Nephron

● Nephron Definition

Nephrons are the functional units of the kidneys and are responsible for the filtration, reabsorption, and secretion processes that help regulate the body's fluid balance, electrolyte levels, and waste elimination. The renal tubules are a crucial component of nephrons and play a significant role in these processes.

●Functions of Nephron

The primary function of nephron is removing all waste products including the solid wastes, and other excess water from the blood, converting blood into the urine, reabsorption, secretion, and excretion of numerous substances.

As the blood passes through the glomerulus with high pressure, the small molecules are moved into the glomerular capsules and travel through a winding series of tubules.

The cell present in each tube absorbs different molecules excluding the glucose, water, and other beneficial molecules which are called as the ultrafiltrate. As the ultrafiltrate molecules travel down the tubules they become more and more hypertonic, which results in more amount of water to be extracted from the ultrafiltrate before it exits the nephrons.

●Diagram of Nephron

● Structure of Nephron

The mammalian nephron is a long tube-like structure, its length varying from 35–55 mm long. At one end, the tube is closed, folded and expanded, into a double-walled, a cuplike structure called the Bowman’s capsule or renal corpuscular capsule, which encloses a cluster of microscopic blood vessels called the glomerulus. This capsule and glomerulus together constitute the renal corpuscle.

The renal tubules consist of several distinct segments, each with specific functions:

1. Glomerular Capsule (Bowman's Capsule): This is where the filtration process begins. Blood is filtered in the glomerulus, a tuft of capillaries, and the filtrate (fluid containing water, electrolytes, and waste products) enters Bowman's capsule. Large molecules like proteins and blood cells are too big to pass through and are retained in the bloodstream.

2. Proximal Convoluted Tubule (PCT): Here, most of the reabsorption occurs. Various substances, such as glucose, amino acids, ions, and the majority of water, are actively or passively reabsorbed from the filtrate back into the bloodstream. This is an essential step in maintaining electrolyte and fluid balance.

3. Loop of Henle: The loop is divided into descending and ascending limbs. It plays a role in creating a concentration gradient in the interstitial fluid surrounding the nephron, which is crucial for the reabsorption of water and electrolytes.

4. Distal Convoluted Tubule (DCT): Further fine-tuning of electrolyte reabsorption and secretion occurs here. The DCT responds to hormones like aldosterone and antidiuretic hormone (ADH), which help regulate sodium and water balance.

5. Collecting Duct: This is where the final adjustments in urine concentration occur. The collecting duct responds to ADH, allowing it to reabsorb more water, concentrating the urine and conserving water in the body. The degree of water reabsorption in the collecting ducts determines the final concentration of urine.

● Note:

Though excretion in human beings takes place through lungs, skin, liver, the kidneys are the main organs of the human excretory system. They are bean-shaped organs, which weigh between 150 to 170 gms and their length ranges from 4 – 5 inches long.