1. Water, Electrolytes, and
Chapter 27
Water, Electrolytes, and
Acid-Base Balance

2. Regulation of Water Content
Sources of water
Ingestion
Cellular metabolism
Routes of water loss
Urine
Evaporation
Perspiration
Respiratory passages
Feces
Content regulated so total volume of water in body remains constant
Kidneys are primary regulators of water excretion
Regulation processes
Osmosis
Osmolality
Baroreceptors
Learned behavior

3. Extracellular Fluid Osmolality
Measure of water vs. solute concentration; the higher the solute concentration, the higher the osmolality
Adding or removing water from a solution changes osmolality
Increased osmolality: triggers thirst and ADH secretion
Decreased osmolality: inhibits thirst and ADH secretion

4. Hormonal Regulation of Blood Osmolality

5. Regulation of ECF Volume
ECF can increase or decrease even if osmolality of extracellular fluid is maintained
Carotid sinus and aortic arch baroreceptors monitor blood pressure, juxtaglomerular apparatuses monitor pressure changes, receptors in walls of atria and large vessels respond to small changes in BP
These receptors activate these mechanisms
Neural: increase in BP recognized by baroreceptors. Decreased sympathetic stimulation leads to increased pressure in glomerulus leading to increased filtration and increased urine output.
Renin-angiotensin-aldosterone
Atrial natriuretic hormone (ANH)
Antidiuretic hormone (ADH)

6. Increase in Blood Volume

7. Decrease in Blood Volume

8. Regulation of ECF Volume

9. Regulation of Electrolytes in ECF
Regulation of Na+ ions
Kidneys major route of excretion
Small quantities lost in sweat {sweat = (in decreasing amounts) water, Na+, urea, Cl-K+, NH3}. Insensible perspiration is water evaporating from skin. Sensible perspiration is secreted by the sweat glands. Contains solutes
Terms
Hypernatremia: elevated plasma Na+
Hyponatremia: decreased Na+
Electrolytes
Molecules or ions with an electrical charge
Ingestion adds electrolytes to body
Kidneys, liver, skin, remove from body
Concentration changes only when growing, gaining or losing weight
Na+ Ions
Dominant ECF cations
Responsible for 90-95% of osmotic pressure

10. Regulation of Chloride, Potassium, Magnesium Ions
Potassium ions
Maintained in narrow range
Affect resting membrane potentials
Aldosterone increases amount secreted
Terms
Hyperkalemia: abnormally high levels of potassium in extracellular fluid
Hypokalemia: abnormally low levels of potassium in extracellular fluid.
Chloride ions
Predominant anions in ECF
Magnesium ions
Capacity of kidney to reabsorb is limited
Excess lost in urine
Decreased extracellular magnesium results in greater degree of reabsorption

11. Potassium Ion Regulation in ECF

12. Regulation of Calcium Ions
PTH increases Ca2+ extracellular levels and decreases extracellular phosphate levels
Vitamin D stimulates Ca2+ uptake in intestines
Calcitonin decreases extracellular Ca2+ levels
Regulated within narrow range
Elevated extracellular levels prevent membrane depolarization
Decreased levels lead to spontaneous action potential generation
Terms
Hypocalcemia
Hypercalcemia

13. Regulation of Calcium Ions

14. Regulation of Blood Magnesium

15. Acids and Bases and Buffers
Buffers: Resist changes in pH
When H+ added, buffer removes it
When H+ removed, buffer replaces it
Types of buffer systems
Carbonic acid/bicarbonate
Protein
Phosphate
Acids
Release H+ into solution
Bases
Remove H+ from solution
Acids and bases
Grouped as strong or weak

16. Comparison of Strong and Weak Acids

17. Regulation of Acid/Base Balance
Buffers: if pH rises, buffers bind H+; if pH falls, buffers release H+
Protein buffer: Intracellular and plasma proteins absorb H+. Provide ¾ of buffering in body. E.g., hemoglobin.
Bicarbonate buffering system: Important in plasma
Phosphate buffer system: important as an intracellular buffer
Respiratory center: if pH rises, respiratory rate decreases; if pH falls, respiratory rate increases
Kidneys: if pH rises, distal tubule decreases H+ secretion into the urine; if pH falls, distal tubule increases H+ secretion into the urine

18. Increase in Blood pH

19. Decrease in Blood pH

20. Respiratory Regulation of Acid-Base Balance
Achieved through carbonic acid/bicarbonate buffer system
As carbon dioxide levels increase, pH decreases
As carbon dioxide levels decrease, pH increases
Carbon dioxide levels and pH affect respiratory centers
Hypoventilation increases blood carbon dioxide levels
Hyperventilation decreases blood carbon dioxide levels

21. Respiratory Regulation of Acid-Base Balance

22. Kidney Regulation of Acid-Base Balance

23. Acidosis and Alkalosis
Acidosis: pH body fluids below 7.35
Respiratory: Caused by inadequate ventilation- reduced elimination of CO2, asthma, damage to respiratory center in brain, emphysema.
Metabolic: Results from all conditions other than respiratory that decrease pH- diarrhea, vomiting, ingesting overdose of aspirin, untreated diabetes mellitus, anaerobic respiration
Alkalosis: pH body fluids above 7.45
Respiratory: Caused by hyperventilation, high altitude (reduced partial pressure of O2
Metabolic: Results from all conditions other than respiratory that increase pH- severe vomiting, too much aldosterone, ingestion of substances like bicarbonate of soda.