Fluid, Electrolyte, & Acid –Base Balance Shurouq Qadous 7/2/2011
ANATOMY AND PHYSIOLOGY Body fluid: As the primary body fluid, water is the most important nutrient of life. Although life can be sustained for many days without food, humans can survive for only a few days without water.
Water in the body functions primarily to: • Provide a medium for transporting nutrients to cells and wastes from cells, and for transporting substances such as hormones, enzymes, blood platelets, and red and white blood cells. • Facilitate cellular metabolism and proper cellular chemical functioning. • Act as a solvent for electrolytes and nonelectrolytes
Help maintain normal body temperature • Facilitate digestion and promote elimination • Act as a tissue lubricant • An insulator and shock absorber
Body Fluid Compartments Fluids are located in two main compartments, or spaces, in the body—the intracellular fluid(ICF) and extracellular fluid (ECF). ICF is the fluid within cells, constituting about 40% of an adult’s body weight, or 70% of the total-body water.
ECF is all the fluid outside the cells. It constitutes about 20% of an adult’s body weight, or 30% of total-body water. ECF includes* intravascular and *interstitial fluids. Intravascular fluid, or plasma, is the liquid component of the blood (ie, fluid found within the vascular system). Interstitial fluid is the fluid that surrounds tissue cells and includes lymph. *Transcellular fluids: CSF, Peritoneal, pericardial, pleural, pancreatic, intraocular, biliary, & synovial fluids.
The term total-body water or fluid refers to the total amount of water in the body expressed as a percentage of body weight. Variations in Fluid Content In a healthy person, total-body water constitutes about 50% to 60% of the body’s weight, depending on such factors as the person’s age, lean body mass, and sex.
Composition of Body Fluids ECF and ICF contain oxygen from the lungs, dissolved nutrients from the gastrointestinal tract, excretory products of metabolism such as carbon dioxide, and charged particles called ions. An ion is an atom or molecule carrying an electrical charge. Substances capable of breaking into electrically charged ions when dissolved in a solution are called electrolytes. Some ions develop a positive charge and are called cations. Others develop a negative charge and are called anions. These charges are the basis of chemical interactions in the body necessary for metabolism and other functions.
Electrolytes are measured in milliequivalent per liter of water (mEq/L) or milligrams per 100 milliliters (mg/100mL).The term milliequivalent refers to the chemical combining power of the ion. The is the unit of measure that describes the chemical activity of electrolytes. One milliequivalent of either a cation or an anion is chemically equivalent to the activity of 1 mg of hydrogen ion. Therefore, 1 mEq of any cation is equivalent to 1 mEq of any anion.
Movement of body Fluids and Electrolytes The body fluid compartments are separated from one another by cell membranes and the capillary membrane.” Selectively permeable”. Small particles such as ions, oxygen, and carbondioxide easily move across these membranes, but larger molecules like glucose and proteinhave more difficulty moving between fluid compartments. The methods by which electrolytes and other solutes move are:-
1-Osmosis is the movement of water across cell membranes, from the less concentrated solution to the more concentrated solutions. Solutes are substances that are dissolved in a liquid.Solutes may be crystalloids ( salts that dissolve readily into true solutions) or colloids (substances such as large protein molecules that do not readily dissolve into true solutions). Solvent is the component of a solution that can dissolve a solute. Water is the primary solvent in the body. The solutes are electrolytes, oxygen and carbon dioxide, glucose, urea, amino acids, and proteins.
Osmolality: concentration of solutes in body fluids (mOsm / kg). An isotonic solution has the same osmolality as body fluids. Normal saline , 0.9% sodium chloride, is an isotonicsolution. Hypertonic solutions have a higher osmolality than body fluids, 3% sodium chloride is a hypertonicsolutions. Hypotonic solutions have a lower osmolality than body fluids, 0.45% sodium chloride (one half normal saline) is ahypotonicsolutions.
Osmotic pressure is the power of a solution to draw water across a semi permeable membrane. Because a hypertonic solution has a greater osmolality, water moves out of the cells and is drawn into the intravascular compartment, causing the cells to shrink. Due to a lower osmolality, a hypotonic solution in the intravascular space moves out of the intravascular space and into intracellular fluid, causing cells to swell and possibly burst.
