I D NO. 35951
•Water accounts for about 60% of the total body
weight in a normal adult, making it the most
abundant constituent of the human body.
• In terms of volume, the total body water in a man
of average weight (70 kg) is roughly 42 L.
•Water provides the medium for the solubilization
and passage of a multitude of nutrients, both
organic and inorganic, from the blood to the cells
and for the return of metabolic products to the
• It also serves as the medium in which the vast
number of intracellular metabolic reactions takes
WATER IN THE PROCESS OF TRANSPORT
Substances produced in organisms often need to be
transported to other parts of the organism.
In humans, blood is used to transport
food, hormones, oxygen, waste products and so on, and
similarly in plants, sap is used to transport food
and other substances.
Both of these mediums for transport (blood and sap)
are mainly water, as this is the solvent which can dissolve
the products to be transported.
Blood allows oxygen to be transported to tissues, and
waste products to be quickly removed.
It also facilitates the transport of hormones, to control
WATER AS A LUBRICANT
Bones meet at joints, and at
these joints lubrication must
be provided to make sure
the bones do not scrape
against each other causing
and enabling free easy
movement by reducing
A synovial membrane at
joints encloses a fluid
called synovial fluid,
which acts at the lubricant.
Water is a major part of this
Many internal organs have watery fluids
surrounding them to give protection, lubrication,
and possibly other functions.
In humans the brain contains cerebro-spinal fluid,
and the lungs have pleural fluid in pleural
Eyes contain aqueous and vitreous humours to
maintain the shape of the eye, help focusing, and to
allow nutrients, oxygen and wastes to diffuse in and
out of the eye.
When a fetus is developing, it is protected by a
watery fluid, known as amniotic fluid.
WATER IN THE PROCESS OF TEMPERATURE
Water has a high specific heat capacity - the approximate
value is 4200J/kg °C. This means that to raise the
temperature of 1kg of water by 1°C, 4200J of energy is
required, so the temperature of water is not easily changed.
Mammals sweat when their body temperature is too high.
Sweat consists of mainly water, and the large amount of
energy required to evaporate the sweat from the surface of
the organisms is taken from the organism, thus giving a large
WATER IN THE ROLE OF
• Each joint within the body carries with it a
thin layer of carpet.
• This natural protection cushions the
areas between the bones.
• When the body is deprived of water,
causing dehydration, arthritis can
• Water also provides protection along the
lining of the gastrointestinal tract, and
protects the spinal cord from damage and
• Metabolic water
– from aerobic metabolism
– from dehydration synthesis
• Preformed water
– ingested in food and drink
• Routes of loss
– urine, feces, expired breath, sweat, cutaneous
• Loss varies greatly with environment and activity
– respiratory loss : with cold, dry air or heavy work
– perspiration loss : with hot, humid air or heavy work
• Insensible water loss
– breath and cutaneous transpiration
• Obligatory water loss
– sweat, feces, minimum urine output (400 ml/day)
WATER BALANCE MECHANISMS IN OUR BODY
Several Mechanisms in the Body Work Together to Maintain
When your body needs water, nerve centres deep within the
brain are stimulated, resulting in the sensation of thirst.
The sensation becomes stronger as the body's need for
water increases, motivating a person to drink the needed
When the body has excess water, thirst is suppressed.
2. VASOPRESSIN – THE ANTI-DIURETIC HORMONE
When the body is low in water, the pituitary gland (located at the base of
the brain) secretes vasopressin – the anti-diuretic hormone, into the
bloodstream, which stimulates the kidneys to conserve water and
excrete less urine.
When the body has excess water, the pituitary gland secretes little antidiuretic hormone, enabling the kidneys to excrete excess water in the
The body can move water from one area to another as needed.
When water loss is severe, the amount of water in the bloodstream
decreases, so the body moves water from inside the cells to the
bloodstream until it can be replaced through increased intake of fluids.
When the body has excess water, the body moves water from the
bloodstream into and around the cells. In this way, blood volume and
blood pressure can be kept relatively constant.
• Electrolytes, or mineral salts, such as sodium and
potassium, are dissolved in the water in the body.
• Water balance and electrolyte balance are closely
• The body works to keep the total amount of water
and the levels of electrolytes in the bloodstream
4.CONTROLLING BLOOD VOLUME
• The total amount of sodium affects the
amount of fluid in blood and around cells.
• The body continually monitors blood volume
and sodium (and other electrolyte)
• When either becomes too high or too low,
sensors in the heart, blood vessels, and
kidneys detect the change and stimulate the
kidneys to respond accordingly.
Most of the body's sodium is located in
blood and in the fluid around cells.
• Sodium helps the body keep fluids in a
normal balance. Sodium plays a key role in
normal nerve and muscle function.
• The body obtains sodium through food and
drink and loses it primarily in sweat and
• Healthy kidneys maintain a consistent level
of sodium in the body by adjusting the
amount excreted in the urine.
