Chapter 27 Fluid, Electrolyte, and Acid-Base Homeostasis

Copyright © John Wiley & Sons, Inc. All rights reserved.  The human body requires constant attention to the methods of regulating body fluid. These processes are necessary to maintain required proportions of water and solutes among body compartments l Water is by far the largest single component of the body making up 55–80% of total body mass (depending on age and sex). Filtration, reabsorption, diffusion, and osmosis continually exchange water and solutes among these compartments Body Fluids

Copyright © John Wiley & Sons, Inc. All rights reserved.  In addition to its job of filtering and excreting waste products from the blood, the kidneys are also charged to take the lead in maintaining the composition of water and salts in the body’s various fluid compartments l This makes the study of fluid balance in this chapter a fitting enjoiner to our recent discussion of renal function Body Fluids

Copyright © John Wiley & Sons, Inc. All rights reserved.  Body Fluids and Water Flow Body Fluids and Water Flow Body Fluids (Interactions Animation) You must be connected to the internet to run this animation

Copyright © John Wiley & Sons, Inc. All rights reserved.  The fluid compartments of the body are all contained in either the intracellular compartment or the extracellular compartment l Intracellular fluid is all fluid contained inside cells, and comprises 2/3 of all body fluids l Extracellular fluid is all fluid outside the confines of the plasma membranes that contain all intracellular contents. 1/3 of all body fluid is contained in the extracellular compartment Fluid Compartments

Copyright © John Wiley & Sons, Inc. All rights reserved.  While the intracellular fluids are all contained within a single space (inside cells), extracellular fluid is found in a number of sub compartments l Most extracellular fluid (¾) is found between the cells of the body (interstitial fluid) l The rest is found in the intravascular fluid space (blood plasma – about ¼), with small amounts existing as lymph, CSF, synovial fluid, aqueous humor, endolymph and perilymph, and pericardial fluid Fluid Compartments

Copyright © John Wiley & Sons, Inc. All rights reserved.  Babies are more “wet” than adults, with water composing about 80% of total body mass Fluid Compartments

Copyright © John Wiley & Sons, Inc. All rights reserved.  Normal fluid intake is through: l Ingestion of liquids and moist foods (2300mL/day) l Metabolic synthesis of water during cellular respiration and dehydration synthesis (200mL/day)  Normal fluid loss is through: l The kidneys (1500mL/day) l Evaporation from the skin (600mL/day) l Exhalation from the lungs (300mL/day) l In the feces (100mL/day) Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Fluid intake and output (I & O) are usually balanced on a daily basis, despite the fact that intake of water and electrolytes are rarely proportional l The kidneys excrete excess water through dilute urine, or excess electrolytes through concentrated urine Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Fluid Balance Animation Fluid Balance Animation Fluid Balance (Interactions Animation) You must be connected to the internet to run this animation

Copyright © John Wiley & Sons, Inc. All rights reserved.  A number of feedback mechanism contribute to balance of daily fluid inpu and output Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Three hormones regulate renal Na + and Cl – reabsorption or excretion l Angiotensin II and aldosterone promote urinary Na + and Cl – reabsorption (and water by osmosis) when dehydrated l Atrial natriuretic peptide (ANP) promotes excretion of Na + and Cl – followed by water excretion to decrease blood volume Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Na + and Cl – balance is regulated by 3 hormones l Aldosterone l Atrial natriuetic peptide l Angiotension II Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  In addition to the hormones that regulate Na + and Cl - homeostasis, antidiuretic hormone (ADH) is a hormone that plays a major role in directly regulating water loss in the collecting ducts of the kidneys l Also known as vasopressin, ADH (from the posterior pituitary) increases permeability of the collecting ducts to water by promoting insertion of aquaporin-2 into the principal cells – producing a concentrated urine Fluid Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Ions form when electrolytes dissolve and dissociate. They have 4 general functions l Control osmosis of water between body fluid compartments l Help maintain the acid-base balance l Carry electrical current l Serve as cofactors Electrolytes

