Understanding Invertebrate and Vertebrate Excretory Systems

Invertebrate Excretory Systems (Protonephridia): These are found in animals with no coelom, such as flatworms. They consist of highly branched tubes that end in a few cells with cilia (flame cells) or flagella (solenocytes). Both cilia and flagella are located in the lumen of the tube, and their movement causes the continuous elimination of waste substances. (Metanephridia): This system occurs in coelomates (annelids, mollusks, etc.). It is made up of a coiled tube surrounded by a capillary network, with two openings: the external (nephridiopore) and internal (nephrostome), which opens into the coelomic cavity. Inside the metanephridia, reabsorption occurs for compounds that are still useful, while waste substances are expelled outside through the Malpighian tubules. (Malpighian Tubules): This is an adaptation of the tubes in insects. They are thin, closed at one end, and open at the other to the gastrointestinal tract. Waste, along with plenty of water and solutes, passes into the tube. While waste is expelled through the anus, water and solutes are recovered in the back of the green glands. (Intestinal Glands): Also called antennal glands, these are located at the base of the antennae in crustaceans. Structurally, they consist of a blind sac, where waste material is collected, a long tubule where useful substances are reabsorbed, and a bladder through which waste material is removed via a hole or nephridiopore.

Urine Formation in Vertebrates: This process occurs in the nephrons and involves three stages: (1) Glomerular Filtration: This occurs in the Bowman's capsule and is driven by pressure differences between the glomerular capillaries and the capsule. The pressure in the glomerular capillaries is a consequence of the caliber of the intake ducts of the blood being greater than that of the outflow. As a result of this pressure, a liquid called filtrate is filtered into Bowman's capsule, which consists of waste substances and usable materials. (2) Tubular Reabsorption: This process occurs after glomerular filtration, where a large amount of usable chemicals in the nephron tubules is reabsorbed. In different sections of the nephron, various substances are reabsorbed through the numerous capillaries surrounding them, which return to the blood. (3) Secretion: This is a process by which some substances pass from the capillaries surrounding the nephron into the nephron, especially within the distal tubule. Secretion is important for maintaining the concentration of certain ions, particularly H⁺ and K⁺. At the end of this process, urine is formed, which passes through the collecting ducts of the kidney. These ducts converge, pouring the urine into the renal pelvis through the ureters, and it reaches the bladder, where it is stored until being expelled to the outside through the urethra.