53. Physiological Challenges Flashcards

Question

Oxygen consumption rises up to 12-fold in exercise. How is this increased consumption emabled by the cardiovascular system?

Answer 1

* 4 times increase due to 4 times increase in cardiac output * 3 times increase due to 3 times as much oxygen being extracted from venous blood (Note: Sa_O2 cannot be increased because arterial blood is almost completely saturated already)

Answer 2

Venous saturation must drop from around 0.73 to around 0.25.

Answer 3

It is a rise of 20-fold (compared to just a 4-fold increase in cardiac output).

Answer 4

* Muscles -\> Increases * Heart and brain -\> Around constant * Viscera -\> Decreases * Skin -\> Depends on need for heat loss

Answer 5

VO₂ increases about 40-fold due to: * Blood flow increases 20-fold due to: * 4-fold increase in blood velocity * 5-fold increase in number of capillaries open * Oxygen extraction from blood doubles 4 x 5 x 2 = 20

Answer 6

Heart rate: * Increases linearly * Increase from around 70 to 190bpm Stroke volume: * Plateaus at around 1/3rd VO₂ * Increase from around 70 to 100ml

Answer 7

BP = CO x SVR (Note that blood pressure here ignores venous pressure because it is so small, but this induces some error. So could consider it as the difference in blood pressure, really.)

Answer 8

BP = CO x SVR * Blood pressure increases are quite small (which is sort of intuitive) * Therefore, the body must accomodate most of the increase in BP by decreasing the systemic vascular resistance * This is done mostly by dilation of arterioles supplying muscles and other respiring tissues

Answer 9

It decreases.

Answer 10

* Systemic vascular resistance falls in exercise due to vasodilatation in active skeletal muscles * Blood pressure is maintained at normal or elevated levels during exercise by increasing cardiac output (CO) and increasing sympathetic vasomotor tone as exercise intensity increases. In other words, skeletal muscle arterioles vasodilate, but the rest of the system vasoconstricts.

Answer 11

12-fold (in proportion to increased oxygen consumption)

Answer 12

Ventilation = Tidal volume x Respiratory rate

Answer 13

* Ventilation increases pretty linearly with oxygen consumption up until a 'break point' where the ventilation increases rapidly (1st graph) * This increase is mostly due to tidal volume at first (2nd graph), and then due to respiratory rate (3rd graph)

Answer 14

* At rest, it is perfusion limited due to the large excess area of lungs * During exercise, the lungs may become diffusion limited at times

Answer 15

The adrenaline protects the heart from dysrhythmias.

Answer 16

* It contributes to the pain and fatigue seen during exercise * It makes the blood more acidic -\> This means that the anaerobic threshold at which lactate starts being produced is often associated with the 'breaking point' of ventilation, where the ventilation rapidly increases

Answer 17

* Total body haemoglobin * Heart size * Flow rates in ventilation are lower than maximally possible * Has to be diffusion gradient for oxygen to diffuse into lungs (so cannot extract too much oxygen from the venous blood) * Heart rate In general, in healthy people it is the cardiovascular system that limits exercise, not the respiratory system.

Answer 18

(Astrand, 1952): * Compared the maximum oxygen consumption in individuals with their total body haemoglobin * A roughly proportional relationship was seen * The thinking behind this was that a larger total body haemoglobin is associated with a higher volume of blood, which is associated with a higher cardiac output

Answer 19

(Hammond, 1992): * Operated on two groups of sheep, cutting the pericardium of one group (since the pericardium limits the size of the heart) * The group with the cut pericardium showed an increased cardiac output upon follow up, while the control group showed a small decrease

Answer 20

Note how the exercise loop is not as large as the maximal loop, suggesting that it may be ventilation that is the limiting factor occasionally. However, it is usually cardiovascular factors that are limiting, not ventilation.

Answer 21

Most of all, stroke volume.

Answer 22

(Scharhag, 2002): * Compared 21 male athletes with untrained controls * Scans showed that heart volume and wall thickness were increased in the athletes * The athletes also had a 62% larger VO₂ max

Answer 23

The medulla, which controls ventilation, receives input from: * Voluntary inputs * Stretch receptors in the lungs * Chemoreceptors: * Central (in medulla) -\> Respond to PaCO₂ mostly * Peripheral -\> Respond to PaO₂ mostly Muscle spindles in thoracic wall also lead to spinal reflexes that affect ventilation.

Answer 24

These curves are at rest. You can see that, during exercise, the carbon dioxide and oxygen do not change much, but the ventilation does, which implies that something else must be at play during exercise!

