Chapter I Introduction
Physiologyis the science that explains how the bodies of living organisms work. Thedevelopment of human physiology as a science depended on integration of anatomywith knowledge gained from experiments on living animals and parts of animals.
FundamentalCharacteristics of Living Organism
MetabolismAll vital processes of the body involve myriad chemical reactions,which collectively constitute the metabolism. It includes all the material andenergy changes that occur in the body. All the changes fall into twocategories: catabolic and anabolic reactions. Catabolic reactions involve thebreakdown of larger, more complex molecules into smaller, simpler molecules,during which energy is released for the activity of the body. Anabolicreactions involve the synthesis of complex molecules from simpler ones.Anabolism and catabolism are two opposing activities taking placesimultaneously in the cell.
ExcitabilityChanges in the external or internal environment can induce changesin the activity of a living organism, or the environmental stimuli can elicitresponse of the tissues of the body. Response can be classified into two types:excitation and inhibition. Excitation signifies an increase in activity, suchas contraction of a muscle, acceleration of the heartbeat. Inhibition is adecrease in activity, such as the slowing of the heartbeat. The property ofliving organisms that permits them to react to stimuli is defined asexcitability. An adequate stimulus applied to excitable tissue such as muscle,nerve or gland can induce an electric change spreading along the muscle, nerveor gland, that is called "action potential". Action potential appearssimultaneously with the conduction of excitation along the nerve fiber, andalways proceeds the contraction of a muscle, thereby is regarded as anindication of excitation.
The excitability of a tissue can beassessed by measuring the intensity of a stimulus (the threshold of stimulus)that is needed for eliciting a minimal reaction or an action potential in anexcitable tissue. The higher the intensity of stimulus is needed, the lower theexcitability is, and vice versa.
ReproductionA process bywhich an animal produces one or more individuals similar to itself is calledreproduction or self-replication. The individual can survive withoutreproduction, but the species can not.
Homeostasis The extracellular fluid (ECF) iscomposed of the blood plasma and the fluid between cells---the interstial fluid(80 percent is interstial fluid and 20 percent is plasma). Interstial fluid andplasma have essentially the same composition except that plasma contains a muchhigher concentration of protein. ECF is in constant motion throughout the bodyand is rapidly mixed by blood circulation and by diffusion between the bloodand the interstitial fluid. It provides all cells with a constant environmentto live in. For this reason, the ECF is often called the internal environmentof the body. Cells are capable of living, growing and performing their specialfunctions as long as the proper concentrations of oxygen, glucose,electrolytes, amino acids, lipids and other essential substances are availablein the internal environment. The maintenance of a relatively constant internalenvironment is called homeostasis. A major part of physiology is concerned withthe manner in which each organ or tissue contributes to homeostasis.
Regulation of Bodily Functions
Nervous regulationThe nervoussystem may be regarded as the main coordinator of the activities of the body,and the main form of activity of the nervous system is reflex. Under themediation of the nervous system, a particular stimulus elicits a particularresponse. For example, burning of the fingertip elicits withdrawal of the hand;acid in the mouth elicits secretion of large amount of saliva. Most of thebasic reflex activities are stereotyped. In fact many reflexes in the adultindividual are not congenitally available, but are learned by the individualthrough life experience. For example, withdrawal of the hand may occur inresponse to the viewing of a fire without being actually burned. These reflexesare categorized as conditioned reflexes to be differentiated from unconditionedreflex, which is congenitally possessed and stereotyped in nature.
Humoral regulation Hormones or endocrine isthe main humoral factors playing a regulatory role on the body function. Someendocrine systems act independently of the nervous system. However, in manycases the endocrine system is so closely related to the nervous system that itcan be regarded as an extension of the efferent limb of the reflex arc. In thisinstance it is called neuro-humoral regulation. Compared with the nervousregulation that is characterized by rapidity and accuracy in response, humoralregulation is, generally speaking, slow in onset, diffuse in nature, and oftenlonger in duration.
Auto-regulation In certain cases a tissue oran organ can respond directly to the environmental change, depending neither onnervous nor on humoral control. For example, distension of the cardiac chamberby surplus amount of venous blood return results in an increase in thecontractile force of the cardiac muscle, thus ensuring the complete emptying ofthe ventricle. This form of regulation is called auto-regulation, forming asupplementary mechanism to the major control systems.
Feedback Control ofthe Body Function
Usually, a constancy of physiological variable requires a feedbackmechanism that feeds the output information back to the control system so as tomodify the nature of the control. Feedback mechanisms consist of two forms:negative and positive feedback.
Negative feedbackNegativefeedback signal always produce an effect opposite to that produced by theinitiating stimulus. When the output of the control system becomes excessive, adetector would send back signals to suppress the control system so as to returnthe variable toward a previously set level or the mean physiological value,thus maintaining homeostasis.
Positive feedbackIncontrast to the negative feedback mechanism that restrains the activity of thecontrol system, a positive feedback circuit intensifies the activity of thecontrol system. For example, starting of urination results in a continuous andeven stronger contraction of the urinary bladder to ensure its completeemptying.
Feed forwardFeedforward regulation anticipates changes in a regulated variable, improves thespeed of the body's homeostatic responses, and minimizes fluctuations in thelevel of the variable being regulated.
