The Fluid Exchange, Fluid Therapy and Fluid Loss Page!

NOTE: There is continuous fluid exchange between body water compartments in healthy persons. Tissue perfusion and exchange are promoted by fluid intake, movement, exercise and change of position. Remember, body fluids do not simply contain water. They also contain electrolytes, proteins, gases, nutrients and metabolic wastes. Major exchange occurs between plasma and ISF (across the capillary endothelium), between plasma and the ICF of circulating cells and between the ISF and the ICF. The value of fluid therapy cannot be overemphasized. Remember, the greatest benefits of exchange are derived when the composition of all body water compartments approach their appropriate physiological values.

Fluid therapy promotes tissue perfusion which, in turn, promotes exchange.

Fluids can be administered directly to the ECF (eg. plasma (iv), digestive tract (per os) and intraperitoneal (ip)). Intravenous (iv) fluid therapy takes advantage of the ready access to the plasma compartment. Remember, however, that plasma represents less than 10% of total body water. Plasma is only 1/4 of ECF and ECF is only 1/3 of total body water. Thus, plasma is only 1/12th (about 8%) of total body water. This means that fluid therapy and plasma analysis, which go hand in hand, often need to be continuous to obtain maximal benefits for hospital patients.  This is really no different than athletes continually taking in small quantities of fluid during, for example, a tennis match.  Before electrolyte sports drinks became popular, Jimmy Connors used to literally fall to the ground during prolonged tennis matches in long tournaments.  An unusually large and powerful tennis player, Connors required tremendous amounts of fluid to run around the court.  When he became dehydrated and "electrolyte imbalanced," he would experience muscle cramping and would sometimes fall and require assistance to walk off the court.  Once he got his fluids straightened out, he was one of the most awesome tennis players ever.

Note that the benefits of i.v. fluid therapy are so great that i.v. fluid bags/bottles/pumps are wheeled about on stands so that ambulatory patients can take their fluid therapy with them. Remember, exercise and movement cause muscles to contract.  When muscles contract, they put pressure on veins and capillaries, assisting blood flow, tissue perfusion and exchange.  One-way valves in veins assure that sketetal muscle contractions move blood forward in the circulatory system.

Stand 

8The average adult drinks about 2.5 liters of fluid everyday. Adults urinate about 1.5 liters of fluid everyday (see figure).

What are the other major physiological routes through which we lose water everyday? (makes for good online discussion)

What are some major pathological routes of water loss? (also makes for good online discussion)

Obtain the composition of 3 electrolyte "sports" drinks (eg. Gatorade). Calculate their osmolarity.

1.4.3 The Extracellular Fluid Transport System

Bulk flow refers to transport driven by pressure or by a potential energy gradient, for example, the flow of water down a hill under the influence of gravity or the flow of blood into the lower appendages when you jump up out of bed in the morning (orthostatic hypotension). Some teenagers who lie around too long and then jump up or elderly persons who stand up too quickly sometimes faint as blood flows to their lower limbs and abdomen.  This is a form of vasovagal syncope that we will talk about in much more detail later.  Bulk flow in the body is generally driven by one of the following two types of pressure:

8Hydrostatic pressure is a physical force exerted by fluids trying to push across a membrane or vessel wall. This is the principle type of pressure generated within blood vessels. An excess of hydrostatic pressure can cause edema by forcing fluid out of blood vessels in the face of high blood pressure.

8Colloid osmotic (protein oncotic) pressure is the osmotic pressure generated by large solute molecules such as proteins or polysaccharides (carbohydrates). This type of colloid osmotic pressure is critical in maintaining the correct volume of fluid within cells and within the plasma compartment.

The following are examples of increases or decreases in which types of pressure?????

a) The majority of plasma proteins are large circulating globular proteins (alpha- and gamma-globulins) which are too large to be filtered from the glomerulus at the kidney. The majority of these proteins are serum albumin, serum globulin, fibrinogen and prothrombin, with molecular weights >69,000.  The largest molecules filtered at the kidney have molecular weights <70,000.  Thus, proteins are concentrated in the unfiltered portion of plasma passing through the kidney. These concentrated proteins provide the principle driving force behind reabsorption from the renal filtrate and back into the plasma compartment in the peritubular capillaries.

(what are the 2 types of pressure in this example - (1) The kidney filters plasma, filtration driven by blood pressure, blood pressure is hydrostatic pressure. (2) The kidney reabsorbs much of the filtrate, reabsorption is due to osmotic pressure, the osmotic pressure is generated by ... the concentrated proteins, protein osmotic pressure.

b) Brisket disease (cor pulmonale) in cattle at high elevations:

Blood vessels in the lungs constrict in any areas of the lungs in which the oxygen concentration is low.  Normally, this prevents perfusion of areas of the lung where gas exchange would be less effective while shunting (bypassing) blood to areas of the lungs where more oxygen can be picked up.  Unfortunately, low oxygen levels at high elevations can cause general activation of this "local vasoconstrictive mechanism" in the pulmonary circulation. This creates increased fluid pressure in the chest and may cause edema around the heart. Edema in the brisket region increases pressure on the heart, increasing pressure within the pericardial sac (in this diagram you can see the visceral and parietal pericardium) and reducing cardiac return and cardiac filling.

