The basic structural and functional unit of a kidney is the nephron. There are more than eight lakh nephrons in each human kidney. It is made up of renal corpuscles and kidney tubules (renal tubules). The nephrons are the main regulators of the entire waste removal process from the body’s blood. It separates the different components present in the blood in order to filter it. It removes the unwanted toxic substances and waste present in the blood and adds back to the blood required molecules. The process by which the nephron performs this function of waste removal is termed as ultrafiltration.
Ultrafiltration takes place when the water molecules and other smaller molecules are forced out of the capillary walls due to blood pressure. The liquid that is forced out of the pores of the capillary walls is called the ultrafiltrate. It lacks red blood cells and other essential giant molecules. The ultrafiltrate traverses through the tubules of the nephron, then collects into the collecting duct and finally into the bladder. A special set of capillaries are present inside the nephron this is what keeps the kidney processes going. A well developed and functional kidney is found only in vertebrates. The excretory material in amphibians and earlier form of vertebrates is ammonia, in aves and reptiles, it is uric acid which is very toxic as it has higher concentrations of ammonia and in mammals it is urea. In order to prevent water loss or to retain as much water in the body as possible, water molecules present in urea is also absorbed to form concentrated urea in the urine. This process helps in the reabsorption of water from the ultrafiltrate enabling more water retention in the body and hence making it possible for certain animals living in dry conditions to survive. The best example of this is the camel living in the desert regions. Camels can easily filter out most of the water present in the blood and store it and use it again and again.
The nephrons in mammals are extended into aU shaped loop called the loop of Henle. The picture above depicts the loop of Henle and hence we can conclude that it is a mammalian nephron. Other than the loop everything else is structurally similar in other vertebrates. The glomerulus, as mentioned above is a set of or a mesh/net of blood capillaries. This is contained in a capsule structure called the glomerular capsule or Bowman’s capsule. The extension of the Bowman’s capsule is what forms the proximal convoluted tubule or PCT. The proximal convoluted tubule extends to form the loop of Henle which later opens into the distal convoluted tubule. The distal convoluted tubule opens into the collecting duct, from there everything is drained out into the urinary bladder. Every different part of the structure is made of specific cell types whose function is to absorb and retain water and other useful molecules inside the renal tubules. In a vertebrate, a single kidney consists of almost hundreds to millions of nephrons in them and each of them takes part in the process of urine formation and making sure it is collected in the urinary bladder. The arrangement of cells is such that cells with the more concentrated substance are placed at the bottom and ones containing less concentrated substances are placed at the top. The highly concentrated cells reabsorb as much water as possible from the ultrafiltrate before it is drained out into the urinary bladder.
The nephron’s main functions involve the filtration of blood, reabsorption of water and other smaller essential molecules from the ultrafiltrate and also the secretion of glutamate which is a neurotransmitter involved in sending excretory function signals. The Bowman's capsule is lined by different layers which separates it from the glomerulus. One of the layers known as the basement membrane is composed of collagen and fibers made up of glycoproteins. These glycoprotein fibers form a net or mesh which filters blood by the process of ultrafiltration. This mesh acts like a filter paper allowing only small molecules to pass through and not the larger molecules. While filtering the blood, a lot of other important solutes like glucose etc gets filtered out too but they are later reabsorbed from the glomerular filtrate and hence why the process of reabsorption is very important. The proximal tubule contains the filtered liquid and this is reabsorbed in the peritubular capillaries. This is where important solutes that were filtered out earlier get added back into the blood. When this happens the solute concentration in the blood is higher and therefore it has to balance out by absorbing an equal amount of water from the proximal tubule. The fluid now enters the distal convoluted tubule. From the distal convoluted tubule, it enters the collecting duct. In the collecting duct, it undergoes another round of ultrafiltration before draining out into the bladder through the ureters. From the urinary bladder, the next transport of urine is out of the body.
Although, there are many mechanisms performed by the body too removes waste from the entire body, the nephron is the most important unit and plays the most important role in waste removal from body and filtration of the body's blood and for the main organs involved in this entire process are the kidneys and failure of the kidneys to perform their function can lead to fatal disorders. Hence, kidneys are one of the most important organs in the human body.