Fluid regulation is essential to homeostasis. If water or electrolyte levels rise or fall beyond normal limits, many bodily functions fail to proceed at their normal rates. Maintaining normal pH levels is also important for normal body functioning because small changes in pH can produce major changes in metabolism.

Water is a constituent of all living things. It is often referred to as the universal biological solvent. Only liquid ammonia is able to dissolve more substances than water.

The importance of water as a biological solvent is due to its polar asymmetry (H⁺- OH^-) and its high affinity for other water molecules. As a polar molecule, water is highly interactive with other polar molecules and acts to compete with them for hydrogen bonding, thus weakening their electrostatic forces. This allows a greater number of polar molecules to coexist and undergo reactions; in a non-polar solvent, these polar molecules would form strong bonds with one another and thus be unavailable for further reaction.

The affinity of water molecules for each other is very high. When non-polar molecules are placed in water, the polar water molecules are attracted to other water molecules. This action tends to leave the non-polar molecules clustered together. This clustering property is important in structuring macromolecules, binding enzymes to their substrates, and in membrane formation.

Water acts to minimise temperature changes throughout the body because of its high specific heat.

A considerable amount of energy is needed to break the hydrogen bonds between water molecules in order to make the water molecules move faster (that is, increase the temperature of water). Therefore, water can absorb much heat without rapidly changing it's own temperature.

The adult human body consists of about 60% water by weight, depending upon age and the amount of body fat. The water content of the tissues of the body varies. Adipose tissue (fat) has the lowest percent of water; the skeleton has the second lowest water content. Skeletal muscle, skin, and the blood are among the tissues that have the highest content of water in the body. See Table 1

Infants have a higher percent of water than adults do as much as 77%. The total water content of the body decreases most dramatically during the first 10 years and continues to decline through old age, at which time it may be only 45% of the total body weight. Men tend to have higher percentages of water (about 65%) than women (about 55%) mainly because of their increased muscle mass and lower amount of subcutaneous fat. Fat has less water content than any other body tissue. This also accounts for a lower than normal water percentage in obese people.

TABLE 1: Distribution of water and percentage of body weight in a 70 kg. (11 stone) man

The water in the body has many important functions, which are listed in table 2 below.

Table 2: Functions of water in human physiology

Water in the body is in a constant state of motion. Shifting between the three major fluid compartments of the body and in addition being continuously lost from, and taken into, the person.

In a normal, healthy human being WATER INPUT = WATER OUTPUT.

Maintaining this ratio is of prime importance in maintaining health.

Approximately 90% of the body's water intake comes via the gastro-intestinal tract. The remaining 10% is called metabolic water and is produced as the result of various chemical reactions in the cells of the body's tissues.

The normal healthy adult loses water via the route shown in table 3 below.

Table 3: Where water is lost from the body (healthy adult)

The amount of water lost via the kidneys is under hormonal control. The average amount of water lost and consumed per day is around 2.5 litres (approx. 4¹/₄ pints) in a healthy adult.

Body fluid is found in three different fluid compartments within the body. These are:

1. Blood plasma

2. Interstitial fluid

3. Intracellular fluid

Number 1 and 2 above make up the portion of body fluid known as extracellular fluid.

In terms of body weight and total percentage of body water, these can be summarised as in table 4 below.

Table 4: Distribution of water in the body

From the table it can be seen that most of the body's water is locked up within tissue cells

The walls of the blood vessels form a barrier to the free passage of fluid between interstitial fluid and blood plasma. At the capillaries, these walls are only one cell thick. These capillary walls are generally permeable to water and small solutes but impermeable to large organic molecules such as proteins. Thus, the blood plasma tends to have a higher concentration of such molecules when compared to the interstitial fluid. Much of this interstitial fluid is taken up by the lymphatic system and eventually finds its way back into the blood stream. This process is discussed in the pages on the lymphatic system.

Because water and small solutes such as sodium (Na⁺⁾, potassium (K⁺⁾, calcium (Ca⁺⁺), etc. can be freely exchanged between the blood plasma and the interstitial fluid, regulation of these electrolytes can be controlled by the action of the kidneys on the blood. This exchange depends mainly upon the hydrostatic and osmotic forces of both of these fluid compartments.

