Describe the structure and functioning of nephrons.

Nephrons are the functional units of the kidneys, responsible for the filtration of blood and the formation of urine. Each kidney contains about one million nephrons. The structure of a nephron is highly specialized and consists of several components, each with a specific function in the process of urine formation.

Structure of a Nephron:
1. Renal Corpuscle:

» Bowman’s Capsule: The nephron begins with a double-walled, cup-shaped structure called Bowman’s capsule. It surrounds a cluster of capillaries called the glomerulus.

» Glomerulus: A network of tiny blood vessels where filtration of the blood occurs. Blood is pushed into the Bowman’s capsule, along with small solutes and water, forming the filtrate.

2. Renal Tubule:

» Proximal Convoluted Tubule (PCT): The filtrate enters the PCT, where most of the reabsorption of water, ions, and nutrients back into the blood occurs.

» Loop of Henle: The renal tubule descends into the medulla, makes a hairpin turn (the loop), and ascends back toward the cortex. The loop plays a crucial role in concentrating urine by creating a concentration gradient in the medulla.

» Distal Convoluted Tubule (DCT): The remaining filtrate enters the DCT, where further selective reabsorption and secretion take place.

» Collecting Duct: The DCT connects to the collecting duct, which receives urine from multiple nephrons. The collecting ducts merge and deliver urine to the renal pelvis.

Functioning of a Nephron:
1. Filtration:

» Blood from the renal artery enters the glomerulus, and the high pressure in the glomerulus forces water, ions, and small solutes (filtrate) into Bowman’s capsule. Larger particles, such as blood cells and proteins, are usually not filtered.

2. Reabsorption:

» As the filtrate moves through the renal tubule, the proximal convoluted tubule reabsorbs the majority of filtered water, glucose, ions, and other essential substances back into the blood. This reabsorption occurs through active transport and passive diffusion.

» The loop of Henle is responsible for creating a concentration gradient in the medulla, allowing for the reabsorption of water in the collecting duct.

3. Secretion:

» The distal convoluted tubule selectively secretes additional substances (e.g., hydrogen ions, drugs) from the blood into the filtrate to be excreted in the urine.

4. Concentration and Dilution:

» The loop of Henle plays a crucial role in concentrating urine. As the filtrate descends into the medulla, water is reabsorbed, making the urine more concentrated.

The collecting duct adjusts the final concentration of urine based on the body’s hydration needs. Antidiuretic hormone (ADH) regulates water reabsorption in the collecting duct, influencing urine concentration.

5. Excretion:

» The final urine, now concentrated and containing waste products, is transported through the collecting ducts to the renal pelvis. From there, it flows into the ureter and is eventually eliminated from the body through the urethra.
Overall, the nephron’s intricate structure and functions ensure the maintenance of water and electrolyte balance, acid-base balance, and the elimination of waste products from the body through the formation of urine.