Kidney Structure

The kidneys are two bean-shaped organs located under the diaphragm in the upper abdomen. They are responsible for osmoregulation, the balancing of water and solutes in blood. The kidneys filter blood several times daily to remove toxins and rebalance water and electrolytes through the renal veins, maintaining homeostasis in the circulatory system.

Blood enters the kidneys through the renal artery at the hilum, the concave side, and flows into smaller arterial branches and capillaries. Within the cortex, the renal corpuscle contains a glomerulus of capillaries surrounded by Bowman's capsule, which filters out nearly all solutes except proteins. This filtration process initiates blood filtration in the nephrons.

The renal tubule contains three main segments: the proximal convoluted tubule (PCT) nearest the corpuscle, the loop of henle with descending and ascending sides in the medulla, and the distal convoluted tubule (DCT). These segments are arranged sequentially to allow selective reabsorption of water and solutes back into surrounding capillaries.

Unabsorbed filtrate passes from the renal tubules into collecting ducts, which join to form medullary pyramids. Each pyramid's apex points toward the hilum and empties filtrate into a renal calyx, which drains into the renal pelvis. The renal pelvis opens into the ureter, allowing urine to pass from the kidney to the bladder for storage.

A nephron is the functional unit of the kidney, composed of the renal corpuscle, proximal and distal convoluted tubules, the loop of henle, and collecting ducts. Each human kidney contains approximately 1 million nephrons that work together to filter blood, reabsorb water and electrolytes, and excrete toxins.

When a person donates a kidney, halving the number of nephrons, the remaining kidney compensates by increasing in size and volume. Donors can lead healthy lives after donation without increased mortality risk. However, some donors may develop high blood pressure as the single kidney must perform the work of two kidneys.

Kidney stones result from solute supersaturation, most commonly of calcium, which crystallizes and aggregates in the loop of henle where calcium is absorbed. If filtrate volume is insufficient, calcium can crystallize as it moves through collecting ducts into the renal pelvis. Most stones pass without symptoms, but large stones blocking the ureter cause substantial pain.