 Now, difference in kidney of different organisms in fresh and marine water are their specific adaptations. As we talked in case of the fish, the freshwater fish and the marine fish, that is freshwater fish produces dilute urine with lot much quantities of water and the marine fish they produces less urine, concentrated urine with very decreased quantity of water. Now, we come on the human beings, the human excretory system. Humans produces carbon dioxide, water, ammonia and other waste products and they have to remove these consistently to maintain their homeostatic balance. Because this homeostatic balance is responsible for performing all the functions of the body. In the case of increase in pH for example, or a decrease in pH, many enzymes of the body may not work. In the case of increase or decrease in pH or some other solute concentration of the body are many cells may not respond as they are doing normally to the environmental situations, to the internal environment, maybe to the external environment. The excretory system is extremely important and the major organ which is involved in human excretory system is the kidney. There are other accessory structures which are associated with kidneys. Let us have a look on a diagram to look at the structure of human excretory system. Let us look at this diagram. This diagram shows the excretory organs in human beings. If we start from the top, we can see on the right side in red color the aorta. Aorta is the main vessel with oxygenated blood coming from the heart. On the left side we can see in blue the vena keva, the major vein which is taking the blood from the body back to the heart. This is deoxygenated blood. We can see that both of these vessels are entering on both sides into two bean shaped structures, the artery and the vein. These bean shaped structures are the kidneys in human beings. We can see that on the top of the kidney there are two light yellow colored bodies. These bodies are called adrenal glands. Then if we come down we can see that towards the inner side of these bean shaped kidneys there is a depression. Through this depression a canal is coming out. This part of the kidney is called hillus and the canal which is coming out which is initially very thick and then it becomes thin. This is called the renal pelvis. When it goes down and becomes a thin canal, a thin tube, this is called the ureter. You can see that one pelvis is coming out of each kidney. It means that both kidneys are giving rise to a pelvis and then to ureter. Both of these ureters, right ureter and the left ureter are going down to an organ called bladder also called urinary bladder. This is the part of human excretory system that stores urine for temporary time. The artery which is going inside each kidney on the both sides are called renal arteries, renal arteries left and right and the veins which are entering in the arteries are called renal vein left and right. This is the generalized diagram of the human excretory system. In human beings a pair of kidneys exist inside the abdominal cavity. These are attached to the dorsal body wall. They are present and they are slightly covered with a fatty layer to protect them but they keep attached to the dorsal body wall so that they are protected in a way that they are attached and they are not exposed to the any type of interaction with other organs. The convex part of the kidney that is the part as we have seen in the previous diagram which have a depression is towards the vertebral column and the more rounded part is towards is facing other than towards the other side of the vertebral column. One kidney as you may have observed if you go back to the diagram, one kidney is slightly higher or anterior in position than the other kidney. Actually and the other kidney is towards more posterior side. This difference is actually due to presence of a large stomach towards that side due to the presence of a large stomach because stomach in human being is a large organ. The kidney's position is slightly posterior in comparison to the other one which is below the liver. The convex part of kidney that is the part towards the vertebral column, the depression give rise to a space or we can say provide a space where the renal vein, renal arteries and the nerves can enter the kidneys. So this depression is actually the place where the renal artery, renal vein and the nerves enters or leaves the kidney. Then comes the ureters and then the urinary bladder. Let's have a look on a diagram, a surface diagram as we previously observed and now we are looking at in a more larger picture. Kidney looks like a beam externally. If we do not observe it under cross section, we can see more clear way that an artery is entering inside the kidney and when it is entering it is branching. We can see that a vein is also entering kidney, actually leaving kidney, it is also branched. Then renal pelvis is coming out or we can say the depression is also giving rise to renal pelvis which is making down there a ureter, a thin duct which is actually for collecting the urine and moving it downwards towards the urinary bladder. Let's have a look on a cross section now. If we cut the kidney and look at the cut section, a human kidney consists of three major regions. One is the cortex, you can see in the diagram that the external most part in pinkish color is called the cortex. Then the next