Review of Fluid Balance
Water is a major component of the body and is found both within and outside the cells. It is critical to homeostasis, the maintenance of a relatively constant and favorable environment for the cells. Water is the medium within which metabolic reactions and other processes take place. It also comprises the transportation system for the body. For example, water carries nutrients into cells and removes wastes, transports enzymes in digestive secretions, and moves blood cells around the body. Without adequate fluid, cells cannot continue to function, and death results. Fluid also facilitates the movement of body parts, for example, the joints and the lungs.
Fluid Compartments
Although the body appears to be a solid object, approximately 60% of an adult's body weight consists of water, and an infant's body is about 70% water. Female bodies, which contain a higher proportion of fatty tissue, have a lower percentage of body weight as water than male bodies. The elderly and the obese also have a lower proportion of water in their bodies. Individuals with the less fluid reserve are more likely to be adversely affected by any fluid imbalance.
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The fluid is distributed between the intracellular compartment (ICF), or fluid inside the cells, and the extracellular compartment (ECF).
ECF includes the:
- Intravascular fluid (IVF) or blood;
- Interstitial fluid (ISF) or intercellular fluid; cerebrospinal fluid (CSF); and
- Transcellular fluids present in various secretions, such as those in the pericardial (heart) cavity or the synovial (joint) cavities.
In an adult male, blood constitutes about 4% of body weight and interstitial fluid about 15%; the remaining transcellular fluids amount to about 1% of total body weight. Water constantly circulates around the body and moves between various compartments. For example, CSF forms continuously from the blood and is reabsorbed back into the general circulation. A large volume of water (up to 8 liters in 24 hours) is present in the digestive secretions entering the stomach and small intestine, and this fluid is reabsorbed in the colon, making up a very efficient water recycling system.
Movement of Water
To maintain a constant level of body fluid, the amount of water entering the body should equal the amount of water leaving the body. The fluid is added to the body through the ingestion of solid food and fluids and as a product of cell metabolism. The fluid is lost in the urine and feces as well as through insensible (invisible) losses through the skin (perspiration) and exhaled air.
Control of fluid balance is maintained by:
- The thirst mechanism in the hypothalamus, the osmoreceptor cells of which sense the internal environment, both fluid volume, and concentration and then promote the intake of fluid when needed;
- The hormone, antidiuretic hormone (ADH), which controls the amount of fluid leaving the body in the urine. ADH promotes reabsorption of water into the blood from the kidney tubules; and
- The hormone aldosterone, which determines the reabsorption of both sodium ions and water from the kidney tubules. These hormones conserve more fluid when there is a fluid deficit in the body.
Fluid constantly circulates throughout the body and moves relatively freely, depending on the permeability of the membranes between compartments, by the processes of filtration or osmosis. Water moves between the vascular compartment or blood and the interstitial compartment through the semi-permeable capillary membranes, depending on the relative hydrostatic and osmotic pressures. Proteins and electrolytes contribute to the osmotic pressure of a fluid and therefore are very important in maintaining fluid volumes in various compartments. Hydrostatic pressure may be viewed as the "push" force and osmotic pressure as the "pull.'
At the arteriolar end of the capillary, the blood hydrostatic pressure (or blood pressure) exceeds the opposing interstitial hydrostatic pressure and the plasma colloid osmotic pressure of the blood, and therefore fluid moves out from (or is "pushed" out of) the capillary into the interstitial compartment. At the venous end of the capillary, the blood hydrostatic pressure is greatly decreased and osmotic pressure higher, and therefore fluid tends to shift (or is "pulled") back into the capillary. It is easier to remember the direction of movement if one thinks of the movement of nutrients and oxygen out of the arterial blood toward the cells and the flow of wastes and carbon dioxide from the cell back into the venous blood. Any change in the relative values of hydrostatic pressure or osmotic pressure in the compartments alters the fluid shift. Excess fluid and any protein in the interstitial compartment can return to the circulation through the lymphatic capillaries.
Many cells have mechanisms to control intracellular volume. A major factor in the movement of water through cell membranes is the difference in osmotic pressure between the two compartments. As the relative concentrations of electrolytes in the interstitial fluid and intracellular fluid change, the osmotic pressure also changes, causing water to move across the cell membrane by osmosis. For example, if an erythrocyte is placed in a dilute solution (low osmotic pressure), water will enter the cell, causing it to swell and eventually rupture.
