Effects of long-term exposure to hot and dry environment Layla Abdulla Physiology – Group 3

Outline Fluid intake & outputBody fluid compartmentsShifts of water between compartmentsDehydration - hypernatremiaOsmoreceptor-ADH feedback systemThirst

Fluid intake and output Total body water is ≈ 60% of body weight - age, adipose tissue, gender Daily water intake - in liquids, food, or oxidation of carbohydrates - depends on climate, habits, level of physical activity Daily water loss - insensible water loss - fluid loss in sweat - water lost in feces - water lost by kidneys

Water loss by kidneys Occurs in form of urine excreted by kidneys Most important way body maintains balance of: - water intake & output - electrolyte intake & output Urine volume: - dehydration: 0.5L/day - drinking  amounts of water: 20L/day Kidneys adjust excretion rate of water and electrolytes: - to precisely match intake of the substances - to compensate for excessive losses

Fluid intake and output

Body fluid compartments Intracellular fluid: - K +, Mg 2+, ATP, ADP, AMP Extracellular fluid: - Na +, Cl -, HCO 3 - Transcellular fluid: - cerebrospinal, pleural, peritoneal, digestive fluids

Water shifts between compartments 1.Volume of body fluid compartment depends on amount of solute it contains. 2.Osmolarity: concentration of solute particles, expressed as osmoles (or milliosmoles) per liter of solution. 3.In steady state, intracellular osmolarity = extracellular osmolarity. 4.NaCl, NaHCO 3, mannitol confined to ECF compartment because they don’t readily cross cell membranes. To maintain this equality, water shifts freely + rapidly across cell membranes.

Dehydration Decrease in ECF volume – water loss, Na + loss Tonicity: effect of concentration of solution on volume of cell - Isotonic (solution doesn’t change volume) - Hypotonic (solution causes cell to swell) - Hypertonic (solution causes cell to shrink) Types: - Isosmotic dehydration (hemorrhage, gastrointestinal fluid loss) - Hyposmotic dehydration (renal loss of NaCl) - Hyperosmotic dehydration (water deficit)

Dehydration Primary measurement is plasma sodium concentration Normal plasma sodium concentration is ≈ 142 mEq/L Hypernatremia = if sodium concentration is elevated above 155 mmol/L  Plasma Na + concentration  ECF Volume  ICF Volume

Dehydration - hypernatremia Associated with  plasma sodium concentration, which causes increased osmolarity Causes: - water loss which concentrates sodium ions - excess sodium in extracellular fluid - decreased ADH effect, renal causes Clinical features: - abnormal urine output, ECF volume is lowered, CNS disorders

Increased concentration of sodium chloride in ECF Dehydration - hypernatremia Kidneys excrete large amounts of dilute urine Diabetes insipidus Inability to secrete or respond to antidiuretic hormone Antidiuretic hormone needed for kidneys to conserve water Dehydration - hypernatremia

When water intake that is less than water loss - Prolonged, heavy exercise - Lost in the desert without adequate drinking water Sweat = loss of NaCl and water - compared to body fluids, sweat has  water than solute In new steady state,  ECF and ICF volumes,  ECF & ICF osmolarities  ECF volume  Osmolarity Difference in osmolarity causes water shift from ICF  ECF Water will flow until ICF osmolarity = ECF osmolarity Water shift causes  ICF volume

Dehydration - hypernatremia Even a small rise in plasma Na + concentration above normal range results in sensation of thirst and correction of abnormality by increase in free water intake So hypernatremia occurs most often in - infants, elderly - people with impared mental status - people with impaired sense of thirst - people who have intact thirst mechanism but unable to obtain water During water deficits, kidneys minimize fluid loss through osmoreceptor-ADH feedback system and increase adequate fluid intake through thirst mechanism.

Osmoreceptor-ADH Feedback System

Thirst Thirst = conscious desire for water Many factors that stimulate ADH secretion also  thirst. Temporarily relieved after drinking water to prevent overhydration and excess dilution of body fluids

Sources Hall, John E., and Arthur C. Guyton. Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA: Saunders/Elsevier, Print. Costanzo, Linda S. Physiology with Student Consult Online Access. 5 th ed. Philadelphia, PA: Saunders Elsevier, Print. Costanzo, Linda S. Board Review Series Physiology. 5th ed. Philadelphia, PA: Wolters Kluwer Health/Lippincott Williams & Wilkins, Print.