Review & Illustrations
Completion requirements
[A] Temperature-decreasing mechanisms (figure):
Functions of skin:
- Protective function → bacteria, toxic substances, ultraviolet rays, mechanical blow
- Sensory function → largest sense organ in the body
- Storage function → fat, water, chloride, sugar, blood
- Synthetic function → Vitamin D3
- Regulation of body temperature → radiation, conduction, convection, evaporation, sweating
- Regulation of water and electrolyte balance → excreting water and salts through sweat
- Excretory function → excretes small quantities urea, salts and fatty substance.
- Absorptive function → absorbs fat-soluble substances and some ointments
- Secretory function → sweat through sweat glands and sebum through sebaceous glands
Glands of skin
1. Sebaceous glands → Sebum (contains free fatty acids, triglycerides, sterols, waxes, paraffin, free fatty acids of sebum have antibacterial and antifungal actions)
- Prevents the infection of skin by bacteria or fungi (due to free fatty acids of sebum)
- Keeps the skin smooth and oily, thus it protects the skin from unnecessary desquamation and injury caused by dryness (due to the lipid nature of sebum)
- Prevent heat loss from the body (due to the lipids of the sebum)
2. Sweat glands:
- Eccrine glands
- Apocrine glands
Body temperature
The core temperature of the body (temperature of the deep tissues of the body, i.e. the temperature of organs in the cranial, thoracic, and abdominal cavities) remains almost exactly constant, within ± 0.6oC, day in and day out (the normal range of human temperature by mouth is between 36.6 to 37.2°C. The core temperature is always more than oral or rectal temperature. It is about 37.8°C ). The surface temperature, in contrast to the core temperature, rises and falls with the temperature of the surrounding.
- Febrile temperature (pyrexia) is above 37.2°C.
- Hyperpyrexia is above 41.6°C.
- Subnormal temperature is blow 36.6°C.
- Hypothermia is below 35°C.
- Age
- Sex
- Diurnal variation
- After meals
- Menstrual cycle
- Sleep
- Emotion
- Exercise
Sources of body heat:
- Metabolic Activities → Metabolism of carbohydrates, proteins, and fats.
- Muscular Activity → During muscular activity (80% of heat of activity is produced by skeletal
muscles.
- Role of Hormones → adrenaline, sympathetic stimulation, Thyroxin
- Hot food and drinks and radiation of Heat from the Environment
- Shivering
- Brown Fat Tissue
Heat is lost from the body by various methods and these are:
- Evaporation → 60%
- Conduction → 3%
- Convection → 15%
- Radiation → 22%
Flow of blood to the skin and heat transfer from the body core:
- Heat transfer can be controlled by venous
plexus (see figure). A
high rate of blood flow causes heat to be conducted from the core of the body
to the skin with great efficiency, whereas reduction in the rate of blood flow
decreases the efficiency of heat conduction from the core.
- Vasoconstriction → controlled almost entirely by the sympathetic
adrenergic nervous system in response to change in the body core temperature
and the environmental temperature.
- Vasodilatation is achieved → inhibition of the sympathetic adrenergic nervous system (and probably by active cutaneous vasodilation occurs via sympathetic cholinergic nerve co-transmission with potential roles for nitric oxide, vasoactive intestinal peptide, prostaglandins, and substance P (and/or neurokinin-1 receptors).
Innervation of sweat glands(see figure):
- Ecchrine sweat gands → All the body → supplied by muscarinic cholinergic nerve fibers + and have adrenergic receptors.
- Ecchrine sweat gands → forehead, axilla, palms, soles → supplied by muscarinic cholinergic nerve fibers + and have adrenergic nerve fibers.
- Mechanism of sweat secretion: [A] When the sweat glands are stimulated only slightly, the primary secretion passes through the duct very slowly and therefore, all the sodium and chloride ions are reabsorbed (this reduces the osmotic pressure of the fluid to such a low level that most of water is also reabsorbed, which concentrates most of the other constituents such as urea, lactic acid, and potassium ions), [B] when the sweat glands are strongly stimulated by the sympathetic nervous system, very large amounts of precursor secretion are formed, and the duct can now reabsorb only slightly more than one half of sodium chloride. Therefore, the concentrations of the sodium and chloride ions then usually rise to maximum levels. Furthermore, the water reabsorption is greatly reduced so that the other constituents of the sweat are only moderately increased in concentration.
Brain centers involved in temperature regulation:
Peripheral thermoreceptors + Central thermoreceptors → Hypothalamus thermostat → temperature-decreasing procedures OR temperature-increasing procedures.[A] Temperature-decreasing mechanisms (figure):
- Vasodilatation and decreased peripheral resistance
- Sweating
- Decrease in heat production
- Skin vasoconstriction
- Piloerection
- Increase in heat production by:
- shivering
- Sympathetic excitation → increase in the rate of cellular metabolism (this is called chemical thermogenesis).
- Thyroxin secretion
- Inhibiting the process of sweating
- Lipolysis of Brown
fat.
- Endogenous pyrogen
- Local release of prostaglandins, interleukins, some interferons, and tumor necrosis factor.
Hyperthermia: An elevation of a person’s body temperature above normally accepted range because of an imbalance between the rates of heat production and heat loss such as exercise, exposure to hot environment. It may lead to:
- Heat cramps
- Heat exhaustion
- Heatstroke
- Malignant hyperthermia