Thermoregulate

Question 1

What is the range of temperatures to support mammalian life?

Answer 1

27-45 C (below 27-29 cardiac fibrillation and death occur, above 45 can cause fatal brain lesions. Below 34C mammals lose the ability to regulate their temperature)

Question 2

How do homeotherms maintain a constant temperature despite the environmental temp?

Answer 2

Requires a higher metabolic rate and therefore higher energy intake

Question 3

Core vs. Peripheral temperature. What is rectal temp compared to core? What changes the range of temperatures that an animal can tolerate?

Answer 3

Usually a bit lower. Animals in temperate climates have a narrower range than animals in more extreme climates.

Question 4

How much does shivering increase the basal metabolic rate?

Answer 4

up to 4 x basal metabolic rate for short periods

Question 5

What increases the basal metabolic rate in the body?

Answer 5

Thyroxine and the effect of catecholamines on fat. Newborn mammals have brown fat (specialized vascular mitochondrial rich fat located between the scapulae; more sensitive to catecholamine induced metabolism; these cells are largely uncoupled and mainly produce heat and very little ATP)

Question 6

What are the sources of heat in the body?

Answer 6

remote from the skin. Liver, heart, brain, limb muscles.
Body tissues are poor conductors (especially fat)- hence the effectiveness of blubber as an insulator. The main type of heat transfer from these tissues is by convection to the circulation.

Question 7

Circulatory convection, what is it? How else can the body cool?

Answer 7

Mechanism allows preferential redistro of heat to conserve core temperature. This can protect the brain and major viscera. Alternatively heat can be preferentially directed to the skin to promote coolings by:

1) dilation of arterioles of the skin vascular beds
2) opening the artiovenous anastamoses in the limbs, ears, and muzzle

Question 8

Countercurrent exchanges

Answer 8

When environmental temp is high, superficial veins are used for venous return. When it is cold, deep veins are used.

Question 9

Carotid rete?

Answer 9

Cartoid artery forms a rete (An interwoven network of nerves, blood vessels or passageways) bathed in a venous sinus that drains the nasal cavity. This cools the blood going to the brain and is especially important during exercise.

Question 10

What is another method of countercurrent exchange in horses?

Answer 10

Air filled guttural pouches of horses surround the internal carotid artery and are believed to have similar function to carotid rete.

Question 11

What are the mechanisms of heat loss?

Answer 11

1) convection- body warms surrounding air or water (rate of loss by convection can be increased by forced convection- where the limbs are moved to move air or water over the skin surface- a breeze or current does this– especially bad for juveniles or small animals)
2) conduction- body warms a surface in direct contact with it (not a major form of heat loss except where poor husbandry or prolonged anaesthesia on a stainless steel table or cage)
3) radiation- body radiated infrared radiation (if ambient temperature approaches body temperature, evaporation is the only effective mechanism for heat loss- this depends on air humidity)
4) evaporation- of respiratory secretions, sweat, saliva

Question 12

How can rate of loss of convection be reduced?

Answer 12

Piloerection (goose bumps)
thickness of hair coat
reducing the exposed SA by curling in a ball or huddling together

Question 13

What controls reabsorption of Na and Cl?

Answer 13

Aldosterone

Question 14

What is sweating under control of?

Answer 14

Sympathetic cholinergic fibres

Question 15

What is panting?

Answer 15

Movement of small tidal volumes over the respiratory dead space. Animals doing this also engorge respiratory and oral mucosa and increase salivation. Because only ventilating dead space, hyperventilation and resp alkalosis largely avoided.

Question 16

How does horses and cattle lose heat?

Answer 16

Sweat is the major form

Question 17

Dogs lose heat?

Answer 17

Pant

Question 18

Sheep lose heat?

Answer 18

sweat and pant

Question 19

Lagomorphs, rabbits?

Answer 19

Smear saliva or water on their fur

Question 20

Where are the central receeptors located?

Answer 20

Hypothalamus. Sensors area lso located throughout the body which feed back to the hypothalamus

Question 21

What sensors predominate?

Answer 21

Cold sensors and central receptors over peripheral sensors. (a rise in core temp increase cooling mechanisms much more than a similar rise in peripheral receptors)

Question 22

How are small changes of set point handled?

Answer 22

Vaso-motor mechanisms

Question 23

How are large changes to set point handled?

Answer 23

Evaporative cooling or thermogenesis

Question 24

What does chronic cold do?

Answer 24

Increase thyroxine production

Question 25

Causes of temperature derangements

Answer 25

Fever (acute phase response), heat stroke, exercise induced hyperthermia, Malignant Hyperthermia (Pathologic and Pharmacologic Hyperthermia)

Question 26

How is fever initiated?

Answer 26

Initiated by exogenous pyrogens that activate endogenous pyrogens. The pyrogens enter the hypothalamus and cause the endothelial cells to produce additional prostaglandins E2 and other arachidonic acid metabolites that reset the body’s setpoint.

Question 27

Why fever?

