Kidney Parts and Functions at a Glance
Each kidney is a compact organ made of repeating functional units that filter blood, regulate fluids, and maintain chemical balance. Understanding kidney parts and functions helps explain how the body manages waste, blood pressure, and electrolyte levels every minute of every day.
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Gross Anatomy: Cortex, Medulla, and Pelvis
A kidney has three main regions visible on cross-section. The outer cortex contains the filtering units and early tubular segments. The inner medulla houses the loops of Henle and collecting ducts arranged in conical pyramids. The renal pelvis sits at the center, collecting urine before it passes into the ureter.
- Cortex: contains glomeruli and proximal tubules
- Medulla: concentrates urine via countercurrent systems
- Pelvis: funnel for urine drainage
Nephrons, the Working Units
Each kidney contains roughly one million nephrons, the microscopic structures that perform the core work. A nephron has two principal parts: the renal corpuscle and the renal tubule.
Renal Corpuscle
The corpuscle includes the glomerulus, a tuft of capillaries, and Bowman's capsule, which surrounds it. Blood pressure forces water and small solutes out of the glomerular capillaries into Bowman's space, producing a filtrate that will become urine.
Renal Tubule
The tubule extends from Bowman's capsule and includes the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. Along its length, the tubule reabsorbs most of the filtered water, glucose, amino acids, and ions, while secreting additional waste products into the lumen.
Blood Supply and Filtration
The renal artery delivers blood to each kidney, branching into segmental, interlobar, arcuate, and interlobular arteries before reaching the glomeruli. The efferent arteriole carries blood away from the glomerulus and forms a second capillary network, the peritubular capillaries, which surrounds the tubules and supports reabsorption and secretion.
| Structure | Primary Role | Context |
|---|---|---|
| Glomerulus | Filtration of blood plasma | Driven by hydrostatic pressure |
| Proximal tubule | Reabsorption of nutrients, ions, water | Highly active metabolically |
| Loop of Henle | Creates medullary osmotic gradient | Enables concentrated urine |
| Distal tubule | Fine-tuning of electrolyte balance | Responsive to aldosterone, ADH |
| Collecting duct | Final water reabsorption, acid-base regulation | Influenced by ADH and aldosterone |
Hormonal Regulation and Kidney Functions
The kidneys respond to hormones that adjust their output. Antidiuretic hormone (ADH) increases water reabsorption in the collecting ducts, concentrating urine. Aldosterone promotes sodium reabsorption and potassium secretion in the distal tubule. Renin release from the kidney helps control blood pressure through the renin-angiotensin-aldosterone system.
Waste Removal and Urine Formation
Kidney parts work together in three steps to form urine: glomerular filtration, tubular reabsorption, and tubular secretion. Filtration produces a protein-free ultrafiltrate. Reabsorption returns essential substances to the blood. Secretion moves additional waste, drugs, and ions from the peritubular capillaries into the tubular fluid. The result is urine, which drains into the renal pelvis and exits via the ureter.
Electrolyte and Acid-Base Balance
The kidneys regulate sodium, potassium, calcium, phosphate, and bicarbonate levels. By adjusting how much of each electrolyte is reabsorbed or excreted, they help maintain fluid volume, blood pressure, and a stable pH. Hydrogen ions are secreted, and bicarbonate is reclaimed or generated to buffer the blood.
Additional Kidney Functions
Beyond filtration, the kidneys produce erythropoietin, which stimulates red blood cell production, and activate vitamin D for calcium absorption. They also contribute to gluconeogenesis during prolonged fasting and help clear metabolic waste products such as urea and creatinine from the bloodstream.