water metabolism

 Water Metabolism

Introduction- Of the three factors, water, salts and food, water is the most important. Fluid balance is an aspect of the homeostasis of organisms in which the amount of water in the organism needs to be controlled via  osmoregulation and behavior, such that the concentrations of electrolytes in the various body fluids are kept within healthy ranges.

The basic principle of fluid balance is that the amount of water lost from the body must equal the amount of water taken in; for example, in humans, the output via  respiration, perspiration, urination, defecation, and expectoration must equal the input via  eating and drinking, or by parenteral intake. Euvolemia is the state of normal body fluid volume, including blood volume, interstitial fluid volume, and  intracellular fluid volume; hypovolemia and hypervolemia are imbalances. Water is necessary for all life on Earth. Humans can survive for 4 to 6 weeks without food but only for a few days without water.

Chemistry of water-Water is actually a polymer of H₂O. Water and oxidane, are two official names for the chemical compound H₂O. Water is the main constituent of Earth's hydrosphere and the fluids of all known living organisms in which it acts as a solvent. It is vital for all known forms of life, even though it provides no calories or organic nutrients.

Each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds. The hydrogen atoms are attached to the oxygen atom at an angle of 104.45°. 

Water is the name of the liquid state of H₂O at standard conditions for temperature and pressure. Water (H₂O) is a polar inorganic compound that is at  room temperature a tasteless and odorless liquid, nearly colorless with a hint of blue. This simplest hydrogen chalcogenide is described as the universal solvent for its ability to dissolve many substances. 

Water as found in nature almost always includes various dissolved substances. Water is the only common substance to exist as a solid, liquid, and gas in normal terrestrial conditions.

Water is also the liquid state of H₂O.The other two common stated of H₂Oare the solid state- ice (H₂O)₄, and the gaseous state (H₂O)₃- water vapor or steam. The addition or removal of heat can cause state transitions- freezing (water to ice), melting (ice to water),vaporization(water to vapor), condensation (vapor to water), sublimation (ice to vapor) and deposition (vapor to ice). The boiling point of water is 100°C, freezing point is 0°C and critical temperature of water is 365°C.

States of Water

Phase	Chemical Formula
Liquid(water)	H2O
Solid(Ice)	(H2O)4
Gas (Water Vapors)	(H2O)3

 

Distribution of water in the body- Total water content is 60%to 70% of the adult body weight, i.e., 45-49 liters, females having somewhat lower values than males. The total body water is distributed throughout body in two main compartments-

1. Intracellular- approximately 50% of the body weight (39 liters)

2. Extracellular- 20% of the body weight, (14 liters), of which 3 liters in plasma and 11 liters in interstitial fluid and lymph. The extracellular component is more heterogeneous and is subdivided into four subcomponent

·        Blood plasma (45%, body water)

·        Interstitial fluid and lymph (8%)

·        Dense connective tissue, cartilage, bones (6%).

·        Transcellular fluid (15%), such as aqueous and vitreous humor, cerebrospinal fluid, endolymph, per lymph, etc.

Water content in various tissues- The water content of the tissues and organs varies from time to time according to the loss and supply of water and the degree of activity.

The percentages of water in various tissues are as follows:

Tissue	Percentage
skin	20%
muscles	75-80%
blood	70%
plasma	92%
connective tissues	60%
corpuscles,	60%
grey matter	85%
white matter	70%
Adipose tissue	20%
dentine	10%
bones without marrow	25%
cerebrospinal fluid	99%

 

Sources- It must be remembered that the water content of the body is derived from two sources-

(a) Exogenous water from food and drink

(b) Endogenous water as end product of metabolism.

Body water states-The body water remains in two states-

(a) Water in Free state-found not combined with anything. Most of the body water remains in this form. Various substances can remain dissolved in this water and be removed by ultra filtration.

