Health Science I - Lecture 8: Water, Electrolytes, and Body Fluid PDF

Lecture 8: Water, electrolytes and body fluid Dr Isabel Hwang Senior Lecturer Year 1 Coordinator (Medicine) Division of Education, School of Biomedical Sciences, Faculty of Medicine, CUHK Email: [email protected] Email: [email protected] Office number: 3943 67...

Lecture 8: Water, electrolytes and body fluid Dr Isabel Hwang Senior Lecturer Year 1 Coordinator (Medicine) Division of Education, School of Biomedical Sciences, Faculty of Medicine, CUHK Email: [email protected] Email: [email protected] Office number: 3943 6795 Important Notice: These slides contain copyright materials. Access is limited to students of MEDF1011, unless otherwise specified. 1 Copyright © 2016 The Chinese University of Hong Kong Lecture Outline Definition of total body water Distribution of body fluid in different fluid compartments Relationship between osmosis and osmotic pressure Definition and application of osmolarity or osmolality Examples of effective and ineffective osmoles in plasma Homeostatic regulation of water balance Thirst reflex mechanism ADH mechanism Fluid movement across the capillary wall Formation of edema and examples of its common causes Copyright © 2024 The Chinese University of Hong Kong 2 Pre-class assignment on Blackboard Micromodules 7 3 Regulation of body fluid and its composition Regulation of composition and volume of body fluids is fundamental to physiology. Our body cells requires a well-defined composition to function normally – Composition of intracellular fluid (ICF) is relatively constant – External environment will affect the internal environment Blood volume (BV) affects venous return and thus mean arterial blood pressure (MAP) – Fluid disturbances are observed in disease states. – A basic understanding of body fluids and its composition is useful for patient management and dosing of drugs. 4 Why water balance so important? The amount of water contained in cells remains fairly constant but can undergo significant change in extreme or pathological conditions Slide recall: Lecture 1 Tissue cells such as brain cells are very sensitive to water status Disturbance of water balance can occur in two types: Dehydration Overhydration More common Less common Due to inadequate fluid intake or Due to excessive water intake excessive fluid loss (e.g. vomiting, Athletes may drink too much water to diarrhaea, sweating, etc.) avoid dehydration Affects people of all ages especially the elderly people 5 Dehydration Loss of excessive water Excessive water exit Stretching of from body Shrinkage of vascular Intracranial Dehydration cells brain cells connections bleeding including to the skull brain cells Symptoms of dehydration: Dry mouth and tongue Thirsty Headache Fatigue (tiredness) Dizziness Confusion 6 Overhydration (water intoxication) Gain of excessive water Excessive water entry Increase in Overhydration Increase in Impaired into body space (water intra-crancial blood flow cells occupied in intoxication) pressure to the brain including the skull brain cells Symptoms of over-hydration: Nausea and vomiting Headache Muscle weakness Confusion Disorientation Seizures Coma 7 Water is kept in different body fluid compartments Cell membrane Body fluid is divided into different fluid compartments (extracellular versus intracellular) Compartments are separated by boundaries such as membranes and blood vessel wall Capillary wall Cell membranes are semi-permeable (selectively permeable) but capillary wall is much more diffusible and not as selective Transport processes occur between compartments via passive and active processes 8 Total body water (TBW) Is the total volume of water (in L) in a person's body expressed as a percentage of their total weight (in kg) TBW accounts for approximately 60% of total body weight in adult Water content (%) Muscle cells (50%); bone cells (22%); Fat cells (10%) Water content declines to 45% in old age and in dehydration state Body water content is the major consideration in determining drug dosage ❖ Women and older people have less body water and will have a higher concentration of drug in the plasma than will young people if all are given an equal dose per kg of body mass 9 Body fluid in different compartments Example: For a healthy person with body weight of 70 kg 1/3 of TBW 2/3 of TBW Total body water = 70 x 60% = 42L Interstitial fluid is also called tissue fluid. 10 Plasma volume is not the same as blood volume Lecture 1: Blood and plasma Hematocrit (Hct) measures the volume of red blood cells (45% used in Lecture 1, 40% used in this example) compared to the total blood volume (red blood cells and plasma). There is a sex difference as men usually has higher % 11 Electro-neutrality of body fluid As a whole, all body fluids have no net charge (electroneutral) meaning that the sum of its positively charged ion (cations) must equal the sum of its negatively charged ions (anions) in aqueous solution. – E.g. plasma and urine are both electrically neutral 12 Definition recall: A solute A solute is a substance that can be dissolved into a solution by a solvent. A solvent is a substance in which a solute is dissolved Solutes can be classified as non-electrolytes or electrolytes Non-electrolytes Electrolytes Mostly organic Mostly inorganic Do not dissociate in water Dissociate