Topic Five - Homeostasis and Response

• Homeostasis is the regulation of conditions inside the body, to maintain a stable internal environment, in response to changes in both internal and external conditions. This is important because your cells need the right conditions in order to function properly.
• We have automatic control systems that regulate our internal environment (e.g. nervous and hormonal systems). These control systems maintain body temperature, blood glucose level and water content.
• Automatic control systems consist of 3 main components: receptors to detect and monitor factors; coordination centres like the brain and spinal cord for the nervous system and the pancreas for the hormonal system; and effectors that cause a change to bring a factor to where it should be.
• You do not have control over automatic responses.
• Your automatic control systems keep your internal environment stable using a mechanism called negative feedback. When the level of something gets too high or too low, your body uses negative feedback to bring it back to normal.
• 1. Receptor detects a stimulus (level is too high or low)
  2. The coordination centre receives and processes the information, and then organises a response.
  3. The effector produces a response, which counteracts the change and restores the optimum level.
• The effectors will just carry on producing the responses for as long as they're stimulated by the coordination centre. This might cause the level to change too much away from the ideal. Luckily the receptor detects if the level becomes too different and negative feedback starts again.
• • When your temperature is too high (above 37 degrees) your body's automatic response is sweating or vasodilation
  • When your temperature is too low the automatic responses are shivering and goosebumps.
• • shivering is contractions that release heat energy
  • goosebumps hold the hair on end so it traps air
  • vasodilation is when blood vessels open up (get bigger) and go closer to the skin to help heat energy radiate out.
• The nervous system allows humans to react to their surroundings and coordinate their behaviour.
• The nervous system is made up of the central nervous system (CNS), sensory neurons, motor neurones and effectors.
• In vertebrates, the Central Nervous System (CNS) consists of the brain and spinal cord only. In mammals, the CNS is connected to the body by sensory neurons and motor neurones.
• Sensory neurones are the neurones that carry information as electrical impulses from the receptors to the CNS.
• Motor neurons are the neurons that carry electrical impulses from the CNS to effectors.
• The effectors in the central nervous system consist of all the muscles and glands, which respond to nervous impulses.
• Receptors are the cells that detect stimuli. There are many different types of receptors such as taste receptors on the tongue and sound receptors in the ears. They form part of larger, complex organs e.g. the retina of the eye is covered in light receptor cells.
• The CNS is a coordination centre, it receives information from the receptors and then coordinates a response that is carried out by effectors.
• Some responses need to be very fast to prevent injury or to control basic functions that we don't need to think about e.g. blinking. Instead of going to a conscious part of the brain, the impulse travels along a sensory neuron which goes to the spinal cord or through an unconscious part of the brain. This speeds up reaction time.
• Synapses are the connections between two neurons. The nerve signal is transferred by chemicals which diffuse across the gap. These chemicals then set off a new electrical signal in the next neuron.
• Reflexes are rapid, automatic responses to certain stimuli that don't involve the conscious part of the brain. e.g. your pupils automatically get smaller when a bright light is shone on them, to prevent damage.
• The passage of information in a reflex (from receptor to effector) is called a reflex arc.
• The neurons in reflex arcs go through the spinal cord or an unconscious part of the brain. Impulses are sent along a sensory neuron to a relay neuron in the CNS.
• Relay neurons connect sensory neurons to motor neurons
• When you are stung by a bee...

  The stimulus (sting) is detected by receptors, impulses are sent along a sensory neuron to a relay neuron in the CNS. The impulses go from the sensory to the relay neuron through a synapse and then from the relay to the motor neuron through another synapse. The impulse then travels along the motor neurone to the effector. The muscle (effector) contracts and moves your hand away from the bee.
• Reaction time is the time it takes to respond to a stimulus. It can be affected by factors such as age, gender or drugs.
• Ruler test

  1. The person being tested should sit with their arm resting on the edge of a table (to stop them moving during the test).
  2. hold a ruler vertically between the thumb and forefinger. Make sure that the zero end of the ruler is level with the thumb and finger then let go without warning
  3. The person being tested should try to catch the ruler. Record the number at the top of the person's thumb after they have caught the ruler.
  4. Repeat the test and calculate a mean.
• To investigate the effect of caffeine on reaction times, carry out the ruler drop test on the person then repeat it ten minutes after they have drunk a caffeine drink.
• Reaction time can be measured with a computer test. This removes the possibility of human error and is more precise as computers can measure in milliseconds. Also in the ruler test the catcher can learn a pattern and predict the drop this is not possible with a computer program.
• Hormones are chemical molecules released directly into the blood. They target particular cells in target organs and control things in organs and cells that need constant adjustment. They are chemical messengers and are secreted by glands. They have long-lasting effects.
• The pituitary gland produces many hormones that regulate body conditions. It is sometimes called the 'master gland' because these hormones act on other glands, directing them to release hormones.
• The thyroid produces thyroxine, which is involved in regulating things like the rate of metabolism, heart rate and temperature.
• The adrenal gland produces adrenaline which is used to prepare the body for a fight or flight response.
• The pancreas produces insulin, which is used to regulate blood glucose level.
• The testes produce testosterone which controls puberty and sperm production in males.
• The ovaries produce oestrogen, which is involved in the menstrual cycle in females.
• Nerves have very fast actions and act for a short time on a precise area. Hormones are slower and act for a long time in a more general way
• Eating food that contains carbohydrates puts glucose into the blood from the gut. The normal metabolism of cells removes glucose from the blood. Excess glucose can be stored as glycogen in the liver and in the muscles.
• The level of glucose in the blood must be kept steady. Changes are monitored and controlled by the pancreas, using the hormones insulin and glucagon, in a negative feedback cycle.
• When blood glucose is too high, insulin is added. It removes glucose from the blood into the liver and muscle cells. In response to insulin, the liver converts glucose into glycogen. Insulin is a hormone.
• When blood glucose is too low, glucagon is secreted by the pancreas and added to the bloodstream. It then goes into the liver and triggers the liver to begin converting stored glucose (glycogen) into a usable format and be released into the bloodstream.