RED BLOOD CELL MASS
The total red blood cell mass (RBCM), where mass refers to the quantity of material, in the circulatory system is regulated very closely by the body. Not only does the body work to make sure that an adequate number of red cells is always available to provide sufficient tissue oxygenation, but the body must also work very hard to make sure that there are not too many RBCs, so that the cells do not become so concentrated that they impede or clog blood flow. Therefore, RBC production is controlled very tightly by the body in a process called negative feedback (Figure 3). Most control systems of the body respond to the body's needs through this kind of process. What it means is that if the body senses too much of something (for example, if the body senses too much oxygen availability), the body will respond by causing a reaction in the opposite direction by decreasing something (for example, a decrease in RBC production). Conversely, if the body senses too little oxygen, the negative feedback mechanisms will cause the body to increase RBC production. So you can really think of a negative feedback mechanism as more of an "opposite reaction" system. In other words, if there is MORE of something than needed, it makes LESS of something, which will cause the body to return to its homeostatic condition. If there is LESS of something than needed, it makes MORE of something else, which will cause the body to return to homeostasis. This concept should become clearer as we discuss the specifics about RBC production next.
|
|
Figure 3. a) A rise in some factor of the internal environment (X) is detected by a sensor. The sensor activates an effector (generally, nerves or hormones), which causes a decrease in this factor (iX). In this way the factor returns to its initial level and homeostasis is maintained. The circled numbers indicate the sequence of events. b) A negative feedback hop in which a decrease in some factor of the internal environment (lX) is reversed by the actions of an effector. c) Negative feedback loops (indicated by negative signs) work to maintain stability in our bodies within a normal range. |