Homeostasis... THE MAINTENANCE OF STATIC OR CONSTANT CONDITIONS IN THE INTERNAL ENVIRONMENT... External Environment External Environment Internal Environment External Environment External Environment
Homeostasis is about staying the same... but... things don t stay the same... They CHANGE! Conditions change here constantly... External Environment External Environment External Environment Internal Environment External Environment Where they cannot change is here... The Internal Environment
What characteristic of life would respond to this situation?
Homeostasis is about staying alive! For the body s cells to survive and function properly, the composition and temperature of the fluids around the cells must remain much the same. An organism is said to be in homeostasis when the internal environment contains: ü The optimal concentration of gases ü The optimal concentration of nutrients ü The optimal concentration of ions and water ü At the optimal temperature
Which increases the amount of CO2 in our blood Activities such as exercise change the rate at which we breathe... Which changes the ph of the blood... Which is dangerous Potentially fatal, unless... The body responds homeostatically by changing the volumes of air we breathe and adjusting blood ph.
Homeostasis operates by means of control systems Sensory receptors Control centre Effectors
Homeostasis is characterized by Negative Feedback...response is negative to the initiating stimulus...the control system initiates a series of changes that return the factor toward a certain mean value... thus maintaining homeostasis.
Oxygen must be delivered to all cells and carbon dioxide (a waste product of cellular respiration) must be removed. Breathing (inhalation and exhalation) brings in oxygen and expels CO2. The rate of breathing is varied according to the oxygen requirement. Both gases are transported around the body in the blood; the oxygen mostly bound to hemoglobin.
All of us are under constant attack from pathogens (disease causing organisms). The body has a number of mechanisms that help to prevent the entry of pathogens and limit the damage they cause if they do enter the body. The skin, the digestive system and the immune system are all involved in limiting damage.
Food and drink must be taken in to maintain the body's energy supplies. Steady levels of energy (as glucose) is available to cells through hormonal regulation of blood sugar levels. Insulin, released by the endocrine cells of the pancreas, causes cells to take up glucose after a meal. Glucagon causes the release of glucose from the liver.
Damage to body tissues triggers the inflammatory response. There is pain, swelling, redness, and heat. Phagocytes and other white blood cells move to the injury site. The inflammatory response is started (and ended) by chemical signals (e.g. from histamine and prostaglandins) released when tissue is damaged.
The levels of water and ions in the body are maintained mainly by thekidneys, although the skin is also important. Osmoreceptors monitor the fluid and ion levels of the blood and bring about the release of regulatory hormones; the kidneys regulate reabsorption of water and sodium from blood in response to levels of the hormones ADH and aldosterone.
The body is constantly bombarded by stimuli from the environment. The brain sorts these stimuli into those that require a response and those that do not. Responses are coordinated via nervous or hormonal controls. Simple nervous responses (reflexes) act quickly. Hormonal responses take longer to produce a response and the response is more prolonged.
What about Positive Feedback? Generally --- but not always --- this is bad news for the body as it generally leads to a vicious cycle. Unlike negative feedback which tends to dampen the original stimulus, positive feedback acts to intensify that stimulus.
Positive Feedback? A system exhibiting positive feedback does the following, "A produces more of B which in turn produces more of A". A vicious cycle! The Tacoma Narrows Bridge collapsed in 1940, due to a design flaw that allowed positive feedback to dominate. Alarm or panic can spread by positive feedback among a herd of animals to cause a stampede In sociology a network effect can quickly create the positive feedback of a bank run. Some examples...
Example # 1
A panic attack... or stress of ANY kind... causes high levels of adrenaline to be released from the adrenal medulla. The body responds by preparing to either run away or fight for its life ( fight or flight response ) If no actual confrontation occurs then more anxiety results and the body responds by releasing ACTH from the anterior pituitary gland which acts on its target gland the adrenal medulla to produce yet more adrenaline.. And hence the vicious cycle continues... Positive feedback for sure!
Example # 2... Insert your own story
The normal heart pumps about 5 L of blood/ min Volume of blood (L) 6 5 4 3 Positive Feedback Negative Feedback With the withdrawal of 2L of blood the heart becomes progressively less effective in terms of its pumping effectiveness. This leads to less blood being delivered to tissues which in turn further weakens the heart making it even less able to effectively pump... A vicious cycle or positive feedback that without intervention quickly leads to eternity. 2 1 0 0 hours 1 hour 2 hours 3 hours 4 hours Note that with only 1L of blood loss the heart is capable of returning (via negative feedback) to normal function.
Example # 3 Childbirth... oxytocin a (usually) non-fatal example See explanation on next page