Chapter 31 Homeostasis and the Organization of the Animal Body 31 1 Homeostasis How Do Animals Regulate Their Internal Environment The cells of the animal body cannot survive if the internal environment constantly deviates from a narrow range of acceptable states Homeostasis is the process by which an organism maintains its internal environment within a narrow range of conditions for optimal cell function in response to a changing external environment The internal environment is maintained in a state of dynamic constancy Although the word homeostasis implies a static unchanging state the internal environment is actually in a state of dynamic constancy The body actively adjusts to ongoing internal and external changes to maintain constant conditions Examples of conditions within an animal s body that are regulated by homeostatic mechanisms include the following Temperature Water and salt concentrations hydration Glucose concentrations blood sugar pH acid base balance O2 and CO2 concentrations respiration Why are cells so particular about their surroundings Animal cells are constantly generating and using large quantities of ATP to sustain life processes Continuous supplies of high energy molecules primarily glucose and O2 are required to carry out the reactions that generate ATP Protein enzymes needed to make ATP must maintain a specific 3 D structure This structure is maintained by hydrogen bonds which can be disrupted if the environment is too hot salty acidic or basic Animals vary in their homeostatic abilities Scientists classify animals according to their major source of body warmth Endotherms generate most of their heat through metabolic reactions Includes birds and mammals Ectotherms derive body heat from the environment and maintain this heat by occupying a constant environment or by behavioral activities such as basking in the sun Includes reptiles amphibians fishes and invertebrates Warm Blooded or Cold Blooded Fig 31 1 Feedback systems regulate internal conditions There are two types of feedback systems Negative feedback where the output is opposite to the input in the system Positive feedback where the output is the same as the input and is enhanced Negative feedback reverses the effects of changes The most important mechanism governing homeostasis is negative feedback where a change in the environment causes responses that feed back and counteract the change The overall result of negative feedback is a return of the system to its original condition There are three principal components to negative feedback systems A sensor which detects the current condition A control center which compares that condition to the desired state called the set point An effector which produces an output that restores the desired condition Negative feedback systems that control the heating of a home can be compared to those that control body temperature Home temperature is regulated by negative feedback In a home the sensor is a thermometer the control center is a thermostat and the effector is a heater The thermometer detects the room temperature and sends that information to the thermostat where the actual temperature is compared to the set point of the desired temperature Negative Feedback Maintains Homeostasis Stimulus cold Condition room temperature Produces an output heat that counteracts the deviation from the set point Measured by Effector heater Sensor thermometer Sends the measurement to If the temperature deviates from the set point sends a signal to Control center thermostat with the set point a Controlling the temperature in a house during cold weather Fig 31 2a Negative feedback maintains body temperature The temperature control center is located in the hypothalamus a part of the brain that controls many homeostatic responses The set point for most people is between 97 and 99 F Nerve endings in the hypothalamus abdomen skin and large veins act as temperature sensors and transmit this information to the hypothalamus If the body temperature falls below the set point the hypothalamus activates effector mechanisms that raise body temperature These mechanisms include shivering blood vessel constriction and increased metabolic rate releases heat When body temperature is restored the sensors signal the hypothalamus to switch off the actions that generate and conserve heat Negative Feedback Maintains Homeostasis Stimulus cold Condition body temperature Generates heat that raises the body temperature Effectors e g skeletal muscles fat stores If the body temperature is lower than the set point activates Measured by Sensor temperature receptors in the body Sends the measurement to Control center hypothalamus in the brain b Controlling body temperature during cold weather Fig 31 2b The body s temperature controls system can also act to lower body temperature if it rises over the set point The hypothalamus sends out signals that cause the blood vessels leading to the skin to dilate allowing warm blood to flow to the skin where heat can be radiated out to the air Sweat glands secrete fluid cooling the body by evaporating water from the skin Fatigue and discomfort cause the body to slow down so the body generates less heat Positive feedback enhances the effects of changes In positive feedback a change produces a response that intensifies the initial change Positive feedback is relatively rare in biological systems but occurs during childbirth Childbirth is an example of positive feedback Labor contractions force the baby s head against the cervix at the base of the uterus causing it to stretch and open Stretch receptor neurons in the cervix signal the hypothalamus The hypothalamus releases a hormone called oxytocin which stimulates more frequent and stronger uterine contractions Stronger contractions cause the baby s head to stretch the cervix even more causing the release of more oxytocin Delivery of the baby relieves the pressure on the cervix halting the positive feedback cycle 31 2 How Is the Animal Body Organized The animal body is based on a simple organizational hierarchy of structures Animal tissues are composed of similar cells that perform a specific function Organs are structures that perform complex functions and include two or more interacting tissue types Organ systems consist of two or more interacting organs that function in a coordinated manner Cells Tissues Organs and Organ Systems Tissues epithelial connective Cells epithelial cells Organ stomach arteriole venule
View Full Document
Unlocking...