There are two kidneys in the human body and it is their job to clean the blood, regulate blood pressure and secrete hormones involved in keeping the blood and bones healthy. It is possible to survive perfectly well with just one kidney but it is not possible to survive with damaged kidneys. The kidneys are situated at the back of the body behind the digestive organs. This location can be referred to as retroperitoneal. The collection of organs in the urinary system also includes the urethra, the bladder and the ureters, whose function is to collect and secrete waste liquid from the body. The kidney comprises of three distinct sections. The renal cortex (the outer layer), the renal medulla (the middle layer) and the renal pelvis (the inner area). The term renal means ‘of the kidney’.
A nephron is the functional unit of the kidney. It is the role of the nephron to produce urine by filtering the blood. There are around one million nephrons in each kidney but they do not work together, the work in isolation as independent filters. The filter lies in an area called the cortex and is called the renal corpuscle. Once filtered the convoluted tubules process the filtrate before collecting the final product in the collecting ducts and then emptying it into the ureters.
There are three main processes involved in producing urine. The first is glomerular filtration, which is the filtering process that creates the substance known as filtrate. This is then processed trough tubular reabsorption and tubular secretion.
Each section of the kidney has its own role and as such has different types of cell to allow this to happen. This diagram shows the structure of the nephron.
The first part of the nephron is the Bowman’s capsule. This is where the first part of urine production takes place. In the Bowman’s capsule the renal artery brings blood into the nephron and then through the afferent arteriole into the capillaries of the glomerulous. The pressure forces a selection of water, ions and molecules into the Bowman’s space from the capillaries and the now cleaner blood leaves via the efferent arterioles. The process is called glomerular filtration and is the most important part of kidney function and the arrangement of glomerulus that allow the process of filtration to be as effective as possible. The pressure in the in the afferent arteriole is a lot higher than in any of the other arterioles in the body, they are also lined with special endothelial cells. They are thin and have many small holes know as fenestrations which are designed to allow small molecules through but not larger molecules like blood cells. The basement membrane is a gel like material and holds a negative charge. This causes a level of repulsion as the molecules in the blood which are also negative in charge. This helps the filtration process by discouraging particles that should be in the blood from filtering out while still allowing unwanted particles through. This is helped by specialised epithelial cells in the Bowman’s space that leave channels that only liquid can pass through. These cells are strange in shape and as such have a special name that means foot cells. They are known as podocytes.
The amount of liquid filtered through glomerular filtration is known as the glomerular filtration rate. For an individual that weighs 70 kg the daily glomerular rate is approximately 180 litres a day which is massive in comparison to the filtration rate of other parts of the body, which is about 4 litres a day.
Once the newly produced filtrate leaves the Bowman’s capsule it enters the proximal convoluted tubule. There are two convoluted tubules separated by the loop of henle. The second convoluted tubule is called the distal convoluted tubule.
The walls of the proximal convoluted tubule are formed by a single layer of epithelial cells. As the filtrate passes through the tubule its composition changes by the reabsorption of certain products back into the blood. Although this reabsorption is heavily controlled some substances such as glucose and organic molecules are not controlled. This is because they are nearly totally reabsorbed and as such their levels are actually maintained during the blood’s passage through the kidneys.
In the proximal convoluted tubule is where the most reabsorption of glucose, amino acids and water occurs. The cells lining the tubules differ in order to allow them to have different functions and absorption rates. The cells in the proximal convoluted tubule are designed to allow the reabsorption of larger molecules such as glucose and amino acids as they have large mitochondria many microvilli increasing their surface area. By the time the filtrate gets to the loop of Henle it needs to facilitate the passive movement of water. The cells are thin and have few microvilli, this is in the thinner descending limb of the loop of henle and osmosis is the filtration method here. By the ascending part of the loop the cells are thicker again because the process changes from osmosis to active transport and signals the start of the secretion process.
It is vital for the body to secrete some of the larger molecules that it was unable to force into the Bowman’s space. This happens in the ascending limb of the loop of Henle and also in the distal convoluted tubule. The main molecules transferred at this point are Hydrogen and Potassium. They use active transport and are attached to sodium reabsorption. Once the filtrate has passed through both convoluted tubules and the loop of Henle it is collected in the various collecting ducts and emptied into the ureters.
It is this multiple processes of absorption and reabsorption that make the kidneys so successful at filtering the waste products of the body. The nephrons are well designed to produce urine because the types of cells are so specialised. The use of a sieve like structure in the renal corpuscle is unique and effective. Then the tubules a designed to carry out the finer sifting afterwards, controlling the reabsorption of different elements dependant on what the body needs. The tubules even strategically pass close to the afferent and efferent arterioles allowing for the transport of larger molecules into the filtrate that are unable to pass through the sieve structure of the Bowman’s capsule. The process is well controlled and repetitive making it highly successful and perfectly suited to urine production.