in water into particles called ions E.g. NaCl → Na+ + Cl− Exerts greater osmotic effect than non- electrolytes i.e. greater ability to cause fluid shifts No charged particles created Charged particle created E.g., glucose, lipids, creatinine, E.g. Na+, K+, Cl−, H+, and some proteins and urea 13 Distribution of fluid in ECF and ICF Fluid and electrolyte can move between compartments according to different driving forces Movement across cell membrane is driven by osmotic pressure (osmotic force) Movement across capillary wall is driven by both hydrostatic pressure and colloid osmotic pressure (oncotic pressure) 14 Osmosis Defined as passive diffusion of water molecules through a membrane down its concentration gradients (i.e. from high water concentration to low water concentration) Characteristics of osmosis: Always a passive process called simple diffusion Unaffected/independent of membrane potentials Movement is driven by water gradient only 15 Compartment 1 and 2 are separated by a semi-permeable membrane like a cell membrane A semi-permeable membrane Compartment 1 Compartment 2 A solute that cannot cross the membrane, i.e. non- penetrating There will be a driving force called osmotic pressure (or osmotic force) that drives water to enter the compartment with more solutes At equilibrium, this osmotic pressure is balanced off by a hydrostatic pressure (force) Hydrostatic pressure is the pressure exerted by fluid 16 Osmotic pressure Compartment 1 Compartment 2 (lower osmotic pressure) (higher osmotic pressure) Concept check: Though water moves from low osmotic pressure to high osmotic pressure. The concept of moving down gradients is still valid as water moves from high conc. to its low conc. across the cell membrane Osmotic pressure reflects total solute concentration rather than the molecular identities of the solutes Example: Na+ ion in plasma Osmotic pressure also reflects the ability to "pull " water towards the region with more solutes Higher solute concentration means higher osmotic pressure 17 Non-penetrating solutes are also called effective osmoles Effective osmoles generate the net driving force to pull water Examples of effective solutes are NaCl (ECF), KCl (ICF), plasma albumin (ECF) Non-penetrating solutes cannot cross the plasma membrane (membrane non-permeant) The relative abundance of effective osmoles in intracellular and extracellular compartments dictates body water distribution between the intracellular fluid (ICF)and extracellular fluid (ECF) 18 Penetrating solutes are called ineffective osmoles At equilibrium Semipermeable membrane “Penetrating” solute water Penetrating solutes can cross the plasma membrane (membrane- permeant) Ineffective osmoles cannot generate net driving force on water movement Examples of ineffective osmoles include urea, glucose (in healthy people), ethanol, etc. 19 Concept recall: Molarity represents solute concentration but….. There is a limitation to use molarity to describe concentration as it can be misleading sometimes…. Lecture 1: Ingredients of life 20 Molarity represents solute concentration but….. The important factor for osmosis to occur is the number of ‘solutes or particles” in a given volume of solution but not the number of molecules As some molecules dissociate into ions when they dissolve in a solution, the number of particles/solutes in solution is not always equivalent to its number of molecules in the same solution Example: dissolving glucose or NaCl in water gives different total number of particles/solutes 21 New concentration concept called ‘osmolarity’ represents the ‘total’ solute concentration in a solution We express the concentration of biological solutions as osmolarity which is defined as the number of particle (ions or intact molecules) per liter of solution – Expressed as Osm/L or OsM (called osmolar) – In physiological condition, it is usually expressed as mOsm/L Mathematically, we can convert molarity into osmolarity: Osmolarity = Molarity x Number of particle(s)/ per molecule 22 22 Osmolarity Osmolarity is a measure of the osmotic pressure of a solution. The more solute dissolved in a solution, the higher osmotic pressure (and thus the higher the osmolarity). Molarity 1mM NaCl Molarity 1mM glucose 1 mM Na+ 1 mM glucose 1 mM Cl− Total osmolarity = 2 mOsm/L Total osmolarity = 1 mOsm/L Normal extracellular fluid (ECF) osmolarity is 285-295 mOsm/L For simplicity, we sometimes round it up to 300 mOsm/L 23 You can kill a patient if you infuse solutions of inappropriate osmolarity! Case studies: Overdose of KCl injection in a local hospital 24 Osmolarity and Osmolality Normal: 300 mOsm/L Normal: 300 mOsm/kg Osmolarity Osmolality Number of dissolved solute particles Number of dissolved solute particles (or molecules) in 1 L of solvent (or molecules) in 1 kg of solvent (mOsm/L). (mOsm/kg). Is temperature dependent but we Is based on the mass of the measure osmolarity at normal body solvent, is temperature temperature independent. For this reason, osmolarity and For this reason, osmolality is osmolality have the same the preferred term in most values (but different units) in a clinical or biologic systems healthy person 25 Self-assessment: How to describe solution of different osmolarities? 