WATER

SUBMITTED BY: AISHWARYA JAISWAL

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 blood.• It also serves as the medium in which the vast number of intracellular metabolic reactions takes place.

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 various organs.   

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 damage, 

and enabling free easy movement by reducing friction. 

A synovial membrane  at joints encloses a fluid called synovial fluid, which acts at the lubricant. Water is a major part of this fluid.

Many internal organs have watery fluids surrounding them to give protection, lubrication, and possibly other functions. For exampleIn humans the brain contains cerebro-spinal fluid, and the lungs have pleural fluid  in pleural membranes. 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 CONTROL 

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 cooling effect.

WATER IN THE ROLE OF PROTECTION

• 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 develop.

• Water also provides protection along the lining of the gastrointestinal tract, and protects the spinal cord from damage and shock.

The Water Balance

Water Gain

• Metabolic water– from aerobic metabolism– from dehydration synthesis

• Preformed water– ingested in food and drink

Water Loss• Routes of loss– urine, feces, expired breath, sweat, cutaneous

transpiration• 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 Water Balance

1.THIRSTWhen 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 fluids.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 anti-diuretic hormone, enabling the kidneys to excrete excess water in the urine.

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.

3.MINERAL SALTS

• Electrolytes, or mineral salts, such as sodium and potassium, are dissolved in the water in the body.

• Water balance and electrolyte balance are closely linked.

• The body works to keep the total amount of water and the levels of electrolytes in the bloodstream constant.

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) concentrations.

• 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.

4.SODIUM

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 urine.

• Healthy kidneys maintain a consistent level of sodium in the body by adjusting the amount excreted in the urine.

5.POTASSIUM• 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 amount lost.

• 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 divisions.

• 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.

ACID AND BASE BALANCE

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 cells.

Hydrogen ion concentration can also affect both the cellular uptake and regulation of metabolites and minerals and the uptake and release of oxygen from hemoglobin.

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

• Buffer systems contain the weak bases and the weak acids that occur naturally in the body.

• 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 the kidneys.

• One liter of urine is the end product of more than 1000 liters of circulating blood processed through the kidneys.

• 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 parathyroid hormone.