Anatomy and physiology of excretory system in vertebrates
Introduction
All vertebrates have kidneys. Like the human kidney, they are made up of many
nephrons. However, there are differences in the structure and functioning of various vertebrate
kidneys that adapt them to the environment in which the animals live.
Nitrogenous wastes are the end breakdown products of proteins and nucleic acids.
The main nitrogen product of metabolism is ammonia (NH3), which is toxic.Aquatic animals
excrete ammonia directly into large amounts water. Mammals use energy to convert ammonia
to the less toxic urea, which is excreted in urine.Birds and insect convert the ammonia to uric
acid, which requires even more energy to produce, but is insoluble in water and can be excreted
as a paste with little water loss.
Figure.1 Types of nitrogenous waste
Freshwater Vertebrates
All animals that live in freshwater must cope with a continual inflow of water from
their hypotonic environment. In order to maintain homeostasis of its extracellular fluid (ECF),
the freshwater fish must excrete this excess water. Contraction of its heart powered by ATP
provides the pressure to force the water, small molecules, and ions into the glomerulus as
nephric filtrate. The essential ingredients are then reclaimed by the tubules, returning to the
blood in the capillaries surrounding the tubules. The blood in these capillaries comes from the
glomerulus and also from the renal portal veins which drain the posterior part of the fish's body.
After solute reabsorption is complete, the urine is little more than water. Most of the
nitrogenous wastes which including large amounts of ammonia, NH3 leave by diffusion out of
the gills. So, the kidney is mostly a device for maintaining water balance in the animal, rather
than an organ of excretion.
, Figure.2 Freshwater kidney
Marine Fishes
Marine fishes face just the opposite problem from that as freshwater fishes. The salt
content of sea water (~3%) is so hypertonic to that of their extracellular fluid that they are in
continual danger of dehydration. The two major groups of marine fishes have solved this
dilemma differently.
Figure.3 Marine kidneys
Cartilaginous Fishes (Chondrichthyes)
The cartilaginous fishes such as sharks, skates, and rays have developed high levels of
urea in their blood. Shark's blood may contain 2.5% urea in contrast to the 0.01-0.03% in other
vertebrates. This high level makes sharks blood isotonic to sea water, so the shark lives in
osmotic balance with its environment and has a kidney that functions like ours with the
exception that far more urea is reabsorbed in the shark's tubules than in ours.
Bony Fishes (Osteichthyes)
Marine bony fishes have solved the problem differently. They do lose water
continuously but replace it by drinking sea water and then desalting it. The salt is returned to
the sea by active transport at the gills. Living in constant danger of dehydration by the
hypertonic sea, there is no reason to pump out large amounts of nephric filtrate at the
glomerulus. The less water placed in the tubules, the less that has to be reabsorbed. So it is not
surprising that many bony fishes have small glomeruli and some have no glomeruli at all. With
a reduction in the filtration-reabsorption mechanism, the marine bony fishes rely more on
tubular secretion for eliminating excess or waste solutes. Tubular secretion requires a good
blood supply to the tubules. Lacking efficient glomeruli, the renal portal system must carry
most of the burden.
Introduction
All vertebrates have kidneys. Like the human kidney, they are made up of many
nephrons. However, there are differences in the structure and functioning of various vertebrate
kidneys that adapt them to the environment in which the animals live.
Nitrogenous wastes are the end breakdown products of proteins and nucleic acids.
The main nitrogen product of metabolism is ammonia (NH3), which is toxic.Aquatic animals
excrete ammonia directly into large amounts water. Mammals use energy to convert ammonia
to the less toxic urea, which is excreted in urine.Birds and insect convert the ammonia to uric
acid, which requires even more energy to produce, but is insoluble in water and can be excreted
as a paste with little water loss.
Figure.1 Types of nitrogenous waste
Freshwater Vertebrates
All animals that live in freshwater must cope with a continual inflow of water from
their hypotonic environment. In order to maintain homeostasis of its extracellular fluid (ECF),
the freshwater fish must excrete this excess water. Contraction of its heart powered by ATP
provides the pressure to force the water, small molecules, and ions into the glomerulus as
nephric filtrate. The essential ingredients are then reclaimed by the tubules, returning to the
blood in the capillaries surrounding the tubules. The blood in these capillaries comes from the
glomerulus and also from the renal portal veins which drain the posterior part of the fish's body.
After solute reabsorption is complete, the urine is little more than water. Most of the
nitrogenous wastes which including large amounts of ammonia, NH3 leave by diffusion out of
the gills. So, the kidney is mostly a device for maintaining water balance in the animal, rather
than an organ of excretion.
, Figure.2 Freshwater kidney
Marine Fishes
Marine fishes face just the opposite problem from that as freshwater fishes. The salt
content of sea water (~3%) is so hypertonic to that of their extracellular fluid that they are in
continual danger of dehydration. The two major groups of marine fishes have solved this
dilemma differently.
Figure.3 Marine kidneys
Cartilaginous Fishes (Chondrichthyes)
The cartilaginous fishes such as sharks, skates, and rays have developed high levels of
urea in their blood. Shark's blood may contain 2.5% urea in contrast to the 0.01-0.03% in other
vertebrates. This high level makes sharks blood isotonic to sea water, so the shark lives in
osmotic balance with its environment and has a kidney that functions like ours with the
exception that far more urea is reabsorbed in the shark's tubules than in ours.
Bony Fishes (Osteichthyes)
Marine bony fishes have solved the problem differently. They do lose water
continuously but replace it by drinking sea water and then desalting it. The salt is returned to
the sea by active transport at the gills. Living in constant danger of dehydration by the
hypertonic sea, there is no reason to pump out large amounts of nephric filtrate at the
glomerulus. The less water placed in the tubules, the less that has to be reabsorbed. So it is not
surprising that many bony fishes have small glomeruli and some have no glomeruli at all. With
a reduction in the filtration-reabsorption mechanism, the marine bony fishes rely more on
tubular secretion for eliminating excess or waste solutes. Tubular secretion requires a good
blood supply to the tubules. Lacking efficient glomeruli, the renal portal system must carry
most of the burden.
