VETS8014 ASSIGNMENT

Faculty of Veterinary Science - University of Sydney Report/Assignment Cover Sheet

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NAME and STUDENT ID: Abby Main 460163657

UNIT NAME AND CODE: VETS8014

ASSIGNMENT TOPIC: Thermoregulation in domestic animals

SUPERVISOR OF ASSESSMENT TASK: Peter White

WORD COUNT: 2445 words (excluding references)

DISCLOSURE STATEMENT: the following IMAGES or OBJECTS were used to develop this DIAGRAM (eg tree – from clipart)

Website Header:

All Paws, (2016). Dog header. [image] Available at: https://www.allpaws.com/ [Accessed 29 Apr. 2016].

Apart from the acknowledgements above, I hereby certify that this assignment is your own work; based on your study and/or research, and that we have acknowledged all material and sources used in the preparation of this assignment. I also certify that no part of the assignment has been previously submitted for assessment and that I have not copied in part or whole or otherwise plagiarised the work of others. I give my permission for the teaching staff in the Faculty of Veterinary Science to use my assignment as a learning tool for other students within the veterinary degree with the due credit given to us. (if you do not give permission, please cross out this statement before signing the document)

Signature: Abby Main

As the assessment had a degree of flexib i l i ty I dec ided to make my ass ignment in the format of a webs ite .

P lease fo l low the l ink below to v is i t the webs ite . The wr i t ten ass ignment is be low and is attached on the webs ite

www.thermoregulat ionindomest ican imals.weebly.com

THERMOREGULATION IN DOMESTIC ANIMALS

IntroductionThe function of the body relies on the maintenance of a relatively stable internal environment despite an ever-changing external environment. Chemical processes within the body need to be performing at an optimal level to ensure the body is functioning efficiently. In order to maintain optimal chemical and physical functioning the body regulates its interconnected network of body components in a process called homeostasis (Sherwood, 2004). Homeostasis is a regulatory mechanism that is utilised by all levels of life (Akers and Denbow, 2013) as it is important for the functioning of each individual cell and its contribution as part of a body system (Sherwood et al., 2013). Neural and endocrine control systems work efficiently in order to stabilise the internal environment and maintain homeostasis. Thermoregulation is one example of a homeostatic mechanism that allows organisms to maintain a body temperature within an optimal range and regulated set point (Campbell, 2011). In order to do so the central thermoreceptors, peripheral thermoreceptors, effectors and control centre work together to regulate negative feedback mechanisms in order to maintain body temperature (Sherwood et al., 2013). Although dogs share many common characteristics of thermoregulation seen in other species, they only have eccrine sweat glands in their foot pads and are limited in their ability to loose heat by evaporation due to their dense hair (Reece et al., 2015). Dogs have specific features to ensure they maintain ideal body temperatures such as panting and salvation.

HomeostasisHomeostasis is the maintenance of a relatively stable internal environment in the face of disturbances in the external environment (Sherwood et al., 2013). This process is under the control of the nervous and endocrine system and their interrelationship (Reece et al., 2015). The nervous system is linked to fast, short-term and immediate responses whilst the endocrine system is linked to long distance, long-term and slow responses (Reece et al., 2015). In order to stabilise the physiological functioning of the body, homeostatic control systems utilise feedback mechanisms that recognise a stimulus, identify any deviations and initiate the appropriate response (Reece et al., 2015, Sherwood, 2004). As seen in figure 1, this process requires

the communication and interrelationship between sensors, integrators and effectors. Sensor receptors recognise a stimulus which is a change in the environment (Reece et al., 2015). This relays information to the integrator control centre via the afferent pathway where it is compared to set points for that variable (Sherwood et al., 2013, Reece et al., 2015). When variations between the set point and stimulus are recognised the control centre sends a signal via the efferent pathway to the effector, which then induces the change in order to maintain homeostasis (Reece et al., 2015). Feedback mechanisms consist of negative feedback systems, positive feedback systems or feedforward systems. Negative feedback induces changes that counteract the stimulus recognised whereas positive feedback induces changes that amplify the stimulus recognised (Sherwood et al., 2013, Reece et al., 2015). A feedforward system acts by predicting a disturbance stimulus before it occurs (Sherwood et al., 2013). One example of homeostasis is thermoregulation, the control of body temperature.

