Cells as the Living Units of the Body;
The basic living unit of the body is
the cell. About 60 per cent of the adult human body is fluid, mainly a water
solution of ions and other substances. Although most of this fluid is inside
the cells and is called intracellular fluid, about one third is in the spaces
outside the cells and is called extracellular fluid.
Difference Between Intracellular and Extracellular Fluid
Definition
Intracellular Fluid: The intracellular
fluid is a fluid found in the cell membrane, containing dissolved ions and
other components, which are essential to cellular processes.
Extracellular Fluid: The extracellular
fluid is the fluid found outside of the cell, aiding the functioning of a
particular tissue.
Significance
Intracellular Fluid: The intracellular
fluid is found inside the cell.
Extracellular Fluid: The extracellular
fluid is found outside the cell.
Components
Intracellular Fluid: The intracellular
fluid comprises the cytosol.
Extracellular Fluid: The extracellular
fluid comprises blood plasma, tissue fluid, and transcellular fluid.
Sodium and Potassium Ion Concentration
Intracellular Fluid: The concentration
of sodium ions is low in intracellular fluid and the concentration of potassium
ions is high.
Extracellular Fluid: The concentration
of sodium ions is high in extracellular fluid and the concentration of
potassium ions is low.
Water
Intracellular Fluid: The intracellular
fluid comprises 55% of body water.
Extracellular Fluid: The extracellular
fluid comprises about 45% of body water.
Body Weight
Intracellular Fluid: The intracellular
fluid comprises 33% of total body weight.
Extracellular Fluid: The extracellular
fluid comprises 27% of total body weight.
Volume
Intracellular Fluid: The intracellular
fluid comprises 19 L of total body fluids.
Extracellular Fluid: The extracellular
fluid comprises 23 L of total body fluids.
Homeostasis
The term homeostasis is used by
physiologists to mean maintenance of nearly constant conditions in the internal
environment.
Essentially all organs and tissues of
the body perform functions that help maintain these constant conditions.
Extracellular Fluid Transport and Mixing System
Extracellular fluid is transported
through all parts of the body in 2 stages.
- Movement of blood through the body in the blood vessels,
- Movement of fluid between the blood capillaries band the intercellular spaces between the tissue cells.
Origin of Nutrients in the
Extracellular Fluid
Respiratory System- each time the
blood passes through the body, it also flows through the lungs.
The blood picks up oxygen in the
alveoli, thus acquiring the oxygen needed by the cells.
Gastrointestinal Tract- A large
portion of the blood pumped by the heart also passes through the walls of the
gastrointestinal tract.
Liver and Other Organs- That Perform
Primarily Metabolic Functions- The liver changes the chemical compositions of
many of these substances to more usable forms, and other tissues of the body
fat cells, gastrointestinal mucosa, kidneys, and endocrine glands.
Musculoskeletal System- The musculosk
eletal system fit into the
homeostatic functions of the body. The musculoskeletal system also provides
motility for protection against adverse surroundings, without which the entire
body.
Regulation of Body Functions
Nervous System-
The nervous system is composed of 3
major parts:
- the sensory input portion,
- the central nervous system
- the motor output portion.
Sensory receptors detect the state of
the body or the state of the surroundings.
The central nervous system is composed
of the brain and spinal cord.
Hormonal System of Regulation
Located in the body are eight major
endocrine glands that secrete chemical substances called hormones.
Hormones are transported in the extracellular
fluid to all parts of the body to help regulate cellular function.
Control Systems of the Body;
Many other control systems operate
within the organs to control functions of the individual parts of the organs;
others
operate throughout the entire body to control the interrelations between the
organs.
For instance,
---the respiratory system, operating in
association with the nervous system, regulates the concentration of carbon
dioxide in the extracellular fluid.
---The liver and pancreas regulate the
concentration of glucose in the extracellular fluid
---the kidneys regulate concentrations
of hydrogen, sodium, potassium, phosphate, and other ions in the extracellular
fluid.
Characteristics of Control Systems
Negative Feedback--Nature of Most Control Systems
Most control systems of the body act
by negative feedback.
The regulation of carbon dioxide
concentration a high concentration of carbon dioxide in the extracellular fluid
increases pulmonary ventilation.
This, in turn, decreases the
extracellular fluid carbon dioxide concentration because the lungs expire
greater amounts of carbon dioxide from the body.