(b) Bound water- This is a very small quantity. In this form water remains combined with the colloids and other substances

Metabolic Water (Endogenous Water)-this type of water is produced during various reactions of metabolism. Almost the whole of H⁺ of solid food is converted into water, only about 3 gm of H⁺ being excreted in the form of ammonia, urea, etc. Different foods produce different quantities of water. Approximate figures are given below per 100 gm –

Fat	107 gm water
Starch	55 gm water
Protein	41 gm water
Alcohol	117 gm water

Functions of Water

Some of the important physiological functions of water are summarized below:

1. Essential constituent of living cell- No living thing can live without water.

2. Solvent action-By its solvent action it forms a great number of crystalloidal and colloidal solutions and thus serves as a universal medium in which the intracellular and extracellular chemical reactions take place.

3. Medium for various physical processes-It acts as a medium for various physical processes, such as osmosis, diffusion, filtration, etc.

4. Hydrolysis- It is an important chemical process involved in digestion and metabolism. In this process the H and OH ions of water are introduced into bigger molecules and these are broken down into smaller units.

5. Dehydration and condensation In these processes water molecule is removed. This takes place in certain synthetic processes in which bigger particles are formed by the union of smaller ones. This action is the reverse of hydrolysis.

6. Ionizing medium. Water is a very good ionizing medium. Its dielectric constant of water is very high, so oppositely charged ions can co exist in water without much interference.

7. Vehicle for various physiological processes- It acts as a vehicle for various physiological processes, such as

(a) for absorption of food material from the intestine;

(b) for re absorption from kidney tubules;

(c) for the transport of the various food stuffs from place to place;

(d) for the drainage and excretion of the end products of metabolism;

(e) for the manufacture of various secretions, such as, digestive juices, etc.;

(f) for carrying the hormones to their places of activity, etc.

(g) The physical and chemical properties of water permit chemical reactions requiring large amounts of heat to take place at a low body temperature.

8. Heat regulation. Body temperature is regulated by water in the following ways:

(a) Heat absorption- Due to high specific heat of water more heat is required to raise the temperature of 1 gm of water through 1°Cthen most of known solids and liquids. By virtue of this property water can absorb large quantity of heat.

(b) Heat conduction and distribution-Heat conducting power of water being very high it acts as a very good agent in carrying away heat from the site of production and distributing it throughout the body. By the two above properties, water acts as an important heat-buffer.

(c) Heat loss-Water helps heat loss through urine and stool and by evaporation from skin, lungs, etc. Water has got the highest latent heat of evaporation. By the help of these properties water takes an important part in regulating body heat.

9. Lubricant action- Water acts as a lubricant to prevent friction and drying. In joints, pleura, peritoneum, conjunctiva, etc., the aqueous solution is practically free from fats and acts as a lubricant against rubbing and drying.

10. Refractive medium- The aqueous humor helps to keep up the shape and tension of the eye ball and acts as a refractive medium for light.

11, Mechanical action- The cerebrospinal fluid which contains99% water acts as a great mechanical buffer preventing injury to nearly all the nervous system.

12. Respiratory function- Although CO₂, and O₂, are only slightly soluble in water, yet this little solubility is of immense importance for the gaseous exchange in the tissues and lungs.

Water Balance in the Body

Water is continuously being supplied and lost from the body. But still the total water content of the body is kept more or less constant, by maintaining a balance between supply and loss. The total water requirement of an adult, under ordinary conditions, is about 2,500-3,000 ml, i.e. about 1 ml per calorie of food intake. Half of this quantity (i.e., about 1,500 ml or half ml per calorie) should be taken as free drinks.

Water Balance

Water Intake		Water out put
Drinking water or fluids	1450 ml	Kidney	1500 ml
Solid Food	800 ml	Skin	600 ml
Oxidation	350 ml	Lungs	400 ml
-	-	Feces	100 ml
Total	2600 ml	Total	2600 ml

Positive and negative water balance

Water balance is said to be positive when water intake exceeds loss under the following conditions-

(1) Growing infants and children

(2) Convalescents,

(3) Athletes

(4) Pregnancy

(5)When diet is changed from high fat to high carbohydrate, water retention takes place and the balance becomes positive

(6) When persons are storing water and building their body tissues. Each gram of protein is deposited with about 3 gm of normal water. Fat and glycogen are deposited with less amount of water.

