The kidneys are bean-shaped organs, about the size of an adult fist, located on either side of the spine, just below the rib cage. Their main job involves filtering waste products and excess fluid from the blood, returning the cleaned blood to the body. This filtering process is continuous, with kidneys processing approximately 200 quarts of fluid from the blood daily, of which about 2 quarts become urine.
Beyond filtration, kidneys regulate blood pressure, aid in red blood cell production, and balance important vitamins and minerals. They also regulate body fluid levels and control body chemistry by balancing salts, water, and other chemicals.
External Features and Entry Point
The kidney is encased by a tough, fibrous layer known as the renal capsule, providing a protective barrier against physical trauma and infection. This smooth, transparent outer covering helps maintain the kidney’s shape and integrity.
On the medial side of the kidney, there is a prominent indentation called the hilum. This concave region serves as the entry and exit point for several structures connected to the kidney. The renal artery, which supplies oxygenated blood, enters here, while the renal vein, carrying filtered blood away, exits. The ureter, a tube that transports urine from the kidney to the bladder, also emerges from the hilum.
The Kidney’s Main Internal Zones
A frontal section of the kidney reveals two distinct internal regions: the renal cortex and the renal medulla. The renal cortex is the lighter, outermost layer, located directly beneath the renal capsule. This region is where the initial filtration of blood takes place, containing millions of tiny filtering units.
Beneath the cortex lies the renal medulla, which appears darker and more striated. The medulla is composed of 8 to 18 cone-shaped structures called renal pyramids. These pyramids have a broad base facing the cortex and a pointed apex directed inward. Extensions of the renal cortex, known as renal columns, project down between these pyramids, further dividing the medulla and providing structural support.
The Urine Collection System
Collection and drainage occur through a series of specialized structures. At the apex of each renal pyramid, urine flows from the collecting ducts into a structure called the renal papilla. From the renal papilla, urine then drains into small, cup-shaped structures called minor calyces.
Each minor calyx collects urine from a single renal papilla. Several minor calyces converge to form larger structures known as major calyces. Two or three major calyces are present within each kidney, collecting urine from multiple minor calyces.
These major calyces then unite to form the renal pelvis, a large, funnel-shaped chamber located within the hilum of the kidney. The renal pelvis acts as a reservoir for urine collected from the major calyces. This funnel-shaped structure narrows progressively as it extends out of the hilum, ultimately forming the ureter. The ureter is a muscular tube that transports urine to the urinary bladder for storage and elimination.
Major Blood Vessels
The kidney receives a substantial blood supply for its continuous filtering function. The renal artery, a large vessel branching directly from the aorta, carries oxygenated blood to the kidney. Upon entering the hilum, the renal artery divides into several segmental arteries. These segmental arteries further branch into interlobar arteries, which ascend through the renal columns, coursing between the renal pyramids.
At the junction of the renal cortex and renal medulla, the interlobar arteries give rise to the arcuate arteries. These vessels curve over the bases of the renal pyramids, forming arches. From the arcuate arteries, smaller interlobular arteries extend into the renal cortex, supplying blood to the filtering units.
After filtration, filtered blood is collected by a corresponding network of veins. Interlobular veins drain into arcuate veins, which then merge to form interlobar veins. These interlobar veins converge to form the renal vein, which empties into the inferior vena cava, returning the cleaned blood to the systemic circulation.