What would we do without body fluids? Fluids are vital to all forms of life. They help maintain body temperature and cell shape and they help transport nutrients, gases, and wastes.
With this post, we want to refresh your knowledge on body fluids. It is the first of three posts, the next two will be about how you can apply this knowledge into your daily practice.
Sensible and insensible fluids
Fluid losses from the skin and lungs are referred to as insensible losses because they can’t be measured or seen. Fluid losses from urination, defecation, wounds, and other means are referred to as sensible losses because they can be measured. A typical adult loses about 150 to 200 ml/day of fluid through defecation.
Average Total Body Water (TBW):
Adult male: 60%
Adult female: 55%
The body holds fluid in two basic areas: inside and outside cells. Fluid found inside cells is called intracellular fluid (ICF); fluid found outside them is extracellular fluid (ECF).
Remember: the risk of suffering a fluid imbalance increases with age. After 60 years old, water content drops to about 45%.
Fluids in the body aren’t generally found in pure forms. They’re usually found in three types of solutions: isotonic, hypotonic, and hypertonic.
Isotonic: an isotonic solution has the same solute concentration as another solution. If two fluids in adjacent areas are equally concentrated, they’re already in balance, so the fluid inside each compartment does not move (e.g. normal saline).
Hypotonic: a hypotonic solution has a lower solute concentration than another solution. If two fluids are in adjacent areas, one solution contains only one part sodium, and the one includes two parts. The first solution is hypotonic compared with the second one. As a result, fluid from the hypotonic solution would move into the second one until the two solutions reach an equal concentration of sodium.
Hypertonic: a hypertonic solution has a higher solute concentration than another solution. For instance, say one solution contains a large amount of sodium and a second solution contains hardly any. The first solution is hypertonic compared with the second solution. As a result, fluid from the second solution would move into the hypertonic one until the two solutions reach an equal concentration.
Maintaining fluid balance
Kidneys play a vital role in fluid balance. If kidneys don’t work properly, the body has a hard time controlling fluid balance. The workhorse of the kidney is nephron. A nephron consists of a glomerulus and a tubule. Glomerulus is a cluster of capillaries that filters blood. Like a vascular cradle, Bowman’s capsule surrounds the glomerulus. Along the length of the tubule, water and electrolytes are either excreted or retained depending on the body’s needs. If the body needs more fluid, for instance, it retains more. If it needs less fluid, less is reabsorbed and more is excreted. Electrolytes, such as sodium and potassium, are either filtered or reabsorbed throughout the same area. The resulting filtrate eventually becomes urine.
Remember: kidneys also secrete renin, an enzyme that activates the renin-angiotensin-aldosterone system.
Several hormones affect fluid balance, among them a water retainer called antidiuretic hormone (ADH). The hypothalamus produces ADH, but the posterior pituitary gland stores and releases it. An imbalance of fluids can stimulate the release of ADH, which, in turn, increases the kidneys’ reabsorption of water. The increased reabsorption of water results in more concentrated urine. The amount of ADH released varies throughout the day, depending on the body’s needs. This up-and-down cycle of ADH release keeps fluid levels in balance all day long.
To help the body maintain a balance of sodium and water as well as a healthy blood volume and blood pressure, special cells (called juxtaglomerular cells) near each glomerulus secrete an enzyme called renin. Through a complex series of steps, renin leads to the production of angiotensin II, a powerful vasoconstrictor.
Angiotensin II causes peripheral vasoconstriction and stimulates the production of aldosterone. Both actions raise blood pressure. Usually, as soon as the blood pressure reaches a normal level, the body stops releasing renin, and this feedback cycle of renin to angiotensin to aldosterone stops.”
The renin-angiotensin-aldosterone system isn’t the only factor at work balancing fluids in the body. A cardiac hormone called atrial natriuretic peptide (ANP) also helps keep that balance. Stored in the cells of the atria, ANP is released when atrial pressure increases. The hormone counteracts the effects of the renin-angiotensin-aldosterone system by decreasing blood pressure and reducing intravascular blood volume
Electrolytes help regulate water distribution, govern acid-base balance and transmit nerve impulses. They also contribute to energy generation and blood clotting.
- Main intracellular fluid (ICF) cation
- Regulates cell excitability
- Permeates cell membranes, thereby affecting the cell’s electrical status
- Helps to control ICF osmolality and, consequently, ICF osmotic pressure
- A leading ICF cation
- Contributes to many enzymatic and metabolic processes, particularly protein synthesis
- Modifies nerve impulse transmission and skeletal muscle response (unbalanced Mg concentrations dramatically affect neuromuscular processes)
- Maintains cell membrane stability (Lobo, Lewington, & Allison, 2013)
- Main ICF anion
- Promotes energy storage and carbohydrate, protein, and fat metabolism
- Acts as a hydrogen buffer
- Main extracellular fluid (ECF) cation
- Helps govern normal ECF osmolality (a shift in Na concentrations triggers a fluid volume change to restore normal solute and water ratios)
- Helps maintain acid-base balance
- Activates nerve and muscle cells
- Influences water distribution (with chloride)
- Main ECF anion
- Helps maintain normal ECF osmolality
- Affects body pH
- Plays a vital role in maintaining acid-base balance; combines with hydrogen ions to produce hydrochloric acid
- A major cation in teeth and bones; found in fairly equal concentrations in ICF and ECF
- Also found in cell membranes, where it helps cells adhere to one another and maintain their shape
- Acts as an enzyme activator within cells (muscles must have Ca to contract)”
- Aids coagulation
- Affects cell membrane permeability and firing level
- Present in ECF
- Regulates acid-base balance
Goals of fluid management
- Maintenance of normal body homeostasis
- Correction of life threatening imbalance
- Integrate fluid and electrolyte therapy with nutrition therapy
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