Introduction to Human Physiology PDF

Introduction to Human Physiology maintain gradients of ions, gases, and organic molecules necessary to provide the energy to accomplish physiological work. Therefore, the properties of these barriers as they relate to the movement of particular substances are the fundamental characteris...

Introduction to Human Physiology maintain gradients of ions, gases, and organic molecules necessary to provide the energy to accomplish physiological work. Therefore, the properties of these barriers as they relate to the movement of particular substances are the fundamental characteristics responsible for the differences in composition observed between the compartments. Exchange and communication are key concepts for understanding physiologic homeostasis. This is the fundamental property driving most physiology mechanisms. For understanding physiologic homeostasis, this is the fundamental property driving most physiologic mechanisms. It would take millennia for scientists to determine what was being balanced and how that balance was achieved. Most cells were recognized as being in contact with the extracellular fluid and that this extracellular fluid was in flux as it exchanged materials with individual cells. It was also determined that common physiologic factors such as blood pressure or body temperature and bloodborne factors such as oxygen, glucose, or sodium are maintained within a predictable range despite changes in the external environment. This represented the concept that a constant internal environment was a prerequisite to good health. Homeostasis is defined as a state of reasonably stable balance among physiologic variables. Observation suggests, however, that no physiologic variable remains constant for very long. Rather, one observes dramatic swings in variables around some average value defined as the set point. So you see here the set point and the variations in blood glucose levels after meals. This occurs because homeostasis is not static but rather is a dynamic process. An example would be fluctuations in blood glucose levels during the typical day. Fluctuations observed for blood glucose often exhibit a rather large variation in the short term in comparison to long-term changes. This concept then describes homeostasis as a state of dynamic constancy. Thus, blood glucose may change in the short term, i.e., after a meal, but it is stable and predictable when averaged over the long term. When homeostasis is maintained, we refer to this as Physiology, and when homeostasis is not maintained, we have pathophysiology. General characteristics of homeostatic control systems: The activities of cells, tissues, and organs must be regulated and integrated with each other so that any change in the composition of the extracellular fluid initiates a reaction to the change. The compensatory mechanisms mediating such responses are called homeostatic control mechanisms. When considering a homeostatic control system for body temperature, we must first understand that the system is in a steady state. This is defined as a system in which a particular variable, in this case, temperature, is not changing, but a system where a constant input of heat Introduction to Human Physiology or loss of heat is required to maintain the constant temperature. Steady state is similar to equilibrium in that the variable is constant but different because energy is required to maintain steady state, whereas equilibrium is static. No energy is required to maintain a constant condition. The steady state temperature is the set point of the thermal regulatory system. This example illustrates a critical generalization about homeostasis. The stability of the internal environmental variable is achieved by the balancing of inputs and outputs. Feedback systems: The thermoregulatory example is an example of a negative feedback system. A negative feedback system detects a change in a particular variable. The system reacts by adjusting the variable back toward the set point. If the internal variable increases relative to the set point, the system will respond with processes that reduce the variable back toward the set point. Negative feedback systems are very common and represent stabilizing actions. One of the most common examples of negative feedback is allosteric regulation of metabolic pathways via end-product inhibition. The end product of a metabolic pathway will allosterically regulate an enzyme mediating one of the first reactions in that pathway. Therefore, excess end product will slow the pathway responsible for its production. So the active product then will negatively impact the ability of this enzyme to catalyze one of the initial reactions within the biochemical pathway. Positive feedback systems tend to accelerate a process, moving the variable further from the set point. Positive feedback systems tend to be destabilizing. Positive feedback systems produce a rapid and powerful effect. An example would be parturition, where the stretching of the cervix by the fetus's head will become large enough to elicit a strong reflexive increase in the contractility of the uterine body. This pushes the baby forward, which stretches the cervix more, and the cycle continues until the baby is expelled. Resetting of set points: Physiologic set points can be altered by changing external conditions or they can be reset by a change in internal physiology, such as might occur with the generation of a fever. Fever is an increase in the body's temperature set point in response to an infection. Many pathogens are at a metabolic disadvantage as temperature rises. The resetting of the body's thermostat is an adaptive mechanism to counter pathogen proliferation. Set points for some variables change in a rhythmic pattern within the body. For example, temperature is higher in the day compared to the nighttime hours. Regulatory mechanisms can sometimes be in competition with one another, and some variables may have multiple regulatory mechanisms. Active product controls the sequence of chemical reactions by inhibiting the sequence's rate-limiting enzyme, Enzyme A.