Elimination: Nitrogenous Wastes | Biology for JAMB PDF Download

Introduction

 Animals produce various waste substances such as ammonia, urea, uric acid, carbon dioxide, water, and ions like sodium (Na+), potassium (K+), chloride (Cl–), phosphate, and sulfate either through metabolic processes or due to excess intake. These substances need to be eliminated from the body either totally or partially. This chapter focuses on the mechanisms of eliminating these substances, with special attention to nitrogenous wastes.

What is Excretion ?

Excretion is the natural process by which the body removes waste products that are produced during metabolism. These waste products consist of substances such as amino acids, urea, uric acid, carbon dioxide, water, and ammonia.

Importance of Excretion

• Excretion plays a role in maintaining homeostasis, which is the process of keeping the body's internal environment stable and balanced. This stability is crucial for the proper functioning and survival of organs and biological systems.
• The organisms regulate the osmotic pressure, which involves maintaining a balance between inorganic ions and water, and they also ensure the proper acid-base balance through this mechanism.

Nitrogenous Wastes

Ammonia, urea, and uric acid are the primary forms of nitrogenous wastes excreted by animals. 

•  Ammonia is the most toxic form of nitrogenous waste and requires a large amount of water for its elimination. 
•  Uric acid, being the least toxic, can be removed with minimal water loss. 

Modes of Excretion In Animals

1. Ammonotelism

•  The process of excreting ammonia is known as ammonotelism. 
•  Many bony fishes, aquatic amphibians, and aquatic insects are ammonotelic in nature. 
•  Ammonia, being readily soluble, is typically excreted by diffusion across body surfaces or through gill surfaces in fish as ammonium ions. 
•  The kidneys do not play a significant role in the removal of ammonia in these animals. 

2. Ureotelism

•  Terrestrial adaptation required the production of less toxic nitrogenous wastes like urea and uric acid to conserve water. 
•  Mammals, many terrestrial amphibians, and marine fishes primarily excrete urea and are called ureotelic animals. 
•  In these animals, ammonia produced by metabolism is converted into urea in the liver, released into the blood, and excreted by the kidneys. 
•  Some animals may retain urea in the kidney matrix to maintain a desired osmolarity. 

3. Uricotelism

•  Reptiles, birds, land snails, and insects excrete nitrogenous wastes as uric acid in the form of pellets or paste with minimal water loss. 
•  These animals are called uricotelic animals. 

Excretory Organs in Animals

 A survey of the animal kingdom reveals a variety of excretory structures. In most invertebrates, these structures are simple tubular forms, while vertebrates have complex tubular organs called kidneys. 

 Some of the excretory structures in animals are as follows: 

• Protonephridia or flame cells are found in Platyhelminthes (flatworms like Planaria), rotifers, some annelids, and the cephalochordate Amphioxus. These structures are primarily involved in ionic and fluid volume regulation, known as osmoregulation. 
• Nephridia are the tubular excretory structures of earthworms and other annelids. They help remove nitrogenous wastes and maintain fluid and ionic balance in the body. 
• Malpighian tubules are the excretory structures of most insects, including cockroaches. These tubules assist in the removal of nitrogenous wastes and play a role in osmoregulation. 
• Antennal glands, also known as green glands, perform excretory functions in crustaceans like prawns.