Body Fluid Compartments PDF

The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema Ankara Yıldırım Beyazıt University Faculty of Medicine Department of Physiology Prof. Dr. Fahri BAYIROĞLU The Body Fluid Compartments: Extracellular and Intracel...

The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema Ankara Yıldırım Beyazıt University Faculty of Medicine Department of Physiology Prof. Dr. Fahri BAYIROĞLU The Body Fluid Compartments: Extracellular and Intracellular Fluids; Edema Body Fluid Compartments Fluid Intake and Output Are Balanced During Steady-State Conditions In case of extensive burns – water loss -up to 3-5 L/day  Constituents of Extracellular and Intracellular Fluids Because of the Donnan effect, the concentration of positively charged ions (cations) is slightly greater (≈2 percent) in the plasma than in the interstitial fluid. Negatively charged plasma proteins tend to bind cations, such as sodium and potassium ions, thus holding extra amounts of these cations in the plasma along with the plasma proteins. Conversely, negatively charged ions (anions) tend to have a slightly higher concentration in the interstitial fluid compared with the plasma, because the negative charges of the plasma proteins repel the negatively charged anions Ionic Composition of Plasma and Interstitial Fluid Is Similar Measurement of Fluid Volumes in the Different Body Fluid Compartments-the Indicator-Dilution Principle This method can be used to measure the volume of virtually any compartment in the body as long as (1) the indicator disperses evenly throughout the compartment, (2) disperses only in the compartment that is being measured, (3) not metabolized or excreted Indicator-dilution method for measuring fluid volumes. The conservation of mass principle- the total mass of a substance after dispersion in the fluid compartment will be the same as the total mass injected into the compartment. Determination of Volumes of Specific Body Fluid Compartments  Regulation of Fluid Exchange and Osmotic Equilibrium Between Intracellular and Extracellular Fluid A frequent problem in treating seriously ill patients is maintaining adequate fluids in one or both of the intracellular and extracellular compartments. The relative amounts of extracellular fluid distributed between the plasma and interstitial spaces are determined mainly by the balance of hydrostatic and colloid osmotic forces across the capillary membranes. The distribution of fluid between intracellular and extracellular compartment is determined mainly by the osmotic effect of the smaller solutes-especially sodium, chloride, and other electrolytes acting across the cell membrane. The reason for this is that the cell membranes are highly permeable to water but relatively impermeable to even small ions such as sodium and chloride.  Basic Principles of Osmosis and Osmotic Pressure Osmosis is the net diffusion of water across a selectively permeable membrane from a region of high water concentration to one that has a lower water concentration Relation Between Moles and Osmoles -The total number of particles in a solution is osmoles. One osmole (osm) is equal to 1 mole (mol) (6.02 × 1023) of solute particles. A solution containing 1 mole of glucose in each liter has a concentration of 1 osm/L. If a molecule dissociates into two ions (giving two particles), such as NaCl ionizing to give Cl and Na ions, then a solution containing 1 mol/L will have an osmolar concentration of 2 osm/L. Osmolality and Osmolarity The osmolal concentration of a solution is called osmolality when the concentration is expressed as osmoles per kilogram of water; it is called osmolarity when it is expressed as osmoles per liter of Solution. milliosmole (mOsm) equals 1/1000 osmole. Calculation of the Osmolarity and Osmotic Pressure of a Solution van't Hoff's law –calculation of the potential osmotic pressure of a solution, assuming that the cell membrane is impermeable to the solute. For example, the osmotic pressure of a 0.9 percent NaCl solution is calculated as follows: A 0.9% solution means that there is 0.9 gram of NaCl per 100 ml of sol., or 9 g/L. Molecular weight of NaCl = 58.5 g/mol, the molarity of the solution = 9 g/L divided by 58.5 g/mol, or about 0.154 mol/L. Because each molecule of NaCl is equal to 2 osmoles, the osmolarity of the solution is 0.154 × 2 = 0.308 osm/L. The osmolarity of this solution is 308 mOsm/L. The potential osmotic pressure of this solution =308 × 19.3 mmHg/mOsm/L= 5944 mm Hg. Osmotic Equilibrium Is Maintained Between Intracellular and Extracellular Fluids  Osmotic Equilibrium Is Maintained Between Intracellular and Extracellular Fluids -Large osmotic pressures can develop across the cell membrane with relatively small changes in the concentrations of solutes in the extracellular fluid. For each milliosmole concentration gradient of an impermeant solute (one that will not permeate the cell membrane), about 19.3 mm Hg osmotic pressure is exerted across the cell membrane. If the cell membrane is exposed to pure water and the osmolarity of intracellular fluid is 282 mOsm/L, the potential osmotic pressure that can develop across the cell membrane is more than 5400 mm Hg. This demonstrates the large force that can move water across the cell membrane when the intracellular and extracellular fluids are not in osmotic equilibrium. As a result of these forces, relatively small changes in the concentration of impermeant solutes in the extracellular fluid can cause large changes in cell volume. Volume and Osmolality of Extracellular and Intracellular Fluids in Abnormal States Some factors that can cause extracellular and intracellular volumes to change markedly; ingestion of water, dehydration, intravenous infusion of different types of solutions, loss of large amounts of fluid from the gastrointestinal tract, loss of abnormal amounts of fluid by sweating or through the kidneys. Basic principles: 1. Water moves rapidly across cell membranes; therefore, the osmolarities of intracellular and extracellular fluids remain almost exactly equal to each other except for a few minutes after a change in one of the compartments. 2. Cell membranes are almost completely impermeable to many solutes; therefore, the number of osmoles in the extracellular or intracellular fluid generally remains constant unless solutes are added to or lost from the extracellular compartment.  Effect of Adding Saline Solution to the Extracellular Fluid Effect of adding isotonic, hypertonic, and hypotonic solutions to the extracellular fluid after osmotic equilibrium. The normal state is indicated by the solid lines, and the shifts from normal are shown by the shaded areas.  Calculation of Fluid Shifts and Osmolarities After Infusion of Hypertonic Saline We can calculate the sequential effects of infusing different solutions on extracellular and intracellular fluid volumes and osmolarities. For example, if 2 liters of a hypertonic 3.0% NaCl solution are infused into the extracellular fluid compartment of a 70-kilogram patient whose initial plasma osmolarity is 280 mOsm/L, what would be the intracellular and extracellular fluid volumes and osmolarities after osmotic equilibrium? -A 3.0 % sol. means 3.0 g/100 ml or 30 g NaCl /L. -(Mol.weight of NaCl is 58.5 g/ mol, 0.513 mole of NaCl /L. For 2 liters, this would be 1.026 mole of NaCl (1 mole of NaCl equals to 2 osmoles). - 2 L. of this sol. means adding 2052 miliosmoles to the extracellular fluid - Thus, the following values would occur instantly after adding the solution. In the third step, we calculate the volumes and concentrations that would occur within a few minutes after osmotic equilibrium develops. Thus, one can see from this example that adding 2 liters of a hypertonic sodium chloride solution causes more than a 5-liter increase in extracellular fluid volume while decreasing intracellular fluid volume by almost 3 liters. Glucose and Other Solutions Administered for Nutritive Purposes Glucose solutions are widely used, and amino acid and homogenized fat solutions are used to a lesser extent. When these solutions are administered, their concentrations of osmotically active substances are usually adjusted nearly to isotonicity, or they are given slowly enough that they do not upset the osmotic equilibrium of the body fluids. After the glucose or other nutrients are metabolized, an excess of water often remains, especially if additional fluid is ingested. Ordinarily, the kidneys excrete this in the form of a very dilute urine. The net result, therefore, is the addition of only nutrients to the body. Clinical Abnormalities of Fluid Volume Regulation: Hyponatremia and Hypernatremia  Consequences of Hyponatremia: Cell Swelling Rapid changes in cell volume as a result of hyponatremia can have profound effects on tissue and especially the brain. A rapid reduction in plasma sodium concentration, for example, can cause brain