Excretion

These products include carbon dioxide and water, excess nitrogen and solutes.

The excretion of respiratory carbon dioxide is done by expiration.

The excretion of nitrogenous wastes is usually associated with the regulation of water and solute (ionic) balance by a physiological process called osmoregulation.

If the osmotic concentration of the body fluids of an animal is equal to that of the medium in which it lives, this animal is called an osmoconformer.

If an animal maintains its body fluids at a different osmotic concentration from that of its surrounding environment it is called an osmoregulator.

Animals living in seawater have body fluids with an osmotic concentration that is about 1/3 less than the surrounding seawater, and water tends to leave their bodies continually.

Land animals have a higher concentration of water in their fluids than in the surrounding air.

They tend to lose water to the air through evaporation, and may use considerable amounts of water to dispose of wastes.

Generally marine invertebrates have about the same osmotic concentration as seawater.

Cnidarians, echinoderms, sponges, all do not have specialized excretory structures because wastes simply diffuse into the surrounding isosmotic water.

In some freshwater species, cells on the body surface actively pump ions into the animal.

Many freshwater species however, have contractile vacuoles that pump out excess water.

Contractile vacuoles are energy requiring devices that expel excess water from individual cells exposed to hypotonic environments.

Protonephridia

Most have nephridia that serve for excretion, osmoregulation, or both.

Probably the earliest type of nephridium to appear in the evolution of animals was the protonephridium.

Among the simplest of the protonephridia are flame cell systems, such as those found in rotifers, some annelids, larval mollusca, and some flatworms.

The protonephridial excretory system is composed of a network of excretory canals that open to the outside of the body through excretory pores.

Bulblike flame cells are located along the excretory canals.

Fluid filters into the flame cells from the surrounding interstitial fluid and is propelled by the beating cilia through the excretory canals and out of the body through the excretory pores.

Flame cell systems function primarily in eliminating excess water.

Nitrogenous waste simply diffuses across the body surface into the surrounding water.

A more advanced type of excretory structure among invertebrates is the metanephridium.

Both open to the outside, but metanephridia are 1) also open internally to the body fluids and 2)multicellular.

Each metanephridium begins with a ciliated funnel, the nephrostome, that opens form the body cavity of a segment into a coiled tubule.

As the fluid is moved through the tubule by beating cilia, ions are reabsorbed and carried away by a network of capillaries surrounding the tubule.

Each tubule leads to an enlarged bladder that empties to the outside of the body through an opening called the nephridiopore.

Insects have an excretory system made up of malpighian tubules attached to the gut.

The malpighian tubules and the gut together serve as the excretory system.

Excretion involves the active transport of potassium ions from the blood surrounding the tubules into the tubules and osmotic movement of water follows.

nitrogenous waste also enter the tubules.

As the fluid moves through the malpighian tubules, some of the water and certain ions are recovered.

All of the uric acid passes into the gut and out of the body.

A variety of mechanisms have evolved in vertebrates to cope with their osmoregulatory problems, and most of them are adaptations of the urinary system.

Vertebrates have a closed circulatory system containing blood that is under pressure.

This pressure forces blood through a membrane filter in a kidney where the following four key functions take place:

1. Filtration, in which blood is passed through a filter that retains blood cells, proteins, and other large solutes but lets small molecules, ions, and urea pass through.

2. reabsorption, in which selective ions and molecules are taken back into the bloodstream from the filtrate.

3. Secretion, whereby select ions and end products of metabolism that are in the blood are added to the filtrate for removal from the body. (K+, H+, NH3)

4. Excretion, in which urine is voided form the body .

The filtration devise of the metanephric kidney consist of over one million individual filtration, secretion, and absorption structures called nephrons.

At the beginning of the nephron is the filtration apparatus called the glomerular capsule.

The capsules are located in the cortical (outermost) region of the kidney.

In each capsule, an afferent arteriole enters and branches into a fine network of capillaries called the glomerulus.

The walls of these glomerular capillaries contain small perforations that act as filters.

Because the perforations are so small, large proteins and blood cells are retained in the blood and leave the glomerulus via the efferent arteriole.

Beyond the glomerular capsule are the proximal convoluted tubule, the loop of the nephron, and the distal convoluted tubule.

At various places along these structures, selective reabsorption of the glomerular filtrate occurs, returning certain ions to the bloodstream.

Potentially harmful compounds, such as hydrogen and ammonium ions, drugs, and various other foreign materials are secreted into the nephron lumen.

In the final portion of the nephron, called the collecting duct, water is reabsorbed so that the urine contains an ion concentration well above that of the blood.

Mammalian, and to a lesser extent avian and reptilian, kidneys can remove far more water form the glomerular filtrate than can the kidneys of amphibians.

mammals and to a lesser extent birds, achieve this remarkable degree of water conservation by a unique, yet simple, evolutionary adaptation the bending of the nephron tube in the form of a loop..

By bending, the nephron can greatly increase the salt concentration in the tissue through with the loop passes and use this gradient to draw large amount water out of the tube.