HUMANUMAN P PHYSIOLOGYHYSIOLOGY (432 pages) by Wikibooks contributors Contents Introduction . 3 CHAPTERS 4 01. Homeostasis .4 02. Cell Physiology . 14 03. The Integumentary System 35 04. The Nervous System . 54 05. Senses 81 06. The Muscular System 107 07. Blood Physiology 122 08. The Cardiovascular System .137 09. The Immune System 162 10. The Urinary System .186 11. The Respiratory System 201 12. The Gastrointestinal System 217 13. Nutrition 244 14. The Endocrine System .262 15. The Male Reproductive System 281 16. The Female Reproductive System .301 17. Pregnancy and Birth 326 18. Genetics and Inheritance . 351 19. Development: Birth Through Death 370 APPENDICES . 397 A. Answers to Review Questions 397 ABOUT THE BOOK 424 History & Document Notes .424 Authors & Image Credits 425 GNU Free Documentation License . 426 Introduction Human physiology is the study of the functioning of the normal body, and is responsible for describing how various systems of the human body work. Explanations often begin at a macroscopic level and proceed to a molecular level. In 1926, Fritz Kahn portrayed the body as a complex chemical plant, as seen in the painting on the right. This textbook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course. As such, some material is deliberately left out (but references will be provided within chapters for students wishing to learn more). Chapter 1 1HOMEOSTASIS live version ã discussion ã edit lesson ã comment ã report an error Overview he human body consists of trillions of cells all working together for the maintenance of the entire organism. While cells may perform very different functions, all the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with all that the cells need to survive (oxygen, glucose, mineral ions, waste removal, and so forth) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis.T What is Homeostasis? Homeostasis in a general sense refers to stability, balance or equilibrium. Maintaining a stable internal environment requires constant monitoring and adjustments as conditions change. This adjusting of physiological systems within the body is called homeostatic regulation. 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. A metaphor to help us understand this process is the operation of a thermostat. The thermostat monitors and controls room temperature. The thermostat is set at a certain temperature that is considered ideal, the set point. The function of the thermostat is to keep the temperature in the room within a few degrees of the set point. If the room is colder than the set point, the thermostat receives information from the thermometer (the receptor) that it is too cold. The effectors within the thermostat then will turn on the heat to warm up the room. When the room temperature reaches the set point, the receptor receives the information, and the thermostat tells the heater to turn off. This also works when it is too hot in the room. The thermostat receives the information and turns on the air conditioner. When the set point temperature is reached, the thermostat turns off the air conditioner. Our bodies control body temperature in a similar way. The brain is the control center, the receptor is our body's temperature sensors, and the effector is our blood vessels and sweat glands in our skin. When we feel heat, the temperature sensors in our skin send the message to our brain. Our brain then sends the message to the sweat glands to increase sweating and increase blood flow to our skin. When we feel cold, the opposite happens. Our brain sends a message to our sweat glands to decrease sweating, decrease blood flow, and begin shivering. This is an ongoing process that continually works to restore and maintain homeostasis. Because the internal and external environment of the body are constantly changing and adjustments must be made continuously to stay at or near the set point, homeostasis can be thought of as a dynamic equilibrium. 4 | Human Physiology Homeostasis Positive and Negative Feedback When a change of variable occurs, there are two main types of feedback to which the system reacts: ãNegative feedback: a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostasis. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signaled to increase their activity and expel more carbon dioxide. Thermoregulation is another example of negative feedback. When body temperature rises (or falls), receptors in the skin and the hypothalamus sense a change, triggering a command from the brain. This command, in turn, effects the correct response, in this case a decrease in body temperature. ãHome Heating System Vs. Negative Feedback: When you are 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. ãPositive feedback: 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. '*Harmful Positive Feedback' Although Positive Feedback is needed within Homeostasis it also can be harmful at times. When you have a high fever it causes a metabolic change that can push the fever higher and higher. In rare occurrences the the body temperature reaches 113 degrees the cellular proteins stop working and the metabolism stops, resulting in death. Summary: Sustainable systems require combinations of both kinds of feedback. Generally with the recognition of divergence from the homeostatic condition, positive feedbacks are called into play, whereas once the homeostatic condition is approached, negative feedback is used for fine tuning responses. This creates a situation of metastability, in which homeostatic conditions are maintained within fixed limits, but once these limits are exceeded, the system can shift wildly to a wholly new (and possibly less desirable) situation of homeostasis. Homeostatic systems have several properties ãThey are ultra-stable, meaning the system is capable of testing which way its variables should be adjusted. ãTheir whole organization (internal, structural, and functional) contributes to the Wikibooks | 5 Chapter 1 maintenance of balance. ãPhysiology is largely a study of processes related to homeostasis. Some of the functions you will learn about in this book are not specifically about homeostasis (e.g. how muscles contract), but in order for all bodily processes to function there must be a suitable internal environment. Homeostasis is, therefore, a fitting framework for the introductory study of physiology. Where did the term Homeostasis come from? The concept of homeostasis was first articulated by the French scientist Claude Bernard (1813-1878) in his studies of the maintenance of stability in the milieu interior. He said, All the vital mechanisms, varied as they are, have only one object, that of preserving constant the conditions of life in the internal environment (from Leçons sur les Phénonèmes de la Vie Commune aux Animaux et aux Végétaux, 1879). The term itself was coined by American physiologist Walter Cannon, author of The Wisdom of the Body (1932). The word comes from the Greek homoios (same, like, resembling) and stasis (to stand, posture). Cruise Control on a car as a simple metaphor for homeostasis When a car is put on cruise control it has a set speed limit that it will travel. At times this speed may vary by a few miles per hour but in general the system will maintain the set speed. If the car starts to go up a hill, the systems will automatically increase the amount of fuel given to maintain the set speed. If the car starts to come down a hill, the car will automatically decrease the amount of fuel given in order to maintain the set speed. It is the same with homeostasis- the body has a set limit on each environment. If one of these limits increases or decreases, the body will sense and automatically try to fix the problem in order to maintain the pre-set limits This is a simple metaphor of how the body operates--constant monitoring of levels, and automatic small adjustments when those levels fall below (or rise above) a set point