plasma, dissolved materials, and small proteins into
Bowman’s capsules; the fluid is now called renal
filtrate.- Tubular reabsorption is selective in terms of useful-
ness. Nutrients such as glucose, amino acids, and
vitamins are reabsorbed by active transport and
may have renal threshold levels. Positive ions are
reabsorbed by active transport and negative
ions are reabsorbed most often by passive trans-
port. Water is reabsorbed by osmosis, and small
proteins are reabsorbed by pinocytosis.
Reabsorption takes place from the filtrate in the
renal tubules to the blood in the peritubular capil-
laries. - Tubular secretion takes place from the blood in the
peritubular capillaries to the filtrate in the renal
tubules and can ensure that wastes such as creati-
nine or excess Hions are actively put into the fil-
trate to be excreted. - Hormones such as aldosterone, ANP, and ADH
influence the reabsorption of water and help main-
tain normal blood volume and blood pressure. The
secretion of ADH determines whether a concen-
trated or dilute urine will be formed. - Waste products remain in the renal filtrate and are
excreted in urine. The effects of hormones on the
kidneys are summarized in Table 18–1 and illus-
trated in Fig. 18–5.
THE KIDNEYS AND
ACID–BASE BALANCE
The kidneys are the organs most responsible for main-
taining the pH of blood and tissue fluid within normal
ranges. They have the greatest ability to compensate
for the pH changes that are a normal part of body
metabolism or the result of disease, and to make the
necessary corrections.
This regulatory function of the kidneys is complex,
but at its simplest it may be described as follows. If
body fluids are becoming too acidic, the kidneys will
secrete more Hions into the renal filtrate and will
return more HCO 3 ions to the blood. This will help
raise the pH of the blood back to normal. The reac-
tions involved in such a mechanism are shown in Fig.
18–6, to which we will return later. First, however, let
us briefly consider how the kidneys will compensate
for body fluids that are becoming too alkaline. You
might expect the kidneys to do just the opposite of
what was just described, and that is just what happens.
The kidneys will return Hions to the blood and
excrete HCO 3 ions in urine. This will help lower the
pH of the blood back to normal.
Because the natural tendency is for body fluids to
become more acidic, let us look at the pH-raising
mechanism in more detail (see Fig. 18–6). The cells
of the renal tubules can secrete Hions or ammonia
in exchange for Naions and, by doing so, influence
the reabsorption of other ions. Hydrogen ions are
obtained from the reaction of CO 2 and water (or other
processes). An amine group from an amino acid is
combined with an Hion to form ammonia.
The tubule cell secretes the Hion and the ammo-
nia into the renal filtrate, and two Naions are reab-
sorbed in exchange. In the filtrate, the Hion and
ammonia form NH 4 (an ammonium radical), which
reacts with a chloride ion (Cl) to form NH 4 Cl
(ammonium chloride) that is excreted in urine.
As the Naions are returned to the blood in the428 The Urinary System
Table 18–1 EFFECTS OF HORMONES ON THE KIDNEYSHormone (gland) Function
Antidiuretic hormone(ADH)
(posterior pituitary)
Parathyroid hormone (PTH)
(parathyroid glands)
Aldosterone
(adrenal cortex)
Atrial natriuretic peptide
(ANP) (atria of heart)- Increases reabsorption of water from the filtrate to the blood.
- Increases reabsorption of Ca^2 ions from filtrate to the blood and excretion of
phosphate ions into the filtrate. - Increases reabsorption of Naions from the filtrate to the blood and excretion of
Kions into the filtrate. Water is reabsorbed following the reabsorption of sodium. - Decreases reabsorption of Naions, which remain in the filtrate. More sodium
and water are eliminated in urine.