2 - Water and Electrolytes (1) PDF

IFS0703 – INTRODUCTION TO CLINICAL BIOCHEMISTRY 1 Water and Electrolytes Dr Lucienne Vassallo Gatt  Objectives Understand the distribution of water in the body Review the molecular structure and bonding found in water Describe the importance of water in humans...

IFS0703 – INTRODUCTION TO CLINICAL BIOCHEMISTRY 1 Water and Electrolytes Dr Lucienne Vassallo Gatt  Objectives Understand the distribution of water in the body Review the molecular structure and bonding found in water Describe the importance of water in humans List the physical and chemical properties of water Explain polarity of bonds and H-bonding Understand that H-bonding is not only important in water Distinguish between the terms hydrophilic and hydrophobic Give examples of electrolytes and explain their uneven distribution in body fluids Explain the nature of osmosis and the need for osmoregulation  Body fluids Are composed of – Water – Dissolved particles Electrolyte Non-electrolyte For normal cell metabolism and normal organ function, the volume, distribution, composition and osmolality of body fluids are essential. Body fluid distribution Body fluids are distributed in two distinct areas: – Intracellular fluid (ICF) 40% body weight – Extracellular fluid (ECF) 20% body weight Interstitial fluid -15% body weight Plasma -5% body weight Fluid compartments are separated by membranes that are freely permeable to water. Movement of fluids due to hydrostatic pressure and osmotic pressure  Water distribution Water taken in = Water lost From the diet In and metabolism From the skin, Out lungs, kidneys and gut  Extracellular fluid Extracellular fluid → Is fluid that is found outside cells. It contains plasma, lymph, interstitial fluid and transcellular fluid. The ECF is the body’s internal environment which bathes all cells. Plasma → a yellowish fluid that contains water, dissolved proteins, carbon dioxide, glucose, clotting factors. Lymph → is an almost colourless fluid that surrounds body tissues. It forms from fluid that filters across blood vessel walls from blood. It contains lymphocytes and circulates in the lymphatic circulatory system which joins the venous bloodstream. Lymph acts to remove bacteria and waste products from tissues. Interstitial fluid → the liquid found between cells of the body that provides much of the liquid environment of the body N.B. Lymph is not exactly the same as interstitial fluid!! Why?? Fluid components in a 70 kg male  Homeostasis It is important that the amount of water in the body is kept at a steady state. Humans deprived of fluids die after a few days from circulatory collapse as a result of the reduction of total body water. Failure to maintain ECF volume, with the consequence of impaired blood circulation, rapidly leads to tissue death due to lack of oxygen and nutrients and failure to remove waste products. Water is lost in urine by the kidneys, by perspiration and respiration. Water loss by the kidney is regulated by the antidiuretic hormone – ADH (vasopressin)  The importance of water The solvent of life (dissolves compounds) Bathes cells Transports compounds in blood Provides a medium for molecule movement in and out of cells and organelles Dissipates heat Takes part in chemical reactions  Properties of water Water is electrically neutral Water is a dipolar molecule Water can H-bond with other molecules  Properties of water The water molecule is electrically neutral  Properties of water Water is a dipolar molecule Partial positive and partial negative charge  Properties of water Water can H-bond with other molecules A hydrogen bond is a weak noncovalent interaction between the hydrogen of a molecule and the more electronegative atom of an acceptor molecule. H-bonding between water molecules  Hydrogen Bonding between Water Molecules Hydrogen bonding contributes to the high specific heat and heat of vaporization of water. The specific heat of a substance is the amount of heat needed to raise the temperature of 1 gram of the substance by 1°C. A relatively high amount of heat is required to raise the temperature of water because each water molecule participates in many hydrogen bonds that must be broken in order for the kinetic energy of the water molecules to increase. The heat of vaporization is the amount of heat required to convert water from a liquid to a gas. A lot of energy is required to break the hydrogen bonds and permit the water molecules to dissociate and enter the gas phase.  Hydrogen bonding in ice Ice contracts on heating! Ice is less dense than water and floats on it. This allows life to exist in frozen lakes, ponds and rivers. Water molecules in ice form an open, hexagonal lattice in which every water molecule is hydrogen-bonded to four others. The geometrical regularity of these hydrogen bonds contributes to the strength of the ice crystal.  Hydrogen Bonds between Other Molecules Hydrogen bonds occur in other molecules as well as water. Ammonia molecules can also stick together when they are cold. Hydrogen bonding can only happen in molecules that have a permanent dipole (as water does) and that also contain the highly electronegative elements fluorine, oxygen or nitrogen. Hydrogen bonding also occurs in organic molecules containing N-H groups - in the same sort of way that it occurs in ammonia. Hydrogen bonding in double stranded DNA Hydrogen bonding In DNA  Hydrophilic and Hydrophobic Molecules Compounds that dissolve readily in water in water are hydrophilic for example NaCI and glucose. Water dissolves most biomolecules that are charged or polar. Nonpolar compounds such as lipids and waxes that do not dissolve in water are hydrophobic. These compounds dissolve in solvents like chloroform and benzene.  Electrolytes A term applied to bicarbonate, inorganic anions and cations. Is found in extracellular and intracellular fluid The major electrolytes in extracellular fluid are Na+ and Cl- The major electrolytes inside the cell are K+ and PO43-. Distribution of ions in body fluids ECF (mmol/L) ICF (mmol/L) Cations Na+ 145 12 K+ 4 150 Anions CI- 105 5 HCO3- 25 12 Inorganic phosphate 2 100 Distribution of sodium and potassium Sodium Potassium 70% is 90% is exchangeable exchangeable Extracellular Intracellular Water and sodium homeostasis  Transport of Water between Compartments The amount of water in both intracellular and extracellular compartments is determined by solute concentration in each compartment. This is referred to as osmolarity. Osmolarity is proportionate to the total concentration of dissolved solutes. Water moves from a compartment with a low solute concentration to a compartment with a high solute concentration until osmolarity is equal.  How Does Water Cross Cell Membranes? Ion channels called aquaporins allow the passage of water across the semi- permeable cell membrane Aquaporins are integral membrane proteins Water also moves across capillaries separating interstitial fluid from plasma  Water depletion Occurs either when: 1. The amount of water taken in is not enough 2. Water loss is excessive [Sodium cannot be excreted from the body without the use of water thus sodium loss is always accompanied by water loss] Water depletion needs to be managed Water depletion  Water excess Usually occurs due to an impairment related to water excretion Per hour kidneys can excrete 1L, so water excess is seen when large amounts of fluid are ingested quickly. Restrict water intake Administration of demeclocycline Infusion of hypertonic saline if patients are symptomatic