i) The Kidney
Kidneys are present in all vertebrates; they playa characteristic and central role
In osmoregulation. While it is doubtful that any species could survive for long in
the absence of this organ, its relative overall importance differs . This is due, as we
have seen, to the presence of other tissues such as 'salt' glands and gills that may
also contribute to the regulation of solute metabolism. DICKER (1970) has recently
given a complete and lucid description of the function of mammalian kidneys.
a) Functions. The kidney is the only organ in vertebrates that can secrete large
volumes of water containing solutes at concentrations less than 1% of their total
level in the body fluids. It thus has a role as an organ for excretion of water, while
at the same time conserving solute. In mammals, the kidney is principally responsible for regulatory secretion of excess mono- and divalent ions . However, among
the birds, reptiles and Chondrichthyes, the 'salt' gland also contributes to regu -
lation of sodium and potassium, wh ile in marine teleost fishes the gills are the principal site for expulsion of sodium and chloride. The kidney nevertheless remains,
in all species, the main avenue for excretion of divalent ions, like Ca2+, SO / +and
HPO/ +.
Solutes that are physiologically undesirable and arise as by -products of metabolism, may be excreted through the kidney, and to some extent through the gut,
skin and gills. These include : by-products of nitrogen metabolism such as urea,
uric acid and ammonia; creatinine, creatine and the remnants of steroid and protein
hormones. A variety of organic molecules may, by fair means or foul, gain access
to the animal from its external environment ; in a valetudinary species like man this
is particularly prevalent, taking the form of various drugs and potions.
Control of the concentration of free hydrogen ion (pH) and the buffering capacity of the body fluids is due to both carbon dioxide exchange by the respiratory
organs and the renal regulation of bicarbonate and phosphate concentrations. This
is a complex process involving directly the passage of hydrogen ions into the urine,
and more indirectly (but vitally) the secretion of ammonia by the renal tubular cells.
The kidney thus has functions as an organ for excretion, conservation and regulation of the constituents of the animal.
fJ) Structure. The vertebrate kidney is composed of a large number of units called
nepbrons. These are tubular structures which have a tuft of capillaries (the glomerulus) invaginated into a capsule (BOWMANS capsule) at the ir anterior end. The total
number of these units differ; in mammals the range is from about 25 000 in a mouse
to 15 million in an elephant (H. SMITH, 1951). The usual pattern is one glomerulus
to each tubule, though in the Agnatha (GERARD, 1954) a large glomerulus may communicate with several tubules while in some marine fishes, such as the goosefish,
Lophius, and the toadfish, Opsanus, glomeruli are absent. Th e size of glomeruli
also shows considerable variation, and they may be cigar-shaped structures 1400
!Jlong in the lamprey (Agnatha) (BENTLEY and FOLLETT, 1963) or, as seen more
usuall y in most other species, oval tufts 60 to 200 fJ in diameter (H. SMITH, 1951).
The renal tubules are segmented into units of varying length and diameter. In
the Agnatha a prominent ciliated neck segment is present at the junction of the
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Kidneys are present in all vertebrates; they playa characteristic and central role
In osmoregulation. While it is doubtful that any species could survive for long in
the absence of this organ, its relative overall importance differs . This is due, as we
have seen, to the presence of other tissues such as 'salt' glands and gills that may
also contribute to the regulation of solute metabolism. DICKER (1970) has recently
given a complete and lucid description of the function of mammalian kidneys.
a) Functions. The kidney is the only organ in vertebrates that can secrete large
volumes of water containing solutes at concentrations less than 1% of their total
level in the body fluids. It thus has a role as an organ for excretion of water, while
at the same time conserving solute. In mammals, the kidney is principally responsible for regulatory secretion of excess mono- and divalent ions . However, among
the birds, reptiles and Chondrichthyes, the 'salt' gland also contributes to regu -
lation of sodium and potassium, wh ile in marine teleost fishes the gills are the principal site for expulsion of sodium and chloride. The kidney nevertheless remains,
in all species, the main avenue for excretion of divalent ions, like Ca2+, SO / +and
HPO/ +.
Solutes that are physiologically undesirable and arise as by -products of metabolism, may be excreted through the kidney, and to some extent through the gut,
skin and gills. These include : by-products of nitrogen metabolism such as urea,
uric acid and ammonia; creatinine, creatine and the remnants of steroid and protein
hormones. A variety of organic molecules may, by fair means or foul, gain access
to the animal from its external environment ; in a valetudinary species like man this
is particularly prevalent, taking the form of various drugs and potions.
Control of the concentration of free hydrogen ion (pH) and the buffering capacity of the body fluids is due to both carbon dioxide exchange by the respiratory
organs and the renal regulation of bicarbonate and phosphate concentrations. This
is a complex process involving directly the passage of hydrogen ions into the urine,
and more indirectly (but vitally) the secretion of ammonia by the renal tubular cells.
The kidney thus has functions as an organ for excretion, conservation and regulation of the constituents of the animal.
fJ) Structure. The vertebrate kidney is composed of a large number of units called
nepbrons. These are tubular structures which have a tuft of capillaries (the glomerulus) invaginated into a capsule (BOWMANS capsule) at the ir anterior end. The total
number of these units differ; in mammals the range is from about 25 000 in a mouse
to 15 million in an elephant (H. SMITH, 1951). The usual pattern is one glomerulus
to each tubule, though in the Agnatha (GERARD, 1954) a large glomerulus may communicate with several tubules while in some marine fishes, such as the goosefish,
Lophius, and the toadfish, Opsanus, glomeruli are absent. Th e size of glomeruli
also shows considerable variation, and they may be cigar-shaped structures 1400
!Jlong in the lamprey (Agnatha) (BENTLEY and FOLLETT, 1963) or, as seen more
usuall y in most other species, oval tufts 60 to 200 fJ in diameter (H. SMITH, 1951).
The renal tubules are segmented into units of varying length and diameter. In
the Agnatha a prominent ciliated neck segment is present at the junction of the
28