Water and Sodium Balance PDF

WATER AND SODIUM BALANCE DR. SHASORE H. O CONSULTANT CHEMICAL PATHOGIST LAGOS STATE UNIVERSITY TEACHING HOSPITAL, IKEJA 24/07/24. OBJECTIVES  To describe the physiologic regulation of wa...

WATER AND SODIUM BALANCE DR. SHASORE H. O CONSULTANT CHEMICAL PATHOGIST LAGOS STATE UNIVERSITY TEACHING HOSPITAL, IKEJA 24/07/24. OBJECTIVES  To describe the physiologic regulation of water and sodium balance  To discuss sodium balance and disorders of sodium. INTRODUCTION  BODY WATER  Water is the most important solvent of living organisms  Total body water (TBW) makes up a major percentage of body weight. Body water is divided into 2 compartments: extracellular and intracellular compartments. BODY WATER DISTRIBUTION  Water is the most abundant constituent of the human body accounting for:  75 - 80% at birth  60% of adult male body weight  55% of adult female (lower due to a relatively higher percentage of body fat) COMPARTMENTS  ICF – ⅔ of TBW – (k+)  ECF – ⅓ of TBW – (NA+ --------- 98%)  Interstitial F ¾ of ECF (ISF) –(low protein)  Intravascular ¼ of ECF (plasma)- ( high protein) Body Water Distribution Body water content in percentage of a body weight is lowest in. (a) Fat woman (b) Fat Man INTRACELLULAR FLUID  Body water bounded by cell membranes.  It becomes the dominant fluid compartment after the first few weeks of life.  Contributes about 30-40% of the body weight; 60% of total body water.  Potassium is the major cation while phosphorus and organic acids are the major anions. EXTRACELLULAR FLUID  This refers to fluid outside the cell membranes  It is the dominant fluid compartment in the fetus and for the first few weeks of life.  Sodium is the major cation while chloride is the major anion.  Plasma and interstitial fluid make up the exchangeable part of the ECF, and both make up 20-25% of body weight  The other aspects of ECF are transcellular fluid (GIT & urine, CSF, pleural, peritoneal fluid) and slowly exchangeable fluid such as in bones and cartilage Functions of Body Water  Involved in Biochemical reactions  Water act as reactant in many hydration hydrolytic reactions of metabolic pathways.  Transporting media of body:  Transportation of nutrients and waste metabolites through aqueous media of blood and tissue floods.  Regulates body temperature  Water transports hormones, enzymes, blood platelets, and red and white blood cells  Water act as a solvent for Electrolytes and Non electrolytes  Water Facilitates digestion and promoting elimination of ingested food  Water serve as a tissue Lubricant WATER BALANCE  To survive, multicellular organisms must maintain their ECF volume.  Humans deprived of fluids die after a few days from circulatory collapse as a result of the reduction in the 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.  The osmolality of the ECF and ICF, in health, is maintained between 285-295 mOsm/Kg H2O.  An individual with a normal diet and normal fluid intake has a urine osmolality of approximately 500-850 mOsm/kg water  Water Balance Water Loss: via the Water Gain: Kidneys: as urine (e.g. renal impairment,  Drinking: driven by thirst diabetes insipidus)  Major stimuli are plasma osmolality (1-2% change) & plasma volume (10% GIT: in faeces (e.g. gastroenteritis change)  Oxidative processes Lungs & Skin: as evaporative losses. This  Example of disease condition affecting is called insensible fluid loss and amounts water gain: coma, vomiting, to 400-500ml/m2/day  RENAL WATER EXCRETION  Each day 130 – 180 litres of water are presented as glomerular filtrate to the proximal renal tubules.  About 20% of the plasma volume passing through the glomerulus at any given time is filtered.  This means that about 180 liters of fluid are filtered by the kidneys every day. Thus, the entire plasma volume (about 3 liters) is filtered 60 times a day!  Only 1 to 2 litres finally appear as urine.  This is due to passive re-absorption of 70-80% in the proximal tubule and further re-absorption in the collection ducts under the influence of ADH. BODY ELECTROLYTES What are Electrolytes?  