Standard Set 9. Physiology (Homeostasis)
From the individual cell to the total organism, each functioning unit is organized according to homeostasis,
or how the body and its parts deal with the changing demands while maintaining a constant internal
environment. In 1859 noted French physiologist Claude Bernard described the difference between the
internal environment of the cells and the external environment in which the organ-ism lives. Organisms are
shielded from the variations of the external environment by the â€œconstancy of the internal milieu.â€�
This â€œsteady stateâ€� refers to the dynamic equilibrium achieved by the integrated functioning of all the
parts of the organism.
American physiologist Walter Cannon called this phenomenon homeostasis, which means â€œstanding still.
â€� All organ systems of the human body contribute to homeostasis so that blood and tissue constituents
and values stay within a normal range. Students will need supportive review of the major systems of the
body and of the organ components of those systems (see Standard Set 2, â€œLife Sciences,â€� for
grade five in Chapter 3 and Standard Set 5, â€œStructure and Function in Living Systems,â€� for grade
seven in Chapter 4). As the prime coordinators of the bodyâ€™s activities, the nervous and endocrine
systems must be examined and their interactive roles clearly defined.
9. As a result of the coordinated structures and functions of organ system, the
internal environment of the human body remains relatively stable (homeostatic)
despite change sin the outside environment. As a basis for understanding this
9. a. Students know how the complementary activity of major body systems provides cells with
oxygen and nutrients and removes toxic waste products such as carbon dioxide.
The digestive system delivers nutrients (e.g., glucose) to the circulatory system. Oxygen molecules move
from the air to the alveoli of the lungs and then to the circulatory system. From the circulatory system
glucose and oxygen molecules move from the capillaries into the cells of the body where cellular
respiration occurs. During cellular respiration these molecules are oxidized into carbon dioxide and water,
and energy is trapped in the form of ATP. The gas exchange process is reversed for the removal of
carbon dioxide from its higher concentration in the cells to the circulatory system and, finally, to its
elimination by exhalation from the lungs.
The concentration of sugar in the blood is monitored, and students should know that sugar can be stored
or pulled from reserves (glycogen) in the liver and muscles to maintain a constant blood sugar level. Amino
acids contained in proteins can also serve as an energy source, but first the amino acids must be
deaminated, or chemically converted, in the liver, producing ammonia (a toxic product), which is
converted to water-soluble urea and excreted by the kidneys. Teachers should emphasize that all these
chemicals are transported by the circulatory system and the cells. Organs at the final destination direct
these chemicals to their exit from the circulatory system.
9. b. Students know how the nervous system mediates communication between different parts of
the body and the bodyâ€™s interactions with the environment.
An individual becomes aware of the environment through the sense organs and other body receptors (e.g.,
by allowing for touch, taste, and smell and by collecting information about temperature, light, and sound).
The body reflexively responds to external stimuli through a reflex arc (see Standard 9.e in this section). (A
reflex arc is the pathway along the central nervous system where an impulse must travel to bring about a
reflex; e.g., sneezing or coughing.) Students can examine the sense organs, identify other body receptors
that make them aware of their environment, and see ways in which the body reflexively responds to an
external stimulus through a reflex arc.
Hormones work in conjunction with the nervous system, as shown, for example, in the digestive system,
where insulin released from the pancreas into the blood regulates the uptake of glucose by muscle cells.
The pituitary master gland produces growth hormone for controlling height. Other pituitary hormones have
specialized roles (e.g., follicle-stimulating hormone [FSH] and luteinizing hormone [LH] control the
gonads, thyroid-stimulating hormone [TSH] controls the thyroid, and adrenocorticotropic hormone
[ACTH] regulates the formation of glucocortiÂ¬coids by the adrenal cortex). This master gland is itself
controlled by the hypothalamus of the brain.
9. c. Students know how feedback loops in the nervous and endocrine systems regulate
conditions in the body.