In the body, Plasma proteins (high molecular weights) exert an osmotic draw called Colloid osmotic pressure or Oncotic pressure, pulling water from the interstitial space into the vascular compartment.
2-Diffusion The solute moves from an area of higher concentration to an area of lower concentration until equilibrium is established. Gases also move by diffusion. Oxygen and carbon dioxide exchange in the lung’s alveoli and capillaries occurs by diffusion.
3-Filtration is a process whereby fluid and solutes move together across a membrane from one compartment to another . Passage is from an area of high pressure to one of lower pressure. An example of filtration is the movement of fluid and nutrients from the capillaries of the arterioles to the interstitial fluid around the cells.
Hydrostatic pressure is the pressure exerted by a fluid within a closed system on the walls of a container in which it is contained. The hydrostatic pressure of blood is the force exerted by blood against the vascular walls.
Filtration pressure is the difference between colloid osmotic pressure and blood hydrostatic pressure. These pressures are important in understanding how fluid leaves arterioles, enters the interstitial compartment, and eventually returns to the venules. The filtration pressure is positive in the arterioles, helping to force or filter fluids into interstitial spaces; it is negative in the venules and thus helps fluid enter the venules.
4-Active Transport Is a process that requires energy for the movement of substances through a cell membrane from an area of lesser solute concentration to an area of higher solute concentration. Active transport can be called “pumping uphill.” Substances believed to use active transport include amino acids, glucose (in certain places only, such as in the kidneys and intestines), and ions of sodium, chloride, potassium, hydrogen, phosphate, calcium, and magnesium.
Regulating body fluids The desirable amount of fluid intake and loss in adults ranges from 1500 to 3500 mL each 24 hours, with most people averaging 2500 mL per day. A person’s intake should normally be approximately balanced by output or fluid loss. A general rule is that in healthy adults, the output of urine normally approximates the ingestion of liquids, and the water from food and oxidation is balanced by the water loss through the feces, the skin, and the respiratory process. The intake–output balance may not always occur in a single 24-hour period but should normally be achieved within 2 to 3 days.
Fluid Sources Water for the body derives from several sources, including ingested liquids, food, and metabolism. • Ingested Liquids The ingestion of liquids makes up the largest amount of water normally taken into the body. Fluid intake is primarily regulated by the thirst mechanism. Located within the hypothalamus, the thirst control center is stimulated by intracellular dehydration and decreased blood volume.
Water in Food The water contained in food is the second largest source of water for the body. The amount ingested depends on the diet. For example, melons and citrus fruit are high in water content, whereas cereal and dried fruits have a relatively low water content. • Water From Metabolic Oxidation Water is an end product of the oxidation that occurs during the metabolism of food substances, specifically, carbohydrates, fats, and protein. This source also varies among different types of nutrients.
Fluid Losses Water is lost from the body through the kidneys as urine, through the intestinal tract in feces, and through the skin as perspiration. These losses are termed sensible water losses. Insensible water loss, that which is imperceptible also occurs. For example, an invisible amount of water is lost from the skin constantly through evaporation. Insensible loss from the lungs is moisture exhaled in breaths. Any deviations from normal ranges for a balanced water intake and output should alert the nurse to potential imbalances.
Maintaining Homeostasis Fluid homeostasis normally functions automatically and effectively. Almost every organ and system in the body helps in some way to maintain fluid homeostasis. Fluid balance is threatened when any organ fails to function properly. • The kidneys, frequently referred to as the master chemists of the body, normally filter 135 to 180 L of plasma daily in the adult, while excreting only 1.5 L of urine; selectively retain electrolytes and water and excrete wastes and excesses.
Antidiuretic Hormone It regulates water excretion from the kidney, is synthesized in the anterior portion of the hypothalamus and acts on the collecting ducts of the nephrons. When serum Osmolality rises, ADH is produced, causing the collecting ducts to become more permeable to water. This increased permeability allows more water to be reabsorbed into the blood. As more water is reabsorbed, urine output falls and serum osmolality decreases because the water dilutes body fluid. If serum Osmolality decreases, ADH is suppressed, the collecting ducts become less permeable to water and urine output increases.
Other factors also affect the production and release of ADH, including blood volume, temperature, pain, stress, and some drugs such as opiates, barbiturates, and nicotine.
