1. Fluid and Electrolyte Balance
Muse
Lecture #10
3/23/11

2. Fluid, Electrolyte, and Acid–Base Balance
Fluid Balance
Is a daily balance between
Amount of water gained
Amount of water lost to environment
Involves regulating content and distribution of body water in ECF and ICF

3. Fluid, Electrolyte, and Acid–Base Balance
The Digestive System
Is the primary source of water gains
Plus a small amount from metabolic activity
The Urinary System
Is the primary route of water loss

4. Fluid, Electrolyte, and Acid–Base Balance
Electrolytes
Are ions released through dissociation of inorganic compounds (Na+ , K+, Cl-, CO3- )
Can conduct electrical current in solution
Electrolyte balance
When the gains and losses of all electrolytes are equal
Primarily involves balancing rates of absorption across digestive tract with rates of loss at kidneys and sweat glands

5. Fluid, Electrolyte, and Acid–Base Balance
Precisely balances production and loss of hydrogen ions (pH)
The body generates acids during normal metabolism
Tends to reduce pH

6. Fluid, Electrolyte, and Acid–Base Balance
The Kidneys
Secrete hydrogen ions into urine
Generate buffers that enter bloodstream
In distal segments of distal convoluted tubule (DCT) and collecting system
The Lungs
Affect pH balance through elimination of carbon dioxide

7. Fluid Compartments Water Exchange
Water exchange between ICF and ECF occurs across plasma membranes by
Osmosis
Diffusion
Carrier-mediated transport

8. Body Fluid Compartments
In lean adults, body fluids constitute 55% of female and 60% of male total body mass
Intracellular fluid (ICF) inside cells
About 2/3 of body fluid
Extracellular fluid (ECF) outside cells
Interstitial fluid between cell is 80% of ECF
Plasma in blood is 20% of ECF
Also includes lymph, cerebrospinal fluid, synovial fluid, aqueous humor, vitreous body, endolymph, perilymph, and pleural, pericardial, and peritoneal fluids

9. Body Fluid Compartments

10. Daily Water Gain and Loss

11. Fluid Compartments Major Subdivisions of ECF
Interstitial fluid of peripheral tissues
Plasma of circulating blood

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

13. Fluid Compartments
Figure 27–1a The Composition of the Human Body.

14. Fluid Compartments
Figure 27–1a The Composition of the Human Body.

15. Fluid Movement
Edema
The movement of abnormal amounts of water from plasma into interstitial fluid

16. Regulation of body water gain
Mainly by volume of water intake/ how much you drink
Dehydration – when water loss is greater than gain
Decrease in volume, increase in osmolarity of body fluids
Stimulates thirst center in hypothalamus

17. Regulation of water and solute loss
Elimination of excess body water through urine
Extent of urinary salt (NaCl) loss is the main factor that determines body fluid volume
Main factor that determines body fluid osmolarity is extent of urinary water loss
3 hormones regulate renal Na+ and Cl- reabsorption (or not)
Angiotensin II and aldosterone promote urinary Na+ and Cl- reabsorption of (and water by osmosis) when dehydrated
Atrial natriuretic peptide (ANP) promotes excretion of Na+ and Cl- followed by water excretion to decrease blood volume

18. Fluid Compartments An Overview of the Primary Regulatory Hormones
Affecting fluid and electrolyte balance:
Antidiuretic hormone
Aldosterone
Natriuretic peptides

19. Fluid Compartments ADH Production Osmoreceptors in hypothalamus
Monitor osmotic concentration of ECF
Change in osmotic concentration
Alters osmoreceptor activity
Osmoreceptor neurons secrete ADH

20. Major hormone regulating water loss is antidiuretic hormone (ADH)
Also known as vasopressin
Produced by hypothalamus, released from posterior pituitary
Promotes insertion of aquaporin-2 into principal cells of collecting duct
Permeability to water increases
Produces concentrated urine

21. Hormonal Regulation of Na+ and Cl-

22. Fluid Compartments Aldosterone
Is secreted by suprarenal 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. Fluid Compartments “Water Follows Salt” salt pumps in PCT and ALOH
High aldosterone plasma concentration
Causes kidneys to conserve salt
Conservation of Na+ by aldosterone
Also stimulates water retention

24. Concentrations in body fluids
Concentration of ions typically expressed in milliequivalents per liter (mEq/liter)
Na+ or Cl- number of mEq/liter = mmol/liter
Ca2+ or HPO42- number of mEq/liter = 2 x mmol/liter
Chief difference between 2 ECF compartments (plasma and interstitial fluid) is plasma contains many more protein anions
Largely responsible for blood colloid osmotic pressure

