Overview
Water is the body’s main ingredient, making up roughly 55 to 60 percent of adult body weight, and the substances dissolved in it must be held within tight limits. This lesson covers where body water sits, how it shifts between compartments, which electrolytes matter most, and how the body defends the acidity of the blood. These are the numbers clinicians watch first in seriously ill patients.
Body Fluid Compartments
Total body water is divided between two main spaces.
| Compartment | Location | Share of body water |
|---|---|---|
| Intracellular fluid (ICF) | Inside cells | About two-thirds |
| Extracellular fluid (ECF) | Outside cells | About one-third |
The extracellular fluid is itself split into interstitial fluid, which bathes the cells, and plasma, the liquid part of blood inside vessels. Water moves freely between these spaces, so a change in one eventually affects the others.
Osmosis and Tonicity
Water crosses cell membranes by osmosis, moving from where solutes are less concentrated toward where they are more concentrated. Because water follows solute, the concentration of dissolved particles controls fluid distribution.
Tonicity describes how a surrounding solution affects a cell:
- Isotonic: equal solute concentration; no net water movement, so the cell keeps its size. Normal saline is used clinically for this reason.
- Hypotonic: lower outside solute; water enters and the cell swells and may burst.
- Hypertonic: higher outside solute; water leaves and the cell shrinks.
This is why the composition of intravenous fluids matters so much: the wrong tonicity can damage cells.
Key Electrolytes
Electrolytes are charged minerals essential to nerve signaling, muscle contraction, and fluid balance.
| Electrolyte | Main location | Key roles |
|---|---|---|
| Sodium (Na+) | Extracellular | Governs extracellular volume; drives water balance; nerve impulses |
| Potassium (K+) | Intracellular | Sets resting membrane potential; critical for heart rhythm |
| Calcium (Ca2+) | Bone and extracellular | Muscle contraction, clotting, nerve function, bone strength |
| Chloride (Cl-) | Extracellular | Main negative ion; helps balance charge and fluid; part of stomach acid |
Sodium and potassium deserve special note. Sodium largely determines how much water stays in the extracellular space, so it drives blood volume. Potassium sets the resting membrane potential of excitable cells, so even small changes can disturb the heartbeat, a point that links directly to the membrane and cardiac lessons.
Dehydration and Fluid Balance
Dehydration is a deficit of body water, from inadequate intake or excess loss through sweating, vomiting, diarrhea, or fever. As water is lost, blood volume falls, blood pressure drops, and the heart rate rises to compensate. Signs include thirst, dry mucous membranes, reduced urine output, and, when severe, confusion and low blood pressure. Treatment restores both water and the electrolytes lost with it, because replacing water alone can dangerously dilute sodium.
Acid–Base Balance
Enzymes work only within a narrow acidity range, so arterial blood pH is held near 7.35 to 7.45. The body defends this with buffers plus two organ systems.
The bicarbonate buffer system is central and can be read as a chemical seesaw:
CO2 + H2O <-> H2CO3 <-> H+ + HCO3-
Add carbon dioxide or acid and the balance shifts to release hydrogen ions, lowering pH. Remove carbon dioxide or add bicarbonate and pH rises. Two organs adjust the two ends of this equation on different timescales:
- The lungs change breathing to alter carbon dioxide within minutes. Breathing faster removes carbon dioxide and raises pH; breathing slower retains it and lowers pH.
- The kidneys adjust bicarbonate reabsorption and acid excretion over hours to days, a slower but powerful correction.
Disturbances are named by cause and direction:
| Disturbance | Primary problem | pH |
|---|---|---|
| Respiratory acidosis | Too little breathing; carbon dioxide builds up | Low |
| Respiratory alkalosis | Too much breathing; carbon dioxide falls | High |
| Metabolic acidosis | Excess acid or loss of bicarbonate | Low |
| Metabolic alkalosis | Loss of acid or excess bicarbonate | High |
When one system is the source of a problem, the other tries to compensate: the lungs speed up to offset a metabolic acidosis, for example.
Clinical Relevance
Fluid and electrolyte disorders are among the most common problems in emergency and hospital care. A patient with severe diarrhea can lose enough water and bicarbonate to develop dehydration and metabolic acidosis together. Diabetic ketoacidosis floods the blood with acids, driving pH down and prompting the deep, rapid breathing that blows off carbon dioxide to compensate. Abnormal potassium, high or low, can trigger fatal heart rhythms, which is why it is checked urgently. And the choice of intravenous fluid rests on tonicity and electrolyte content, so understanding osmosis, electrolytes, and acid–base balance is foundational to safe patient care.