St. George's University Lecture Notes: Basic Principles of Medicine 1 - PDF

# Basic Principles of Medicine 1 ## Module: Cardiovascular, Pulmonary, Renal ### Lecture No: 46 ## Distribution of Body Fluids - **Dr. Noah Leton** - **[email protected]** - **St. George’s University** ## Copyright - All year 1 courses materials, whether in print or online, are protected by copyri...

# Basic Principles of Medicine 1 ## Module: Cardiovascular, Pulmonary, Renal ### Lecture No: 46 ## Distribution of Body Fluids - **Dr. Noah Leton** - **[email protected]** - **St. George’s University** ## Copyright - All year 1 courses materials, whether in print or online, are protected by copyright. - The work, or parts of it, may not be copied, distributed or published in any form, printed, electronic or otherwise. - As an exception, students enrolled in year 1 of St. George's University School of Medicine and their faculty are permitted to make electronic or print copies of all downloadable files for personal and classroom use only, provided that no alterations to the documents are made and that the copyright statement is maintained in all copies. - View only files, such as lecture recordings, are explicitly excluded from download and creating copies of these recordings by students and other users are strictly illegal. - The author of this document has made the best effort to observe current copyright law and the copyright policy of St George's University. - Users of this document identifying potential violations of these regulations are asked to bring their concern to the attention of the author. ## Learning Objectives - **SOM.MK.II.BPM1.3.CPR.2.PHYS.0293.** List major routes and normal ranges for water intake and loss. - **SOM.MK.II.BPM1.3.CPR.2.PHYS.0294.** Given the body weight estimate the a) total body water, b) extracellular fluid volume, c) intracellular fluid volume, d) interstitial fluid volume, e) plasma volume, and f) blood volume. - **SOM.MK.II.BPM1.3.CPR.2.PHYS.0295.** Demonstrate the ability to use the indicator dilution principle to measure plasma volume, blood volume, extracellular fluid volume, and total body water, and identify compounds used to measure each volume. - **SOM.MK.II.BPM1.3.CPR.2.PHYS.0296.** Explain the water movement between intracellular and extracellular fluid compartments caused by increases or decreases in extracellular fluid osmolality. ## Recommended Reading - Medical Physiology: Principles for Clinical Medicine with Companion Website Access, R. Rhoades & D.Bell. 6th ed., 2023; Chapter 23 ## Water Gain and Water Loss | **Water Gain** | **Water Loss** | |---|---| | Liquid consumption 1-2 L | Kidneys 1-2 L | | Water in food 0.8-1 L | Skin 0.6 L | | Metabolic water 0.3-0.4 L | Lungs 0.3 L | | | Fecal loss 0.2 L | - The intake of water is controlled via thirst mechanisms, whereas the removal of water is regulated mainly by the kidneys. - The excretion of water by the kidneys is proportional to the volume of water in the body. - Antidiuretic hormone (ADH) controls the excretion of water by the kidneys. ## Body Fluid Distribution - Total body water (TBW) for females is 55% of body weight, and males 60% of body weight. - The intracellular fluid (ICF) makes up 55% of TBW and extracellular fluid (ECF) 45%. - TBW is less in females compared to males, due to the presence of more subcutaneous fat, and less muscle mass. - TBW is less in obese individuals. - TBW also decreases with age. | **Adult Female** | **Adult Male** | |---|---| | 45% Solids | 40% Solids | | 55% TBW | 60% TBW | | | TBW 55% ICF | | | 45% ECF | ## Fluid Spaces of the ECF - The 45% ECF is derived from, interstitial fluid (ISF) 20%, plasma 8%, and fluid in bone, cartilage, connective tissue and transcellular fluid – 17%. - ICF plus ISF and plasma are rapidly interchangeable, while fluid in bone, cartilage, etc are poor accessible. - As a result, swift changes in the distribution of water occur between the ICF and rapidly interchangeable ECF (plasma and ISF) making up 83% of TBW (55 +20 + 8). - Therefore, ICF = 2/3rds TBW; rapid ECF = 1/3rd TBW | **55% ICF** | **45% ECF** | |---|---| | | 20% ISF | | | 8% Plasma | | | 17% Bone, cartilage, connective tissue | ## Example 1 - In a 70 kg man with a hemotocrit of 45%, determine the TBW, ICF, ECF, ISF, plasma volume(PV) and blood volume(BV) - TBW = 60% of body weight = 0.6 x 70 = 42 L - ICF = 55% of TBW = 0.55 x 42 = 23.1 L - ECF = 45% of TBW = 0.45 x 42 = 18.9 L - ISF = 20% of TBW = 0.20 x 42 = 8.4 L - PV = 8% of TBW = 0.08 x 42 = 3.4 L - BV = 3.4/1-0.45 = 3.4/0.55 = 6.2 L ## Fluid Compartment Volume Measurement - An indicator-dilution method is utilized. - In this case, indicators can be used to estimate the volumes of various compartments. | **TBW** | **ECF** | **Plasma** | |---|---|---| | Isotopic