USMLE Step 1: Physiology 29. Answer: B All choices involve a fluid imbalance. The decrease in blood pressure indicate a fluid loss. This eliminates water intoxication(choice D), which is a gain in fluid. The fact that plasma osmolarity did not change means a loss of isotonic fluid. This eliminates hypotonic diuresis(choice C)and alcohol-induced dehydration(choice E), which is also a hypotonic diuresis. Hemorrhage and diarrhea are both isotonic Buid losses; however, hemorrhage would not increase the hematocrit. The decrease in K is due to the fact there is a net secretion of K in the colon, which, along with the kidney, is a normal route for eliminating excess K. 32出 edical
USMLEStep 1: Physiology 32 UPLANd . . me IcaI 29. Answer: B.All choices involvea fluid imbalance. The decrease in blood pressure indicates a fluid loss.This eliminates water intoxication (choice D), which is a gain in fluid. The fact that plasma osmolarity did not change means a loss of isotonic fluid. This eliminates hypotonic diuresis (choice C) and alcohol-induced dehydration (choice E), which is also a hypotonic diuresis. Hemorrhage and diarrhea are both isotonic fluid losses; however, hemorrhage would not increase the hematocrit. The decrease in K is due to the fact there is a net secretion of K in the colon, which, along with the kidney, is a normal route for eliminating excessK
SECTION II Excitable tissue
SECTIONII ExcitableTissue
lonic Equilibrium and Resting Membrane potential ELECTROCHEMICAL POTENTIAL What the usmle Requires you to Know Membrane conductance That the three general Definition channel groupings are wn as ungated, voltage- Membrane conductance refers to the number of channels that are open in a membrane For example, Nat conductance is proportional to the number of open channels that will allow the Natto pass through the membrane. It does not indicate if there will be a net diffusion of ior The information gained through the channel from a calculation using the General Properties The condusions drawn If conductance is increasing, channels are opening, and if conductance is decreasing, channels bout the dynamic behavior of an ion when the membrane potential Gu he rate at which ions move across a membrane depends on the number of open channels and m of the cell is known and the equilibrium When ions Alow through channels, the cells membrane potential changes. However, under tential for a particular ion hysiologic conditions, too few ions flow to produce a significant effect on the ions extracellu X(Ex) has been calculated lar concentration or the concentration gradient across the membrane from the Nemst equation Channels are classified into three main groups Ungated channels: Because these channels have no gates, they are always open. Fc potassium ions and sodium example, all celis possess ungated potassium channels. This means there will be a net ons across the membrane flux of potassium ions through these channels unless potassium is at equilibrium of a typical resting excitable Voltage-gated channels: In these channels, the gates open and/or close in response to a cell in vivo membrane voltage change. For example, many excitable cells possess voltage-gated sodium channels. The channels are closed under resting conditions, but membrane depolarization causes them to quickly open and then quickly close stance(ligand ) It is the interaction of the ligand with the receptor that regulates the opening and closing of the channel. For example, post- junctional membranes of chemical synapses possess ligand-gated channels, and transmission depends on the nteraction of the transmitter and the ligand-gated channel medical 35
IonicEquilibriumandResting MembranePotential ELECTROCHEMICALPOTENTIAL MembraneConductance Definition Membrane conductance refers to the number of channels that are open in a membrane. For example, Na+ conductance is proportional to the number of open channels that will allow the Na+ to pass through the membrane. It does not indicate if there will be a net diffusion of ions throughthe channels. . General Properties If conductance is increasing, channels are opening, and if conductance is decreasing, channels are closing. The rate at which ions move across a membrane depends on the number of open channels and the net force. When ions flow through channels, the cell's membrane potential changes. However, under physiologic conditions, too few ions flow to produce a significant effect on the ion's extracellular concentration or the concentration gradient across the membrane. Channels are classifiedinto three main groups: . Ungated channels: Becausethese channels have no gates, they are alwaysopen. For example, all cellspossess ungated potassium channels. This means there will be a net flux of potassium ions through these channels unless potassium is at equilibrium. . Voltage-gated channels: In these channels, the gates open and/or close in response to a membrane voltage change. For example, many excitable cellspossess voltage-gated sodium channels. The channels are closed under resting conditions, but membrane depolarization causes them to quickly open and then quickly close. . Ligand-gated channels: The channel complex includes a receptor to a specific substance (ligand). It is the interaction of the ligand with the receptor that regulates the opening and closing of the channel. For example, post-junctional membranes of chemical synapses possess ligand-gated channels, and transmission depends on the interaction of the transmitter and the ligand-gated channel. What the USMLE ~equires You to Know . Thathethreegeneral channelgroupingsare knownasungated,voltagegated,andligand-gated . Theinformationgained fromacalculationusingthe Nernstequation . Theconclusionsdrawn abouthedynamic behaviorofanionwhen themembranepotential (Em)ofthecellisknown andtheequilibrium potentialfora particularion x(Ex)hasbeencalculated fromtheNernstequation . Thedynamicsofthe potassiumionsandsodium ionsacrossthemembrane ofatypicalrestingexcitable cellinvivo KAPLA~. meulCa I 35
