Excretory Systems and Circulatory Fluids in Animals

Excretory Organs

Malpighian Tubes

These are typical of insects. They consist of thin tubular outgrowths of the digestive tract with a closed end in contact with the body cavity and an open end in the digestive tract. Hemolymph is filtered through them, and water is recovered in the upper intestine.

Green Glands

Exclusive to crustaceans, these are located at the base of the antennae or jaws. They consist of a sac that collects waste products, a tubule where useful substances are reabsorbed, and a bladder from which waste is removed through a pore called the nephridiopore.

Metanephridia

Found in annelids and mollusks, these are tubes open at both ends. The inner end opens to the body cavity and is shaped like a funnel surrounded by cilia (nephrostomes), while the other end opens to the outside via a nephridiopore. Fluid containing waste products is drawn by cilia through the nephrostomes into the tubules, where useful substances are reabsorbed before waste is expelled.

Nephrons

The nephron consists of a renal corpuscle where blood filtration occurs, and a convoluted tube of variable length with different morphological and functional specializations where resorption and active removal of solutes occur. Tubules of several nephrons meet in collecting ducts and eventually empty into the ureter.

Protonephridia

Common in less evolved animals like flatworms and nematodes, these consist of a series of branched closed tubes whose inner ends terminate in ciliated cells. Flagellated cells circulate the excretory fluid inside the tube. Once excess water is absorbed, the final product is removed through excretory pores.

Circulating Fluids

• Hydrolymph: Proper to echinoderms. Its composition is similar to seawater and it does not transport gases. Its functions are limited to the transport of nutrients and metabolic waste.
• Hemolymph: Found in many invertebrates, such as arthropods and mollusks. It contains the respiratory pigment hemocyanin.
• Lymph: Unique to vertebrates. Its composition is similar to blood but lacks erythrocytes and platelets.
• Blood: Contains small cells without nuclei. In humans, the concentration is higher in men than in women. Red blood cells contain hemoglobin, an iron-rich protein responsible for the red color of blood and the transport of O2 and CO2.

Vessels and Nodes

Vessels are composed of a capillary network. They are highly permeable and cluster into nodes along their route. Lymph nodes are scattered throughout the body and are tissues where lymphocyte maturation concludes.