Blood transports essential substances like oxygen and nutrients to every cell. It also collects waste products generated by cellular activity. To maintain health, blood undergoes continuous purification, removing harmful substances and excess water while retaining beneficial components.
Initial Blood Separation
Blood purification begins in the kidneys, bean-shaped organs positioned below the rib cage. Each kidney contains approximately one million tiny filtering units called nephrons. Blood enters these nephrons through a network of small blood vessels, reaching a cluster of capillaries known as the glomerulus.
In the glomerulus, blood undergoes initial filtration. The thin walls of these capillaries act like a sieve, allowing smaller molecules, waste products, and water to pass into an adjacent tubule, forming a fluid called filtrate. Larger components such as red blood cells, white blood cells, platelets, and large proteins remain in the bloodstream. The kidneys filter about 150 to 180 quarts of blood daily.
Processing the Filtered Fluid
The fluid initially filtered, known as filtrate, contains waste products and many substances the body needs, such as water, glucose, amino acids, and essential salts. This filtrate enters renal tubules, where a two-step process of reabsorption and secretion takes place. This system ensures the body reclaims valuable substances while actively removing additional waste.
Reabsorption is the process by which the body selectively takes back beneficial substances from the filtrate and returns them to the bloodstream. For instance, most filtered water, glucose, and amino acids are reabsorbed, along with electrolytes like sodium, potassium, calcium, and bicarbonate. This reabsorption occurs through both passive movement and active transport mechanisms that require cellular energy.
Conversely, secretion involves the active removal of waste products and excess ions from the blood into the filtrate within the tubules. Substances like urea, creatinine, hydrogen ions, and various drugs are actively secreted for their efficient elimination from the body. This selective reabsorption and secretion allows the kidneys to regulate the body’s fluid volume, electrolyte balance, and acid-base levels, fine-tuning the fluid’s composition before it becomes urine.
Return to Circulation
While the filtrate undergoes processing in the renal tubules, the blood that was not initially filtered flows through a network of tiny blood vessels surrounding the nephrons. These peritubular capillaries retrieve the reabsorbed water and essential solutes, returning them to the body’s general circulation.
This purified blood, cleansed of waste and replenished with necessary substances, exits the kidneys through large blood vessels called renal veins. Each kidney has its own renal vein, which merges to drain into the inferior vena cava, a major vein that carries blood back to the heart. From the heart, this clean blood is then pumped throughout the body, ready to deliver oxygen and nutrients, and continue its vital functions.
Excretion of Waste Products
The fluid remaining in the renal tubules, after reabsorption and secretion, becomes concentrated urine. This final waste product is collected from various nephrons into collecting ducts. From the collecting ducts, the urine flows into the renal pelvis, a funnel-shaped structure, before moving into the ureters.
The ureters are two muscular tubes that transport the urine from each kidney to the bladder through rhythmic contractions. The bladder serves as a storage reservoir for urine, expanding as it fills. When the bladder is sufficiently full, urine exits the body through the urethra. Urine is primarily composed of water (91% to 96%), along with waste products such as urea, uric acid, creatinine, and excess salts and electrolytes. This excretion is crucial for maintaining the body’s overall fluid and electrolyte balance.