Describe the structure and functioning of nephron

The structural and functional units of the kidney are called nephrons. The role of the nephron in kidney is to separate water and other molecules from the blood and help filter wastes and toxic substances and reabsorb the essential molecules back into the blood. 

Structure of Nephron

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What is Nephron?

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Nephrons are the coiled structure of the kidneys and are held tightly by tough fibrous tissues. They are the functional units of kidneys that help in filtering the blood from impurities.

• Nephrons also help in regulating the pH of the blood, and blood pressure, and in controlling electrolytes.
• Typically, a human kidney consists of about 1.2 to 1.5 million nephrons. 
• They are thin tubes (30-35 mm) that consist of a renal corpuscle, renal tubule, and a collecting tubule. 

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Structure of Nephron

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The kidney has nearly 1 million nephrons which are thin tube-like structures of about 1.2 to 2.2 inches. They have a complex structure and are divided into three parts: Renal Corpuscle, Renal Tubule, & Collecting Duct. 

Renal Corpuscle

Renal corpuscles, also called malpighian bodies, constitute Bowman’s Capsule and Glomerulus. Together they form a cortex region where the formation of urine takes place. The renal corpuscle is also called the first filtration component of the nephron.

• Bowman’s capsule is a cup-shaped sac that envelops or surrounds the glomerulus.
• The glomerulus is a network of very fine blood capillaries responsible for filtering blood in the kidneys.
• In the initial stage, the blood passes through the glomerulus and the waste products and excess components are filtered by Bowman’s capsule.

Renal Tubule

A long tubule attached to the bowman’s capsule is called a renal tubule. It is categorized into three main components: Proximal Convoluted Tubule (PCT), Loop of Henle, & Distal Convoluted Tubule (DCT).

Proximal Convoluted Tubule (PCT)

Proximal Convoluted Tubule or PCT is the most important part of the nephron. It is lined with columnar epithelial cells that help in increasing the surface area for absorption.

• The major role of PCT is to maintain the fluids present in our body.
• It receives the filtrate from the Bowman’s capsule.
• It is that part of the nephron where substances like glucose, protein, and amino acids, are reabsorbed.
• Through tubular secretion, PCT also secretes hydrogen, ammonium, and potassium into the filtrate and absorbs HCO3- from it.

Henle’s Loop

As the name suggests, Henle’s loop is a U-shaped structure of the renal tubule responsible for maintaining the fluid and ion balance.

• These are of two types: The descending limb and the ascending limb of the loop of Henle.
• The descending limb is water-permeable and helps in the reabsorption of filtrate.
• The ascending limb only allows ion reabsorption because it is not permeable in water.

Distal Convoluted Tubule (DCT)

The Distal Convoluted Tubule is located next to the glomerulus. It is somewhat similar to PCT as it secretes ions into the filtrate.

• They help in maintaining blood pH and levels of sodium and potassium salts.
• DCT is the final segment of the nephron and passes its content to the collecting duct.

Collecting Duct

The last part of the nephron is the collecting duct which receives the filtrate from the renal tubules.

• It is a long tube that is lined with the cuboidal epithelium.
• After passing through all parts of nephron, the filtrate enters into the collecting tube where it is called urine.
• This duct reabsorbs water and ions from the urine thereby concentrating the urine.

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Types of Nephrons

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There are two types of nephrons present that differ in structure and function, these are:

1. Cortical Nephrons
2. Juxtamedullary Nephrons

Cortical Nephrons

Cortical Nephrons are those which are present in the outer layer of the kidneys called the cortex.

• Such types of nephrons contain a short Henle loop that extends only to the outer renal medulla.
• The human kidney comprises 85% of the total nephrons.

Juxtamedullary Nephrons

Juxtamedullary Nephrons have a long Henle loop that extends deeply in the renal medulla which helps to efficiently concentrate urine.

• These are commonly found in animals such as birds.
• Only 15% of the human kidney contains this nephron. 

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Functions of Nephron

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Nephrons perform several functions that are essential in maintaining internal health of the body. Some of these functions are given below:

• Nephrons play an essential role in filtering wastes, toxic substances, and other compounds like glucose, water, salt, etc from the blood and form a filtrate.
• To maintain overall health, they also function in reabsorbing essential components like amino acids, water, glucose, etc.
• They also help in regulating the pH and balancing electrolytes in the body, by secreting substances like hydrogen and potassium ions into the filtrate. 
• All the excess components and waste materials are excreted from the body in the form of urine. 
• Nephrons also help in regulating blood pressure and help in maintaining the fluid balance in the body. 

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Things to Remember

• Nephrons produce an enzyme called renin which helps in controlling blood pressure.
• They release erythropoietin, a hormone that aids in promoting the creation of red blood cells.
• These structures' main purposes are excretion, reabsorption, and filtering.
• Tubular reabsorption is the process by which necessary substances (proteins, glucose, etc.) are reabsorbed.
• Nearly 75% of the water in the urine is taken up again by the collecting duct.