3 types of solutions: Iso-osmotic, hyper-osmotic and hypo-osmotic How to compare with each solution? Solution A Solution B Solution C (1 Osm/L glucose) (2 Osm/L glucose) (1 Osm/L NaCl) A is hypo-osmotic to B B is hyper-osmotic to A C is iso-osmotic to A A is iso-osmotic to C B is hyper-osmotic to C C is hypo-osmotic to B 26 Abundance of extracellular Na+ ion Because Na+ ion is the major cation of the extracellular fluid, and thus it is the major determinant of plasma osmolarity ECF osmolarity can be roughly obtained by doubling the [ Na+] Plasma osmolarity = 2 x plasma [ Na+] Plasma osmolarity = 270-290 mOsm/L Therefore, clinically we frequently refer to the plasma (or serum) Na+ concentration as an index of plasma osmolarity/osmolality. 27 Homeostatic control of plasma osmolarity Represents Plasma osmolarity = mOsm / L volume, i.e., water Represents number of solute especially Na+ ion solute Plasma osmolarity = water 1. Thirst reflex 2. ADH solute Plasma osmolarity = water 28 Homeostatic control of plasma osmolarity Both thirst reflex and secretion of antidiuretic hormone (ADH, also called vaspressin) is stimulated by an increase in plasma (ECF) osmolarity Increase in plasma osmolarity can be due to dehydration Dehydration is caused by not drinking enough fluid or by losing more fluid than you take in Examples: sweating, vomiting, diarrhea, etc. unchanged 29 The thirst reflex Components of reflex: Stimulus Is the primary defense against an Receptor increase in plasma osmolarity Integrator Integrated by the hypothalamus Hypothalamic osmoreceptors Effectors detect ECF osmolarity and activated by Increased plasma osmolarity (of 1-2%) Dry mouth Decreased blood volume or MAP X Inhibitory feedback signals X include Relief of dry mouth Activation of stomach and intestinal stretch receptors 30 Anti-diuretic hormone (ADH) Also called vasopressin because it can act as a vasoconstrictor at high plasma levels Is secreted by posterior pituitary in response to dehydration. Stimulates insertion of water channels (called aquaporin-2) into epithelial membranes of the collecting duct in the kidneys. When [ADH] is high, H2O is drawn out of collecting duct and then reabsorbed water can return to the plasma. 31 Movement of fluid across compartments Intracellular fluid Plasma Capillary Interstitial fluid F A Fluid movement between plasma and interstitial fluid depends on the balance between 1. Hydrostatic pressure forces fluid out of the capillary (Filtration) 2. Plasma colloid osmotic pressure (oncotic pressure) draws fluid back to the capillary (Absorption) 32 Distribution of the extracellular fluid by bulk flow The concentrations of all plasma solutes except proteins are virtually the same in the filtered fluid as in plasma A A A The wall of A A systemic capillaries A A are highly diffusible A to all solutes except A A A plasma proteins A A A such as albumin A = Albumin Albumin acts as effective osmoles and draw water into the plasma compartment by absorption This osmotic pressure generated by the plasma proteins is called colloid osmotic pressure or oncotic pressure 33 A higher blood hydrostatic pressure at the arteriolar end of the capillary that favors filtration 34 The albumin in plasma creates colloid osmotic pressure and favors absorption at the venule end of the capillary 35 The lymphatic system returns excessive interstitial fluid filtered from capillaries due to Starling forces The lymphatics Offers one-way route for return of lymph (interstitial) fluid from the tissues to the circulatory system About 4L of fluid per day is returned to the blood Prevents edema formation 36 Summary: distribution of fluid between plasma and interstitial fluid At the arterialar end, hydrostatic pressure is higher than oncotic pressure ✓ fluid move out of blood vessel (called filtration) At the venule end, hydrostatic pressure is lower than oncotic pressure ✓ fluid moves into blood vessel (called absorption) Overall, there is a net movement of fluid out (i.e. a net filtration) of the blood vessel which will return to the venous system via the lymphatic system 37 Edema Accumulation of excessive fluid in the interstitial space Loss of balance between hydrostatic and oncotic pressure Formation of edema Decrease in oncotic Increase in hydrostatic pressure pressure E.g. Malnutrition, loss of E.g. Hypertension plasma proteins in kidney due to renal disease 38 Decrease in plasma oncotic pressure in malnutrition 39 Application of your concepts: Post-class assignment on Blackboard 40 Learning outcomes ⚫ Define total body water for a person of 60kg. ⚫ Calculate your own total body water, intracellular, extracellular, blood and plasma volumes. ⚫ Recall that osmotic pressure reflects total solute concentration and the ability to cause fluid shift across a compartment ⚫ Name examples of effective and non-effective osmoles ⚫ Write down the formula that roughly gives the osmolarity of ECF fluid ⚫ Write a flow diagram summarizing the thirst reflex mechanism ⚫ Write a flow diagram summarizing the ADH mechanism ⚫ Define edema and state common cause of edema with examples ⚫ Describe how hydrostatic blood pressure and oncotic pressure (colloid osmotic pressure) help regulate fluid exchange across the capillary wall 41 Required reading: Basic Concepts in biomedical sciences I Chapter 4, Water, electrolytes and body fluids Page 55-98 42 42  End of this lecture Copyright © 2023 The Chinese University of Hong Kong 43