WATER AND ELECTROLYTES: A TEACHER’S COMPREHENSIVE GUIDE
Welcome, future healthcare professionals!
Water forms the major component of body mass. Lack of water increases blood pressure, malfunctions heart, and also results in failure of kidneys. On an average 60% (range 45-70%) of total body weight consists of water. It occupies around 74% of muscle and organ weight but only 20-30% of adipose tissue; this is because water content in a muscular body is much more than in a fatty body. Due to the high percentage of lean body mass, men in comparison to women have a higher percentage of water in their total body weight.
Dpharmguru’s exam insights:
In my years of teaching physiology and biochemistry, I have observed that students often get confused about water distribution in the body. Remember: Water makes up about 60% of body weight in men and 50-55% in women. Why the difference? Women have more adipose tissue, which contains less water. This is a common exam question!
DISTRIBUTION OF WATER IN THE BODY
Total body water can broadly be classified into intracellular and extracellular water. Water found inside the cell membrane is called the intracellular water, while that outside the cell membrane is called the extracellular water, e.g., water in the plasma (intravascular water), water around the cells (interstitial fluid), the humour of eye, cerebrospinal fluid, and water present in gastrointestinal and urinary tracts.
Extracellular water provides the medium for metabolic exchanges, while intracellular water provides the medium for chemical reactions of cellular metabolism.
Table 10.1: Distribution of Water in the Body
| Compartment | Litres | % Total Body Water |
|---|---|---|
| Total Body Water | 40 | 100 |
| Intracellular | 23 | 57 |
| Extracellular | 17 | 43 |
| Plasma | 2.8 | 7 |
| Interstitial | 8.0 | 20 |
| Bone | 2.8 | 7 |
| Dense Connective Tissue | 2.8 | 7 |
| Trans-cellular | 1.6 | 4 |
Dpharmguru’s exam insights:
A common exam question is: “What percentage of total body water is intracellular versus extracellular?” The answer is approximately 57% intracellular and 43% extracellular. Also, remember that plasma is about 7% of total body water—this is a frequently tested value!
FUNCTIONS OF WATER IN THE BODY
1. Provides Protection
Water regulates the body temperature and keeps the tissues moist. It keeps the body hydrated, maintains an optimum level of moisture around the sensitive areas of body, tissues, blood, bones, and the brain. It acts as a lubricant and cushion, thus protects joints and spinal cord.
2. Eliminates Waste from the Body
The body eliminates waste through perspiration, urination, and defecation; therefore adequate water intake is essential. Excretory organs of the body (sweat glands, kidneys, liver, and intestines) utilise water to flush out waste from the body.
3. Aids in Digestion
Digestion is the process of degradation through enzymes, the medium of which is water. Water dissolves many minerals, fibres, and nutrients. It also provides medium for absorption of the breakdown products of digestion.
4. Prevents Dehydration
Body loses fluids excessively during vigorous exercise, sweat in the high heat, fever, and in illness causing vomiting or diarrhoea (loss of excess fluid).
5. Keeps Temperature Moderate
Within the body various enzymatically catalysed chemical reactions occur. These enzymes show optimum activity on a specific range of temperature. Water helps in keeping the range of temperature in the human body moderate for these enzymes to act properly.
WATER TURNOVER AND BALANCE
Body water turnover is the replacement of body water in a day or other given period, and shows body water homeostasis. Water turnover is considered in terms of external balance and internal fluxes.
External Balance
Sensitive mechanisms that respond to variations in osmolality or intravascular volume regulate total body water volume. Daily water requirements are calculated using a variety of criteria, the most accurate of which is metabolic rate.
Estimation of Daily Water Requirements in Unstressed Healthy Adults
- Based on metabolic rate: 80-110 ml/100 kcals
- Based on body surface area: 1.5 L/m²/day
- Based on weight: 30-40 ml/kg/day
Components of Daily Obligatory Water Loss
| Way of Water Excretion | Typical Values |
|---|---|
| Insensible Loss | 800 ml |
| Minimal Sweat Loss | 100 ml |
| Faecal Loss | 200 ml |
| Minimal Urine Volume | 500 ml |
| Total | 1,600 ml |
Dpharmguru’s exam insights:
The daily obligatory water loss is approximately 1,600 ml. This is a common exam question! Remember: Insensible loss (through skin and lungs) is the largest component at 800 ml/day. Also, the minimum urine volume required to excrete solutes is 500 ml/day.
