Flow Through The Kidney - Biofluidmechanics: An Introduction to Fluid Mechanics Macrocirculation and Microcirculation - page 314

is ingested versus what is excreted from the body (e.g., after a person eats five bananas
there will be a large excess of potassium ions within the body, and this excess must be
removed by the kidneys). The first function occurs through a filtration of the blood plasma.
The second function occurs through reabsorption of molecules from the filtrate at a rate
that is dependent on the need of the body combined with an added secretion of unneces-
sary compounds. Following the banana example, when potassium ions are in excess, the
reabsorption of potassium will be low; however, if the potassium ion concentration is
depleted within the blood plasma, then the reabsorption of potassium will be higher.
The kidneys are located along the posterior wall of the abdomen and are approximately
the size of a clenched fist. Each kidney is indented at its interior medial location, and this
location is termed the hilum ( Figure 12.1 ). At the hilum, the renal artery and the renal
nerve enter the kidney and the renal vein, lymphatic vessels, and the ureter leave the kid-
ney. The kidney is then divided into two main sections: the outer cortex and the medulla.
The outer cortex is the location where plasma is filtered, while reabsorption and excretion
occurs within the medulla. Each renal artery enters the kidney at the hilum and branches
to form smaller arteries until they penetrate into the entire outer cortex of the kidney. At
this point, the vessels are the size of arterioles. The smallest arterial blood vessel within
the kidney circulation is termed the afferent arteriole, which supplies each nephron (the
functional unit of the kidney) with blood. The afferent arterioles branch into the glomeru-
lar capillaries, which then converge to an efferent arteriole. A second capillary network,
termed the peritubular capillaries, surround the nephron and then converge to venules.
These venules converge to larger veins and eventually the renal vein, which exits the kid-
ney at the hilum. Interestingly, slightly more than 20% of the cardiac output is directed to
both kidneys under normal resting conditions.
A unique characteristic of the renal circulation is that it has two capillary beds, which
are separated by an arteriole. Interestingly, the efferent arteriole primarily regulates the
pressure in both capillary beds and hence the flow rate throughout each capillary network.
The afferent arteriole, to a lesser extent, regulates the pressure and flow throughout both
capillary networks. Unlike the systemic and respiratory circulatory systems, the hydro-
static pressure within the glomerular capillaries is very high, around 60 mmHg. This
allows for a rapid filtration of blood, because there is a high hydrostatic pressure gradient
between the blood and the filtrate in the nephrons. The pressure within the peritubular
capillaries is much lower, close to 15 mmHg, which allows for rapid absorption of any

FIGURE 12.1 Anatomical location (posterior view)

of the kidneys, showing their location in relation to
other structures in the abdomen. The kidneys are pro-
tected by the eleventh and twelfth ribs (both shown).
Urine formed in the kidneys passes through the left
and right ureter into the bladder. As the bladder fills,
urine can be removed from the body via the urethra.
Adapted from Martini and Nath (2009).

Left kidney

Right kidney

Abdominal
aorta

Ureter
Inferior vena
cava

Bladder
Urethra