Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 318

from plasma, but enter the tubule system of the nephron. It is important to note that for
each substance in the blood (except for cells and proteins), there is a particular combina-
tion of filtration, tubule reabsorption, and tubule secretion that occurs to remove the com-
pound from the body or maintain the compound in the body ( Figure 12.4 ).
The glomerulus acts as a filtrate for all of the substances within plasma. The glomerular
filtrate contains all of the compounds in plasma, except for proteins, at nearly the same
concentration as found in plasma. Glomerular filtration is a bulk flow process, where
water and all of the dissolved components can move freely between the vascular space
and Bowman's capsule. Proteins do not move with this filtrate process because most of
the pores within the glomerular capillaries are negatively charged and the majority of
plasma proteins also have a partial negative charge. Also, most high molecular weight
compounds cannot move through the pore space themselves due to a size restriction. As
stated above, the majority of low molecular weight compounds can move freely into
Bowman's capsule except for those molecules that are associated with plasma proteins.
For instance, the majority of plasma calcium ions is associated with plasma proteins and is
therefore not filtered by the glomerulus. Calcium ions can be actively secreted into the
nephron, if necessary.
The glomerular filtrate is formed as a balance between the hydrostatic pressure within
the glomerular capillary, the hydrostatic pressure within Bowman's capsule, and the
osmotic pressure within the glomerular capillary. The glomerular capillary hydrostatic
pressure favors filtration while the hydrostatic pressure of Bowman's capsule and the
osmotic pressure of the glomerular capillaries resist filtration. Under normal conditions,
the glomerular capillary hydrostatic pressure is approximately 60 mmHg, the Bowman's
space hydrostatic pressure is approximately 15 mmHg, and the capillary osmotic pressure
is approximately 30 mmHg; therefore, there is a net movement of fluid from the glomeru-
lar capillaries into Bowman's capsule. Under normal conditions, the net filtration pressure
(approximately 15 mmHg into Bowman's capsule) is always positive, and this initiates
urine formation by forcing the filtrate into and through the tubule system.

Glomerulus

Bowman's
capsule

Urine Urine

Urine

FIGURE 12.4 Three possible scenarios for glomerular filtration, tubule secretion, and tubule reabsorption. In

the first case, a small amount of substance is filtered and a large amount is secreted, so that there is a minimal
concentration left in the peritubular capillaries. In the second scenario, some of the substance is filtered and reab-
sorbed. In the last case, all of the filtered substance is reabsorbed. Urea would fall under the first case (although it
is not completely secreted), bicarbonate would be an example for the second case, and glucose would be an exam-
ple for the third case. Adapted from Widmaier, Raff, and Strang (2007).