CHAPTER 1 SUMMARY
Introduction
•Physiology is the study of biological functions, which at the highest level include selforganization using energy and raw materials from the surroundings, maintaining integrity in the
face of disturbances (homeostasis), and reproduction.
•Physiology is the result of evolution. Consequently, there are two levels of scientific explanation
for biological functions: the mechanistic (or “how it works”), and the evolutionary (“how did it
get to be this way?”).
•Teleological explanations for physiological processes assume that every organismal feature is an
adaptation with a useful purpose. This is often a valid approach, but because of the constraints
of evolution, not all features are logical or optimized.
•Physiology is inherently integrative, requiring knowledge of physics, chemistry, genomics and
molecular biology, anatomy, and evolution.
•Comparative physiology provides broad insights into understanding life’s functions by studying
all types of organisms.
Methods in Physiology
•The hypothetico-deductive method is the most widely accepted version of “the scientific
method.” It consists of asking questions and making initial discoveries, proposing testable and
falsifiable explanations called hypotheses, then making predictions from those hypotheses and
testing the predictions.
•Physiological experiments use techniques from the molecular to the behavior level in order to
integrate organismal functions across all levels of biology..
Levels of Organization in Organisms
•An organism is composed of an interactive society of cells, which are the basic units of both
structure and function. Each cell performs basic functions essential for its own survival, such as
obtaining O2 and nutrients, eliminating wastes, synthesizing needed cellular components,
reacting to changes in the surrounding environment, controlling movement of materials within
the cell and between the cell and its environment, and reproducing.
•In multicellular organisms, each cell performs an additional specialized activity that contributes
to the function of the whole organism, and that is usually an elaboration of one of the basic cell
functions. Animal cells are highly organized into functional groupings, with cells of similar
structure and specialized activity organized into tissues. There are four primary types of tissue:
(1) muscle tissue, which is specialized for contraction, movement and force generation; (2)
nervous tissue, which is specialized for initiation and transmission of electrical impulses for
rapid communication; (3) epithelial tissue, which lines and covers various body surfaces and
cavities and also forms secretory glands; and (4) connective tissue, which connects, supports,
and anchors various body parts.
•Tissues are further organized into organs, which are structures composed of several types of
primary tissue that act together to perform one or more functions. Organs make up body
systems, which are collections of organs that perform related functions and interact to
accomplish a common activity essential for survival of the whole body. Organ systems, in turn,
compose the whole body.
Size and Scale
•Larger organisms have lower surface-area-to-volume ratios that smaller ones. This has major
consequences for many features such as diffusion, metabolism, and limb size.
Homeostasis and Regulated Change (Rheostasis)
•Homeostasis refers to the maintenance of a dynamic steady state within the internal environment
(the ECF, extracellular fluid) in which the multicellular organism’s cells live. The factors of the
internal environment that are often homeostatically maintained in animals are its (1)
concentration of nutrient molecules, (2) concentration of O2 and CO2, (3) concentration of
waste products, (4) pH, (5) concentration of water, salt, and other electrolytes, and (6) volume
and pressure. Some animals also maintain (7) temperature.
•Most intrinsic and extrinsic control systems generally operate on the principle of negative
feedback: a change in a regulated variable triggers a response that drives the variable in the
opposite direction of the initial change, thus opposing the change. Often there is a built-in set
point that indicates the ideal state. Effector control can be antagonistic (that, is can correct for
disturbances in both directions), and can be behavioral as well as based on individual internal
organs.
•Inadequacies in basic negative feedback systems can be improved with feedforward or
anticipation systems (which take corrective action before a disturbance occurs) and
acclimatization systems (which improve an existing system response to chronic or long-term
change). Some organisms use enantiostasis, achieving functional homeostasis by countering
change in one physiological state with an offsetting change in another.
•Not all states must be homeostatic all of the time. Useful regulated changes (rheostasis) can be
simply turned on or off as needed, or regulated by reset systems (which alter the set point of a
negative feedback system) or by positive feedback (which acts to enhance a change once it has
started).
•Pathophysiological states ensue when one or more of organisms systems fail to function properly
so that an optimal internal environment can no longer be maintained. Serious homeostatic
disruption leads to death.
Organization of Regulatory and Organ Systems
•Regulation is hierarchically arranged from cell to organ to whole-animal levels. Cells are able to
regulate some of their own states and processes, without suffering from the delays that would
occur if they had to await commands from a distant “higher” integrator. However, higher
integrators are needed to coordinate responses of multiple organs, for needs that often override
those of individual cells.
•Control systems that regulate body systems’ various activities to maintain homeostasis can be
classified as (1) intrinsic controls, which are inherent compensatory responses of an organ to a
change, and (2) extrinsic controls, which are responses of an organ that are triggered by factors
external to the organ, primarily by nervous and endocrine systems.
•The functions performed by most of the animal body systems—which fall in the categories of
Control, Support and Movement, Maintenance, and Reproductive systems--are often directed
toward maintaining homeostasis. Each body system’s functions ultimately depend on the
specialized activities of the cells composing the system. Thus, homeostasis is essential for each
cell’s survival, and each cell contributes to homeostasis.