• Most of body's potassium is located inside the cells.
• Potassium is necessary for the normal functioning of
cells, nerves, and muscles, but the body must maintain
the potassium level in blood within a narrow range.
• A potassium level that is too high or too low can have
serious consequences, such as an abnormal heart
rhythm or even stopping of the heart (cardiac arrest).
• Body can use the potassium stored within cells to help
maintain a constant level of potassium in blood,
matching the amount of potassium consumed with the
• We get potassium from food and drinks that contain
electrolytes (including potassium).
• We lose potassium through urine and sweat, and also
through the digestive tract.
• Healthy kidneys can adjust potassium excretion to
match changes in consumption.
Fluid Compartments in the Body
• The body's water is effectively
compartmentalized into several major
• Intracellular Fluid (ICF) comprises 2/3 of the
body's water. If your body has 60% water, ICF
is about 40% of your weight. The ICF is
primarily a solution of potassium and organic
anions, proteins etc.The cell membranes and
cellular metabolism control the constituents of
this ICF.ICF is not homogeneous in our body. It
represents a conglomeration of fluids from all
the different cells.
• Extracellular Fluid (ECF) is the remaining 1/3 of your
body's water.ECF is about 20% of our weight.The ECF is
primarily a NaCl and NaHCO3 solution.The ECF is further
subdivided into three subcompartments:
Interstitial Fluid (ISF) surrounds the cells, but does not
circulate. It comprises about 3/4 of the ECF.
Plasma circulates as the extracellular component of
blood. It makes up about 1/4 of the ECF.
Transcellular fluid is a set of fluids that are outside of
the normal compartments. These 1-2 liters of fluid make
up the Digestive Juices, Mucus, etc.
The hydrogen ion concentration in body fluids must
be controlled within a narrow range.
In fact, its regulation is one of the most important
aspects of homeostasis, because merely slight
deviations from normal acidity can cause marked
alteration in enzyme-catalyzed reaction rates in the
Hydrogen ion concentration can also affect both the
cellular uptake and regulation of metabolites and
minerals and the uptake and release of oxygen from
ROLES OF LUNGS AND KIDNEY IN
CONTROLLING THE BLOOD pH
• Lungs can control an increase in blood pH by releasing a
higher level of carbon dioxide than usual.
• Carbon dioxide is mildly acidic in nature, which causes
the acidity of the blood to increase. By controlling the
depth and speed of breathing, the human brain controls
the release of carbon dioxide.
• The kidneys also help in controlling the blood pH by
regulating the release of acid or base. However, it takes
more time for the kidneys to control the pH because it
releases acids and bases in very minute amount.
• Buffer systems contain the weak bases and
the weak acids that occur naturally in the
• When the pH is at its optimum level, these are
secreted in balance.
• But once the balance inclines towards acidity
or basicity, the weak bases or the weak acids
start excreting correspondingly.
• As a result, the pH of the blood comes at or
near its optimum level. The two principal
buffer systems of the blood are carbonic acid
and bicarbonate ions.
ROLE OF KIDNEY IN
EXCRETION OF WATER
The functional unit of the kidney is the
nephron. Each kidney contains about 1 to
1.5 millions nephrons.
The five components of the nephron are the
Bowman’s capsule, proximal convoluted
tubule, loop of Henle, distal convoluted
tubule, and collecting duct.
The excretion process starts in the Bowman’s
capsule, the blind, dilated end of the
renal tubule, which encapsulates a tuft of
about 50 capillaries linking the afferent
(flowing into the capsule) and efferent
(flowing from the capsule) arterioles that
surround the tubule segments after they
leave the capsule.
The capillary network in the Bowman’s
capsule is called the glomerulus,and it
accounts for the particularly rich blood
supply to kidney.
• A large amount of circulating blood flows through
the kidneys. Approximately 25% of the cardiac
output or 1200 ml of blood per minute is received by
• One liter of urine is the end product of more than
1000 liters of circulating blood processed through
• Urine formation begins in the glomerular
capillaries, with dissolved substances passing into
the proximal tubule as a result of the force of blood
pressure in the large afferent arteriole and the
pressure in Bowman's capsule.
• The renal tubule is responsible for reabsorption and secretion.
Reabsorption is the process of moving solutes from the tubules
and reabsorbing or returing them to the bloodstream.
• Some substances such as glucose and sodium are reabsorbed
until the plasma level reaches a specific concentration known as
the renal threshold.
• Secretion is the process of transporting solutes into the renal
tubule so that they can be excreted in the urine. Secretion
allows substances such as hydrogen ions to be eliminated at a
rate that exceeds glomerular filtration.
• Both reaborption and secretion are controlled by the selective
permeability of different areas of the renal tubule to water,
sodium, and urea (a by-product of protein metabolism) and the
response of the distal collecting tubules in the kidney to
hormones such as aldosterone, antidiuretic hormone, and