Session 5

REGULATION OF BODY FLUID COMPARTMENTS

The membranes of individual cell, the vascular capillary walls and the lymphatic capillary walls are semipermeable membranes that separate fluid compartments. Movement between compartments is constant and necessary for cell to operate. Water and some electrolytes move easily across these semipermeable membranes, but larger molecules, such as proteins, are less able to move across capillary walls.

Diffusion

1.    Diffusion is the process of molecules moving from an area of higher concentration to an area of lower concentration.

2.    Diffusion is the most important physical phenomenon that governs the movement of substances across various body membranes.

3.    Diffusion is the process to maintain electrochemical balance, a state in which the numbers of anions and cations are balanced within each fluid compartment. If any compartment contains an excess of cations, then an identical number of anions must diffuse into the compartment so that the charge is balanced. The same is true for anions.

4.    Movement by diffusion is much more rapid than movement by other forces.

5.    Diffusion of electrolytes into and out of cells and fluid compartments occurs via the kinetic energy of molecular motion.

Osmosis

1.    Osmosis is the movement of a fluid through a semipermeable membrane. A semipermeable membrane allows substances to travel through but not others.

2.    Osmosis will occur is one compartment contains a greater concentration of a dissolved substance (hyper-osmolar) than the other compartment of lower concentration (hypo-osmolar).

3.    Water passes through the membrane to the area of greater concentration. The goal of osmosis is to equalize solution concentration on both sides of the membrane.

Filtration

1.    Filtration is the movement of water and dissolved substances through a permeable membrane from a region of high pressure to a region of low pressure.

2.    Filtration depends on hydrostatic pressure, or the pressure exerted by fluid against the walls of its compartment, promoting the flow of fluid out of the capillaries.

3.    Filtration occurs within the glomerular capillaries of the kidney and in tissue capillaries.

Sodium-Potassium Pump

1.    Since sodium concentration is greater in ECF than in ICF, there is a tendency for sodium to enter the cell by diffusion.

2.    This tendency is balance out by the sodium-potassium pump, which sits in the cell membrane waiting to move sodium from the cell into the ECF.

3.    Also the reverse is true, the high intracellular potassium concentration is maintained by pumping of potassium into the cell.

4.    When substances are moved across a cell membrane against a concentration gradient, or uphill, the cell must work and expend energy to accomplish this task. Such movement is called active transport because the cell must make active efforts for the movement to take place.

PRESSURES AFFECTING FLUID AND ELECTROLYTE MOVEMENT

Fluid movement between and within the ICF and the ECF are determined by the pressures surrounding them and what causes change in these pressures.

Osmotic Pressure

1.    Osmotic pressure is the water-pulling pressure. The greater the solute concentration within a solution (osmolarity), the higher the osmotic pressure of that solution.

2.    Plasma proteins contribute to the osmotic pressure because they attract water.

3.    A solution that has the same osmotic pressure or osmolarity as blood plasma is called isotonic, an example is normal saline (0.9%). When an isotonic solution enters the circulation, there is no net movement of water across the membrane, so cells retain their normal size.

4.    Hypotonic solution has a concentration of solute that is less than blood plasma. When a hypotonic solution, such a water, surrounds cells, water will cross the membrane into the cells, causing them to swell.

5.    Hypertonic solution has a concentration of solute that is greater than blood plasma. When a hypertonic solution, such as 3% sodium chloride, is infused, water will leave cells, causing the cells to decrease in size.

Hydrostatic Pressure

1.    Hydrostatic pressure is the pressure exerted by the fluid on the walls of the blood vessel at both the arterial and the venous ends of the vessel.

2.    It promotes filtration of fluid from an area of higher pressure to an area of lower pressure.

3.    Factors that affect hydrostatic pressure include the arterial blood pressure, the force with which the heart pumps blood, the rate of blood flow, and venous pressure.

4.    Hydrostatic pressure in the arteriole is approximately 32 mm Hg. As the pressure becomes lower there is more chance of damage to tissue cells. The pressure would become lower as disease enters the body, such as cardiac disease and diabetes.

