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:

Body fluid compartments

Movement of water between body compartments

Renal clearance