homeostasis and control systems

• Homeostasis = existence of a stable internal environment. Dynamic constancy of internal environment despite changes to the external
• Homeostatic regulation= adjustment of physiological systems to preserve homeostasis via two mechanisms:

  1. Autoregulation = cell, tissue or organ adjusts response to some environmental change (ie. O2 decreased in tissue, cells release chemicals from vasodilation)
  2. Extrinsic regulation = activities in the nervous or endocrine system. Environmental change leading to change in electrical or chemical messages

  • Nervous response = short-term specific response
  • Endocrine response = long-term response, hormones released into the blood
• Components of homeostatic regulation

  1. Receptor = sensitive to stimulus or environmental change
  2. Control centre = received and processes information from receptor
  3. Effector = cell or organ and responds to commands from control centre(opposes or enhances stimulation)
• Negative feedback in homeostasis

  • Minimises change, makes majority of homeostatic mechanisms
  • Maintains range rather than fixed value
  • Aims to maintain variable at its set point
  • Homeostatic set points determined by genetic factors, age, gender, general health and environmental conditions
• Example: thermoregulation

  • Control centre in hypothalamus receiving information from temperature temperature sensors in skin and hypothalamus
  • Rise in temperature:
  • Hypothalamus targets (1) effector in muscle tissue lining of walls of blood vessels supplying skin and (2) sweat glands
  • Muscle tissues dilate → blood flow through vessels near the body surface and sweat glands speed up secretion of sweat.
  • Decrease in temperature:
  • Effectors: blood vessels, sweat glands and skeletal muscles
  • Decrease blood flow to skin
  • Decrease sweating, increase shivering
• Positive feedback in homeostasis

  • Amplifies/enhances original changes
  • Found when dangerous process must be completed
  • example: blood clotting
• State of equilibrium = when opposing forces are in balance (ie. rate of heat loss = rate of heat production)
• Dynamic equilibrium = physiological systems continually adapt and adjust to changing conditions
• The nervous system

  • Central = brain
  • Peripheral = everything else (ie. somatic nerves ((skeletal muscle) allconscious) and autonomic nerves (unconscious))
  • Autonomic nerves = parasympathetic and sympathetic
• General structure of autonomic nervous system

  1. CNS: preganglionic neuron originating from spinal chrome
  2. Peripheral ganglion: postganglionic neuron
  3. Target cell: effector cell
• Differences between sympathetic and parasympathetic

  • Parasympathetic = active in vegetative situations
  • Sympathetic = active in stressful situations
  • Antagonistic effects

  
• ve feedback in hormonal control example

  *Regulation in the release of cortisol (raised blood glucose and metabolism)

  1. Hypothalamus releases corticotropin release hormone
  2. At night CRH travels to anterior pituitary gland and stimulates releaseof ACTH into blood stream
  3. ACTH to the adrenal gland
  4. Adrenal cortex to cells that make cortisol
  5. Increase in cortisol feedback to hypothalamus to stop CRH release aswell as pituitary to stop ACTH