| Author: Estella Wetzel
|
| Author: Estella Wetzel
|
Where would we be without body fluids? Fluids are vital to all forms of life. They help to maintain body temperature, cell shape, as well as transport nutrients, gases, and wastes. Let's take a closer look at fluids and the way the body balances them.
Making gains equal losses
Just about all major organs work together to maintain the proper balance of fluid. To support that balance, the amount of fluid gained throughout the day must equal the amount lost. Some of those losses can be measured; others can't.
How insensible
Fluid losses from the skin and lungs are referred to as insensible losses because they can't be measured or seen. Losses from evaporation of fluid through the skin are constant but depend on a person's total body surface area. For example, the body surface area of an infant is greater than that of an adult relative to the respective weights. Because of this difference in body surface area—a higher metabolic rate, a larger percentage of extracellular body fluid, and immature kidney function—infants typically lose more water than adults do.
Changes in environmental humidity levels also affect the amount of fluid lost through the skin. Likewise, respiratory rate and depth affect the amount of fluid lost through the lungs. Tachypnea, for example, causes more water to be lost; bradypnea, less. Fever increases insensible losses of fluid from both the skin and lungs.
Now that's sensible
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 100 to 200 ml/day of fluid through defecation. In cases of severe diarrhea, losses may exceed 5,000 ml/day. (See Sites involved in fluid loss.)
Following the fluid
The body holds fluid in two basic areas, or compartments—inside the cells and outside the cells. Fluid inside the cells is intracellular fluid (ICF); fluid outside the cells is extracellular fluid (ECF). Capillary walls and cell membranes separate the intracellular and extracellular compartments. (See Fluid compartments.)
To maintain proper fluid balance, the distribution of fluid between the two compartments must remain relatively constant. In an average adult, the total amount of fluid is 42 L, with the total amount of ICF averaging 66% of the person's body weight, or about 28 L. The total amount of ECF averages 33% of the person's body weight, or about 12 L.
Memory JoggerTo help you remember which fluid belongs in which compartment, keep in mind that inter means between (as in interval—between two events) and intra means within or inside (as in intravenous—inside a vein).
ECF can be broken down further into interstitial fluid, which surrounds the cells, and intravascular fluid or plasma, which is the liquid portion of blood. In an adult, interstitial fluid accounts for about 75% of the ECF. Plasma accounts for the remaining 25%.
The body contains other fluids, called transcellular fluids, in the cerebrospinal column, pleural cavity, lymph system, joints, and eyes. Transcellular fluids generally aren't subject to significant gains and losses throughout the day, so they aren't discussed in detail here.
Water here, water there
The distribution of fluid within the body's compartments varies with age. Compared with adults, infants have a higher percentage of body water stored inside interstitial spaces. About 75% to 80% (40% ECF, 35% ICF) of the body weight of a full-term neonate is water. About 90% (60% ECF and 30% ICF) of the body weight of a premature (23 weeks' gestation) infant is water. The amount of water as a percentage of body weight decreases with age until puberty. In a typical 154-lb (70 kg) lean adult male, about 60% (93 lb [42 kg]) of body weight is water. (See The evaporation of time.)
Skeletal muscle cells hold much of that water; fat cells contain little of it. Female adults, who normally have a higher ratio of fat to skeletal muscle than male adults, typically have a somewhat lower relative water content. Likewise, a person with obesity may have a relative water content level as low as 45%. Accumulated body fat in these individuals increases weight without boosting the body's water content.
Fluids in the body generally aren't found in pure forms. There are three types of solutions fluids are found in: isotonic, hypotonic, and hypertonic.
Isotonic: Already at match point
An isotonic solution has the same solute (matter dissolved in solution) concentration as another solution. For instance, if two fluids in adjacent compartments are equally concentrated, they are already in balance, so the fluid inside each compartment stays put. No imbalance means no net fluid shift. (See Understanding isotonic fluids.)
For example, normal saline solution is considered isotonic because the concentration of sodium in the solution nearly equals the concentration of sodium in the blood.