part which is light pinkish in color is called renal medulla. This part also have various structures which are looking dark brown in color. These are called the pyramids, pyramids of the kidney because their shape resembles the pyramids. There are many pyramids inside and then these pyramids are entering inside the renal pelvis. A funnel shape or an extended portion, we can call it a funnel shape portion. All of the pyramids are entering or attached to this funnel shape portion and this funnel shape portion is actually called the renal pelvis. These pyramids have those units which are responsible for producing the urine and then all of these functional units release that urine or collect that or send that towards the renal pelvis which lead towards the ureter and the urine goes down. Let us have a look on these functional units. Let us look at the next diagram. If we cut a section of kidney through from cortex through medulla, we can see one functional unit as we are observing in the diagram towards right. This functional unit is called a nephron. The kidney have so many nephrons to produce urine. Let us have a look on different parts of the nephron. Nephron is the functional unit of kidney. This is responsible. All the nephrons are collectively responsible for producing a urine and conducting various processes for the production of urine. The first part as you can see is called a renal corpuscle. There is a small corpuscle like structure, a capsule like structure from which this nephron actually starts where the artery actually a part of artery enters. Then comes the next part, a highly convoluted part which is called a proximal tibule. This proximal tibule is convoluted, highly convoluted. Then it goes down in the form of the third part, it makes the loop of Henle. As you can see that renal corpuscle and the proximal tibule are present inside the cortex. Then the loop of Henle is entering inside the medulla. Now there are two types of nephrons. In one type of nephron the loop of Henle is not that long that it enters too deep in the medulla. We call them cortical nephrons. There are other nephrons like this which you are observing here who have long loops of Henle and their loop of Henle goes deep down into the medulla. These are called medullary nephrons. So nephrons themselves are of two types, cortical nephrons and the medullary nephrons. Cortical nephrons whose loop of Henle is short and the medullary nephrons whose loop of Henle is very long. After the loop of Henle, loop of Henle you can see have two parts. One is descending going down towards medulla and then making a u down there in the medulla. You can say it takes a u turn and then it goes up. This is the proximal part. Then this goes back into the cortex, convoluted again and makes the next part which is called a distal tibule. Distal tibule remains in the cortex. After distal tibule it is distal tibule as you can see in the diagram is again convoluted. It goes down and make a comparatively larger duct called the collecting duct. The collecting duct as you can see goes down till the end of the medulla and this actually produces the final u turn towards the u return. So a nephron number one, nephrons are of two types, cortical nephrons and the medullary nephron. Cortical whose loop of Henle is short, medullary whose loop of Henle is very long and goes deep in the medulla. Then various parts of the nephron are corpuscle, the renal corpuscle. Then proximal tibule, then a loop of Henle, then a distal tibule and then the collecting duct. These are various parts of the nephron. Let us have a look on another diagram for some more details of the renal corpuscle. The renal corpuscle actually consists of an extension of the convoluted tibule in the form of a bow or a capsule. There is as you can see in the diagram the efferent arteriole, the artery is coming inside which is giving rise to an arteriole and this arteriole is making a complex network of capillaries in the form of a circular structure, a rounded structure. This rounded structure convoluted network of capillaries is called a glomerulus. So glomerulus is a network of capillaries which is surrounded by an extension of the renal tibule and makes it a whole corpuscle or a capsule. This is sometimes also called termed as the bowman's capsule in the name of the discoverer. So it means that artery enters inside the kidney and gives rise to arteriole and arteriole enters into the glomerulus, actually the bowman's capsule and makes the glomerulus. The glomerulus is surrounded by the bowman's capsule and then you can see next that this artery, the same artery on the other side from the glomerulus is moving up and moving from the top of the convoluted tibules. It is making a whole network around the convoluted tibule, proximal tibule and the loop of Henley and then it is making a capillary network down there and after that this capillary network is joining with the venule which is going back in the vein which is coming inside the kidney. So it means that each nephron is surrounded from every side, almost every side except for the collecting duct. It is surrounded by the blood vessel, by the artery first then a capillary network and then going back towards the venule. So nephron is richly supplied with blood vessel and this is a requirement for exchange of the materials, the waste products between the blood and the renal tibule.