In this article we will discuss about:- 1. Introduction to Water Metabolism 2. Distribution of Water in the Body 3. Water Content in Various Tissues 4. Functions of Water 5. Water Balance.
Introduction to Water Metabolism:
Of the three factors, water, salts and food, water is the most important. If we remember that life evolved first in an aquatic medium, there will be nothing to be astonished that water is the most essential substance for life. Deprivation of water will kill a subject much earlier than deprivation of salt or food. With water starvation death takes place in the shortest possible time (in about one week), when only 20% of the total body weight is lost.
Our former idea about the chemical constitution of water, having the formula H2O with a molecular weight 18, is fast changing and is probably no more tenable now. By studying the various properties of water, modern chemistry suggests that water is actually a polymer of H2O.
Critical temperature of water is 365°C. Boiling water is (H2O)3, ice is (H2O)4 and ordinary water is a mixture of the two. Experiments with heavy water (D2O) have clarified many aspects of water metabolism.
Distribution of Water in the Body:
Total water content is 60% to 70% of the adult body weight, i.e., 45—49 litres, females having somewhat lower values than males. Accumulated evidences suggest that using of body weight as a parameter of reference; body water content is inversely related to the adiposity of the organism.
Thus increase of fatty tissue in the body will automatically result in a reciprocal decrease in total water content when expressed in terms of percent, body weight. Behnke, to minimise the errors in the concept of total body water, introduced the term lean body mass which is made up of functional tissue, containing only essential fat.
The total body water calculated on a fat free basis, according to the concept of lean body mass, on a large number of animals, e.g., rat, guinea-pig, cat, dog, monkeys, etc., averaged 73.2% range being 70—76%. This measurement is applicable only to normal adults. In very young, and in those having other abnormalities, deviations are to be expected.
The total body water is distributed throughout two main compartments:
(1) Intracellular, approximately 50% of the body weight (i.e., 39 litres), and
(2) Extracellular —20% of the body weight, i.e., 14 litres, of which 3 litres in plasma and 11 litres in interstitial fluid and lymph.
Recent investigations indicate that although the concept of a single intracellular water component is still useful, the extracellular component is more heterogeneous and is subdivided into four subcomponents:
1. Blood plasma (4.5% body water).
2. Interstitial fluid and lymph (8%).
3. Dense connective tissue, cartilage, bones (6%).
4. Transcellular fluids (1.5%), such as aqueous and vitreous humour, cerebrospinal fluid, endolymph, perilymph, etc.
A man weighing 11 stones (70 kg) contains about 47 litres of water in the body. Of this, 20 litres (about half) are in the muscles and 10 litres (about one-fifth) in the skin. Blood contains about one-fourteenth part of the total body water. In young animals and in very active tissues, the water content is much higher. A baby contains water much over 70% of its body weight. Water content is maximum in the foetus and diminishes with age.
Water Content in Various Tissues:
The percentage of water in various tissues are as follows- skin, 20%; muscles, 75—80%; blood, 76%; plasma, 92%; connective tissues, 60%; corpuscles, 60%; (total amount of water in blood is 4—5 litres); nervous tissue -grey matter, 85% (more than in blood, yet it is solid); white matter, 70%; adipose tissue, 20%; dentine, 10% (least and therefore hardest); bones (without marrow), 25%; cerebrospinal fluid, 99%.
These figures are approximate and average. 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.
It must be remembered that the water content of the body is derived from two sources—
(a) From food and drink,
(b) As end product of metabolism.
It is better to call the former as exogenous water and the latter as endogenous water.
The body water remains in two states:
(a) in the free state, i.e., 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 ultrafiltration,
(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 water comes as an end product of metabolism. Almost the whole of H of solid food is converted into water, only about 5 gm of H being excreted in the form of ammonia, urea, etc. Different foodstuffs yield different quantities of water.
Approximate figures are given below:
Functions of Water:
Some of the important physiological functions of water are summarised below:
i. It is an essential constituent of living cell. No living thing can resist drying.
ii. By its 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. Probably no chemical reaction inside the body can take place without water.
iii. It Acts as a Medium for Various Physical Processes:
It acts as a medium for various physical processes, such as osmosis, diffusion, filtration, etc.
