Homeostasis

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Homeostasis

• Definition
• Regulating Blood Glucose Level
• Source
• Raven P. and G. Johnson 1992 Biology. Third Ed.
  United States Mosby Year Publishing

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Homeostasis

• Mechanisms to ensure the maintenance of a
  constant internal condition
• Temperature, pH, glucose concentration and
  oxygen must stay very constant for cell to
  function efficiently (Raven, 1992)
• To maintain homeostasis the body must constantly
  monitor itself and act to correct any deviation
  feedback loop

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Feedback Loops

• Feedback loops are important components of the
  nerve impulse labor and delivery and resistance
  to disease
• Negative Feedback
• Changes that are detected and reversed involves
  negative feedback mechanisms
• Positive feedback
• Occurs when a disturbance is accentuated

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Negative Feedback

• Prevents the departure of a controlled variable,
  e.g glucose level, pH, temperature, blood
  pressure, from its normal value or setpoint
• In negative feedback the actual value of a
  controlled variable is continuously compared with
  the set point
• Changes in the controlled variable trigger
  responses that tend to oppose the change and
  restore the variable to its setpoint

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• Stimulus (input)
• Receptor Integrator Effector
• Response leads to change. Change is feedback.
• In negative feedback, the response of the system
• cancels of counteracts the original stimulus

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Positive Feedback

• Set in motion a chain of events that intensify an
  original condition, after a limited time, the
  intensification reverses the change
• Associated with instability in a system

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Example of Positive Feedback

• Childbirth (stimulus) fetus exerts pressure on
  the
• wall
  of its mothers uterus
• Hypothalamus (integrator)
• Production of oxytocin (effector)
• Muscles contract exerting pressure on the fetus
• Fetus is expelled

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Regulating Blood Glucose Level

• Excess glucose is absorbed by liver cells which
  convert it to glycogen
• When blood glucose levels drop below normal level
  (set point) the liver breaks down glycogen
  liberating glucose to the bloodstream
• Glucose level in the fluid surrounding cells
  changes little over time, even if the bodys
  intake of glucose is concentrated within a short
  period of time

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The Endocrine System

• The endocrine system is a regulatory system that
  includes a set of glands that release chemical
  messengers -hormones- directly into body fluids.
• Neurosecretory cells in endocrine organs and
  tissues secrete hormones.
• Chemical signals bind to specific receptor
  proteins (plasma membrane proteins) within target
  cell or on their surface and initiate a metabolic
  pathway.
• Only target cells respond.

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The Role of the Endocrine and Nervous System in
Homeostasis

• Some animals have neurosecretory cells in their
  brain that secrete hormones into the blood.
• There are several chemicals in the body that
  serve as hormones of the endocrine system and as
  signals in the nervous system.
• Epinephrine functions as the so-called fight or
  flight hormone and as a neurotransmitter that
  transmits messages between neurons.
• Epinephrine is produced by the adrenal medulla,
  an endocrine gland. (Campbell et al,
  p.893-94)

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Endocrine and Nervous System Interaction

• Example 1 release of milk
• Suckling (stimulus)
• stimulate sensory cells in the nipples
• nervous signals to Hypothalamus
• Hypothalamus stimulates the Pituitary gland
• Pituitary releases Oxytocin

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• Example 2
• Ca level rises above set point (stimulus)
• Thyroid gland releases calcitonin
• Blood calcium level falls
• If Ca level falls below setpoint the Parathyroid
  gland release PTH
• Blood Ca level rises