Answer 25

* At rest, they are important * However, during exercise, blood oxygen and carbon dioxide do not change much, while ventilation must increase, meaning that they cannot be the only signals

Answer 26

* Phase 1 -\> Rapid increase in breathing within seconds of starting the exercise * Phase 2 -\> More slow increase in breathing later * Phase 3 -\> Plateau in breathing

Answer 27

* Central command -\> This would allow a faster response since there is no need to wait for feedback. Learning plays a major role in this * Peripheral chemoreceptors: * Mean PaCO₂ does not change majorly, but there are oscillations in PaCO₂, which increase during exercise. Chemoreceptors could detect this increased oscillation. * Lactate, potassium and adrenaline all stimulate peripheral chemoreceptors. * Reflex feedback from muscle 'work receptors' via the spinal cord (experiments in dogs suggest this is the case, but experiments in paraplegics suggest it is not) After these fast responses, the feedback mechanisms, such as from the peripheral chemoreceptors, can also assist in ventilation control.

Answer 28

* Parasympathetic -\> Vagus * Sympathetic -\> Superior, middle and inferior cardiac nerves

Answer 29

(Donald, 1968): * Exercised 4 groups of greyhounds: 2 with all heart innervation removed and 2 with beta blockade due to propranolol * The heart rate increased more slowly in the black line on the right graph than the left graph, showing that circulating catecholamines work more slowly * The heart rate was slightly lower in the red graph on the left than the black line, which is due to partial blockage of the noradrenaline by propranolol * The red line on the right was flat due to complete blockage by propranolol

Answer 30

The phases are called phases 1, 2 and 3, just like with ventilation changes.

Answer 31

* Central command -\> This would allow a faster response since there is no need to wait for feedback. Learning plays a major role in this * Feedback from muscles from 'work receptors' -\> Potassium and H⁺ are probably sensed After these fast responses, the feedback mechanisms, such as from the peripheral chemoreceptors and the baroreflex, can also assist in ventilation control.

Answer 32

Chemoreceptors

Answer 33

(Rowell, 1976): * A cuff was used to trap metabolites in the thighs after exercise * This showed that the trapped metabolites maintained blood pressure and (to a lesser extent) heart rate after exercise, but not ventilation * Release of occlusion showed a spike in all three, due to stimulation the peripheral chemoreceptors, etc.

Answer 34

At rest: * Mostly controlled by sympathetic vasoconstrictor nerves * Blood flow is normally 30ml/kg/min, but can go as low as 6ml/kg/min when there is haemorrhage During exercise: * Mostly due to local metabolic vasodilation (increases flow up to 20 times) * Not certain which factors produce this vasodilation, but could include: K⁺, H⁺, ADP and low PO₂ * Flow to muscle is also increased by vasoconstriction of blood vessels supplying other tissues, such as voscreal organs * Adrenaline also increases vasodilation by acting on beta-2 receptors * The 'muscle pump' -\> Contraction of muscles pushes blood through the veins

Answer 35

(Ponikowski, 2015): * Showed that iron supplementation was beneficial to heart failure patients, even without any anaemia * Supplementing iron was shown to improve the distance the patient could walk in 6 minutes

Answer 36

McArdles' disease: Patients cannot break down glycogen and become easily fatigued

Answer 37

* Temperature * Humidity * Solar radiation * Remoteness * Hypoxia

Answer 38

(Peacock, 1998)

Answer 39

(Peacock, 1998)

Answer 40

* Breathing increases so the alveolar partial pressure of CO₂ decreases * Heart rate increases * Red blood cell production increases * Systolic pulmonary blood pressure increases

Answer 41

* Peripheral chemoreceptors (carotid body) -\> Samples arterial blood for decreases in P_O2 or to a lesser extent increases in pH and P_CO2 * Central chemoreceptors (medullary) -\> Sample CSF pH, which is dependent on blood P_CO2and HCO₃^- These communicate with medullary respiratory neurons, which control breathing.

Answer 42

Mostly the carotid body (peripheral).

Answer 43

Central chemoreceptors (medullary)

Answer 44

Assuming that there is a sudden change of altitude: * P_O2 drops and this hypoxia drives hyperventilation via the carotid bodies * This increases P_O2 and decreases P_CO2 * The fall in P_CO2 leads to alkalosis, which reduces ventilation via reduced stimulation of the central chemoreceptors * This leads to a balance between hypoxia and hypocapnia * Over several days, acclimatisation occurs, causing a shift towards hyperventilation

Answer 45

It could be either by: * Reduced inhibition of ventilation by hypocapnia * Increased stimulation of ventilation by hypoxia We are not sure about the balance of these two.

