CHAPTER 27  FLUIDS & ELECTROLYTES

 

Body fluid consists of body water and its dissolved substances, and makes up 55 - 60% of the body weight on average.

   1. Intracellular fluid--inside cells (2/3 of total)

   2. Extracellular fluid--all other body fluids (1/3 total)

      a. Plasma--20%--liquid part of blood

      b. Interstitial fluid--80%--mostly between cells, also includes:        

      c. Also:

                     Lymph                        Pericardial fluid

                     Cerebrospinal fluid      Peritoneal fluid

                     GI tract fluids              Glomerular filtrate

                     Synovial fluids              Fluids of the eye

                     Pleural fluid                 Fluids of the ear

 

Body fluids are separated into compartments by selectively permeable membranes, the plasma membranes of cells and blood vessel walls. Molecules are constantly moving from one compartment to another, but in homeostasis the total volume in each compartment remains fairly constant.

 

 

 

 

 

 

 

Fluid balance--the body contains the required total amount of water and the water is properly distributed in the various compartments. Since water moves from one compartment to another mainly by osmosis, fluid balance requires electrolyte balance.

 

Typical daily fluid intake:                      Daily output:

  Ingested liquids      1600 ml                  Kidneys         1500 ml

  Water in food          700 ml                    Skin               600 ml

  Metabolic water        200 ml                  Lungs              300 ml

                                2500 ml                   GI tract           100 ml

                                                                                    2500 ml

 

Normally intake and output balance in every 24 hour period. If water output exceeds intake, even for a brief period, dehydration results.

 

REGULATION OF WATER INTAKE     

 

Thirst is the major regulator. We feel thirst for 3 reasons:

 

Increased blood osmotic                Decreased flow of         Decreased blood

   pressure                                        saliva                          volume

 

              All stimulate thirst center of hypothalamus

 

In addition, decreased blood pressure (due to decreased blood volume) leads to activation of the RAA system, and angiotensin II also stimulates the thirst center.

 

We drink water and get immediate wetting of the oral mucosa, distention of the stomach and then, as absorption occurs, increased blood volume and lowered blood osmotic pressure. All of these effects inhibit the thirst center.

 

Metabolic water is produced in the reactions of aerobic cellular respiration. This is added to the water we take in.

 

REGULATION OF OUTPUT

 

   1. Angiotensin II and aldosterone increase the reabsorption of Na+ in the DCT and collecting tubule. Since water follows Na+, these hormones also promote increased reabsorption of water. If water output exceeds intake, these 2 hormones are secreted. If water intake is high, the hormones are inhibited.

   2. Atrial natriuretic peptide is secreted to increase water and Na+ loss when blood volume rises.

   3. ADH regulates water reabsorption by changing the permeability of tubule cells so that more water is reabsorbed.

   4. Dehydration leads to decreased blood volume and decreased blood pressure. The glomerular filtration rate (GFR) therefore falls and urine output decreases.

   5. Vomiting and diarrhea or increased loss through the skin can lead to dehydration.

 

In dehydration, water first leaves the extracellular fluids, but the increased concentration of solutes in plasma soon causes fluid loss from interstitial fluid and inside cells to follow. Excess intake of plain water can lead to large amounts of water entering cells. This can interfere with normal function of neurons and is called water intoxication.

 

ELECTROLYTES

 

Body fluids contain a number of dissolved chemicals, which can be put into 2 major groups:

   Organic compounds such as glucose, creatinine, urea dissolve but do not dissociate (form ions) in water    

   Most other solutes DO dissociate when dissolved in water and are therefore called electrolytes (because water containing ions will conduct an electrical current). These are mostly inorganic molecules.

 

Electrolytes are essential to the body in many ways:

   1. Control movement of water by osmosis between various body compartments.

   2. Help maintain acid-base balance.

   3. Carry electrical currents required for nerve impulse conduction, heartbeat, etc.

   4. Essential minerals required for various body processes such as activating enzymes, blood clotting, etc.

  

 

 

Electrolyte content of the body fluids:

   Extracellular fluid:

      Plasma--main solutes are Na+ and Cl-  as well as considerable protein. This is the fluid we test for levels of the various electrolytes. Even though they are present in tiny amounts, the proper amount in blood of substances such as Ca+, K+, Mg+, etc. are essential.

