Maintaining a normal level of metabolism in the body, called homeostasis, is carried out by means of neuro-humoral regulation of the processes of respiration, digestion, blood circulation, excretion and reproduction. This article will consider the system of organs of excretion of humans and animals, their structure and functions, as well as the importance in the metabolic reactions of living organisms.
The biological significance of the organs of excretion
As a result of the metabolism that occurs in every cell of a living organism, a large number of toxic substances accumulate: carbon dioxide, ammonia, and salts. To remove them, a system is needed that removes toxins to the external environment. The structure and functions of the organs of the excretory system are studied by anatomy and physiology.
For the first time, a separate excretory organ appears in invertebrate animals having bilateral symmetry. The walls of their body consist of three layers: exo-meso- and endoderm. These organisms include flat and roundworms, and the excretory system in them is represented by protonephridia.
How excretory organs function in flatworms and nematodes
Protonephridia is a system of tubular formations extending from the main longitudinal channel. They are formed from the external germinal leaf - exoderm. Toxins and excess ions are removed to the surface of the helminths body through the pores.
The inner end of protonephridia is equipped with a group of processes - cilia or flagella. Their undulating movements mix the intercellular fluid, which helps to strengthen the filtration functions of the excretory tubes.
Progressive complications of excretory organs in annelids
Ringworms, for example, earthworm, nereis, sand gland, remove metabolic products from their body using methanephridia - the organs of excretion of worms. They have the form of tubules, one end of which is leukiformly expanded and equipped with cilia, and the other goes to the integument of the animal and has a hole - the pore. The complication of excretory organs in earthworms is explained by the appearance of a secondary body cavity - coelom.
Features of the structure and function of malpigium vessels
In representatives of the arthropod type, the excretory organ has the appearance of branching tubes into which dissolved metabolic products and excess water are absorbed from the hemolymph - intracavitary fluid. They are called malpighian vessels and are characteristic of representatives of the classes of arachnids and insects. The latter, in addition to the excretory tubules, have another organ - the fatty body, in which metabolic products accumulate. Malpighian vessels, which received toxic substances, flow into the posterior intestine. From there, metabolic products are secreted out through the anus.
The excretory organ in crustaceans - crayfish, lobsters, spiny lobsters - is represented by green glands, which are altered metanephridia. They are located on the cephalothorax of the animal, behind the base of the antennas. Under the green glands of the crustaceans, there is a bladder that opens with a secretory pore.
Fish excretion organ
In representatives of the class of bone fish, a further complication of the excretory system occurs. It has the appearance of dark red ribbon-like bodies - trunk kidneys located above the swimming bladder. The ureter departs from each of them, through which urine flows into the bladder, and already from it into the urogenital opening. In representatives of the class of cartilaginous fish (sharks, stingrays), the ureters fall into the cloaca, and the bladder is absent.
Based on the structure of the excretory system, all bone fish are divided into three groups: living in fresh water, in salt ponds, as well as a group of so-called migratory ones living in both salt and fresh water due to the characteristics of spawning.
Freshwater fish (perch, crucian carp, carp, bream), in order to avoid excessive intake of water in their bodies, are forced to remove a large amount of fluid through the renal tubules and malpighian glomeruli of the kidneys. So, carp releases up to 120 ml of water per 1 kg of its mass, and catfish - up to 380-400 ml. So that the body does not experience a lack of salts, the gills of freshwater fish act as pumps that pump sodium and chlorine ions from the water. Sea inhabitants - cod, flounder, mackerel - on the contrary, suffer from a lack of water in the body. To avoid dehydration and maintain normal osmotic pressure inside the body, they are forced to drink sea water, which, being filtered in the kidneys, is cleared of salt. Excess sodium chloride is removed through the gills and excreted.
In migratory fish, for example, European eel, there is a “switching” of osmoregulation methods carried out by the kidneys and gills, depending on what water they are in.
Excretory system in amphibians
Being cold-blooded inhabitants of the surface-aquatic environment, amphibians, like fish, remove harmful metabolic products through bare skin and trunk kidneys. In frogs, tritons, Ceylon fish snakes, the excretory organ is represented by paired kidneys located on both sides of the spine, with ureters extending from them, flowing into the cloaca. Partially gaseous metabolic products are removed from them through segments of the lungs, which, along with the skin, perform an excretory function.
Pelvic kidneys - the main organs of excretion of birds and mammals
In the process of evolutionary development, the trunk kidneys are modified into a more progressive form of the excretory organ - the pelvic kidneys. They are located deep in the pelvic cavity, almost next to the cloaca in reptiles and birds, and near the reproductive glands (testes and ovaries) - in mammals. The weight and volume of their kidneys are reduced, but the filtration capacity of renal nephron cells increases significantly, and this leads to the fact that the excretory organs in animals belonging to the classes of birds and mammals purify blood from decomposition products much more effectively and protect the body from dehydration.
In addition, in birds, unlike all other terrestrial vertebrates, there is no bladder, so their urine does not accumulate, and from the ureters it enters immediately into the cloaca, then out. This is a device that reduces the body weight of birds, which is important, given their ability to fly.
Filtration and adsorption functions of human kidneys
In humans, the excretory organ - the kidneys - reaches its highest development and specialization. It can be considered as a very compact (the weight of both kidneys of an adult does not exceed 300 g) biological filter that passes through its cells - nephrons, up to 1500 liters of blood per day. In physiology and medicine, the normal functioning of this organ is given special importance. And in the Chinese health system, Wu-Xing kidneys are the main life-supporting element.
The renal parenchyma contains about 2 million nephrons, consisting of Bowman-Shumlyansky capsules, in which the blood is filtered and primary urine is formed, and convoluted tubules (Henle loop) that provide reabsorption - the selective extraction of glucose, vitamins and low molecular weight proteins from primary urine, and returning them to the bloodstream. As a result of reabsorption, secondary urine is formed. It contains an excess of water, salts, urea. It flows into the renal pelvis, and from them into the ureters, and then into the bladder. This is about 2 l / day. From it, it is removed through the urethra to the outside.
Thus, accumulation of fluid in the cavity of internal organs is not allowed and intoxication of the body is prevented.
Additional organs for the release of metabolic products
In addition to the kidneys, which play a major role in osmoregulation and removal of excess salts and toxins, the lungs, skin, sweat and digestive glands partially perform the excretory function in the human body. So, as a result of gas exchange carried out by the alveoli, which make up the segments of the lungs, carbon dioxide, water vapor, and toxic substances, for example, ethanol decomposition products, are removed. By excretion of sweat glands, urea, excess salts and water are removed. The liver, in addition to the leading role in the digestion process, inactivates the toxic decomposition products of proteins, drugs, alcohol, cadmium and lead salts contained in venous blood.
The work of all organs (kidneys, lungs, skin, digestive and sweat glands), which have an inherent excretory function, ensures the normal course of all metabolic reactions and homeostasis.