Body Fluid Compartments

Kidneys = homeostasis of volume, pressure, composition of blood

Ins and Outs
  
60:40:20 rule   60% of body weight = total water; 40% intracellular: 
20% extracellular (= 15% interstitial fluid + 5% plasma)

Kidneys - 1 million nephrons per kidney

glomerulus - has two arterioles in series; afferent and efferent arterioles

Kidney functions

• filtration
• secretion
• reabsorption

• endocrine functions
• renin (enzyme) - long term control of blood volume/pressure
• EPO - stimulated by Hypoxia-inducible factor (HIF-1)
• Calcitriol - active form of Vitamin D

Regulation of Body Fluid Levels

• water moves by osmosis between inside and outside of cells - sodium is the most important "osmolyte"
• water moves by Starling forces between inside and outside of capillaries - this includes osmosis due to colloid osmotic pressure difference

Calculating the volume of fluid compartments

Uses definitions of concentration (C), amount (A), and volume (V)

C = A/V  A = CV   V = A/C     for substances trapped in a compartment; i.e., Amount in = Amount after dilution.   Amount in = VinCin  =  Amount after = VaCa

Va = ViCi/Ca     
Example:  1 mg of Evans blue dye is dissolved in 1 ml saline and injected into a patient's vein.  After 10 minutes, a sample of the patient's blood is taken and the Evans blue concentration if found to be 0.1 mg/L plasma.  
Va = ViCi/Ca  = 1mg = 0.1 mg/L = 10 L

Plasma osmolality calculated from sodium, glucose, and BUN  (2 x Na is good approximation)

normal values:  Na = 140; glucose = 100; BUN = 20.

osmolality = 2x140 + 100/18 +20/2.8 = 280 + 5.5 + 7.1 = 292.6