MEDICAL PHYSIOLOGY Body Fluids Conference Quiz 1A August 27, 2001 1. What is the osmolarity of a 1.8% NaCl solution? Justify your answer. 600 mOsm Justification Option #1: A 0.9% NaCl solution is 300 mOsm, so a solution twice as strong would be 1.8% NaCl, or 600 mOsm. Justification Option #2: = 1.8% NaCl = 1.8 g NaCl / 100 g H2O = 1.8 g NaCl / 100 ml H2O = 18 g NaCl / 1000 ml H2O = 18 g NaCl / 1 liter H2O = 18 g NaCl * (1 mole NaCl / 58.5 g NaCl) / 1 liter H2O = 0.3 mole NaCl / 1 liter H2O = 300 mM NaCL = 600 mOsm NaCl 2. Which of the following values are abnormal? What is the normal value for each that you identify as abnormal? Blood Na+: 100 mM Blood K+: 4.5 mM Plasma osmolality: 250 mOsm Blood (plasma) Na+ is normally about 140 mM. A range of about 137 - 147 mM is also a valid way of specifying normal plasma Na+ concentration. Blood (plasma) osmolalaity is normally about 300 mOsm (more accurately about 280 mOsm). A range of about 280 - 310 mOsm is also a valid way of specifying normal plasma osmolality. 3. If a patient is overhydrated, which of the following urine osmolalities would you expect to be part of a normal compensatory response? (That is, which will help compensate for the overhydration?) 150 mOsm, 300 mOsm, 600 mOsm Briefly explain your reasoning. Normal osmolality can be restored to an overhydrated patient by removing excess water. Ideally, you would remove just water with no solute. However, removing any solution that has less solute than the patient's body fluids will help to concentrate the remaining solutes. The only choice which can be hypoosmotic to the body fluids of an overhydrated patient is that at 150 mOsm. 4. What consequence of the tap water enemas is most likely to have produced the seizures and coma in the patient discussed in conference? Increased intracranial pressure resulting from cerebral (brain) edema. The major culprit is the increased intracellular volume in the restricted cranial cavity. Extracellular fluid, because it communicates with other parts of the body, can be "squeezed our" of the brain when intracranial pressure rises. However, the only force that can cause water to leave cells in the brain is an osmotic driving force. 5. What would be the directional changes in body water compartment volumes in an overly hydrated patient 2 minutes after an intravenous infusion of 5% glucose solution? Would this be better than the intravenous infusion given to the conference patient? A 5% glucose solution has an osmolarity of about 300 mOsm. An overly hydrated patient has body fluids that have an osmolarity below normal, i.e. less than 300 mOsm. Thus, the infusion will be slightly hyperosmotic compared to this patient's body fluids. Two minutes after infusion, glucose behaves as an impermeable solute. Therefore, the infusion will draw a small amount of water from the cells of the patient. This would NOT be better than the strongly hypertonic solution used in conference because it will not significantly relieve the major immediate threat of increased intracranial pressure. ------------------------------------------------------------------------ MEDICAL PHYSIOLOGY Body Fluids Conference Quiz 1B August 27, 2001 1. What is the osmolarity of a 10% glucose solution? Justify your answer. 600 mOsm Justification Option #1: A 5% glucose solution is 300 mOsm, so a solution twice as strong would be 10% glucose, or 600 mOsm. Justification Option #2: 5% glucose = 5 g glucose / 100 g H2O = 5 g glucose / 100 ml H2O = 50 g glucose / 1000 ml H2O = 50 g glucose / 1 liter H2O = 50 g glucose * (1 mole glucose / 180 g glucose) / 1 liter H2O = 0.6 mole glucose / 1 liter H2O = 600 mM glucose = 600 mOsm glucose 2. Which of the following values are abnormal? What is the normal value for each that you identify as abnormal? Blood Na+: 140 mM Blood K+: 2.5 mM Plasma osmolality: 350 mOsm Blood (plasma) K+ is normally about 4.5 mM. A range of about 3.7 - 5.0 mM is also a valid way of specifying normal plasma K+ concentration. Blood (plasma) osmolalaity is normally about 300 mOsm (more accurately about 280 mOsm). A range of about 280 - 310 mOsm is also a valid way of specifying normal plasma osmolality. 3. If a patient is dehydrated, which of the following urine osmolalities would you expect to be part of a normal compensatory response? (That is, which will help compensate for the dehydration?) 150 mOsm, 300 mOsm, 600 mOsm Briefly explain your reasoning. Normal osmolality can be restored to a dehydrated patient by removing excess solute. Ideally, you would remove just solute with no water. However, removing any solution that has a higher solute concentration than that of the patient's body fluids will help to dilute the remaining body fluids. The only choice which can be hyperoosmotic to the body fluids of a dehydrated patient is that at 600 mOsm. 4. What consequence of the tap water enemas is most likely to have produced the abnormal renal function (high urine osmolality) in the patient discussed in conference? Increased intracranial pressure resulting from cerebral (brain) edema. The major culprit is the increased intracellular volume in the restricted cranial cavity. Extracellular fluid, because it communicates with other parts of the body, can be "squeezed our" of the brain when intracranial pressure rises. However, the only force that can cause water to leave cells in the brain is an osmotic driving force. (The increased pressure probably causes inappropriate antidiuretic hormone (ADH) secretion from the brain.) 5. What would be the directional changes in body water compartment volumes in an overly hydrated patient after an intravenous infusion of 0.9% saline solution? Would this be better than the intravenous infusion given to the conference patient? A 0.9% glucose solution has an osmolarity of about 300 mOsm. An overly hydrated patient has body fluids that have an osmolarity below normal, i.e. less than 300 mOsm. Thus, the infusion will be slightly hyperosmotic compared to this patient's body fluids. Sodium chloride (the solute in saline) is an impermeable solute. Therefore, the infusion will draw a small amount of water from the cells of the patient. This would NOT be better than the strongly hypertonic solution used in conference because it will not significantly relieve the major immediate threat of increased intracranial pressure.