From the Guidelines
The nephron's segments, including the glomerulus, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct, perform distinct functions in urine formation through specialized processes of filtration, reabsorption, secretion, and excretion. The glomerulus handles filtration, where blood pressure forces water and small solutes through the glomerular membrane into Bowman's capsule, while retaining blood cells and proteins 1. The key differences between each segment are:
- Filtration: primarily occurs in the glomerulus
- Reabsorption:
- Proximal convoluted tubule: massive reabsorption of about 65% of filtrate including glucose, amino acids, sodium, and water through active and passive transport mechanisms
- Loop of Henle: creates a concentration gradient in the medulla, with the descending limb allowing water to exit while the ascending limb actively pumps out sodium and chloride but is impermeable to water
- Secretion: primarily occurs in the proximal and distal tubules, where substances like hydrogen ions, potassium, and medications are actively transported from blood into the tubular fluid for elimination
- Excretion: the final step in urine formation, where the collecting duct makes final adjustments to urine concentration under antidiuretic hormone (ADH) control, becoming water-permeable when ADH is present to create concentrated urine, or producing dilute urine when ADH is absent. The distal convoluted tubule fine-tunes electrolyte balance through hormone-regulated sodium reabsorption and potassium secretion, particularly under aldosterone influence 1. It is essential to understand these distinct functions to appreciate the complexity of kidney function and the importance of each segment in maintaining proper electrolyte balance, acid-base balance, and overall fluid homeostasis.
From the Research
Nephron Segments and Their Functions
The nephron is the basic structural and functional unit of the kidney, responsible for maintaining the body's internal environment through the processes of filtration, reabsorption, and secretion 2, 3, 4. The key differences between each segment of the nephron are:
- Filtration: The glomerulus acts to filter the blood, producing an ultrafiltrate composed of smaller circulating elements 2.
- Reabsorption: The tubule is highly specialized at various segments to remove substances from the tubular fluid, keeping the organism in balance in terms of water, minerals, electrolytes, and hydrogen ion concentration 2, 3, 4.
- Secretion: The tubule also adds substances to the tubular fluid, eliminating toxic substances produced by the body 2, 3, 4.
- Excretion: Unwanted fluid and substances are excreted from the body as urine 3, 4.
Segment-Specific Functions
Each segment of the nephron has specific functions, including:
- Glomerulus: filtration of blood to produce ultrafiltrate 2
- Proximal convoluted tubule: reabsorption of glucose, amino acids, and other substances 2
- Loop of Henle: concentration and dilution of urine 2
- Distal convoluted tubule: reabsorption of calcium and magnesium, and secretion of hydrogen ions 2
- Collecting duct: regulation of water and electrolyte balance, and excretion of urine 2
Regulation of Body Fluid and Solutes
The regulation of body fluid and solutes is governed by the processes of filtration, reabsorption, and secretion, which are critical for maintaining the body's internal environment 3, 4. Dysfunction of these processes can lead to electrolyte and acid-base disturbances, such as hyperkalemia, metabolic acidosis, and hypocalcemia 5.