USMLE Step l: Physiolog The net force acting on an ion across a membrane is the sum of two independent forces Concentration Force Determined by the concentration difference across the membrane The greater the concentration difference, the greater the concentration force E|e The size of this force is determined by the electrical difference across the membrane(usually measured in millivolts (mVI). The in vivo magnitude is determined by the membrane potential (Em), which is a value that must be measured or given The direction of the force is based on the fact that like charges repel and opposite charges attract. For example, if the membrane potential is -70 mv, this represents a force of 70 mv that attracts all positive ions and repels all negative ions. The two forces(concentration and electrical )can be represented by separate vectors. If the two vectors act in the same direction the net force is the sum of the individual If the two vectors act in opposite directions, the net force is the difference between the two forces and is directed along the axis of the larger force If the two forces are equal but opposite, there is Do net force and the ion is in a state of At equilibrium, the two forces are always equal but opposite in direction A simple model demonstrating concentration and electrical forces is illustrated in Figure 1I-1-1 A-B Concentration Gradien 10 mM 1 mM A-B Electrical Gradient -60mV A-B Electrical Gradient The concentration force for both sodium ions and chloride ions is directed from a to B Because they have the same concentration gradient, they have the same concentration force (-60 mV) attracts the positive sodium ion with a force of bo mk ' gative membrane potential The electrical force on sodium ion is directed toward B. the ne with open membrane sodium channels, positive sodium ion charges flow from A to B, driven by the concentration force and the electrical force The electrical force on the chloride ion is also 60 mv, but because like charges repel, this force is directed from B to A. Because the forces on the chloride ion are opposite in direction, the greater force determines the direction of the net force. 36 medical
USMLEStep 1: Physiology 36 KAPLAlfil - me leaI NetForce The net force acting on an ion across a membrane is the sum of two independent forces. ConcentrationForce Determined by the concentration difference across the membrane. The greater the concentration difference, the greater the concentration force. Electrical Force The size of this force is determined by the electrical difference across the membrane (usually measured in millivolts [mY]). The in vivo magnitude is determined by the membrane potential (Em), which is a value that must be measured or given. The direction of the force is based on the fact that like charges repel and opposite charges attract. Por example, if the membrane potential is -70 mV;this represents a force of 70mV that attracts all positive ions and repels all negative ions. The two forces (concentration and electrical)can be represented by separate vectors. If the two vectors act in the same direction, the net force is the sum of the individual forces. If the two vectors act in opposite directions, the net force is the differencebetween the two forces and is directed along the axis of the larger force. If the two forces are equal but opposite, there is no net force and the ion is in a state of equilibrium. At equilibrium, the two forces are alwaysequal but opposite in direction. A simple model demonstrating concentration and electrical forces is illustrated in Figure II-1-1. Na+- A~B Concentration Gradient A~ B Electrical Gradient CI --A ~ B Concentration Gradient A~ B Electrical Gradient Figure 11-1-1 The concentration force for both sodium ions and chloride ions is directed from A to B. Because they have the same concentration gradient, they have the same concentration force. The electrical force on sodium ion is directed toward B. The negative membrane potential (-60 mV) attracts the positive sodium ion with a force of 60 mV. With open membrane sodium channels, positive sodium ion charges flow from A to B, driven by the concentration force and the electrical force. The electrical force on the chloride ion is also 60 mV;but because like charges repel, this force is directed from B to A. Because the forces on the chloride ion are opposite in direction, the greater force determines the direction of the net force. A B 10 mM 1 mM Na+ Na+ CI- CI- -60 mV