Hydrostatic forces

This is due to the weight of water pushing against a surface, such as a dam in a river or the wall of a blood vessel. The pressure of blood in the capillaries serves as a major hydrostatic force in the human body. The capillary hydrostatic pressure (CHP) is a filtration force. This is due to the pressure of the fluid being higher at the arterial end of the capillary than at the venous end. The pressure at the arterial end is about 30mm/Hg. At the venous end, this pressure has fallen to about 10mm/Hg.

The pressure of the interstitial fluid (IFHP) is a negative one of -5mm/Hg. due to the lymphatic system continuously taking up the excess fluid forced out of the capillaries.  

Figure 1. Fluid exchange

Osmotic pressure

This is due to the attraction of water to large molecules such as proteins. As already stated these are more abundant in the blood vessels than outside them. This tends to attract water in from the interstitial space.

Figure 2. Fluid dynamics at the capillary

CHP = Capillary Hydrostatic Pressure COP = Capillary Osmotic Pressure

IFHP = Interstitial Fluid Hydrostatic Pressure IFOP + Interstitial Fluid Osmotic Pressure

The overall nett forces acting upon the extracellular fluid at the capillary can be seen in figure 2 above. These can be calculated as follows (in mm of Hg):

Arterial end of capillary

CHP + IFOP = Pressure taking fluid out of capillary

30 + 6 = 36

COP + IFHP = Pressure putting fluid into capillary

28 - 5 = 23

Therefore

30 - 23 = 13

Thus, there is an overall outward pressure exerted of 13 mm/Hg. resulting in water from the plasma being forced into the interstitial space.

Venous end of capillary

CHP + IFOP = Pressure taking fluid out of capillary

10 + 6 = 16

COP + IFHP = Pressure putting fluid into capillary

28 - 5 = 23

Therefore

23 - 16 = 7

Thus, there is an overall pressure into the capillary of 7mm/Hg resulting in water from the interstitial fluid entering the plasma at the venous end of the capillary.

Overall, there is near equilibrium between fluid forced out of the capillaries and the fluid absorbed back in. This is because the lymphatic system collects the excess fluid forced out at the artery end and eventually drains it back into the veins at the base of the neck.

This is known as Starling's Law of the Capillaries.

A similar situation exists between the interstitial fluid and the intracellular fluid although it is complicated by the presence of ion pumps and carriers. Generally water movement is substantial in both directions but ion movement is restricted and dependent upon active transport via the pumps. Nutrients and oxygen, because they are dissolved in water, move passively into cells whilst waste products and carbon dioxide move out.

The mechanisms for the regulation of body fluids are centred in the hypothalamus. (See Fig 3). The hypothalamus also receives input from the digestive tract that helps in the control of thirst.

The regulation of body fluid volume and extracellular osmolarity is under the control of Anti Diuretic Hormone (ADH) which has already been discussed in previous lectures on the endocrine system.

ADH has many areas of influence in the body. One of the major functions of ADH is to increase the permeability of the distal convoluted tubules and the collecting tubules in the kidneys. This allows more water to be reabsorbed in the kidneys. This is manifest in times of drought or if the body is short of fluid intake (such as during sleep) and results in a concentrated, darker coloured urine of reduced volume. Absence of ADH occurs when the individual is over-hydrated such as at a party if a lot of beer, cider, alcopops etc. are being drunk Here the urine is dilute, pale or colourless and of high volume.

 Primary factors involved in the triggering of ADH production are osmoreceptors and baroreceptors (pressure receptors). Secondary factors include, stress, pain, hypoxia, severe exercise, surgery (especially anaesthetics such as cyclopropane and nitrous oxide).

Figure 3. The hypothalamus and thirst response

Osmoreceptors

These trigger ADH production in response to the following:

Dehydration produced by either water loss or lack of fluid intake. This increases the osmolarity of the plasma.

Relative dehydration in which there is no overall loss of the body's water content but the gain of sodium ions. The effect is again to raise the osmolarity of the plasma.

The precise location of the osmoreceptors is as yet unclear but it seems likely that they are to be found in the hypothalamus or the third ventricle of the brain (see lecture notes for the CNS in post module 3).

Baroreceptors

ADH secretion is also stimulated by changes in the circulating volume of body fluid that results in an increase or decrease of internal pressure.

A reduction of around 8-10% from the normal body volume of water due to haemorrhage or excess perspiration in desert conditions will result in ADH secretion. Pressure receptors are located in the atria of the heart and the pulmonary artery and vein and relay their messages to the hypothalamus via the vagus nerve. (see lecture notes on the autonomic nervous system from module 1).