      Interstitial fluid--mainly Na+ and Cl-, little protein

 

      Intracellular fluid--mainly K+, phosphate, large amounts of proteins

 

All electrolytes are important in osmosis and fluid balance. Also, each electrolyte has its particular specific use or uses in the body. If a person has an electrolyte imbalance, certain signs and symptoms may suggest that an imbalance exists and what the problem is, but normally a lab test would be run before much treatment is given--we do not usually depend on signs and symptoms alone. Several different imbalances may cause similar symptoms, and there is  no guarantee that only one electrolyte level at a time will be disturbed. However, if a person in a situation that commonly leads to a particular electrolyte imbalance begins to show typical symptoms, quick action may prevent the problem from becoming more serious. Electrolyte imbalances can  occur in healthy young adults, but are seen more often in babies & children, the elderly, and patients with predisposing conditions.

                   

SPECIFIC ELECTROLYTES

 

1. SODIUM (Na+)--most abundant extracellular ion, 90% of extracellular  cations (+)

      USES: Generation and transmission of nerve impulses

            Muscle contraction

            Major regulator of osmosis and water distribution

      CONTROL: Mainly by aldosterone, ADH, atrial natriuretic peptide. Most of us take in more Na each day than we require and the kidneys excrete the excess.

      IMBALANCES: Common

         HYPONATREMIA—Decreased Na intake or excess Na loss from perspiration, vomiting, diarrhea, burns, etc., especially when these losses are replaced by plain water. Also aldosterone deficiency, certain diuretics, and excessive water intake. Muscular weakness, hypotension, confusion, stupor, coma.

         HYPERNATREMIA--most commonly due to inadequate water intake, but also excessive salt intake, possibly from unexpected sources such as Alka-Seltzer. Thirst, hypertension, edema, convulsions resulting from cellular dehydration of the neurons.

 

2. CHLORIDE (Cl-)--major extracellular anion (-)

      USES: Regulating fluid distribution

            Forms HCl in stomach

      CONTROL: Indirect by aldosterone--when Na is reabsorbed, Cl follows

      IMBALANCES: Rare

         HYPOCHLOREMIA--Occasionally occurs due to excessive vomiting and certain diuretics. Muscle spasms, metabolic alkalosis, shallow respirations, hypotension.

          HYPERCHLOREMIA—Even more rare, possibly occurs with severe dehydration, excessive chloride intake, severe renal failure. Lethargy, weakness, metabolic acidosis, deep breathing.

 

3. POTASSIUM (K+)--most abundant cation in intracellular fluid

      USES: Necessary for function of nerve and muscle tissue, including the heart

            Regulation of pH and fluid volume in cells

       CONTROL: Mineralocorticoids, secreted into DCTs and collecting ducts when excess is present

       IMBALANCES: Very common, occurring naturally and also related to medications

          HYPOKALEMIA--vomiting, diarrhea, kidney disease, high Na+ intake, poor K+ intake, certain diuretics. Muscular weakness, mental confusion, decreased bowel motility, cardiac irregularities.

          HYPERKALEMIA--rare with normal kidney function, but common in renal failure, esp. if the patient takes in excessive K+ such as from salt substitutes. May occur with severe crushing type injuries. Irritability, nausea, diarrhea, weakness, cardiac fibrillation and death.

 

4. BICARBONATE (HCO3 - )—Second most plentiful extracellular ion. Levels vary between arterial and venous blood because of its function in CO2 transport.

      USES:  CO2 transport

                  Maintaining acid/base balance

      CONTROL: Kidneys, which can regulate reabsorption of HCO3 - and even make additional HCO3 - ions and secrete them into the blood when needed.