In other words, the high concentration of
carbon dioxide initiates events that decrease the concentration toward normal,
which is negative to the initiating stimulus.
Conversely, if the carbon dioxide
concentration falls too low, this causes feedback to increase the
concentration. This response also is negative to the initiating stimulus
Example Negative Feedback
- Human body temperature - The hypothalamus of a human responds to temperature fluctuations and responds accordingly. If the temperature drops, the body shivers to bring up the temperature and if it is too warm, the body will sweat to cool down due to evaporation.
- Human blood pressure - When blood pressure increases, signals are sent to the brain from the blood vessels. Signals are sent to the heart from the brain and heart rate slows down, thus helping blood pressure to return to normal.
- When a human is hungry, metabolism slows down to conserve energy and allow the human to continue living with less food.
- Regulation of blood sugar in humans - When blood sugar rises, insulin sends a signal to the liver, muscles and other cells to store the excess glucose. Some is stored as body fat and other is stored as glycogen in the liver and muscles.
- Production of human red blood cells (erythropoiesis) - A decrease in oxygen is detected by the kidneys and they secrete erythropoietin. This hormone stimulates the production of red blood cells.
Positive Feedback—Can Sometimes Cause Vicious Cycles and Death
In some instances, the body uses
positive feedback to its advantage.
Blood clotting is an example of a
valuable use of positive feedback.
When a blood vessel is ruptured and a
clot begins to form, multiple enzymes called clotting factors are activated
within the clot itself.
Childbirth is another instance in
which positive feedback plays a valuable
role.
Another important use of positive
feedback is for the generation of nerve signals.
That is, when the membrane of a nerve
fiber is stimulated, this causes slight leakage of sodium ions through sodium
channels in the nerve membrane to the fiber’s interior.
----- In each case in which positive
feedback is useful, the positive feedback itself is part of an overall negative
feedback process.
For example, in the case of blood
clotting, the positive feedback clotting process is a negative feedback process
for maintenance of normal blood volume.
Also, the positive feedback that
causes nerve signals allows the nerves to participate in thousands of negative
feedback nervous control systems.
Examples of Positive Feedback
-Blood Clotting When a part of the
body is injured, it releases chemicals that activate blood platelets.
Platelets are responsible for stopping
bleeding by forming clots.
An activated platelet in turn activates more
platelets, which group together to form a blood clot.
(In individuals with hemophilia, the
blood lacks enough blood-clotting proteins, causing excessive bleeding after an
injury.)
-The Menstrual Cycle Before a woman
ovulates, the hormone estrogen is released by the ovary.
The estrogen travels to the brain, which
causes gonadotropin-releasing hormone (GnRH) to be released from the
hypothalamus and luteinizing hormone (LH) to be released from the pituitary
gland.
LH causes more estrogen to be released from
the ovary, which in turn causes an increase in GnRH and LH in the bloodstream
through positive feedback.
The rise in these hormones, along with
follicle-stimulating hormone (FSH), causes ovulation to occur.
-Labor and Childbirth The process of
labor and childbirth is perhaps the most-cited example of positive feedback.
In childbirth, when the fetus’s head
presses up against the cervix, it stimulates nerves that tell the brain to
stimulate the pituitary gland, which then produces oxytocin.Oxytocin causes the uterus to
contract.
This moves the fetus even closer to
the cervix, which causes more oxytocin to be produced until childbirth occurs
and the baby leaves the womb.
Breastfeeding
is also a positive feedback loop; as the baby suckles, the mother’s pituitary
gland produces more of the hormone prolactin, which causes more milk to be
produced.Adaptive Control---More Complex Types of Control Systems
Some movements of the body occur so
rapidly that there is not enough time for nerve signals to travel from the
peripheral parts of the body all the way to the brain and then back to the
periphery again to control the movement.
Therefore, the brain uses a principle
called feed-forward control to cause required muscle contractions.
That is, sensory nerve signals from
the moving parts apprise the brain whether the movement is performed correctly.
If not, the brain corrects the feed-forward
signals that it sends to the muscles the next time the movement is required.
Then, if still further correction is
needed, this will be done again for subsequent movements. This is called
adaptive control.
Adaptive control, in a sense, is
delayed negative feedback.
Example of adptive control
- Heat stress
- Cold stress
- Exercise
- Oxidative stress
- Body response mechanism to hypoxia
- The changes that take place under food deprivation (glycogenolysis,ketone body synthesis in starvation, etc.)