The heart must work harder to maintain sufficient cardiac output against this additional pressure and the heart may fail.  This buildup of fluid pressure around the heart is known as "cardiac tamponade."  A similar condition can occur in humans. (What type of pressure is discussed in this example - at high elevation reduced oxygen in the lung,hypoxic vasoconstriction triggered unfortunately throughout lung, increase in blood pressure in lung which is hydrostatic pressure leads to the fluid edema in chest)

c) We may see massive (life-threatening) pulmonary edema in humans with left ventricular failure or pulmonary embolisms. (What type of pressure is this - decide first - this is tough - then check your answer below)

d) Central edema (meaning edema within the CNS) may result due to massive vasoconstriction due to sympathetic outpouring and adrenalin release following electrocution.  Severe central edema (edema within the cranium) leads to increased pressure within the cranium.  The pressure in the cranium may get so high that it is impossible to pump enough blood to the brain (Cushing Reaction).  This may lead to death following electrocution, lightning strikes or concussions. What kind of pressures are discussed here and why do we closely monitor a patient who feels O.K. after being electrocuted? (when you have decided - check your answer below)

e) Marasmic Kwashiorkor (protein-energy malnutrition) is associated with massive abdominal edema due to protein starvation and reduced plasma protein concentration. With the reduction in protein in the ICF and plasma, there is an increase in the volume of the ISF and peritoneal fluids in the abdomen. (what kind of pressures are involved here? - less colloid osmotic pressure to hjold ICF and plasma)

f) Hypovolemia (hypovolemic shock) may be associated with intestinal blockage, trauma with plasma loss, internal bleeding or severe dehydration. Intestinal blockage leads to rapid fluid transfer (secretion) into the digestive tract and may lead to hypovolemic shock. (when you think you know what the problem is here, check it out)

g) Dextran is a large carbohydrate (polysaccharide) often incorporated in i.v. fluid/electrolyte therapy. Large dextran molecules (polysaccharides) are not readily lost from the vascular compartment (the polysaccharide molecules are too large to diffuse out of the vasculature and too large to be filtered at the kidney). Dextran, thus, represents a solute particle which will attract or hold water in the vascular compartment. This prevents the fluids being replaced from simply diffusing out of the vascular compartment, preventing a rapid return of the patient to the hypovolemic state (and possibly shock). (what kind of pressure are we talking about here? - once you have decided, check your answer)

h) Burn victims are often treated with globular proteins (i.v.) to help hold fluid in their vascular compartment.  As a major role of skin is protection from dehydration, burn victims can quickly become hypovolemic.

i)  When you are working outside on a very hot day, you may perspire at greatly increased levels.  In fact, you may lose fluid at rates exceeding several hundred mls per hour.  Despite possibly tremendous fluid losses, we do not immediately become hypovolemic.  Note that we have a lot of protein in our cells and in our plasma, but very little in our interstitium.  As we begin to dehydrate, globular proteins in plasma and ICF will be concentrated.  The resulting increase in protein concentration in these compartments, even though slight, will help protect the volume of the plasma and ICF, while fluid will be lost more quickly from the less immediately critical ISF which contains little or no measurable protein. (you should know the type of pressure being discussed here - check your answer)

Diffusion and Fluid Diffusion

8Diffusion is passive transport driven by a concentration gradient. Diffusion is usually defined as random movement of solute particles from an area of high solute concentration to an area of low solute concentration. Once the solute particles are evenly distributed, the solute particles still move randomly, but diffusion is said to have stopped. The rate of diffusion depends on several factors, some of which are listed below:

i) molecular weight (size)
ii) valence (charge)
iii) sphere of hydration (charge density)
iv) prevailing concentration gradient
v) temperature

We've already discussed how all of these factors affect lipid solubility, and they affect diffusion the same way.  The bigger a molecule is, the more slowly it will diffuse.  Charged molecules carry a sphere of hydration and are, therefore, larger than the uncharged version of the same molecule.  Given 2 ions or molecules with the same charge, the smaller one will have a greater sphere of hydration (due to its greater charge density) and will, therefore, diffuse more slowly.  The "steeper" the concentration gradient, the faster the movement of solute particles from an area of high solute concentration to an area of low solute concentration.

Solvents also diffuse and this can be thought of as solvent molecules moving from an area of low solute concentration (high solvent concentration) to an area of high solute concentration (low solvent concentration). Assuming that a membrane will allow passage of both solute and solvent, solutes and solvents essentially diffuse simultaneously in opposite directions.