Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 317

Afferent arteriole

Juxtaglomerular cells

Efferent arteriole

Macula densa

FIGURE 12.3

Anatomical arrangement of the juxtaglomerular apparatus, which can monitor and regulate the
glomerular filtration rate, through the production of renin. The thick ascending Loop of Henle passes between the
afferent and efferent arterioles of the same nephron to form the juxtaglomerular apparatus. Depending on the
concentration of sodium ions and chloride ions, the macula densa cells can either increase or decrease the produc-
tion of renin. Adapted from Martini and Nath (2009).

arteriole specialized cells are termed the juxtaglomerular cells. This location provides a
feedback mechanism within each nephron to control the filtration rate of the glomerular
capillaries of that nephron. The exact details of the feedback mechanism are unknown, but
what is clear is that when the flow decreases within the Loop of Henle, there is an
increased reabsorption of sodium and chloride ions into the peritubular capillaries (more
details regarding reabsorption can be found in Section 12.3 ). An increased reabsorption of
these ions decreases the concentration of these ions within the plasma filtrate passing
through the ascending Loop of Henle. The macula densa cells are sensitive to the sodium
and chloride concentrations, and when either concentration reduces, a signal is sent from
the macula densa that has two effects on nephron function. The first effect is that the resis-
tance of the afferent arteriole is reduced (e.g., the diameter is increased), thereby increas-
ing the amount of filtrate within the tubule system. The second effect is the production of
renin from the juxtaglomerular cells. Renin is an enzyme that causes an increased produc-
tion of angiotensin I, which is rapidly converted to angiotensin II in the kidney.
Angiotensin II constricts the efferent arteriole, which decreases glomerular capillary out-
flow. Again, the net effect of this is to increase the rate of glomerular filtration. If both of
these processes are functioning properly, the glomerular filtration rate changes marginally
with large changes in systemic arterial pressure, and therefore, the kidneys continue to
produce urine at a normal rate. Molecular concentrations, which partially dictate osmotic
pressure, are the more important contributor to glomerular filtration rate.

12.2 GLOMERULAR FILTRATION

The basic function of the nephron can be divided into three processes: filtration, reab-
sorption, and secretion. Filtration occurs within the glomerulus and will be the focus of
this section. Reabsorption and secretion occur within the tubule section of the nephron
and will be discussed in Section 12.3 . Filtration is the process where some components of
plasma enter the nephron. Reabsorption is when compounds are removed from the filtrate
and re-enter the peritubular capillaries. Secretion is when compounds are again removed