PHYSIOLOGIC RESPONSES TO COLD
1. How are responses to cold activated?
2. What is accomplished by the countercurrent flow of blood in the limbs of animals?
3. What are some behavioral responses for reducing heat loss?
4.
What is piloerection?5. Which farm animals have the lowest critical temperature?
6. What is accomplished by shivering?
7. What is the role of thyroid hormone in adaptation to cold?
Cold activates body heating mechanisms, just as excess heat activates body cooling mechanisms. With excess cooling, heat is either conserved by reducing heat loss or is generated to compensate for that which is lost. The physiologic responses to cold are activated by blood temperature and local reflexes, as are the responses,to heat.
Reduction of Heat Loss
In an attempt to reduce heat loss, animals instinctively curl up when they lie down. This behavioral response reduces the surface area exposed to the cold. To increase the insulation value of their hair or fur, piloerection occurs. In this process the hair is made to become more erect by the arrector pili muscle of the hair follicle (see Figure 13-4). With sustained exposure to cold, the hair coat thickens and the amount of subcutaneous fat increases.
In contrast to vasodilation, which occurs to accommodate heat loss, the peripheral vessels are constricted by an increase in vasoconstrictor tone. Heat is also conserved by the arrangement of the deep blood vessels that supply the legs of animals. Blood returning in the veins from the colder legs is close to the warmer blood in the arteries going to the legs. Because of the temperature differences, heat is transferred from the arteries to the veins; this decreases the gradient for heat loss from the arterial blood to the environment. This arrangement of blood vessels is known as a countercurrent system.
Increase in Heat Production
When the ability to reduce heat loss is not adequate to maintain a normal body temperature, heat must be produced.
The temperature to which body temperature decreases before heat generation begins is known as the critical temperature. Among farm animals, cattle and sheep have the lowest critical temperature, which means that they are better suited to withstand cold.Shivering is one means by which heat is generated for withstanding cold. Shivering is a generalized rhythmic contraction of muscles. Because 30% to 50% of the energy of muscle contraction is converted to heat, the seemingly spasmodic contraction of muscle serves a useful purpose.
Other methods are used to generate heat in addition to shivering. Epinephrine and norepinephrine (see Chapter 6) are both released in increased amounts in the cold. Brown fat is an important source of thermogenesis (see the later subsection Brown Fat versus White Fat). Epinephrine and norepinephrine are the stimuli for increased metabolism of brown fat. In addition to hibernating animals, brown fat is also found in newborn mammals. Epinephrine and norepinephrine have calorigenic effects with other cells as well, and the calorigenic effects are potentiated by thyroid hormone. Thyroid hormone (see Chapter 6) is secreted in increased amounts during periods of cold.
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