Kidneys
The kidneys are a paired bean-shaped organ about 11 cm long and weighing 150 g, located retroperitoneally on the sides of the spine at the level of the XII thoracic - III lumbar vertebrae. The right kidney is usually 1–2 cm lower than the left due to pressure from the liver.
Macrostructure
In a longitudinal section the kidneys are visible:
- Cortical substance - outer layer containing glomeruli of nephrons
- Medulla - the inner layer that forms pyramids (8–18 pieces), the tops of which are directed towards the renal pelvis
- Renal pelvis - funnel-shaped cavity that collects urine
At the hila of the kidney the renal artery and nerves enter, the renal vein and ureter exit.
Nephron - functional unit
Each kidney has about 1 million nephrons. Each consists of:
Renal corpuscle (Shumlyansky-Bowman glomerulus) is a capillary glomerulus in a double-walled capsule. Filtration occurs here: water, glucose, amino acids, urea, electrolytes pass into the capsule, and proteins and blood cells remain in the vessel.
Proximal tubule - reabsorbs 65% of water, all glucose, amino acids, most electrolytes
Loop of Henle - creates an osmotic gradient in the medulla necessary for concentrating urine
Distal tubule - fine regulation of Na⁺, K⁺, Ca²⁺ under the influence of hormones
Collecting duct - reabsorption of water under the influence of ADH (antidiuretic hormone)
Filtering numbers
Every day, the kidneys filter about 180 liters of primary urine, but 99% is reabsorbed back. As a result, only 1.5–2 liters of final urine is excreted.
Renal blood flow - 1,200 ml/min (20-25% of cardiac output). During the day, all the body’s blood passes through the kidneys about 300 times.
Hormonal function
The kidneys are not only a filter, but also an endocrine organ:
- Renin - regulation of blood pressure through the renin-angiotensin-aldosterone system
- Erythropoietin - stimulation of red blood cell formation in the bone marrow
- Active form of vitamin D – regulation of calcium metabolism
Chronic kidney disease leads to anemia, hypertension and osteoporosis precisely because these hormonal functions are disrupted.
Would you like to see a cross-section of the kidney - the cortex and medulla, the pyramids and the pelvis? Open 3D atlas Humio and study the structure of the kidney interactively.