Feedback loops are the means through which the nervous system uses the endoÂ¬crine system to regulate
body conditions. The presence or absence of hormones in blood brought to the brain by the circulatory
system will trigger an attempt to regulate conditions in the body. To make feedback loops relevant to
students, teachers can discuss the hormone leptin, which fat cells produce as they become filled with
storage reserves. Leptin is carried by the blood to the brain, where it normally acts to inhibit the appetite
center, an example of negative feedback. When fat reserves diminish, the concentration of leptin
decreases, a phenomenon that in turn causes the appetite center in the brain to start the hunger stimulus
and activate the urge to eat.
9. d. Students know the functions of the nervous system and the role of neurons in transmitting
Transmission of nerve impulses involves an electrochemical â€œaction potentialâ€� generated by gated
ion channels in the membrane that make use of the countervailing gradients of sodium and potassium ions
across the membrane. Potassium ion concentration is high inside cells and low outside; sodium ion
concentration is the opposite. The sodium and potassium ion concentration gradients are restored by an
active transport system, a pump that exchanges sodium and potassium ions across the membrane and uses
ATP hydrolysis as a source of free energy. The re-lease of neurotransmitter chemicals from the axon
terminal at the synapse may iniÂ¬tiate an action potential in an adjacent neuron, propagating the impulse to
a new cell.
9. e. Students know the roles of sensory neurons, interneurons, and motor neurons in sensation,
thought, and response.
The pathways of impulses from dendrite to cell body to axon of sensory neuÂ¬rons, interneurons, and
motor neurons link the chains of events that occur in a reflex action. Students should be able to diagram
this pathway. Similar paths of neural connections lead to the brain, where the sensations become
conscious and conscious actions are initiated in response to external stimuli. Students might also trace the
path of the neural connections as the sensation becomes conscious and a response to the external stimulus
is initiated. Students should also be able to idenÂ¬tify gray and white matter in the central nervous system.
9. f.* Students know the individual functions and sites of secretion of digesÂ¬tive enzymes
(amylases, proteases, nucleases, lipases), stomach acid, and bile salts.
To bring about digestion, secretions of enzymes are mixed with food (in the mouth and as the food
proceeds from the mouth through the stomach and through the small intestines). For example, salivary
glands and the pancreas secrete amylase enzymes that change starch into sugar. Stomach acid and gastric
enzymes begin the breakdown of protein, a process that intestinal and pancreatic secretions continue.
Lipase enzymes secreted by the pancreas break down fat molecules (which contain three fatty acids) to
free fatty acids plus diglycerides (which contain two fatty acids) and monoglycerides (which contain one
fatty acid). Bile secreted by the liver furthers the process of digestion, emulsifying fats and facilitating
digestion of lipids. Students might diagram the digestive tract, labeling important points of secretion and
tracing the pathways from digestion of starches, proteins, and other foods. They can then outline the role
of the kidney nephron in the formation of urine and the role of the liver in glucogenesis and glycogenolysis
(glucose balance) and in blood detoxification.
9. g.* Students know the homeostatic role of the kidneys in the removal of nitrogenous wastes
and the role of the liver in blood detoxification and glucose balance.
Microscopic nephrons within the kidney filter out body wastes, regulate water, and stabilize electrolyte
levels in blood. The liver removes toxic materials from the blood, stores them, and excretes them into the
bile. The liver also regulates blood glucose.
9. h.* Students know the cellular and molecular basis of muscle contraction, including the roles
of actin, myosin, Ca+2 , and ATP.
Controlled by calcium ions and powered by hydrolysis of ATP, actin and myoÂ¬sin filaments in a
sarcomere generate movement in stomach muscles. Striated muscle fibers reflect the filamentous makeup
and contraction state evidenced by the banding patterns of those fibers. A sketch of the sarcomere can be
used to indicate the functions of the actin and myosin filaments and the role of calcium ions and ATP in
9. i.* Students know how hormones (including digestive, reproductive, osmoregulatory) provide
internal feedback mechanisms for homeostasis at the cellular level and in whole organisms.
Hormones act as chemical messengers, affecting the activity of neighboring cells or other target organs.
Their movement can be traced from their point of origin to the target site. The feedback mechanism works
to regulate the activity of hormones and promotes homeostasis.