cell edema and neurological symptoms, including headache, nausea, lethargy, and disorientation. If plasma sodium concentration rapidly falls below 115 to 120 mmol/L, brain swelling may lead to seizures, coma, permanent brain damage, and death. Brain cell volume regulation during hyponatremia. During acute hyponatremia, caused by loss of Na+ or excess H2O, there is diffusion of H2O into the cells (1) and swelling of the brain tissue. This stimulates transport of Na+, K+, and organic solutes out of the cells (2), which then cause water diffusion out of the cells (3). With chronic hyponatremia, the brain swelling is attenuated by the transport of solutes from the cells.  Consequences of Hypernatremia: Cell Shrinkage Hypernatremia is much less common -severe symptoms usually occur only with rapid and large increases in plasma sodium concentration above 158 to 160 mmol/L. One reason for this is that hypernatremia promotes intense thirst that protects against a large increase in plasma and extracellular fluid sodium. However, severe hypernatremia can occur in patients with hypothalamic lesions that impair their sense of thirst, in infants who may not have ready access to water, or elderly patients with altered mental status. Edema: Excess Fluid in the Tissues -Excess fluid in the body tissues, -Mainly in extracellular fluid compartment, -sometimes intracellular level. Intracellular edema: (1) hyponatremia, (2) depression of the metabolic systems of the tissues; (3) lack of adequate nutrition to the cells.  Causes of Extracellular Edema (1) abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries, (2) failure of the lymphatics to return fluid from the interstitium back into the blood, often called lymphedema. The most common clinical cause of interstitial fluid accumulation is excessive capillary fluid filtration.  Factors That Can Increase Capillary Filtration Any one of the following changes can increase the capillary filtration rate: 1.Increased capillary filtration coefficient. 2. Increased capillary hydrostatic pressure. 3. Decreased plasma colloid osmotic pressure.  Lymphedema-Failure of the Lymph Vessels to Return Fluid and Protein to the Blood When lymph vessel function is greatly impaired, due to blockage or loss of the lymph vessels, edema can become especially severe because plasma proteins that leak into the interstitium have no other way to be removed. The rise in protein concentration raises the colloid osmotic pressure of the interstitial fluid, which draws even more fluid out of the capillaries. Causes of Extracellular Edema SAFETY FACTOR to prevent the development of edema The development of edema is not an easy process. First the safety factor has to be overcome to cause edema. Safety factor is equal to 17 mm of Hg. It has three components i.e. 1.Negative Interstitial Fluid Hydrostatic pressure (caused by pumping of lymphatic system): which contributes -3 mm of Hg. 2.Capacity of Lymphatic System: Increased amount of tissue fluid formed by 7 mm of Hg increase in Capillary pressure can be drained by lymphatic flow. 3.Increased Washout of proteins from Interstitium: it contributes 7 mmHg. When there is increased formation of tissue fluid there is increased amount of fluid along with proteins that will enter the interstitium and then lymphatics for washing out. So the interstitial fluid colloid osmotic pressure can decrease from 8 mm of Hg upto 1mm of Hg before edema can start. Safety Factor Caused by Low Compliance of the Interstitium in the Negative Pressure Range Relation between interstitial fluid hydrostatic pressure and interstitial fluid volumes, including total volume, free fluid volume, and gel fluid volume, for loose tissues such as skin. Note that significant amounts of free fluid occur only when the interstitial fluid pressure becomes positive.  Fluids in the "Potential Spaces" of the Body When edema occurs in the subcutaneous tissues adjacent to the potential space, edema fluid usually collects in the potential space as well and this fluid is called effusion - Fluid Is Exchanged Between the Capillaries and the Potential Spaces - Lymphatic Vessels Drain Protein from the Potential Spaces Some examples of “potential spaces” are pleural cavity, pericardial cavity, peritoneal cavity, and synovial cavities, including both the joint cavities and the bursae The abdominal cavity is especially prone to collect effusion fluid, and in this instance, the effusion is called ascites