HORMONAL INFLUENCES

               There are several hormones that influence the regulation of fluid balance in the body. They work by regulating the urine output and concentration to bring balance between fluid and electrolytes.

Renin-Angiotensin-Aldosterone System

1.    This system regulates the ECF volume.

2.    Renin release is stimulated by decreased arterial blood pressure, decreased renal blood flow, increased renal sympathetic nerve activity, or a low-salt diet.

·        Renin is secreted by the kidney.

·        Renin splits angiotensinogen into angiotensin.

3.    Angiotensinogen is produced by the liver and circulates in the blood.

·        Angiotensin is a very strong vasoconstrictor.

·        Angiotensin stimulates aldosterone secretion.

4.    Aldosterone is produced by the adrenal cortex.

·        It regulates sodium reabsorption in the distal tubules and collecting ducts of the kidney.

·        The result of aldosterone action on the kidney is the reabsorption of saline, which is ECF.

Antidiuretic Hormone (ADH)

1.    ADH is manufactured by the hypothalamus and is stored in the pituitary gland.

2.    When plasma osmolarity increases, activating the hypothalamus, ADH is released into the circulatory system.

3.    ADH maintains the osmolarity of the blood within normal limits by making adjustment in the amount of water excreted in the urine.

4.    When ADH is increased, the urine becomes more concentrated as the body strives to conserve water.

5. When ADH is inhibited, the urine becomes more diluted as there is an excess of water in the body.

Parathyroid Hormone (PTH)

1.    PTH helps to regulate the calcium and phosphate balance in the body.

2.    In the presence of PTH there is a reciprocal relationship between calcium and phosphorus levels.

·      ­ PTH ® ­ serum calcium and ¯ serum phosphate

·      ¯ PTH ® ¯ serum calcium and ­ serum phosphate.

INFLUENCES OF BODY FLUIDS

1.    Fluids constitute approximately 60% of adult weight and consist of the ECF, ICF, and a small amount of transcellular water.

2.    The ECF compartment makes up about 43% of total body water and is comprised of the following fluids: interstitial fluid (tissue), blood plasma, lymph, bone and connective tissue water.

3.    Transcellular water makes up about 2-3% of total body water, consisting primarily of cell membrane water.

4.    The ICF compartment makes up the rest at about 55% of the total body water.

5.    Infants have more body weight in water than that of adults (80% body water).

6.    Men have proportionately more body water weight than do women because women have a higher ratio of body fat than do men. The more fat present in the body the less water present.

Sources of Fluid Intake

1.    Normal fluid intake is regulated by thirst. The thirst center is located in the hypothalamus and is stimulated by the rise in the plasma osmolarity or by a decrease in the plasma volume.

2.    Most fluids consumed are hypotonic to the ECF.

3.    The average daily fluid intake for an adult is 1,300 ml of water (about six glasses). About another 1000 ml is obtained from foods, especially from fruits and vegetables.

Routes of Fluid Loss

Water and electrolytes can be lost from the body in four ways: through the kidneys as urine, through the skin as perspiration, through the lungs as insensible water loss, and through the gastrointestinal tract in stool or vomit.

Kidneys

1.    The kidney is the main organ regulating fluid balance. The process is regulated through the hormonal regulation of ADH and aldosterone.

2.    The general rule is that output is 1 ml of urine per kilogram of body weight per hour (1ml/kg/hr), bringing the normal urine output to approximately 1,500 ml for a 24 hour period.

Skin

1.    Loss of fluid from the skin in the form of perspiration is approximately 100 – 200 ml per day. In addition to perspiration, insensible fluid loss can be significant.

2.    Insensible fluid or water loss occurs when continuous water is lost by evaporation caused by changes in the environmental atmosphere or by decompensating conditions of the body such as fever, burns, trauma, or extreme stress. These conditions greatly increase insensible water loss through the skin and the lungs.

