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 : 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 ml (6%)
Can be significant in vomiting and diarhhea
From skin (sweat) ml (6%)
From lungs and skin 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. Decrease in blood proteins caused by: Dietary deficiency in proteins
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
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+.

51. Control of Acid-Base Balance
Buffer systems
Exhalation of carbon dioxide
Kidney excretion

52. Buffers
Are pairs of chemical substances that prevent a sharp change in the pH of a solution.
Buffers exchange strong acids for weaker acids that do not release as much H+ and thus change the pH less.

53. Bicarbonate Buffer System
NaHCO H2CO3
sodium bicarbonate carbonic acid
Addition of a strong acid:
HCl + NaHCO3 → H2CO3 + NaCl
Carbonic acid does not dissociate completely, and pH is changed much less.

54. Addition of a strong base:
NaOH + H2CO3 → NaHCO3 + H2O
Water dissociates very little, and pH remains nearly the same.

55. Usually the body is called upon to buffer weaker organic acids, such as lactic acid.
Carbonic acid is formed, and amount of bicarbonate ion decreases.
Blood needs to maintain a 20:1 ratio of bicarbonate ion : carbonic acid.
H+ concentration increases slightly
pH drops slightly

56. Carbonic acid is the most abundant acid in the body because it is constantly being formed by buffering fixed acids and by:
H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-

57. Phosphate Buffer System
Is present in extracellular and intracellular fluids, most important in intracellular fluids and renal tubules.
H+ + HPO42- → H2PO4-
monohydrogen dihydrogen
phosphate phosphate
OH- + H2PO4- → H2O + HPO42-

58. Protein Buffer System The most abundant in body cells and plasma.
Carboxyl group -COOH ↔ -COO- + H+
Amino group –NH2 ↔ -NH3+

59. Respiratory Mechanisms – Exhalation of CO2
Because carbonic acid can be eliminated by breathing out CO2 it is called a volatile acid.
Body pH can be adjusted this way in about 1-3 minutes
pH also affects breathing rate
Powerful eliminator of acid, but can only deal with carbonic acid.

61. Kidney excretion of H+
Metabolic reactions produce large amounts of fixed acids.
Kidneys can eliminate larger amounts of acids than the lungs
Can also excrete bases
Can excrete acids while conserving bicarbonate ion
Can produce more bicarbonate ion
Kidneys are the most effective regulators of pH; if kidneys fail, pH balance fails

63. The regulators work at different rates
Buffers are the first line of defense because they work almost instantaneously.
Secondary defenses take longer to work:
Respiratory mechanisms take several minutes to hours
Renal mechanisms may take several days

65. pH imbalances The normal blood pH range is 7.35 – 7.45
Any pH below this range is considered to be a condition of acidosis
Any pH above this range is considered to be a condition of alkalosis
The body response to acid-base imbalance is called compensation: Compensation may be complete if the blood pH is brought back to normal, or partial if it is still outside the norms.

67. Respiratory problems
Respiratory acidosis is a carbonic acid excess (blood CO2 is too high)
Respiratory alkalosis is a carbonic acid deficit (blood CO2 is too low)
Compensation would occur through the kidneys

70. Metabolic problems Metabolic acidosis is a bicarbonate deficit
Metabolic alkalosis is a bicarbonate excess
Compensation would occur through changes in the depth and rate of respiration.