 

5. CALCIUM (Ca2+)--Most abundant ion of the body but mostly tied up in bone

      USES: Structural element of bones and teeth

            Transmission of nerve impulses

            Muscle contraction including the heartbeat

            Blood coagulation

      CONTROL: Mainly calcitonin/ parathyroid hormone, also calcitriol (Vit D)

      IMBALANCES: Slightly low levels over a long period of time lead to osteoporosis. These levels usually produce no symptoms except weakened bones. Many acutely ill patients have more severe hypocalcemia, except those with a malignancy, who tend to have hypercalcemia.

         HYPOCALCEMIA--hypoparathyroidism, malabsorption, Vitamin D deficiency--  Numbness, muscle cramps and spasms, mental confusion, convulsions, cardiac arrhythmias. Longterm leads to bone fractures due to demineralization.

         HYPERCALCEMIA--hyperparathyroidism, prolonged immobilization, cancer. Weakness, confusion, psychotic behavior, cardiac arrest.

 

6. PHOSPHATE (H2PO4-, HPO42-, PO4 3-)

      USES: Mostly structural in bones and teeth

            Phospholipids in membranes

            Phosphate in DNA & RNA

            ATP

      CONTROL: Indirect by calcitonin/ parathyroid hormone

      IMBALANCES:

         HYPOPHOSPHATEMIA: alcohol withdrawal, diabetic ketoacidosis, severe burns, debilitated patients placed on TPN or feeding tubes, patients on prolonged high levels of antacids. Irritability, weakness, confusion, memory loss, seizures, coma.

         HYPERPHOSPHATEMIA: renal failure, chemotherapy, high intake such as very large amounts of milk and Fleet's enema. Short-term effects are tingling, numbness, muscle spasms. Long-term effects can involve deposition of calcium phosphate in sites such as the kidneys, arteries, joints.

 

7. MAGNESIUM (Mg2+)

      USES: Activates enzymes in metabolism of carbohydrates and proteins

            Required for operation of Na/K pump

            Regulates neuromuscular activity

      CONTROL: Poorly understood but probably aldosterone plays a role

      IMBALANCES: Common in the very ill & alcoholics, seldom seen otherwise.

          HYPOMAGNESEMIA: Mimics and often accompanies hypokalemia. Alcoholism is main cause, along with diabetes mellitus, certain diuretics, and refeeding after starvation. Neuromuscular irritability, tremors, convulsions, cardiac arrhythmias.

         HYPERMAGNESEMIA: Renal failure is main cause but occasionally occurs due to excessive intake (Mg-containing antacids)). Flushing of skin, hypotension, bradycardia, coma.

 

          TABLE 27.2   P.  1045  SUMMARY OF ELECTROLYTE

                                          IMBALANCES

 

 

 

ACID-BASE BALANCE

 

Acid-base balance in the body is maintained by controlling the concentration of H+ ions in body fluids. We ingest some H+ ions, but most are produced by cellular metabolism. Normal metabolism produces more acids than bases. High protein diets produce especially large amounts of acids when broken down. The pH of extracellular fluid must remain between 7.35 and 7.45. If the pH drops below 7.35, this is acidosis; above 7.45 is alkalosis. The pH is maintained by:

   1. Buffer systems

   2. Respiration

   3. Kidney

 

1. Buffer systems maintain a fairly constant level of H+ ions in body fluids. Buffers work very quickly to change strong acids to weak acids and strong bases to weak bases. Most buffer systems in the body consist of a weak acid and the salt of that acid.

 

   a. Protein buffer system—most abundant buffer in intracellular fluid and plasma. One example is hemoglobin inside RBC. Albumin is the main one in plasma. Proteins can buffer both acids and bases.

   b. Bicarbonate buffer system--the body's major buffer system, based on bicarbonate ions (HCO3-). This is the major regulator of blood pH. It consists of H2CO3 (carbonic acid, a weak acid) and NaHCO3 (its salt, which acts as a weak base). If a strong acid or a strong base enter the blood, the following occur:

 

  HCl   +   NaHCO3 ---->  NaCl   +   H2CO3

 

  NaOH   +   H2CO3 ---->  H2O   +   NaHCO3

 

Once the strong acid or base is changed to a weak acid or base, there is little dissociation and much less effect on pH. Respiratory problems can interfere with this buffer system.

