Fluids in the Body Notes

Contents

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Fluids in the Body essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Fluids in the Body:

Body fluid compartments

Movement of water between body compartments

Renal clearance

NOTES NOTES FLUIDS IN THE BODY BODY FLUID COMPARTMENTS osms.it/body-fluid-compartments GENERAL CHARACTERISTICS ▪ Fluid divisions in body ▫ Includes intracellular fluid, extracellular fluid ▪ “60-40-20 rule” ▫ Total body water is 60% of body weight, of which two thirds is intracellular → total intracellular fluid is 40% of body weight, total extracellular fluid is 20% of body weight ▪ Due to macroscopic electroneutrality principle, fluid compartments have same concentration of positive charges as negative charges INTRACELLULAR & EXTRACELLULAR FLUID ▪ Large difference between intracellular fluid and extracellular fluid (e.g. Na+K+ ATPases establish high concentration of K+ inside cell and high concentration of Na+ outside cell) Intracellular fluid ▪ Dissolves cations (esp. K+ and Mg2+) and anions (esp. proteins and organic phosphates e.g. ATP) Extracellular fluid ▪ Includes interstitial fluid (around cells) and plasma (aqueous part of blood, containing about 10% proteins e.g. albumin) ▫ Both dissolve cations (esp. Na+) and anions (esp. Cl- and HCO3-) ▫ Solutes and water travel between the interstitial fluid and plasma through pores in endothelial cells of capillaries ▫ Negative plasma proteins are too big to travel through pores; electroneutrality is maintained by repelling small anions into interstitial fluid and attracting small cations into plasma (Gibbs–Donnan effect) → interstitial fluid has ↑ small anion concentration (e.g. Cl-) and ↓ small cation concentration (e.g. Na+) VOLUMES OF BODY FLUID COMPARTMENTS ▪ Determined by administering and measuring concentration of substances that are known to settle in specific compartments (dilution method) ▫ Radiolabeled albumin for plasma (cannot pass into interstitial fluid) ▫ Smaller molecules like mannitol and inulin for interstitial fluid (cannot pass through cell membranes) ▫ Heavy water (D2O) for total body water (knowing this and above, intracellular fluid can be calculated too) ▫ Measuring concentration of these substances in their respective body fluid compartments allows us to calculate AmountGiven volume (= Concentration ) ▫ To account for loss of these substances in urine, subtract amount lost from amount given and use this value in formula OSMOSIS.ORG 523
Figure 59.1 A sample problem demonstrating how to solve for total body water, extracellular fluid, and intracellular fluid volumes using information gained from D2O and mannitol. 524 OSMOSIS.ORG
Chapter 59 Renal Physiology: Fluids in the Body WATER SHIFTS BETWEEN BODY FLUID COMPARTMENTS osms.it/water-shifts-between-body-fluid-compartments Key features ▪ Movement of water between body fluid compartments to maintain constant osmolarity ▪ Shifts are characterized by change in volume and concentration of extracellular fluid ▫ ECF volume: ↑ = expansion; ↓ = contraction ▫ ECF osmolarity: ↑ = hyperosmotic; ↓ = hyposmotic; no change = isosmotic ▪ Six possible combinations VOLUME CONTRACTION Isosmotic volume contraction ▪ Loss of isosmotic fluid from ECF ▪ Volume ↓ but osmolarity is constant → no water shift ▪ ↓ plasma volume and arterial pressure; ↑ plasma protein concentration and hematocrit ▪ E.g. diarrhea Hyperosmotic volume contraction ▪ Loss of hyposmotic fluid from ECF ▪ Volume ↓ and osmolarity ↑ → water shifts from ICF (net effect is still volume contraction) ▪ ↓ plasma volume and arterial pressure; ↑ plasma protein concentration but hematocrit is unchanged (since red blood cells lose volume too) ▪ E.g. heavy sweating (sweat is hyposmotic relative to ECF) ▪ E.g. adrenal insufficiency (deficiency in several hormones, including aldosterone). Aldosterone important for sodium reabsorption from kidneys; ↓ aldosterone = ↑ sodium loss in urine VOLUME EXPANSION Isosmotic volume expansion ▪ Gain of isosmotic fluid in ECF ▪ Volume ↑ but osmolarity is constant → no water shift ▪ ↑ plasma volume and arterial pressure; ↓ plasma protein concentration and hematocrit ▪ E.g. receiving an infusion of isotonic NaCl solution Hyperosmotic volume expansion ▪ Gain of solutes or hyperosmotic fluid in ECF ▪ Volume ↑ and osmolarity ↑ → water shifts from ICF ▪ ↑ plasma volume and arterial pressure; ↓ plasma protein concentration and hematocrit ▪ E.g. eating salty chips Hyposmotic volume expansion ▪ Gain of hyposmotic fluid in ECF ▪ Volume ↑ and osmolarity ↓ → water shifts to ICF (net effect is still volume expansion) ▪ ↑ plasma volume and arterial pressure; ↓ plasma protein concentration but hematocrit is unchanged ▪ E.g. too much antidiuretic hormone causing excessive water reabsorption Hyposmotic volume contraction ▪ Loss of solutes/hyperosmotic fluid from ECF ▪ Volume ↓ and osmolarity ↓ → water shifts to ICF ▪ ↓ plasma volume and arterial pressure; ↑ plasma protein concentration and hematocrit OSMOSIS.ORG 525
Figure 59.2 Visualization of the types of volume contraction. 526 OSMOSIS.ORG
Chapter 59 Renal Physiology: Fluids in the Body Figure 59.3 Visualization of the types of volume expansion. OSMOSIS.ORG 527
RENAL CLEARANCE osms.it/renal-clearance ▪ Rate at which kidneys clear blood plasma of substance ▪ For substance “x”, renal clearance C= [U ]x ×V [ P]x ▫ [U]x: urine concentration of x ▫ [P]x: plasma concentration of x ▫ V: urine flow rate ▪ To measure reabsorption/secretion of substance in kidneys, inulin can be used as reference point ▫ Inulin is freely filtered ▫ Inulin is not reabsorbed/secreted ▪ Clearance ratio for substance x is Cx Cinulin ▫ = 1 → x is freely filtered, not secreted ▫ > 1 → x is freely filtered, secreted ▫ < 1 → x is not freely filtered/is reabsorbed 528 OSMOSIS.ORG ▪ Free water clearance is renal clearance of pure water CH O = V − 2 U osm Posm V ▫ Uosm: urine osmolarity ▫ Posm: plasma osmolarity
Chapter 59 Renal Physiology: Fluids in the Body Figure 59.4 Sample questions solving for renal clearance of a solute and free water clearance. OSMOSIS.ORG 529

Osmosis High-Yield Notes

This Osmosis High-Yield Note provides an overview of Fluids in the Body essentials. All Osmosis Notes are clearly laid-out and contain striking images, tables, and diagrams to help visual learners understand complex topics quickly and efficiently. Find more information about Fluids in the Body by visiting the associated Learn Page.