Blood Flow To Skin From Body Core Biology Essay

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The insulator system of the body is important in maintaining normal body core temperature. The body is insulated by the skin, subcutaneous tissue and subcutaneous fat. (Rutherford, 2010)

When heated, blood from the core doesn't reach the periphery of the body, the subcutaneous fat acts as a conductor of heat. Fat insulation is equal to three quarters of a male body in a suit of clothing. Females generally contain more fat, therefore have better insulation. The insulation also allows for temperature changes according to the environment. (Hall & John, 2011) 

BLOOD FLOW TO SKIN FROM BODY CORE:

The body's core temperature is naturally higher and transfers heat to the rest of the body. This transfer of heat takes place due to an extensive venous plexus, arteriovenous anastomosis and capillaries located in the skin. The venous plexus is located in the subcutaneous tissue and receives an inflow of blood from the capillaries in the skin. Blood from the hands, feet and ears reaches the plexus via the arteriovenous anastomosis.

The rate of the blood flow determines the increase or decrease of conduction from the body core. The higher the rate of blood flow, the more heat is transferred. The lower the blood flow rate, the less heat is transferred. This rate varies in between a little above zero to thirty percent of total cardiac output. This rate is also affected by the vasoconstriction and vasodilation of the arterioles and the arteriovenous anastomosis. The system that regulates these changes is the autonomic nervous system. The rates are regulated in response to changes in core temperature or temperature changes in the environment. (Hall & John, 2011) 

Role of Hypothalamus in Thermoregulation

The Hypothalamus is a highly complex cone shaped structure that weighs about 4gm and is roughly the size of an almond that is located just below the thalamus, and just above the brain stem. (Saper, 2010)

The body maintains the core temperature at an optimal temperature of 37ï‚° C by physiological adjustments by the hypothalamus.

The hypothalamus works together with autonomic and higher nervous thermoregulatory centers to maintain the optimal core body temperature, the thermoregulatory responses are involuntary, mediated by the autonomic nervous system, some are neurohormonal and others are voluntary behavioural responses.

RESPONSE TO COLD:

When the body is exposed to cold (wind, ice, too little clothing), body temperature drops which will stimulate the skins cold receptors, mucous membrane receptors and the blood flowing into the skin will drop in temperature, the information will be sent to the hypothalamus and the higher cortical centers.

This initiates responses that promote heat gain and inhibits centers that promote heat loss. The activation of Sympathetic Centers results in several responses including:

Brown fat which is found in infants and some animals, oxidation increases causing thermogenesis.

Piloerection occurs which traps air close to skin to retain heat.

A Shivering Centre in the hypothalamus is also activated which activates the Brainstem Motor Centres to initiate involuntary contraction of skeletal muscles causing shivering, which generates heat.

Cold also activates some compensatory behavioural responses including huddling, voluntary physical activity (hand rubbing, pacing), sheltering next to a heat source and wearing warm clothing.

Norepinephrine (NE) release from sympathetic fibres which causes vasoconstriction.

Epinephrine (E) secretion from adrenal medulla increases thermogenesis by increasing cellular metabolism.

The hypothalamus releases Thyrotropin Releasing Hormone which activates the anterior pituitary gland to release Thyroid Stimulating Hormone (TSH). TSH induces the thyroid gland to produce the thyroid hormone (T3 and T4) into the blood. Thyroid hormone increases metabolic rate, which increases the amount of body heat production.

As the body gets warmer, the hypothalamic sensors detect the warmth and diminish the heat producing and heat loss prevention responses. (Dabrowski, 2008)

RESPONSE TO HEAT:

When the body is exposed to heat (sun, fire, too much clothing), body temperature rises which will stimulate skin warmth receptors and blood temperature increases this information will be sent to the hypothalamus, which results in several responses including:

The hypothalamus inhibits the adrenergic activity of the sympathetic nervous system causing cutaneous vasodilatation and reducing BMR. This causes an increase in heat loss via the skin and a decrease in heat production in the core.

The cholinergic sympathetic fibres, which innervate sweat glands release Ach stimulating sweat production.