water (heavy water, D₂O) | Radiactive sodium Sulfate | Radioacitve albumin | | Tritiated water (HTO) | Mannitol | Evans blue | | Antipyrine | Inulin | | - For ICF and ISF, estimation is done indirectly: - ICF = TBW - ECF - ISF = ECF - PV ## Compartment Volume Formula - After administration of a known amount of the indicator (dye), and allowing adequate time for equilibration, a plasma sample is collected and the palsma concentration determined. - The compartment volume distribution can then be calculated using the formula: - $V = \frac{Q}{C}$ - Where: - V = Compartment volume - Q = Amount injected – amount excreted - C = Concentration of the indicator in plasma after equlibration ## Example 2 - A 60 kg man is administered 100 mL of D20 of which 10% is excreted. After equilibration, the plasma concentration is 2.5 mL/L. - What body fluid compartment volume can be calculated? - Calculate the compartment volume. - **Answer:** - 1. TBW - 2. V = Q/C = 100-10/2.3 = 90 mL/2.5 mL/L = 36 L - A TBW of 36L in a 60 kg man implies that TBW is 60% of his body weight (36/60 x 100 = 60%) ## Example 3 - Following the adminstration of 5 mCi of (radioactive) 131|-albumin to a patient, 0.5% is metabolized and excreted. After equilibration, plasma concentration of the marker is 2 mCi. - Which fluid compartment can be measured? - Calculate the compartment volume. - **Answer:** - 1. Plasma volume - 2. V = Q/C = 5-0.025 / 2 (0.5% of 5 is 0.025) = 4.475/2 = 2.5 L ## The Darrow-Yannet Diagram - The Darrow-Yannet diagram is a schematic that allows for the visualization of volume and osmolality changes in the ICF and ECF. - Volume changes affect the ECF volume first. - ICF volume changes only when there is a change in ECF osmolality. - The osmolality of the ICF and ECF are equal due to osmotic equilibration. ## Volume and Osmolality Changes in Fluid Compartments - Volume changes take the form of expansion or contraction (reduction) in fluid compartments. - These changes can be isosmotic, hyperosmotic, or hyposmotic | **Type** | **Volume Expansion** | **Volume Contraction** | |---|---|---| | **Isosmotic** | Isosmotic volume expansion | Isosmotic volume contraction | | **Hyperosmotic** | Hyperosmotic volume expansion | Hyperosmotic volume contraction | | **Hyposmotic** | Hyposmotic volume expansion | Hyposmotic volume contraction | - Plasma osmolality = 2 x Na (mEq/L) + glucose (mg/dL) /18 + BUN (mg/dL)/2.8 ## Isosmotic Volume Expansion - Expansion of ECF volume by addition of isotonic saline to it. - Example: the administration of 0.9% NaCl solution (isotonic saline) to the ECF results in the expansion of ECF volume only. - The osmolality does not change since this solution has the same osmolality as normal body fluids. ## Hyperosmotic Volume Expansion - Expansion of ECF volume by addition of hypertonic saline to it. - Example: High salt intake or hypertonic saline (3% NaCl) leads to an increase in ECF osmolality. - Water moves from the ICF to the ECF, resulting in the expansion of ECF volume. ## Hyposmotic Volume Expansion - Expansion of ECF volume by addition of hypotonic fluid to it. - Example: Ingestion of large amounts of water or SIADH (increased reabsorption of water) increases the volume of water in the ECF while decreasing osmolality. - Water movement into the ICF also increases ICF volume and decreases osmolality. ## Isosmotic Volume Contraction - Reduction in ECF volume by loss of isosmotic fluid from it. - Acute blood loss, vomiting, and diarrhea (certain types), are examples of conditions that lead to reduced fluid volume. - The osmolality stays the same because isotonic fluid is lost. ## Hyperosmotic Volume Contraction - Reduction in ECF volume by loss of hypotonic fluid from it. - Water deprivation and severe sweating are examples of conditions that can cause hyperosmotic volume contraction. - The loss of hypotonic fluids leads to an increase in the ECF osmolality. This results in the movement of water from the ICF to the ECF to equilibrate both compartments. ## Hyposmotic Volume Contraction - Reduction in ECF volume with a decrease in sodium concentration. - The lack of aldosterone in adrenal insufficiency can lead to hyposmotic volume contraction. - The reduced reabsorption of sodium leads to a decrease in ECF osmolality. Water then moves from the ECF to the ICF. ## Summary - Volume and Osmolality Changes - **Volume Expansion** - **Isotonic Saline:** Osmolality (mmos/kg H₂O) 285 ICF ECF 0 28 42 Volume (L) - **Volume Contraction** - **Hemorrhage:** Osmolality (mmos/kg H₂O) 285 ICF ECF 0 28 42 Volume (L) ## Intravenous Fluids | **Type** | **Fluid** | |---|---| | **Isotonic** | 0.9% NaCl, Ringers solution | | **Hypotonic** | 0.45% NaCl, 5% dextrose | | **Hypertonic** | 5% dextrose in 0.9% NaCl, 3% NaCl |