rates of glomerular filtration affect tubular absorption to some extent, a group in
Copenhagen have put forth evidence showing the rever se is so (KRUHOFFER, 1960).
Reabsorption of fluid in the tubule may increase the GFR by reducing tubular
back-pressure, thus facilitating the hydrostatic pressure in the glomerulus. The
magnitude and ph ysiological importance of such effects have not been agreed upon.
b) Renal Tubular Function. The glomerular filtrate is progressively modified as
it moves along the renal tubule. These changes involve active and passive
reabsorption, tubular secretion of solutes, as well as the osmotic transfer of water.
The precise details of such processes, both with respect to their regional location
in the nephron and interactions with each other, are continually being reconsidered
so that the present summary is not a final one. The reader should consult H . SMITH
(1951), KRUHOFFER (1960), and MALNIC, KLOSE, and GIEBISCH (1966 a and b) for
a detailed description of the available evidence, which is summarized in Fig . 1.4.
Sodium is actively transported, from the glomerular filtrate back into the blood,
from most regions of the nephron as well as the collecting ducts. The exception
appears to be the descending loop of HENLE in mammals, which is relatively impermeable to sod ium and water. Active potassium tr ansfer takes place from the
luminal fluid in both the proximal and distal tubules.
Hydrogen ions are secreted from the renal tubular cells into the filtrate in both
the proximal and distal tubule. These ions are derived from carbonic acid, which
is formed from carbon dioxide and water under the influence of the enzyme carbonic anhydrase. The ability to secrete H+ is limited at a urinary pH of about 4.5
to 4.8, but these ions may be neutralized in the distal tubule by NH 3 (to form
NH+ 4)' The ammonia arises in the tubular cells by deamination of amino acids
such as glutamine.
The principal anions present in the filtrate are chloride and bicarbonate, and
these move out of the tubule down the ir electro-chemical gradients. The movement
of HCO - 3 is facilitated by its interaction with secreted H + to form CO 2 and water.
Bicarbonate, equivalent to the H +, is then transferred from inside the tubular cell
to the blood.
Potassium excretion under certain circumstances may exceed the rate at which
it is filtered across the glomerulus, indicating its tubular secretion. Such secretion
takes place in the distal tubule, and down an electro-chemical gradient, which partly
results from the active sodium transport out of the tubule.
The quantity of the various other solutes in the tubular fluid is also modified ;
calcium, magnesium, sulphate, phosphate and glucose are reabsorbed, but in aglomerular fish such ions are presumably also secreted. In birds and reptiles uric
acid is secreted acros s the tubular epithelium. In mammals about 40 to 70% of the
filtered urea diffuses out of the tubules, but in species that utilize urea for their osmotic equ ilibrium with sea-water, little may be lost (1%), as seen in the frog , Rana
cancrivora (SCHMIDT-NIELSEN and LEE, 1962), while in the marine Chondrichthyes urea may even be conserved by active reabsorption (H. SMITH, 1936).
In mammals and amphibians water mo ves osmotically out of the proximal tubule, so that isoosmoticity with the bathing fluids exists. In the more distal regions
of the nephron and collecting ducts, such osmotic equilibration mayor may not
occur. In the tetrapod vertebrates, such adjustments are largely under the control
of the neurohypophysial hormones from the pituitary gland .
32
Copenhagen have put forth evidence showing the rever se is so (KRUHOFFER, 1960).
Reabsorption of fluid in the tubule may increase the GFR by reducing tubular
back-pressure, thus facilitating the hydrostatic pressure in the glomerulus. The
magnitude and ph ysiological importance of such effects have not been agreed upon.
b) Renal Tubular Function. The glomerular filtrate is progressively modified as
it moves along the renal tubule. These changes involve active and passive
reabsorption, tubular secretion of solutes, as well as the osmotic transfer of water.
The precise details of such processes, both with respect to their regional location
in the nephron and interactions with each other, are continually being reconsidered
so that the present summary is not a final one. The reader should consult H . SMITH
(1951), KRUHOFFER (1960), and MALNIC, KLOSE, and GIEBISCH (1966 a and b) for
a detailed description of the available evidence, which is summarized in Fig . 1.4.
Sodium is actively transported, from the glomerular filtrate back into the blood,
from most regions of the nephron as well as the collecting ducts. The exception
appears to be the descending loop of HENLE in mammals, which is relatively impermeable to sod ium and water. Active potassium tr ansfer takes place from the
luminal fluid in both the proximal and distal tubules.
Hydrogen ions are secreted from the renal tubular cells into the filtrate in both
the proximal and distal tubule. These ions are derived from carbonic acid, which
is formed from carbon dioxide and water under the influence of the enzyme carbonic anhydrase. The ability to secrete H+ is limited at a urinary pH of about 4.5
to 4.8, but these ions may be neutralized in the distal tubule by NH 3 (to form
NH+ 4)' The ammonia arises in the tubular cells by deamination of amino acids
such as glutamine.
The principal anions present in the filtrate are chloride and bicarbonate, and
these move out of the tubule down the ir electro-chemical gradients. The movement
of HCO - 3 is facilitated by its interaction with secreted H + to form CO 2 and water.
Bicarbonate, equivalent to the H +, is then transferred from inside the tubular cell
to the blood.
Potassium excretion under certain circumstances may exceed the rate at which
it is filtered across the glomerulus, indicating its tubular secretion. Such secretion
takes place in the distal tubule, and down an electro-chemical gradient, which partly
results from the active sodium transport out of the tubule.
The quantity of the various other solutes in the tubular fluid is also modified ;
calcium, magnesium, sulphate, phosphate and glucose are reabsorbed, but in aglomerular fish such ions are presumably also secreted. In birds and reptiles uric
acid is secreted acros s the tubular epithelium. In mammals about 40 to 70% of the
filtered urea diffuses out of the tubules, but in species that utilize urea for their osmotic equ ilibrium with sea-water, little may be lost (1%), as seen in the frog , Rana
cancrivora (SCHMIDT-NIELSEN and LEE, 1962), while in the marine Chondrichthyes urea may even be conserved by active reabsorption (H. SMITH, 1936).
In mammals and amphibians water mo ves osmotically out of the proximal tubule, so that isoosmoticity with the bathing fluids exists. In the more distal regions
of the nephron and collecting ducts, such osmotic equilibration mayor may not
occur. In the tetrapod vertebrates, such adjustments are largely under the control
of the neurohypophysial hormones from the pituitary gland .
32