 

   c. Phosphate buffer system--important regulator of pH within cells, also functions in extracellular fluids and urine. This system consists of dihydrogen phosphate ions (H2PO4 -), a weak acid, and monohydrogen phosphate, (HPO4 2-), a weak base. 

 

2. Respiration--an increase in CO2 in body fluids lowers the pH; a decrease in CO2 raises pH

      Increased CO2 ----> Increased H+ ions  ----> More acid pH

      Decreased CO2 ----> Decreased H+ ions ----> More alkaline pH

     

Slower, shallow breathing tends to increase CO2 levels and cause a more acid pH.

 

Rapid, deep breathing tends to decrease CO2 levels and cause a more alkaline pH.

 

The respiratory system can quickly (in minutes) to eliminate more acid or base than all the buffers combined, but it is only effective for H2CO3.

 

3. Kidneys--kidneys are relative slow (hours to days) but still play a major role in pH regulation. This is the only way to eliminate acids other than carbonic.

 

      Acidosis--kidneys secrete H+ ions which pass out in urine. At the same time, bicarbonate ions are reabsorbed to further balance out the excess acidity. The kidneys can even manufacture new bicarbonate ions and absorb them into the blood.

 

      Alkalosis--kidneys stop secreting H+ ions; also stop reabsorbing bicarbonate--both of these actions tend to shift blood back to a more acid pH.

 

 

 

ACID-BASE IMBALANCES

 

Normal blood pH: 7.35 to 7.45

Acidosis: 7.35 to 6.8 (any lower would rapidly result in death)

Alkalosis: 7.45 to 8 (any higher would rapidly result in death)

 

Acidosis causes depression of the CNS, leading to coma and even death due to depression of transmission at synapses.

 

Alkalosis causes overexcitability of the CNS and peripheral nerves, muscle spasms, convulsions and death

 

Compensation---if blood pH shifts, homeostatic mechanisms go into action to try to correct the problem.

 

RESPIRATORY ACIDOSIS/ALKALOSIS--in these, the problem begins as CO2 levels go above or below normal

 

RESPIRATORY ACIDOSIS--high CO2 in arterial blood is the problem. May be caused by anything that interferes with removal of CO2 from the body, such as hypoventilation of reduced exchange of gases (emphysema, pulmonary edema, airway obstruction, injury to respiratory center, etc.)

 

   BODY'S WAY OF COMPENSATING: Kidneys increase excretion of H+ ions and increase reabsorption of bicarbonate ions

 

   MEDICAL TREATMENT: Correct respiratory problem and possibly IV bicarbonate if severe

 

RESPIRATORY ALKALOSIS--low CO2 in arterial blood. Caused by hyperventilation due to anxiety, high altitude, aspirin overdose, etc.

 

   BODY'S WAY OF COMPENSATING: Kidneys decrease excretion of H+ ions and decrease reabsorption of bicarbonate ions.

 

   MEDICAL TREATMENT: Breathe in paper bag.

 

METABOLIC ACIDOSIS/ALKALOSIS--these are due to abnormal levels of bicarbonate in the blood. The problem is somewhere besides the lungs.

 

METABOLIC ACIDOSIS--low (acid) pH due to a decrease in bicarbonate ions or a gain in H+ ions from an acid other than carbonic. Low bicarbonate levels may be caused by severe diarrhea or kidney dysfunction. Gain in H+ ions may be due to ketoacidosis or kidney failure.            

 

   BODY'S WAY OF COMPENSATING: Hyperventilation

 

   MEDICAL TREATMENT: Correct cause, IV bicarbonate

 

METABOLIC ALKALOSIS--high (alkaline) pH due to a gain in bicarbonate ions or a loss of H+ ions. Gain of bicarbonate could be caused by very high intake of antacids. Loss of H+ ions could be due to gastric suction, severe vomiting, certain diuretics, endocrine disorders.

 

   BODY'S WAY OF COMPENSATING: Hypoventilation

 

   MEDICAL TREATMENT: Correct cause, fluids to restore balance