Internal Balance
According to Starling’s hypothesis, the net movement of water between the intravascular and interstitial compartments across the capillary membrane is determined by the balance of hydrostatic pressure and oncotic pressure.
Internal Fluxes Include
- Water turnover in the body’s capillaries via diffusion is huge, i.e., about 80,000 liters/day.
- With normal activity, lymph flow ranges from 1-2.5 liters/day.
- Glomerular filtration rate is 180 litres/day, with renal tubules reabsorbing the great majority.
- Bowel fluid turnover rate is approximately 8-9 litres/day.
ELECTROLYTES
Any substance that contains free ions with the property of electrical conductance is known as an electrolyte. An electrolyte is simply a material that undergoes dissolution in water to form a solution conducting electric current.
Dietary Intake of Electrolytes
| Electrolyte | Primary Roles | Target Dose per 250mL | Daily Intake |
|---|---|---|---|
| Sodium | Muscle contraction, Nerve transmission | 150-250 mg | 1500-4500 mg |
| Chloride | Peak muscle function | 45-75 mg | 3000 mg |
| Potassium | Muscle contraction, Nerve transmission, Glycogen formation | 50-80 mg | 2500-4000 mg |
| Magnesium | Muscle relaxation, ATP Production | 20-30 mg | 400-800 mg |
| Calcium | Bone health, Nerve transmission, Muscle contraction | 10-15 mg | 1200-1600 mg |
Electrolyte Composition of the Body Fluids
| Electrolyte | ECF (mEq/L) | ICF (mEq/L) |
|---|---|---|
| Sodium | 142 | 14 |
| Potassium | 4 | 140 |
| Calcium | 5 | 4 |
| Magnesium | 2 | 30 |
| Chloride | 104 | 2 |
| Bicarbonate | 25 | 8 |
| Phosphate | 2.3 | 40 |
Dpharmguru’s exam insights:
The difference between ECF and ICF electrolyte composition is frequently tested. Remember: Sodium is the major cation in ECF (142 mEq/L), while Potassium is the major cation in ICF (140 mEq/L). Also, Chloride is the major anion in ECF (104 mEq/L), while Phosphate is the major anion in ICF (40 mEq/L).
Physiological Importance of Electrolytes
- Regulation of blood oxygen and pH level, i.e., acidity.
- Controlling the fluid distribution and water balance in the body.
- Maintaining an electrical gradient across the cell membranes, crucial for nerve transmission and muscle contraction.
- Allowing the movement of nutrients within the cells and the waste products out of the cells.
Normal Serum Electrolyte Values
| Electrolyte | Reference Intervals |
|---|---|
| Bicarbonate (HCO₃⁻) | 22-26 mmol/L |
| Chloride (Cl⁻) | 96-106 mmol/L |
| Phosphate (PO₄³⁻) | 0.78-1.42 mmol/L |
| Potassium (K⁺) | 3.5-5.0 mmol/L |
| Magnesium (Mg²⁺) | 0.75-1.25 mmol/L |
| Sodium (Na⁺) | 135-145 mmol/L |
| Calcium (Ca²⁺) Total | 2.15-2.55 mmol/L |
| Calcium (Ionised) | 1.16-1.32 mmol/L |
DEHYDRATION
Dehydration is a condition in which the body’s intravascular, interstitial and intracellular fluid levels drop when body fluids are deficient in water and electrolytes. Dehydration either occurs alone or along with other electrolytes or acid-base imbalances.
Causes of Dehydration
- Extreme loss of gastrointestinal fluids due to vomiting, diarrhoea, gastrointestinal suctioning, and intestinal drainage is the most common cause.
- Hypovolemia: Loss of extracellular fluid volume, thus reducing circulating blood volume.
- Isotonic Dehydration: Loss of electrolytes along with fluid.
- Third Spacing: A shift of fluid from the vascular space into an area where it does not support normal physiologic processes.
