Describe how urine is formed in the nephron through filtration, reabsorption and secretion.

Urine formation in the nephron, the functional unit of the kidney, involves three main processes: filtration, reabsorption, and secretion.

Filtration:
Filtration occurs in the renal corpuscle, which consists of the glomerulus and Bowman's capsule. Blood enters the glomerulus through the afferent arteriole and exits via the efferent arteriole. The high pressure in the glomerulus forces water, ions, and small molecules such as glucose, amino acids, urea, and electrolytes out of the blood and into the Bowman's capsule. This fluid, called filtrate, contains waste products as well as essential substances that the body needs to reabsorb.

Reabsorption:
Reabsorption occurs primarily in the renal tubules, which include the proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting ducts. As the filtrate passes through these tubules, various substances are selectively reabsorbed back into the bloodstream to maintain the body's homeostasis. For example:

The PCT is highly permeable and reabsorbs most of the filtered water, electrolytes, glucose, amino acids, and other essential substances back into the bloodstream.
The loop of Henle creates a concentration gradient in the medulla of the kidney, facilitating the reabsorption of water and electrolytes.
The DCT and collecting ducts further fine-tune the reabsorption of water and electrolytes under the influence of hormones such as aldosterone and antidiuretic hormone (ADH).
Secretion:
Secretion involves the active transport of additional waste products and excess substances from the blood into the renal tubules. These substances include ions (e.g., potassium, hydrogen ions) and certain drugs or toxins that were not adequately filtered in the glomerulus. Secretion primarily occurs in the DCT and collecting ducts and helps to further regulate the composition of urine.

As the filtrate passes through the nephron, the processes of filtration, reabsorption, and secretion work together to produce urine, which is eventually drained into the renal pelvis, then into the ureter, and finally into the bladder for storage until elimination through the urethra.