25. Fluid Movement

26. ICF differs considerably from ECF
ECF most abundant cation is Na+, anion is Cl-
ICF most abundant cation is K+, anion are proteins and phosphates (HPO42-)
Na+ /K+ pumps play major role in keeping K+ high inside cells and Na+ high outside cell

27. Electrolyte and protein anion concentrations

28. Electrolytes in body fluids
Ions form when electrolytes dissolve ad dissociate
4 general functions
Control osmosis of water between body fluid compartments
Help maintain the acid-base balance
Carry electrical current
Serve as cofactors

29. Sodium Na+ Most abundant ion in ECF 90% of extracellular cations
Plays pivotal role in fluid and electrolyte balance because it account for almost half of the osmolarity of ECF
Level in blood controlled by
Aldosternone – increases renal reabsorption
ADH – if sodium too low, ADH release stops
Atrial natriuretic peptide – increases renal excretion

30. Potassium K+ Most abundant cations in ICF
Key role in establishing resting membrane potential in neurons and muscle fibers
Also helps maintain normal ICF fluid volume
Helps regulate pH of body fluids when exchanged for H+
Controlled by aldosterone – stimulates principal cells in renal collecting ducts to secrete excess K+

31. Chloride Cl- Most prevalent anions in ECF
Moves relatively easily between ECF and ICF because most plasma membranes contain Cl- leakage channels and antiporters
Can help balance levels of anions in different fluids
Chloride shift in RBCs
Regulated by
ADH – governs extent of water loss in urine
Processes that increase or decrease renal reabsorption of Na+ also affect reabsorption of Cl-

32. Bicarbonate HCO3- Second most prevalent extracellular anion
Concentration increases in blood passing through systemic capillaries picking up carbon dioxide
Carbon dioxide combines with water to form carbonic acid which dissociates
Drops in pulmonary capillaries when carbon dioxide exhaled
Chloride shift helps maintain correct balance of anions in ECF and ICF
Kidneys are main regulators of blood HCO3-
Can form and release HCO3- when low or excrete excess

33. Calcium Ca2+ Most abundant mineral in body
98% of calcium in adults in skeleton and teeth
In body fluids mainly an extracellular cation
Contributes to hardness of teeth and bones
Plays important roles in blood clotting, neurotransmitter release, muscle tone, and excitability of nervous and muscle tissue
Regulated by parathyroid hormone
Stimulates osteoclasts to release calcium from bone – resorption
Also enhances reabsorption from glomerular filtrate
Increases production of calcitrol to increase absorption for GI tract
Calcitonin lowers blood calcium levels

34. Phosphate
About 85% in adults present as calcium phosphate salts in bone and teeth
Remaining 15% ionized – H2PO4-, HPO42-, and PO43- are important intracellular anions
HPO42- important buffer of H+ in body fluids and urine
Same hormones governing calcium homeostasis also regulate HPO42- in blood
Parathyroid hormone – stimulates resorption of bone by osteoclasts releasing calcium and phosphate but inhibits reabsorption of phosphate ions in kidneys
Calcitrol promotes absorption of phosphates and calcium from GI tract

35. Magnesium
In adults, about 54% of total body magnesium is part of bone as magnesium salts
Remaining 46% as Mg2+ in ICF (45%) or ECF (1%)
Second most common intracellular cation
Cofactor for certain enzymes and sodium-potassium pump
Essential for normal neuromuscular activity, synaptic transmission, and myocardial function
Secretion of parathyroid hormone depends on Mg2+
Regulated in blood plasma by varying rate excreted in urine

36. Acid-base balance
Major homeostatic challenge is keeping H+ concentration (pH) of body fluids at appropriate level
3D shape of proteins sensitive to pH
Diets with large amounts of proteins produce more acids than bases which acidifies blood
Several mechanisms help maintain pH of arterial blood between 7.35 and 7.45
Buffer systems, exhalation of CO2, and kidney excretion of H+

37. Acid–Base Balance
Figure 27–6 The Basic Relationship between PCO2and Plasma pH.

38. Acid–Base Balance Three Major Buffer Systems Protein buffer systems:
Help regulate pH in ECF and ICF
Interact extensively with other buffer systems
Carbonic acid–bicarbonate buffer system:
Most important in ECF
Phosphate buffer system:
Buffers pH of ICF and urine

