 The urine formation by the nephron. Urine formation is quite a long and complex process that consists of few steps. We start from step 1. Step 1 is filtration. It is also called pressure filtration because when blood enters in the kidney, it is entering through the artery, the renal artery. The renal artery enters inside the kidney and give rise to the arterioles. These arterioles as we previously seen in the various diagrams, that the arterioles enters inside the renal capsule and makes the glomerulus. Then it goes out to cover the nephron except for collecting duct. In the renal corpuscle, pressure filtration occurs because there is a network of capillaries inside and there is a corpuscle made by the renal tibials. This is an extension of the renal tibials. Here the process of pressure filtration occurs. Why we call it a pressure filtration? Because we know that in arteries the blood is pushed by the heart, by the contraction of the heart, the pumping action of the heart. Blood is coming with the pressure. So, in the arterioles and in the capillaries, blood is coming with the pressure. And because the glomerular capillaries are in a close contact with the renal capsule, the moments capsule, the exchange of materials, not many materials, occur between the blood capillaries, the glomerulus and the renal capsule. This makes the initial filtrate which is also called Bowman's filtrate because this capsule on the name of discoverer, the renal capsule is called, this capsule is called the Bowman's capsule. We sometimes call it a Bowman's filtrate. This is a very initial filtrate and this filtration occur under high pressure because arterioles coming giving rise to the capillaries, pressure is high. This filtrate have many ingredients except for proteins and the red blood cells. Proteins, they are large, they cannot come out of the capillaries and the RBCs, they are also large, they do not also come out but glucose, amino acid, salts and water comes out of the blood vessels and enter inside the Bowman's capsule and makes the Bowman's filtrate. So, the pressure filtration is the first step in urine formation. Blood is filtered, we can say that blood is filtered under the high blood pressure. Not high blood pressure in those terms that we use normally for a disease but pressure of blood is high here in the renal capsule. So, the things are filtered and enter inside the corpuscle. Let us have a look on the diagram of very close diagram of the renal corpuscle. We can see that there is an efferent arterioles which is entering inside the Bowman's capsule, the glomerulus and the other one is going out which is called the efferent arterioles. Glomerulus consists of the capillaries which are extensive and which have a higher pressure and you can see that the parietal layer of the glomerular capsule is so close to this network of capillaries that due to the pressure in the blood, the pressure in this network the blood is filtered and water, urea, glucose and some amino acids goes out of this fluid and enters the renal capsule. We call it a Bowman's filtrate. Now, we know that this Bowman's filtrate have some essential important ingredients which are required by the body like glucose and amino acids and various salts and even excessive amounts of water. Body cannot afford losing all of these ingredients. So, this initial filtrate is highly filtered again and many of these things which are present inside this filtrate access water, salts, many salts not all, glucose, amino acids, these are reabsorbed by the renal tibule, different parts of the renal tibule. This process is called reabsorption. Reabsorption is highly important and this is actually the major important function of the renal tibule. Now, as we know that renal tibule is highly convoluted and this urine have to pass from all of these convoluted parts of this tibule and this tibule, second fact is this that this tibule is surrounded by a network of capillaries. So, it is richly supplied with blood vessels except for the collecting duct. So, this is easy, the reabsorption in this way is easy in this functional unit of kidney. This occur through this network of capillaries surrounding these tibules. We also can call it a selective reabsorption because of course all of the ingredients are not reabsorbed. Water because due to high pressure in the arterioles is filtered in excessive quantity. So, water is reabsorbed but not all, not much water remains as urine, part of urine. Many salts are reabsorbed because salts are also again just like the water removed in excesses quantities or we can say filtered in excessive quantities due to higher pressure. So, many salts are also reabsorbed for example, the sodium chloride, sodium and chloride ions. These are reabsorbed. Glucose is essentially reabsorbed because glucose is a required molecule by the body, it is reabsorbed. Only we know that only the diabetic patients they have glucose in their urine and this situation is called hyperglycemia. But this is in the case of a disease condition when glucose is present in the body in excessive amount and due to the patient have diabetes it is not