Water and Body Fluid Compartments Authors JeanPaul Kovalik

Water and Body Fluid Compartments Author(s): Jean-Paul Kovalik, MD, Ph. D Ruediger Lehrich, MD Level: Basic Academic Affiliation : Duke University Medical Center Submitted: April 2012 Version No: 1. 0 Editors’ Review:

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4 Learning Objectives • Review the different body fluid compartments • Understand how water determines osmolarity • Recognize that the composition of IV fluid determines how it gets distributed to the body compartments

5 Outline • Water as a major component of body mass • Three major fluid compartments • Composition of fluid compartments • Water and body fluid osmolarity • Clinical cases highlighting the effects of fluid administration on body fluid compartments

6 Total Body Water Non-Water Weight 40% Total Body Mass Total Body Water 60%

7 Body Fluid Compartments Interstitial Intravascular (Plasma) 3/4 of Extracellular (10. 5 L) Intracellular 2/3 of TBW (28 L) 1/4 of Extracellular (3. 5 L) Extracellular 1/3 of TBW (14 L)

8 Composition of Body Fluid Compartments Intracellular Interstitial Na Na K Sodium: 12 m. Eq/L Potassium: 150 m. Eq/L K Sodium: 145 Potassium: 4. 1 Intravascular (Plasma) Na K Sodium: 142 Potassium: 4

9 Water and Osmolarity Water Loss Normal Serum Osmolarity Water Gain Hyperosmolar state Hypoosmolar state

10 Response to Hyperosmolarity Increased thirst Dilution of concentrated serum Water retention by the kidneys Hyperosmolar state Normal Serum Osmolarity

11 Response to Hypoosmolarity Thirst shut off Concentration of dilute serum Water excretion by the kidneys Hypoosmolar state Normal Serum Osmolarity

12 ADH Regulates Water Balance Serum Osmolarity and ADH Release 20 18 16 ADH 14 Normal Osmolarity 12 10 8 6 4 2 0 200 220 240 260 280 300 Serum Osmolarity m. Osm/L 320 340

13 Case Number 1: Volume Depletion • History: 82 year old man with 4 days nausea, vomiting and diarrhea • Physical Exam: – Temperature 38. 5 C, blood pressure 85/45 mm. Hg pulse 124, weight 70 kg – Moaning incoherently, mucous membranes dry, neck veins flat, chest clear, abdomen soft and mildly tender to palpation, no pitting edema, tenting of skin • Diagnosis: Viral gastritis with severe volume depletion • Treatment: 0. 9% Saline given as a 2 L bolus followed by additional 2 L over the following several hours • Follow-up Exam: – Temperature 37. 5 C, blood pressure 124/60 mm. Hg pulse 85, weight 74 kg – Awake, able to answer questions

14 Case Number 1: Volume Depletion • Distribution of the 4 L 0. 9% saline administered to patient: – Intracellular: 0 m. L (sodium is primarily an extracellular cation) – Extracellular: 4 L Interstitial ¾ of extracellular compartment: 3 L Intravascular ¼ of extracellular compartment: 1 L • Only ¼ (1 L) of the infused 0. 9% saline remains in the intravascular compartment • Expansion of the intravascular fluid compartment requires large volumes of 0. 9% saline since ¾ of the IV fluid moves into the interstitial space

15 Case Number 2: Dehydration • History: 23 year old man with head trauma and polyuria • Physical Exam: – Temperature 37. 1 C, blood pressure 122/75 mm. Hg pulse 82, weight 70 kg – Comatose, mucous membranes moist, chest clear, abdomen soft, no pitting edema – Urine output 4. 2 L per 24 hours, Serum sodium 156 m. Eq/L • Diagnosis: Central diabetes insipidus and dehydration • Treatment: – Administration of ADH – Sterile water containing 5% Dextrose infused at 100 m. L/hr over 48 hrs • Follow-up Exam: – Serum sodium 142 m. E/L, Urine output decreased to 1. 2 L per 24 hours

16 Case Number 2: Dehydration • Calculating water deficit – Deficit=Total body water x (Measured serum sodium/Ideal sodium -1) – Deficit=42 L x (156/140 -1) – Deficit=42 L x 0. 11= 4. 8 L • Distribution of infused water (4. 8 L total) – Intracellular=2/3 of water=3. 2 L – Extracellular=1/3 of water=1. 6 L Interstitial= ¾ of extraceulluar= 1. 2 L Intravascular=1/4 of extracellular=0. 4 L • Out of almost 5 L IV water administered only 400 cc stays in intravascular space

17 Summary • Water makes up 60% of the average body mass • The three major fluid compartments are intra-cellular, interstitial and intra-vascular • Water balance determines osmolarity of the body fluids • Composition of IV fluids determines how it gets distributed to the various compartments

18 Key References • Briggs JP, Kriz W and Schnermann JB: Overview of Kidney Function and Structure. In Greenberg A (ed): Primer on Kidney Diseases. Philadelphia, Elsevier, 2009, pp 2 -18. • Guyton AC and Hall JE: The Body Fluids and kidney. In Guyton AC and Hall JE (eds): Textbook of Medical Physiology, 11 th ed. Philadelphia, Elsevier, 2006.

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