Answer 27

Fish and reptiles seek higher temps to raise their body temp, fever enhances leucocyte activity, reduces morbidity and mortality from many infectious diseases, many viruses are heat sensitive and cannot replicate in high temps

Question 28

Exogenous pyrogens

Answer 28

Infectious agents: bacterial, fungi, viruses, etc.

Nonmicrobial agents: soluble antigen-antibody complexes, bile acids, pharmacological agents, etc.

Question 29

Endogenous pyrogens

Answer 29

Cytokines released from the immune system, some neoplastic cells also release cytokines

Question 30

Heat stroke

Answer 30

Hyperthermia associated with systemic inflammatory response leading to a syndrome of multi-organ dysfunction in which encephalopathy predominates. In other words the hallmark of heat stroke is severe CNS derrangement (comatose, poorly responsive, disorientated, depends on the stage- if you have this and high temp- then you have to assume heat stroke)

Question 31

What is heat stroke a result of?

Answer 31

Inadequate heat dissipation, larger breed dogs, obesity, brachycephalic breeds predisposed, may occur rapidly in a closed environment

Question 32

Heat stroke consequences?

Answer 32

Once temperature exceeds 41.5C to 42.5C cellular function is seriously impaired and organ damage occurs. there is an exaggerated acute phase response and alteration of heat shock proteins. This leads to the inflammatory cascade, multi-organ failure and death.
* oxygen consumption exceeds oxygen delivery
* cellular energy production cannot keep up with need
* cellular function and integrity deteriorate
** Renal failure (acute, uremia)
**Hepatic failure (hypoglycemia, hyperbilirubinemia)
**GI failure (epithelial desquamation, endotoxin absorption, haemorrhage)
** DIC (thrombosis and bleeding)
(hypoxemia, metabolic acidosis, hyperkalemia, myocardial failure, skeletal muscle cytolysis (break down), tachycardia and hypotension, tachypnoea and hyperventilation, cerebral oedema- waste products and vasodilation of the brain- also getting blood vessel volume to go down)

Question 33

How do you treat heat stroke?

Answer 33

Total body cooling (owner calls- hose the animal down- allows for evaporation), does not respond to anti-pyretic agents, appropriate supportive care is also required for successful treatment
** coagulation problems- one way to test how severe

Question 34

What is the pathway of exercise induced hyperthermia?

Answer 34

Similar pathway to heat stroke, causes include inappropriate amounts of exercise and secondary to seizures including seizures from eclampsia (seizure because of low calcium). How high is the temp? Do I need to do active cooling now?

Question 35

Active cooling

Answer 35

Quick variations aren’t great. Rectal temp lags behind core temperature. Temp at 42C- cool it do 40C instead of regular temp because core temp is lagging behind.

Question 36

Malignant hyperthermia

Answer 36

Rare. Rapid and relentless progressive increase in body temp. Has to do with intracellular calcium metabolism. Pigs and people- dogs, cats, and horses (grey hounds). Hard to treat. Can be triggered by drugs. Pathological lesions affecting hypothalamus can also cause hyperthermia. These include neoplasia curshing the hypothalamus

Question 37

Hypothermia

Answer 37

Heat loss exceeds production. Primary hypothermia- caused by excessive exposure to low env. temp. Secondary hypothermia- result of disease, trauma, surgery, or drug-induced (happens sooner than primary)

Question 38

Secondary hypothermia

Answer 38

<32C is critical (i.e. in surgery)

Question 39

Hypothermia CV and resp effects

Answer 39

Initially increased HR and BP from catecholamine release. Vasodilation leading to hypotension and low CO. Oxygen curve shifts left- Hb does not give up O2 as easily- hypoxia. Slower HR eventually because the heart is cold and not depolarizing properly. Lower stimulus to breathe and shallowered. Pulmonary oedema. Bronchopneumonia. Mucociliary action decreased.

Question 40

Hypothermia neurological

Answer 40

decreased mentation, unconsciousness, cerebral metabolism decreases,

Question 41

Hypothermia metabolic effects

Answer 41

Peripheral vasoconstriction, body perceives increased BV, decreases production of ADH and leads to diuresis. Can cause dehydration and azotemia. If this continues- renal blood flow decreases, glomerular filtration decreases and along with ischemia, acute renal failure can occur.

• decrease liver activity- if patient is cold during surgery, can mean that patient wont’ get rid of anaesthetic agents as quickly so post op recovery is prolonged.
• Acidosis from respiratory depression (hypoventilation) and tissue hypoxia and increased muscle activity from shivering
• primary immune functions are impaired through in animals mild hypothermia has not been found to be a risk factor in post op infection
• Coagulation stops working properly- this can be bad in surgery- looks like they are bleeding from a lot of places- likely not to do with clotting factors- likely the patient is just cold.
• DIC can occur from impairment of inhibitors of fibrinolysis

Question 42

Hypothermia treatment

Answer 42

Rewarm them slowly. Don’t heat extremities (vasodilation and then lose heat that way).