Water, Electrolyte and Acid-Base Balance

1. Water, Electrolyte and Acid-Base Balance Chapter 21

2. Balance – a state of equilibrium – substances are maintained in the right amounts and in the right place in the body

3. Water Balance • Osmosis is the primary method of water movement into and out of body fluid compartments. • Osmosis is the net movement of water molecules through a selectively permeable membrane from an area of high water concentration to an area of lower water concentration.

4. The concentration of solutes determines the direction of water movement. • Most solutes in the body are electrolytes – inorganic compounds which dissociate into ions in solution. • “Where sodium goes, water follows.”

5. About 40 Liters (10.56 gallons) of body water • Babies – 75% water • Men – 63 % • Women – 52%

7. Fluid compartments • Separated by selectively permeable membranes • Intracellular – 2/3 (63%) of total body water • Extracellular – 1/3 (37%) • Interstitial fluid – 80 % of extracellular water • Blood plasma – 20 % of extracellular water

9. Composition of compartments • Extracellular fluids: • High in Na+, Cl-, Ca++, HCO3- • Blood plasma has more protein than interstitial fluid and lymph • Intracellular fluids: • High in K+, phosphate, Mg++, and more protein than plasma

11. Movement of water • Hydrostatic pressure – pressure of fluids • Osmotic pressure – solute concentration (often Na+) • In blood referred to as colloid osmotic pressure (COP)

12. Water intake = Water loss • Average adult takes in about 2,500 ml/day • Sources of water: • Preformed water: 2,300 ml • Drinking water: 1,500 ml (60%) • Moist food : 750 ml (30%) • Water of metabolism: 250 ml (10%) • Cellular respiration • Dehydration synthesis

14. Regulation of water intake • Main regulator is thirst. • Dehydration (output>intake) as little as 1% decrease in body water causes: • Decreased production of saliva • Increased blood osmotic pressure – stimulates osmoreceptors in the hypothalamus • Decreased blood volume – renin is produced

15. The thirst center in hypothalamus is stimulated ( or mistakenly, the hunger center) and person feels thirsty • Wetting of the mouth and stretching of stomach or intestines decrease thirst before we take in too much water. • Water is absorbed, and blood osmotic pressure decreases.

16. Sources of water loss • Through kidneys in urine – 1500 ml (60%) • Through intestines - 150 ml (6%) • Can be significant in vomiting and diarhhea • From skin (sweat) - 150 ml (6%) • From lungs and skin 700 ml (28%) • Last is called insensible loss • (menstruation)

18. Regulation of Water Output • Through regulating urine formation • ADH – production stimulated by ↑ blood tonicity of decrease in volume. • Acts on distal convoluted tubules and collecting ducts of kidney – permits reabsorption of water

19. Aldosterone – production is stimulated by angiotensin II through renin production • Causes sodium ( and water) to be reabsorbed • ANP – causes sodium (and water) loss when pressure in right atrium is too high

20. Water imbalances • Dehydration is the imbalance seen most often. • Prolonged diarrhea or vomiting • Excessive sweating

22. Water toxicity • If lose water by sweating, we also lose sodium. • Rapidly drinking large quantities of water decreases plasma sodium concentration initially, then see decrease in ISF as well. • Water is drawn into cells • This increases ISF tonicity, and water is drawn from blood • Add salt when replacing fluids like this!

24. Overhydration • Can occur if I.V. fluids are given too rapidly or in too large amounts. • Extra fluid puts strain on heart

25. Water that moves back into capillaries depends on concentration of plasma proteins. • Decrease in blood proteins caused by: • Dietary deficiency in proteins • Liver failure • Blockage of lymphatic system • Increased capillary permeability • Burns, infection

27. Fluid moves from the blood to the interstitial fluid. • Get large amounts of fluid in the intercellular spaces – Edema

29. Of the three main compartments (IVF, ICF and ISF) the interstitial fluid varies the most.

31. Edema • Can be caused by: • Decrease in plasma proteins • Retention of electrolytes, esp. Na+ • Increase in capillary blood pressure

32. Electrolyte Balance • Cations – positively charged ions • Anions – negatively charged ions • Body fluids also contain charged organic molecules • Only a small percentage of molecules in fluids are non-electrolytes: glucose, urea, creatinine

33. Functions of electrolytes • Certain ions control the osmosis of water between body compartments • Ions help maintain the acid-base balance necessary for cellular activity • Ions carry electric current, which allows for action potentials and secretion of neurotransmitters • Several ions are cofactors needed for the optimal activity of enzymes

34. Electrolyte intake • Food and water • Produced by metabolism • Salt craving

35. Electrolyte loss • Sweat • Feces • Urine

37. Osmolarity • The total concentration of dissolved particles determines osmolarity. • Glucose – one dissolved particle • NaCl – dissolves into two particles • One mole of NaCl = 2 osmoles • Osmoles/L = osmolarity of solution

38. Sodium (Na+) • 90 % of extracellular cations and half the osmolarity of extracellular solutions • Necessary for action potentials in nerve & muscle cells • Aldosterone increases reabsorption from DCT and collecting ducts • ↓ blood volume, ↓ extracellular Na+ ,↑ extracellular K+ • ANP causes loss of Na+

39. Potassium (K+) • Most numerous intracellular cation • Membrane potential and repolarization • Controlled by aldosterone – causes loss of K+ in urine

41. Calcium (Ca++) • Part of bone, most abundant mineral in body. 98% of Ca is in bone • Extracellular cation • Needed for blood clotting, nerve and muscle function • PTH causes reabsorption of bone and increases reabsorption from G.I tract and glomerular filtrate • Calcitonin inhibits osteoclasts and stimulates osteoblast, so calcium is removed from blood

42. Chloride (Cl-) • Most common extracellular anions • Cl- diffuses easily between compartments – can help balance charges (RBC’s) • Parietal cells in stomach secrete Cl- & H+ • Aldosterone indirectly adjusts Cl- when it increases the reabsorption of Na+ - Cl- follows the Na+

43. Bicarbonate (HCO3-) • Part of the body’s chief buffer and transports CO2 in blood stream. • CO2 + H2O ↔H2CO3↔ H+ + HCO3- • The kidneys are the main regulators of bicarbonate: they form bicarb when levels are low and excrete it when levels are high.

44. Phosphate (HPO42-) • Like calcium, most of the phosphate is found in the bones. • 15% is ionized • Found in combination with lipids, proteins, carbohydrates, nucleic acids and ATP. • Three different forms • Part of the phosphate buffer system • PTH causes phosphate to be released from bones and to be excreted by the kidneys. Calcitonin removes phosphate by encouraging bone formation.

45. Acid-Base Balance • pH – negative log of H+ concentration • Affects functioning of proteins (enzymes) • Can affect concentrations of other ions • Modify hormone actions (proteins)

47. Acid intake • Foods • Produced by cellular metabolism

49. Strengths of Acids and Bases • Acids and bases that ionize (break apart) completely are strong acids and bases. (HCl; NaOH) • Acids and bases that do not completely dissociate in solution are weak acids and bases. (lactic acid, carbonic acid)

50. Remember, blood needs to stay between 7.35 and 7.45 for the body to function properly. • Since more acids than bases are formed, pH balance is mainly a matter of controlling excess H+.