Homeostasis and response

• Homeostasis is the maintenance of a constant internal environment. Mechanisms are in place to keep optimum conditions despite internal and external changes. This is needed for enzyme action and all cell functions.
• In the human body homeostasis controls :

  • Blood glucose concentration
  • Body temperature
  • Water levels
• Nervous and hormonal communication is involved in the automatic control systems, which detect changes and respond to them.
• All control systems have :

  • Receptors
  • Coordination centres
  • Effectors
• Receptors - cells that detect stimuli(changes in the environment)
• coordination centres - process the information received from the receptors, e.g. brain, spinal cord and pancreas
• Effectors - bring about responses to bring the conditions in the body back to optimum levels, e.g. muscles or glands
• The nervous system allows us to react to our surroundings, and coordinate actions in response to stimuli.
• Response in the nervous system :

  • Receptor cells convert a stimulus into an electrical impulse.
  • This electrical impulse travels along cells called sensory neurons to the central nervous system(CNS).
  • Here, the information is processed and the appropriate response is coordinated, resulting in an electrical impulse being sent along motor neurones to effectors.
  • The effectors carryout the response(this may be muscles contracting or glands secreting hormones).
• Automatic responses which take place before you have time to think are called reflexes.
• Reflexes are important as they prevent the individual from getting hurt. This is because the information travels down a pathway called a reflex arc, allowing vital responses to take place quickly. This pathway is different from the usual response to stimuli because the impulse does not pass through the conscious areas of your brain.
• How a reflex arc works :

  • A stimulus is detected by the receptors.
  • Impulse are sent along a sensory neuron.
  • In the CNS the impulse passes to a relay neuron.
  • Impulses are sent along a motor neuron.
  • The impulse reaches an effector resulting in the appropriate response
• Examples of reflex arcs are: pupils getting smaller to avoid damage from bright lights, moving your hand from a hot surface to prevent damage.
• Synapse are the gaps between two neurons.
• Here is how a synapse works:

  • When the impulse reaches the end of the first neuron, a chemical is released into the synapse.
  • This chemical diffuses across the synapse.
  • When the chemical reaches the second neuron, it triggers the impulse to begin again in the next neuron.
• Your reaction time is how long it takes you to respond to a stimulus. It can be measured with the drop ruler test.
• The human body has two communication systems - the nervous system and the endocrine system. The endocrine system sends hormones(chemical messengers) around the body. When they reach a target tissue they produce a response.
• The endocrine system is made up of glands which secrete hormones directly into the blood stream.
• Pituitary gland:

  • The master gland.
  • Secretes hormones into the blood to either have an effect on the body or act on other glands to stimulate them to produce different hormones.
• Pancreas:

  • Secretes insulin.
  • Controls blood glucose levels.
• Thyroid:

  • Secretes thyroxine
  • Controls metabolic rate, heart rate and temperature
• Adrenal gland:

  • Secretes adrenaline
  • Involved in the flight or fight response(the body's response to stressful situations)
• Ovary:

  • Secretes oestrogen
  • Is involved in the menstrual cycle and the development of female secondary sexual characteristics(different features that develop during puberty that distinguish a female from a male)
• Testes:

  • Secretes testosterone
  • Is involved in the production of sperm and the development of male secondary characteristics
• The blood transports the hormone to a target organ or tissue where it has an effect
• Compared to the nervous system, the hormonal system is much slower but it acts for longer
• The concentration of glucose in your blood needs to be kept within a certain limit because glucose is needed by cells for respiration. It is controlled by the pancreas
• Eating foods that contain carbohydrates increases the glucose levels in the blood
• If the glucose levels are too high, the pancreas produces the hormone insulin
• Insulin binds to cells in target organs (muscles and liver) causing:

  • Glucose to move from the blood into muscle cells for respiration
  • Excess glucose to be converted into glycogen which is stored in the liver
• Rigorous activity, for example exercise, uses glucose for respiration and therefore there is less in the blood
• If glucose levels decrease, the pancreas produces the hormone glucagon
• • Glucagon binds to the liver cells causing glycogen to be broken down into glucose
  • Glucose is released unto the blood, increasing the blood glucose concentration
• Your blood glucose concentration is kept constant through using the hormones insulin and glucogon. They work in a negative feedback loop.
• When blood glucose levels increase/decrease, a hormone is secreted to oppose the change.
• The action of this hormone cannot occur continually because when the blood arrives at a certain glucose concentration the other hormone is produced, resulting in the opposite effect.
• When you have diabetes you cannot control your blood glucose level
• Type 1 diabetes: the pancreas cannot produce enough insulin
• In type 1 diabetes blood glucose level can rise to a fatal amount
• In type 1 diabetes :

  • Glucose is excreted with urine and lots of urine is produced leaving the individual very thirsty