 Assalamu alaikum. Today we are going to talk about the excretory system, excretion. Excretion means to remove something. In organisms, when we are dealing with the living organisms, then excretion means removal of different materials from the body of organisms. As we know that organisms, the living organisms always conduct carry out the metabolism. They have to break down various things and they have to make various things. Metabolism is a combination of catabolism and anabolism. Anabolism is a process which is making of things. Whenever body need to make certain things, this process is called anabolism. In this process, energy is utilized. When body is performing some functions, energy is also utilized. This energy has to be produced. Body conduct carry out catabolism to produce energy. As we know that the major molecule which is involved in production of energy is the glucose molecule. By the breakdown of glucose molecules, we produce, the organisms produce energy for them. This is the major catabolism, the major catabolic pathways. But there are lot many other metabolic pathways which are always carried out inside the body. All of these catabolic pathways releases some byproducts, some extra products or we can say the waste products. These are those things, those metabolites we must say in a more proper term. These are those metabolites or the end products of the metabolism which are not required by the body. And because catabolism is always, it is always in action, it is always carried out. So, these products keep producing and if they keep producing, they tend to be accumulated inside the body if they are not removed on time. This is very important that organisms get rid of these byproducts or waste products. Let us look at some major reactions that occur inside the body. Have a look on the slide. It shows that carbohydrates when react with oxygen produces carbon dioxide, water and energy. As we know that carbohydrates, proteins and lipids are three major food components of the food. The proteins, when they react with oxygen, they are catabolized, they are broken down to carbon dioxide, water, ammonia and energy. Lipids, when they react with oxygen, they produce carbon dioxide, water and energy. Energy is useful for body. Body utilizes energy for its anabolic reactions and all the functions. But carbon dioxide, water and ammonia, as we observe in all of the three reactions with carbohydrates, with proteins or lipids, three things or three products, waste products are produced, carbon dioxide, water and ammonia. If these reactions goes on and these metabolic byproducts keep producing and they are not removed, they will be accumulated inside the body and their accumulation is not good or we must say these are dangerous or hazardous for the body. Rise in carbon dioxide may result in decrease in the pH of the body. Carbon dioxide, if it is increased in the body, it causes the decrease in pH, which means that the pH of the body will become more acidic. If more carbon dioxide is produced in the blood and this more acidic pH could be very dangerous for different organs and the vessels of the blood itself, for the vessels of the blood itself. Water, if it is present in excess, this may give rise to various problems. For example, if water is water normally as in the plasma is about 92% of the plasma. If its concentration increases, then it may result in removal of the fluid, the water content from the blood vessels towards different tissues, which results in accumulation of water content in different tissue fluids of the body, which is called dropsy, which may lead towards you can say fluid retention in different parts of the body swelling. If the water content is decreased, decreased than normal, then this is called dehydration. This may also give rise to various problems. For example, decrease in blood pressure. If water content, because we know that there is a specific volume of the blood inside the body and the vessels, they are also in a specific number and their sizes are specific. So, if the volume of water decrease, then the volume of plasma will decrease inside the vessels and this will result in drop of blood pressure, which could result in serious problems. This has to be taken seriously and have to be corrected. Ammonia is highly toxic, very very toxic for the body tissues. If the concentration of ammonia goes high, then this is very risky for the tissues because it is toxic means this is poisonous for the tissues of the body. Body readily convert ammonia into Julia, which is less toxic product, but even this less toxic product also has to be removed from the body. So, for removal of all of these ingredients, these waste products excretory systems are required. As we know that carbon dioxide is also removed by the lungs during the exhalation process. During the inhalation and exhalation process, the respiration, organismic respiration, carbon dioxide is removed from the body. Ammonia is converted by the liver into Julia, but then Julia has to be removed, particularly the nitrogenous waste materials and water has to be removed by a system called excretory system. We talk about excretion in different vertebrate land animals as an example. As we know that land animals, the vertebrates are in a huge diversity There are a lot many types of vertebrates present on the earth in the biosphere. They are present on land, they are present in water, they are present in fresh water, they are present in marine water, live on the trees, live in the deserts. So, the vertebrates are living in diverse environments and they are of diverse types. Let us look at the different waste products that the vertebrates produce. Carbon dioxide, mineral salts, Julia, creatinine, uric acid, excess water, ammonia. So, the vertebrates produces a variety of waste products. To remove these waste