23.2: Kidney Structure
The kidneys are two large bean-shaped organs located in the upper abdomen. They filter the blood several times a day to remove toxins and rebalance water and electrolytes of the circulatory system via the renal veins. The kidneys receive blood directly from the heart via the renal arteries. These arteries enter the kidney at the hilum, the concave surface of the bean, where they branch and divide into smaller vessels and capillaries.
The renal cortex is the thick outer layer of the kidney. It houses renal corpuscles, where capillaries come into close contact with the end of a renal tubule. The end of the tubule, or Bowman’s capsule, surrounds a net of capillaries that looks like a ball, the glomerulus. This unusual arrangement of the capillaries increases the surface area where the end of the renal tubule and the capillaries interact. From the Bowman’s capsule, the convoluted tubules extend into the Loop of Henle that lay in the renal medulla, the tissue beneath the renal cortex.
Cortical intrusions structure the medulla into multiple renal pyramids. The apex of each pyramid points towards the hilum area, thus draining the collecting ducts into calyces in the renal pelvis. As the pelvis fills, urine is emptied into the ureter. The ureter connects the kidneys to the bladder, where urine is stored before being eliminated.
The renal corpuscle, proximal and distal convoluted tubules, the Loop of Henle, and collecting ducts form a nephron, the functional unit of the kidney. Nephron filter blood to reabsorb water and electrolytes, and excrete toxins. There are approximately 1 million nephrons in each human kidney.
When a person donates a kidney, thus halving the number of nephrons, the remaining kidney compensates, increasing in size and volume. Donors can lead healthy lives after donation without an increased risk of mortality. However, some donors may develop high blood pressure as the lone kidney has to perform the job of two.
Kidney stones are a result of solute supersaturation—most commonly of calcium, which crystallizes and aggregates. Supersaturation of calcium can occur in the Loop of Henle, where calcium is absorbed. If there is not enough filtrate volume, the calcium can crystallize and aggregate as it moves through the collecting ducts and into the renal pelvis. Most stones pass without symptoms; however if a stone is large enough to block the ureter, it may cause substantial pain.