The excretory system functions in ridding the body of nitrogenous (nitrogen-containing, discussed below) and other wastes.
It also regulates the amount of water and ions present in the body fluids.
Review of Excretory Systems in Animals
The concentration of solutes in isotonic animals is approximately equal to that of their environment. As a result, they do not gain or lose water.
Only marine invertebrates and cartilaginous fish (chondrichthyes) are isotonic.
The concentration of solutes in the tissues of isotonic animals is approximately equal to that of the ocean. This is 100 times higher than that found in the mammalian bodies. The high concentration of solutes in chondrichthyes is due mostly to the presence of urea.
The rate of water loss is high in marine bony fish. They drink water seawater at a rate of approx. 1% of their body weight/hour.
Specialized cells in the gills excrete excess salt.
Freshwater Bony Fish
Freshwater bony fish tend to gain water from their environment due to osmosis.
They produce large quantities of dilute urine (approx. 1/3 of their body weight/day) and do not drink water.
Salt-absorbing cells in he gills use active transport (energy is required) to pump salts into their body.
Birds and reptiles near the sea
Birds and reptiles living near the sea consume a large amount of salt in their diet. Nasal salt glands remove this excess salt from their body by secreting a concentrated salt solution.
The kidneys of sea mammals (ex: seals, whales, porpoises) are able to maintain a constant salt concentration in their bodies by producing urine that has a high concentration of salt.
They are able to drink seawater because the salt concentration of their urine is higher than that of sea water.
Most terrestrial animals drink water, some do not.
Metabolic water produced from cellular respiration may be sufficient to meet the needs of some animals. The equation below shows that six molecules of water are produced for every molecule of glucose oxidized.
Example: Kangaroo Rat
Kangaroo rats of southwestern US deserts never have to drink. Their water comes from metabolic water released during cellular respiration and water present in their food.
They emerge from their burrows only at night, when the air is cooler and more humid than during the day. This helps prevent water loss from their bodies and reduces the amount needed to keep cool (sweating).
They avoid movement while in their burrow. This minimizes heat production and thus, the need for sweating.
Dry food stored in their burrows absorbs moisture lost in breathing. This moisture is taken back in when the food is eaten.
Their noses become cooled during inhalation as a result of evaporating water but the cooled membranes cause the moisture to condense during exhalation.
Their large intestine absorbs almost all water present in the digestive tract. Their feces are dry, hard pellets.
Their kidneys conserve water by excreting concentrated urine.
Organs of Excretion
Cells use amino acids to construct proteins and other nitrogen-containing molecules. Amino acids can also be oxidized for energy or converted to fats or carbohydrates.
When amino acids are oxidized or converted to other kinds of molecules, the amino (NH2) group must be removed. The nitrogen-containing compounds produced as a result of protein breakdown are toxic and must be removed by the excretory system.
Nitrogenous wastes of animals are excreted in form of ammonia, urea, or uric acid.
Ammonia is formed immediately after the amino group is removed from an amino acid. This process requires very little energy.
Ammonia is highly soluble in water but very toxic. Aquatic animals such as bony fishes, aquatic invertebrates, and amphibians excrete ammonia because it is easily eliminated in the water.
Terrestrial amphibians and mammals excrete nitrogenous wastes in the form of urea because it is less toxic than ammonia and can be moderately concentrated to conserve water.
Urea is produced in the liver by a process that requires more energy to produce than ammonia does.
Insects, reptiles, birds, and some dogs (Dalmatians) excrete uric acid. Reptiles and birds eliminate uric acid with their feces. The white material seen in bird droppings is uric acid.
It is not very toxic and is not very soluble in water. Excretion of wastes in the form of uric acid conserves water because it can be produced in a concentrated form due to its low toxicity.
Because it is relatively insoluble and nontoxic, it can accumulate in eggs without damaging the embryos.
The synthesis of uric acid requires more energy than urea synthesis.
Structures of the excretory system
Regions of the Kidney
renal pelvis (innermost chamber)- collects the urine
microscopic; about 1 million/kidney
some are primarily in the cortex, others dip down into the medulla
glomerulus- a capillary tuft from which fluid leaves the circulatory system (filtration)
Bowman's capsule- a funnel-like structure that collects filtrate from the glomerulus
proximal convoluted tubule
loop of the nephron
distal convoluted tubule
collecting duct- delivers urine to renal pelvis
The path of blood flow through a kidney is listed below.
Blood enters the kidney through a branch of the aorta called the renal artery.
Branches of the renal artery within the kidney produce afferent arterioles.
Each afferent arteriole leads to a network of capillaries called a glomerulus. Fluid leaks out of the capillaries of the glomerulus but large molecules and cells do not fit through the pores. This process is called filtration.
Blood leaves the capillaries of the glomerulus via an efferent arteriole and enters capillaries in the medulla called peritubular capillaries, which collect much of the water that was lost through the glomerulus.
Venules from the peritubular capillaries lead to the renal vein, which exits the kidney and returns blood to the inferior vena cava.
Pressure filtration occurs in the glomerulus.
Blood enters the glomerulus via an afferent arteriole where blood pressure forces water and small molecules out through pores in the glomerular capillaries.
The filtrate has approximately the same composition as tissue fluid.
Blood leaves the glomerulus via the efferent arteriole.
Approximately 45 gallons of liquid per day are filtered from the blood in the glomerulus.
Proximal Convoluted Tubule
Loop of Henle
In mammals, the loop of Henle conserves water resulting in concentrated urine.
This is done by the gut in birds and reptiles.
Water moves out of the descending loop as it passes through the area of high salt concentration produced by the ascending loop.
The descending loop is not permeable to ions.
Salt is actively pumped out in the ascending loop.
This part of the loop is impermeable to water reentry.
This creates a concentration gradient with a higher concentration in the medulla (interior region).
Countercurrent Mechanism, Collecting Duct
The movement of sodium out of the ascending loop and into the medulla results in water loss and concentrated urine in the descending loop. The water loss and increased salt concentration that occurs in the descending loop further enhances the ability of the ascending loop to pump more salt out into the medulla. High salt in the medulla acts to help remove water in the descending loop. This phenomena is called the countercurrent multiplier.
Urea is concentrated in the fluid; some is able to move out of the lower portion of the collecting duct. It does not enter the blood stream, however, so little urea is lost once a concentration gradient is established.
The combination of urea and salt produces a high osmotic concentration in the medulla.
Length of the Loop of Henle
A longer loop of Henle will function to produce a greater concentration of urea and salt in the medulla. The higher concentration gradient enables the removal of more water as fluid moves through the collecting duct.
The length of the loop of Henle varies among mammals. The beaver, which does not need to conserve water, has a relatively short loop.
Desert-dwelling mammals have very long loops and are capable of producing extremely concentrated urine resulting in very little water loss.
Distal Convoluted Tubule
Hormones that Regulate Water Loss
Antidiuretic Hormone (ADH)
Atrial Natriuretic Hormone
pH of the Blood