products, different organs play their role. For example, carbon dioxide is removed by respiration, the respiratory system. Water is removed by sweating through skin, but the major role is played by an excretory system helped by a structure called kidney. Kidneys are the most important structures and they are the major contributors in excretory systems in vertebrates. Let us have a look on excretion generally in animal just briefly. There are organisms which are unicellular, there are small animals, there are large animals. In very small animals, the unicellular animals, because the body consists of just a single cell like an amoeba. This is very easy to exchange their waste material or the other materials with the environment. Just through their plasma membrane, the cell membrane, these organisms exchange their materials with the environment. They can remove their waste products outside. For example, the amoeba have to face the excess of water because it is living in a hypotonic environment in fresh water. Amoeba have contractile vacuole inside which continuously removes water from its body. Amoeba removes ammonia directly through the skin, through the cell membrane inside the water because it is living in the water. There are small animals which have simpler structures. They can remove their waste products directly in the water. For example, hydro, small organism, multi-cellular organism can easily remove its waste products in the water coming in and going out through its gastrovascular cavity. It have a cavity inside the body in the center which is called a gastrovascular cavity. There is an opening of that cavity and water enters in and fill that cavity and this organism can remove its waste products directly in the water. That is because water is so close to the cells that they can directly exchange their waste products or maybe other products like oxygen and other materials with that water and then water goes out. In complex organisms like vertebrates, these mechanisms are more and more complex and their habitats as we previously talked about are very diverse. There are adaptations to the habitat in the excretory systems. Vertebrates live in water, in fresh water, in brackish water, like for example marine water. They live in deserts, they live in terrestrial environment, even aerial. The birds live in air, they fly in air for most of their time. Organisms live in water, fresh water, the fish, the fresh water fish have to face with lots and lots of quantities of water. The organisms living in brackish water like marine water, the sea fish, have to face a very high concentration of salts. The animals living in desert, because in desert there is a shortage of water, they always have a problem of water shortage. That is they don't have good quantities of water for drinking. The terrestrial animals in comparison are in a balanced position. These organisms according to their habitats and according to the availability of water and availability of other foods and other things, adapted different types of kidneys and related structures. For example the desert animals, they have their kidneys with the comparatively say smaller nephrons in a way, their kidneys are designed in a way that they produce a very concentrated urinary or we can say excretory products, mostly in the form of crystals, which have minimum quantity of water. The fresh water fish, because they are exposed to larger quantities of water, they have to excrete large quantities of water from their body, urine, water in the form of urine. So their kidneys are adapted accordingly for removal of excess of water. The marine fish on the other hand, because they have to face brackish water, water with very high salt concentrations and there is a reduced quantity of water in that say solution, we can say the brackish water solution. Their kidneys are designed in this way that they produce minimum urine, just like the terrestrial animals. They have a very adaptive and we must say advanced form of kidney we will talk more about the human kidney, which is the most complicated and which is designed according to the needs and the requirements of the human beings. Some other organs also act as excretory organs. For example skin, skin also act as an excretory structure. As we say that mineral salts and water are the common metabolic waste products and skin act as excretory organ. It removes mineral salts and water for example in summer in the form of sweat. We experience sweat commonly and the sweat actually consists of the mineral salts and water. So the skin also sometimes act as an excretory organ. Some organisms which are particularly living in very high salt environment have a specific organ called salt gland. This is a gland inside their head behind or behind or close the eyes which continuously remove the salts from their body and in some birds this salt goes out just like in the form of tears. People says that this bird cries. Actually those birds have salt glands and they continuously excrete salt from their through their eyes and it goes out. So this is how they remove their excess salt with the help of a salt gland. This is a very specific adaptation of some birds which lives in very high brackish water which the water which have a very high concentration of salts. In some organisms intestine also act as an excretory organ. Through intestine water is many times removed mostly intestine act as an excretory organ for excess water. Sometimes it may act for certain salts. Then comes the kidney. Kidney is the major organ which is involved in excretion invertebrates. It also helps in osmoregulation because osmoregulation means the regulation of water inside the body because water is the water regulation is also extremely important because water potential is one of the most important homeostatic balances in the body.