Cardiorespiratory Physiology (PDF)

**[Cardiorespiratory Physiology]** **Cardiorespiratory System** - Learning Objectives - Locate and describe the cardiovascular centre - **The cardiovascular system consists of four components** - A pump - High-pressure distribution system...

**[Cardiorespiratory Physiology]** **Cardiorespiratory System** - Learning Objectives - Locate and describe the cardiovascular centre - **The cardiovascular system consists of four components** - A pump - High-pressure distribution system - Exchange vessels - Low-pressure collection system - **Systemic circulation** - **Arterial distribution** - Oxygenated blood - **Venous return** - Deoxygenated blood - **Pulmonary circulation** - Blood going to and from lungs - **Cardiac output (CO)** - is the volume of blood pumped per minute - CO = SV x HR - Measured in mL/minute - **Stroke volume (SV)** - is the volume of blood ejected from the left ventricle per heart beat - Measure in millilitres (mL) - Influenced by contractility, preload, and afterload - **Contractility** - Relative ability of the heart to eject blood from the left ventricle - **Preload** - Left ventricular end-diastolic pressure - **Afterload** - Resistance against which the heart has contract to eject blood - Vascular resistance - **Frank-Starling Law** - The Frank-Starling law states that SV will increase as the left ventricular volume increases. - Represents the relationship between stroke volume and end diastolic volume - Function of venous return - *Length-tension relationship of cardiac smooth muscle* - **Heart rate (HR)** - is the frequency of heartbeat contractions per minute - Measured in beats per minute (bpm) - Diagram of a diagram of a heart and stroke output Description automatically generated with medium confidence - **Intrinsic Control** - Unlike other tissues, cardiac muscle maintains its own rhythmic contractions - The sinus node (SA) acts as a pacemaker and generates nerve impulses to set resting HR at approx. 100 bpm - **Extrinsic control** - The **cardiovascular centre** exerts extrinsic control of the cardiovascular system by modulating - **Heart rate** - **Myocardial contractility** - **Vascular smooth muscle** - In doing so, the cardiovascular centre regulates - **Cardiac output** - **Blood flow distribution** - The cardiovascular centre is located in the **ventrolateral aspect** of the **medulla** - 'Centre' refers to a group of nerve cells located close together - The cardiovascular centre continuously receives input from **higher brain centres** and **sensory receptors** - **Higher brain centres** - Higher brain centres include - **Cerebral cortex** - **Amygdala** - **Hippocampus** - **Hypothalamus** - Higher brain centers have descending control of the cardiovascular centre - Sending info to cardiovascular centre through afferent nerves - Descending control from higher brain centres explains - Why cardiac activity increases in preparation for physical activity - Why cardiac activity increases in response to anger, fear, anxiety, and stress - Identify and describe cardiovascular sensory receptors - Sensory receptors include - **Chemoreceptors** - Detect changes in arterial O~2~, CO~2,~ and pH - **Aortic body** - Located at the **aortic arch** - Sends information through the **vagus**  - **Carotid body** - Located at the **carotid sinus** - Carotid sinus is located at the junction of the internal and external carotid arteries - Sends information through the **glossopharyngeal nerve** - - **Baroreceptors** - Detect changes in blood pressure - **Arterial** - Located in the **aortic arch** and in the **carotid sinus** - **Cardiopulmonary** - Located in the **myocardial** and **pulmonary vasculature** - - **Muscle and joint receptors** - **Muscle and joint receptors** - **Mechanoreceptors** - Specialized sensory receptors in muscle and joint that detect mechanical stimuli such as joint movement and muscle tension - **Metaboreceptors** - Specialized sensory receptors in muscle that detect production and accumulation of metabolites - Mechanoreceptors and metaboreceptors send afferent information to the cardiovascular centre to increase HR and SV - **Exercise Pressor Reflex** - The exercise pressor reflex refers to an increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner due to the stimulation of mechanoreceptors and metaboreceptors - **Occlusion training or blood-flow restriction** training increases the EPR affect - Explain how the cardiovascular centre exerts sympathetic and parasympathetic influence - Based on the information from the higher brain centres and sensory receptors, the Cardiovascular centre will exert **sympathetic** or **parasympathetic