Copyright © John Wiley & Sons, Inc. All rights reserved.  The term “milliequivalent” (mEq) is used to measure the number of electrical charges (electrolytes) in blood serum and other solutions. Denoting the number of mEq per liter of solution gives the concentration of anions or cations in a given volume of solution l One equivalent is the charge in 1 mole of H + ions. A milliequivalent is simply 1/1000 of an equivalent ◦Sodium mEq/L ◦Potassium mEq/L Electrolytes

Copyright © John Wiley & Sons, Inc. All rights reserved.  The size of a substance does not determine its osmotic contribution – that is determined by the number of milliequivalents. For example: l 1 millimole NaCl = 2 mEq (1mEq of Na + and 1mEq of Cl - Electrolytes

Copyright © John Wiley & Sons, Inc. All rights reserved.  As we have seen, osmotic forces have a definite influence on movement of water between body compartments. Osmotic pressure exerted by proteins on either side of the capillary membrane is called oncotic pressure. It is not, however, the only force in play - hydrostatic forces are another major factor to consider l Net movement of fluids is controlled by all forces favoring filtration minus all forces opposing filtration Starling Forces

Copyright © John Wiley & Sons, Inc. All rights reserved.  The Starling equation compares the forces at the arterial end of a capillary with those at the venous end l Forces favoring filtration are the capillary hydrostatic pressure (pressure against the capillary wall) and the interstitial oncotic pressure l Forces favoring reabsorption are the plasma oncotic pressure (water-pulling) and the interstitial hydrostatic pressure Starling Forces

Copyright © John Wiley & Sons, Inc. All rights reserved.  Normal Starling forces favor a small amount of fluid flowing out of the capillary which is drained by the lymphatic system Starling Forces

Copyright © John Wiley & Sons, Inc. All rights reserved.  Edema occurs when excess interstitial fluid collects, causing swelling in the tissues. Edema occurs anytime filtration exceeds reabsorption l The most important causes of edema are: ◦increased blood pressure (increased blood hydrostatic pressure) ◦an increase in the capillary permeability ◦a decrease in the concentration of plasma proteins ◦an obstruction in lymphatic drainage Starling Forces

Copyright © John Wiley & Sons, Inc. All rights reserved.  A major homeostatic challenge is keeping the H + concentration (pH) of body fluids at an appropriate level. Because metabolic reactions often produce a huge excess of H +, failure of homeostatic mechanisms would cause the pH of body fluids to quickly fall to a lethal level l In a healthy person, chemical buffers, the lungs, and the kidneys help maintain the pH of systemic arterial blood between 7.35 and 7.45 Acid-Base Balance

Copyright © John Wiley & Sons, Inc. All rights reserved. 1. Buffer systems act quickly to temporarily bind excess H + or OH -, sequestering (hiding) the highly reactive ions until they can be permanently excreted 2. By increasing the rate and depth of breathing, CO 2 is exhaled or retained, and blood pH is corrected 3. Kidney excretion/reabsorption of acidic ions (H + and NH 4 + ) or basic ions (HCO3 – or OH - ) is the slowest mechanism; but is the only way to eliminate acids other than carbonic acid Acid-Base Balance

Copyright © John Wiley & Sons, Inc. All rights reserved.  Regulation of pH Regulation of pH Acid-Base Balance (Interactions Animation) You must be connected to the internet to run this animation

Copyright © John Wiley & Sons, Inc. All rights reserved.  Respiratory acidosis occurs whenever CO 2 accumulates because of hypoventilation  Metabolic acidosis occurs whenever non-respiratory acids accumulate, as seen in diabetic ketoacidosis or aspirin overdose  Respiratory alkalosis occurs whenever too much CO 2 is lost because of hyperventilation  Metabolic alkalosis occurs whenever non-respiratory acids are lost, which happens infrequently Acid-Base Imbalances

Copyright © John Wiley & Sons, Inc. All rights reserved.  The homeostatic correction for states of acidosis (which are much more common and serious than states of alkalosis) are depicted in this flowchart Acid-Base Imbalances

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