Behavioural responses to heat, such as lethargy, resting in shaded areas or lying down with limbs spread out, decreases heat production and increases heat loss.

Wearing loose and lightly coloured clothing, fanning and consuming cold liquids also help with heat loss. (Dabrowski, 2008)

Role of cutaneous vasodilation & vasoconstriction in thermoregulation

All mammals are homeothermic, meaning that despite their external environment they are able to maintain a constant internal temperature. (The free dictionary, ND)

The human body contains its own temperature regulating system similar to that of a thermostat and can be located in an area of the brain called the hypothalamus. Information regarding core temperature as well as surface temperature is sent to the anterior hypothalamus to be processed. (Lloyd, ND)

Vasoconstriction is the physiological process whereby peripheral resistance increases. This occurs when the walls of the smooth muscle layer in the blood vessels constrict, resulting in decreased blood flow to a specific area. (HealthScout, 2009)

Vasodilation is the opposite of this process and refers to the widening of blood vessels to increase blood supply to an area. Both of these temperature regulating mechanisms in conjunction with other mechanisms play a vital role in maintaining homeostasis of the body. (MedicineNet, 2011)

During vigorous physical activities such as exercise, the core temperature of the body increases causing cutaneous blood vessels such arteries and arterioles to dilate. The dilation of these superficial blood vessels allow for an increase of heat transfer through convection so that core temperature of the body can be transferred to the peripheral surfaces of the skin. Heat then radiates from the skin to the external environment causing body temperature to decrease until normal temperature resumes. Although increased cardiac output is required to carry out this process, oxygen supply is not compromised.

A set point is a term used to describe the target value of a variable that the body tries to maintain. When the body's set point temperature increases it leads to an increase in core temperature resulting in a common condition known as fever. Unlike fever hyperthermia is caused by an increase in body temperature due to external factors and not because of a change in the control centers of the brain, this condition arises when heat conservation exceeds heat loss. Environmental conditions such as extreme hot temperatures, hypothalamic lesions and drug abuse are a few of many examples that are responsible for the occurrence of this condition. (eNotes, ND)

How heat is lost from the skin surfaces

Hypothalamic thermoregulatory center receives and regulates temperatures that occur at the core and body surface. This center will then adjust the temperatures so as to dissipate the heat. This will involve an increase in cardiac output and redistribution of blood from the organs not involved like the GIT, to the muscle and skin that is more active. As evaporation increases, sweat glands become more active. There are four routes that are responsible for heat exchange or heat loss, mainly conduction, convection, evaporation and radiation. (Brookes & Fahey, 1984)

CONDUCTION

This is the transfer of heat from the warm surface of the skin to a cooler, solid object if there is direct contact between them. If the cooler object is a better conductor of heat then there will be a greater heat loss from the skin surface. The best way to prevent heat loss in this instance is to take precautions such as wearing thick wool socks, hand gloves, using warm packets and also closed cell foam pads or cushions for the body. These will provide an excellent barrier to prevent heat loss through conduction. (Brookes & Fahey, 1984)

RADIATION

At rest, it is a primary method for giving off the body's excess heat. This heat is given off as infrared rays, as all heat is lost in the form of waves. Since these rays cannot travel through thick material and thus this radiation generated by the body can be trapped by clothing so that it is not lost out to the environment. Radiation heat loss or gain is dependent upon temperature gradient between the skin and the environment. (Brookes & Fahey, 1984)

EVAPORATION

Evaporation occurs when a liquid, sweat in this case, changes to vapour. This is the most vital heat dissipation mechanism in warmer environments even though it accounts for a lesser percentage for body heat loss than when the temperatures are higher than 20 degrees Celsius. It is very important in dissipating heat during exercise. Rate of evaporation is determined by air velocity and the water vapour pressure gradient between the skin and the environment. When the temperature of the surroundings becomes greater than that of the skin, heat is lost by evaporation. (Brookes & Fahey, 1984) (Hall & John, 2011) (Jordan, 2003)

CONVECTION

It transfers heat from the muscles and skin surface. This is all dependent on the difference in temperature between the skin and the environment and the amount of heat transfer coefficient that is different with available body surface area and wind velocity. And the best way to minimize convective heat loss is to wear windproof clothing. (Brookes & Fahey, 1984)

EFFECT OF CLOTHING ON HEAT ABSORPTION & LOSS

During cold weather conditions layers of clothing help to prevent heat loss while windproof garments prevent heat transfer through convection. Dark colored clothing absorbs more heat on sunny days and light colored clothing reflects heat.

