Water and Electrolytes Water Sodium Potassium Acid base balance Chloride 1

2 Composition of Body Fluids Water is the universal solvent Solutes are broadly classified into: Electrolytes–inorganic salts, all acids and bases, and some proteins Nonelectrolytes–examples include glucose, lipids, creatinine, and urea Electrolytes have greater osmotic power than nonelectrolytes Water moves according to osmotic gradients The major electrolytes include Na +, K +, Ca 2+, Mg 2+, Cl -, SO 4 2-, bicarbonate and lactate, as well as few other organic anions and the trace elements The major electrolytes occur primarily as free ions; the trace elements occur primarily in combination with proteins

3 Electrolyte Balance Electrolytes are important for: Neuromuscular excitability Secretory activity Membrane permeability Controlling fluid movements Fluid and electrolytes balance is central to the management of any patient that is ill Measurement of serum sod, pot, chloride and bicarbonate is the most commonly requested biochemical profile The dietary requirements for electrolytes vary widely; some need to be consumed only in small amounts. Others, such as calcium, potassium and phosphorus, are excreted continuously and must be ingested regularly to prevent deficiency Salts enter the body by ingestion and are lost via perspiration, feces, and urine

Body Water Content: Fluid Compartments Water occupies two main fluid compartments Intracellular fluid (ICF)–about two thirds by volume, contained in cells (28 L) Extracellular fluid (ECF) (14 L)–consists of two major subdivisions 1. Plasma–the fluid portion of the blood (3.5L) 2. Interstitial fluid (IF)–fluid in spaces between cells (10.5L) Ex. A man weight is 70kg TBW = 70X 0.6 = 42L ICF = 0.4 X 70= 28 L ECF = 0.2X 70 = 14 L

5 Body compartments are separated by semipermeable membranes through which water moves freely Osmotic pressure must be the same on both sides of a cell membrane and water moves to keep the osmolality the same even this water causes cells to shrink or expand in volume The osmolality of ICF is normally the same as the ECF  the two compartments contain isotonic solution Plasma osmolality = 1.86 ([Na+]) + [urea]/2.8 + [glucose]/18 Plasma osmolality = 2 ([Na+]) + [urea]/3 + [glucose]/20 Or simply plasma osmolality = 2 ([Na+]) Osmolality

Extracellular and Intracellular Fluids Each fluid compartment of the body has a distinctive pattern of electrolytes Extracellular fluids are similar (except for the high protein content of plasma) Sodium is the chief cation Chloride is the major anion Intracellular fluids have low sodium and chloride Potassium is the chief cation Phosphate is the chief anion Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites This reflects the activity of cellular ATP-dependent sodium- potassium pumps

Electrolyte Composition of Body Fluids Figure 26.2

Water Balance and ECF Osmolality To remain properly hydrated, water intake must equal water output Water intake sources Ingested fluid (60%) and solid food (30%) Metabolic water or water of oxidation (10%) Water output Urine (60%) and feces (4%) Insensible losses (Skin and lung) (28%), sweat (8%) Obligatory water losses include: 1.Insensible water losses from lungs and skin 2.Water that accompanies undigested food residues in feces Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH)

9 Amount needed to give the proper osmotic concentration Amount needed to replace water lost excretion What Determine Your Need for Water?

Water Intake and Output Under normal conditions, the amounts of water taken into the body and lost from it are equal over a period of time. Water is obtained from the diet and oxidative metabolism and is lost through the kidneys, skin, lungs and gut. The minimum volume of urine necessary for normal excretion of waste products is about 500 ml/24 hr but, as a result of obligatory losses by other routes, the minimum daily water intake necessary for the maintenance of water balance is approximately 1100 ml.

11 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.

12  Water intake largely depends on social habits and is very variable. Some people drink less than half a liter each day, and others may drink more than 5 L in 24 hours without harm  Water losses are equally variable and are normally seen as changes in the volume of urine produced. The kidneys can respond quickly to meet the body's need to get rid of water. The urine flow rate can vary widely in a very short time Water Intake and Output

13 Changes in body water content will alter the osmolality: Loss of water from the ECF  increase its osmolality  movement of water from the ICF to the ECF. A Slight increase in ECF osmolality  stimulating the hypothalamus thirst centre, which promotes a desire to drink, and the hypothalamic osmoreceptors, which causes the release of Arginine vasopressin (AVP) (antidiuretic hormone or ADH). Vasopressin (ADH) makes the renal collecting ducts permeable to water  water re- absorption and concentration of the urine If the ECF osmolality falls, there is no sensation of thirst and vasopressin secretion is inhibited. A dilute urine is produced  water loss and restoration of the ECF osmolality Other stimuli affecting vasopressin secretion include angiotensin II, arterial and venous baroreceptors and volume receptors. If there is a decrease in plasma volume of more than 10%, hypovolaemia becomes a powerful stimulus to vasopressin release Water and ECF osmolality

14 Regulation of Water Intake: Thirst Mechanism

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Influence and Regulation of ADH Water reabsorption in collecting ducts is proportional to ADH release Low ADH levels produce dilute urine and reduced volume of body fluids High ADH levels produce concentrated urine Hypothalamic osmoreceptors trigger or inhibit ADH release Factors that specifically trigger ADH release include prolonged fever; excessive sweating, vomiting, or diarrhea; severe blood loss; and traumatic burns

Disorders of Water Balance: Dehydration Water loss exceeds water intake and the body is in negative fluid balance Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, and diuretic abuse Signs and symptoms: cottonmouth, thirst, dry flushed skin, and oliguria Prolonged dehydration may lead to weight loss, fever, and mental confusion Other consequences include hypovolemic shock and loss of electrolytes

Disorders of Water Balance: Dehydration Excessive loss of H 2 O from ECF ECF osmotic pressure rises Cells lose H 2 O to ECF by osmosis; cells shrink (a) Mechanism of dehydration

Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration, or water intoxication ECF is diluted–sodium content is normal but excess water is present The resulting hyponatremia promotes net osmosis into tissue cells, causing swelling These events must be quickly reversed to prevent severe metabolic disturbances, particularly in neurons Disorders of Water Balance: Hypotonic Hydration

Excessive H 2 O enters the ECF 1 2 ECF osmotic pressure falls 3 H 2 O moves into cells by osmosis; cells swell (b) Mechanism of hypotonic hydration

21 Symptoms Weight gain & edema Cough, dyspnea [fluid congestion in lungs] bounding pulse, neck vein engorgement [fluid excess in the vascular system]  Hg and Hct Nausea & vomiting Management Restrict fluids to lower fluid volume Diuretics or hypertonic saline Continuous assessments to prevent skin breakdown Overhydration