BASIC FUNCTIONS AND HOMEOSTASIS
Blood enters the
kidneys in a series of branching vessels that
give rise to afferent arterioles. Each afferent arteriole leads to a tuft of
glomerular capillaries. Plasma and small, non–protein bound solutes are filtered
across the walls of the glomerular capillaries into Bowman’s space, the initial
portion of the nephron. From there, the filtrate is conveyed through the
remaining segments of the nephron which
include the proximal tubule, thin limb, distal tubule, and collecting duct before being excreted in the final urine.
In the various segments of the renal tubules, there is extensive exchange
of material with the surrounding capillaries. Such exchange is known as
“reabsorption” if materials are transferred from the tubular lumen to the
capillaries and/or interstitium, and as “secretion” if they are transferred in
the opposite direction.
By continuously adjusting the contents of blood, the kidneys make
critical contributions to the maintenance of fluid and salt homeostasis, as well
as to the excretion of unwanted chemicals and waste products. In addition, the
kidneys contribute to the regulation of arterial pressure, acid-base status,
erythropoiesis, and vitamin D synthesis.
Mechanisms Of Homeostasis
To maintain homeostasis, the kidneys must adjust their retention or
excretion of fluid and filtered solutes so that, in cooperation with other
excretory organs (lungs, skin, bowel), overall output equals intake.
Water, for example, accounts for approximately 60% of total body weight.
Approximately two thirds of this volume is intracellular, whereas the remaining
third is extracellular. Each day, the average individual consumes approximately
2000 to 2500 mL of water, while carbohydrate oxidation produces another 200 to
300 mL of water. At baseline, these input volumes must be offset by an equal
amount of output. On an average day, the kidneys excrete approximately 1500 mL
of water, while sweat and feces each contain approximately 100 mL of water. The
remaining water is insensibly lost through the skin and lungs.
During significant physical exertion, a greater amount of water is lost as
sweat and insensible losses. As a result, the relative amount of fluid excreted
as urine decreases. Likewise, a person who is severely dehydrated needs to
produce far less urine than a person who consumes a large volume of water. A
reduction in urine volume could be effected by reducing the rate of plasma
filtration from the glomerular capillaries into nephrons; however, this would be
an impractical response because the kidneys would consequently be unable to
excrete other unwanted substances. Instead, the kidneys continue to filter a
large amount of plasma, but they increase the rate of fluid reabsorption from
the tubules so that the final urine volume remains low.
The same basic mechanism applies to solutes, such as potassium, calcium,
and other salts, the concentrations of which are maintained in very narrow
ranges in the extracellular and
intracellular spaces. The kidneys filter
these solutes at a largely constant rate, but they alter their rate of excretion based on input from homeostatic
sensor mechanisms. Solute excretion can be adjusted by altering the rate of
either reabsorption or secretion. Many substances are reabsorbed or secreted
using active, transcellular mechanisms that can be very finely tuned.
The signals that modulate these processes differ depending on the
substance in question. For example, aldosterone
is released in response to elevated extracellular potassium levels and promotes
increased potassium secretion (see Plate 3-10). In contrast, parathyroid hormone
(PTH) is released in response to decreased calcium levels and promotes a net
increase in calcium reabsorption (see Plate 3-11). The details of these
homeostatic mechanisms, as well as some of the complications that occur when
they are are discussed in detail later in this section.