Fluid Excess-Edema
Fluid excess occurs in the extracellular compartment and maybe isotonic, hypotonic, or hypertonic, depending on the cause. The tonicity or concentration of solute in the fluid affects fluid shifts between compartments, including the cells.
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Edema refers to an excessive amount of fluid in the interstitial compartment, which causes swelling or enlargement of the tissues. Edema may be localized in one area or generalized throughout the body. Depending on the type of tissue and the area of the body, edema may be highly visible or relatively invisible, or not accurately reflect the amount of fluid hidden in the area; for example, facial edema is usually visible but edema of the liver or a limb may not be. Edema is usually more severe in dependent areas of the body, where the force of gravity is greatest, such as the buttocks, ankles, or feet of a person in a wheelchair. Prolonged edema can interfere with venous return, arterial circulation, and cell function in the affected area.
Causes of Edema
Edema has four general causes.
1/ The first cause is increased capillary hydrostatic pressure (equivalent to BP or blood pressure), which prevents the return of fluid from the interstitial compartment to the venous end of the capillary, or forces excessive amounts of fluid out of the capillaries into the tissues. The latter is a cause of pulmonary edema, in which excessive pressure, often due to increased blood volume, can force fluid into the alveoli, interfering with respiratory function. Specific causes of edema related to increased hydrostatic pressure include increased blood volume (hypervolemia) associated with kidney failure, pregnancy, and congestive heart failure.
In pregnancy, the enlarged uterus compresses the veins, and when a pregnant woman must stand still for long periods of time, the pressure in the leg veins can become quite elevated, causing edema in the feet and legs. In some people with congestive heart failure, the blood cannot return easily through the veins to the heart, raising the hydrostatic pressure in the legs and abdominal organs and causing ascites, or fluid in the abdominal cavity.
2. Secondly, edema may be related to the loss of plasma proteins, particularly albumin, which results in a decrease in plasma osmotic pressure. Plasma proteins usually remain inside the capillary and seldom move through the semipermeable capillary membrane. The presence of fewer plasma proteins allows more fluid to leave the capillary and less fluid to return to the venous end of the capillary.
Protein may be lost in the urine through kidney disease, or synthesis of protein may be impaired in patients with malnutrition and mal-absorption diseases or with liver disease. Protein levels may drop acutely in burn patients who have large areas of burned skin; the subsequent inflammation and loss of the skin barrier allow the protein to leak out of the body.
Frequently excessive sodium levels in the extracellular fluid accompany the two causes just mentioned. When sodium ions are retained, they promote the accumulation of fluid in the interstitial compartment by increasing the osmotic pressure and decreasing the return of fluid to the blood.
Blood volume and blood pressure are usually elevated as well. High sodium levels are common in patients with heart failure, high blood pressure, kidney disease, and increased aldosterone secretion.
3. Edema may result from obstruction of the lymphatic circulation. Such an obstruction usually causes localized edema because excessive fluid and protein are not returned to the general circulation. This situation may develop if a tumor or infection damages a Lymph node or if lymph nodes are removed, as they may be in cancer surgery.
4. The fourth cause of edema is increased capillary permeability. This usually causes localized edema and may result from an inflammatory response. In this case, histamine and other chemical mediators released from cells following tissue injury cause increased capillary permeability and increased fluid movement into the interstitial area. Protein also leaks into the interstitial compartment, increasing the osmotic pressure in ISF and thus holding more fluid in the interstitial area. A general increase in capillary permeability can result from some bacterial toxins, or from large burn wounds, both leading to hypovolemia and shock.
Effects of Edema
- A local area of swelling may be visible and may be very pale or red in color, depending on the cause.
- Pitting edema occurs in the presence of excess interstitial fluid, which moves aside when firm pressure is applied by the fingers. A depression or "pit" remains after the finger is removed.
- In people with generalized edema there is a significant increase in body weight, which may indicate a problem before there are other visible signs of it.
- Functional impairment due to edema may occur, for example, when it restricts the movement of joints. Edema of the intestinal wall may interfere with digestion and absorption. Edema or accumulated fluid around the heart or lungs impairs the movement and filling of the organ.
- Pain may occur if edema exerts pressure on the nerves locally, as with the headache that develops in patients with cerebral edema. If cerebral edema becomes severe, the pressure can impair brain function because of ischemia and can cause death. When viscera such as the kidney or liver are edematous, the capsule is stretched, causing pain.