influence** on the cardiovascular system? - **Cardiovascular Centre** - The cardiovascular centre exerts parasympathetic influence through the vagus nerves - The vagus nerve innervates SA and AV - Decreases heart rate - The cardiovascular centre exerts sympathetic influence through the cardiac plexus - The cardiac plexus innervates SA and AV nodes, atria, and ventricles - Increases heart rate and stroke volume - **Parasympathetic influence** - Parasympathetic nerves release acetylcholine (ACh) - On the heart, ACh binds to a type of cholinergic receptor called muscarinic receptors - Once activated by ACh, muscarinic receptors decrease rate of SA and AV node firing rate -  - **Resting Heart rate** - At rest, the SA and AV nodes receive parasympathetic influence, which decreases resting HR to approx. 50-70 bpm - **Heart Rate during physical activity** - Heart rate increases due to reduced parasympathetic influence and increased sympathetic influence - Higher brain centres - Sensory receptors - **Muscarinic receptors** - Muscarinic receptors are also located on **sweat glands** - On sweat glands, muscarinic receptors act as sympathetic receptors and once activated, increase sweat production - **Sympathetic influence** - Sympathetic influence increases heart rate - This is known as the **chronotropic effect** - Sympathetic influence also increases myocardial contractility, which increases stroke volume - This is known as the **inotropic effect** - Explain how neurotransmitters and hormones are involved in the sympathetic response - Sympathetic nerves release **norepinephrine** - As a neurotransmitter, norepinephrine exerts sympathetic influence directly on the heart - As a neurotransmitter, norepinephrine stimulates the adrenal medulla to release catecholamine hormones - **Norepinephrine** - **Epinephrine** - As a neurotransmitter and as catecholamines, epinephrine and norepinephrine bind to **adrenergic receptors** - There are two types of adrenergic receptors - Alpha-adrenergic receptors - Arteries - Beta-adrenergic receptors - Heart, lungs, adipocytes - As a hormone, norepinephrine has a greater effect on alpha receptors and thus, *arteries* - During exercise, norepinephrine produces vasoconstriction - As a hormone, norepinephrine has a greater effect on alpha receptors and thus, arteries - During exercise, norepinephrine produces vasoconstriction - **Beta Adrenergic receptors** - There are three types of beta adrenergic receptors - Beta 1 - Heart ---\> heart rate and contractility - Beta 2 - Bronchioles ---\> bronchodilation - Beta 3 - Adipocytes ---\> lipolysis - A diagram of a nervous system Description automatically generated - **During exercise, Active muscle experiences localized vasodilation to increase blood flow and thus, oxygen supply to support energy demands** - **Vasodilation** - At rest, approx. 1 in every 30 or 40 capillaries around muscle tissue remain open - At the onset of exercise, metabolic factors stimulate a rapid response to coordinate vasodilation of capillaries perfusing working tissues - **Vasodilation** occurs when the smooth muscle located in the blood vessel walls relaxes - **CO~2~, H^+^, lactate,** and **potassium ions** act as potent stimuli for local vasodilation - During exercise, these metabolites present in the extracellular space and stimulate the vascular endothelium to produce **nitric oxide (NO)** - NO causes vasodilation through the activation of **Guanylyl Cyclase** - NO is synthesized from l-arginine in an enzymatic reaction with O~2~ and NO synthase - *This is why many pre-workout supplements contain l-arginine and/or arginine's precursor l-citrulline* **Respiratory System** - **Respiratory system** - The respiratory system consists of two components - **Airways** - **Alveoli** - Differentiate between ventilation and respiration - **Ventilation** is the movement of air in and out of the respiratory lungs - **Minute ventilation** - Minute ventilation (V~E~) is the volume of air that enters the respiratory system per minute - V~E~ = Respiratory Rate (RR) x Tidal Volume (TV) - RR is the frequency of breaths per minute - TV is the volume of air that enters the respiratory system per breath - **Respiration is the exchange of gases at the alveoli or cell** - **External respiration** is the exchange of gases between the alveoli and blood - **Internal respiration** is the exchange of gases between blood and cells - **External respiration** - **Gas exchange occurs at the alveoli** - Gas exchange occurs down concentration gradients of O~2~ and CO~2~ - - Differentiate between quiet and active breathing - **Quiet Breathing** - At rest - Inspiration - active, contraction of diaphragm