39. Acid–Base Balance
Figure 27–7 Buffer Systems in Body Fluids.

40. Acid–Base Balance
Figure 27–8 The Role of Amino Acids in Protein Buffer Systems.

41. Buffer systems Act to quickly temporarily bind H+
Raise pH but do not remove H+
Most consist of weak acid and salt of that acid functioning as weak base
Protein buffer system
Most abundant buffer in ICF and blood plasma
Hemoglobin in RBCs
Albumin in blood plasma
Free carboxyl group acts like an acid by releasing H+
Free amino group acts as a base to combine with H+
Side chain groups on 7 of 20 amino acids also can buffer H+

42. Buffer Systems Carbonic acid- bicarbonate buffer system
Based on bicarbonate ion (HCO3-) acting as weak base and carbonic acid (H2CO3) acting as weak acid
HCO3- is a significant anion in both ICF and ECF
Because CO2 and H2O combine to form this buffer system cannot protect against pH changes due to respiratory problems in which there is an excess or shortage of CO2
Phosphate buffer system
Dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO42-)
Phosphates are major anions in ICF and minor ones in ECF
Important regulator of pH in cytosol

43. Acid–Base Balance The Hemoglobin Buffer System
CO2 diffuses across RBC membrane
No transport mechanism required
As carbonic acid dissociates
Bicarbonate ions diffuse into plasma
In exchange for chloride ions (chloride shift)
Hydrogen ions are buffered by hemoglobin molecules

44. Exhalation of carbon dioxide
Increase in carbon dioxide in body fluids lowers pH of body fluids
Because H2CO3 can be eliminated by exhaling CO2 it is called a volatile acid
Changes in the rate and depth of breathing can alter pH of body fluids within minutes
Negative feedback loop

45. Acid–Base Balance The Hemoglobin Buffer System
Is the only intracellular buffer system with an immediate effect on ECF pH
Helps prevent major changes in pH when plasma PCO2 is rising or falling

46. Acid–Base Balance Carbonic Acid–Bicarbonate Buffer System
Carbon Dioxide
Most body cells constantly generate carbon dioxide
Most carbon dioxide is converted to carbonic acid, which dissociates into H+ and a bicarbonate ion
Is formed by carbonic acid and its dissociation products
Prevents changes in pH caused by organic acids and fixed acids in ECF

47. Acid–Base Balance
Figure 27–9 The Carbonic Acid–Bicarbonate Buffer System

48. Acid–Base Balance Phosphate Buffer System
Consists of anion H2PO4- (a weak acid)
Works like the carbonic acid–bicarbonate buffer system
Is important in buffering pH of ICF

49. Acid–Base Balance Limitations of Buffer Systems
Provide only temporary solution to acid–base imbalance
Do not eliminate H+ ions
Supply of buffer molecules is limited

50. Acid–Base Balance Renal Responses to Acidosis Secretion of H+
Activity of buffers in tubular fluid
Removal of CO2
Reabsorption of NaHCO3

51. Kidney excretion of H+ Metabolic reactions produce nonvolatile acids
One way to eliminate this huge load is to excrete H+ in urine
In the proximal convoluted tubule, Na+ /H+ antiporters secrete H+ as they reabsorb Na+
Intercalated cells of collecting duct include proton pumps that secrete H+ into tubule fluid
Urine can be up to 1000 times more acidic than blood
2 other buffers can combine with H+ in collecting duct
HPO42- and NH3

52. Secretion of H+ by intercalated cells in the collecting duct

53. Metabolic alkalosis Abnormally high HCO3- in systemic arterial blood
Nonrespiratory loss of acid - vomiting of acidic stomach contents, gastric suctioning
Excessive intake of alkaline drugs (antacids)
Use of certain diuretics
Severe dehydration
Hypoventilation can help
Give fluid solutions to correct Cl-, K+ and other electrolyte deficiencies and correct cause of alkalosis

54. Respiratory alkalosis
Abnormally low PCO2 in systemic arterial blood
Cause is hyperventilation due to oxygen deficiency from high altitude or pulmonary disease, stroke or severe anxiety
Renal compensation can help
One simple treatment to breather into paper bag for short time

55. Regulation of blood pH by the respiratory system

56. Acid–Base Balance Renal Responses to Alkalosis
Rate of secretion of H+ at kidneys declines
Tubule cells do not reclaim bicarbonates in tubular fluid
Collecting system transports HCO3- into tubular fluid while releasing strong acid (HCl) into peritubular fluid

57. Acid–Base Balance Disturbances
Figure 27–15 A Diagnostic Chart for Acid–Base Disorders.