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 the latter are broken down into smaller units.
v. 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. For instance, glycogen from glucose. This action is the reverse of hydrolysis.
vi. Ionising Medium:
Water is a very good ionising medium. The dielectric constant of water being very high, oppositely charged ions can coexist in water without much interference.
vii. It Acts as a Vehicle for Various Physiological Processes:
(a) For absorption of food material from the intestine;
(b) For reabsorption 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.
The physical and chemical properties of water permit chemical reactions requiring large amounts of heat to take place at a low body temperature.
viii. 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°C, than most of known solids and liquids. By virtue of this property water can mop up 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 part in regulating body heat.
ix. 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.
x. Refractive Medium:
The aqueous humour helps to keep up the shape and tension of the eye-ball and acts as a refractive medium for light.
xi. Mehanical Action:
The cerebrospinal fluid which contains nearly 99% water acts as a great mechanical buffer preventing injury to the nervous system.
xii. 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. The fish derive oxygen almost exclusively from dissolved O2 in water.
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. This indicates that there must be efficient machinery for maintaining water balance.
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.
The above figures are average and approximate. Water loss by any one of the channels may rise or fall under various conditions. Loss through skin varies according to the temperature and humidity of the atmosphere and also upon the amount of muscular exercise done. In hot climates and with exercise, excretion through skin may vary from 3—10 litres per day. Higher atmospheric humidity reduces water loss through the skin.
Water excretion, by lungs also increases in hot dry weather. In diarrhoea, dysentery, cholera, etc., more water is lost in the faeces, while in conditions of diuresis more is passed out by the kidneys. The water secreted in the digestive juices is not lost water. Because it is almost completely reabsorbed and about 5—7 litres of water circulate in this way per day. The loss in saliva and lachrymal secretion is negligible under normal conditions.
Positive and Negative Water Balance:
Water balance is said to be positive (intake exceeds loss) in growing infants and children, in convalescents, athletes and pregnant women who are storing water and building their body tissues. Each gram of protein is laid down with about 3 gm of water. Fat and glycogen are deposited with less amount of water. When diet is changed from high fat to high carbohydrate, water retention takes place and the balance becomes positive.
Water balance is negative (loss exceeds intake) under the following conditions:
(a) When the subject is thirsty,
(b) When a pre-existing oedema is clearing up due to diuresis, and
(c) 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 constantly appearing in and disappearing from the body, a fairly accurate balance is maintained between its gain and loss, which indicates that there must be a strong regulating machinery. The mechanism which regulates water balance is very intricate and is not yet fully known.
The following factors are closely involved in it:
(ii) Autonomic nervous system—hypothalamus and the vasomotor system,
(iii) Renal regulation,
(iv) Respiration and skin,
(v) Phenomenon of thirst.
The roles of these factors are discussed below:
A number of endocrines take part in water regulation.
They are as follows.
a. Posterior Pituitary:
It manufactures two hormones, e.g., antidiuretic hormones or vasopressin and oxytocin, of which antidiuretic hormone has got influence upon water balance (Fig. 10.118). The antidiuretic hormone—This increases the reabsorption of water from the distal renal tubules and thus reduces urine volume. It is very interesting to note that 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. In the thirsting animals the presence of an antidiuretic substance has been demonstrated in the urine.
b. 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 aldosterones regulate the water balance through ADH release from posterior pituitary, causing retention of water and thus increase of blood volume (Fig. 10.118).
In adrenal cortical insufficiency there is decreased reabsorption of Na+ and as a result more Na+ is lost in the urine. There is increased reabsorption of K+. The reabsorbtion of CI is also depressed. There is a consequential change in the body fluids. The intracellular crystalloid osmotic pressure exceeds that of the extracellular crystalloid osmotic pressure, and water flows from the extracellular fluid to the intracellular fluid. Plasma volume decreases, and there is anhydraemia and haemoconcentration.
c. Adrenal Medulla:
Injection of adrenaline reduces renal circulation by causing constriction of renal vessels and thus decreases the volume of urine.
