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Chapter 6 Biology Notes Cont. (The Endocrine System (The Endocrine System…

- - - - Heat can be lost from the human body to the external environment through the process of radiation, conduction and convection when the body temperature is higher than the external environment. Evaporation is another process by which the body can lose a major source of heat.
        
        Conduction
        Involves heat transfer immediate physical contact with another object at a different temperature. The direction of heart transfer is from the warmer object to the cooler object. Heat loss by conduction is typically small and is limited to parts of the body that come into immediate contact with external objects that are good conductors of heat.
        
        Evaporation
        The conduction of liquid water to vapour, a process that requires an input of heat energy. As a result, evaporation is a source of heat loss from the body. The source of the water that evaporates maybe external, as for example, with the evaporation of water droplets from the skin of a person who has been swimming, or evaporation from wet clothing that a person is wearing. Both of these can be significant sources of heat loss from the body.
        
        In addition, the water that evaporates may come from internal sources:
        
        the insensible water that is lost as vapour from the body via the skin pores
        
        From the internal moist surface of the lungs and airways, and from the moist membranes of the mouth and nose.
        
        the sweat from the sweat glands of the skin.
        
        Convection
        The process of heat transfer resulting from the mas movement of air (or water) past exposed areas of the body when each is at a different temperature. The greater the rate of movement of air or water past the body, the greater the rate of transfer of heat.
        
        Sweating
        Sweating comes into operation only when a person is exposed to external temperatures of about 37 degrees or higher, or when a person retains excessive internal heat generated through metabolism, such as may occur during strenuous exercise or manual labour. The evaporation of water is most effective when the air is dry and if there is a wind to assist cooling. As the surrounding air becomes more humid and as the air movement slows, evaporation is less effective as a cooling mechanism because the humid air cannot remove as much water as dry air.
        
        Radiation
        
        Requires no physical contact between object for transfer of heat.
        
        Heat is transferred by radiation from a warmer object to a cooler object.
        
        On a hot day, when you are standing in the sun, your body will gain heat by radiation.
        
        On a cold day, you radiate body heat to the environment and so lose heat.
    - - Physiological processes are not under a person's conscious control but occur automatically. These processes are in the hypothalamus of the brain. For example, the hypothalamus has a centre with a temperature set point against which changes in body temperature are monitored. In a normal healthy person, this set point is about 37 degrees.
        
        Heat produced by metabolism
        Basal Metabolic rate is the minimum amount of heat generated internally.
        The level of metabolism needed to maintain the living state in a person at rest, fasting and in a thermo-neutral environment. The increase in metabolic rate is initiated by a centre in the hypothalamus. It releases a hormone that causes the release of other hormones first from the pituitary gland then by the thyroid gland. Motor neurons from the hypothalamus also cause the adrenal glands to secrete the hormones adrenaline and noradrenaline. These hormones increase the basal metabolic rate, particularly in the skeletal muscles also in brown fat, a special kind of fat of particular importance in young babies. Brown fat is also present in adults, but in very small amounts in the neck and the upper chest. Because of the brown fat, babies can produce five times as much heat fro metabolic pathways.
        
        Varying blood flow to the skin surface
        When cold is detected by cold receptors in the skin, the hypothalamus sends nerve impulses that cause the vasoconstriction of arterioles that lead to capillary beds close to the skin. In hot conditions, the hypothalamus sends nerve impulses that cause vasodilation. The contraction or the relaxation of this muscle band determines whether or not blood will flow to the capillaries just below the skin surface. Piloerection means 'hair standing on end'. Although it is not important for the conservation of heat in humans, the ability to rise hair is important for mammals that have a covering of hair over most of their body surface. A layer of air becomes trapped in the erect hair above the skin and acts as a covering of hair above the skin and acts as an insulation layer between the skin of the animal and the external environment. Nerve impulses from the hypothalamus travel to a muscle at the base of each hair, causing the muscle to contract and the hair to rise. Fluffed up feathers play a similar role for a bird.
        
        Shivering
        Shivering is the alternate contraction and relaxation of small muscle groups and is an involuntary action. The hypothalamus contains a centre that controls shivering. This centre activates nerves that control the muscles in the upper limbs and body trunk. When the muscles shiver, almost all the energy of contraction is converted into heat energy.
        
        Human Skin
        Various features of the skin that are involved in homeostatic control of the body temperature:
        
        Erector pili muscles that contract in response to nerve stimuli fro the hypothalamus and raise the hair.
        
        Hot and cold temperature receptors that register skin temperature and transmit that information via nerves to the hypothalamus.
        
        Sweat glands that produce sweat for evaporative cooling by the hypothalamus which can either constrict of relax a band of muscle.
        
        Capillaries that supply blood to the skin with their blood flow controlled around the arterioles that supply these capillaries.