Lungs

1.    Fluid is lost through the lungs during respiration.

2.    Exhalation not only contains carbon dioxide, but also water vapor.

3.    The loss of water through respiration is approximately 300 - 400 ml daily.

4.    The water vapor expelled is an insensible water loss and the loss increases with increased respiratory rate or depth, or both, usually as a result of fever or trauma.

Gastrointestinal Tract

1.    The usual loss through the gastrointestinal tract is only 100 – 200 ml per day, even though approximately 8 liters of fluid circulate through the GI system every 24 hours.

2.    The bulk of the fluid that circulates daily through the GI tract is reabsorbed, so it is apparent that large losses can develop if diarrhea or fistulas occur.

3.    In the healthy person, the daily intake and output are approximately equal.

ASSESSMENT FACTORS OF FLUID AND ELECTROLYTE BALANCE

Physical Assessment

Hydration is defined as the normal state of fluid balance. A normally hydrated adult is alert, has moist eyes and mucous membranes, has a urinary output proportional to fluid intake, and an adequate state of skin hydration. These are all signs of ECF hydration.

Skin and Mucous Membranes

1.    Turgor is the way to measure the hydration of the skin. It is the ability of the skin to return to its normal condition when it is pinched. In a healthy state, the skin should return immediately to its original shape after it is released.

2.    Decreased turgor is present when the pinched skin remains in elevated in shape after being released and rebounds slowly. This is a sign of dehydration.

3.    The elderly population generally has poor skin turgor due to loss of tissue fluids that comes with aging.

4.    Skin hydration assessment also includes the evaluation for dryness.

5.    Mucous membranes and eye conjunctiva are usually visibly moist. A dry, sticky mouth and/or throat, the absence of tearing eyes, and dark-sunken eyes are sign of dehydration.

Urine Output

1.    Urine output is usually proportional to the fluid intake. When the urine output is decreased, there is an indication of dehydration.

2.    Volume and specific gravity of urine are also important to assess.

3.    Specific gravity is a measurement of density, the solute concentration (osmolarity).

4.    Normal specific gravity of urine is 1.010 to 1.025 g/mL.

5.    In dehydrated states, water is conserved and reabsorbed at the renal tubules by the action of ADH so that the volume of urinary output decreases and the specific gravity rises.

6.    The specific gravity of urine increases with a rise in any urine solute concentration. This means that if there is blood, protein or glycogen in the urine it will be reflected by an increased specific gravity value.

Plasma

1.    Assessment can also focus on signs of plasma volume deficits without deficits in the above mentioned tissue fluid space.

2.    Signs of plasma volume deficit include decreased pulse volume or pulse deficit, elevated heart rate, decreased venous filling, lowered peripheral perfusion, and lowered or orthostatic blood pressure. Orthostatic or postural changes in blood pressure occur when the individual changes from the lying to standing position. The blood pressure usually decreases and the pulse may be elevated due to the dehydrated state.

3.    Blood urea nitrogen (BUN) is made up of urea, which is an end product of protein metabolism and is excreted in the urine. The normal BUN is 10 to 20 mg/dl.

·         The BUN is increased with fever, sepsis and dehydration.

·         The more elevated the BUN the more dehydrated is the client.

4.    Creatinine is the end product of muscle metabolism.

·      It is a better indicator of renal function that BUN because it does not vary with protein intake and metabolic state.

·      The normal serum creatinine is 0.6 to 1.5 mg/dl. In dehydration, the concentration of creatinine is greater in the blood because of the decrease in fluid circulating.

5.    Hematocrit measures the volume percentage of red blood cells (RBC) in whole blood. The normal range is 40% to 54%. In dehydration, fluid is decreased and the concentration of RBCs in the blood is greater thereby elevating the hematocrit.

6.    Serum sodium and serum potassium levels are usually elevated due to the greater concentration of both in relation to the fluid available.

Behavioral and Neurologic Changes

1.    Changes is fluid balance can result in alteration of neurologic function.

2.    In hypertonic states, or dehydration, all cells shrink including the neuronal cell and this may cause serious nervous system excitability and hyperactivity, even convulsions.