Thirst response

This is connected to the response of the osmoreceptors. How in fact it actually works is not yet completely understood. We know that it takes up to one hour for ingested fluid to be completely absorbed and distributed in the body. So how is it that a person seems to know instinctively how much to drink? Why is thirst satisfied so quickly after taking in a fluid? Why are some fluids more thirst quenching than others are?

It would appear that the moistening of the mucosal linings of the mouth and pharynx initiates some sort of neurological response, which sends a message to the thirst centre of the hypothalamus. Perhaps more importantly stretch receptors in the gastro-intestinal tract also appear to transmit nerve messages to the thirst centre of the hypothalamus, which inhibit the thirst response.

Clinical problems with fluid balance

There are many ways in which the body's fluid balance can be upset resulting in severe problems and even death.

Dehydration will obviously occur in conditions of drought where the individual is unable to obtain water. However, conditions such as diarrhoea, severe vomiting, excessive sweating, bleeding, and surgical removal of body fluids can also result in dehydration. Three types of dehydration may occur as a result of the above. Hypertonic dehydration occurs when the fluid loss results in an increase in the electrolyte levels. This causes the blood pressure to fall and the blood to become thicker and can result in heart failure. Isotonic dehydration results in no perceptible difference from the normal electrolyte balance. This may lead to hypotonic dehydration where the fluid and electrolyte losses keep pace with each other. Any intake of pure water will alter the fluid electrolyte balance (too much water not enough electrolyte). This is why it is important in cases of severe diarrhoea to replace the body fluid with a balanced preparation of salts and water.

Problems with the production of urine can also lead to dehydration. Impaired ability to concentrate urine can be caused by:

Damage to the medulla of the kidneys. This can occur in chronic renal disorders such as pyelonephritis. The problem here results in damage to the Loop of Henle. Consequently inadequate water re-absorption in the loop occurs and the urine is too dilute resulting in fluid loss.

Inadequate ADH production. This can occur in the condition of diabetes insipidus. Individuals suffering from this may eliminate as much as 5 to 20 litres (8¹/₂pints to 34 pints) of urine per day. A psychological disorder known as polydipsia may occur where the sufferer is obsessed with drinking (usually water). This results in dilution of the plasma causing artificial lowering of the osmolarity and decreasing ADH secretion.

Solute diaresis occurs in individuals suffering from diabetes mellitus. Here elevated blood sugar levels brings about an inability of the kidney to re-absorb water resulting in excess fluid loss.

In any of the above conditions, fluid balance must be maintained otherwise dehydration or even hypovolaemic shock (due to insufficient volume of body fluid) may occur.

Oedema is a condition in which there is an excess of fluid within the interstitial compartment. This often results in tissue swelling (see fig. 4) it is common whenever there is a lymphatic blockage as in elephantiasis where a parasitic worm blocks the lymph vessels. Other causes result from an impaired ability of the body to dilute the urine.
 
 

Renal failure can lead to this problem especially the early stages of acute renal failure and the later stages of chronic renal failure.

Liver failure can result in inefficient metabolism of aldosterone a hormone that controls sodium (Na⁺) levels. Because the aldosterone is not properly broken down by the body the levels of this hormone will rise resulting in increased Na⁺ reabsorption and consequently ware reabsorption as water will be attracted to the sodium ions. This results in a concentrated dark coloured urine and fluid retention.

Heart failure means that the production of aldosterone is enhanced due to the lowering of the BP. The result is the same as in liver failure.

Excessive ADH secretion is a rare condition which may occur as a result of tumours in the lung, brain or pancreas resulting in increased reabsorption of water.

Fig. 4 Test for Oedema

Steady pressure of thumb onto lower leg for 10-20 seconds. If depression remains after removal of pressure fluid retention is indicated.

In any condition where there is fluid retention as in kidney failure, heart failure or liver failure as described above or as at rave parties where excess amounts of water are consumed a condition known as water intoxication may occur. This can lead to severe vomiting and diarrhoea. In more severe cases this can result in neurological dysfunction, circulatory shock and even death.
Click here to view a table of problems and nursing care associated with excess or insufficient body water.

Electrolyte Balance

Although some mention has been made of sodium, potassium etc., we have not, yet, explored the importance of maintaining the correct levels of electrolytes in the body.

An electrolyte is actually any chemical that dissociates into ions when dissolved in a solution. Ions can be positively charged (cations) or negatively charged (anions).