Answer 46

* Parallel shift of line to the left (shown on the left graph) * This is due to increased sensitivity of the central chemoreceptors to P_CO2 * This change in sensitivity may be due to prolonged alkalinisation of the body as P_CO2falls * The result is increased ventilation * Increased slope of the line (shown on the right graph) * This is due to an increase in the sensitivity of the peripheral chemoreceptors to P_O2 * The result is increased ventilation (Remember that this change only occurs over a few days as the body acclimatises to high altitude)

Answer 47

(Kellogg, 1963): * Saw that the graph shifted to the left AND increased in slope * Therefore, both types of shift occur

Answer 48

Hypocapnia

Answer 49

Evidence that peripheral chemoreceptors detecting P_O2is important: * (Howard, 1995) -\> Varied P_O2 without changing P_CO2. It turns out that there is a large increase in ventilation upon hypoxia when P_CO2 is kept constant. * (Bisgard, 1986) -\> Altered the blood supply to a goat's brain so that the peripheral chemoreceptors can be independently perfused. Induced hypoxia at the carotid bodies but not at the central chemoreceptors. It turns out that ventilation increases when only the peripheral chemoreceptors are hypoxic. Evidence that central chemoreceptors detecting P_CO2is important: * (Ren, 1999) -\> Asked patients to hyperventilate while keeping their P_O2 constant. In patients where P_CO2was allowed to drop, the line shifted to the left. In patients where CO₂ was supplied to keep P_CO2 constant, the line did not shift.

Answer 50

* It may be reduced CSF bicarbonate. * This can be explained this way: * At high altitude, there is hyperventilation * This leads to falls in P_CO2 in the body (respiratory alkalosis) * The kidneys excrete increased amounts of bicarbonate to compensate for this * So there are low levels of bicarbonate in the blood and CSF

Answer 51

There are 3 main hypothesis: * Renal compensation * At high altitude, there is hyperventilation * This leads to falls in P_CO2 in the body (respiratory alkalosis) * The kidneys excrete increased amounts of bicarbonate to compensate for this * There is redistribution of bicarbonate from the CSF * Active transport out of the CSF (not fully believed) * At high altitude, there is hyperventilation, leading to respiratory alkalosis * As pH of the CSF fluid falls, bicarbonate is actively transported out of it * Lactacidosis * At high altitudes, there is cerebral tissue hypoxia * This leads to acidosis, which is buffered using bicarbonate

Answer 52

* When the body hyperventilates and leads to respiratory alkalosis, the kidneys excrete increased amounts of bicarbonate to recover. * This leads to drops in CSF bicarbonate, which leads to increased sensitivity of the central chemoreceptors to P_CO2 * This leads to adaptation to high altitude by hyperventilation

Answer 53

* Increased sensitivity of the peripheral chemoreceptors to P_O2 is likely to be the first response that kicks in, responsible for most of the acclimatisation * Increased sensitivity of the central chemoreceptors to P_CO2 is likely to be slower because it relies on the kidneys excreting bicarbonate in order for CSF bicarbonate to fall

Answer 54

* Acetazolamide (a carbonic anhydrase inhibitor) * It increases renal bicarbonate loss * Therefore, it gives a head start to acclimatisation to high altitudes since renal excretion of bicarbonate is required for central chemoreceptor adaptation and it is usually slow

Answer 55

* Sickness that occurs during an ascent, typically before you acclimatise * Risk factors include: * Speed of ascent * Final altitude * Individual predisposition * Defined as headache and Lake Louise Score \>3, in setting of recent ascent

Answer 56

A small degree of cerebral oedema.

Answer 57

* Severe swelling of the brain due to fluid * Causes: * Hypoxia leads to increased cerebral blood flow, increased cerebral blood volume and increased permeability of the BBB * Symptoms: * Headache, malaise, ataxia, confusion and coma * Treatment: * Descent * Decompression * Oxygen * Dexamethasone [IMPORTANT]

Answer 58

* There is periodic (Cheyne Stokes) breathing * This is where there are short bursts of fast, intense ventilation and then periods of very little ventilation * This is due to instability of the breathing system caused by its high gain

Answer 59

It increases due to vasoconstriction.