Hypotonic: Get the lowdown
A hypotonic solution has a lower solute concentration than another solution. For instance, say one solution contains only one part sodium and another solution contains two parts; the first solution is hypotonic compared with the second solution. As a result, fluid from the hypotonic solution would shift into the second solution until the two solutions have equal concentrations of sodium. Remember that the body continually strives to maintain a state of balance, or equilibrium (also known as homeostasis). (See Understanding hypotonic fluids.)
Half-normal saline solution is considered hypotonic because the concentration of sodium in the solution is less than the concentration of sodium in the patient's blood.
Hypertonic: Just the highlights
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 hardly any; the first solution is hypertonic compared with the second solution. As a result, fluid from the second solution would shift into the hypertonic solution until the two solutions had equal concentrations. Again, the body continually strives to maintain a state of equilibrium (homeostasis). (See Understanding hypertonic fluids.)
For example, a solution of dextrose 5% in normal saline solution is considered hypertonic because the concentration of solutes in the solution is greater than the concentration of solutes in the patient's blood.
Just as the heart continually beats, fluids and solutes constantly move within the body. That movement allows the body to maintain homeostasis, a state of balance the body seeks. (See Fluid tips.)
Within the cells
Solutes within the intracellular, interstitial, and intravascular compartments of the body move through the membranes, separating those compartments in different ways. The membranes are semipermeable, meaning that they allow some solutes to pass through but not others. In this section, you'll learn the different ways fluids and solutes move through membranes at the cellular level.
Going with the flow
In diffusion, solutes move from an area of higher concentration to an area of lower concentration, which eventually results in an equal distribution of solutes within the two areas. Diffusion is a form of passive transport because no energy is required to make it happen. Like fish swimming with the current, the solutes go with the flow. (See Understanding diffusion.)
Giving that extra push
In active transport, solutes move from an area of lower concentration to an area of higher concentration. Like swimming against the current, active transport requires energy to make it happen.
The energy required for a solute to move against a concentration gradient comes from a substance called adenosine triphosphate or ATP. Stored in all cells, ATP supplies energy for solute movement in and out of cells. (See Understanding active transport.)
Some solutes, such as sodium and potassium, use ATP to move in and out of cells in a form of active transport called the sodium-potassium pump. (For more information on this physiologic pump, see chapter 5, When sodium tips the balance.) Other solutes that require active transport to cross cell membranes include calcium ions, hydrogen ions, amino acids, and certain sugars.
Letting fluids through
Osmosis refers to the passive movement of fluid across a membrane from an area of lower solute concentration and comparatively more fluid into an area of higher solute concentration and relatively less fluid. Osmosis stops when enough fluid has moved through the membrane to equalize the solute concentration on both sides of the membrane. (See Understanding osmosis.)
Within the vascular system
Within the vascular system, only capillary walls are thin enough to let solutes pass through. The movement of fluids and solutes through capillary walls plays a critical role in the body's fluid balance.
The pressure is on
The movement of fluids through capillaries—a process called capillary filtration—results from blood pushing against the walls of the capillary. That pressure, called hydrostatic pressure, forces fluids and solutes through the capillary wall.
When the hydrostatic pressure inside a capillary is greater than the pressure in the surrounding interstitial space, fluids and solutes inside the capillary are forced out into the interstitial space. When the pressure inside the capillary is less than the pressure outside of it, fluids and solutes move back into the capillary. (See Fluid movement through capillaries.)
Keeping the fluid in
A process called reabsorption prevents too much fluid from leaving the capillaries no matter how much hydrostatic pressure exists within the capillaries. When fluid filters through a capillary, the protein albumin remains behind in the diminishing volume of water. Albumin is a large molecule that generally can't pass through capillary membranes. As the concentration of albumin inside a capillary increase, fluid begins to move back into the capillaries through osmosis.
Think of albumin as a water magnet. The osmotic, or pulling, force of albumin in the intravascular space is called the plasma colloid osmotic pressure. The plasma colloid osmotic pressure in capillaries averages about 25 mm Hg. (See Albumin magnetism.)
