Excretory structures of animal kingdom

Hyderabad: This article is in continuation to the previous article that focused an introduction to the excretory system of living beings. In this article, we will discuss the excretory structures of the animal kingdom, and the human excretory system with a focus on kidneys.

• Animal kingdom presents a variety of excretory structures.
• In most of the invertebrates, these structures are simple tubular forms whereas vertebrates have complex tubular organs called kidneys. Some of these structures are mentioned here.

Protonephridia or flame cells

• Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate – Amphioxus.

• Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation.
Nephridia

• Nephridia are the tubular excretory structures of earthworms and other annelids.
• Nephridia help to remove nitrogenous wastes and maintain a fluid and ionic balance.

Malpighian tubules

• Malpighian tubules are the excretory structures of most of the insects, including cockroaches.
• Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation.
Antennal glands or green glands

• Antennal glands or green glands perform the excretory function in crustaceans like prawns.

Human excretory system
• In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.

Kidneys

• Kidneys are reddish brown, bean-shaped structures situated between the levels of last thoracic and third lumbar vertebra close to the dorsal inner wall of the abdominal cavity.

• Each kidney of an adult human measures 10-12 cm in length, 5-7 cm in width, 2-3 cm in thickness with an average weight of 120-170 g.

• Towards the centre of the inner concave surface of the kidney is a notch called hilum through which ureter, blood vessels and nerves enter.

• Inner to the hilum is a broad funnel-shaped space called the renal pelvis with projections called calyces.

• The outer layer of kidney is a tough capsule.
• Inside the kidney, there are two zones, an outer cortex and an inner medulla.
• The medulla is divided into a few conical masses (medullary pyramids) projecting into the calyces (sing.: calyx).

• The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.
— To be continued…