Answer 60

* It occurs in hypoxic areas of the lungs * This preserves flow to better oxygenated parts of the lungs, optimising ventilation/perfusion matching * This is useful when it is local (e.g. in pneumonia) but it is harmful when it is widespread (e.g. at altitude)

Answer 61

* Severe swelling of the lungs due to fluid accumulation * Causes: * Widespread pulmonary vasoconstriction upon ascent to high altitudes * Strong individual predisposition * Leading cause of death at high altitudes * Symptoms: * Dry cough, then pink frothy sputum and progressive hypoxia * Treatment: * Descent * Oxygen * Nifedipine [IMPORTANT] -\> Calcium channel blocker that reduces vasoconstriction

Answer 62

* Altitude sickness -\> Acetazolamide: * This is a carbonic anhydrase inhibitor * It increases renal bicarbonate loss * Therefore, it gives a head start to acclimatisation to high altitudes since renal excretion of bicarbonate is required for central chemoreceptor adaptation and it is usually slow * High altitude cerebral oedema -\> Dexamethasone: * This is a steroid * It acts to decrease inflammation and thus it prevents oedema, but the exact mechanism of action is not known * High altitude pulmonary oedema -\> Nifedipine * This is a calcium channel blocker, quite specific to the lungs * It acts to decrease vasoconstriction of the lungs, which prevents oedema

Answer 63

Stress failure -\> This is when uneven vasoconstriction can lead to very high pressures in certain capillaries, so that the pressure becomes high enough to damage them.

Answer 64

Hypoxic pulmonary vasoconstriction

Answer 65

High altitude leads to high levels of haemoglobin.

Answer 66

Much lower than normal

Answer 67

* Increased [H⁺] * Increased P_CO2 * 2-3DPG * Temperature

Answer 68

* At high altitudes, the low P_CO2 in the blood and atmosphere causes the haemoglobin dissociation curve to shift to the left * This makes it easier for haemoglobin to pick up oxygen, but it also makes it more difficult for oxygen to dissociate from haemoglobin at tissues

Answer 69

* Usually, at sea level, arterial and alveolar P_O2 are very similar * However, at high altitudes, the arterial blood has lower P_O2 than the alevoli * This is due to diffusion limitation in the lungs, which results from a smaller diffusion gradient

Answer 70

Hypoxia inducible factor

Answer 71

It is a transcription factor that regulates many genes in hypoxia.

Answer 72

* HIF is continuously being produced * In the presence of oxygen and iron, HIF is broken down * When there is no oxygen, HIF cannot be broken down so it accumulates and leads to gene transcription

Answer 73

* Erythropoietin (EPO) * Thus, creation of RBCs is increased during hypoxia

Answer 74

A condition where there is an increased number of RBCs.

Answer 75

HIF upregulation (or other changes in the HIF pathway), which leads to increased EPO production and thus RBC production.

Answer 76

* Chuvash polythemia * Upregulation of HIF pathways not only causes erythropoiesis, but also mimics cardiorespiratory effects of high altitude (hyperventilation and increased pulmonary arterial pressure)

Answer 77

Polycythemia slows blood flow, which decreases oxygen delivery to tissues.

Answer 78

* No, it is not, and it can be dangerous * (Petousi, 2014) found that Tibetans (a high-altitude population) had reduced HPV in response to altitude compare to Han Chinese (a low-altitude population)

Answer 79

* Differences in expression of genes within the HIF-pathways. * These HIF differences also underlie different haemoglobin concentrations in these populations.

Answer 80

* The alveolar gas equation * You can understand the changes by remembering that P_atmdecreases, P_H20stays the same and Pa_CO2 increases slightly. * This means that the P_AO2 greatly falls and therefore haemoglobin saturation falls too. CO and ventilation can be increased to compensate. * Ventilation decreases Pa_CO2, which helps.

Answer 81

ADH levels decrease, so blood volume decreases and thus [Hb] increases.

Answer 82

NB The parameters for core body temperature are very tight, and there is a good reason for this!

Answer 83

* Two compartment system: * The core (brain, thoracic and abdominal compartments containing the heat producing organs) * This region is held at ~37.0+/- 0.5\*C * The peripheral shell (this is cooler and allows for regulation of temperature) * Communication between the two compartments is achieved via the circulation as blood is allowed to move between the two compartments, and can do so in either direction

Answer 84

* Circadian rhythms * Daily variations in body temperature occur, +/-0.5\*C * Minimum occurs in the early hours of the morning, peak in the late afternoon * Menstrual cycle * Rise of +1\*C is seen following ovulation

Answer 85

* To provide optimal conditions for cellular enzymatic reactions * When body temperature exceeds 42\*C, cellular proteins are damaged * If the temperature drops, these enzymes and proteins become less efficient/effective * Energy expenditure falls 13% for every 1\*C drop in core temperature (as drop in body temperature results in decreased enzymatic rates)

Answer 86

* This is an organism that maintains a constant body temperature through **metabolic processes** * This body temperature is often above that of the surrounding temperature * Humans are homeotherms

Answer 87

* External conditions can vary greatly and cause some challenge * Environmental conditions mean that external temperatures can vary by more than 100°C * The highest temperature recorded was 56.7°C in Death Valley, California * The lowest temperature recorded was -89.2°C at Vostok Station in Antarctica * Internal changes in temperature in response to changes in metabolism * This can range from something as simple as eating to exercise

