Excretory System Help maintain homeostasis by regulating water balance and removing harmful substances. Osmoregulation – The absorption and excretion of water and dissolved substances so that proper water balance is maintained between the organism and its surroundings. – Example: Marine fish – Constantly drink, rarely urinate, & secrete accumulated salts. Freshwater fish – Rarely drink, constantly urinate, & absorb salts through their gills.

Various excretory mechanisms 1. Contractile vacuoles 1. Found in various protists, such as paramecia and amoebas. 2. Vacuoles accumulate water, merge with the plasma membrane, and release the water to the environment. 2. Flame cells 1. Found in Platyhelminthes, such as planaria. 2. Flames cells are distributed along a branched tube system that permeates the flatworm. 3. Body fluids are filtered across the flame cells. 4. Wastes (water and salts) are excreted from the tube system through pores that exit the body.

Various excretory mechanisms 3. Nephridia 1. Occurs in pairs within each segment of most annelids, such as earthworm. 2. Interstitial fluids enter a nephridium through a ciliated opening called a nephrostome. 3. Fluids are concentrated as they pass through the collecting tubule due to selective secretion of materials into the surrounding coelomic fluid. 4. Blood capillaries that surround the tubule reabsorb the secreted material. 5. At the end of the collecting tubule, the concentrated waste materials are excreted through an excretory pore.

Various excretory mechanisms 4. Malpighian tubules 1. Found in many arthropods, such as terrestrial insect 2. Tubes attached to the midsection of the digestive tract collect body fluids from the hemolymph. 3. The fluids, include both nitrogen waste and materials to be retained (water and salts), are deposited into the digestive tract. 4. As the fluids pass the digestive tract, materials to be retained pass back out through the walls of the digestive tract. 5. Wastes continue in the tract and are excreted through the anus.

Various excretory mechanisms 5. Kidney 1. Produce waste fluids called urine. 2. Cortex; Medulla; Renal pelvis 3. Nephrons a. Bowman’s capsule b. Glomerulus c. Convoluted tubule 1. Proximal tubule 2. Loop of Henle 3. Distal tubule d. Collecting duct

Operation of human nephron consists of three processes. Filtration – Bowman’s capsule – Glucose; salts; vitamins; nitrogen wastes filtrate Secretion – Movement of material from the interstitial fluids joins the filtrate. Reabsorption – Water moves out of descending portion of the loop of Henle and filtrate becomes more concentrated. – Filtrate becomes more diluted due to passive and active transport of salts out of the ascending portion of the loop of Henle. – Water moves out of the filtrate as it flows down the collecting tubule through salts concentrated interstitial fluids of the medulla.

Hormonal Influence of Osmoregulation Antidiuretic hormone (ADH) – Increases the reabsorption of water in the collecting tubule. – Increases the concentration of salts in the urine Aldosterone – Increases both the reabsorption of water and the reabsorption of Na+. – Increases the permeability of the distal tubule to Na+.

Nitrogen waste Aquatic animals excrete NH 3 directly into the surrounding water. Mammals convert NH 3 to urea in their livers. Birds, insects, and many reptiles convert urea to uric acid. – Uric acid is mostly insoluble in water, it precipitates and forms a solid. – Allows considerable water conservation by permitting the excretion of nitrogen waste as a solid.