Thyroxine increases urine volume along with increased elimination of salt, probably not by a direct effect on the kidney but by raising the general metabolism and thus increasing nitrogenous end products which acted as diuretics. In myxoedema there is increased fluid retention in the extracellular tissue.
ii. The Autonomic Nervous System:
The hypothalamus controls the secretion of antidiuretic hormone of the posterior pituitary through the supra-opticohypophyseal tract. Lesion of this tract or the corresponding region of hypothalamus or disease of the posterior pituitary causes intense polyuria known as diabetes insipidus.
The function of hypothalamus may be controlled in the following way: the water content of the body- excess water dilutes blood and reduces osmotic pressure as a result of which the hypothalamus is depressed leading to less secretion of anti-diuretic hormone and consequently diuresis is produced. When body water is reduced, the osmotic pressure of blood increases, hypothalamus is stimulated— more antidiuretic hormone is secreted and consequently urine volume is reduced.
b. 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 rises, such as by excess water intake, or saline injections, etc. kidneys excrete more water.
This effect may be ascribed to:
(a) Increased blood volume and consequent rise of blood pressure and thereby increased filtration pressure,
(b) Dilution of plasma proteins, reducing colloidal osmotic pressure and consequently increasing the available filtration pressure,
(c) Increasing the number of active glomeruli, and
(d) Depressing the degree of water reabsorption by the renal tubules. It has been shown that the first two effects are negligible. In man no increase of glomerular filtration takes place until the urine volume exceeds 900 ml per hour.
Regarding the third factor it has been proved that in certain species of animals, there is some increase in the number of active glomeruli under such conditions. It is doubtful whether in man this change at all takes place. But even if it is assumed that it do take place, yet this cannot explain the huge increase of urine volume which may be as much as twenty times its normal value,
(e) It has been observed that the increase of central blood volume enhances the urine output through the inhibition of secretion of ADH. It is suggested that the inhibition of ADH secretion takes place reflexly through the stimulation of stretch receptors present in the left atrial wall, and
(f) Besides these, angiotensin II which is formed by the kidney-reinn, takes an important part in regulation of water balance through the secretion of aldosterone (Fig. 10.118).
The fourth factor is, therefore, the chief agent in regulating water excretion by the kidneys under physiological conditions. It has been already explained how the water content controls the secretion of antidiuretic hormone from the posterior pituitary upon which the degree of renal reabsorption depends.
iv. Respiration Lungs and Skin:
These channels also take considerable part in regulation of water balance by excreting variable amounts of water.
v. Phenomena of Thirst:
When more fluid is lost, such as, in diarrhoea, vomiting, diuresis, sweating, haemorrhage, etc. the subject 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. In hibernating animals metabolism is so low that the water produced by the oxidation of foodstuffs is enough to equalise the water loss. Hence, under such condition no thirst is felt.
Drinking is stimulated by three types of stimuli:
a. A rise in vascular tonicity even without any change in blood volume.
b. A fall in blood volume even when unacompained by a rise in osmolarity.
c. A third factor which operates in some animals is a rise in temperature which can stimulate drinking even before there is any obvious change in body water content.
Little is known about the receptors which mediate the sensation of thirst. Presumably the initial sensation of thirst depends on blood volume and osmolarity changes—when appropriate amount of water has been drunk, the sensation vanishes because of the activity of oral and gastric receptors. The thirst centre is situated in the midhypothalamic region near the paraventricular nucleus (caudal to the osmoreceptors).
Thus when the water content of the body increases, the water balance is maintained in two ways:
(i) By Reducing Water Intake:
The subject does not feel thirsty and takes no water.
(ii) By Increasing Water Loss:
This is done by reducing secretion of antidiuretic hormone through hypothalamus and thus causing diuresis.
When the water content of the body is reduced (by loss, etc.)—exactly opposite processes take place and the balance is thus maintained.