The nephron, also sometimes called the uriniferous tubule, is the fundamental functional unit in the kidneys. It is in the nephrons that urine is separated from blood, and some of the water and salts in it are reabsorbed. It is responsible for maintaining the pH and temperature of the bloodstream. It is what makes kidneys one of the five vital organs, along with the heart, the brain, the lungs, and the liver; without healthy nephrons, urine may not be formed at all, and nitrogenous wastes would build up inside the body. Alternately, if the process of reabsorption broke down, we would lose vital ionic salts and glucose at a dangerous rate. Due to this, healthy functioning of the kidneys is essential for life.
Did You Know?Marcello Malpighi, a seventeenth-century Italian anatomist, discovered and described the renal pyramid and the renal corpuscle. In honor of Malpighi, the two were named Malpighian pyramid and Malpighian corpuscle, respectively. This nomenclature is now only rarely in use, and the term 'renal' is considered standard.
The human kidney contains more than 1 million nephrons. Numbers as low as 800,000 are still considered healthy, and up to 1.5 million may be present.
Nephrons are found in structures known as renal pyramids, located in the outward (from the center of the body) curve of either kidney. The location of renal pyramids within the kidneys is shown in the adjoining figure.
Location of the renal pyramids
Each renal pyramid contains thousands of nephrons. Minute blood capillaries reach each and every one of them, and excess water and some waste is extracted from the bloodstream in each one. The urine formed in the individual nephrons is collected in the collection duct of the renal pyramid. The ducts of every individual renal pyramid empty into the renal pelvis, which leads to the ureter, which connects the kidneys to the urinary bladder.
Most of the nephron lies in the outer region of the kidney: the renal cortex. Only one part, the loop of Henle, enters the central part of the kidney, the renal medulla.
Considering that a whole kidney takes up only approx. 20 cubic inches, and that the one million nephrons in it are only found in about three quarters of that, you can only imagine the minuscule scale of this process.
Internal structure of nephrons
A nephron is made up of two parts: The renal corpuscle and the renal tubule.
► The renal corpuscle, also called the Malpighian corpuscle, is made up by the glomerulus and the Bowman's capsule. This is where water, glucose, and some ionic salts are filtered from the bloodstream. The filtrate formed in this part of the nephron later undergoes a process of reabsorption to form urine.
The glomerulus is simply a mesh of tiny blood capillaries contained in the Bowman's capsule. The afferent arteriole branches to form the glomerulus in the Bowman's capsule, and, with the filtration process done, rejoins to form the outgoing efferent arteriole. The efferent arterioles from various nephrons later rejoin to form the renal vein.
The efferent arteriole is slightly narrower than the afferent arteriole; combined with the arterial blood pressure, this hydrostatic pressure causes smaller molecules such as water, glucose, and salts to be filtered out of the arteriole and into the space created by the Bowman's capsule. Special epithelial cells on the inner surface of the Bowman's capsule, called podocytes, prevent the extraction of larger molecules such as proteins. This primary filtering process forms what is known as the glomerular filtrate.
The renal tubule is responsible for the reabsorption of glucose, salts, and some water. As the name indicates, the renal tubule is shaped like a tube. The regions where specific substances are reabsorbed are given in the diagram below.
Reabsorption zones of the renal tubule
The two convoluted tubules are so named due to their tangled (convoluted) nature. The 'proximal' and 'distal' part signifies the relative distance of each from the renal corpuscle, 'proximal' meaning closer and 'distal' meaning farther.
Due to the differing nature of cells along the renal tubule, the function of the various zones is different. The proximal convoluted tubule and the descending arm of the loop of Henle features passive reabsorption by simple osmosis (natural flow of a solvent from a lower concentration of the solute to a higher concentration of the solute). The descending arm of the loop of Henle absorbs water but leaves salts in the solution. The ascending arm, on the other hand, doesn't allow the passage of water, but absorbs salts.
The distal convoluted tubule, unlike the proximal, has mitochondria (power plants of cells, in simple words) that allow active filtration. Various ions, such as Na+, K+, and Cl- are reabsorbed into the bloodstream here.
The efferent arteriole, which exits the glomerulus having deposited the glomerular filtrate, remains close to the renal tubule in the form of vasa recta, and the reabsorbed substances are integrated into the bloodstream in this fashion.
The collecting duct allows the absorption of water when the antidiuretic hormone (ADH) is present. If ADH is present, more than half of the water in urine can be reabsorbed in the collecting duct.
Through nephrons, kidneys are responsible for maintaining the viscosity, and pH of the bloodstream, and thus, the fluid balance in the body. As mentioned before, this makes it one of the five vital organs.