Human Physiology 1 Lecture 2 Homeostasis Week 2

Human Physiology 1 Lecture 2 Homeostasis Homeostasis A person who is in good health may be said to be in a state of homeostasis. Homeostasis reflects the ability of the body to maintain a relatively stable metabolism and to function normally despite many constant changes. The changes t...

Human Physiology 1 Lecture 2 Homeostasis Homeostasis A person who is in good health may be said to be in a state of homeostasis. Homeostasis reflects the ability of the body to maintain a relatively stable metabolism and to function normally despite many constant changes. The changes that are part of normal metabolism may be internal or external, and the body must respond appropriately. Homeostasis Eating breakfast, for example, brings about an internal change. Suddenly there is food in the stomach, and something must be done with it. What happens? The food is digested or broken down into simple chemicals that the body can use. The protein in a hard- boiled egg is digested into amino acids, its basic chemical building blocks; these amino acids can then be used by the cells of the body to produce their own specialized proteins. Homeostasis An example of an external change is a rise in environmental temperature. On a hot day, the body temperature would also tend to rise. However, body temperature must be kept within its normal range of about 97 to 99F (36 to 38C) in order to support normal functioning. What happens? One of the body’s responses to the external temperature rise is to increase sweating so that excess body heat can be lost by the evaporation of sweat on the surface of the skin. This response, however, may bring about an undesirable internal change, dehydration. What happens? As body water decreases, we feel the sensation of thirst and drink fluids to replace the water lost in sweating. Homeostasis Homeostasis Homeostasis Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control center and 3) the effector. The receptor receives information that something in the environment is changing. The control center or integration center receives and processes information from the receptor. And lastly, the effector responds to the commands of the control center by either opposing or enhancing the stimulus. This is an ongoing process that continually works to restore and maintain homeostasis. For example, in regulating body temperature there are temperature receptors in the skin, which communicate information to the brain, which is the control center, and the effector is our blood vessels and sweat glands in our brain. Homeostasis Homeostatic regulation involves three parts or mechanisms: 1) the receptor, 2) the control center and 3) the effector. Work in groups of 3 to describe this mechanism for regulating body temperature when it is too warm For example, in regulating body temperature there are temperature receptors in the skin, which communicate information to the brain, which is the control center, and the effector is our blood vessels and sweat glands in our brain. Homeostasis Homeostasis Negative Feedback Mechanism Notice that when certain body responses occur, they reverse the event that triggered them. In the preceding example a rising body temperature stimulates increased sweating, which lowers body temperature, which in turn decreases sweating. Unnecessary sweating that would be wasteful of water is prevented. This is an example of a negative feedback mechanism, in which the body’s response reverses the stimulus (in effect, turning it off for a while) and keeps some aspect of the body metabolism within its normal range. Negative Feedback Mechanism As metabolic rate decreases, the hypothalamus (part of the brain) and pituitary gland detect this decrease and secrete hormones to stimulate the thyroid gland (on the front of the neck just below the larynx) to secrete the hormone thyroxine. Thyroxine stimulates the cellular enzyme systems that produce energy from food, which increases the metabolic rate. The rise in energy and heat production is detected by the brain and pituitary gland. They then decrease secretion of their hormones, which in turn inhibits any further secretion of thyroxine until the metabolic rate decreases again. Metabolic rate does rise and fall, but is kept within normal limits. Negative Feedback Mechanism Negative Feedback Mechanism “back towards the mean position” – negative feedback “wide oscillations are prevented” – stability  Home Heating System Vs. Negative Feedback When you are at home, you set your thermostat to a desired temperature. Let's say today you set it at 70 degrees. The thermometer in the thermostat waits to sense a temperature change either too high above or too far below the 70 degree set point. When this change happens the thermometer will send a message to the "Control Center", or thermostat which in turn will then send a message to the furnace to either shut off if the temperature is too high or kick back on if the temperature is too low. In the home-heating example the air temperature is the "NEGATIVE FEEDBACK." When the Control Center receives negative feedback it triggers a chain reaction in order to maintain room temperature Home Heating System Vs. Negative Feedback Positive Feedback Mechanism They are rare in the body and quite different from a negative feedback mechanism. In a positive feedback mechanism, the response to the stimulus does not stop or reverse the stimulus, but instead keeps the sequence of events going. A good example is child birth, in which the sequence of events, simply stated, is as follows: Stretching of the uterine cervix stimulates secretion of the hormone oxytocin by the posterior pituitary gland. Oxytocin stimulates contraction of the uterine muscle, which causes more stretching, which stimulates more oxytocin and, hence, more contractions. The mechanism stops with the delivery of the baby and the placenta. This is the “brake,” the interrupting event Positive Feedback Mechanism The rise of a fever may also trigger a positive feedback mechanism. Notice in Fig. 1–3 that bacteria have affected the body’s thermostat in the hypothalamus and caused a fever. The rising body temperature increases the metabolic rate, which increases body temperature even more, becoming a cycle. Where is the inhibition, the brake? For this infection, the brake is white blood cells destroying the bacteria that caused the fever. An interruption from outside the cycle is necessary. It is for this reason, because positive feedback mechanisms have the potential to be self- perpetuating and cause harm, that they are rare in the body. Positive Feedback Mechanism Positive Feedback Mechanism A response is to amplify the change in the variable. This has a destabilizing effect, so does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications. For example, in nerves, a threshold electric potential triggers the generation of a much larger action potential. Blood clotting and events in childbirth are other types of positive feedback. Homeostasis Throughout the Body