Fluid and Electrolyte Balance

LECTURE OUTLINE (CHAPTER 20)

LECTURE OBJECTIVES

1. Explain how fluid balance is maintained in the major fluid compartments
2. Describe how the pH is regulated in the body.

LECTURE OUTLINE

I.  FLUID AND ELECTROLYTE BALANCE

  A. Major body fluid compartments  
     1. Total fluid volume
          a. ECF 
               1) Plasma 
               2) Interstitial Fluid 
               3) Lymph 
          b. ICF 
          c. Movement of fluid in one compartment affects the others
     2. Ionic composition of ECF and ICF
          a. Cation and anion distribution differ between ECF and ICF
  B. Regulation of ECF volume
     1. Why regulate ECF volume?
     2. ECF volume and plasma volume are closely associated
          a. ECF volume and blood pressure
          b. Avenues of water gain and loss
     3. Aldosterone increases Na+ reabsorption/K+ secretion (Na+ > K+)
          a. This regulated uptake of solute in the distal nephron/
              cortical collecting duct encourages water reabsorption.
 C. Regulation of ECF osmolarity
     1. Why regulate ECF osmolarity?
     2. ECF osmolarity and cell turgidity
          a. Avenues of NaCl gain and loss
     3. Vasopressin's (ADH) acts on aquaporin channels
          a. Collecting duct reabsorbs water changing urine osmolarity

II. pH BALANCE

  A. Introduction to pH
     1. Some basics
           a. Strong and weak acids
           b. What is a buffer?
                1) Buffers in physiological systems
     2. pH range is fairly narrow for physiological systems  
          a.  pH is a log scale (pH = log 1/[H+])
          b. Why is pH regulation important?          
          c. Plasma/ECF pH are related to [HCO3-] and [CO2]
               1) The Henderson-Hasselbalch equation calculates pH 
  B. Two systems regulate blood pH
     1. Lungs: Pulmonary ventilation
	      a. pH is proportional to [CO2] 
     2. Kidney: Regulation of H+/HCO3- I cells of the collecting duct
          a. pH is proportional to [HCO3-]
          b. Secretion of H+ occurs during acidosis
          c. Reabsorption of H+ occurs during alkalosis
          d. Urinary buffer systems
               1) Bicarbonate in the urine
               2) Phosphate in the urine
               3) Deamination and the ammonium ion (NH4+)
  C. Acidosis and alkalosis 
     1. Respiratory changes--due largely to changes in [CO2]
     2. Metabolic changes--due largely to changes in [HCO3-]  
Reading Assignment. Please read Chapter 21 for the next lecture.



STUDY QUESTIONS ON FLUID-ELECTROLYTE BALANCE (CHAPTER 20)

    BASIC FACTS AND TERMS

  1. Compare the ionic composition of plasma, ECF, and ICF. Which of the following is present within cells (ICF) at a higher concentration than is present in ECF:  Na+, K+, Ca++, Cl-, HCO3-, and protein

  2. What the sources of water loss from the body? Which one is the most important? What are the sources of water gain? What are the sources of salt loss from the body? Which one is the most important?

  3. What is the relationship between H+ and pH (Review pages 37-38)? Why is regulating pH important? What is a strong acid? What is a weak acid? Give an example of each. What is a base? What is a buffer? Give some examples of physiological buffers.


    CONCEPTS

  4. What is the Henderson-Hasselbalch equation? How do the [HCO3-] and [CO2] components in the modified equation (i.e., pH = pK + log [HCO3-]/[CO2]) relate to kidney and lung function? At a normal blood pH of 7.4, what is the ratio of [HCO3-] to [CO2] ?

  5. How does the kidney regulate blood pH?   How does the lung regulate blood pH?

  6. What is ECF hypertonicity? ECF hypotonicity? Give examples of physiological situations causing these effects.

  7. What causes respiratory acidosis? respiratory alkadosis? metabolic acidosis? metabolic alkadosis? How is carbon dioxide or bicarbonate ion involved in each? Give one or two examples of physiological situations that could cause each. Also, review Table 20-2.

  8. Why are blood K+ levels so carefully regulated? See text.

  9. Read about how the proximal tubule retains filtered bicarbonate and secretes H+ into the urine. See Figure 20-22.

  10. What is the role of aldosterone in electrolyte regulation and how does it act?

  11. How do the Type A and Type B intercalated cells in the distal nephron function in regulating acidosis and alkalosis, respectively.

  12. Contrast the effects of the renin-angiotensin-aldosterone cascade with the natriuretic peptides. See Figures 20-14 and 20-16.


    REASONING AND PROBLEM SOLVING

  13. The plasma Na+ concentration in a person with severe congestive heart failure (and associated low blood pressure) may be below normal even though the amount of Na+ in the ECF is greatly increased. Explain this apparent contradiction.
  14. Strenuous exercise in a hot environment poses some serious problems associated with plasma volume. What is the nature of the problem with regard to exercise and cooling mechanisms? What adjustments are made with time to minimize this condition?

  15. Faith is a 55 year old woman with a history of a duodenal ulcer. She comes to the emergency room because she has been vomiting intermittently for the last several days. Lab tests are performed and reveal the following values:
            Blood pH        7.45       (normal 7.40) 
            Blood pCO2      43 mm Hg   (normal 40 mm Hg)
            Blood pO2       100 mm Hg  (normal 100 mm Hg)
            Blood [HCO3-]   38 mEq/l   (normal 24 mEq/l)
            Urine pH        6.0        (normal range 2 to 6)
    Given these data, what type of acid-base disorder does Faith have? Explain your answer.



Last revised: April 3, 2008