and external intercostals - Expiration - passive, elastic recoil of diaphragm and external intercostals - **Active Breathing** - During exercise - Inspiration -- active, contraction of the diaphragm and external intercostals - Expiration -- active, contraction of the internal intercostals and abdominal musculature - Locate and describe the respiratory centre - **Extrinsic control** - The **respiratory centre** exerts extrinsic control of the respiratory system by modulating - **Breathing frequency** - **Breathing depth** - In doing so, the respiratory centre regulates - **Respiratory rate** - **Tidal volume** - The respiratory centre is located in the **ventrolateral aspect** of the **medulla** and the **pontine tegmentum** in the **pons** - The respiratory centre is comprised of thee groups - **Dorsal respiratory group (DRG)** - **Ventral respiratory group (VRG)** - **Pontine respiratory group (PRG)** - Dorsal respiratory group and ventral respiratory group regulate **respiratory rate** - Pontine respiratory group regulates **tidal volume** - Higher brain centres can also exert descending control and override ventilatory processes - **Cerebral cortex** (Motor Cortex) - Holding your breath and talking - *Talking while playing sports* - *Valsalva maneuver* - **Limbic system** (Amygdala, hippocampus, and hypothalamus) - Fear and emotion - *Hyperventilating during stressful situations* - **Dorsal Respiratory Group** - The DRG regulates inspiration during quiet and active breathing - Axons of the phrenic nerves descend the spinal cord, synapse with lower motor neurons of the primary respiratory muscles - Diaphragm - External intercostals, - **Ventral Respiratory Group** - The VRG regulates expiration and facilitates inspiration during active breathing - Axons of the **phrenic nerves** descend the spinal cord, synapse with lower motor neurons of the accessory respiratory muscles - Internal intercostals - Abdominal musculature - **Pontine Respiratory group** - The PRG exerts descending control of the DRG to regulate TV - Comprised of two centres - **Apneustic centre** - Stimulates the DRG to prolong inspiration, producing deeper breathes - **Pneumotaxic centre** - Inhibits the DRG to shorten inspiration, producing shallower breathes - Identify and describe respiratory sensory receptors - Sensory receptors - **The respiratory centre continuously receives input from sensory receptors** - **Chemoreceptors** - **Chemoreceptors** detect changes in arterial O~2~, CO~2~, and pH as well as cerebrospinal fluid Ph - **Central chemoreceptors** - Located on the **ventrolateral surface** of the medulla - The blood brain barrier is permeable to CO~2~ - Detect changes in pH of cerebrospinal fluid - Carbon dioxide reacts with water and produces carbonic acid - Carbonic acid dissociates into bicarbonate and hydrogen ions - Accumulation of hydrogen ions decreases pH - **Peripheral chemoreceptors** - Located on the aortic arch and carotid sinus - **Carotid Bodies** - Sends information through the **glossopharyngeal nerves** - **Aortic Body** - Sends information through the **vagus nerves** - **Pulmonary stretch receptors** - **Slowly adapting Pulmonary stretch receptors (SaR)** - SAR are mechanoreceptors located in the pulmonary smooth muscle - SAR detect lung stretch and terminate inspiration, which promotes smooth, controlled breathing - During inhalation, SAR detect lung stretch and send inhibitory signals to the respiratory through the vagus nerve - **Hering-Breuer reflex** - SAR initiate the Hering-Breuer reflex to prevent over-inflation of the lungs, particularly during intense physical activity - SAR respond to the degree and duration of lung inflation - The Hering-Breuer reflex decreases TV through inhibition of the apneustic centre and DRG - **Rapidly adapting pulmonary stretch receptors** - RAR are mechanoreceptors located in the pulmonary epithelium - RAR respond to large, rapid inflations of the lungs and irritants (ex. Dust, smoke, pollution, cold air) - **RAR's transmit excitatory signals through the vagus nerve** - **Irritant receptors** - RAR act as irritant receptors and produce several protective respiratory reflexes - Coughing - Bronchoconstriction - Increased breathing rate (Tachypnea) - **Muscle and joint receptors** - Mechanoreceptors and metaboreceptors send afferent information to the respiratory centre to increase RR and TV **Acute Response** - **Explain how oxygen and carbon dioxide are transported in blood** - **Gas Exchange** - O~2~ and CO~2~ diffuse down the concentration gradient from high to low concentration - **External respiration** - At rest and at sea level - Alveolar PO~2~ is approx.100 mmHg - Alveolar PCO~2~ is approx. 