- The basic structural and functional unit of the kidney, also known as the Nephron is a microscopic structure that is made up of both the renal tubule and renal corpuscle.
- The nephron is a Greek word that means kidney and one kidney can have millions of them.
- There are different stages of the development of the nephron. The most primitive one known as pronephros is found in primitive fish, amphibian larvae, and embryos of vertebrates.
- Mesonephrons are found in most fish and amphibians. They are slightly more advance in development than pronephrons.
- Metanephron is the most advanced type of nephron found in adult kidneys of land vertebrates such as mammals, birds, and reptiles.
- In the process of waste removal from blood, the nephron produces urine.
Structure of Nephron
The kidney of a mammal contains a nephron which is a 35-55mm long tube-like structure. At one end of the tube, Bowman’s capsule is present which is formed by closing, expanding, and folding one end of the tube into the double-walled cup-like structure.
It also contains several blood capillaries known as the glomerulus. This capsule and glomerulus combined to form the renal corpuscle.
Nephron structure has 2 major parts:
- Renal tubule
- Renal corpuscle
Renal Tubule of Nephron
A structure that arises from the glomerulus which is long and convoluted in structure is known as a renal tubule. On the basis of its function, it can be divided into 3 major parts:
- Proximal convoluted tubule (PCT): it derived its name from its proximity to the glomerulus.
- Loop of Henle or nephritic loop: it creates a loop-like structuring with descending and ascending limbs and passes through the renal medulla.
- Distal convoluted tubule (DCT): same as PCT, it’s just present at a distance from the glomerulus.
- Bowman’s capsule forms a coiled tubule aka PCT, which further forms a loop of Henle and it extends to DCT, which has its opening into the collecting tube.
- Reabsorption is the major tubule function and it can occur through either active or passive transport. To not affect the balance of electrolytes in the body, urine formation by tubules helps.
a. Proximal convoluted tubule (PCT)
- Maximum reabsorption takes place here
- It passes the blood that is filled in the glomerulus and filtered prior by the renal artery.
- It is a part of renal tubule reabsorption of substances like proteins, amino acids, electrolytes, water, and glucose takes place.
- It is lined by simple cuboidal epithelium which helps in increasing surface area for reabsorption.
- It is an active process.
- Hydrogen, ammonia, and potassium are ions that are selectively secreted by PCT into the filtrate and HCO3 absorb from it. This way, it maintains the electrolyte and acid-base balance of fluids.
b. Loop of Henle
- It has ascending and descending limbs.
- Different permeability is shown by both of them.
- While water is permeable through descenf=ding limb, the electrolyte is impermeable.
- On the other hand, the electrolyte is permeable to ascending limbs, but water is impermeable.
- The filtrate gets diluted because of the reabsorption of electrolytes in the ascending loop of Henle and passes toward ascending limb.
c. Distal convoluted tubule (DCT)
- Multiple nephrons connect to the collecting tube and fuse together and empty their content in it. Further, it enters into the papillae of the renal medulla.
- Just like PCT, it also secreted hydrogen, potassium, and NH3 and passes into the filtrate, and reabsorbs HCO3.
- Maintains the ph and sodium-potassium channel in blood.
To maintain the balance of electrolytes in the blood, hydrogen and potassium are secreted by a long, straight tube known as the collecting duct.to produce urine, it is a place where maximum reabsorption takes place.
B. Renal corpuscle of Nephron
The glomerulus surrounded by a bowman’s capsule forms the renal corpuscle. Glomerulus carries its content through afferent arteriole and takes it to efferent arteriole and exited there. The high blood pressure of the glomerulus is maintained by the small diameter of the efferent arteriole.
The Bowman’s capsule is divided into three layers:
- Outer Parietal layer: epithelial cells make up the outer parietal layer of the bowman’s capsule and it has small pores of diameter 12nm.
- Middle Basement membrane: it is a selectively permeable membrane
- Inner Visceral Layer: it is made up of podocytes which are large nucleated cells and has bear finger-like projections known as podocel.
Types of Nephron
- Cortical nephron: these are short nephron found in the cortex region and is about 80% of total nephrons
- Juxtamedullary nephron: these types of nephrons extended up to the medulla and have a long Henle’s loop. They are 20% of total nephrons.
Functions of Nephron
Since nephron takes care of the entire waste removal process of blood in the body and its basic function is to remove waste from the blood and add any other substance required by blood into it. So ultrafiltration is the filtration of water and other small molecules by collecting it into the tubules of the nephron and then collecting it into the collecting tube before flushing out into the bladder. It does not remove RBC and other important large molecules.
Functioning of the kidney
Since the nephron comprises a special collection of blood capillaries that are essential for kidney functioning, thus nephron helps in urine formation and later collects it into the urinary bladder.
Secretion of glutamate
Glutamate is a neurotransmitter secreted by the nephron required for sending excretory function signals.
Reabsorption and urine formation
- There are many layers present in Bowman’s capsule which separate it from the glomerulus, so among these layers, a membrane is known as the basement layer which is formed from collagen and fibers and creates a mesh-like structure that acts as a sieve to filter the blood.
- But during the filtration process, glucose also gets filtered put and it is again reabsorbed later by the glomerulus.
- The proximal tubule contains all the filtered liquid and it is later reabsorbed using peritubular capillaries.
- Now, the important molecules which were earlier sieved out by the basement membrane are added again.
- An equal amount of water is added to the proximal tubule to balance out this increased solute concentration.
- A distal convoluted tubule collects all the fluid and passes it to the collecting tube.
- Another round of ultrafiltration is carried out into the collecting duct, which drains it into the bladder through the ureter. And from there, urine flushes out.
- Britannica, The Editors of Encyclopaedia. “nephron”. Encyclopedia Britannica, 12 Sep. 2019, https://www.britannica.com/science/nephron. Accessed 30 April 2022.