They are substances when dissolved in solution dissociates into ions  These ions are able to carry an electrical current  An electrolyte is also any substance that develops an electrical charge when dissolved in water  Salts like NaCl and KCl in aqueous solutions gets dissociated to Charged ions Na+ and Cl- called as Electrolytes.  The concentration of these Electrolytes is expressed as mEq/L. TYPES OF ELECTROLYTE  CATION - Positively charged Electrolyte  ANION - Negatively charged Electrolyte  Water molecules completely surround these dissociated ions  These prevents union of Cations and Anions.  ECF Cations ECF Anions Na+ ( 140 mEq/L) Cl- (103 mEq/L) K+ HCO3- Ca+ HPO4-- Mg+ SO4-- Total Cations Total Anions 155 mEq/L 155 mEq/L  ICF Cations ICF Anions Na+ Cl- K+ (150 mEq/L) HCO3- Ca+ HPO4- - (140 mEq/L) Mg+ SO4-- Total Cations Total Anions 195 mEq/L 195 mEq/L Functions of Body Electrolytes  Electrolytes are well distributed in the body compartments.  Electrolytes in the various compartments produce osmotic pressure.  This osmotic pressure helps in maintaining water balance.  Na+: Most abundant electrolyte in the ECF. Where Sodium goes, Water follows.  K+: Essential for normal membrane excitability for nerve impulse  Cl-: Regulates osmotic pressure and assists in regulating acid-base balance  Ca2+: Promotes nerve impulse and muscle contraction/relaxation  Mg2+: Plays role in carbohydrate and protein metabolism, storage and use of intracellular energy and neural transmission. Important in the functioning of the heart, nerves, and muscles REGULATION OF WATER AND SODIUM BALANCE 1. Neural Mechanism- Thirst Mechanism 2. Antidiuretic Hormone/Vasopressin 3. Renin Angiotensin System 4. Aldosterone 5. Atrial Natriuretic Peptide(ANP) 6. Kinins ( Increases Salt and Water excretion) Neural Mechanism- Thirst Mechanism  Normal functioning of this centre is influenced by:  ECF tonicity: hyper-tonicity increases thirst.  Blood volume: decreased volume increases thirst.  Miscellaneous factors: pain and stress, for example increase thirst. When the body water is lowered due to:  No intake of fluids  Body fluids lost through obligatory losses (Urine and Feces).  The ECF volume decreases and becomes hypertonic.  This tends to draw water from ICF causing cellular dehydration.  The major factor determining intake is thirst which is under the control of the thirst centre located in the hypothalamus. Neural Mechanism- Thirst Mechanism  The cellular dehydration stimulates the thirst center located in hypothalamus. In response to the stimulus to thirst center there occurs dryness of mouth and Pharynx.  Feeling of thirst makes one to drink water and water ingested orally quench the thirst to regulate the body water. Antidiuretic Hormone (ADH) (also called vasopressin, arginine vasopressin (AVP) or argipressin)  A 9-aminoacid peptide. half-life of 15-20 minutes.  The synthesis of ADH occurs in the supraoptic and paraventricular nuclei in the hypothalamus.  It is then transported to the posterior pituitary gland via the neurohypophysial capillaries.  In the posterior pituitary gland, its synthesis is completed and it is stored here until it is ready to be secreted into the circulation.  Major stimuli for release are rise in plasma osmolality and a decrease in plasma volume  Other stimuli include nausea & vomiting, pain, stress, hypoglycemia  It acts on the collecting duct cells in the nephrons to facilitate the passive re-absorption of water from the kidneys. ACTIONS OF ANTI-DIURETIC HORMONE  When plasma osmolality or plasma volume is decreased ADH helps to restore it to physiologic state.  Antidiuretic hormone, by regulating aquaporin 2, enhances water reabsorption from the collecting ducts of the kidney.  Aquaporins are cell membrane proteins acting as water channels that regulate water flow.  When ADH secretion is a response to a high extracellular osmolality with the danger of cell dehydration, this is an appropriate response.  