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What is Nephron?

A nephron is the basic structural and functional unit of the kidney. They are microscopic structures composed of a renal corpuscle and a renal tubule. There are millions of nephrons in each human kidney.

Structure of Nephron

The structure of the nephron comprises two major portions:

  1. Renal Tubule
  2. The Renal Corpuscle

 Nephron of Structure

Renal Tubule

The renal tubule is a long and convoluted structure that emerges from the glomerulus and can be divided into three parts based on function.

  • The first part is called the proximal convoluted tubule due to its proximity to the glomerulus; it stays in the renal cortex.
  • The second part is called the loop of Henle, or the nephritic loop because it forms a loop that goes through the renal medulla.
  • The third part of the renal tubule is called the distal convoluted tubule and this part is also restricted to the renal cortex.

The major function of tubules is reabsorption and the process can either be through active transport or passive transport. In addition, secretions by tubules help in urine formation without affecting the electrolyte balance of the body.

  • Proximal Convoluted Tubule (PCT): The blood brought by the renal artery is filtered by the glomerulus and then passed to the PCT. Maximum reabsorption takes place in the PCT of the nephron. PCT is the region of the renal tubule where reabsorption of essential substances like glucose, proteins, amino acids, a major portion of electrolytes and water takes place. Reabsorption takes place at the expense of energy, i.e., the process is active. PCT selectively secretes ions such as hydrogen, ammonia, and potassium into the filtrate and absorbs HCO3from it. Thus, PCT maintains the electrolyte and acid-base balance of the body fluids.
  • Henle’s Loop: Henle’s loop has a descending and an ascending limb. Being parts of the same loop, both the descending and ascending limbs show different permeability. The descending limb is permeable to water but impermeable to an electrolyte, while the ascending limb is permeable to electrolytes but impermeable to water. Since the electrolytes get reabsorbed at the ascending loop of Henle, the filtrate gets diluted as it moves towards the ascending limb. But reabsorption is limited in this segment.
  • Distal Convoluted Tubule (DCT): The DCT, which is the last part of the nephron, connects and empties its contents into collecting ducts that line the medullary pyramids. Similar to PCT, DCT also secretes ions such as hydrogen, potassium, and NH3 into the filtrate while reabsorbing the HCO3from the filtrate. Conditional reabsorption of sodium ions and water takes place in DCT. Thus, it maintains the pH and sodium-potassium level in the blood cells.
  • Collecting Duct: The collecting duct is a long, straight tube where H+ and K+ ions are secreted to maintain the electrolyte balance of the blood. This is also the region where the maximum reabsorption of water takes place to produce concentrated urine.

Renal Corpuscle

It consists of a glomerulus surrounded by a Bowman’s capsule. The glomerulus arises from an afferent arteriole and empties into an efferent arteriole. The smaller diameter of an efferent arteriole helps to maintain high blood pressure in the glomerulus.

The Bowman’s capsule is divided into three layers:

  1. Outer Parietal layer: It is made up of epithelial cells with minute pores of diameter 12nm.
  2. Middle Basement membrane: This layer is selectively permeable.
  3. Inner Visceral Layer: It consists of large nucleated cells called podocytes which bear finger-like projections called podocel.

Types of Nephron
There are two types of the nephron:

  • Cortical nephron: These are the nephrons present within the cortex. These are short and comprise about 80% of the total nephrons.
  • Juxtamedullary nephron: These have long loops of Henle and extend into the medulla. These are about 20%.

Functions of Nephron

The nephron is the main and basic structural and functional unit of kidneys. They regulate the water present in our body and also the soluble substances that are present in the blood. This is done by filtering the blood and reabsorbing all the necessary substances required by the body again, excreting out the waste. The waste is called urine. The nephrons also have their functions in the homeostasis of blood volume and pressure, the osmolarity of the plasma. It is regulated by the neuroendocrine systems present in the body. The hormones are antidiuretic hormone or vasopressin, parathyroid hormones, and aldosterone.

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