Filtration, reabsorption, secretion, and excretion are crucial processes carried out by the kidneys to maintain homeostasis in the body.
Filtration: This is the first step in urine formation, which occurs in the glomerulus of the nephron. Blood pressure forces water and small solutes (like glucose, salt, and urea) from the blood into Bowman’s capsule, while larger molecules like proteins and blood cells remain in the bloodstream.
Reabsorption: Following filtration, reabsorption takes place primarily in the proximal convoluted tubule, where essential nutrients, water, and electrolytes are reabsorbed back into the bloodstream. For instance, glucose and amino acids are almost completely reabsorbed, ensuring that these vital nutrients are not lost.
Secretion: This process occurs mainly in the proximal and distal convoluted tubules, where additional waste products and excess ions (like potassium and hydrogen ions) are secreted from the blood into the tubular fluid. Secretion helps regulate acid-base balance and remove substances that were not filtered initially.
Excretion: Finally, excretion is the process of eliminating waste products from the body through urine. What remains in the tubular fluid after filtration, reabsorption, and secretion is collected in the renal pelvis and funneled into the ureter, eventually leading to the bladder.
In summary, filtration brings substances into the nephron, reabsorption retrieves useful substances, secretion adds more waste items, and excretion is the final removal of waste.