Renin- angiotension- Aldestrone system Receptors found in the justaglomerular cells of the kidney nephrons. If blood flow or pressure to the kidney ↓→renin released →convert angiotensinogen to angiotension 1 →angiotension 2 by angiotension – converting enzyme. Angiotension 2 acts directly on the nephrons to promote sodium and water retention also stimulates the release of aldosterone from the adrenal cortex. The net effect of the renin-angiotension- aldosterone system is to restore blood volume (and renal perfusion) through sodium and water retention.
Atrial Natriuretic factor ANF is released from cells in the atrium of the heart in response to excess blood volume and stretching of the atrial walls. Acting on the nephrons, ANF promotes sodium wasting and acts as a potent diuretic, thus reducing vascular volume.ANF also inhibits thirst, reducing fluid intake.
Regulating electrolytes Electrolytes are important for: • Maintaining fluid balance • Contributing to acid-base regulation • Facilitating enzyme reactions • Transmitting neuromuscular reactions.
Most electrolytes enter the body through dietary intake and are excreted in the urine. Some electrolytes, such as sodium and chloride, are not stored by the body and must be consumed daily to maintain normal levels. Potassium and calcium, on the other hand, are stored in the cells and bones, when serum levels drop, ions can shift out of the storage into the blood to maintain adequate serum levels for normal functioning.
Sodium (Na+) - ECF. - Major contributor to serum osmolality. - Sodium functions in controlling and regulating water balance. - Aldestrone increases Na+ reabsorption in collecting duct of nephrons. Function: Regulating ECF volume and distribution Maintaining blood volume Transmitting nerve impulses and contracting muscles.
Potassium (K+) • ICF • Vital electrolyte for skeletal, cardiac, and smooth muscle activity. • Maintaining acid-base balance • Contributor of intracellular enzyme reactions. • Aldestrone increases K+ excretion • Insulin helps move K+ into cells tissues damage and acidosis shift K+ out of cells into ECF.
Function: • Maintaining ICF osmolality. • Transmitting nerve and other electrical impulses. • Regulating cardiac impulses transmission • Skeletal and smooth muscle function • Regulating acid-base balance
Calcium (Ca2+) - Is found in the skeletal system. - Small amount in Extracellular fluids. - Regulating muscle contraction and relaxation, neuromuscular function, and cardiac function • ECF calcium is regulated by complex interaction of parathyroid hormone, calcitriol, calcitonin and metabolite of vitamin D. - When calcium levels in the ECF fall, parathyroid hormone and calcitriol , causes calcium to be released from the bone into ECF and increase absorption of calcium in the intestine.
Function: • Forming bones and teeth • Transmitting nerve impulses • Regulating muscle contractions • Maintaining cardiac pacemaker • Blood clotting • Activating enzymes such as pancreatic lipase and phospholipase.
Bicarbonate (HCO3-): • Bicarbonate (HCO3-) an anion that is the major chemical base buffer within the body; found in both ECF and ICF. Function • Is essential for acid–base balance; bicarbonate and carbonic acid constitute the body’s primary buffer system.
Acid–Base Balance Body fluids must maintain an acid–base balance to sustain health and life. Acidity or alkalinity of a solution is determined by its concentration of hydrogen ions (H+). An acid is a substance containing hydrogen ions that can be liberated or released. An alkali, or base, is a substance that can accept or trap hydrogen ions.
An acid releases hydrogen, as follows: A base traps hydrogen, as follows:
The unit of measure used to describe acid–base balance is pH, which is an expression of hydrogen ion concentration and the resulting acidity or alkalinity of a substance. Normal blood plasma is slightly alkaline and has a normal pH range of 7.35 to 7.45.
The narrow range of normal pH is achieved through three major homeostatic regulators of hydrogen ions: Buffer systems. Respiratory mechanisms. Renal mechanisms.
Buffer Systems A buffers prevent excessive changes in PH by removing or releasing hydrogen ions. If excess hydrogen ion is present in body fluids, buffers bind with the hydrogen ion, minimizing the change in pH. When body fluids become too alkaline, buffers can release hydrogen ion, minimize the change in pH . The major buffer system in ECF is the bicarbonate (HCO3-) and carbonic acid (H2co3).
An acid releases hydrogen, as follows: A base traps hydrogen, as follows: