Circulatory and Excretory Systems in Animals
Lymphatic Circulatory System
The lymphatic circulatory system, characteristic of vertebrates, is made up of lymphatic vessels and lymph nodes. Lymph is the circulating fluid. The lymphatic vessels are formed as terminal hairs, with one end closed. They are found in almost all tissue spaces and merge to form larger lymphatic vessels containing valves inside that prevent the backward flow of lymph. The lymphatic vessels drain into the blood circulatory system.
Lymph nodes are structures located along lymphatic vessels. Their mission is to produce lymphocytes, a type of white blood cell involved in defending the body. Lymph recovers part of the extracellular fluid, primarily high molecular weight proteins.
Transport of Gases
Oxygen Transport in Blood
The oxygen transfer from the respiratory surface is carried to every cell through the circulatory system. Since the solubility of oxygen in water is low, the transport of gas dissolved in it is only effective in animals with a simple organization and no major metabolic demands. For animals with high metabolic activity, this oxygen transport system is inadequate. Therefore, along the evolutionary process of metazoans, substances capable of capturing and releasing oxygen were incorporated into the circulating fluids. These are called respiratory pigments. The respiratory pigments are heteroproteins containing metal, copper, or iron, that reversibly bind with oxygen.
Transport of Carbon Dioxide in Blood
The CO2 produced during cellular respiration passes into body fluids by diffusion. Because of its higher solubility compared to oxygen, it is transported dissolved directly in them. In higher vertebrates, carbon dioxide is also bound to hemoglobin in small proportions. This union does not interfere with hemoglobin and oxygen, as the binding sites are different for both gases. However, most of the carbon dioxide combines with water in the blood plasma and is transported as bicarbonate. When the blood reaches the lungs, the reverse process takes place, and bicarbonate dissociates, releasing CO2.
Elimination of Waste Products: Excretion
Nitrogenous substances can be expelled in three ways:
- Ammonia: Ammonium salts are the nitrogenous product of animals living in aquatic environments, which have a large amount of water to dilute and expel it quickly. These animals are called ammoniotelic. In terrestrial animals, ammonia, which is toxic, is converted in liver cells into uric acid (uriotelic animals) or urea (ureotelic animals).
- Uric acid: It is insoluble, so it is excreted as a semisolid. It is typical of insects, terrestrial reptiles, and terrestrial birds. Oviparous animals, whose embryos form inside an egg, accumulate their excretion products in it.
- Urea: It is the major nitrogenous waste of ureotelic animals. As it is less toxic than ammonia, it requires a smaller amount of water for disposal. This group includes amphibians and mammals.
Biological Significance of Excretion: Internal Environmental Regulation
The term excretion is usually associated with the elimination of waste products. This, however, is only one of its functions. Excretion is a regulatory system of the internal environment that precisely determines the amount of water and salts in the body at all times and expels the excess abroad. Excretion involves several processes:
- The elimination of waste products.
- Osmoregulation or osmotic pressure regulation.
- Ionoregulation or regulation of the ions of the internal environment.
Excretion Without Specialized Structures
Sponges and coelenterates perform excretion by diffusion.
Excretion Through Specialized Structures
In multicellular animals, whose cells are not in direct contact with the external environment, excretion must be carried out through specialized excretory apparatuses that take the waste products from the circulating fluids and expel them out of the animal’s body. They all perform three main processes:
- Filtration: Fluids pass over the body inside the excretory tubes by diffusion.
- Resorption: It is the return to body fluids of large amounts of water and substances useful for the organism.
- Secretion: Transfer of certain substances from the body to the excretory tubes. The fluid thus obtained is the final urine, which will be expelled to the outside.