ThermoregulationThermoregulation is under the control of the hypothalamus and is the regulation of body temperature within a set range. Maintaining optimal temperature is critical in ensuring the chemical and physical functioning of the body is efficient. Changes in body temperature can alter the biological functioning of the body as it can alter chemical reaction rates and denature proteins (Sherwood et al., 2013), therefore it is extremely important to maintain body temperature within a strict range. Poikilotherms, animals that depend on external heat sources, and homeotherms, animals that depends on internal heat sources differ in the way in which they thermoregulate (Reece et al., 2015). As domestic animals, such as dogs and cats, are homeotherms this will be discussed in further detail.

There is two types of thermoreceptors: central thermoreceptors and peripheral thermoreceptors which integrate to regulate body temperature. Central thermoreceptors are located in the hypothalamus and within the central nervous system and abdominal cavity of the body (Sherwood et al., 2013) and are important in sensing changes in core body temperature. Peripheral thermoreceptors are within the periphery of the body, including the skin that provides the thermoregulatory afferent signals for homeostasis (Schepers and Ringkamp, 2010). Peripheral thermoreceptors are important in sensing changes in skin temperature (Sherwood et al., 2013). Once detected the signals are directed, via the afferent pathway, to the hypothalamus (control centre).

The anterior portion of the hypothalamus initiates responses to increased body temperature by triggering mechanisms for heat loss such as sweating, vasodilation and panting (Campbell, 2011, Sherwood et al., 2013). The posterior portion of the hypothalamus is activated by decreased body temperature and as a result directs mechanisms to increase heat production, such as vasoconstriction and shivering (Campbell, 2011). The body has a thermal neutral zone (TNZ) that is a range of temperatures that an animal doesn’t require energy expenditure for thermoregulation (Sherwood et al., 2013). In order to maintain temperature within these critical, narrow limits the body has various compensatory mechanisms in order to counteract changes in metabolic heat production and environmental temperature (Sherwood, 2004).

The body uses four mechanisms of heat transfer: radiation, conduction, convection and evaporation. Heat always moves down a thermal gradient from a warmer to cooler area (Sherwood, 2004).

Radiation is the emission or absorbance of heat in an electromagnetic wave form (Sherwood, 2004) from a warmer object to a cooler object.

Conduction is the movement of heat energy between objects in direct contact (Sherwood et al., 2013), this is through molecular heat transfer (Sherwood, 2004).

Convection is the movement of heat energy through current of water and air (Sherwood et al., 2013) through the rising of less dense warm air molecules and the replacement of cool air molecules (Sherwood, 2004).

Evaporation is the used for cooling because when water evaporates the process of converting water from liquid to vapour requires energy that is directly absorbed from the skin, therefore losing heat from the body (Campbell, 2011).

Several homeostatic mechanisms are used to ensure the regulation of core body temperature. These can be categorised as responses to cold (gaining external heat, retaining internal heat and generating internal heat) and responses to heat (lose excess internal heat).

Response to heatThe body has various mechanisms in order to maintain homeostasis in the face of heat. Thermoregulation in response to heat exposure is under the control of the anterior hypothalamus (Sherwood, 2004) and responds by either decreasing the production of heat through decreases muscle tone or by increasing heat loss through vasodilation, evaporation by sweating, and panting and through counter-current exchange (Sherwood et al., 2013). Table 1 summarises the responses to heat.

VasodilationIn response to heat the hypothalamus receives signals from the peripheral skin receptors via the afferent pathway (Akers and Denbow, 2013). The anterior portion of the hypothalamus initiates responses to increased body temperature through the activation of the sympathetic nervous system (Campbell, 2011). The sympathetic nervous system decreases blood vessel tone through the relaxation of smooth muscle cells within vessel walls, therefore causing vasodilation (Reece et al., 2015). Vasodilation allows heated blood flow to increase through the skin, which accommodates for an increased heat loss (Sherwood, 2004). Heat from the circulatory system is lost to the environment via radiation, convection and conduction (Sherwood et al., 2013). This

mechanism is predominately under the control of the hypothalamic thermoregulatory centre, however control of arterioles is also under the control of the cardiovascular control centre in the medulla (Sherwood, 2004). As the hypothalamus thermoregulatory centre overrides the cardiovascular control centre, blood pressure and heart rate fall as a result of vasodilation (Sherwood, 2004). Another cardiovascular response to heat is that venous return from the skin takes places in more superficial veins in order to increase heat loss at the periphery (Reece et al., 2015).