Signs and Symptoms of Dehydration
- Dryness of mucous membrane
- Decreased tongue size
- Anxiety and restlessness
- Reduced alertness
- Dry skin
- Pale and cool extremities
- Fatigue
- Hypouresis (decreased urine output)
- Falling systolic/diastolic pressure (severe dehydration)
Treatment and Management
- For those who are able to take oral fluids, oral rehydration is the safest and most effective treatment.
- For mild fluid shortages with low electrolyte loss, water alone is effective.
- For severe fluid deficits with electrolyte loss (by vomiting and diarrhoea), electrolyte solutions (sports drinks and rehydrating solutions) are effective.
- For severe fluid deficit where patient is unable to take fluids, replacement fluids are administered intravenously.
Oral Rehydration Therapy (ORT)
ORT is a simple, cheap and effective therapy used for treating dehydration caused by diarrhoea or gastroenteritis (caused by cholera or rotavirus). Under ORT, a fluid containing a suitable combination of carbohydrates and electrolytes is given through oral route. It is being used all over the world, but mainly in developing countries, where it has saved millions of children’s lives suffering from diarrhoea.
ORS Composition (WHO/UNICEF Recommended)
| Component | Amount | Ions | Concentration |
|---|---|---|---|
| NaCl | 2.6 g | Na⁺ | 75 mM |
| KCl | 1.5 g | K⁺ | 20 mM |
| Trisodium citrate | 2.9 g | Cl⁻ | 65 mM |
| Glucose | 13.5 g | Citrate | 10 mM |
| Water | 1 L | Glucose | 75 mM |
| Total Osmolarity | — | — | 245 mOsm/L |
Dpharmguru’s exam insights:
ORS is a lifesaving therapy for diarrhoea. The WHO/UNICEF recommended ORS has a total osmolarity of 245 mOsm/L (reduced from 310 mOsm/L). Remember: The key components are sodium, potassium, chloride, citrate, and glucose. Potassium is the chief constituent because large amounts of K⁺ are lost during diarrhoea. This is a common exam question!
Uses of ORT
- Infectious diarrhoea
- Short bowel syndrome
- Diarrhoeal illness in nursing home residents
- Paediatric viral illnesses
- AIDS
- Salt wasting nephropathies
SUMMARY
Water is the major component of the human body, making up about 60% of body weight in men and 50-55% in women. Total body water is distributed between intracellular (57%) and extracellular (43%) compartments. Water performs vital functions including protection, waste elimination, digestion, temperature regulation, and prevention of dehydration.
Electrolytes are ionic substances that conduct electricity in solution. Key electrolytes include sodium, potassium, calcium, magnesium, chloride, bicarbonate, and phosphate. Each has specific functions in the body, and their balance is essential for normal physiological function. Dehydration occurs when fluid loss exceeds intake, and can be managed with oral rehydration therapy or intravenous fluids.
Dpharmguru’s exam insights:
Understanding water and electrolyte balance is essential for understanding clinical conditions like dehydration, electrolyte disturbances, and acid-base disorders. In exams, pay special attention to: (1) The distribution of body water, (2) The normal serum electrolyte values, (3) The difference between ECF and ICF composition, and (4) The components and uses of ORS. Remember: “Water is the medium of life—know its balance!”
REFERENCES AND FURTHER READING
- Park, K. (2022). Park’s Textbook of Preventive and Social Medicine (26th ed.). Banarsidas Bhanot Publishers.
- Guyton, A. C., & Hall, J. E. (2020). Guyton and Hall Textbook of Medical Physiology (14th ed.). Elsevier.
- Murray, R. K., Bender, D. A., Botham, K. M., et al. (2018). Harper’s Illustrated Biochemistry (31st ed.). McGraw-Hill.
- World Health Organization (WHO). (2022). Oral Rehydration Therapy Guidelines. Retrieved from https://www.who.int.
- National Institutes of Health (NIH). (2022). Water and Electrolyte Balance Resources. Retrieved from https://www.nih.gov.
Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult qualified healthcare professionals for medical concerns.
written by:
Dr. N. Sujith Kumar
