K a na key leak channel countertransport exchange

K +
A
Na +
KEY
Leak channel
Countertransport
Exchange pump
Diffusion
Reabsorption
Secretion
Cotransport
K
+
Aldosterone-
regulated pump
Aldosterone-regulated reabsorption of
sodium ions, linked to the passive loss
of potassium ions.
A
b
Na
+
Aldosterone
• Hormone produced by
adrenal cortex
• Stimulates synthesis and
incorporation of Na
+
pumps
and channels (In plasma
membranes along DCT
and collecting duct)
• Reduces Na + lost in urine
Variable reabsorption of water,
sodium ions, and calcium ions
(under hormonal control)
Atrial natriuretic peptide (ANP)
opposes secretion of
aldosterone and its actions on
DCT and Collecting System
Variable reabsorption of water, sodium ions, and calcium ions
(under hormonal control)
à Parathyroid
hormone Regulates
calcium ion
reabsorption at DCT
Parathyroid
Glands
Parathyroid hormone (PTH) is secreted by
the Parathyroid glands in response to low
concentrations of Ca
2+
in blood

37
Secretion at DCT
– Blood entering peritubular
capillaries still contains
undesirable substances that did
not cross filtration membrane at
glomerulus
– Rate of K + and H + secretion
rises or falls according to
concentrations in peritubular
fluid
• Higher concentrations lead to
higher rates of secretion
Figure 26–13c Tubular Secretion and Solute Reabsorption by the DCT.
Tubular fluid
CO
2
Cl – H + NH
4
+ Cl –
Cl –
Na + Na +
NH
4
+
CO
2
H
2
O
H
2
CO
3
HCO
3
–
H
+
Amino acid
deamination
HCO
3
–
Cl –
Cl – H
+
HCO
3
–
HCO
3
–
KEY
Leak channel
Countertransport
Exchange pump
Diffusion
Reabsorption
Secretion
Cotransport
Na
+
HCO
3
– Na +
H +
HCO
3
–
Aldosterone-
regulated pump
A
Hydrogen ion secretion in
response to acidosis
– Hydrogen ions generated by
dissociation of carbonic acid
– Bicarbonate ions diffuse into
bloodstream
This system helps prevent
changes in plasma pH:
- Respiratory acidosis
- Ketoacidosis
- Lactic acidosis …
à Aldosterone
stimulates H
+
secretion
Prolonged aldosterone
stimulation can cause
alkalosis

38
Figure 26–9 An Overview of Urine Formation.
Proximal Convoluted Tubule
• Reabsorption of water, ions,
and all organic nutrients
Distal Convoluted Tubule
• Secretion of ions, acids, drugs, toxins
• Variable reabsorption of water,
sodium ions, and calcium ions (under
hormonal control)
Renal Corpuscle
• Production of
filtrate
Glomerulus
Glomerular capsule
Collecting duct
Nephron Loop
Descending thin limb
• Further reabsorption of water
Thick ascending limb
• Reabsorption of sodium
and chloride ions
KEY
Filtration occurs exclusively in the renal corpuscle, across
the filtration membrane.
Water reabsorption occurs primarily along the PCT and the
descending thin limb of the nephron loop, but also to a variable
degree in the DCT and collecting system.
Variable water reabsorption occurs in the DCT and collecting
system.
Solute reabsorption occurs along the PCT, the thick ascending
limb of the nephron loop, the DCT, and the collecting system.