Answer 88

* This is the temperature range over which there is no 'active' temperature gain or loss * This means that the surrounding temperature is pretty well matched with our typical heat generation * If temperature drops below this zone, we start to increase heat production to maintain homeostasis * If the temperature increases above the thermoneutral zone, we begin to actively dissipate heat and begin cooler temperature-seeking behaviours

Answer 89

* Naked = 25-30\*C * Light clothing = 18-22\*C * Affects heat loss from the body therefore reduces the range

Answer 90

* 'For the vast majority of animals, an animal's metabolic rate scales to the 3/4 power of the animal's mass' * I.e. if q₀ is the animal's metabolic rate, and M the mass, then Kleiber's law states that q₀ =~M^3/4 * Essentially, larger animal = larger metabolic rate * Named after Max Kleiber, Swiss agricultural biologist * There is still some argument over the exact value of the power, with some claiming it to be 2/3, but the constant is relatively arbitrary

Answer 91

* Most eventually ends up as heat * Metabolic reactions are frequently inefficient, and so end up releasing the byproduct of heat

Answer 92

* In calories * One calorie is the energy required to heat 1g of water by 1\*C * The Calorie (or kilocalorie) is 1000 calories

Answer 93

* Average 70kg man at rest will use 1650 kcal/day

Answer 94

* Thyroid hormone * Testosterone * Growth hormone

Answer 95

Physical exercise * ~7% of daily energy expenditure is due to 'non-exercise' activity (e.g. fidgeting, non-specific exercise, muscle tone required for holding posture) * As exercise increases/becomes more significant, there is a rapid increase in energy expenditure requirements

Answer 96

* Thyroid hormones (**thyroxine**) can raise BMR to 50-100% above normal * Total loss of thyroxine will decrease BMR to 40-60% of normal levels [EXTRA] * Adaptation of the thyroid gland is seen in people living in different geographical locations * People living in arctic regions have a BMR 10-20% above those in tropical regions, allowing an increased generation of heat to deal with the colder environment

Answer 97

* Adipose tissue rich in mitochondria * Also contain the uncoupling protein 1 (UCP-1) * Heat is generated through the dissipation of the proton gradient across the inner mitochondrial membrane via UCP-1 * Role in adults is a topic of ongoing research (particularly in isolating treatments for obesity) but it has been shown to play a vital role in neonatal thermogenesis (as neonates are unable to shiver) * Found primarily between the shoulder blades and around the kidneys

Answer 98

~10-50% increase in BMR

Answer 99

* BAT is activated in resposne to low temperatures * Stimulation occurs via the release of noradrenaline from the sympathetic nervous system * Activation acts via multiple transcription factors, including PGC1alpha and the PPARs to increase mitochondrial proliferation and fatty acid metabolism * PPARs = peroxisome proliferation activated receptor

Answer 100

~15-20% increase in BMR

Answer 101

Stefan-Boltzmann law * Emissivity is the measure of a material's effective ability to emit and absorb thermal radiation, black objects have a high emissivity, white objects do not * **NB** That the temperature difference between the object and the environment is to the power of 4, therefore has a large effect on the equation * Also note that energy can only be lost from the body via radiation if the temperature gradient favours energy loss to the surroundings (i.e. they are cooler than the body) * If the surrounding temperature increases to exceed that of the body, radiation will occur in the other direction/heat is transferred to the body

Answer 102

* **Thermogenic** * Heat is generated as a byproduct of the chemical reactions including digestion, absorption and storage * NB That the actual temperature of the food will also have an effect (e.g. food is often hot, and so will warm you up that way regardless)

Answer 103

* Fat/carbohydrate meals cause BMR to increase by ~4% * Protein heavy meals can raise BMR up to 30% (hence the meat sweats due to raised body temp after raised BMR)

Answer 104

The process of heat generation in warm-blooded animals (and some plants). Includes the actions of shivering and brown adipose tissue in animals.

Answer 105

Between 36.1-37.8\*C in normal physiological parameters NB that this can differ from the peripheral temperature, which is (understandably) cooler

Answer 106

This is heat loss due to the emission of a spectrum of electromagnetic radiation due to an object's temperature. Requires a thermal gradient.

Answer 107

* This is the removal of heat from the body by air currents (also liquids) * Initial loss is through conduction of heat to the surrounding air, causing these particles to gain heat, become less dense and rise, introducing new, cooler air to the skin and then transferring energy to the surroundings * Requires a thermal gradient

Answer 108

* Magnitude is dependent on the movement of air over the surface of the body * Conductive heat loss ∝ √wind velocity * Wind = greater movement of particles around the body, therefore windy conditions remove warmed particles from around the body/replacing them with cold particles * This increases heat loss * This explains wind chill * Rate is also heavily affected by clothing trapping air close to the skin, as this prevents the warmed air from moving away from the body * [EXTRA] Convection works on all fluids, therefore this is a principle that contributes partly in the function of wetsuits (traps warmed layer of water around the body to limit conductive and convective heat losses)

Answer 109

The transfer of heat through direct contact with objects. Requires a thermal gradient.

