Introduction

Nitrogenous wastes, and how different animals get rid of them

Before looking at the human system in detail, it helps to see the bigger picture: what exactly needs to leave the body, and how different animals across the animal kingdom manage it.

Animals accumulate a mix of substances that must be removed, either totally or partially -- ammonia, urea, uric acid, carbon dioxide, water, and ions such as sodium, potassium, chloride, phosphate and sulphate. These build up through ordinary metabolic activity or, sometimes, simply from taking in more than the body needs. Among these, the nitrogen-containing wastes -- ammonia, urea and uric acid -- get the most attention, because how an animal handles them says a lot about where it lives.

Three strategies for nitrogenous waste

• Ammonotelism -- excreting nitrogenous waste as ammonia itself. Ammonia is highly toxic, but also extremely soluble, so it can only be disposed of safely by diffusing out across a body surface or gill surface into a large volume of surrounding water; the kidneys barely participate in this route. Many bony fishes, aquatic amphibians and aquatic insects rely on this strategy -- animals that are never short of water.
• Ureotelism -- converting ammonia into the far less toxic urea before releasing it. This conversion happens in the liver; the urea then travels in the blood to be filtered and excreted by the kidneys, though some is deliberately retained within the kidney tissue to help maintain the right osmotic balance there. Mammals, many terrestrial amphibians and marine fishes are ureotelic.
• Uricotelism -- converting nitrogenous waste into uric acid, which is only slightly soluble and can be voided as a semi-solid pellet or paste using very little water. This is the strategy of choice for animals that genuinely cannot afford to lose much water -- reptiles, birds, land snails and insects.

The pattern across all three is really about water economy: the more toxic the waste form, the more water its removal costs, so terrestrial animals -- which cannot count on a ready supply of water the way aquatic animals can -- shift toward the less toxic, more water-efficient forms.

Excretory structures across the animal kingdom

Away from humans, the animal kingdom shows a real variety of excretory organs, mostly simple tubular structures in invertebrates and more elaborate kidneys in vertebrates.

• Protonephridia (flame cells) -- found in flatworms (Platyhelminthes, e.g. Planaria), rotifers, some annelids, and the cephalochordate Amphioxus. These are concerned mainly with regulating ionic and fluid balance (osmoregulation) rather than bulk nitrogenous-waste removal.
• Nephridia -- the tubular excretory organs of earthworms and other annelids, which remove nitrogenous wastes while also helping to maintain fluid and ionic balance.
• Malpighian tubules -- the excretory structures of most insects, including cockroaches, handling both nitrogenous-waste removal and osmoregulation.
• Antennal glands (green glands) -- found at the base of the antennae in crustaceans such as prawns, carrying out the excretory role that kidneys perform in vertebrates.