Biology Chapter 19: Excretory Products

Study Notes

Animals accumulate various excretory products including ammonia, urea, uric acid, carbon dioxide, water, and ions such as Na+, K+, Cl–, phosphate, and sulphate due to metabolic activities and excess ingestion.
These waste products must be eliminated to maintain homeostasis in the body.

Major forms of nitrogenous wastes include ammonia (most toxic), urea, and uric acid (least toxic).
Ammonia requires large amounts of water for its elimination; uric acid can be excreted with minimal water loss.
Ammonotelism refers to the excretion of ammonia, primarily seen in bony fishes, aquatic amphibians, and aquatic insects.

Ammonotelic animals excrete ammonia by diffusion across body surfaces or through gills, with kidneys playing a minor role.
Terrestrial animals have adapted by producing less toxic wastes such as urea and uric acid, which conserve water.
Ureotelic animals, such as mammals and many terrestrial amphibians, convert ammonia to urea in the liver before it is filtered by the kidneys.
Uricotelic animals like reptiles, birds, and some insects excrete uric acid in a semi-solid form, minimizing water loss.

Invertebrates typically have simple tubular excretory structures, while vertebrates possess complex kidneys.
Protonephridia (or flame cells) are found in flatworms (e.g., Planaria), rotifers, certain annelids, and cephalochordates, focusing on osmoregulation.
Nephridia are present in earthworms and help eliminate nitrogenous wastes while maintaining fluid balance.
Malpighian tubules, found in most insects like cockroaches, assist in nitrogenous waste removal and osmoregulation.
Antennal (green) glands in crustaceans (e.g., prawns) function in excretory processes.

The human excretory system consists of two kidneys, two ureters, a urinary bladder, and a urethra.
Kidneys are bean-shaped, reddish-brown structures located between the last thoracic and third lumbar vertebrae, measuring 10-12 cm in length and weighing 120-170 g.
Each kidney has a hilum for the entry of ureters, blood vessels, and nerves, leading into the renal pelvis, which has projections known as calyces.
The kidney structure includes an outer cortex and an inner medulla, with the medulla divided into conical masses called medullary pyramids.