Water balance is said to be negative when water loss exceeds intake under the following conditions:

(1) When the person is thirsty.

(2) When a pre-existing edema is clearing up due to diuresis.

(3) When diet is changed from high carbohydrate to high fat.

In any condition of increased water loss, the relative proportion of Na and K content of the fluid excretion will indicate whether the water is coming chiefly from the extracellular or intracellular sources. High Na content will indicate extracellular source, whereas high K content will indicate intracellular source, provided intake remains constant.

Regulation Of Water Balance In spite of large amount of water is constant appearing from the body, a fairly accurate balance is maintained between appearing in its gain and loss, which indicates that there must be strong regulating machinery. The mechanism which regulates water balance is very complex The following factors are closely involved in it-

I. Endocrines- A number of endocrines takes part in water regulation. They are as follows:

1. Posterior pituitary. It manufactures two hormones, e.g., antidiuretic hormones or vasopressin and oxytocin, of which anti diuretic hormone has got influence upon water balance.

The antidiuretic hormone- This increases the reabsorption of water from the distal renal tubules and thus reduces urine volume. The secretion of this hormone is controlled by the water content of the body. Excess of water depresses, while dehydration stimulates the secretion of this hormone.

2. Adrenal cortex- Adrenal cortex secretes Aldosterone which plays an important part in maintenance of water balance. The secretion of Aldosterone is controlled by the angiotensin II and also by high serum K and low serum Na. The Aldosterone regulates the water balance through ADH release from posterior pituitary, causing retention of water and thus increases blood volume.

3. Adrenal medulla-Adrenaline reduces renal circulation by causing constriction of renal vessels and thus decreases the volume of urine.

4. Thyroid-Thyroxin increases urine volume along with increased amount of salt, probably not by a direct effect on the kidney but by increasing the BMR and thus increasing nitrogenous end products which act as diuretics.

II. The autonomic nervous system

1. Hypothalamus. The hypothalamus controls the secretion of antidiuretic hormone ADH of the posterior pituitary through the supra-opticohypophyseal tract by stimulating or decreasing the secretion of DH as per the concentration of blood plasma.

2. Vasomotor system. The vasoconstrictor and vasodilator nerves also play an important role in the regulation of renal circulation and general blood pressure.

III. Renal regulation- When water content of the body increases kidneys excrete more water. This effect may be due to following mechanisms-

(1) Increased blood volume and consequent rise of blood pressure and thereby increased filtration pressure.

(2) Dilution of plasma proteins, reducing colloidal osmotic pressure and consequently increasing the filtration pressure.

(3) Increasing the number of active glomeruli.

(4) Decreasing the degree of water reabsorption by the renal tubules. This factor is the chief agent in regulating water excretion by the kidneys under physiological conditions.

 (5) It has been observed that the increase of central blood volume enhances the urine output through the inhibition of secretion of ADH. The inhibition of ADH secretion takes place reflexly through the stimulation of stretch receptors present in the left atrial wall of heart.

(6) Angiotensin II which is secreted by the kidney, takes an important part in regulation of water balance through the secretion of Aldosterone

IV. Lungs and Skin- These organs also take considerable part in regulation of water balance by excreting variable amounts of water.

V. Thirst-When more fluid is lost, such as, in diarrhea, vomiting, diuresis, sweating, hemorrhage, etc., the person feels thirsty, and takes water. Thirst may be defined as the specific hunger for water. In this way the amount of lost water is replenished. Drinking is stimulated by these stimuli-

1. A rise in vascular tonicity even without any change in blood volume.

2. A fall in blood volume even when unaccompanied by a rise in osmolarity.

Thus when the water content of the body increases, the water balance is maintained in two ways:

(1) By reducing water intake-The subject does not feel thirsty and takes no water.

(2) By increasing water loss-this is done by reducing secretion of antidiuretic hormone through hypo thalamus and thus causing diuresis.