Page 331 - Color Atlas of Biochemistry
P. 331
322 Tissues and organs
Kidney: functions transported back into the blood by resorption,
to prevent losses of valuable metabolites and
A. Functions of the kidneys electrolytes. In the proximal tubule, organic
metabolites (e. g., glucose and other sugars,
The kidneys’ main function is excretion of
water and water-soluble substances (1). This amino acids, lactate, and ketone bodies) are
is closely associated with their role in regulat- recovered by secondary active transport (see
ing the body’s electrolyte and acid–base bal- p. 220). There are several group-specific
ance (homeostasis, 2; see pp. 326 and 328). transport systems for resorbing amino acids,
Both excretion and homeostasis are subject to with which hereditary diseases can be
hormonal control. The kidneys are also in- associated (e. g., cystinuria, glycinuria, and
–
+
volved in synthesizing several hormones (3; Hartnup’s disease). HCO 3 ,Na ,phophate,
see p. 315). Finally, the kidneys also play a role and sulfate are also resorbed by ATP-depend-
in the intermediary metabolism (4), particu- ent (active) mechanisms in the proximal tu-
larly in amino acid degradation and gluconeo- bule. The later sections of the nephron mainly
genesis (see p. 154). serve for additional water recovery and regu-
–
+
The kidneys are extremely well-perfused lated resorption of Na and Cl (see pp. 326,
organs, with about 1500 L of blood flowing 328). These processes are controlled by hor-
through them every day. Approximately 180 L mones (aldosterone, vasopressin).
of primary urine is filtered out of this. Re- Secretion. Some excretable substances are
moval of water leads to extreme concentra- released into the urine by active transport in
+
tion of the primary urine (to approximately the renal tubules. These substances include H
+
one-hundredth of the initial volume). As a and K ions, urea, and creatinine, as well as
drugssuch aspenicillin.
result, only a volume of 0.5–2.0 L of final
urine is excreted per day. Clearance. Renalclearanceisusedasaquan-
titative measure of renal function. It is defined
as the plasma volume cleared of a given sub-
B. Urine formation
stance per unit of time. Inulin, a fructose poly-
The functional unit of the kidney is the neph- saccharide with a mass of ca. 6 kDa (see p. 40)
ron. It is made up of the Malpighian bodies or that is neither actively excreted nor resorbed
renal corpuscles (consisting of Bowman’s cap- butisfreely filtered, hasa clearanceof
sules and the glomerulus), the proximal tu- 120mL min –1 in healthy individuals.
bule, Henle’s loop, and the distal tubule,
which passes into a collecting duct. The hu- Further information
man kidney contains around one million
nephrons. The nephrons form urine in the Concentrating urine and transporting it
following three phases. through membranes are processes that re-
Ultrafiltration. Ultrafiltration of the blood quire large amounts of energy. The kidneys
plasma in the glomerulus gives rise to primary therefore have very high energy demands. In
urine, which is isotonicwithplasma.The pores the proximal tubule, the ATP needed is ob-
in the glomerular basal membrane, which are tained from oxidative metabolism of fatty
made up of type IV collagen (see p. 344), have acids, ketone bodies, and several amino acids.
an effective mean diameter of 2.9 nm. This al- To a lesser extent, lactate, glycerol, and citric
lows all plasma components with a molecular acid arealso used. In thedistaltubuleand
mass of up to about 15 kDa to pass through Henle’s loop, glucose is the main substrate
unhindered. At increasing masses, molecules for the energy metabolism. The endothelial
are progressively held back; at masses greater cells in the proximal tubule are also capable
than 65 kDa, they are completely unable to of gluconeogenesis. The substrates for this are
enter the primary urine. This applies to almost mainly the carbohydrate skeletons of amino
all plasma proteins—which in addition, being acids. Their amino groups are used as ammo-
anions, are repelled by the negative charge in nia for buffering urine (see p. 311). Enzymes
the basal membrane. for peptide degradation and the amino acid
Resorption. All low-molecular weight metabolism occur in the kidneys at high lev-
plasma components enter the primary urine els of activity (e. g., amino acid oxidases,
via glomerular filtration. Most of these are amine oxidases, glutaminase).
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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