unit1 Introduction to Disease 疾病简介.ppt

What Is Disease?;Structure;1 The human body is a masterpiece of art. The more one understands the functioning of the body, the greater appreciation one has for it. Even in disease, the body is quite remarkable in attempting to right what is wrong and compensate for it. Changes constantly occur within the body, and yet a steady state called homeostasis is generally maintained.;A significant disturbance in the homeostasis of the body triggers a variety of responses that often produce disease signs and symptoms. Athletes, for example, develop abnormally high red blood cell counts due to their increased need for oxygen. This is a natural compensatory mechanism to circulate more hemoglobin, but it is a disease symptom in polycythemia, which will be discussed later.;2 An organ will often enlarge, hypertrophy, when it is required to do extra work. The heart enlarges with prolonged high blood pressure as it must continue to pump blood against great resistance. Heart muscle also hypertrophies when the valves are defective because valves that are either too narrow or too wide require extra pumping action. If one kidney fails the other enlarges to meet the needs of the body and compensate for the defective one. When blood flow to the kidneys is inadequate, the kidneys help raise the blood pressure by means of a hormonal secretion. If, however, an organ or body part is not used, it will atrophy or, that is, decrease in size or function.; 3 Blood plays several roles in maintaining homeostasis. When tissue is traumatized, injured, or becomes infected, blood flow increases to the damaged site. This is vital because the blood carries cells that are specialized to remove harmful substances and cellular debris. Other cells in the blood produce antibodies against invading organisms that cause disease.;4 Disease is the unhealthy state of a body part, a physiologic system, or the body as a whole; there is a disordered structure or function. Disease often begins at th