Homeostasis: Thermoregulation

Question 1

Homeostasis

Answer 1

• Homeostasis is the process of keeping the environment inside the body fairly constant
• The important aspects of the internal environment that the body needs to regulate include:
• Core body temperature
• pH and concentration of dissolved substances in the body fluids
• concentration of glucose in the blood
• concentration of oxygen and carbon dioxide in the blood and other body fluids
• blood pressure
• concentration of metabolic wastes

Question 2

Stimulus response feedback

Answer 2

• A feedback system, or feedback loop, is a situation where the response to a stimulus modifies the stimulus.
• Things like the concentration of blood glucose and body temperature fluctuate around a normal level.
• This fluctuation is called a dynamic equilibrium.
• The point around which conditions fluctuate is called the set point.
• In homeostatic mechanisms the response has the effect of reducing or eliminating the stimulus that cause it. This is called negative feedback.
• When positive feedback occurs, the response to a stimulus reinforces and intensifies the stimulus.
• The intensified stimulus results in an even greater response, and so on.
• Obviously, positive feedback could not result in homeostasis, but there are a few situations where it does occur in the human body.
• It is important in controlling processes that must be completed quickly.

Question 3

Thermoregulation

Answer 3

• Human body temperature remains remarkably constant at about 36.9C.
• Maintaining the balance between heat production and heat loss is called thermoregulation.
• Constancy of body temperature is important because the chemical reactions occurring in cells are very heat sensitive.
• A temperature around 37C is the optimum for cellular reactions so that cells maintained at this temperature function in a stable manner.
• The heat produced from metabolic activity helps to maintain this higher level.
• However, during exercise and other strenuous activity, the increase in metabolic rate generates more heat than the body needs to keep temperature constant.
• The excess heat needs to be removed or body temperature will rise.
• Increased body temperature can cause nerve malfunction, change in the structure proteins and death.

Question 4

Heat Production

Answer 4

• The rate at which energy is released by the breakdown of food is called metabolic rate.
• Many factors, such as exercise, stress, and body temperature, affect the metabolic rate of an individual.
• Metabolic rate also increases in times of stress because of the activities of the autonomic division of the nervous system.
• Stimulation of sympathetic nerves releases noradrenaline from the nerve endings; noradrenaline increases the metabolic activity of cells.
• Strong sympathetic stimulation may cause dramatic increases in the metabolic rate, but usually for only a few minutes.

Question 5

Heat Transfer (IMPORTANT)

Answer 5

• Radiation: transfer of heat from one object to another without physical contact.
• Heat can be gained and lost to the surrounding environment through radiation. For example, standing in the sun on a cool day to warm you.
• Conduction: transfer of heat from one object to another through physical contact.
• For example, using a heat/ice pack.
• Convection: flow of air can transfer heat.
• When cool air makes contact with your body, it becomes warmed and is pushed away (rises). This is replaced by cooler air, which then does the same thing. This movement of air is called a convection current.
• The faster the flow of air, the faster the rate of heat transfer.
• Evaporation: the conversion of a liquid to a gas.
• This requires a lot of heat to change state, therefore the heat given off to cause evaporation, is lost from the body.
• This in turn makes the surface temperature of the skin cooler (it has lost heat).
• Heat loss from evaporation is loss through the evaporation of sweat on the skin through heat energy produced in the body reducing the heat of a person.
• Evaporation is ineffective in humid environment and effective in dry environments.

Question 6

Temperature Receptors

Answer 6

• The body has temperature receptors, or thermoreceptors.
• Peripheral thermoreceptors: located in the skin and in some mucous membranes.
• Central thermoreceptors: located in the hypothalamus.
• Peripheral thermoreceptors provide the hypothalamus with information about the external environment.
• There are two types of peripheral thermoreceptors:
• Cold receptors are stimulated by environmental temperatures lower than normal.
• Heat receptors detect temperatures higher than normal.
• Peripheral thermoreceptors are not efficient in regulating body temp (it’s the core temp that must be maintained).
• In addition to the central thermoreceptors located in the hypothalamus, there are others at various internal locations such as the spinal cord and the abdominal organs.
• All are connected to the hypothalamus, which is the body’s main temperature-regulating centre.
• Nerve impulses sent out by the hypothalamus control activities that help to either increase or decrease body temperature.