40 mmHg - Deoxygenated PO~2~ is approx. 40 mmHg - Deoxygenated PCO~2~ is approx. 45 mmHg - **Arterial blood** - Normal resting arterial blood at sea level - O~2~Sat 95-100% - PO~2~ 80-100 mmHg - PCO~2~ approx. 40 mmHg - **pH 7.35-7.45** - **During intense efforts and including intense intermittent efforts, arterial PO~2~, PCO~2~, and O~2~sat remain stable.** - Increases in - Breathe faster - HR increase - Cardiac Output increases linearly but not with HR - Tidal volume (depth) deeper breaths - Circulate Oxygen and remove carbon dioxide to keep stability - **Oxygen in blood** - Blood transports oxygen in one of two ways - Dissolved in plasma - Approx. 2% total O~2~ - Establishes PO~2~ - Bound to hemoglobin - **Hemoglobin** - Each red blood cell contains approx. 250 million molecules of hemoglobin - Each molecule of hemoglobin consists of two alpha globin subunits and two beta globin subunits - Each globin subunit contains a heme subunit, which contains iron - **Iron** - Each molecule of hemoglobin has the capacity to carry **four molecules of oxygen** bound to iron - **Internal respiration** - CO~2~ diffuses down the concentration gradient from the muscle cell into blood - At rest and at sea level - Arterial PCO~2~ -- 40 mmHg - Tissue PCO~2~ -- 45 mmHg - **Carbon Dioxide in blood** - Blood transports carbon dioxide in one of three ways - Dissolved in plasma - Approx. 5% total CO~2~ - Establishes plasma PCO~2~ - Bound to hemoglobin - Approx. 20-30% - Forms carbamino-hemoglobin compound - Bicarbonate buffering system - Approx. 65-75% - **Describe the oxygen-hemoglobin dissociation curve** - **Oxygen-Hemoglobin dissociation curve** - The **oxygen-hemoglobin dissociation** curve illustrates the relationship between the partial pressure of O2 and hemoglobin oxygen saturation - Oxygen binds to hemoglobin through **cooperative binding** - Hemoglobin's affinity for oxygen increases as more O2 binds - **Explain how carbon dioxide and hydrogen ions are processed by the bicarbonate buffering system** - **Bicarbonate buffering system** - **Red blood cells (RBC)** contain an enzyme called **carbonic anhydrase (CA)** - CA catalyzes a reaction between CO~2~ and water and produces **carbonic acid (H~2~CO~3~)** - Carbonic acid is unstable in aqueous solutions and dissociates into **bicarbonate ions** **(HCO~3~^-^)** and **hydrogen ions (H^+^)** - H^+^ ions associate with the amino acids in hemoglobin and cause a conformational change in the hemoglobin protein structure, reducing hemoglobin's affinity for oxygen binding - Hemoglobin offloads oxygen - - **Explain how carbon dioxide is offloaded at the lungs** - **Describe the Bohr and Haldane effects** - **Bohr effect** - The Bohr effect illustrates decreases in hemoglobin's affinity for O~2~ due to increases in PCO~2~ - In acidic environments, hemoglobin has a propensity for releasing O~2~ in favour of the attachment of H^+^ ions - *This leads to increased O2 offloading to and CO2 uptake from working tissues to meet demands* - **Hamburger phenomenon** - The hamburger phenomenon, also known as the chloride shift, refers to exchange of chloride and bicarbonate in and out of RBC - This process allows for the continued uptake of CO~2~ even when the production of CO~2~ increases during intense efforts - **Deoxygenated blood** - When deoxygenated blood reaches the lungs, the highly oxygenated, pH neutral environment favours the dissociation of the CO~2~ molecules and H^+^ ions from hemoglobin in exchange for oxygen binding - **Haldane effect** - The Haldane effect illustrates decreases in hemoglobin's affinity for CO~2~ due to increases in PO~2~ - O~2~ binding progressively increases hemoglobin's affinity for O~2~ at each of the remaining binding sites through cooperative binding - **Reverse reaction** - The dissociation os H+ ions from hemoglobin initiates the reverse reaction - Bicarbonate is shuttled into red blood cells in exchange for chloride - The H^+^ ions combine with bicarbonate to form carbonic acid - Carbonic anhydrase catalyzes the conversion of carbonic acid into CO~2~ and H~2~O - CO~2~ diffuses down its concentration gradient and into the alveoli - **Myoglobin** - Myoglobin is an iron-containing protein in muscle fibres with approx 240x greater affinity for oxygen than hemoglobin - Each molecule of myoglobin contains one molecule of iron - Myoglobin enables muscle to 'store' oxygen and offers an oxygen buffer for fluctuations in exercise intensity - Muscle fibres that have a high concentration of myoglobin (oxidative fibres) appear red - Muscle fibres that have a low concentration of myoglobin (glycolytic fibres) appear pale - **Explain hyperventilation** - **Hyperventilation is breathing that is deeper and more rapid than normal. During intense exercise including intense intermittent efforts, individuals may hyperventilate.