L2 - homeostasis and feedback

• homeostasis:

  -the maintenance of a stable internal environment within a narrow range despite environmental changes
• what internal conditions must be kept constant in the human body:

  -body temperature

  -blood glucose concentration

  -blood salt concentration

  -water potential of blood

  -blood pressure

  -$CO_2$ concentration
• homeostasis is not a specific fixed amount but a normal range exists around a set point

  -for example, body temperature has a set point of 37*C and the normal range is between 35 and 40*C

  
• advantages of having a normal fluctuation parameter rather than a fixed point:

  -if homeostasis was to operate at a fixed point, deviation from the set point would immediately lead to damage and death

  -having a range allows deviation to occur without immediate death such as when we have fever or in early stages of hypothermia
• homeostasis is a dynamic process. it is the ability to return to the set point as quickly as possible so that a balanced equilibrium is reached. the more efficient the homeostatic mechanism, the more the organism is independent of its external environment and can operate in less hospitable environments thus enabling a greater rate of survival
• enzyme activity:

  -enzymes operate best over a specific range of conditions

  -by maintaining pH and temperature in the body all enzyme linked reactions proceed efficiently
• cell size:

  -changes in the water potential of the blood will affect the amount of water in the tissue fluid and cells

  -this could cause animal cells to desiccate or swell and burst
• independence from external conditions:

  -animals with a constant internal environment can maintain a constant level of activity regardless of their environment
• homeostasis is effectively a series of responses to stimuli co ordinated by cell signalling, the requirements are very similar to reflexes. the standard response pathway is:

  stimulus -> receptor -> communication pathway (cell signalling) -> effector -> response
• receptor:

  -able to detect changes in the internal or external environment of the organism

  -e.g. beta cells in the pancreas detect glucose
• effector:

  -a muscle or a gland containing cells that bring about a change in response to the stimulus

  -e.g. the pancreas responds to changes in glucose levels
• sensory receptors:

  -these may be on the surface of the body to monitor external environment (such as temperature receptors) or internal receptors to monitor changes within internal environment
• communication system/controller:

  -communicate via neuronal or hormonal pathways using cell signalling

  -this system coordinates the communication between receptor and effector
• effector cells:

  -muscles or glands that carry out a response (feedback)
• types of homeostatic mechanisms:

  -positive feedback

  -negative feedback
• negative feedback:

  -a change away from the optimum causes a return to the original value

  -same method of control as a home central heating system

  
• postitive feedback:

  -a change away from the optimum causes an increase in the original change

  -positive feedback destabilises systems and is dangerous except for a few scenarios where a small stimulus causes a large and rapid response

  -for example, if body temperature was controlled by positive feedback; a drop in temp would cause a greater drop and a rise in temp would cause a greater rise
• positive feedback examples:

  -breast feeding

  -propagation of an action potential

  -release of oxytocin during childbirth

  -mentrual cycle

  -blood clotting
• negative feedback examples:

  -temperature control

  -blood pressure

  -blood water level

  -blood glucose level

  -calcium ion level

  -mentrual cycle
• useful positive feedback:

  -in neurones; a stimulus causes a small influx of sodium ions. positive feedback leads to a further increase in sodium ions entering, allowing further "signalling"

  -in labour; oxytocin hormone causes uterine wall contraction. positive feedback leads to release of more oxytocin therefore increasing size and frequency of uterus contractions
• example of positive feedback; OXYTOCIN CONCENTRATION:

  -stimulus is baby moving further into birth canal

  -receptor is cervix of uterus stretched so sends nerve impulses to hypothalamus

  -set point is 'normal' oxytocin concentrations

  -effector is posterior pituitary releases oxytocin so uterus contracts more vigorously

  -controller feedback loop is positive feedback

  -positive feedback cycle is broken at birth so stretching of cervix lessens