However, if its secretion is in response to a normal circulating volume, it is an inappropriate response to the osmolality. (SIADH) ACTIONS OF ANTI-DIURETIC HORMONE  Antidiuretic hormone, by regulating aquaporin 2, enhances water reabsorption from the collecting ducts of the kidney and so dilutes the extracellular osmolality.  Aquaporins are cell membrane proteins acting as water channels that regulate water flow.  When ADH secretion is a response to a high extracellular osmolality with the danger of cell dehydration, this is an appropriate response.  However, if its secretion is in response to a normal circulating volume, it is inappropriate to the osmolality ALDOSTERONE  The major factors controlling sodium balance are renal blood flow and aldosterone.  This hormone controls loss of sodium from the distal tubule.  Aldosterone is a mineralocorticoid hormone, is secreted by the zona glomerulosa of the adrenal cortex.  It affects sodium–potassium and sodium–hydrogen ion exchange across all cell membranes.  Aldosterone stimulates sodium reabsorption from the lumen of the distal renal tubule in exchange for either potassium or hydrogen ions.  The net result is the retention of more sodium, and the loss of potassium or hydrogen ions The renin–angiotensin –aldosterone -system (R.A.A.S)  Renin is an aspartyl protease secreted by the juxtaglomerular apparatus.  Renin is derived from prorenin and secretion increases in response to a reduction in renal artery blood flow.  Plasma renin then carries out the conversion of angiotensinogen, released by the liver, to angiotensin 1  Another proteolytic enzyme, angiotensin-converting enzyme (ACE), which is located predominantly in the lungs but is also present in other tissues such as the kidneys, splits off a further two amino acid residues to form angiotensin II R.A.A.S  The remaining octapeptide, angiotensin II, has a number of important actions:  It acts directly on capillary walls, causing vasoconstriction, and so probably helps to maintain blood pressure and alter the glomerular filtration rate (GFR).  Vasoconstriction may raise the blood pressure before the circulating volume can be restored.  It stimulates the cells of the zona glomerulosa to synthesize and secrete aldosterone.(Na and water retention )  It stimulates the thirst centre and so promotes oral fluid intake. ATRIAL NATRIURETIC PEPTIDE(ANP)  Atrial natriuretic peptide is released from the atrium in response to stretching (e.g. increased blood volume, hypervolaemia); it causes:  Increased GFR  Increased glomerular filtration fraction  Natriuresis  Diuresis  Decreased renin and aldosterone secretion  It is unclear how ANP induces natriuresis but the most likely mechanism is variation of the intrarenal blood flow causing increased GFR and increased filtration fraction (constriction of the efferent glomerular arterioles).  Its action in inhibiting renin and aldosterone secretion may also be a factor.  Influence of Other Hormones on Sodium Balance Estrogens: Enhance NaCl reabsorption by renal tubules cause water retention during menstrual cycles responsible for edema during pregnancy Progesterone: Decreases sodium reabsorption Acts as a diuretic, promoting sodium and water loss Glucocorticoids - enhance reabsorption of sodium. OSMOLALITY vs OSMOLARITY  Osmolarity refers to the number of solute particles per 1 L of solvent, whereas osmolality is the number of solute particles in 1 kg of solvent.  Osmolarity is affected by temperature, pH, but osmolality is not affected by temperature and Ph which is why it can be calculated.  Measurements of osmolarity are temperature dependent because the volume of solvent varies with temperature (i.e., the volume is larger at higher temperatures).  In contrast, osmolality, which is based on the mass of the solvent, is temperature independent. OSMOLALITY  The osmolality of serum, urine, or other body fluids depends on the number of osmotically active ions and molecules dissolved in a kilogram of body water.  Sodium, potassium, chloride, bicarbonate, glucose and urea are the osmotically important body fluid solutes.  