surgical incision.
• There are several overlapping stages in the
repair process:
– Inflammation
– Proliferation
– Maturation
 Cont
• Secondary healing (healing by second intention)
• This type of healing follows extensive tissue
destruction or when the edges of a wound
cannot be brought into apposition, e.g. varicose
ulcers and pressure (decubitus) ulcers. The
stages of secondary healing are the same as in
primary healing (see above); healing time
depends on effective removal of the cause and
the size of the wound.
Thank u