** - **Cardiac drift** - Cardiac drift is the progressive increase in heart rate and decrease in stroke volume during prolonged steady-state exercise - Fluid loss through sweat - Decreased blood volume - Competition for blood flow distribution - Heat stress **Maximal Rate of Oxygen Consumption** - **Define VO~2~, VCO~2~, and VO~2~max** - **Vo~2~max** - VO~2~max represents the maximal rate of oxygen consumption during incremental exercise - VO~2~max can be expressed in absolute terms (L/min) or in relative terms (ml/min/kg) - **Metabolic cart** - The composition of ambient air remains relatively constant - A metabolic cart analyses the composition of expired air to determine the quantity of oxygen that has been removed and the quantity of carbon dioxide that has been cardio added - As exercise intensity increases, expired air will contain less oxygen and more carbon dioxide - **Oxygen Consumption** - Oxygen consumption (VO~2~) is the volume (V) of oxygen (O~2~) consumed by tissue - VO~2~ is expressed as L/min or ml/kg/min - **Carbon dioxide production** - Carbon dioxide production (VCO~2~) is the volume (V) of carbon dioxide (CO~2~) produced by tissue - VCO~2~ is expressed as L/min or ml/kg/min - **Respiratory Exchange Ratio** - Respiratory Exchange Ratio (RER) is the volume ratio of carbon dioxide production and oxygen consumption (VCO~2~/VO~2~) - RER is used to indirectly determine the relative contribution of energy from fat and carbohydrate metabolism at rest and during low-moderate intensity exercise - RER is typically between 0.7 and 1.0 - **VO2max vs. Vo2peak** - **Does VO2max predict performance?** - **Criteria for VO~2~max test** - Heart rate - ≤10 beats/min or ≤5% of the age-predicted (220-age) maximum - Blood lactate concentration - \>8mmol/L - RER - \>1.00 - What can you do with VO2 max data - Track training data (progress) - Exercise prescription intensity **Cardiorespiratory Adaptations** - Outline specific adaptations of the cardiovascular system - **Cardiovascular system** - Stress to the Cardiovascular system - Exercise and training must be intense enough to sufficiently overload cardiac output and thus, stroke volume - Intervals of repeated moderate-to-high intensity exercise and continuous, long-duration efforts stimulate adaptive changes in cardiovascular physiology - **Adaptive Changes in Aerobic Capacity** - Vo2 Max - **Overview of Adaptations**  tab11\_03a.jpg - Cardiac Output - Few changes at rest and during submaximal intensities - During high-intensity and maximal exercise, there is a noticeable improvement in cardiac output due to improved stroke volume - Stroke volume  - Improvements in stroke volume are the result of  - **Increased ventricular compliance** - Preload - **Increased internal ventricular dimensions** - Volume - **Increased myocardial contractility** - **Increased venous return** - Increased plasma volume - - Heart Rate - Heart Rate recovery fig11\_05.jpg - Heart Remodelling - Increased myocardial contractility - Left ventricular mass - Left ventricular wall thickness - Blood Volume - Increases in plasma volume improves - **Venous return** - **End-diastolic volume** - **Temperature regulation** - Alterations in red blood count - Erythropoiesis, bone marrow, hypoxia, kidneys, and erythropoietin (EPO) - Gas exchange  - Training the cardiovascular system enhances external and internal respiration through increased capillary density, **angiogenesis** - More capillaries per alveoli - More capillaries per muscle fibre - Outline specific adaptations of the respiratory system - **Respiratory system** - Stress to the Respiratory system - Exercise and training must be intense enough to sufficiently overload minute ventilation and thus, tidal volume and respiratory Rate - Intervals of repeated moderate-to-high intensity exercise and continuous, long-duration efforts stimulate adaptive changes in respiratory physiology - Respiratory Endurance - Training the respiratory system enhances muscular strength and endurance of the primary and accessory respiratory musculature - Reducing relative exercise energy demands - Does Ventilation Limit Aerobic Endurance? - *Pulmonary function does not form a "weak link" in the O~2~ transport system* - Breathing capacity does not reach its maximum even during strenuous exercise and is not responsible for the limitation in oxygen delivery to muscles seen during high intensity activity - Hemoglobin continues to be fully saturated with oxygen throughout exercise in people with normal