EvaporationWhen water evaporates from the skin it requires heat to convert it from a liquid to a gaseous state (Sherwood, 2004). This heat is utilised and absorbed from the skin, therefore increasing heat loss and cooling the body (Sherwood, 2004). The three main mechanisms seen in animals for evaporative heat loss are sweating, panting and saliva. Sweating is under sympathetic nervous system control as a response to increased blood temperature (Reece et al., 2015) and is mediated by the post-ganglionic, cholinergic nerves that terminate onto glands (Kurz, 2008). Evaporation by sweating can be limited by humidity as high humidity has increased air saturation by water wherefore has a decreased capacity to take up additional water molecules (Sherwood et al., 2013). Evaporation through sweating is primarily used in animals with large surfaces areas of hairless skin and is less effective in some animals that have a limited distribution of sweat glands (Morrison and Nakamura, 2011). Panting is another evaporative mechanism to increase heat loss and provide a cooling effect (Sherwood et al., 2013). Panting allows for the evaporation of heat from the upper respiratory tract (Goldberg et al., 1981) through an increase in dead space ventilation (Bicego et al., 2007). Hyperventilation is prevented due to decreased tidal volume, increased respiratory frequency and no change in respiratory alveolar ventilation (Reece et al., 2015). Increases in water loss by evaporation decreases blood volume and the heart compensates by increasing heart rate (Morrison and Nakamura, 2011)

Counter-current exchangeAlthough the counter-current exchange mechanism is more utilised by the response to cold, it is also used to prevent overheating in various organs (Sherwood et al., 2013). As discussed further in the response to cold section, the counter-current exchange mechanism is the movement of heat between veins and arteries at the rete mirabile (Sherwood et al., 2013). This allows the veins (blood directed back to the heart) to move heat into the blood in the arteries directed towards the periphery, therefore moving heat away from the core.

Arrector pili muscle In response to heat, the sympathetic nervous system relaxes the arrector pili muscles of the skin allowing air circulation and increasing cooling capacity therefore lower core body temperature (Sherwood, 2004)

Behavioural thermoregulationBehaviour in response to increased temperature include seeking cooler environments, stretching out on a cooler surface and increased drinking. Stretching out on a cooler surface allows for the movement of heat away from the body through the mechanism of conduction (Campbell, 2011)

Response to coldThe mechanisms for thermoregulation in response to the cold can be categorised into two sections: the mechanisms for retaining heat and the mechanisms for generating heat. Thermoregulation in response to the cold is under the control of the posterior portion of the hypothalamus (Sherwood, 2004). The process of retaining heat includes vasoconstriction, arrector pili muscle contraction, counter-current exchange and behavioural thermoregulation. The process of generating heat includes increased muscular activity, including shivering, and non-shivering thermogenesis, including epinephrine and norepinephrine secretion, thyroid secretion and brown adipose tissue. Table 2 summaries the mechanisms that occur in order to maintain temperature in response to the cold.

Retaining heat Vasoconstriction

In response to the cold, the hypothalamus receives signals from the peripheral skin receptors along the afferent pathway (Akers and Denbow, 2013). The posterior portion of the hypothalamus initiates responses to counteract the decreased body temperature through sympathetic nervous system activation (Campbell, 2011). The sympathetic nervous system sends signals via the efferent pathway to constrict the skin blood vessels therefore reducing heat loss at the skin surface (Morrison and Nakamura, 2011). The process of vasoconstriction is secondarily controlled by the release of norepinephrine by the adrenal medulla, the binding of it to a1-adrenergic receptors to cause vasoconstriction (Akers and Denbow, 2013). The process of vasoconstriction of skin blood vessel results in decreased heat loss at the periphery and an increased blood flow

to the core of the body (Morrison and Nakamura, 2011). As mentioned previously, the hypothalamic thermoregulatory centre takes precedence over the cardiovascular control centre in the medulla (Sherwood, 2004). Therefore vasoconstriction increases blood pressure due to increased muscle tone in the blood vessels causing increased peripheral resistance (Akers and Denbow, 2013)

Piloerection of arrector pili musclesIn response to the cold, the arrector pili muscles at the base of the hair follicle are contracted which increases the insulating barrier between the body and the environment (Sherwood et al., 2013), decreasing heat loss.

