Homeostasis

Previous


Homeostasis is the tendency of an organism or cell to maintain a constant internal environment within tolerance limits 

Internal equilibrium is maintained by adjusting physiological processes, including:

  • Body temperature  (normally 36 - 38°C)
  • Blood pH  (normally 7.35 - 7.45)
  • Carbon dioxide concentration  (normally 35 - 45 mmHg)
  • Blood glucose concentration  (normally 75 - 95 mg / dL)
  • Water balance  (varies with individual body size)


Negative Feedback

  • Most homeostatic control mechanisms operate through a negative feedback loop
  • When specialised receptors detect a change in an internal condition, the response generated will be the opposite of the change that occurred
  • When levels have returned to equilibrium, the effector ceases to generate a response 
  • If levels go too far in the opposite direction, antagonistic pathways will be activated to restore the internal balance


Negative Feedback Loop


Positive Feedback


Homeostatic Control Systems

  • Homeostasis is maintained by the concerted effort of body systems communicating via both electrical (nervous) and chemical (hormonal) systems
  • Both nerves and hormones are specific in their actions - nerves terminate in specific parts of the organism, while hormones only produce activity in specific target cells
  • The actions of both nerves and hormones involve chemical substances - hormones are chemicals themselves, while nerves use chemicals called neurotransmitters to facilitate electrical signalling
  • Nerves tend to bring about a response very rapidly, while hormonal responses are much slower but tend to be longer lasting
  • The initiation of homeostatic responses results from an external or internal stimulus, which is detected by a specific type of receptor


Types of Receptors


Homeostatic Control via the Nervous System

Thermoregulation

Animals capable of temperature regulation within a given range are called homeotherms and maintain a constant body temperature through a negative feedback loop

  • The hypothalalmus acts as a control centre in thermoregulation by detecting fluctuations in body temperature
  • The skin also possesses thermoreceptors and relays this information to the hypothalamus, which coordinates corrective measures


When body temperature rises, the following cooling mechanisms may occur:

  • Vasodilation:  The skin arterioles dilate, bringing blood into closer proximity to the body surface and allowing for heat transfer (convective cooling)
  • Sweating:  Sweat glands release sweat, which which is evaporated at the cost of latent heat in the air, thus cooling the body (evaporative cooling)


When body temperature falls, the following heating mechanisms may occur:

  • Vasoconstriction:  The skin arterioles constrict, moving blood away from the surface of the body, thus retaining the heat carried within the blood 
  • Shivering:  Muscles begin to shake in small movements, expending energy through cell respiration (which produces heat as a by-product)


Other mechanisms through which homeotherms may regulate their body temperature include:

  • Piloerection:  Animals with furry coats can make their hair stand on end (piloerection), trapping pockets of warm air close to the body surface
  • Behavioural responses:  Animals may physically respond to environmental conditions in a bid to regulate temperature (e.g. bathing, burrowing, etc.)


Thermoregulation by the Nervous System


Homeostatic Control by the Endocrine System

Blood Glucose Regulation

The body requires volumes of glucose in order to make ATP, however the amount of ATP demand will fluctuate according to need and thus the body regulates its release of glucose into the bloodstream as high levels of glucose in the bloodstream can damage cells (creates hypertonicity)

  • Two hormones, insulin and glucagon, are responsible for controlling blood glucose concentration (they have antagonistic functions)
  • These hormones are released from different groups of cells with pancreatic pits (called the islets of Langerhans) and act principally on the liver


When blood glucose levels are high (e.g. after feeding):

  • Insulin is released from beta cells in the pancreas and causes a decrease in blood glucose concentration
  • This may involve stimulating glycogen synthesis in the liver (glycogenesis), promoting glucose uptake into the liver and adipose tissue or increasing the rate of glucose breakdown (increase cell respiration)


When blood glucose levels are low (e.g. after strenuous exercise):

  • Glucagon is released from alpha cells in the pancreas and cause an increase in blood glucose concentration
  • This may involve stimulating glycogen breakdown in the liver (glycogenolysis), promoting glucose release from the liver and adipose tissue or decreasing the rate of glucose breakdown (decrease cell respiration)

 

Blood Glucose Regulation by the Endocrine System


Homeostatic Control by Nervous and Endocrine Systems

Osmoregulation

All terrestrial animals regulate their body fluid levels by controlling the amount of water released from the body as urine

  • The medullary region of the kidneys is hypertonic and will draw water out of the collecting ducts and back into the circulating blood
  • Osmoreceptors in the hypothalamus detect water levels in the blood and coordinate the release of the neurohormone, anti-diuretic hormone (ADH) 
  • Neurohormones are hormones released from nerve cells that target distant cells (as opposed to neurotransmitters which target nearby neurons)


When blood water levels are low (e.g. dehydration):

  • More ADH is released from the posterior pituitary
  • ADH stimulates the production of aquaporins in the collecting ducts of the kidneys, making them more permeable to water
  • More water is reabsorbed into the bloodstream and less water is lost in urine


When blood water levels are high:

  • Less ADH is released from the posterior pituitary
  • Less aquaporins are produced in the collecting ducts of the kidneys, making them less permeable to water
  • Less water is reabsorbed into the bloodstream and more water is lost in urine


Osmoregulation by the Nervous and Endocrine Systems