1. Ch. 27 – Fluid, Electrolyte, and Acid-Base Balance
These notes are a brief overview of the main themes of Ch. 27 only
The new information in these notes and in Ch. 27 is FYI only, and will not be fair game for the Final Exam
While Ch. 27 repeats a lot of information we have discussed during the past 9 months, it does a nice job of tying these topics together under the umbrella concept of homeostasis

2. Fluid (water) balance Water gain must = water loss
Water ≈ 50-60% of our body weight
There are no active transporters for water molecules, just water channels (aquaporins)
“Water follows salt” by osmosis
Major players in the maintenance of fluid balance: the renin-angiotensin system, ADH, aldosterone, natriuretic peptides, baroreceptors in the heart and blood vessels, and the thirst center in the hypothalamus
Fig. 27-3, p. 1003

3. Electrolyte balance Electrolyte gain must = electrolyte loss
Electrolytes = ions released when soluble inorganic compounds dissociate; they can conduct an electrical current in solution
So essentially, electrolytes = ions in solution
E.g. Na+, K+, Cl-, Ca2+, etc.
Electrolytes are lost and gained in most (but not all) of the same ways as water
In (gain): diet
Out (loss): urine, feces, and sweat

4. Some important electrolytes in body fluids
Fig. 27-2, p. 1001

5. Acid-base balance H+ production must = H+ loss
Normal body fluid pH =
Major players in the maintenance of acid-base balance:
The kidneys via the reabsorption and/or secretion of H+ and HCO3-
Remember, HCO3- = bicarbonate, which acts with carbonic acid (H2CO3) as a buffer system: H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3-
The elimination of CO2 via external respiration and exhalation at the lungs
Since most of the CO2 in the blood is carried as bicarbonate, exhaling CO2 drives the above equation to the left, ↓ bicarbonate and H+ levels (which ↑ pH)
Other buffer systems…
See the next slide for examples of buffer systems including and in addition to the carbonic acid-bicarbonate buffer system

6. Buffer systems in body fluids
Fig , p. 1014

7. How an amino acid (and thus a protein) can act as a buffer
Fig , p. 1014

8. Acid-base imbalances
Table 27-4, p. 1026
Severe acidosis causes ↓ neuron excitability (which can lead to disorientation, coma, and death)
Severe alkalosis causes ↑ neuron excitability (which can lead to nervousness, muscle spasm, convulsions, and death)

9. Respiratory acidosis and alkalosis
Respiratory acidosis is the most common type of acid-base imbalance
Respiratory alkalosis is relatively rare
Fig , p. 1021

10. Metabolic acidosis
This is the 2nd most common type of acid-base imbalance
E.g. lactic acidosis and ketoacidosis
Fig , p. 1023

11. Metabolic alkalosis Is relatively rare
The loss of HCl via severe vomiting is the most common cause
Fig , p. 1024

12. A diagnostic chart for acid-base disorders
Fig , p. 1025