Question 7

The role of skin in thermoregulation

Answer 7

• The diameter of arterioles to the skin is controlled by autonomic nerves, which can act to increase or decrease flow of blood to the surface, thereby increasing or decreasing rate of heat loss (through radiation).
• When large amounts of body heat must be lost and skin blood vessels are already at maximum dilation, sweating must occur.
• The production and transport of sweat to the skin surface is stimulated by sympathetic nerves.
• Evaporation of sweat from the skin has a cooling effect: heat is removed from the skin when liquid sweat changes into vapour.
• Cooling of the skin results in cooling of the blood flowing through the skin.

Question 8

Preventing Body Temp from falling

Answer 8

• If the environmental temperature falls, or if a person goes from a warm room into a cold environment outside, the cold receptors in the skin send messages to the hypothalamus.
• The hypothalamus then sends out impulses aimed at reducing heat loss and increasing heat production so that body temperature is maintained.
• The body can respond by physiological changes (changes in body functioning) and behavioural changes.
• One physiological response is vasoconstriction of arterioles in the skin.
• Impulses from the hypothalamus stimulate sympathetic nerves that cause blood vessels in the skin to constrict.
• The skin becomes cooler because there is less warm blood flowing through it.
• Less heat will then be lost from the body surface.
• In cold conditions, heat may be produced by a second response – shivering.
• A fall in body temperature will cause the hypothalamus to send stimuli to the parts of the brain that increase skeletal muscle tone.
• These tremors are known as shivering and can increase body heat production.
• A third response initiated by the hypothalamus is the stimulation of the adrenal medulla by sympathetic nerves.
• This stimulation results in the medulla secreting adrenaline and noradrenaline into the blood.
• These hormones bring about an increase in cellular metabolism that leads to an increase in heat production.
• In cold conditions, heat may be trapped by another response – piloerection (goosebumps)
• The contraction of piloerector muscles under the skin, causes the hair to stand on end, trapping an insulating layer of air.
• This helps to prevent heat lost to the surrounding environment (radiation).
• However, this is relatively ineffective in human due to the lack of body hair.
• Behavioural responses:
• Putting on an extra jumper.
• Sheltering from a cold wind.
• Turning on a heater.
• Reducing the surface area of the body from which heat can be lost.
• The fourth response to a fall in body temperature is an increase in the production of thyroxine, which increases metabolic rate.
• This response is slower to have an effect, but it is longer-lasting than other responses.
• The small change in metabolic rate that occurs between the warm summer months and cool winter ones is a result of this response.

Question 9

Preventing body temperature from rising

Answer 9

• Excess heat needs to be lost from the body otherwise core temperature will rise.
• The following responses ensure that body temperature does not rise.
• Vasodilation of arterioles in the skin increases blood flow through the skin. The skin becomes reddish in colour, surface temperature rises, and there is greater heat loss through radiation and convection.
• Above environmental temperatures of about 28°C, sweating is needed to increase heat loss from the body.
• The cooling effect of sweating is effective only in environments that are fairly dry.
• Behavioural responses can also help to prevent body temperature from rising.
• Actions such as turning on a fan or air conditioner, removing external clothing and reducing physical activity can all help to keep temperature constant.
• In the long term there can be a decrease in metabolic rate, which means less heat is produced in the body.
• Such a decrease is brought about by reduction in the secretion of thyroxine, a response that occurs in summer when there is much less heat loss than in winter.

Question 10

Temperature tolerance

Answer 10

• When body temperature rises and the regulatory mechanisms cease it is called heat stroke and it can be very serious, or even fatal, if brain cells are affected.
• Heat exhaustion, occurs as a result of extreme sweating and vasodilation to lose heat. The loss of water in sweating reduces the volume of blood plasma and the vasodilation reduces resistance to blood flow. Blood pressure is thus reduced and output of blood from the heart decreases. The person may, therefore, collapse. Unlike heat stroke, the body temperature is almost normal.
• If a person’s core temperature falls below 33°C, the metabolic rate is so low that heat production is unable to replace the heat lost and body temperature continues to fall. This condition is called hypothermia.