ECF CONSTITUENTS REGULATED IN HOMEOSTASIS, CONTROL
SYSTEMS, ROLES OF BODY SYSTEMS
The actual environment of the cells of the body is the interstitial component of the ECF. Normal cell
function depends on the constancy of this fluid. Several regulatory mechanisms have evolved to maintain
it. The various physiologic arrangements which serve to restore the normal state of this internal
environment of cells are collectively called homeostasis. The buffering properties of the body fluids and
the renal and respiratory adjustments to the presence of excess acid or alkali are examples of homeostatic
mechanisms. There are countless other examples, and a large part of physiology is concerned with
regulatory mechanisms that act to maintain the constancy of the internal environment. Many of these
regulatory mechanisms operate on the principle of negative feedback; deviations from a given normal set
point are detected by a sensor, and signals from the sensor trigger compensatory changes that continue
until the set point is again reached (Ganong, 2005).
MUST BE REGULATED TO MAINTAIN HOMEOSTASIS
CHARACTERISTICS OF CONTROL SYSTEMS
Negative Feedback Nature of Most Control Systems
Most control systems of the body act by negative feedback. The response is negative to the initiating
stimulus. Therefore, in general, if some factor becomes excessive or deficient, a control system initiates
, negative feedback, which consists of a series of changes that return the factor toward a certain mean
value, thus maintaining homeostasis.
Positive feedback can sometimes cause vicious cycles and death. Positive feedback can sometimes be
useful.
FLUID EXCHANGE AND CELLULAR COMMUNICATION
Fluid Exchange between the ICF and ECF
Water moves freely and often rapidly between the various body fluid compartments. Two forces
determine this movement: hydrostatic pressure and osmotic pressure. Hydrostatic pressure from
pumping of the heart (and the effect of gravity on the column of blood in the vessel) and osmotic
pressure exerted by plasma proteins (oncotic pressure) are important determinants of fluid movement
across the capillary wall. By contrast, because hydrostatic pressure gradients are not present across the
cell membrane, only osmotic pressure differences between ICF and ECF cause movement of fluid into
and out of cells.
Osmotic pressure differences between ECF and ICF are responsible for movement of fluid between these
compartments. Because the plasma membrane of cells contains water channels (aquaporins), water can
easily cross the membrane. Hence, a change in the osmolality of either ICF or ECF results in rapid
movement (i.e., minutes) of water between these compartments. Thus, except for transient changes, the
ICF and ECF compartments are in osmotic equilibrium.
In contrast to water, the movement of ions across cell membranes is more variable from cell to cell and
depends on the presence of specific membrane transport proteins (Koeppen & Stanton, 2008).
How can this knowledge about fluid exchange help?
This is an example. Neurosurgical procedures and cerebrovascular accidents (strokes) often result in the
accumulation of interstitial fluid in the brain (i.e., edema) and swelling of neurons. Because the brain is
enclosed within the skull, edema can raise intracranial pressure and thereby disrupt neuronal function,
eventually leading to coma and death. The blood-brain barrier, which separates the cerebrospinal fluid
and brain interstitial fluid from blood, is freely permeable to water but not to most other substances. As
a result, excess fluid in brain tissue can be removed by imposing an osmotic gradient across the blood-
brain barrier. Mannitol can be used for this purpose. Mannitol is a sugar (molecular weight of 182 g/mol)
that does not readily cross the blood-brain barrier and membranes of cells (neurons, as well as other
cells in the body). Therefore, mannitol is an effective osmole, and intravenous infusion results in the
movement of fluid from brain tissue by osmosis (Koeppen & Stanton, 2008).
SYSTEMS, ROLES OF BODY SYSTEMS
The actual environment of the cells of the body is the interstitial component of the ECF. Normal cell
function depends on the constancy of this fluid. Several regulatory mechanisms have evolved to maintain
it. The various physiologic arrangements which serve to restore the normal state of this internal
environment of cells are collectively called homeostasis. The buffering properties of the body fluids and
the renal and respiratory adjustments to the presence of excess acid or alkali are examples of homeostatic
mechanisms. There are countless other examples, and a large part of physiology is concerned with
regulatory mechanisms that act to maintain the constancy of the internal environment. Many of these
regulatory mechanisms operate on the principle of negative feedback; deviations from a given normal set
point are detected by a sensor, and signals from the sensor trigger compensatory changes that continue
until the set point is again reached (Ganong, 2005).
MUST BE REGULATED TO MAINTAIN HOMEOSTASIS
CHARACTERISTICS OF CONTROL SYSTEMS
Negative Feedback Nature of Most Control Systems
Most control systems of the body act by negative feedback. The response is negative to the initiating
stimulus. Therefore, in general, if some factor becomes excessive or deficient, a control system initiates
, negative feedback, which consists of a series of changes that return the factor toward a certain mean
value, thus maintaining homeostasis.
Positive feedback can sometimes cause vicious cycles and death. Positive feedback can sometimes be
useful.
FLUID EXCHANGE AND CELLULAR COMMUNICATION
Fluid Exchange between the ICF and ECF
Water moves freely and often rapidly between the various body fluid compartments. Two forces
determine this movement: hydrostatic pressure and osmotic pressure. Hydrostatic pressure from
pumping of the heart (and the effect of gravity on the column of blood in the vessel) and osmotic
pressure exerted by plasma proteins (oncotic pressure) are important determinants of fluid movement
across the capillary wall. By contrast, because hydrostatic pressure gradients are not present across the
cell membrane, only osmotic pressure differences between ICF and ECF cause movement of fluid into
and out of cells.
Osmotic pressure differences between ECF and ICF are responsible for movement of fluid between these
compartments. Because the plasma membrane of cells contains water channels (aquaporins), water can
easily cross the membrane. Hence, a change in the osmolality of either ICF or ECF results in rapid
movement (i.e., minutes) of water between these compartments. Thus, except for transient changes, the
ICF and ECF compartments are in osmotic equilibrium.
In contrast to water, the movement of ions across cell membranes is more variable from cell to cell and
depends on the presence of specific membrane transport proteins (Koeppen & Stanton, 2008).
How can this knowledge about fluid exchange help?
This is an example. Neurosurgical procedures and cerebrovascular accidents (strokes) often result in the
accumulation of interstitial fluid in the brain (i.e., edema) and swelling of neurons. Because the brain is
enclosed within the skull, edema can raise intracranial pressure and thereby disrupt neuronal function,
eventually leading to coma and death. The blood-brain barrier, which separates the cerebrospinal fluid
and brain interstitial fluid from blood, is freely permeable to water but not to most other substances. As
a result, excess fluid in brain tissue can be removed by imposing an osmotic gradient across the blood-
brain barrier. Mannitol can be used for this purpose. Mannitol is a sugar (molecular weight of 182 g/mol)
that does not readily cross the blood-brain barrier and membranes of cells (neurons, as well as other
cells in the body). Therefore, mannitol is an effective osmole, and intravenous infusion results in the
movement of fluid from brain tissue by osmosis (Koeppen & Stanton, 2008).
