Negative Feedback | Biology for Grade 10 PDF Download

Body Temperature and the Thermoregulatory Centre

Homeostasis

Greg Foot explains the role of the hormone ADH on the permeability of the kidney tubules - regulating the water levels in the body

The conditions inside our body must be carefully controlled if it is to function effectively. Homeostasis is the maintenance of a constant internal environment in the body. The nervous system and hormones are responsible for controlling this.

The body control systems are all automatic, and involve both nervous and chemical responses. It has many important parts, including:

• Receptors detect a stimulus, which is a change in the environment, such as temperature change.
• Coordination centres in the brain, spinal cord and pancreas. They receive information from the receptors, process the information and instigate a response.
• Effectors, such as muscles or glands create the response. Glands often release a hormone, which would restore the optimum condition again.

Body Temperature

Too hot

• Sweat glands in the skin release more sweat. The sweat evaporates, transferring heat energy from the skin to the environment.

Blood vessels leading to the skin capillaries become wider - they dilate - allowing more blood to flow through the skin, and more heat to be lost to the environment. This is called vasodilation.

Too cold

• Skeletal muscles contract rapidly and we shiver. These contractions need energy from respiration, and some of this is released as heat. Blood vessels, which lead to the skin capillaries, become narrower - they constrict – which allows less blood to flow through the skin and conserve the core body temperature. This is called vasoconstriction.

The hairs on the skin also help to control body temperature. The hairs lie flat when we are warm, and rise when we are cold.
If we are too cold nerve impulses are sent to the hair erector muscles which contract. This raises the skin hairs and traps a layer of insulating air next to the skin.
Skin hairs lie flat when we are hot and stand upright when we are cold
The control of body temperature is an example of a negative feedback mechanism. It regulates the amount of:

• shivering (rapid muscle contractions release heat)
• sweating (evaporation of water in sweat causes cooling)
• blood flowing in the skin capillaries
  
  Negative feedback in temperature regulation

Vasoconstriction and Vasodilation

These diagrams show the processes that take place when vasoconstriction and vasodilation occur.

Vasoconstriction – a response to being too cold
Generally when the body temperature is too low a variety of processes happen: vasoconstriction, sweating stops and shivering starts.

Vasodilation – a response to being too hot
When the temperature is too high, different processes happen: Vasodilation, sweat production, which both transfer energy from skin to the environment, resulting in a cooling effect.

Higher Tier Only

• Negative feedback mechanisms in homeostasis help to maintain conditions in the body within an optimal narrow range; any movement away from ideal conditions results in changes occurring which bring them back
• This involves detecting that the level of a substance or a condition has gone above or below normal levels, which triggers a response to bring the level back to normal again
• Blood glucose level and core body temperature control are examples of negative feedback

Adrenaline

Higher tier only

• Adrenaline is produced by the adrenal glands in times of fear or stress
• The adrenal glands are located just above the kidneys (they also make cortisol)
• In response to stressful or scary situations, the brain triggers the release of adrenaline from the adrenal glands
• Adrenaline increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’
  • Increased glucose and oxygen are needed by the cells for respiration to release energy; the delivery of more enables more energy to be released (to fuel the muscles to move/run away for example!)

Thyroxine

Higher tier only

• Thyroxine is a hormone that is released from the thyroid gland (which is located in the neck)
• Thyroxine has a number of important roles in the body:
  • It stimulates the basal metabolic rate (BMR); this is the speed at which chemical reactions occur in the body when it is at rest
  • It also stimulates protein synthesis in cells, which is important for growth and development
• Thyroxine levels are also controlled by negative feedback; with levels of TSH (thyroid-stimulating hormone) released from the pituitary gland responsible for maintaining normal levels in the bloodstream
  • If the level of thyroxine is too high; the release of TSH is inhibited, so less thyroid is released from the thyroid gland
  • If the level of thyroxine falls below a normal level, the release of TSH from the pituitary gland is increased, which stimulates the thyroid to release more thyroxine
• Two conditions related to the thyroid gland are:
  • Hyperthyroidism, caused by an overactive thyroid gland secreting too much thyroxine into the bloodstream which causes an increase in BMR and protein synthesis
  • Hypothyroidism caused by an underactive thyroid gland secreting too little thyroxine into the bloodstream which can lead to heart and nerve problems, and death

Exam Tip: You should be able to interpret and explain simple diagrams of negative feedback control in the exam, recognising what happens when a change away from the normal level is detected.