The major electrolytes found in the human body are:

Sodium (Na⁺)

Potassium (K⁺)

Calcium (Ca⁼⁼)

Magnesium (Mg⁺⁺)

Chloride (Cl^-)

Phosphate (HPO₄^--)

Sulphate (SO₄^--)

Bicarbonate (HCO₃^-)

Interstitial fluid and blood plasma are similar in their electrolyte make up. Na⁺ and Cl^- being the major electrolytes. In the intracellular fluid, K⁺ and HPO₄^- are the major electrolytes.

Sodium Balance

This plays a crucial role in the excitability of muscles and neurones. It is also of crucial importance in regulating fluid balance in the body. Sodium levels are extremely closely regulated by kidney function. Sodium is easily filtered in the glomerular portion of the kidneys and most of it is reabsorbed in the kidney tubules. (see lecture notes for the session on the kidneys for a fuller explanation of this process.) The rate of excretion is directly affected by the rate of filtration of sodium in the glomerulus. (GFR). Major factors that control the GFR include the blood pressure at the glomerulus and the stimulation of renal arteriole by the sympathetic nervous system.

The hormone aldosterone (as has already been mentioned) controls the rate of reabsorption of sodium in the distal convoluted tubules and the collecting ducts of the kidneys.

The amount of sodium reabsorbed in the proximal convoluted tubule remains almost constantly at around 67%.

Release of aldosterone occurs as a result of a complex process known as the renin-angiotensin-aldosterone pathway.

If the arterial BP falls, renin is released. This readily converts angiotensinogen into it's active form angiotensin I. This then travels to the capillary beds where it becomes angiotensin II which is one of the most powerful vasoconstrictors in the body. It is also a stimulator for aldosterone release from the adrenal glands. Because water has a close chemical affinity for sodium, it will follow that more water is reabsorbed in the kidney as well and this will put up the BP to a normal level.

An increase in the arterial BP will result in the release of atrial natriuretic factor (ANF) from the l. and r. atria of the heart. This hormone actually inhibits renin and aldosterone release. By so doing the loss of sodium by the kidneys is enhanced by the decrease of aldosterone stimulated reabsorption. As we have already seen that water will follow sodium, it follows that water is lost from the body allowing the BP to drop to a normal level.

Click here to view a table of problems and nursing care associated with sodium imbalance

Potassium Balance

Potassium is the major cation of intracellular fluid. Concentration within the cells is 28x that of the extra cellular fluids. As with sodium it is extremely important in the correct functioning of excitable cells such as muscles, neurones, sensory receptors etc. It is also importantly involved in the regulation of fluid levels within the cell and in maintaining the correct pH balance within the body.

Potassium output is usually equal to potassium input. Sodium reabsorption by aldosterone is usually in exchange for either hydrogen ions or potassium ions. Therefore if sodium ions are reabsorbed more potassium is lost and vice versa. Thus, high levels of potassium in the interstitial fluid stimulate aldosterone response. Diseases such as Cushing's disease (over production of ACTH) and hyperaldosteronism (overproduction of aldosterone) can lead to a condition known as hypokalaemia (symptoms caused by low potassium levels) which manifests in muscle weakness, flaccid paralysis, cardiac arrhythmias and alkalosis.

The pH balance of the body also affects potassium levels. In acidosis potassium excretion is decreased (leads to hyperkalaemia higher than normal levels of potassium) whereas the opposite occurs in alkalosis.

Click here to view a table of problems and nursing care associated with potassium imbalance

Calcium and Phosphorous Balance

Calcium is found mainly in the extracellular fluids whilst phosphorous is found mostly in the intracellular fluids.

Both are important in the maintenance of healthy bone and teeth.

Calcium is also important in the transmission of nerve impulses across synapses, the clotting of blood and the contraction of muscles. If the levels of calcium fall below normal level both muscles and nerves become more excitable.

Phosphorous is required in the synthesis of nucleic acids and high-energy compounds such as ATP. It is also important in the maintenance of pH balance.

If the levels of calcium in the body fall the parathyroid gland is stimulated to secrete parathyroid hormone (PH). This causes an increase in both the calcium and the phosphate levels of the interstitial fluids by releasing them from the reservoirs of these minerals lodged in the bones and the teeth. PH also decreases calcium excretion by the kidneys.

If the levels of calcium in the body become too high the thyroid gland releases a hormone called calcitonin. This inhibits the release of calcium and potassium from the bones. It also inhibits the absorption of calcium from the gastro-intestinal tract and increases calcium excretion by the kidneys.

