Page 17 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

!
them throughout the body. The lung is re- Although behavioral science, sociology, and
sponsible for the exchange of gases (O 2 intake, psychology are disciplines that border on
CO 2 elimination), the liver and kidney for the physiology, true bridges between them and
excretion of waste and foreign substances, and physiology have been established only in ex-
the skin for the release of heat. The kidney and ceptional cases.
lungs also play an important role in regulating
the internal environment, e.g., water content, Control and Regulation
osmolality, ion concentrations, pH (kidney, In order to have useful cooperation between
lungs) and O 2 and CO 2 pressure (lungs) (! B).
Fundamentals and Cell Physiology long distances (circulation, respiratory tract), In other words, the organs must be subject to
The specialization of cells and organs for
the specialized organs of the body, their func-
specific tasks naturally requires integration,
tions must be adjusted to meet specific needs.
which is achieved by convective transport over
control and regulation. Control implies that a
controlled variable such as the blood pressure
humoral transfer of information (hormones),
and transmission of electrical signals in the
is subject to selective external modification,
for example, through alteration of the heart
nervous system, to name a few examples.
These mechanisms are responsible for supply
rate (! p. 218). Because many other factors
and disposal and thereby maintain a stable in-
also affect the blood pressure and heart rate,
ternal environment, even under conditions of
the controlled variable can only be kept con-
blood pressure, comparing it with the refer-
they control and regulate functions that en-
ence signal (set point), and continuously cor-
sure survival in the sense of preservation of the
1 extremely high demand and stress. Moreover, stant by continuously measuring the current
species. Important factors in this process in-
recting any deviations. If the blood pressure
clude not only the timely development of re- drops—due, for example, to rapidly standing
productive organs and the availability of fertil- up from a recumbent position—the heart rate
izable gametes at sexual maturity, but also the will increase until the blood pressure has been
control of erection, ejaculation, fertilization, reasonably adjusted. Once the blood pressure
and nidation. Others include the coordination has risen above a certain limit, the heart rate
of functions in the mother and fetus during will decrease again and the blood pressure will
pregnancy and regulation of the birth process normalize. This type of closed-loop control is
and the lactation period. called a negative feedback control system or a
The central nervous system (CNS) processes control circuit (! C1). It consists of a controller
signals from peripheral sensors (single with a programmed set-point value (target
sensory cells or sensory organs), activates out- value) and control elements (effectors) that can
wardly directed effectors (e.g., skeletal adjust the controlled variable to the set point.
muscles), and influences the endocrine glands. The system also includes sensors that continu-
The CNS is the focus of attention when study- ously measure the actual value of the con-
ing human or animal behavior. It helps us to lo- trolled variable of interest and report it (feed-
cate food and water and protects us from heat back) to the controller, which compares the ac-
or cold. The central nervous system also plays a tual value of the controlled variable with the
role in partner selection, concern for offspring set-point value and makes the necessary ad-
even long after their birth, and integration into justments if disturbance-related discrepancies
social systems. The CNS is also involved in the have occurred. The control system operates
development, expression, and processing of either from within the organ itself (autoregula-
emotions such as desire, listlessness, curiosity, tion) or via a superordinate organ such as the
wishfulness, happiness, anger, wrath, and central nervous system or hormone glands.
envy and of traits such as creativeness, inquisi- Unlike simple control, the elements of a con-
tiveness, self-awareness, and responsibility. trol circuit can work rather imprecisely
This goes far beyond the scope of physiology— without causing a deviation from the set point
which in the narrower sense is the study of the (at least on average). Moreover, control circuits
4 functions of the body—and, hence, of this book. are capable of responding to unexpected dis-
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
All rights reserved. Usage subject to terms and conditions of license.