As long as capillary blood pressure (hydrostatic pressure) exceeds plasma colloid osmotic pressure, water and solutes can leave the capillaries and enter the interstitial fluid. When capillary blood pressure falls below plasma colloid osmotic pressure, water and diffusible solutes return to the capillaries.
Normally, blood pressure in a capillary exceeds plasma colloid osmotic pressure in the arteriole end and falls below it in the venule end. As a result, capillary filtration occurs along the first half of the vessel; reabsorption, along the second. As long as capillary blood pressure and plasma albumin levels remain normal, the amount of water that moves into the vessel equals the amount that moves out.
Coming around again
Occasionally, extra fluid filters out of the capillary. When that happens, the excess fluid shifts into the lymphatic vessels located just outside the capillaries and eventually returns to the heart for recirculation.
Many mechanisms in the body work together to maintain fluid balance. Because one problem can affect the entire fluid-maintenance system, it's important to keep all mechanisms in check. Here's a closer look at what makes this balancing act possible.
The kidneys
The kidneys play a vital role in fluid balance. If the kidneys don't work correctly, the body has a hard time controlling fluid balance. The workhorse of the kidney is the nephron. Nephrons do the work of filtering out waste from the body's fluid.
A nephron consists of a glomerulus and a tubule. The tubule, sometimes convoluted, ends in a collecting duct. The glomerulus is a cluster of capillaries that filter blood. Like a vascular cradle, Bowman capsule surrounds the glomerulus.
Capillary blood pressure forces fluid through the capillary walls and into Bowman capsule at the proximal end of the tubule. Along the length of the tubule, water and electrolytes are either excreted or retained depending on the body's needs. For instance, when the body needs more fluid, it retains more. If it requires less, less is reabsorbed, and more fluid gets excreted.
Electrolytes, such as sodium and potassium, are either filtered or reabsorbed throughout the same area. The resulting filtrate flows through the tubule into the collecting ducts and eventually into the bladder as urine.
Superabsorbent
Nephrons filter about 125 ml of blood every minute, or about 180 L/day. This is called the glomerular filtration rate and usually leads to the production of 1 to 2 L of urine per day. The nephrons reabsorb the remaining 178 L or more of fluid, an amount equivalent to more than 30 oil changes for the family car!
A strict conservationist
If the body loses even 1% to 2% of its fluid, the kidneys take steps to conserve water. Perhaps the most crucial step involves reabsorbing more water from the filtrate, which produces more concentrated urine.
The kidneys must continue to excrete at least 20 ml of urine every hour (about 500 ml/day) to eliminate body wastes. Usually, a urine excretion rate that's less than 20 ml/hour indicates renal disease and impending kidney failure. The minimum excretion rate varies with age. (See The higher the rate, the greater the waste.)
The kidneys respond to fluid excesses by excreting urine that is more dilute, which rids the body of fluid and conserves electrolytes.
Antidiuretic hormone
Several hormones affect fluid balance, among them, a water retainer called antidiuretic hormone (ADH). (You may also hear this hormone called vasopressin.) The hypothalamus produces ADH, but the posterior pituitary gland stores and releases it. (See How antidiuretic hormone works.)
Adaptable absorption
Increased serum osmolality, or decreased blood volume, 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.
Likewise, decreased serum osmolality, or increased blood volume, inhibits the release of ADH and causes less water to be reabsorbed, making the urine less concentrated. 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. Like a dam in a river, the body holds water when fluid levels drop and releases it when fluid levels rise.
Memory JoggerRemember what ADH stands for—antidiuretic hormone—and you'll remember its job: restoring blood volume by reducing diuresis and increasing water retention.
Renin-angiotensin-aldosterone system
To help the body maintain a balance of sodium and water as well as a healthy blood volume and blood pressure, specialized 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 potent vasoconstrictor.
Angiotensin II causes peripheral vasoconstriction and stimulates the production of aldosterone. Both actions raise blood pressure. (See Aldosterone production.)
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 ends.
