Understanding Excretory Systems in Animals
The excretory apparatus in cell metabolism expels a number of substances from the body, some of which are very toxic, such as nitrogenous waste. Others are not toxic but can pose a problem for the animal, depending on its habitat, such as mineral salts for aquatic animals. Many metabolic wastes are excreted through the skin, even in highly evolved animals. However, there are specialized structures in the leak of the internal environment that, in addition to expelling toxic substances, control the parameters of water, minerals, and nutrients inside the animal. Nitrogen removal is performed by different molecular forms, such as ammonia, urea, or uric acid. The expulsion in the form of ammonia involves the ability to capture large volumes of water steadily, since ammonia must be removed immediately and dissolved in water. If this were not the case, the animal would die. Therefore, animals that expel ammonia nitrogen as a waste product are those that live in water, such as bony fishes. These animals are called ammonotelic. Sharks and rays, amphibians in adulthood, turtles, and mammals expel urea as a nitrogenous waste product. These animals are called ureotelic. Urea is formed when amine radicals attach to carbon. This substance, despite being toxic, can be stored inside the animal as long as it is dissolved in water. Animals that need to restrict water loss, such as insects or reptiles, or those that cannot accumulate large amounts of water due to their lifestyle, such as birds, expel uric acid as nitrogenous waste. These animals are called uricotelic. This substance is expelled in solid form, resulting in minimal water loss.
Excretory systems in invertebrate diblastic animals eliminate nitrogenous substances by diffusion. This system is also followed by simple triblastic animals. However, it is more common to find specific structures to perform this function. We find protonephridia, metanephridia, Malpighian tubes, glands, and green coxal glands. Protonephridia are simple structures found in acoelomates or pseudocoelomates. There are two types of protonephridia: flame cells, which are large cells with cilia. Connecting cells inside the body lead to the outside through a small canal. Nitrogenous products pass from one cell to another until they reach the flame cell, which discharges them into the environment, thanks to the current created by the movement of the cilia. Solenocytes are large, flagellated cells with a collar. They are associated with other cells in a chamber from which nitrogenous substances are ejected, leaving the body through the action of the flagella. Metanephridia appear in annelids, mollusks, and some arthropods. They are coiled tubes with two openings. One end is the nephrostome, which is in contact with the coelomic cavity and removes all types of substances. In the metanephridial tube, called the nephroduct, useful compounds are reabsorbed. Toxic substances are expelled outside through the nephropore. Malpighian tubes (or Malpighian) are structures found in insects. They are tubules with one end closed and another opening into the final leg of the animal’s gut. Substances are captured from the internal cavity and expelled into the intestine. In this area, useful substances are reabsorbed and nitrogenous waste is expelled to the outside. Green glands (or antennal) appear in crustaceans. They are located under the antennae and consist of a bag that contains toxic compounds, a long tube that ends in the bladder, which is a widened zone where nitrogenous substances accumulate and are ejected through the nephridiopore. Coxal glands are structures similar to the green glands of crustaceans, which appear in arachnids. They are found near the coxae, which are the first joints of the legs.
Excretion in vertebrates can be performed by many body structures. These include the skin and exocrine glands, which can expel dissolved substances. Additionally, the respiratory system discharges CO2, a metabolic waste product of cellular activity, along with other substances the body does not want, dispersed in the air humidity. However, vertebrates possess specific organs for the elimination of nitrogenous substances. In addition, as in other animals, the excretory system maintains a constant internal environment regarding the levels of certain substances essential for life. The organs responsible for carrying out these functions are the kidneys. They are paired organs formed by renal tubules. There are three types of filtering structures: Pronephros are structures that appear in vertebrate embryos. They consist of large numbers of nephrostomes that connect to a larger tube called the ureter. The filtrate contained in the nephrostome is a glomerulus formed by capillaries. Mesonephros appear in fish and amphibians in the adult stage and in embryos of reptiles, birds, and mammals. The kidney consists of a large number of tubules that, in their initial area in contact with the circulatory system, have an expanded section called Bowman’s capsule. About this capsule, the nephrostome is atrophied. Bowman’s capsule absorbs liquid leaking from the capillaries of the glomerulus. Amphibians, like other animals, use their kidneys and skin glands to expel toxic substances. Metanephros appears in reptiles, birds, and mammals. The kidney is composed of tubules called nephrons. The nephrons are tubes that are divided into the following parts: Bowman’s capsule: an initial widened zone that collects the liquid seeping from the capillaries of the glomerulus. Proximal tubule: an area that produces tortuous reabsorption of solutes in the filtrate that are necessary for the body, returning them to the blood. Loop of Henle: a narrow, curved section that concentrates the liquid flowing through the nephron, surrounded by blood vessels. Distal convoluted tubule: another tortuous area where reabsorption of substances continues and increases the concentration of circulating fluid. It flows into the collecting tubule.
The Role of Relationship The environment in which animals live is constantly changing. Many of these changes are detected by the animals through their sense organs. Observed changes that induce the development of a response are called stimuli. The stimulus can come from inside the animal, such as hunger or pain, or be produced externally, such as changes in temperature or light. They can be produced by animals of the same species, such as cries of distress or colorful displays of the opposite sex, or produced by animals of different species, such as the production of odorants to mark territory or distinctive sounds. Responses to a stimulus can be positive if the animal approaches the stimulus, or negative if the animal moves away from it, whether external, as a defense or attack, or internal, such as hormone production. To detect these stimuli, the animal has senses that collect visual, tactile, auditory, or chemical information, and target organs for appropriate responses. Coordination systems integrate the information received and prepare the response to be carried out by effector organs. These coordinating systems are the nervous system and the endocrine system.
In animals with a highly developed nervous system, there are protective cells for neurons that feed them. These cells form a supporting framework or prevent the spread of nerve impulses from unwanted areas. They are called glia. The nerve impulse transmits information by polarity changes in the membranes of cells, due to the presence of neurotransmitters that alter the ion concentration within the cell. In poorly developed animals, the nerve impulse is generated without the presence of neurotransmitters. Furthermore, within the neuron, proteins and ions are negatively charged. This difference in ion concentration also produces a potential difference between the outside and inside of the cell membrane. The value reached is about -70 millivolts (negative inside with respect to the positive charges on the outside). This variation between the exterior and the interior is achieved by the pump function of sodium/potassium (Na+/K+) ATPase. It expels three sodium ions that were inside the neuron and introduces two potassium ions from outside. Sodium ions cannot re-enter the neuron because the membrane is impermeable to sodium. Therefore, the concentration of sodium ions outside is high. In addition, three positive charges are lost each time the Na+/K+ pump runs, but only two charges of potassium are gained. This causes the outside to have more positive charges than the inside, creating a potential difference. It is said that the neuron is at resting potential, ready to receive a nerve impulse. When the nerve impulse reaches a resting neuron, the membrane is depolarized, opening channels for sodium. As the sodium concentration is very high outside, when the sodium channels open, the polarity is reversed, causing the inside of the neuron to reach an electropositive value with respect to the exterior. If the depolarization causes a change in potential of 120 mV over the resting potential, it is said to have reached the action potential, which involves the transmission of nerve impulses to the next neuron, as it creates conditions in the cell interior to secrete the neurotransmitter to the area of contact between neurons. The nerve impulse follows the law of all or nothing. This means that if the membrane depolarization does not reach a minimum potential, called the threshold potential, a nerve impulse is not transmitted. However, if this potential is exceeded, a nerve impulse is sent, always with the same intensity. Synapses between neurons in most animals are not physically connected. There is a small space between them, called the synaptic cleft, into which the neurotransmitter is released from the presynaptic membrane, the membrane of the neuron that sends nerve impulses, to the postsynaptic membrane, the membrane of the neuron that receives nerve impulses. The neurotransmitter is the molecule responsible for depolarizing the membrane of the neuron that receives nerve impulses, opening the previously closed sodium channels. Once the neuron emits a nerve impulse, it must return to its initial resting potential. To achieve this, the membrane is repolarized, closing the sodium channels that were opened by the presence of the neurotransmitter. The neurotransmitter is destroyed by enzymes, and the resting potential is achieved by the action of the Na+/K+ pump.
Nervous System The nervous system is a coordination system. It collects information received by the senses, processes it, and produces the appropriate response to be made by effector organs. The nervous system generates rapid responses that transmit nerve impulses to muscles, whether smooth or striated, producing movement. This movement can be applied to bones or internal organs like the heart, bowel, or glands. The nervous system consists of a set of cells called neurons that are connected via synapses to transmit information from one to another. The neuron is the structural and functional unit of the nervous system. Its structure can be distinguished as follows: the cell body, which is the wider area containing almost all cellular organelles; the dendrites, which are extensions of the cell body that tend to be numerous and receive nerve impulses from other neurons; and the axon, which is an extension of the cell body that usually occurs as one per neuron, although it can branch at the end. The axon sends a nerve impulse to another neuron or effector organ. According to the functions they perform, neurons can be classified into sensory neurons, which receive information and transfer it to the central nervous system; association neurons, which connect with other neurons; motor neurons, which connect to an effector organ; and mixed neurons, which perform both sensory and motor functions. Types of Nervous Systems Animals have different types of nervous systems. They range from simple systems, such as those in cnidarians, to complex systems, such as those in vertebrates. The possibilities lie in the presence of a diffuse network, a ventral ganglionic nervous system, a radial system, or a system consisting of a dorsal neural tube. Cnidarians have diffuse nerve cells located in the epidermis. The nerve impulse expands in all directions because the neuron transmits information in both directions. Higher animals have polarized neurons, with one part that collects information and another that sends it. The ventral ganglionic nervous system is located in the ventral area, in the same plane where the mouth is situated. It consists of nodes, which are clusters of neurons, and nerve cords, which are formed by extensions of neurons. In platyhelminths, there are two nodes in the anterior body, known as the cephalic ganglia. These are connected to nerve cords that innervate the entire body along the belly of the animal. There are secondary cords, called commissures, which innervate a pair of nodes in each area of the body. The complete system provides a step-shaped structure of knots, with steps formed by the corners and connective railings. The lymph nodes are nervous. Mollusks have a ring of periesophageal nerves around the digestive tract, with three cerebroidal nodes. From this area, a pair of nerve cords innervate the foot and another pair innervates the visceral mass. In cephalopods, the nervous system is more evolved and has only two nerve cords leading to a highly advanced brain. In annelids, cerebroidal nodes are present, which are continued by a ventral ganglionic chain formed by the fusion of pairs of nodes in each metamerism, losing the appearance of “knots on a step.” In the arthropod nervous system, nodal concentration increases, mainly in the cephalic region, due to the development of sensory organs. A brain is formed by three nodes together, called the protocerebrum, which supplies the eyes; the deutocerebrum, which receives information from the antennas and olfactory organs; and the tritocerebrum, which controls the mouthparts. After the third node, a highly concentrated ventral ganglion chain continues, which controls parts of the body independently of the brain. The radial system is found in echinoderms, animals with radial symmetry. They have an oral ring of five branches that start receiving information from the ambulacral system. A second oral ring, deeper, gives rise to five other branches that control the movement of the arms. Finally, an aboral ring supplies five other nerve branches, which innervate the skin between the skin plates. The dorsal neural tube is characteristic of chordates, reaching its maximum development in vertebrates. The system consists of a tube that widens in the anterior part of the animal in the head and continues along the dorsal area, back of the animal. The anterior zone is enlarged into the brain, and then the tube is called the spinal cord. From this central structure, the central nervous system, nerves start that innervate the entire body and form the peripheral nervous system. The capabilities that facilitate a nervous system as perfect as that possessed by vertebrate animals make them very versatile. Accurately seeing and looking, smelling and recognizing odors, hearing, and even understanding are tasks involving the prior existence of a complex nervous system.