Answer 110

* Conduction has very little effect on heat loss in air as air is a poor conductor * Water is **25x** better at conducting heat, therefore conduction is significantly more important * Immersion in 10\*C water can lead to death in ~2 hours * Wet suits work by preventing conduction losses as air bubbles in the material act as poor conductors

Answer 111

This is the conversion of water from a liquid to a gas - energy is required to overcome the latent heat of vaporisation

Answer 112

This is the amount of energy required to convert one unit (e.g. mol or kg) of liquid into a gas For water, this value is 538kcal/kg

Answer 113

600-700ml/day, contributing 16-19 kcal/hour of heat loss

Answer 114

Between approx. 12\*C and 54\*C (this is larger than the range for the thermoneutral zone as heat loss and gain processes are considered)

Answer 115

Body heat = Metabolic Heat Production +/- Conductive, Convective and Radiant Heat Exchange - Evaporative Heat Loss

Answer 116

* The only way is through evaporative losses * Other processes require a temperature gradient (going from high to low temps) and so will actually cause heat gain if the surroundings are too warm * This means that **sweating** and **evaporative heat loss** become key at high temperatures

Answer 117

* These are methods of allowing a separation between nutritional and thermoregulatory blood flow * These vessels bypass the capillaries and can increase or decrease blood flow to the periphery to facilitate or limit heat loss respectively * Flow through shunts can be 'on' or 'off' * Blood flow through the shunts can be **100-fold** greater than nutritional requirements * Shunts are controlled by **centrally mediated alpha-1 adrenergic receptors** * Although this is augmented by local alpha-2 adrenergic receptors in the periphery

Answer 118

* Primarily through nervous feedback systems from the periphery to the CNS * Centre in the brain: temperature regulating centers in the hypothalamus

Answer 119

Hypothalamus is area through which almost all temperature regulating mechanisms work

Answer 120

* The preoptic and anterior hypothalamus operates as the body's thermostat * This part of the brain has greater sensitivity to heat than to cold

Answer 121

* Spinal cord, abdominal viscera and great veins (**deep body sensors**) * Skin * Greater sensitivity to cold than hot

Answer 122

Through sweat production and its subsequent evaporation facilitates heat loss from the body, particularly in hot conditions

Answer 123

* Sweat production can be increased by cholinergic sympathetic nerve fibres * The glands can also be stimulated by adrenaline/noradrenaline, e.g. during exercise to increase heat dissipation * This elicits the secretion of a primary fluid into the sweat duct * This fluid has a composition similar to that of plasma, just without plasma proteins * As regulation/reabsorption occurs along the tubule, if the demand for secretion increases and therefore the transit time decreases, then the time over which this secretion can be modified is rapidly decreased

Answer 124

1. Hot 2. Cold

Answer 125

* They alter their discharge frequency * Sensors are rapidly adapting/will rapidly adapt to new static levels * This means that they are bad at sensing absolute temperature, but are very good at sensing changes in temperature

Answer 126

* Two bowls of water, one quite warm (35-40\*C), the other very cold (add ice, ~0\*C) * Put a hand in each boel and leave it there for 3-4 mins, then put the hand into a bowl of water at room temperature * Hand in ice water thinks that the bowl of room temperature water is really hot, hand in bowl of warmer water thinks that room temperature bowl is very cold * This shows that the change in temperature has shaped the response, not the actual temperature, or else the hands would experience the same sensation

Answer 127

* This part of the brain collates and combines the signals from the peripheral sensors and from the central sensors (in the anterior hypothalamus) * This leads to control over the heat-conserving and heat-producing reactions of the body

Answer 128

* Uncontrolled losses from the core are minimised using insulation * Therefore a layer of fat is seen around the core to produce this insulation and to minimise heat loss from fluctuations in the core body temperature * Flow of heat from the core to the shell is also controlled through regulating blood flow

Answer 129

* These systems control the flow of heat from the core to the shell * Arteries flow in one direction, carrying warm blood from the core, in close proximity to veins flowing in the other direction, carrying cooler blood from the periphery * This allows the arteries to warm up blood returning to the core in the veins, therefore minimising heat loss at the periphery through maximising heat transfer between the warm arteries and cooler veins * This also has the added benefit of warming the veins to prevent them from taking too much heat away from the viscera * Overall, this **minimises heat loss from the core**