The ups and downs of renin
The amount of renin secreted depends on blood flow and the level of sodium in the bloodstream. If blood flow to the kidneys diminishes, as happens in a patient who is hemorrhaging, or if the amount of sodium reaching the glomerulus drops, the juxtaglomerular cells secrete more renin. The renin causes vasoconstriction and a subsequent increase in blood pressure.
Conversely, if blood flow to the kidneys increases, or if the amount of sodium reaching the glomerulus increases, juxtaglomerular cells secrete less renin. A drop-off in renin secretion reduces vasoconstriction and helps to normalize blood pressure.
Sodium and water regulator
The hormone aldosterone also plays a role in maintaining blood pressure and fluid balance. Secreted by the adrenal cortex, aldosterone regulates the reabsorption of sodium and water within the nephron. (See How aldosterone works.)
Triggering active transport
When blood volume drops, aldosterone initiates the active transport of sodium from the distal tubules and the collecting ducts into the bloodstream. When sodium is forced into the bloodstream, more water is reabsorbed, and blood volume expands.
Atrial natriuretic peptide
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 pressure in the atria increases. The hormone counteracts the effects of the renin-angiotensin-aldosterone system by decreasing blood pressure and reducing intravascular blood volume. (See How atrial natriuretic peptide works.)
This powerful hormone:
Stretch that atrium
The amount of ANP that the atria release rises in response to some conditions; for example, chronic kidney failure and heart failure.
Anything that causes atrial stretching can also lead to increases in the amount of ANP released, including orthostatic changes, atrial tachycardia, high sodium intake, sodium chloride infusions, and use of medications that cause vasoconstriction.
Thirst
Perhaps the simplest mechanism for maintaining fluid balance is the thirst mechanism. Thirst occurs with even small losses of fluid. Losing body fluids or eating highly salty foods leads to an increase in ECF osmolality. This increase leads to drying of the mucous membranes in the mouth, which in turn stimulates the thirst center in the hypothalamus. In an older adult, the thirst mechanism is less effective than it is in a younger person, leaving the older person more prone to dehydration. (See Dehydration in older adults.)
Quench that thirst
Usually, when a person is thirsty, they drink fluid. The ingested fluid is absorbed from the intestine into the bloodstream, where it moves freely between fluid compartments. This movement leads to an increase in the amount of fluid in the body and a decrease in the concentration of solutes, thus balancing fluid levels throughout the body.
If you answered all five questions correctly, congratulations! You're a fluid whiz.
If you answered four correctly, take a swig of water; you're just a little dry.
If you answered fewer than four correctly, pour yourself a glass of sports drink and enjoy an invigorating burst of fluid refreshment!
References
Ambalavanan, N., & Rosenkrantz, T. (Eds.). (2018). Fluid, electrolyte, and nutritional management of the newborn. Medscape.
Felsenfeld, A. J., & Levine, B. S. (2015). Calcitonin, the forgotten hormone: Does it deserve to be forgotten? Clinical Kidney Journal, 8(2), 180–187.
Ferri, F. F. (2019). Ferri's best test: A practical guide to clinical laboratory medicine and diagnostic imaging (4th ed.). Elsevier.
Kear, T. M. (2017). Fluid and electrolyte management across the age continuum. Nephrology Nursing Journal, 44(6), 491–497.
Luft, F. C. (2020). Did you know? Fluid-and-electrolyte replacement and the uncertainty principle. Acta Physiologica, 230(4), 1–8.
Merrill, G. (2021). Our intelligent bodies. Rutgers University Press Medicine.
Potter, P. A., Perry, A. G., Stockert, P. A., Hall, A. M., & Felver, L. (2021). Chapter 42: Fluid, electrolyte and acid-base balance. In Ostendorf, W. R. (Ed.), Fundamentals of nursing (10th ed., pp. 943–991). Elsevier Mosby.
Tkacs, N., Herrmann, L., & Johnson, R. (2021). Advanced physiology and pathophysiology: essentials for clinical practice. Springer Publishing Company.
In this chapter, you'll learn: