JAMES N. PASLEY N USMLE ROAD MAP PHYSIOLOGY HIGH-YIELD FACTS ) ILLUSTRATIONS D CLINICAL PROBLEMS CLINICAL CORRELATIONS 口m=m==TheBestRoutetostep1suces
The McGraw- Hill Companies opyright o 2003 by The McGraw-Hill Co Inc. All rights reserved. Manufactured in the United States of America. Except as permitted under the United States Copyright Act of 1976. no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. 0-07-143556-5 The material in this e Book also appears in the print version of this title: 0-07-140076-1 All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every rence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trade wner, with no intention of infringement of the trademark. Where such designations appear in this bool have been printed with initial caps. McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. For more information, please contact George Hoare, Special Sales, at george hoare @mcgraw- mor(212)904-4069 TERMS OF USE This is a copyrighted work and The McGraw-Hill Companies, Inc ("McGraw-Hill) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms Except as permitted under the Copyright Act of 1976 and the right to store and one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish sublicense the work or any part of it without McGraw-Hill's prior consent. Yo se the work own noncommercial and personal I other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. THE WORK IS PROVIDED"AS IS". MCGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE AND EXPRESSLY DISCLAIM ANY WAR. RANTY EXPRESS OR IMPLIED. INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your nents or that its will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for racy, error or omission, regardless of cause, in the work or for any damages resulting therefrom McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no cir- cumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, cons uential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatso- ever whether such claim or cause arises in contract. tort or otherwise DOI:10.10360071435565
Copyright © 2003 by The McGraw-Hill Companies, Inc. All rights reserved. Manufactured in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. 0-07-143556-5 The material in this eBook also appears in the print version of this title: 0-07-140076-1 All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps. McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. For more information, please contact George Hoare, Special Sales, at george_hoare@mcgraw-hill.com or (212) 904-4069. TERMS OF USE This is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. THE WORK IS PROVIDED “AS IS”. McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK, INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operation will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise. DOI: 10.1036/0071435565 ebook_copyright 8 x 10.qxd 8/27/03 9:33 AM Page 1
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For more information about this title. click here GONTENNIS Using the Road Map Series for Successful Review 1. Cell Physiology IL Ion channels 4 II. Cell signaling 8 IV Membrane potential 11 V. Structure of Skeletal Muscle 13 I Neuromuscular and Synaptic Transmission 18 Ⅵ II. Smooth muscle22 Clinical Problems 24 2. Cardiovascular Physiology L. General Principles 27 IL Hemody Ill. Electrophy V Cardiac Muscle and Cardiac Output 37 V. Cardiac Cycle with Pressures and ECG 42 VI. Regulation of Arterial Pressure 44 VIl. Control Mechanisms and Special Circulations 44 VIlL. Integrative Function 48 Clinical Problems 5 3. Respiratory Physiology. L Lung volumes and Capacities 56 II. Muscles of Breathing 58 Ill. Lung Corm, sf Lung Recoil 61 nce IV Components V. Airway Resistance 62 VIL. Carbon Dioxide Transport o port 63 VI Gas Exchange and Oxygen Transp VIll. Respiration Control 68 IX Pulmonary Blood Flow 70 X. Ventilation-Perfusion Differences 73 XL. Special Environments 74 Clinical Problems 75 4. Body Fluids, Renal, and Acid-Base Physiology L. Body Fluids 79 II Kidney Function 83 Ill. Renal Anatomy 84 IV Renal Blood Flow and Glomerular Filtration 87 Copyright o 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Us
CONTENTS Using the Road Map Series for Successful Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii 1. Cell Physiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 I. Plasma Membrane 1 II. Ion Channels 4 III. Cell Signaling 8 IV. Membrane Potential 11 V. Structure of Skeletal Muscle 13 VI. Neuromuscular and Synaptic Transmission 18 VII. Smooth Muscle 22 Clinical Problems 24 Answers 25 2. Cardiovascular Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 I. General Principles 27 II. Hemodynamics 27 III. Electrophysiology 32 IV. Cardiac Muscle and Cardiac Output 37 V. Cardiac Cycle with Pressures and ECG 42 VI. Regulation of Arterial Pressure 44 VII. Control Mechanisms and Special Circulations 44 VIII. Integrative Function 48 Clinical Problems 51 Answers 53 3. Respiratory Physiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 I. Lung Volumes and Capacities 56 II. Muscles of Breathing 58 III. Lung Compliance 60 IV. Components of Lung Recoil 61 V. Airway Resistance 62 VI. Gas Exchange and Oxygen Transport 63 VII. Carbon Dioxide Transport 67 VIII. Respiration Control 68 IX. Pulmonary Blood Flow 70 X. Ventilation-Perfusion Differences 73 XI. Special Environments 74 Clinical Problems 75 Answers 77 4. Body Fluids, Renal, and Acid-Base Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 I. Body Fluids 79 II. Kidney Function 83 III. Renal Anatomy 84 IV. Renal Blood Flow and Glomerular Filtration 87 iii 5506fm01.qxd_sw 2/17/03 2:09 PM Page iii For more information about this title, click here. Copyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use
V. Transport Mechanisms of Nephron Segments 91 VI Regulation of NaCl Excretion 95 VIl. Potassium Regulation 98 VIll. Renal Handling of Glucose 98 IX. Urea Regulation 98 XI. Renal Calcium Regulation 99 XIl Magnesium Regulation 100 XIIL Co 100 XIV. Acid-Base Balance 101 XV. Diagnostic Hints for Acid-Base Disorders 10 L Selected Acid-Base Disorders 106 Clinical Problems 108 Answers 110 .113 I. Regulation: Muscle, Nerves, and Hormones of the Gut 113 Il Salivary Secretion 114 Ill. Swallowing 116 IV Gastric Motor Function 117 V. Gastric Secretion 119 VI. Motility of the Small Intestine 123 VIl Exocrine Pancreas 125 VIll. Biliary Secretion 126 IX. Digestion and Absorption 1 X Motility of the Colon and Rectum 133 6 Endocrine Physiology∴∴……∵ ...139 I. General Principles 139 IL Adrenal Cortex 142 Ill. Adrenal Medulla 147 V Glucagon 7, eas 148 VI Human Growth Hormone 154 VIl. Hormonal Calcium Regulation 155 VIII. Thyroid Hormones 158 IX Male Reproductive Hormones 161 X Female Reproductive Hormones 164 Clinical Problems 170 7. Neurophysin 174 I Autonomic Nervous System 174 II. Sensory Syster IIL Motor Pathy ion of the Cerebral Cortex 201 in Barrier and Cerebrospinal Fluid 203 VI Body t
V. Transport Mechanisms of Nephron Segments 91 VI. Regulation of NaCl Excretion 95 VII. Potassium Regulation 98 VIII. Renal Handling of Glucose 98 IX. Urea Regulation 98 X. Phosphate Regulation 99 XI. Renal Calcium Regulation 99 XII. Magnesium Regulation 100 XIII. Concentrating and Diluting Mechanisms 100 XIV. Acid-Base Balance 101 XV. Diagnostic Hints for Acid-Base Disorders 104 XVI. Selected Acid-Base Disorders 106 Clinical Problems 108 Answers 110 5. Gastrointestinal Physiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 I. Regulation: Muscle, Nerves, and Hormones of the Gut 113 II. Salivary Secretion 114 III. Swallowing 116 IV. Gastric Motor Function 117 V. Gastric Secretion 119 VI. Motility of the Small Intestine 123 VII. Exocrine Pancreas 125 VIII. Biliary Secretion 126 IX. Digestion and Absorption 128 X. Motility of the Colon and Rectum 133 Clinical Problems 134 Answers 136 6 Endocrine Physiology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 I. General Principles 139 II. Adrenal Cortex 142 III. Adrenal Medulla 147 IV. Endocrine Pancreas 148 V. Glucagon 151 VI. Human Growth Hormone 154 VII. Hormonal Calcium Regulation 155 VIII. Thyroid Hormones 158 IX. Male Reproductive Hormones 161 X. Female Reproductive Hormones 164 Clinical Problems 170 Answers 172 7. Neurophysiology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 I. Autonomic Nervous System 174 II. Sensory System 177 III. Motor Pathways 192 IV. Language Function of the Cerebral Cortex 201 V. The Blood-Brain Barrier and Cerebrospinal Fluid 203 VI. Body Temperature Regulation 205 iv Contents N 5506fm01.qxd_sw 2/17/03 2:09 PM Page iv
Cont Clinical problems 208 210 213
Clinical Problems 208 Answers 210 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Contents v 5506fm01.qxd_sw 2/17/03 2:09 PM Page v
USG HE UsⅣ LE RAⅣ AP SERIES FOR SUGGESSFUL REVIEW What Is the Road Map Series? Short of having your own personal tutor, the USMLE Road Map Series is the best source for efficient review of major concepts and information in the medical Why Do You Need A Road Map? It allows quickly and easily through your physiology course notes and textbook and prepares you for USMLE an How Does the Road Map Series Work? Outline Form: Connects the facts in a conceptual framework so that you understand the ideas and retain the information. Color and Boldface: Highlights words and phrases that trigger quick retrieval of concepts and facts. Clear Explanations: Are fine-tuned by years of stu udent interaction. The material is written by authors selected for their excellence in teaching and their experience in preparing students for board examinations. Illustrations: Provide the vivid impressions that facilitate comprehension and recall Clinical Correlations: Link all topics to their clinical applications, promoting fuller understanding and memory retention Clinical Problems: Give you valuable for the clinical vignette-based USMLE questions Explanations of Answers: Are learning tools that allow you to pinpoint your trengths and weaknesses. Copyright o 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Us
USING THE USMLE ROAD MAP SERIES FOR SUCCESSFUL REVIEW vii What Is the Road Map Series? Short of having your own personal tutor, the USMLE Road Map Series is the best source for efficient review of major concepts and information in the medical sciences. Why Do You Need A Road Map? It allows you to navigate quickly and easily through your physiology course notes and textbook and prepares you for USMLE and course examinations. How Does the Road Map Series Work? Outline Form: Connects the facts in a conceptual framework so that you understand the ideas and retain the information. Color and Boldface: Highlights words and phrases that trigger quick retrieval of concepts and facts. Clear Explanations: Are fine-tuned by years of student interaction. The material is written by authors selected for their excellence in teaching and their experience in preparing students for board examinations. Illustrations: Provide the vivid impressions that facilitate comprehension and recall. Clinical Correlations: Link all topics to their clinical applications, promoting fuller understanding and memory retention. Clinical Problems: Give you valuable practice for the clinical vignette-based USMLE questions. Explanations of Answers: Are learning tools that allow you to pinpoint your strengths and weaknesses. CLINICAL CORRELATION 5506fm01.qxd_sw 2/17/03 2:09 PM Page vii Copyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use
P三 CELL PHYSIOLOGY I Plasma membrane A. The structure of the plasma membrane allows the separatio reation of listinct molecular environments within cells. The lipid bilay lar to thin layers of oil surrounding fluid ozone. Thus, the lipid bilayer cell into functional compartments B. The fluid mosaic model is the accepted view of the molecular nature of plasma 1. The model proposes that proteins traverse the lipid bilayer and are incorpo rated within the lipid 2. Proteins and lipids can move freely in the plane of the membrane, producing C. The plasma membrane is composed of phospholipids and proteins. 1. Membrane lipids can be classified into three major classes: phospholipids, a. Phospholipids are the most abundant membrane lipids (1)They have a bipolar(amphipathic) nature, containing a charged head d two hydrophobic(water-insoluble, noncharged )tails. (2)The hydrophobic tails face each other, forming a bilayer and exposing the polar head group to the aqueous environment on either side of th membrane allows them to insert into membranes. These lipids can be modified by the addition of carbohydrate units at their polar end, creating glycosphingo- lipids in brain cells. c. Cholesterol is the predominant sterol (unsaturated alcohols found in animal and plant tissues) in human cells; it increases the fluidity of the membrane by rting itself between phospholipids, improving membrane stabilit TAY-SACHS DISEASE The accumulation of glycosphingolipid associated with Tay-Sachs disease causes paralysis and impair- 2. Membrane proteins that span the lipid bilayer are known as integral mem- brane proteins, whereas those associated with either the inner or the outer Copyright o 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Us
N CHAPTER 1 CHAPTER 1 CELL PHYSIOLOGY 1 I. Plasma Membrane A. The structure of the plasma membrane allows the separation and creation of distinct molecular environments within cells. The lipid bilayer is similar to thin layers of oil surrounding fluid ozone. Thus, the lipid bilayer divides the cell into functional compartments. B. The fluid mosaic model is the accepted view of the molecular nature of plasma membranes. 1. The model proposes that proteins traverse the lipid bilayer and are incorporated within the lipids. 2. Proteins and lipids can move freely in the plane of the membrane, producing the fluid nature of the membrane. C. The plasma membrane is composed of phospholipids and proteins. 