Counter-current exchangeAs mentioned previous, the counter-current exchange occurs between the veins and arteries (or venules and arterioles) that are in close proximity in the rete mirabile (Sherwood et al., 2013). In order to retain core temperature, there is an increased heat movement from the arteries to the vein that are being directed back to the heart (Sherwood et al., 2013). The retes are found in difference regions of different animals, for example in the nasal passage of birds and mammals such as dogs (Sherwood et al., 2013)

Behavioural thermoregulationThe processes of behavioural thermoregulation in response to the cold are activated to reduce heat loss. Examples of these are postural changes to decrease exposed surface area, finding shelter or seeking warmer environments.

Generating heat Increasing muscular activity

Muscle contractions are the primary mechanism for heat generating and is achieved by shivering (involuntary) or voluntary muscular movement (Morrison and Nakamura, 2011). Increased muscular activity boosts heat production as heat is a by-product of aerobic respiration, which is the process of energy production in order to sustain muscular activity. Producing heat through shivering results from the inefficiency of ATP utilisation (due to lack of external work) in the cross-bridge cycle and is under the control of the central nervous system (Bicego et al., 2007)

Non-shivering thermogenesisThermogenesis is the heat production in response to environmental changes or diet that includes increasing basal metabolic rate through hormone secretion and heat production through brown adipocyte mechanisms (Bicego et al., 2007)

- Brown adipose tissue (BAT)Heat production via BAT is a mechanism found in newborn animals and small mammals, in particular hibernators (Sherwood et al., 2013). It utilises the BAT cells, which contain adipocytes and deposits of triglycerides that have

specialised uncoupling protein channels in their mitochondria that allow free fatty acid oxidation to produce heat instead of ATP (Bicego et al., 2007, Sell et al., 2004). This heat is important for newborn animals as their surface area to volume ratio is higher than adults (Sherwood et al., 2013).

- Epinephrine and norepinephrineRelease of epinephrine and norepinephrine from the adrenal glands under sympathetic stimulation (Sherwood, 2004). This prompts the mobilisation of fats and carbohydrates for utilisation in energy production, therefore increasing the production of heat through increased metabolic rate (Sherwood, 2004). Norepinephrine increases lipid oxidation in BAT (Bicego et al., 2007)

- Thyroid hormoneThyroid hormone is important in increasing basal metabolic rate which increases heat production (Sherwood, 2004) as it stimulates cellular respiration and increases RNA synthesis leading to protein synthesis (Reece et al., 2015). The hypothalamus signals the anterior pituitary to release thyroid-stimulating hormone to signal the thyroid gland to secrete thyroid hormone (Sherwood, 2004)

Thermoregulation in dogsAs dogs only have eccrine sweat glands in their foot pads, the effectiveness of evaporation through sweating is decreased (Reece et al., 2015). As a result dogs are highly dependent on panting as a primary means of heat loss (Goldberg et al., 1981). Alongside this, the presence of the Steno’s gland and rete-like anastomoses allow a dog to effectively control their temperature (May and Tucker, 2015, Baker et al., 1974).

Panting in dogsPanting allows the upper respiratory tract to evaporate heat (Goldberg et al., 1981). In particular, dogs have a large surface area of their tongue which allows for increased evaporation (Goldberg et al., 1981), alongside the tongue being outside of the mouth often, which increases heat loss. Increases saliva of the mouth also assists heat loss through evaporation

Steno’s glandThe steno’s gland is a gland that opens near the nostril, beneath the maxillary sinus wall, and provides the evaporative water for cooling in dogs (Goldberg et al., 1981, May and Tucker, 2015)

Counter-current heat exchangerThe dog has a nasal counter-current heat exchanger that normally enables them to conserve heat through the heating and humidifying of inhaled air directed towards the lungs and into the body (Goldberg et al., 1981). In response to heat, a dog is able to bypass the heat exchanger through exhaling through the mouth in order to increase heat loss (Schmidt-Nielsen et al., 1970).

Short-nosed dogs (brachycephalic)

The throat and breathing passage of some brachycephalic dogs are undersized or flattened resulting in brachycephalic airway obstruction syndrome (Akers and Denbow, 2013). This results in constant opened-mouth breathing, heat intolerance and a bluish tongue (Akers and Denbow, 2013). These kinds of dogs are less capable of thermoregulation in heat than other dogs.

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