The osmolality of a body fluid increases as the ratio of solute to water molecules increases.  Osmolality is expressed as milliosmoles per kilogram of water (mOsm/kg water). OSMOLALITY  The osmolality of a fluid can be calculated by adding the values of its constituent solutes.  A common simplified formula for serum osmolality is: Calculated osmolality = 2 x serum sodium + serum glucose + serum urea (all in mmol/L).  OR plasma osmolality = 2[Na+] + [Glucose]/18 + [BUN]/2.8 where [Glucose] and [BUN] are measured in mg/dL. OSMOLALITY  This calculation is not valid if gross hyperproteinaemia or hyperlipidaemia is present or an unmeasured osmotically active solute, such as mannitol, methanol, ethanol or ethylene glycol, is circulating in plasma.  Usually between 285 and 295mOsm/Kg of water.  Osmolality can also be measure by an osmometer.  The difference between the calculated value and measured value is known as the osmolar gap. DISTRIBUTION OF SODIUM AND WATER HOMEOSTASIS  Isotonic Solution— equal concentrations of solute and water  Hypertonic Solution—More solute and less water  Hypotonic Solution—More water and less solute  The salt is the solute, and the water is the solvent. Example of Isotonic Solutions 0.9% Sodium Chloride Solution / Normal Saline  Ringer’s Solution Lactated Ringer’s Solution  5% dextrose in water (D5W) 39 HYPOTONIC SOLUTIONS Cell in hypotonic solution 5% DEXTROSE WATER 2.5% DEXTROSE WATER 0.45% SODIUM CHLORIDE 0.33% SODIUM CHLORIDE HYPERTONIC SOLUTIONS 3% SODIUM CHLORIDE Cell in hypertonic solution 5% SODIUM CHLORIDE WHOLE BLOOD ALBUMIN TOTAL PARENTERAL NUTRITION TUBE FEEDINGS CONCENTRATED DEXTROSE (>10%) DISORDERS OF TOTAL BODY WATER AND SODIUM  Clinically total body water (TBW) deficiency, or dehydration, presents with the classical picture of thirst, dry skin and mucous membranes, loss of skin turgor, decreased urinary output and circulatory disturbances reflecting a low blood volume (high pulse rate, hypotension, shock, etc).  TBW excess can present as peripheral oedema, pulmonary oedema, ascites, and circulatory overload with cardiac failure.  The serum (Na) depends on the body sodium content and can be either high, low, or normal. CAUSES OF TOTAL BODY WATER DEFCIENCY  The basic cause of water deficiency, which presents as dehydration, is a negative water balance, i.e. input less than output.  Losses can be readily replaced if the patient has access to water and the thirst mechanism is intact.  Depending on the amount of concomitant sodium loss, water depletion is usually classified on the basis of the lost fluid into three types: predominant water depletion, hypotonic fluid loss, and isotonic fluid loss. Predominant water depletion.  In ‘pure’ water depletion the problem is inadequate fluid intake (oral or iv) when homeostatic mechanisms (e.g. the thirst reflex) fail, or in the face of gross depletion, either due to inadequate intake or to excessive loss by other routes.  It may occur in: Subjects too old, too young, or too sick to drink  Inappropriate iv therapy  Disturbances of thirst centre  Such patients develop hypernatraemia (due to water loss being greater than sodium loss) which can be quite severe , e.g. 160 to 170 mmol/L. Hypotonic fluid loss  Dehydration due to loss of fluid containing significant amounts of sodium (coupled with inadequate fluid intake) may be due to:  Skin losses: excessive sweating  Gut losses: vomiting, diarrhoea, drainage into fistulae  Renal losses: diuretic therapy, Addison’s disease, salt-losing nephritis, diabetes insipidus Isotonic fluid loss.  This is unusual but may occur in:  Loss of blood: haemorrhage, accidents  Loss of fluid: burns  ‘Third space’ accumulations: ileus, pancreatitis, peritonitis, crush injury  The hypovolaemia stimulates: a) Renal sodium retention resulting in a low urinary sodium concentration (20 mmol/L) no other cause for hyponatraemia (no diuretic use and no suspicion of hypothyroidism, cortisol deficiency, marked hyperproteinaemia, hyperlipidaemia or hyperglycaemia).  