During warm conditions thin layered garments should be worn to decrease the barrier between the skin and the external environment so that heat loss through conduction takes place more effectively. (Dawson & Pyke 1988)

Sweating & Shivering

SWEATING

When the body temperature increases, the central and peripheral thermoreceptors pick up changes, they send the information to the hypothalamus and the cerebral cortex. When the body heat is above normal, the hypothalamus stimulates the sweat glands to secrete sweat to moisten the skin. Heat loss at an increased rate of 10 times more is obtained when this sweat evaporates. An increase in reliability on evaporation also increases sweat production. Sweat production is also increased by secreting aldosterone. (King, 2004) (Hall & John, 2011)

The stimulated hypothalamus conveys the impulse via the sympathetic nerve fibers throughout the body. The sweat glands are tubular in structure and they extend to the dermis, epidermis and open in to the surface of the skin. These glands have secretory portions. During light sweating, sweat moves slowly through the tubule giving more time for sodium and chlorine to be reabsorbed. Therefore the sweat contains less sodium and chlorine ions. However in heavy sweating, the filtrate moves fast and sweat reaches the surface of the skin with more of these ions. (King, 2004)

SHIVERING

Shivering is an involuntary muscle action which causes heat production. (eHow, 1999)

The Primary motor center for shivering is located in the posterior part of the hypothalamus. It is usually inhibited by the heat center in the anterior hypothalamus. (Hall & John, 2011) 

When signals are received from heat receptors, this center activates and transmits signals through to the anterior motor neurons in the spinal cord. It usually causes contractions of both, the flexor and extensor muscles. These signals are non-rhythmical and cause a gradual increase of the tone of the muscle rather than actual shaking. The purpose of shivering is to produce heat and this is achieved since there is an increase in energy consumption which causes more heat to be produced. Shivering also produces friction which generates heat. (Marjoniemi, 2001) ( Hall & John, 2011)

Hormones involved in thermoregulation

THYROXINE

The hypothalamus controls the thermoregulatory center of the body. Any deviances cause the hypothalamus to react. The endocrine reaction of the hypothalamus is to stimulate the release of TRH (Thyrotropin releasing hormone) which in turn causes the release of TSH (Thyroid stimulating hormone) from the Anterior Pituitary Gland. (King, 2004)

The effects of thyroid hormones include:

Increased rates of oxygen & energy consumption; Rise of body temperature, in children.

Increased heart rate and force of contraction which causes blood pressure to increase

(Martini & Nath, 2009)

Thyroxine increases the rate of carbohyradte metabolism resulting in heat production as a by-product. As your BMR and cellular output increases, so does your temperature. This mechanism however, is seen as a long-term temperature control mechanism as endocrine and hormone related mechanisms are not effective immediately and usually last longer. (Hall & John, 2011)

OTHER HORMONES

Other hormones involved include:

Seratonin

When increased, increases body temperature. (Psych Central Staff, 2012)

Norepinephrine

When increased, increases body temperature by causing vasoconstriction (Dohrman, ND)

Epinephrine

When increased, increases body temperature by increasing cellular metabolism (Dabrowski, 2008)

Abnormalities of temperature control

Heat related injuries may be caused by too high or low body temperatures. These include the following:

HEAT RASH

This is also known as the prickly heat rash. The person gets a red rash with prickling and stinging sensation during sweating. This develops on covered parts of the body. It may be prevented by drying yourself continuously with a towel. (King, 2004)

HEAT SYNCOPE

This is the heat collapse associated with physical fatigue due to overexposure to heat. Peripheral vasodilation occurs on the superficial blood vessels on the skin, resulting in hypotension (pulling of the blood vessels to the extremities.).This causes dizziness, fainting and nausea. It can be prevented by lying on a cool surface or moving to a cooler area. (King, 2004)

HEAT STROKE

This disorder is characterized by a rapid pulse and increased respiration, increased body temperature above 104áµ’ F/ 40áµ’ C, cessation of sweating, hypertension, confusion and eventually unconsciousness. This results from complete collapse of thermoregulatory mechanisms. (King, 2004)

HYPERTHERMIA

This is due to abnormally high internal temperatures.

