The kidneys contain roughly one million microscopic units called nephrons that filter waste from the blood. These structures are the functional component of the kidneys. Each day, the entire blood volume in the human body is filtered multiple times, a process that removes waste and helps maintain a stable internal environment.
Anatomy of a Nephron
The nephron’s structure consists of two primary parts: the renal corpuscle and the renal tubule. The process begins in the renal corpuscle, located in the kidney’s outer region, the renal cortex. This structure is composed of a tangled cluster of capillaries called the glomerulus, enclosed by a cup-shaped sac known as Bowman’s capsule. Blood enters the glomerulus through a small artery, where initial filtration occurs.
From the renal corpuscle, the filtered fluid, now called filtrate, moves into the renal tubule. This long, coiled tube has several segments, starting with the proximal convoluted tubule. It then leads into the Loop of Henle, a U-shaped segment that dips into the renal medulla. After the loop, the filtrate passes into the distal convoluted tubule, also in the cortex, before entering a collecting duct.
The Three Steps of Filtration
The nephron produces urine by filtering blood in three steps. The first is glomerular filtration, which takes place in the renal corpuscle. Here, blood pressure forces water, small solutes, and waste from the glomerulus into Bowman’s capsule. The filtration membrane is selective, preventing larger components like red blood cells and proteins from passing through, ensuring they remain in the bloodstream. This resulting fluid, or filtrate, will eventually become urine.
The second step is tubular reabsorption, which occurs as the filtrate flows through the renal tubule. The body reclaims necessary substances from the filtrate before they are lost as urine. The cells lining the tubule transport materials such as glucose, amino acids, vitamins, water, and ions back into adjacent capillaries.
The final step is tubular secretion, which works in the opposite direction of reabsorption. Waste products and excess ions are transported from the blood in surrounding capillaries into the renal tubule. Substances like hydrogen ions, potassium ions, and certain drugs are secreted into the filtrate. This final adjustment helps fine-tune blood composition before the fluid, now urine, moves to the collecting ducts.
Systemic Regulatory Roles
Beyond filtering waste, nephrons maintain the body’s internal balance, or homeostasis. By adjusting the amount of water and salt reabsorbed, nephrons help regulate blood volume and blood pressure. When blood pressure drops, the kidneys produce an enzyme called renin. This initiates the renin-angiotensin system, a hormonal cascade that leads to increased water and sodium retention to raise blood pressure.
Nephrons also maintain the balance of electrolytes in the blood, such as sodium, potassium, and calcium. They control the reabsorption and secretion of these ions to keep them in an optimal range for cellular function. Nephrons also impact blood pH. They manage the acid-base balance by secreting excess hydrogen ions (H+) into the urine and reabsorbing bicarbonate into the blood to prevent it from becoming too acidic or alkaline.
When Nephrons Are Damaged
Nephrons can be damaged by conditions like chronic high blood pressure and diabetes. High blood sugar from uncontrolled diabetes can damage the small blood vessels in the glomeruli, impairing filtration. Sustained high blood pressure can also exert excessive force on these structures, leading to physical damage.
This damage is significant because nephrons have a limited ability to regenerate. Once a significant number of nephrons are destroyed, the kidneys’ overall filtering capacity diminishes, a condition that can progress to chronic kidney disease. As the condition worsens, the decline in function can lead to a buildup of toxins in the body. This may eventually require medical interventions like dialysis or a kidney transplant.