Objectives To understand homeostasis in more depth To

Objectives �To understand homeostasis in more depth. �To compare and contrast positive and negative feedback. �To list examples of the two types of feedback in our body.

Principles of Homeostasis. . . THE MAINTENANCE OF STATIC OR CONSTANT CONDITIONS IN THE INTERNAL ENVIRONMENT. . . External Environment Internal Environment External Environment

Homeostasis is about staying the same. . . but. . . things don’t stay the same. . . They CHANGE! Conditions change here constantly. . . External Environment Internal Environment External Environment Where they cannot change is here. . . The Internal Environment

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 (“interstitial fluid”) must remain much the same. • An organism is said to be in homeostasis when the internal environment contains: üThe optimal concentration of gases üThe optimal concentration of nutrients üThe optimal concentration of ions and water üAt the optimal temperature

Which increases the amount of CO 2 in our blood Activities such as exercise change the rate at which we breathe. . . Which changes the p. H of the blood. . . • Which is dangerous • Potentially fatal, unless. . . The body responds homeostatically by changing the volumes of air we breathe and adjusting blood p. H with buffers (HCO 3 , Hb and others) Our lab will be on this!!

Homeostasis operates by means of control systems • Sensory receptors • Control center • 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.

Maintaining Homeostasis The various organ systems of the body act to maintain homeostasis through a combination of hormonal and nervous mechanisms. In everyday life, the body must regulate respiratory gases, protect itself against agents of disease (pathogens), maintain fluid and salt balance, regulate energy and nutrient supply, and maintain a constant body temperature. All these must be coordinated and appropriate responses made to incoming stimuli. In addition, the body must be able to repair itself when injured and be capable of reproducing (leaving offspring).

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 CO 2. 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 the kidneys, 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.

Negative Feedback �Scenario: You are outside enjoying your last moments of summer on the beach DTS… unfortunately, you fall asleep! Your body temperature starts to rise… what does your body do to maintain homeostasis? ? ? �Something called “thermoregulation”

Increase in body temperature leads to increase in blood temperature. Increase in blood temperature detected by receptors in the hypothalamus. . . it is the hypothalamus that causes various responses in the body designed to lower temperature. Blood vessels carrying blood to the skin dilate, carrying more blood to the surface. Heat loss from the blood by radiation Sweat glands become active and produce sweat Loss of heat by radiational cooling and through evaporation of sweat lead to a decrease in blood temperature (negative feedback) Decrease in blood temperature will be detected by the receptors in the hypothalamus which will decrease dilation of blood vessels in the skin and stop sweat production.

Example # 1 Temperature Control

Example # 2 Blood Sugar Control

**What would be the consequences if this negative feedback mechanism for thermoregulation failed? ** • This would result in positive feedback • A positive feedback mechanism reinforces the existing state, i. e. would cause a further increase in temperature. • A further increase in temperature would cause enzymes to work faster, increasing heat production and leading to denaturation of enzymes and ultimately death.

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, in response to perturbation, acts to increase the magnitude of the perturbation. That is, "A produces more of B which in turn produces more of A". A vicious cycle! In contrast, a system that responds to a perturbation in a way that reduces its effect is said to exhibit negative feedback. 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

The normal heart pumps about 5 L of blood/ min Volume of blood (L) 6 Positive Feedback Negative Feedback With the withdrawal of 2 L 2 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 of positive feedback that without intervention quickly leads to eternity. 1 Note that with only 1 L of 5 4 3 0 0 hours 1 hour 2 hours 3 hours 4 hours blood loss the heart is capable of returning (via negative feedback) to normal function.

Example # 3 Childbirth. . . oxytocin See explanation on next page a (usually) non-fatal example

Now this is somewhat of an exception in our examples of positive feedback. Depending on your point of view, you see, positive feedback in this case results in nothing more disastrous than a wee crying baby. What happens is that labour is set in motion by contractions of the walls of the uterus. These contractions are stimulated by the hormone oxytocin (a posterior pituitary hormone). More contractions result in more oxytocin release causing more and deeper contractions until the little baby is finally born. Bravo for positive feedback! If the uterus is not contracting sufficiently to expel a fully developed fetus, commercial preparations of oxytocin are sometimes used to stimulate uterine contractions, thus inducing labour. Also, such preparations are often administered to the mother following childbirth to ensure that the uterine muscles will contract enough to squeeze broken blood vessels closed, minimizing the danger of haemorrhage.

So what have we learned today? �What is homeostasis and how is it important? �What is negative feedback? ◦ Give an example �What is positive feedback? ◦ Give an example