67. Heat balance regulation

Question 1

Central thermoregulation

Answer 1

The core and peripheral temperature of the body always represent a well-determined “current value”. The peripheral and central receptors sens this and forward it encoded as an afferent neural signal into the hypothalamic center.
The hypothalamic center as a thermostat determines a certain set point value for the core temperature. The effector systems (heat production and heat dissipation) are used to maintan body temperature near to the hypothalamic setpoint.
If the set point is higher than the current value of the core temperature, then the center commences heat producing and conserving procedures. These processes adjust the conserved and produced heat quantity untill the current value reaches the set point. If the set point value is lower than the current value reverse processes will result in heat dissipation.

Question 2

Hypothermia

Answer 2

In case of hypothermia, body temperature decreases below the controllable range. The homeothermia ceases, metabolism slows down and heart and respiratory frequency drops. It is followed then by circulatory failure with hypercapnia and hypoxia. Disorders of digestion, absorption and kidney function, and fainting, occasionally ventricular fibrillation can occur as well. These disorders lead to cold death in extreme conditions.

Question 3

The fever:

Answer 3

At the beginning of fever process the pyrogenic substance sets the hypothalamic set point to a higher temperature. This induces heat production and heat conserving processes (shivering). As the current value reaches the set point the fever temperature becomes stabilized at a higher value. After removing the pyrogenic substance the set point drops back to the previous normal value: the cooling mechanisms start to work (sweating) and the fever becomes solved.

Question 4

Pyrogens:

Answer 4

These are endotoxins of Gramnegative bacteria, which are heat stable polysaccharides of high molecular weight, membrane debris and lipopolysaccharide fragments.

Question 5

Acclimatization

Answer 5

The “relatively slow” – generally seasonal – adaptation capability to extreme environmental temperature conditions is a basic criterion for the survival of different species. Adaptation can shift the lower or higher critical temperature (shift of the thermoneutral zone) to higher or lower levels. This is the phenomenon of acclimatization.It is very likely that besides the above-mentioned processes the kinetic changes of the enzymatic reactions play an important role in the mechanisms of acclimatization. One of the manifestation is the appearance of new enzyme isoforms by enzyme induction. This can change significantly the previous activity of metabolic enzymes so it results in the suitable long-term increase or decrease of the metabolic processes.

Question 6

Hyperthermia

Answer 6

Hyperthermia is a consequence of insufficient operation of heat dissipating mechanisms mainly at high environmental temperature.

• During hyperthermia metabolic processes decrease as well. The inflow of excessively large amount of blood into the blood vessels can lead to corruption of circulation.
• This is the so called overheating shock.
• It is followed by hypercapnia, loss of water and salts, hypovolemia, and muscle seizures.
• Warm blood damages respiratory and circulatory centers and the central nervous system, as well.
• Animals bear hyperthermia harder because their body temperature is closer to the upper limit of durable temperature.
• The most sensitive species is swine, but hyperthermia is not tolerated well in horse, cattle, and chicken. The heat stress sensitivity must be taken into consideration especially at transport of animals because it can cause heat-death.