respiratory function - Overview of adaptations - Vital Capacity - the maximal volume of air that can be expired following maximum inspiration  - Residual Volume - the volume of air remaining in the lungs after maximum forceful expiration - Compare and contrast the adaptive changes in cardiorespiratory physiology from bed rest and aging - Dallas bed rest and training study - In 1966, the Dallas Bed Rest and Training study investigated changes in cardiorespiratory performance from extreme changes in physical activity - 5 healthy 20 year-old male participants - 3 weeks of complete bed rest with no weight bearing - 8 weeks of endurance training - VO~2~max was assessed at baseline, after bed rest, and after endurance training - VO~2~max declined 27% and cardiac output decreased 26% after bed rest - VO~2~max increased 45% and cardiac output increased 40% after endurance training - There were no significant changes in maximal arterial venous oxygen difference (A-VO~2~ diff) (ie. Not attributable to peripheral alterations) - Changes in VO~2~max were attributable to changes in cardiac output as a result of changes in stroke volume - Maximal stroke volume decreased 31% after bed rest and increased 48% after endurance training - Additional important observations were the impact of bed rest and exercise training on submaximal exercise performance - A submaximal workload of 1.5 L/m represented 45% of baseline maximal workload, 63% after bed rest, and 38% after endurance training - A submaximal workload of 1.5 L/m represented heart rates of 129 bpm at baseline, 164 bpm after bed rest, and 115 bpm after endurance training - Therefore, at any submaximal workload, myocardial demands were higher after bed rest and lower after exercise training - 8-week endurance training protocol - The endurance training protocol was comprised of - 4-5 sessions per week - Moderate-vigorous aerobic physical activity - Stationary cycle - 30-40 minutes per session - Progressive overload was applied by increasing duration and cycling intensity as cardiorespiratory fitness improved - Final weekly volume of 250 minutes per week - - follow ups - The same 5 participants were studied 30 and 40 years later to evaluate the effects of the age-related changes in cardiorespiratory fitness - 30-year follow up - Age 50 - Baseline VO~2~max declined by 12% over the 30 year period - Thus, 3 weeks of bed rest at age 20 decreased cardiorespiratory fitness more than 30 years of aging - Participants completed similar endurance training to the original 1966 study - 6 months instead of 8 weeks - Achieved final weekly duration of 250 minutes per week - VO~2~max increased by 14% with training and achieved levels similar to baseline testing in 1966 - Thus, endurance training in middle-aged men effectively reversed the effects of 30 years of aging - 40-year follow up - The same 5 participants were studied again 10 years later at the age of 60 - 3 participants had developed hypertension, 2 had arterial fibrillation, and 1 had disabling back pain due to renal cancer - Only baseline testing was performed with no training component - VO~2~max declined an additional 17% over the 10-year period, and 27% in total over the 40-year period - VO~2~max was the same after bed rest in 1966 as it was at the 40 year follow-up - 3 weeks of bed rest at the age of 20 was as detrimental as 40 years of aging - Cardiorespiratory fitness declined over time primarily because of an impaired efficiency of maximal peripheral oxygen extraction by working tissues (ie. Attributable to changes in A-VO~2~ diff) - Key takeaway - 3 weeks of bed rest at age 20 had a more profound impact on cardiorespiratory fitness than 30 years of aging and had the same impact as 40 years of aging **Thermoregulation** - Define thermoregulation - **Thermoregulation** - **Thermoregulation** represents the ability of an organism to maintain its body temperature within certain boundaries in varying environmental conditions. - In humans, core body temperature is typically **36.5 to 37.5°C** but may deviate considerably when exposed to extreme conditions. - Thermoregulation involves **behavioural** and **autonomic** processes that maintain a steady internal body temperature despite changes in external conditions. - The drive to maintain thermal balance remains so strong that it readily triggers a sweating rate of 2L/hr during exercise in heat or an oxygen consumption of 1200 ml/min from shivering in severe cold - **Skin temperature** - Skin acts as the interface with the environment - Skin temperature is not regulated and varies across the body in response to the thermal environment - Mean skin temperature can be categorized as cool (\