Answer 130

* When naked: ~12\*C to ~54\*C * When this range is exceeded, other methods of regulating core body temperature must be found * Using clothing, this range can be extended somewhat, and it is easier to deal with colder temperatures in this way

Answer 131

* Shivering can increase heat production by approx 200%, but this is small compared to exercise-induced increases which can be \>10 fold * This leads to shivering and exercise-seeking behaviours in cold conditions

Answer 132

In newborn infants - this makes them far more susceptible to the cold and they instead rely upon the action of BAT

Answer 133

* Rapid (~50Hz) and unsynchronised muscular activity * This results in ATP cross-bridge cycling to facilitate contraction and production of heat as a byproduct of the metabolic reaction * Shivering is likely to result from a feedback oscillation of the muscle spindle stretch reflex mechanism * Random oscillation is overlaid on a **centrally controlled 4-8 cycle/min** of 'waxing and waning' * E.g. shivering cycles between more and less severe bouts

Answer 134

* Posterior hypothalamus signals the need for shivering after collating inputs from peripheral receptors and central receptors (anterior and preoptic hypothalamus) * This signal is translated to the anterior motor neurons

Answer 135

* Severe form of hypothermia * Occurs when surface areas (commonly on peripheral parts of the body) freeze and there is formation of ice crystals * Commonly affects the hands, feet and ear lobes * If extensive formation of ice crystals occurs, this can lead to cell death * Therefore gangrene often follows thawing * Surgical removal of affected areas is necessary

Answer 136

* With light stimulation, almost all of the Na+ and Cl- is reabsorbed (along with a lot of water) as the secretions travel along the duct * This leads to a concentrated solution made up of **urea, lactic acid** and **potassium** * With heavy stimulation, large volumes of sweat are produced * This results in poor reabsorption of NaCl and increased fluid loss (due to the subsequently less concentrated liquid secreted)

Answer 137

2 litres/hour

Answer 138

* Individuals feel 'cold' despite core body temperature being above the normal level * This results in the recruitment of normal physiological responses to being cold, such as shivering * Upon restoration of a normal set-point, individuals experience a febrile 'rush' or 'crisis' where the body tries to restore a normal core temperature through sweating and vasodilation * This is seen at the end of illness, but there is also some evidence to suggest that this occurs cyclically throughout fever (bouts of breaking and shivering)

Answer 139

* Over 1-6 weeks of exposure to a hot climate, sweat production is upregulated * In response to the increased loss of NaCl caused by this change, the body also upregulates **aldosterone** secretion to increased Na+ reabsorption

Answer 140

Cerebral cortex provides the necessary inputs

Answer 141

* Alter the balance with external sources of radiation * If too hot: alter clothing to change emissivity of the individual * If too cold: find an alternate source of radiation, e.g. sit in front of a fire/lie out in the sun

Answer 142

* Control with clothing where possible * E.g. if hot, will remove layers, if cold, will put them on * Remember that air is a poor conductor, so trapping layers of air close to the skin can act as insulation * Cold individuals will avoid getting clothes wet, as this will increase conduction

Answer 143

* Convection can be controlled using clothing and flow of air over the skin * E.g. if hot, increasing air flow will aid heat losses via convection - this can be achieved using a fan * If cold, individuals will try and find shelter/get out of the wind

Answer 144

* If hot, can spray/douse themselves with water * This will have the same effect as sweating and facilitate transfer of energy to the surroundings through the evaporation of this added water off of the skin

Answer 145

* There are many methods, all give slightly different values depending on how close you can actually get to the body temperature * Tympanic membrane or nasopharyngeal sites can give readings relatively close to that of the central body temperature, but can be uncomfortable and risk damage * Rectal thermometers are more peripheral and therefore give a lower reading but can perhaps be tolerated for a longer time

Answer 146

* Core body temperature \>38.3\*C * Classified as severe once the temperature reaches 40\*C (heat stroke = \>40.6\*C)

Answer 147

* Core body temperature \<35\*C

Answer 148

* This is where continued exposure to cold temperatures causes the core body temperature to drop below **35\*C** * Energy expenditure **falls 13%** for every **1\*C drop** in core temperature * Loss of consciousness occurs at core body temp of **30\*C**

Answer 149

* Changes can be seen on the ECG, including bradycardia and prolongation of the PR and QT intervals * Ventricular fibrillation occurs at **28\*C**

Answer 150

* Neonates are particularly susceptible due to reduced behavioural adaptations possible (e.g. cannot put on clothes or move) and have a greater SA:Vol ratio than adults * They also cannot shiver, but do have BAT for thermogenesis * The elderly are also susceptible due to potentially reduced mobility and less efficient thermoregulatory mechanisms * Lessened mobility also makes them less able to move for heat generation and also slows the process to find and put on clothes

