Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 330

crystallized and therefore are no longer in solution. As the solid crystals pass through the
kidney tubule system, they may become lodged in a region or simply form in the larger
tubules. When this happens, urine cannot be processed and the calculi must be removed
by natural means (e.g., passing a kidney stone) or through the use of a laser to break apart
the salt deposits. Approximately 75% of calculi are formed from calcium because there is a
high daily calcium intake that cannot be handled by the kidney when it also experiences a
high-salt environment. The easiest treatment for calculi is to decrease the intake of salts,
which will reduce the load on the kidney. However, once a stone has formed, it must pass
through the urinary system or be removed by some other means.

END OF CHAPTER SUMMARY
12.1 The kidneys remove metabolic waste products and foreign particles from the body, as well
as maintain the water volume and the concentration of various ions within the body.
Urine formed within the kidneys passes through the ureters into the bladder, where it is
held until it is excreted from the body. Blood is supplied to the kidneys via the renal arter-
ies. Interestingly, the renal circulation is composed of two capillary beds that are separated
by an arteriole. The efferent arteriole predominantly regulates the pressure in both arter-
ioles. The first set of capillaries form the glomerulus, which is the site for blood filtration.
The second set of capillaries, the peritubular capillaries, is involved in the reabsorption and
secretion of compounds. The juxtaglomerular apparatus forms a direct feedback mechanism
within each nephron to control the filtration rate based on the concentration of sodium ions
within the nephron.
12.2 Glomerular filtration is the process by which compounds are removed from the blood and
enter the nephron. Filtration is a bulk flow process which is dependent on the hydrostatic
pressure within the glomerulus, the hydrostatic pressure within Bowman's capsule, and the
osmotic pressure within the glomerulus. Under normal conditions, the total blood volume
is filtered approximately 60 times per day. By knowing the plasma concentration of a com-
pound, it is possible to calculate the nephron's filtered load for that ion.
12.3 Tubule reabsorption is a process by which compounds within the filtrate can be brought back
into the blood. Secretion is a process by which compounds that were not filtered can be
brought into the nephron. Most compounds are reabsorbed or secreted by both passive
and active transport mechanisms. The reabsorption of many molecules is coupled to sodium
movement within the nephron. The amount of any substance that is secreted can be calculated
from

Amount Excreted Amount Filtered Amount Secreted Amount Reabsorbed

5 1 2

12.4 Sodium reabsorption is regulated by the amount and type of channels present within the

luminal surface of the nephron epithelial cells. Sodium movement into the epithelial cells is
coupled to glucose or hydrogen movement. Sodium movement out of the epithelial cells is
regulated by the sodium-potassium ATPase. The balance between sodium reabsorption/
secretion is coupled to both the juxtaglomerular apparatus and the renin-angiotensin sys-
tem. Water movement out of the nephron is either by bulk diffusion or is facilitated through
the presence of aquaporin channels.