INTRODUCTION
1. The extracellular fluid contains approximately one-third of the water in the body. The ECF includes the interstitial fluid and the plasma.
2. The intracellular fluid contains two-thirds of the body water and is entirely enclosed by cell membranes.
3. Exchange occurs between the ECF and ICF due to the permeability of cell membranes, but the two fluid compartments retain their distinctive characteristics.
4. Homeostatic mechanisms that monitor and adjust the composition of body fluids respond to changes in the ECF, not the ICF. Maintenance of normal fluid volume, composition, and pH in the ICF and ECF can be viewed as three integrated processes: fluid balance, electrolyte balance, and acid-base balance.
FLUID AND ELECTROLYTE BALANCE
1. Receptors initiating the homeostatic adjustments are sensitive to changes in the composition of the plasma.
2. The body content of water or electrolytes will rise if intake exceeds outflow and fall if losses exceed gains. Homeostatic adjustments primarily address the rates of dietary absorption and urinary excretion. The responses may be behavioral or physiological in nature.
3. Three hormones are involved in the regulation of fluid and electrolyte balance, ADH, aldosterone, and ANF.
4.ADH encourages water resorption at the kidneys and creates a desire to drink. Aldosterone increases the rates of sodium resorption at the kidneys. ANF opposes these actions and promotes fluid and electrolyte losses in the urine.
Fluid Balance
1. Water losses are normally balanced by gains through eating, drinking, and metabolic generation.
2. Although the ECF and ICF differ in terms of their ionic composition, their osmolarities are identical and no net movement of water occurs between the two. Alterations in the osmolarity of either compartment results in fluid shifts that eliminate the differences.
3. Gaining water without electrolytes will lower the osmolarities of the ECF and ICF. Losing water without electrolytes will increase the osmolarities of both compartments.Compensation occurs through changes in the rate of ADH secretion.
Electrolyte Balance
Sodium ion regulation:
1. Problems with electrolyte balance are generally attributable to an imbalance between sodium gains and losses.
2. The rate of sodium uptake across the intestinal epithelium is directly proportional to the amount included in the diet. Sodium losses primarily occur in the urine and through perspiration.
3. The rate of urinary loss is regulated by the rate of sodium reabsorption along the distal convoluted tubules and collecting segments. These activities are regulated by the levels of circulating aldosterone.
4. Sodium reabsorption is tied to absorption of a corresponding anion, usually chloride or bicarbonate, and the secretion of either hydrogen or potassium ions.
5. Changes in the rates of sodium uptake or excretion do not alter the sodium concentration of the ECF because osmotic water movements maintain equilibrium. As a result the volume of the ECF changes but the osmolarity remains relatively constant. Reference:
6. The volume changes are corrected by the secretion of ADH and aldosterone, if the volume is inadequate, or ANF if the volume is excessive. Identical homeostatic adjustments occur in response to volume depletion caused by blood or tissue fluid losses.
Potassium Ion Regulation:
1. Potassium ion concentrations in the ECF are very low, and they are not as closely regulated. Potassium excretion increases as ECF concentrations rise, under aldosterone stimulation, and when the pH rises.Potassium retention occurs when the pH falls.
ACID-BASE BALANCE
1. A neutral solution contains hydrogen and hydroxyl ions in equal concentrations. The pH represents the negative log of the hydrogen ion concentration, and it ranges from 0 to 14.
Acids, Bases, and pH
1.Acids dissociate in water and release hydrogen ions. Bases remove hydrogen ions or introduce hydroxyl ions. Salts dissociate with little effect on hydrogen or hydroxyl ion concentrations.
2. Acids and bases may be categorized as strong or weak depending on their behavior in solution.
3. Normal pH ranges from 7.35-7.45.Variations outside of this relatively narrow zone produce acidosis or alkalosis.
The Maintenance of Normal pH:
1. The regulation of pH occurs through a combination of buffers and the pulmonary and renal mechanisms.
2. Most threats to homeostasis result from the generation of volatile, fixed, and organic acids.
3. Carbon dioxide concentration represents the single most important factor influencing the pH of body fluids. In solution carbon dioxide reacts with water molecules to form carbonic acid, and the dissociation of carbonic acid releases hydrogen ions. Carbon dioxide is a volatile acid because it readily diffuses out of solution into the alveolar air.
4. Sulfuric and phosphoric acids are produced during the catabolism of amino acids and compounds containing phosphate groups. These are fixed acids.
5. Organic acids include metabolic products such as lactic acid or ketone bodies.
6. Buffers provide or remove hydrogen ions to stabilize pH.A buffer system includes weak acids and weak bases. The bicarbonate buffer system accounts for most of the buffering capabilities in the ECF. The phosphate buffer system is particularly important in the ICF, and protein buffer systems are distributed in both compartments.
7. The bicarbonate buffer system consists of carbonic acid and sodium bicarbonate. The combination stabilizes the concentrations of hydrogen ions and bicarbonate ions in solution, absorbing or removing them as needed.
8. The phosphate buffer system resembles the bicarbonate buffer system in its basic operation.Phosphate concentrations are quite high in the ICF, where this system provides important buffering capabilities.
9. Protein buffer systems depend upon the abilities of amino acids to bind or release hydrogen ions. The hemoglobin buffer system within the red blood cells plays a role in carbon dioxide transport.
10. The lungs and kidneys participate in pH regulation primarily through their influence on the bicarbonate buffer system.
11. The loss of carbon dioxide at the lungs lowers the concentrations of hydrogen ions and bicarbonates in solution. Increasing or decreasing the rate of respiration thus has a direct effect on pH.
12. The kidneys vary their rates of hydrogen ion secretion and bicarbonate ion resorption depending on the pH of the extracellular fluids. In acidosis the rates of H+ secretion and bicarbonate ion reabsorption and generation increase.
Disturbances in Acid-Base Balance:
1. Disturbances of acid-base balance may result from physical or physiological disorders affecting circulating buffers or the performance of the respiratory, urinary, cardiovascular, or nervous systems.
2. Respiratory disorders result from abnormal concentrations of carbon dioxide in body fluids.Metabolic disorders are caused by the generation of organic or fixed acids or by the loss of bicarbonate ions.
3. Respiratory acidosis results from the excessive accumulation of carbon dioxide in body fluids. The usual cause is hypoventilation, normally corrected by chemoreceptor reflexes.In the absence of normal homeostatic adjustments acute respiratory acidosis develops.
4. Respiratory alkalosis is an uncommon condition associated with hyperventilation.
5. Metabolic acidosis results from any condition that depletes the normal reserves of bicarbonate ions. This can result from a loss of bicarbonate ions or their utilization in neutralizing quantities of organic or fixed acids.
6. The most frequent cause of metabolic acidosis is an inability to excrete hydrogen ions at the kidneys. A less common cause is the production of large numbers of fixed and organic acids. It can also result from the bicarbonate loss that accompanies chronic diarrhea.
7. Metabolic alkalosis occurs when bicarbonate ion concentrations become elevated. Cases are rare, but they may occur following extended periods of vomiting due to the alkaline tide that accompanies gastric acid secretion.