104 -105áµ’ F/ 40áµ’ C: The person gets cold sensations over the stomach and back with piloerection, commonly known as goose bumps.

105-106áµ’ F/ 41áµ’ C: Muscles become weak, disorientation and postural equilibrium is lost.

106-107áµ’ F/ 42áµ’ C: The sweat gets diminished and unconsciousness occurs.

When it is above 108áµ’ F, death results. (King, 2004)

HYPONATREMIA

This is defined as abnormally low blood-Sodium levels. This may be caused by excessive ingestion of water resulting in a lower concentration of sodium. The person has a high sweat excretion rate. This condition compromises the central nervous system. (King. J, 2004)

Table of Figures

Figure

Caption

Source

Figure 1.1

Arterial & Venous plexus located in skin and subcutaneous tissues

Graepel, S (10 April 2012). Outdoor Gear and Thermodynamics: How to layer for optimal comfort… Even on Everest. Available at:

http://adventureblog.nationalgeographic.com/2012/04/10/outdoor-gear-and-thermodynamics-how-to-layer-for-optimal-comfort-even-on-everest/ .Last Accessed 2 Aug 2012

Figure 2.1

Location of Hypothalamus

Curnow, B. (12 July 2010). What is the hypothalamus. Available: http://www.news-medical.net/health/What-is-the-Hypothalamus.aspx . Last accessed 3 Aug 2012.

Figure 2.2

Hypothalamic Response to cold

Dabrowski, D. (2008). NEUROPHYSIOLOGY OF THERMOREGULATION. Available: http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html . Last accessed 03 Aug 2012

Figure 2.3

Hypothalamic Response to heat

Dabrowski, D. (2008). NEUROPHYSIOLOGY OF THERMOREGULATION. Available: http://dwb.unl.edu/teacher/nsf/c01/c01links/www.science.mcmaster.ca/biology/4s03/thermoregulation.html . Last accessed 03 Aug 2012

Figure 4.1

Heat transfer processes

Emergency Essentials. (2010). Emergency Warmth. Available: http://preparednesspantry.blogspot.com/2010/02/emergency-warmth.html . Last accessed 31 Jul 2012.

Figure 4.2

Evaporation during heat loss

Lisa Mitchell. (2012). 5 reasons to practice Hot Yoga. Available: http://www.mindbodygreen.com/0-5985/5-Reasons-to-Practice-Hot-Yoga.html . Last accessed 02 Sep 2012.

Figure 4.3

Heat Transfer

Zubeita, P. Chapter 21: Thermo-Regulation, Temperature and Radiation. Available: http://www.zuniv.net/physiology/book/chapter21.html . Last accessed 05 Sep 2012

Figure 5.1

Sweating & Shivering control mechanisms

Duobaorulai. (2011). Thermoregulation. Available: http://bio119homeostasis.blogspot.com/2011/03/energy-and-homeostasis.html . Last accessed 25 Aug 2012

Figure 6.1

TRH-TSH Feedback loop

Zoe. (2006). TRH-TSH-Thyroid Feedback Loop. Available: http://iodine4health.com/body/thyroid/trh_tsh_th_loop_diagram.htm . Last accessed 2 Aug 2012.

Figure 7.1

Heat rash

Ratini, M. (2012). Understanding heat rash -- the basics. Available: http://www.webmd.com/skin-problems-and-treatments/understanding-heat-rash-basics . Last accessed 2 Sept 2012.

Figure 7.2

Circulatory Response to heat

Rowel L.B. Human circulation. Regulation during physical stress. OUP, 1986

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