Fluid, Electrolyte and Acid-Base Balance

1. Fluid, Electrolyte and Acid-Base Balance Chapter 27

2. Composition of the Human Body Figure 27–1a

3. Composition of the Human Body Figure 27–1b

4. Water content varies with age & tissue type Fat has the lowest water content (~20%). Bone is close behind (~22 – 25%). Skeletal muscle is highest at ~65%.

5. Fluid Compartments • ECF (extra cellular fluid) and ICF (intracellular fluid) are called fluid compartments: • because they behave as distinct entities • are separated by cell membranes and active transport

6. Water Composition • Is 60% percent of male body weight • Is 50% percent of female body weight • Mostly in intracellular fluid

7. Water Exchange • Water exchange between ICF and ECF occurs across cell membranes by: • osmosis • diffusion • carrier-mediated transport

8. Major Subdivisions of ECF • Interstitial fluid of peripheral tissues • Plasma of circulating blood

9. Minor Subdivisions of ECF • Lymph, perilymph, and endolymph • Cerebrospinal fluid (CSF) • Synovial fluid • Serous fluids (pleural, pericardial, and peritoneal) • Aqueous humor

10. Cations Body Fluids Figure 27–2 (1 of 2)

11. Anions in Body Fluids Figure 27–2 (2 of 2)

12. 4 Principles of Fluids and Electrolyte Regulation • All homeostatic mechanisms that monitor and adjust body fluid composition respond to changes in the ECF, not in the ICF • No receptors directly monitor fluid or electrolyte balance

13. 4 Principles of Fluids and Electrolyte Regulation • Cells cannot move water molecules by active transport • The body’s water or electrolyte content will rise if dietary gains exceed environmental losses, and will fall if losses exceed gains

14. Electrolyte concentrations are calculated in milliequivalents mEq/L = ion concentration (mg/L) x number of charges on one ion atomic weight Na+ concentration in the body is 3300 mg/L Na+ carries a single positive charge. Its atomic weight is approximately 23. Therefore, in a human the normal value for Na+ is: 3300 mg/L= 143 mEq/L 23 Note: One mEq of a univalent is equal to one mOsm whereas one mEq of a bivalent ion is equal to ½ mOsm. However, the reactivity of 1 mEq is equal to 1 mEq.

15. Regulation of water balance • It is not so much water that is regulated, but solutes. • osmolality is maintained at between 285 – 300 mOsm. • An increase above 300 mOsm triggers: • Thirst • Antidiuretic Hormone release

16. 3 Primary Regulatory Hormones • Affect fluid and electrolyte balance: • antidiuretic hormone • aldosterone • natriuretic peptides

17. Antidiuretic Hormone (ADH) • Stimulates water conservation at kidneys: • reducing urinary water loss • concentrating urine • Stimulates thirst center: • promoting fluid intake

18. ADH Production • Osmoreceptors in hypothalamus: • monitor osmotic concentration of ECF • Change in osmotic concentration: • alters osmoreceptor activity • Osmoreceptor neurons secrete ADH

19. ADH Release (1 of 2) • Axons of neurons in anterior hypothalamus: • release ADH near fenestrated capillaries • in posterior lobe of pituitary gland

20. ADH Release (2 of 2) • Rate of release varies with osmotic concentration: • higher osmotic concentration increases ADH release

21. The Thirst Mechanism An increase of 2 – 3% in plasma osmolality triggers the thirst center of the hypothalamus. Secondarily, a 10 – 15% drop in blood volume also triggers thirst. This is a significantly weaker stimulus.

22. Aldosterone • Is secreted by adrenal cortex in response to: • rising K+ or falling Na+ levels in blood • activation of renin–angiotensin system • Determines rate of Na+ absorption and K+ loss: • along DCT and collecting system

23. Dehydration • Chronic dehydration leads to oliguria. • Severe dehydration can result in hypovolemic shock. • Causes include: • Hemorrhage • Burns • Vomiting • Diarrhea • Sweating • Diuresis, which can be caused by diabetes insipidus, diabetes mellitus and hypertension (pressure diuresis).

24. Overhydration (Hypotonic hydration) • Also called water excess • Occurs when excess water shifts into ICF: • distorting cells • changing solute concentrations around enzymes • disrupting normal cell functions

25. Causes of Overhydration • Ingestion of large volume of fresh water • Injection into bloodstream of hypotonic solution • Endocrine disorders: • excessive ADH production

26. Causes of Overhydration • Inability to eliminate excess water in urine: • chronic renal failure • heart failure • cirrhosis

27. Signs of Overhydration • Abnormally low Na+ concentrations (hyponatremia) • Effects on CNS function (water intoxication)

28. Hyponatremia • Hyponatremia results in: • Cerebral edema (brain swelling) • Sluggish neural activity • Convulsions, muscle spasms, deranged behavior. • Treated with I.V. hypertonic mannitol or something similar.

29. Hypotonic hydration

30. Blood pressure, sodium, and water

31. Atrial Naturetic Peptide:The heart’s own compensatory mechanism.

32. Fluid Gains and Losses Figure 27–3

33. Water Balance Table 27–1

34. Sources of intake & output

35. Importance of Electrolyte Balance • Electrolyte concentration directly affects water balance • Concentrations of individual electrolytes affect cell functions

36. Sodium • Is the dominant cation in ECF • Sodium salts provide 90% of ECF osmotic concentration: • sodium chloride (NaCl) • sodium bicarbonate

37. Normal Sodium Concentrations • In ECF: • about 140 mEq/L • In ICF: • is 10 mEq/L or less

38. Potassium • Is the dominant cation in ICF

39. Normal Potassium Concentrations • In ICF: • about 160 mEq/L • In ECF: • is 3.8–5.0 mEq/L

40. 2 Rules of Electrolyte Balance • Most common problems with electrolyte balance are caused by imbalance between gains and losses of sodium ions • Problems with potassium balance are less common, but more dangerous than sodium imbalance

41. Sodium Balance in ECF • Sodium ion uptake across digestive epithelium • Sodium ion excretion in urine and perspiration

42. Sodium Balance in ECF • Typical Na+ gain and loss: • is 48–144 mEq (1.1–3.3 g) per day • If gains exceed losses: • total ECF content rises • If losses exceed gains: • ECF content declines

43. Changes in ECF Na+ Content • Do not produce change in concentration • Corresponding water gain or loss keeps concentration constant

44. Homeostatic Regulation of Na+ Concentrations in Body Fluids Figure 27–4

45. Na+ Balance and ECF Volume • Na+ regulatory mechanism changes ECF volume: • keeps concentration stable • When Na+ losses exceed gains: • ECF volume decreases (increased water loss) • maintaining osmotic concentration

46. Large Changes in ECF Volume • Are corrected by homeostatic mechanisms that regulate blood volume and pressure • If ECF volume rises: • blood volume goes up • If ECF volume drops: • blood volume goes down

47. Fluid Volume Regulation and Na+ Concentrations Figure 27–5 (1 of 2)

48. Fluid Volume Regulation and Na+ Concentrations

49. Homeostatic Mechanisms (1 of 2) • A rise in blood volume: • elevates blood pressure • A drop in blood volume: • lowers blood pressure

50. Homeostatic Mechanisms (2 of 2) • Monitor ECF volume indirectly by monitoring blood pressure: • baroreceptors at carotid sinus, aortic sinus, and right atrium