Click here to see a table of problems and nursing care associated with calcium imbalance

Magnesium Balance

Most magnesium is found in the intracellular fluid and in bone. Within cells magnesium functions in the sodium-potassium pump and as an aid to the action of enzymes. It plays a role in muscle contraction, action potential conduction, and bone and teeth production. Aldosterone controls magnesium concentrations in the extracellular fluid. Low Mg⁺⁺ levels result in an increased aldosterone secretion, and the aldosterone increases Mg⁺⁺ reabsorption by the kidneys.

Chloride Balance

Chloride (Cl^-) is the most plentiful extracellular anion with an extracellular concentration 26 times that of its intracellular concentration. Chloride ions are able to diffuse easily across plasma membranes and their transport is closely linked to sodium movement, which also explains the indirect role of aldosterone in chlorine regulation. When sodium is reabsorbed, chlorine follows passively. It helps to regulate osmotic pressure differences between fluid compartments and is essential in pH balance. The chloride shift within the blood helps to move bicarbonate ions out of the red blood cells and into the plasma for transport. In the gastric mucosa, chlorine and hydrogen combine to form hydrochloric acid.

pH Balance

pH is a measurement of the hydrogen concentration of a solution. Lower pH values indicate a

higher hydrogen concentration, or a higher acidity. Higher pH values indicate a lower hydrogen concentration, or higher alkalinity.

Therefore, hydrogen ion balance is often referred to as pH balance or acid-base balance. Hydrogen ion regulation in the fluid compartments of the body is of critical importance to health. Even a slight change in hydrogen ion concentration can result in a marked alteration in the rate of chemical reactions. Changes in hydrogen ion concentration can also affect the distribution of ions such as sodium, potassium, and calcium. It also can affect the structure and function of proteins.

The normal pH of the arterial blood is 7.4, whereas that of the venous blood is 7.35. The lower pH of the venous blood is due to the higher concentration of carbon dioxide in the venous blood, which dissolves in water to make a weak acid, called carbonic acid. When the pH changes in the arterial blood, two conditions may result: acidosis or alkalosis. Acidosis is a condition occurring when the hydrogen ion concentration of the arterial blood increases and, therefore, the pH decreases. Alkalosis is the condition occurring when the hydrogen ion concentrations in the arterial blood decreases and the pH increases.

Sources of hydrogen ions in the body include: carbonic acid formed as mentioned above, sulphuric acid (a by-product in the breakdown of proteins), phosphoric acid (a by-product of protein and phospholipid metabolism), ketone bodies from fat metabolism, and lactic acid (a product formed in skeletal muscle during exercise).

About half of all the acid formed or introduced into the body is neutralised by the ingestion of alkaline foods. The remaining acid is neutralised by three major systems of the body. Namely chemical buffers, the respiratory system and the kidneys.

Chemical buffers have an instantaneous effect on pH changes. They are very effective in minimising pH changes but do not entirely eliminate the change. Within cells chemical buffer generally take about 2 to 4 hours to minimise changes in pH. The respiratory system also helps to minimise pH changes; the effects occur within minutes. Renal regulation of pH is able to completely return the pH to normal and requires from hours to several days.

A fuller discussion of these systems will be given during the relevant sessions during the course.

Nursing responsibilities

Nursing observation and interventions are essential to the detection and management of disturbances in fluid and electrolyte balance. There are a variety of situations in which this vital balance can be disturbed. The balance is so precarious (especially in children and old people) that it can result in extremely rapid changes in the individual's condition. Situations where recipients of nursing care may suffer fluid and electrolyte imbalance occur across all branches of nursing. The alcohol or drug abuser in mental health nursing can suffer fluid imbalance. Administration of medications can alter the balance. Compulsive behaviours can affect the intake of vital fluids. Vomiting, diarrhoea, skin problems ( the latter are relatively common in people with a learning disability) can result in excess fluid loss via the normally impermeable surface of the body. Burns, major surgery, traumatic accidents etc. Because of this, nurses of all branches need to be comfortable with recognising the early signs of dehydration or over-hydration and should be familiar with assessment and monitoring of fluid and electrolyte balance.
The next page has much more information relevant to nursing the person with fluid and electrolyte imbalances.

This page last updated on 03-May-1999 22:04 +0100

e-mail any queries or comments to johnross@cwcom.net