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The absence of proteins in the glomerular space (the lumen within the glomerular capsule) results in a capsular osmotic pressure near zero.

This pressure acting to draw water into the glomerulus is called blood colloid osmotic pressure. Most proteins cannot pass into the filtrate resulting in water’s movement out of the capsule towards the glomerulus. The concentration of plasma solutes in the glomerulus is greater than the concentration of the filtrate in the glomerular capsule since the filtration membrane limits the size of particles crossing the membrane. Water moves across a membrane from areas of high water concentration (low dissolved solute concentration) to areas of low water concentration (high dissolved solute concentration) through the process of osmosis. However, the concentration of the solutes in the fluids affects net movement of fluid as well. Net fluid movement will be in the direction of the lower pressure. These fluids exert pressures in opposing directions. This is the capsular hydrostatic pressure. The fluid in the glomerular capsule creates pressure pushing fluid out of the glomerular capsule back into the glomerulus, opposing the glomerular hydrostatic pressure. The blood inside the glomerulus creates glomerular hydrostatic pressure which forces fluid out of the glomerulus into the glomerular capsule. Hydrostatic pressure is the pressure produced by a fluid against a surface. Recall that filtration occurs as pressure forces fluid and solutes through a semipermeable barrier with the solute movement constrained by particle size. GFR is influenced by multiple factors, like those seen at tissue capillary beds (see chapter 19). However, 99% of this filtrate is returned to the circulation through reabsorption resulting in only about 1–2 liters of urine per day. This amount equates to a volume of about 180 L/day in men and 150 L/day in women. The work of the kidneys produces about 125 mL/min filtrate in men (range of 90 to 140 mL/min) and 105 mL/min filtrate in women (range of 80 to 125 mL/min). Approximately 20% of your cardiac output is filtered by your kidneys per minute under resting conditions. The volume of filtrate formed by both kidneys per minute is termed glomerular filtration rate (GFR). Recall that the filtration membrane lies between the blood in the glomerulus and the filtrate in the Bowman’s (glomerular) capsule and this filtration membrane is highly fenestrated allowing the passage of small molecules such as water, sodium, glucose, etc. Glomerular filtration is a passive process as cellular energy is not used at the filtration membrane to produce filtrate.