Physiology Topics
Homeostasis is the condition of equilibrium in the body's internal environment produced by the continuous interplay of all the body's regulatory processes. An important aspect of homeostasis is maintaining the volume and composition of body fluids.
A feedback system is a cycle of events in which the status of a body condition is continually monitored, evaluated and changed. Each monitored variable is a controlled condition. Any disruption that changes a controlled condition is called a stimulus. In this context, a receptor may be defined as a body structure that monitors changes in a controlled condition and sends input in the form of nerve impulses or chemical signals to a control center. A control center sets the range of values within a controlled condition should be maintained, evaluates the input it receives from receptors and generates output commands when they are needed. An effector is a body structure that receives output from the control center and produces a response or effect that changes the controlled condition.
A negative feedback system reverses a change is a controlled condition. A positive feedback system reinforces a change in the controlled condition. Most homeostatic feedback systems are negative. A positive feedback system needs a termination mechanism. An example of negative feedback is the regulation of blood pressure. An example of positive feedback is the control of contractions during childbirth.
Body tissues can be classified into four basic types: epithelial, connective, muscle and nerve tissues. Epithelial tissue covers body surfaces, lines hollow organs and cavities and forms glands. Connective tissue protects and supports organs. Muscle tissue generates the physical force needed to make body structures move. Nerve tissue detects changes in a variety of conditions inside and outside the body, and responds by generating nerve impulses. Nerve tissue in the brain helps maintain homeostasis..
Muscle tissue consists of fibers constructed to generate force. Skeletal muscle is striated, i.e. the fibers contain alternating light and dark bands, and is voluntary because it can be made to contract or relax by conscious control. Cardiac muscle forms the bulk of the heart wall, and is striated and involuntary. Smooth muscle is located in the walls of hollow internal structures such as blood vessels, and it is non-striated and involuntary.
Nervous tissue consists of two principal kinds of cells. Neurons convert stimuli into nerve impulses and conduct the impulses to other neurons or tissues. Neuroglia do not generate or conduct nerve impulses but have a variety of other functions..
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