No cardiac, adrenal, pituitary, or thyroid dysfunction  No drug or diuretic therapy SODIUM AND ITS DISORDERS  Sodium is an electrolyte and a mineral  Transmit electrical impulses in heart and nervous system  Regulates water distribution and fluid balance through entire body  Help regulate acid/base balance  Extracellular level: 135-145 mEq/L  Intracellular level: 10-12 mEq/L  Sodium imbalances normally related to changes in total body water, not changes in sodium.  Sodium imbalances can lead to hypovolemia or hypervolemia.  Hypernatraemia suggests a negative balance (input less than output), and hyponatraemia, a positive 58 water balance. HYPONATREMIA  Most common electrolyte disorder  More common in very young or very old CAUSES: 1. Sometimes caused by water intoxication (too much water intake causes sodium dilution in the blood—will overwhelm the kidney's compensation mechanism). 2. Can be caused by a syndrome of inappropriate anti-diuretic hormone secretion (SIADH). 3. Often caused by increased in Antidiuretic Hormone (ADH). 4. Other causes: profuse diaphoresis, vomiting, diarrhea, draining wounds (burns), excessive blood loss (trauma, GI bleeding), some medication side effects, renal disease 59 HYPONATREMIA Causes:  Severe diarrhea, vomiting, drainage of GIT secretions, fistulas  Excessive sweating – exercise, fever  Burns  Renal diseases  Adrenal insufficiency – hypoaldosteronism (Addison’s disease)  Diuretic therapy  Osmotic diuresis – DM  SIADH  Excess fluid replacement e.g. hypotonic (5% dextrose)  Psychogenic polydipsia  Sick cell syndrome – shock  Artefactual- drip arm from dextrose solution Three Main Types Hyponatremia Hypovolemic Euvolemic Hypervolemic Total body water Total body water Total body sodium decreases increases increases Extracellular fluid Extracellular fluid Extracellular fluid increases minimally increases markedly volume decreases (no edema) (with edema) *Most Common* 61 Management of hyponatremia  Need to determine and treat underlying cause  Get good IV access  Send blood work to lab  Start.0.9%NS IV fluid or lactated Ringer’s solution.  Check capillary blood glucose—hyperglycemia can cause false hyponatremia lab results  (Hyperglycemia causes hyperosmolality, and the water moves from intracellular space to extracellular space, which in turn produces a dilutional decrease in serum sodium level.)  No more than 10-12mEq/L in first 24 hours and no more than 18mEq/L in first 48 hours 62 HYPERNATREMIA  Very rare to see over 145mEq/L, 50% of cases are fatal at that level  Leads to intercellular dehydration because water follows sodium into extracellular spaces Signs and Symptoms  Restlessness, change in mental status, irritability, seizure activity  Nausea, vomiting, increased thirst  Ataxia, tremors, hyper-reflexia  Flushed skin, increased capillary refill time  Decreased cardiac output related to decreased myocardial contractility 63 CAUSES OF HYPERNATREMIA  Usually associated with dehydration--there is too little water.  Water loss can occur from vomiting and/or diarrhea, excessive sweating, or from drinking fluid with high salt concentrations.  Decreased thirst drive: elderly, dementia patients, newborns  Decreased water intake--leads to free water deficit--leads to high sodium  Artefactual – Na heparin as anticoagulant in specimen container or collection of blood sample from a normal saline infusion site.  Unconsciousness  Diarrhoea, vomiting, major burns  Diabetes insipidus (cranial or nephrogenic)  Excessive sodium intake  Accidental salt ingestion  Drugs containing sodium  Excessive mineralocorticoid - Conn’s syndrome, Cushing’s syndrome 64 LABORATORY INVESTIGATION OF HYPERNATREMIA  Serum osmolality  urine osmolality  and urinary sodium concentration—are essential in the evaluation of patients with hyponatremia.  The first step in the diagnostic approach is to estimate the volume status (intravascular volume) of the hypernatremic patient. MANAGEMENT OF HYPERNATREMIA  Intravenous fluids and determine the underlying cause. ANY QUESTIONS??????