- With sustained edema the arterial circulation may be impaired. The increased interstitial pressure may restrict arterial blood flow into the area, preventing the fluid shift that carries nutrients into the cells. This can prevent normal cell function and reproduction and eventually results in tissue necrosis or the development of ulcers. This situation is evident in individuals with severe varicose veins in the legs-large, dilated veins that have high hydrostatic pressure. Varicose veins can lead to fatigue, skin breakdown, and varicose ulcers. The ulcers do not heal easily because of the continued insufficient blood supply.
- Other complications: For example, in dental practice, it is difficult to take accurate impressions when the tissues are swollen; dentures do not fit well, and sores may develop that often are slow to heal and become infected because the blood flow is impaired.
Fluid Deficit-Dehydration
Dehydration refers to insufficient body fluid resulting either from inadequate intake or excessive loss or a combination of the two. Losses are more common and affect the extracellular compartment first. Water can shift within the extracellular compartments. For example, if the fluid is lost from the digestive tract because of vomiting, water shifts from the vascular compartment into the digestive tract to replace the lost secretions. If the deficit continues, eventually fluid is lost from the cells, impairing cell function.
As a general guide to extracellular fluid loss, a mild deficit is defined as a decrease of 2% in body weight, a moderate deficit as a 5% weight loss, and severe dehydration as a decrease of 8%. This figure should be adjusted for the individual's age, body size, and condition. Dehydration is a more serious problem for infants and elderly people, who lack fluid reserves and the ability to conserve fluid quickly.
Also, infants experience not only greater insensible water losses through their proportionately larger body surface area but also an increased need for water owing to their higher metabolic rate. The vascular compartment is rapidly depleted in an infant (hypovolemia), affecting the heart, brain, and kidneys. This is indicated by decreasing urine output and increasing lethargy.
Water loss is often accompanied by a loss of electrolytes and sometimes of proteins, depending on the specific cause. For example, sweating results in a loss of water and sodium chloride. Electrolyte losses can influence water balance significantly because osmotic pressures change between compartments. To restore balance, electrolytes, as well as fluid, must be replaced. Isotonic dehydration refers to a proportionate loss of fluid and electrolytes, hypotonic dehydration to a loss of more electrolytes than water, and hypertonic dehydration to a loss of more fluid than electrolytes. The latter two types of dehydration cause signs of electrolyte imbalance and influence the movement of water between the intracellular and extracellular compartments.
Causes of Dehydration
Common causes of dehydration include:
- Vomiting and diarrhea, both of which result in the loss of numerous electrolytes and nutrients such as glucose, as well as water. Drainage or suction of any portion of the digestive system can also result in deficits;
- Excessive sweating with loss of sodium and water;
- Diabetic ketoacidosis with loss of fluid, electrolytes, and glucose in the urine; and
- Insufficient water intake in an elderly or unconscious person.
Effects of Dehydration
Initially, dehydration involves a decrease in interstitial and intravascular fluids. These losses may produce direct effects such as:
- Dry mucous membranes in the mouth or decreased skin turgor or elasticity;
- Lower blood pressure, weak pulse, and a feeling of fatigue; and
- Increased hematocrit, indicating a higher proportion of red blood cells compared to water in the blood.
The body attempts to compensate for the fluid loss by:
- Increasing thirst;
- Increasing the heart rate; constricting the cutaneous blood vessels, leading to pale and cool skin; and
- Decreasing urine output (water retention), leading to the high specific gravity of urine (more concentrated) as a result of renal vasoconstriction and increased secretion of ADH and aldosterone.
As the brain cells lose water, decreasing mental function, confusion, and loss of consciousness develop.
Third-Spacing: Fluid Deficit and Fluid Excess
Third spacing refers to a situation in which fluid shifts out of the blood into a body cavity or tissue where it is no longer available as circulating fluid. Examples include peritonitis, the inflammation and infection of the peritoneal membranes, and burns, in which extensive inflammation of the skin and underlying tissues causes fluid to shift out of the blood, causing edema. The result of this shift is a fluid deficit in the vascular compartment (hypovolemia) and a fluid excess in the interstitial space. Until the cause is removed, fluid remains in the "third space"-in the body but is not a functional part of the circulating fluids.
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