Answer 151

* Recovery from cold injury * Anna Båagneholm survived a skiing accident that left her trapped in freezing water under a layer of ice for 80 minutes * Took her \>2 hours to get to the hospital, core body temp. so low that doctors thought she was dead * One doctor believed that the low temp may have saved her/prevented ischaemic damage, so slowly brought her temperature back up * Restarted her heart and she somehow survived * https://www.youtube.com/watch?v=jLr15BBBtrc * Deliberate hypothermia * Used in neurosurgery while on extracorporeal ('out of body') circulation to allow circulation to be stopped for ~15 minutes without incurring ischaemic injury to tissue * Due to reduction of metabolic rate, damage by active enzymes within the ischaemic tissue is reduced * Cold cardioplegia – cooling heart with cold solution high in potassium to reduce metabolism and depolarize membrane to prevent cardiac excitation allows cardiac surgery for around an hour without ischemic damage

Answer 152

* Infection can lead to an elevation of the core body temperature set point, and therefore hyperthermia * This is caused either through the direct action of proteins/ protein breakdown products/lipopolysaccharide toxins released from bacterial cells (**pyrogens**) or by their action on macrophages, leukocytes and lymphocytes * This causes the release of cytokines (IL-1 and IL-6) * These pyrogens act to elevate **prostaglandin E2** synthesis in the hypothalamus (this raises the set point)

Answer 153

* Prostaglandin E2 (PGE2) * Acts on E2 receptors in the anterior hypothalamus to raise the set point of body temperature

Answer 154

* Drugs that act to reduce the elevated set point * Examples include: * Acetaminophen (paracetamol) * Aspirin and other NSAIDs

Answer 155

* Principally through their inhibition of the enzyme cyclooxygenase (COX), preventing the synthesis of inflammatory prostaglandins (including PGE2) * Recent studies on the mechanism of action have revealed effects independent of COX inhibition as well

Answer 156

* Heat exhaustion * Heat stroke

Answer 157

* AKA heat syncope * Occurs due to hypovolaemia following excessive sweating * Symptoms include hyperthermia, excessive sweating, dizziness, nausea, headache

Answer 158

* This is where body temperature \>40.6\*C * Thermoregulatory mechanisms fail at this point, causing the patient to become dizzy and confused * Loss of consciousness follows rapidly, then death * Treatment involves rapidly reducing body temperature * There is some disagreement over whether cold or cool water should be used * Cold water can induce vasoconstriction and shivering, which will instead elevate temperature

Answer 159

* Malignant hyperthermia is a pathological state where body temperature is overwhelmed by heat production * It is often fatal * Caused by drugs include the muscle relaxant suxamethonium and volatile anaesthetics including halothane * This is very rare (1 case per 100,000 in adult general anaesthetics) * Can be caused by mutations in ryanodine receptor 1 (RYR1), inherited in an AR fashion * **Dantrolene** is the reversal agent for MHS caused by anaesthetic agents, and so will be found in the collection of all anaesthetists * Some individuals with this condition can occasionally enter malignant hyperthermia after extreme exercise or heat stroke, as the condition is caused by over-activation of skeletal muscle and its contraction, causing excessive heat release

Answer 160

* Erection or 'bristling' of hairs along the body due to the involuntary contraction of erector pili muscles at the base of the hair follicles * This is a reflexive response to cold, shock or fear, mediated by the sympathetic nervous system * Thought to trap a layer of air close to the surface of the skin, providing some small amount of insulation, and also raises water on the hairs away from the skin

Answer 161

* Burns cause cell damage and a hypermetabolic state * This is characterised by increased energy expenditure and increased muscle protein catabolism, which can lead to a hyperthermia of sorts * If there is extensive skin loss, however, hypothermia can occur as the skin is essential for many energy- and heat-preserving behaviours * With increased energy expenditure, more heat will be lost and so burns can rapidly lead to hypothermia

Answer 162

* Stimulation of the arterial baroreceptors inhibits respiration and this has been demonstrated experimentally either by applying pressure to an isolated carotid sinus or by stimulation of the carotid sinus nerve (which contains both baroreceptor and chemoreceptor afferents). * However, this is a controversial effect that is not well defined.

Answer 163

* Hypoxia inducible factor (HIF) stimulates erythropoiesis via transcription of EPO. The result of this is increased haematocrit, which is contributed to by increased urination. * Along with increases in capillary density, this means that it is easier to oxygenate the blood in the lungs and deliver sufficient oxygen to respiring tissues.

53. Physiological Challenges Flashcards

(205 cards)