Animal Excretory Systems and Kidney Anatomy

Invertebrate Excretory Mechanisms

Simplest animals: Excretion occurs by direct diffusion through the skin.

• Protonephridium: Found in Platyhelminthes (flatworms) and annelids. This is a closed system consisting of two highly branched duct systems distributed throughout the body. At the closed ends, waste molecules are taken up by ciliated cells (flame cells); at the open end, they feature a pore (nephridiopore) for waste elimination.
• Metanephridium: Found in annelids and molluscs. This consists of a tube that allows fluid to enter the tubule through a ciliated, funnel-like opening. It is surrounded by blood vessels that reabsorb water, salts, sugars, and amino acids.
• Antennal glands: Found in crustaceans. Located in the ventral part of the head, these represent an advanced design of the basic nephridial organ.
• Malpighian tubules: Found in insects and spiders. These operate in conjunction with specialized glands in the wall of the rectum.

The Vertebrate Kidney and Nephron Structure

Vertebrates possess a kidney system consisting of two kidneys with two conducting tubes (ureters) that communicate with a urinary bladder. The functional units of the kidney are the nephrons.

The Renal Corpuscle

The renal corpuscle is responsible for the filtration of blood plasma. It consists of:

• Glomerulus: A web of capillaries.
• Bowman’s capsule: Surrounds the glomerulus and collects the product of filtration.

The Renal Tubule

The renal tubule handles the reabsorption of advantageous products and the excretion of toxic products. It includes:

• Proximal convoluted tubule: Located proximal to the renal corpuscle.
• Loop of Henle: Enters the kidney’s medulla and then returns to the cortex.
• Distal convoluted tubule: Communicates with the collection duct.
• Collection ducts: Lead to the renal medulla and collect urine from different nephrons.

Excretion Across Different Vertebrate Classes

• Fish: Possess two elongated kidneys. Urine is stored in a urinary bladder that opens into a urogenital hole.
• Amphibians: The kidneys communicate with the cloaca via two ureters. Amphibians have a primitive urinary bladder that serves as a water reservoir.
• Reptiles: Kidneys communicate with the cloaca through two ureters. Some turtles and lizards have an accessory urinary bladder connected to the cloaca.
• Birds: They have well-developed kidneys that end in the cloaca, but they do not have a urinary bladder.
• Mammals: Feature well-developed kidneys that end in a urinary bladder.

Specialized Glands and Alternative Excretion

• Salt glands: These excrete an excess of salts. They are found in elasmobranchs (sharks, rays, and skates), seabirds, and some reptiles.
• Calciferous glands: These eliminate calcium and are found in some roundworms.
• Bile: Secreted by the liver in mammals (aiding in fat digestion), it also excretes hemoglobin from dead erythrocytes.
• Sweat glands: Produce sweat, which has a composition similar to urine but is more diluted.