utilized by the body. So, this is released in the urine and due to this situation is called hyperglycemia. Diabetes was a disease which was indicated as a sweet urine and people have sweet urine disease. Anyway, then amino acids are essentially reabsorbed because we know that amino acids are used as parts of proteins. They have to make proteins or amino acids are essential ingredients. These are reabsorbed by the phenyl tipu. Let us have a look on an animation which shows you process of reabsorption from the filtrate. See this animation shows that true glomerulus filtrate which is actually the Bormans filtrate which have quite a high concentration of sodium amino acids and glucose which are all required the body. They are present in the filtrate and when they go down these convoluted tibials these are absorbed by the nearby capillary present. The capillary is closed in the closer vicinity and due to difference in the concentration gradients these sodium amino acids and glucose they are reabsorbed from this filtrate and go back to the blood. So, they are returned to the blood. So, from very first part of the convoluted tibule the renal tibule these important ingredients or these important materials molecules go back to the blood. Now have a look on the next diagram. This diagram shows small part of our collecting duct which shows that the lumen of collecting duct and the epithelial cells of these collecting duct which are in close vicinity with the medulla and exchange of different ions and materials occur. Actually in medulla and cortex of the kidney these are the tissues and they have different concentrations of various ions and salts. Because these collecting ducts they are close to those tissues which have different concentrations that we can say have different concentration gradients in relation with the collecting duct and the convoluted and other ducts. The exchange of different salts, water and other materials takes place. Let us have a look on the next diagram which shows that different parts of the nephron proximal tibule for example there is a reabsorption of sodium chloride there is a reabsorption of water there is a reabsorption of various nutrients, potassium ions but you can see that from cortex ammonia and hydrogen ions are going towards or they are actually entering inside this proximal tibule. This process actually occur against the concentration gradient and it occurs by diffusion then you can see down there in the medullary region in the loop of Henley that water is going up, water and sodium chloride are removed and they go back to the medulla and they are absorbed nearby vessels the blood vessels and go back and return back to the blood. If we go up and look at the distal tibule then there is even more removal of a reabsorption of sodium chloride water, bicarbonate ions and some of the potassium and hydrogen are also added to the these tibules the renal tibule. The entrance of this potassium and hydrogen ions in the renal tibule from the surrounding tissue is called is termed as sometimes secretion it says that hydrogen ion and potassium ions are secreted from the tissue of the kidney inside the renal tibule and when they enter inside they are actually included and become part of the urine. We can see at the end in the collecting duct even there is again some reabsorption of some more sodium chloride but at the end it makes the final volume of the urine which have urea, water and some salts. This last part the collecting duct as we know previously that it enters this collecting duct we can say at the end it is extended this extended part reaches at the end of the medulla and enters inside the pelvis the renal pelvis. Now all of the nephrons they are doing this same process at the same time they are filtering lot much of blood at the same time and through whole of this process they are all forming some part of urine and then this urine is entered inside the renal pelvis. Through renal pelvis urine produced by all of these renal tibules all of these nephrons is through renal pelvis it goes down towards the ureters and then this is stored inside the urinary bladder. So, all of the this is the process of urine formation. So, through this process all of the nephrons the functional units are producing urine all the time and releasing that urine into the renal pelvis and then storing this through the ureters into the urinary bladder and when urinary bladder is filled then the reflex is the reflex arises due to which this is released through the external opening called the urethra. The urinary bladder when it is filled it gives rise to a reflex. This reflex is actually there are two controls upon this urinary bladder and its path towards urethra. One is involuntary that is when it is filled it is activated and gives a reflex but the other part is muscular. As we know that the urination is not controlled in the infants. This is an instinct which is learned. Later on in life a human being a child even quite a toddler can learn to consciously control the urination. But in infants this is not yet learned. So, in infants this is not under the conscious control.