1. Membrane lipids can be classified into three major classes: phospholipids, sphingolipids, and cholesterol. a. Phospholipids are the most abundant membrane lipids. (1) They have a bipolar (amphipathic) nature, containing a charged head group and two hydrophobic (water-insoluble, noncharged) tails. (2) The hydrophobic tails face each other, forming a bilayer and exposing the polar head group to the aqueous environment on either side of the membrane. b. Sphingolipids have an amphipathic structure similar to phospholipids that allows them to insert into membranes. These lipids can be modified by the addition of carbohydrate units at their polar end, creating glycosphingolipids in brain cells. c. Cholesterol is the predominant sterol (unsaturated alcohols found in animal and plant tissues) in human cells; it increases the fluidity of the membrane by inserting itself between phospholipids, improving membrane stability. TAY-SACHS DISEASE The accumulation of glycosphingolipid associated with Tay-Sachs disease causes paralysis and impairment of mental function. 2. Membrane proteins that span the lipid bilayer are known as integral membrane proteins, whereas those associated with either the inner or the outer CLINICAL CORRELATION 5506ch01.qxd_ccII 2/17/03 2:08 PM Page 1 Copyright © 2003 by The McGraw-Hill Companies, Inc. Click here for Terms of Use
2 USMLE Road Map: Physiology surface of the plasma membrane are known, respectively, as peripheral or lipid-anchored membrane proteins a. The majority of integral membrane proteins span the bilayer through the formation of a-helices, a group of 20-25 amino acids twisted to ex- pose the hydrophobic portion of the amino acids to the lipid environment in the membrane( Figure 1-1) b. Protein content of membranes varies from less than 20% for myelin,a substance that helps the propagation of action potentials, to more than 60%in liver cells, which perform metabolic activities C. Cellular proteins act as receptor sites for antibodies as well as hormone -, neurotransmitter-,and drug-binding sites d. Enzymes bound to the cell membrane are often involved in phosphoryla- tion of metabolic intermedi Carrier proteins in the membrane transport materials across the cell mem- f. Membrane channels allow polar charged ions(Na, K, Cl, and Ca")to flow across the plasma rane.lon channel gates regulate ion passage and are controlled by voltage (voltage gated, ligands(ligand gated) mechanical means(mechanically gated D. The plasma membrane acts as a selective barrier to maintain the composition of the intracellular environment 1. Passive transport, or diffusion, involves transport of solutes across the plasma membrane due to the substance's concentration gradient a. The term passive implies that no energy is expended directly to mediate the transport process. b. Passive transport is simple diffusion of substances that can readily pene trate the plasma membrane, as is the case for O2 or CO C. Passive transport is the only transport mechanism that is not carrier medi- egral membrane Figure 1-l. Membrane proteins
2 USMLE Road Map: Physiology N Integral membrane protein Peripheral Cholesterol membrane protein Figure 1–1. Membrane proteins. surface of the plasma membrane are known, respectively, as peripheral or lipid-anchored membrane proteins. a. The majority of integral membrane proteins span the bilayer through the formation of -helices, a group of 20–25 amino acids twisted to expose the hydrophobic portion of the amino acids to the lipid environment in the membrane (Figure 1–1). b. Protein content of membranes varies from less than 20% for myelin, a substance that helps the propagation of action potentials, to more than 60% in liver cells, which perform metabolic activities. c. Cellular proteins act as receptor sites for antibodies as well as hormone-, neurotransmitter-, and drug-binding sites. d. Enzymes bound to the cell membrane are often involved in phosphorylation of metabolic intermediates. e. Carrier proteins in the membrane transport materials across the cell membrane. f. Membrane channels allow polar charged ions (Na+ , K+ , Cl− , and Ca2+) to flow across the plasma membrane. Ion channel gates regulate ion passage and are controlled by voltage (voltage gated), ligands (ligand gated), or mechanical means (mechanically gated). D. The plasma membrane acts as a selective barrier to maintain the composition of the intracellular environment. 1. Passive transport, or diffusion, involves transport of solutes across the plasma membrane due to the substance’s concentration gradient. a. The term passive implies that no energy is expended directly to mediate the transport process. b. Passive transport is simple diffusion of substances that can readily penetrate the plasma membrane, as is the case for O2 or CO2. c. Passive transport is the only transport mechanism that is not carrier mediated. 5506ch01.qxd_ccII 2/17/03 2:08 PM Page 2
d. substances diffuse because of their inherent random molecular movement (ie, following the principle of Brownian motion) e. Diffusion across membranes occurs if the membrane is permeable to the f. The net rate of diffusion ) is proportional to the membrane area(A)and lute concentration difference(C1-C2) and the permeability(p)of the membrane g. Diffusion is measured using the formula]=PA(C1-C2) 2. Facilitated diffusion is the transport of a substrate by a carrier protein down a. Facilitated diffusion is required for substrates that are not permeable to the lipid bilayer and is faster than simple diffusion b. Facilitated diffusion is used cellular survival, including glucose and amino acids. 3. Osmosis is the movement of water across a semipermeable membrane due to a water concentration difference. Osmosis follows the same principles as diffu of any solute For example, if two solutions, A and B, are separated by a membrane im- permeable to solute but permeable to water and A contains a higher solute concentration than B, a driving force exists for water movement from B to A to equilibrate water concentration differences. Thus, water moves toward a solution with a higher osmolal b. Osmolality is a measure of the total concentration of discrete solute parti les in solution and is measured in osmoles per kilogram of water c. Because it is much more practical to measure the volume than the weigh of physiological solution, the concentration of solute particles is typicall expressed as osmolarity, which is defined as osmoles per liter: Osmolarity =gx C here number of particles in solution(Osm/mol) concentration(mol/L) d. Consider the following example: What is the osmolarity of a 0. 1 mol/L NaCl solution(for NaCl, g= 2)? Osmolarity =2 Osm/mol x 0. 1 mol/L=0.2 Osm/L or 200 mOsm/L e. Two solutions that have the same osmolarity are described as isosmotic. 4. An isotonic solution is one in which the volume of cells incubated in it does not change, implying that there is no movement of water in or out of the cell a. Under normal conditions. an isotonic solution is isosmotic with intracell lar fluid, which is isosmotic with plasma(290 mOsm/L) b. Not all isosmotic solutions are isotonic. A 290 mM(millimolar)solution of urea will be isosmotic(290 mOsm/L) but not isotonic because urea is meable to the cell membrane and will diffuse inside the cell. this cause increased concentration of urea inside the cell, which induces water influx and an increase in cell volume
Chapter 1: Cell Physiology 3 N d. Substances diffuse because of their inherent random molecular movement (ie, following the principle of Brownian motion). e. Diffusion across membranes occurs if the membrane is permeable to the solute. f. The net rate of diffusion (J) is proportional to the membrane area (A) and solute concentration difference (C1−C2) and the permeability (P) of the membrane. g. Diffusion is measured using the formula J = PA (C1−C2). 2. Facilitated diffusion is the transport of a substrate by a carrier protein down its concentration gradient. a. Facilitated diffusion is required for substrates that are not permeable to the lipid bilayer and is faster than simple diffusion. b. Facilitated diffusion is used to transport a variety of substances required for cellular survival, including glucose and amino acids. 3. Osmosis is the movement of water across a semipermeable membrane due to a water concentration difference. Osmosis follows the same principles as diffusion of any solute. a. For example, if two solutions, A and B, are separated by a membrane impermeable to solute but permeable to water and A contains a higher solute concentration than B, a driving force exists for water movement from B to A to equilibrate water concentration differences. Thus, water moves toward a solution with a higher osmolality. b. Osmolality is a measure of the total concentration of discrete solute particles in solution and is measured in osmoles per kilogram of water. c. Because it is much more practical to measure the volume than the weight of physiological solution, the concentration of solute particles is typically expressed as osmolarity, which is defined as osmoles per liter: where g = number of particles in solution (Osm/mol) C = concentration (mol/L) d. Consider the following example: What is the osmolarity of a 0.1 mol/L NaCl solution (for NaCl, g = 2)? e. Two solutions that have the same osmolarity are described as isosmotic. 4. An isotonic solution is one in which the volume of cells incubated in it does not change, implying that there is no movement of water in or out of the cell. a. Under normal conditions, an isotonic solution is isosmotic with intracellular fluid, which is isosmotic with plasma (290 mOsm/L). b. Not all isosmotic solutions are isotonic. A 290 mM (millimolar) solution of urea will be isosmotic (290 mOsm/L) but not isotonic because urea is permeable to the cell membrane and will diffuse inside the cell. This causes an increased concentration of urea inside the cell, which induces water influx and an increase in cell volume. Osmolarity = 2 Osm/ mol 0.1 mol/ L = 0.2 Osm/ L or 200 mOsm/ L × Osmolarity g = × C 5506ch01.qxd_ccII 2/17/03 2:08 PM Page 3
